Reversible single crystal photochemistry and spin state switching in a metal-cyanide complex

Reversible single crystal photochemistry and spin state switching in a metal-cyanide complex | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Reversible single crystal photochemistry and spin state switching in a metal-cyanide complex Dawid Pinkowicz, Michał Magott, Mirosław Arczyński, Leszek Malec, and 10 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5611691/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 29 Sep, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Manipulating the physical properties of solid matter using only photons is a major challenge in materials science. In this study, we present the photochemistry occurring in a single crystal of a simple cyanide complex, K 4 [Mo III (CN) 7 ]·2H 2 O. Upon exposure to visible light at different wavelengths, a reversible breaking and reformation of dative bonds is triggered, resulting in a photoswitching of the Mo III coordination geometry between 6- and 7-coordinate. This transformation, in turn, induces a spin state change. The observed solid-state photochemical reactivity is robust, quantitative and occurs at a record-high temperature. It paves the way for the development of new photo-switchable high-temperature magnets and nanomagnets. Physical sciences/Materials science Physical sciences/Chemistry/Coordination chemistry Physical sciences/Chemistry/Inorganic chemistry/Solid-state chemistry Physical sciences/Chemistry/Photochemistry Figures Figure 1 Figure 2 Figure 3 Introduction The absorption of visible light induces substantial changes in the electronic structure of molecules, playing a crucial role in photo-activated processes such as photosynthesis, solar energy harvesting, and photocatalysis. In most cases, the lifetime of the photoexcited state is extremely short. However, certain systems can be thermally trapped in a metastable state for hours, days or even months before eventually relaxing back to the ground state. This phenomenon enables photoswitching between states with distinct physical and chemical properties. The potential applications of photoswitchable materials are vast, including light-responsive molecular junctions, 1 molecular valves and machines, 2 – 4 information storage and processing devices, 5 , 6 and solar energy harvesting. 7 A well-known example of photoswitching is the photoisomerization of rhodopsin, the protein responsible for vision in humans and many animals, which continues to be an active area of research. 8 , 9 , 10 Many current research efforts in the field of photoswitchable materials focus on light-induced isomerization of purely organic molecules. 11 – 14 While these systems dominate the field, they do not exhibit changes in magnetic properties upon light absorption with only a few exceptions. 15 – 17 To achieve the photomagnetic response, these organic photoswitchable molecules must be coupled with metal centers. 18 – 23 However, this approach has thus far only produced paramagnetic systems. In contrast, inorganic photomagnetic compounds typically rely on spin crossover (SCO) sites, which display light-induced excited spin state trapping (LIESST) originating at the metal center. 24 – 27 Unfortunately, the photoexcited state of SCO systems suffers from rapid thermal relaxation above 80 K, which limits their practical applications as photomagnetic switches at room temperature. Nevertheless, a careful analysis of the microscopic origins of their photomagnetism offers clues for designing high-temperature magnetic photoswitches. For example, in the [Fe II (LN 5 )(CN) 2 ]·MeOH SCO complex, the formation and breaking of a coordination bond extends the lifetimes of its photoexcited state that can be observed up to 130 K. 28 Building on these insights, we propose a strategy towards high-temperature photomagnetic systems based on photochemistry – specifically, the photodissociation of cyanometallates. While cyanide photodissociation has been extensively studied in aqueous solutions 29 – 32 and, to a lesser extent, in aprotic solvents or solids, 33 , 34 it has always been found to be irreversible. Herein, we demonstrate a fully reversible metal-cyanide photodissociation occurring in single crystals of potassium heptacyanomolybdate(III) dihydrate 35 – 37 K 4 [Mo III (CN) 7 ]·2H 2 O ( 1 ), and compare it to the photodissociation observed in its solution. The single crystals of 1 undergo a complete metal-cyanide bond photodissociation upon absorption of violet light, resulting in the formation of a hexacoordinate complex, K 4 [Mo III (CN) 6 ]·CN·2H 2 O ( 2 ). Remarkably, compound 1 is fully recovered through red light absorption by 2 , representing an unprecedented solid-state ligand photoassociation reaction. These transformations are non-destructive and maintain the crystallinity of 1 throughout the process. If the unique properties of [Mo III (CN) 7 ] 4− could be extended to its bimetallic assemblies, 35 – 41 this would enable the light control of the long-range magnetic ordering, 38 – 41 or slow relaxation of the magnetization 42 – 44 in these compounds. The photochemical switching behavior of 1 was thoroughly characterized using a combination of techniques, including single-crystal X-ray diffraction (scXRD), infrared (IR) and electron paramagnetic resonance (EPR) spectroscopies, optical reflectivity studies and X-ray absorption spectroscopy (XAS), complemented by magnetization measurements and X-ray magnetic circular dichroism (XMCD). For comparison, the product of the irreversible photodissociation of 1 in acetonitrile (MeCN) solution, [K(crypt-222)] 3 [Mo III (CN) 6 ]·2MeCN ( 3 ), was also isolated and fully characterized. Results and discussion Synthesis, solid state photochemistry and crystallography 1 was obtained by reacting MoCl 3 (THF) 3 with KCN in deoxygenated H 2 O, followed by crystallization from a H 2 O/MeOH mixture, similar to the method reported by Young. 35 The identity and purity of 1 were confirmed through scXRD, PXRD (Fig. S1 ), IR spectroscopy (Fig. S2) and elemental analysis (see Methods). The crystal structure of 1 was investigated by using synchrotron radiation (Diamond Light Source, I19) at 30 K. The experiments were conducted on two single crystals, both before and after 405 nm irradiation, followed either by 638 nm irradiation or by heating to 200 K (Fig. 1 and Table S1 with crystallographic details). Before irradiation, the [Mo III (CN) 7 ] 4− anion adopts a capped trigonal prism (CTPR-7) geometry with seven cyanide ligands coordinated to the Mo III center via carbon atoms (Fig. 1 a and Table S2). Structural analysis after 20 minutes of 405 nm irradiation reveals major structural changes in 1 . The most striking feature is the ‘disappearance’ of one cyanide ligand from the Mo coordination sphere. As a consequence, the Mo coordination number decreases from seven to six, forming K 4 [Mo III (CN) 6 ]·CN·2H 2 O ( 2 ; Fig. 1 b and 1 e, Animation 1). In this photoinduced structure, the remaining six cyanide ligands are arranged at the vertices of a distorted octahedron. The displaced C7N7 cyanide is located 4.195(10) Å from the Mo center, compared to 2.143(9) Å before irradiation (Fig. 1 d), and is now trapped between two water molecules. Notably, all other Mo-C bonds in the octahedral complex 2 with an average length of 2.172(6) Å, become more uniform and slightly longer than 2.153(8) Å observed in the pristine 1 (Fig. 1 d). Indeed, the bond lengths in 2 are very similar to those found in Li 3 [Mo III (CN) 6 ]·6DMF reported by Beauvais and Long. 45 The lattice volume changes from 759.04(13) Å 3 for 1 to 819.08(15) Å 3 for 2 , representing a 7.9% expansion — the largest volume increase in response to light irradiation ever reported (black symbols in Fig. 1 f and Table S1 ). This is also reflected in the change of the crystal size upon 405 nm irradiation at 8 K (Movie 1). Despite the large displacement of the C7N7 cyanide upon photodissociation to form 2 , the original crystal structure is fully restored either through 638 nm irradiation or by heating the crystal to 200 K (Fig. 1 c and Table S1 ). Both restored structures closely resemble the original 1 in terms of molecular features: bond lengths, supramolecular arrangement and unit cell parameters. The observed photo-induced changes suggest that the photodissociation of [Mo III (CN) 7 ] 4− leads to the formation of [Mo III (CN) 6 ] 3− and a free CN − anion trapped between H 2 O molecules. This process appears to occur without any change in the Mo oxidation state, as indicated by minimal variation in Mo-C bond lengths (Fig. S3). The C7-N7 bond length of the photodissociated cyanide in 2 measures 1.19(1) Å, which is typical for a 'free' cyanide, as seen in anhydrous KCN or NaCN. 46 – 48 This value is only slightly longer than the average C–N bond length observed for cyanide ligands coordinated to Mo in 1 (1.16(1) Å) or in other cyanometallates. 49 Solution photochemistry Several transition metal cyanides are known to undergo photolysis in solution. 29 , 31 , 50 However, compound 1 has never been studied in this context, despite reports suggesting that heptacyanomolybdate(III) anion may 'transform' into hexacyanomolybdate(III). 51 – 53 Encouraged by the quantitative and reversible photochemical reactivity of 1 in the solid-state, we decided to test its behavior in a non-aqueous solution. Since 1 is insoluble in organic solvents, [2.2.2]cryptand (crypt-222) 54 was added to dissolve it in anhydrous acetonitrile. The resulting yellow solution was irradiated with either violet or white light (Fig. S4), and in both cases, bleaching occured within minutes (Fig. S5). This is consistent with the photobleaching of a single crystal of 1 observed at 8 K (Movie 1). Colorless plate crystals of [K(crypt-222)] 3 [Mo III (CN) 6 ]·2CH 3 CN ( 3 ) were isolated as the sole Mo-based product from this solution (Table S3 and Figs. S6 and S7) while the by-product [K(crypt-222)]CN remains in solution. The isolated compound showed no electronic transitions in the visible range of the UV-vis spectrum, either in the solid state or in MeCN solution (Fig. S8), supporting that bleaching results from the light-induced dissociation of a metal-cyanide bond. The scXRD analysis of 3 revealed a nearly identical geometry of the [Mo III (CN) 6 ] 3− anion to that observed in 2 (see Supplementary Information for detailed discussion). Thus, the photochemical reactivity of [Mo III (CN) 7 ] 4− in aprotic solution mirrors the solid state behavior, though it is irreversible in solution. Spectroscopic characterization Optical reflectivity measurements were conducted on 1 in the solid state, revealing broad bands that are nearly temperature independent (Figs. S9a-b). Exposure to specific wavelengths within the 365–1050 nm range at 10 K produced an increase in reflectivity below 800 nm, particularly under 365, 385 and 405 nm irradiations (Figs. S9c-f), which again is in line with the bleaching of the green crystals of 1 upon 405 nm irradiation producing nearly colorless 2 (Movie 1). To obtain the spectrum of the photoexcited state, which persists up to 150 K (Fig. S9f), 1 was irradiated for 2 hours at 405 nm — the same wavelength used in photocrystallography (Fig. S9e). Above 150 K, the characteristic bands of the photoexcited state begin to fade, disappearing entirely above 160 K as the original spectrum of 1 is restored (Fig. S9e-f). Additionally, 1 and its photo-induced state 2 can be reversibly cycled using 385 and 660 nm irradiations over multiple cycles (Fig. S10). The reversible photodissociation of the metal-cyanide bond was also confirmed by IR spectroscopy (Fig. 2 a) in a polycrystalline sample of 1 . The C ≡ N stretching vibrations give strong IR absorption bands, highly sensitive to the metal center's electronic configuration and the complex's geometry and symmetry. At 100 K, the ν (C ≡ N) band for 1 appears as two peaks: a main one at 2069 cm − 1 and a weaker component at 2105 cm − 1 , consistent with the literature values. 36 Following 405 nm irradiation, the band maximum shifts to 2104 cm − 1 with a shoulder at 2073 cm − 1 . The main peak position in 2 aligns well with the cyanide stretching observed in Li 3 [Mo III (CN) 6 ]·6DMF, reported at 2115 cm − 1 , 45 with a small difference highlighting the influence of the second coordination sphere of [Mo III (CN) 6 ] 3− . Further support comes from the IR spectrum of 3 , which shows a single, narrow ν (C ≡ N) band at 2088 cm − 1 (Fig. S11). It is worth noting that heating 2 to 200 K fully restores the initial spectrum typical for 1 (Fig. 2 a). XAS measurements at 4 K before and after irradiation support also the photo-induced formation of [Mo III (CN) 6 ] 3− (Fig. 2 b). In the initial Mo L 2,3 -edge spectra of 1 , prominent 'white line' resonances arise from 2 p→ 4 d excitations, with fine structures reflecting ligand field splitting of the 4 d states. Following 405 nm irradiation, the two primary signals weaken, while lower-energy peaks intensify. This aligns with the capped trigonal prism geometry of the [Mo III (CN) 7 ] 4− anion, characterized by a single electron hole in the [(4 d x2 − y2 )(4 d xz )] 3 orbital pair, 55 leading to a lower intensity of the 2 p →[(4 d x2y2 )(4 d xz )] transition (Fig. 2 c). In contrast, the nearly octahedral geometry of the photo-induced [Mo III (CN) 6 ] 3− corresponds to the t 2g 3 e g 0 configuration with three holes in the t 2g orbitals. This results in a higher intensity of the low energy pre-edge region at the expense of the main resonances, mirroring the comparison between the XAS spectra of 1 and 3 (Fig. S12). The integrated white-line intensities for 1 , 2 and 3 are nearly identical, indicating consistent 4 d occupancy across these compounds. Further insights into the unoccupied Mo 4 d orbitals were provided by the XMCD measurements, showing that the dichroic signals for 2 are predominantly concentrated at the low energy peaks for both Mo L 2 and L 3 edges (light blue line in Fig. 2 b). Application of the magneto-optical sum rules 56 , 57 to the XMCD spectra reveals contributions from both spin magnetic moment ( M S ) and orbital magnetic moment ( M L ) in the magnetization of the photo-product 2 . The calculated values of M S = 2.86 µ B and M L = -0.16 µ B agree well with the those expected for high-spin hexacyanomolybdate(III), where the t 2g 3 e g 0 electron configuration implies a minimal orbital contribution. The solid-state EPR spectrum of powdered 1 is consistent with the findings of Rossman et al. 36 showing an S = ½ system with slight g -factor anisotropy ( g z = 2.108, g xy = 1.964, black line in Fig. 2 d). Upon 405-nm irradiation at 10 K, the S = ½ signal gradually decreases and vanishes within tens of minutes, while a new broad resonance appears around 160 mT, corresponding to g ’ ≈ 4.3 (blue line in Fig. 2 d). This EPR signal is attributed to the anticipated S = 3/2 state of 2 , as inferred from the XMCD study. Following thermal relaxation at room temperature, the initial S = ½ spectrum of 1 is recovered, while the 160 mT signal vanishes (orange line in Fig. 2 d), further confirming the reversible transformation observed in crystallographic, optical reflectivity, and IR studies. Computational studies To investigate the photoswitching mechanism, ab initio calculations (CASSCF/CASPT2) of the electronic transitions in 1 and 2 were conducted. For 1 , the light-driven transformation to 2 likely involves either a spin-allowed transition calculated at 361.5 nm or spin-forbidden ones at 373.4 and 460.9 nm (Table S5). However, in the solid state, the absorption bands are broad and heavily overlap, allowing efficient photoconversion to 2 experimentally at 365, 385 and 405 nm, as observed in optical reflectivity studies (Fig. S9c). In contrast, the calculated spin-allowed transitions for 2 fall within the deep UV region, while the 4 A 2g → 2 T 2g transitions appear at 587.6 and 604.8 nm, matching the de-excitation wavelengths (Table S6 and Fig. S10a). These spin-forbidden transitions gain intensity due to a slight distortion in the octahedral geometry of [Mo III (CN) 6 ] 3− observed in 2 , potentially enabling the light-induced reverse metal-cyanide bond association process. Periodic DFT geometry optimizations accurately reproduce changes in the metric parameters of the crystal structures and the molecular geometry of the complex associated with a spin-state change at the Mo III center. The 1 → 2 transformation was reproduced by re-optimizing the low-spin state structure of 1 , fixing the number of unpaired electrons per Mo to 3 within the first optimization step, as inferred for the high-spin state structure of 2 from EPR and XMCD (see Methods and Supplementary Information for details). The reverse 2 → 1 transformation was similarly modeled by re-optimizing the structure of 2 for one unpaired electron per Mo. Figs. S13-S16 and Table S7-S8 compare the experimental and optimized geometries of the [Mo III (CN) 7 ] 4− and [Mo III (CN) 6 ] 3− anions in their respective crystal structures. Photoswitching of the magnetization The magnetic and in situ photomagnetic studies of 1 were conducted using variable temperature/magnetic field magnetometry with light delivered to the sample chamber through an optical fiber. The temperature dependence of the χT product (product of the molar magnetic susceptibility, χ , and temperature, T ) measured at 0.1 T for pristine 1 shows behavior typical of an S = ½, g = 2.0 paramagnetic system (Fig. S17a), in agreement with the EPR results (Fig. 2 d). Upon cooling below 50 K, the χT ( T ) data deviates from the constant value of 0.36 cm 3 ·K·mol − 1 , ultimately decreasing to 0.03 cm 3 ·K·mol − 1 at 1.8 K, indicating supramolecular antiferromagnetic interactions between S = ½ Mo III spins. Below 3.5 K, these couplings lead to an antiferromagnetically ordered state, as evidenced by the M ( H ) data ( M - molar magnetization, H - magnetic field strength; Fig. S17b) and corroborated by low-temperature heat capacity measurements, discussed in detail in the Supplementary Information (Figs. S17-S19). Photomagnetic experiments (Fig. 3 ) conducted on 1 at 10 K show an increase in χT from 0.30 to 1.79 cm 3 ·K·mol − 1 upon 405-nm irradiation (ca. 5 mW·cm − 2 , 2 h, violet and black symbols in Fig. 3 a). The χT ( T ) profile recorded in the dark, immediately after irradiation, reveals a plateau around 1.84 cm 3 ·K·mol − 1 in the 20–100 K range (blue symbols in Fig. 3 b) close to 1.875 cm 3 ·K·mol − 1 expected for a magnetically isotropic S = 3/2 state (with g = 2). This aligns with the 1.82 cm 3 ·K·mol − 1 value observed for 3 (green symbols in Fig. 3 b; for details see Supplementary Information, Figs. S20-21). Additionally, the M ( H ) dependence for 2 shows a magnetization close to 3.0 µ B at 7 T, indicating a high-spin S = 3/2 configuration (Fig. 3 c), in good agreement with XMCD results and the magnetic data for 3 (Fig. S21). Collectively, these photomagnetic experiments at 10 K suggest thermal trapping of the S = 3/2 metastable state, analogous to spin-crossover systems, confirming the formation of light-induced high-spin hexacyanomolybdate(III) complex. As the temperature rises (at 2 K·min − 1 ), the photoinduced state relaxes to the original S = ½ ground state of the [Mo III (CN) 7 ] 4− complex around 150 K, indicated by a sharp drop in the χT product (blue symbol in Fig. 3 b). Notably, the relaxation temperature of 150 K for 2 surpasses that of previously reported spin-crossover systems, 18 , 19 , 58 suggesting an unusually long lifetime of the photoinduced state in this molybdenum cyanide complex. To quantify the characteristic time of the thermally induced 2 → 1 relaxation, time-dependent magnetization studies were performed at several temperatures in the 132.5-147.5 K range (Fig. 3 e). The temperature dependence of the estimated relaxation time (Table S9) follows an Arrhenius behavior with an activation energy of 4770(90) K (Fig. 3 f). The relaxation behavior of 2 is comparable with that reported for the record-holding photomagnetic molecular Prussian blue analogue {Co 4 Fe 4 }. 59 This result indicates that considerable thermal energy is needed for the complex to overcome the energetic barrier between the high-spin and low-spin states. This aligns with the substantial structural reconfiguration upon the 2 → 1 relaxation — specifically, the displacement of the C7N7 cyanide group by about 2 Å back towards the Mo center. As shown in Fig. 3 d, the robustness and reversibility of the 405 nm irradiation at 10 K ( 1 → 2 ) and thermal relaxation ( 2 → 1 ) processes was confirmed across multiple cycles by magnetization measurements. Moreover, similar behavior is observed at the record-high temperature of 100 K over at least four consecutive cycles of 405 nm irradiation and thermal relaxation (Fig. S22) with only slight radiation damage to the sample. Importantly, the substantial energy barrier of the thermally activated metal-cyanide association ( 2 → 1 ) does not prevent this process from happening under a 638 nm irradiation at low temperatures as demonstrated by scXRD at 30 K (Fig. 1 ) or optical reflectivity at 10 K (Fig. S10). Photomagnetic experiments at 10 K further support this observation (Fig. 3 a): following the initial 405 nm irradiation of 1 at 10 K, which photoconverts 1 into 2 , a subsequent 638 nm irradiation fully restores the magnetic susceptibility of 1 (red symbols in Fig. 3 a). Conclusions A single crystal of the inorganic compound K 4 [Mo III (CN) 7 ]·2H 2 O demonstrates a reversible photochemical reaction, enabling the molybdenum center to switch between the 7-coordinate low-spin and 6-coordinate high-spin geometries. Exposure to 405 nm light causes dissociation of one of the seven metal-cyanide bonds, resulting in a 2 Å displacement from the Mo center, while subsequent irradiation at 638 nm reverses this effect, restoring the original structure. This bidirectional photoswitching is remarkably robust and non-destructive, yielding colossal structural, electronic, and magnetic changes. Interestingly, photodissociation in solution is irreversible, permitting isolation of the [Mo III (CN) 6 ]³⁻ complex. K 4 [Mo III (CN) 7 ]·2H 2 O stands out as a unique example of bidirectional photoswitching in an inorganic coordination compound in the crystalline phase, with its high performance highlighted by three key factors: (i) photo-induced ligand dissociation achieves 100% efficiency, (ii) the photodissociation is fully reversible with light of a different wavelength resulting in photo-association, and (iii) the record-high relaxation temperature allows photoswitching at temperatures exceeding 100 K. These exceptional photochemical properties set the stage for designing new photomagnetic materials that could switch between paramagnetic and ferromagnetic (or ferrimagnetic) states at room temperature. By leveraging similar mechanisms in coordination polymers and materials based on the [Mo III (CN) 7 ] 4− building-block, or its analogs, this approach holds promise for advanced high-temperature photomagnetic applications. Methods Syntheses MoCl 3 (THF) 3 precursor was synthesized under an argon gas atmosphere inside the Inert PureLab HE glovebox (O 2 < 0.1 ppm and H 2 O < 0.5 ppm), in a direct, two-step reduction of MoCl 5 according to a literature method. 60 All other reagents were commercially available and used without further purification. The distilled water used in the synthesis of 1 was deoxygenated by refluxing under an argon atmosphere for at least 12 hours. Deoxygenated methanol, acetonitrile and diethyl ether were prepared by passing HPLC grade solvents through the Inert PureSolv EN7 solvent purification system. K 4 [Mo III (CN) 7 ]·2H 2 O ( 1 ) was synthesized using a modified procedure based on established literature methods. 35 , 61 Inside the oxygen-free LABCONCO glovebox (O 2 < 0.1%), a Schlenk flask was charged with MoCl 3 (THF) 3 (1.7 g, 4.1 mmol), KCN (3.0 g, 46.0 mmol; Sigma-Aldrich, ≥ 96%) and 20 mL of deoxygenated water. The flask was sealed with a glass stopper, removed from the glovebox and connected to the Schlenk line. The mixture was subjected to vigorous magnetic stirring and slowly evacuated until it began to boil gently, at which point the connection to the Schlenk line was sealed. The flask was wrapped in an aluminum foil to prevent light exposure and heated to 50°C for 24 hours. Then the reaction mixture was transferred to an oxygen-free glovebox and vacuum-filtered through a sintered-glass funnel (G4) to remove any insoluble impurities. The filtrate was treated with 13 mL of deoxygenated methanol (MeOH), sealed in the glass jar and kept at 4°C for more than 24 hours. After that time, the precipitation of dark-green crystals was observed, which were collected by vacuum filtration inside oxygen-free glovebox and washed with MeOH:H 2 O = 7:3, MeOH:H 2 O = 9:1 and pure MeOH, then dried in vacuo for 15 minutes. The final product (1.5 g) was obtained in 78% yield, based on MoCl 3 (THF) 3 . Anal. Calcd. (found) for C 7 H 4 K 4 MoN 7 O 2 , C: 17.87% (18.01%), H: 0.857% (0.873%), N: 20.84% (20.94%). [K(crypt-222)] 3 [Mo III (CN) 6 ]·2MeCN ( 3 ) was synthesized via a photochemical route, similar to that utilized in photochemical preparation of [K(crypt-222)] 3 [Mo IV (CN) 7 ]·3MeCN and [K(crypt-222)] 3 [W IV (CN) 7 ]·4MeCN. 34 , 62 In the Inert PureLab HE glovebox (O 2 < 0.1 ppm and H 2 O < 0.5 ppm), K 4 [Mo III (CN) 7 ]·2H 2 O (0.24 g, 0.51 mmol) was placed in a vial filled with 7 mL of deoxygenated CH 3 CN in the presence of [2.2.2]cryptand (crypt-222; 0.9 g, 2.39 mmol; Merck, ≥ 99%). The yellow suspension was irradiated using white light (Photonic LED-Light-Source F3000) until it turned into a nearly colorless solution. Then it was subjected to slow diffusion of diethyl ether vapor for two days, resulting in the precipitation of colorless plate crystals, which were collected by vacuum filtration. The purity of the product varies from batch to batch; at elevated ambient temperatures, increased evaporation of diethyl ether can lead to the precipitation of a by-product, [K(crypt-222)]CN. In this case, the product can be purified by dissolving it in 4 mL of CH 3 CN and then performing repeated diffusion of diethyl ether vapors. The final product (0.7 g) is obtained in 87% yield based on K 4 [Mo III (CN) 7 ]·2H 2 O. The purity of the compound was checked by PXRD, with the experimental pattern (Fig. S7) matching perfectly the simulated one from the scXRD structural model obtained at 293 K (CCDC 2352266). Structure determination and refinement. Single-crystal X-ray diffraction experiments for the 1st crystal of pristine 1 (CCDC 2352257), after 405 nm irradiation 2 (CCDC 2352263), after 638 nm irradiation 1 (2352259), for the 2nd crystal of pristine 1’ (CCDC 2352264), after 405 nm irradiation 2’ (CCDC 2352261) and after thermal relaxation 1’ (CCDC 2352262) were all performed at 30 K using synchrotron X-ray radiation (Diamond Light Source, United Kingdom) at the I19 beamline (EH2) equipped with Newport 4-circle κ-diffractometer, monochromatic X-ray synchrotron radiation (λ = 0.6889 Å), Pilatus 300K detector and an N-Helix Oxford Cryosystems gas cooler. 63 In situ irradiation experiments were conducted with 405 nm (50 mW·cm − 2 ) and 638 nm (100 mW ·cm − 2 ) laser diodes (Thorlabs). Absorption corrections, data reduction and unit cell refinement were performed using Xia2 and CrysAlisPRO software (Rigaku Oxford Diffraction, 2019). 64 Introduction of hydrogen atoms in the structures of 2 and 2’ led to the instability of the refinement; therefore, to maintain consistency, we decided to remove hydrogen atoms from the corresponding structural models. Diffraction experiments for 3 were performed at 100 (CCDC 2352265) and 293 K (CCDC 2352266) using Bruker D8 Quest Eco diffractometer (Mo Kα radiation, Triumph® monochromator). Absorption corrections, data reduction and unit cell refinements were performed using SADABS and SAINT programs included in the Apex3 suite. All the structures were solved using direct methods and refined anisotropically using weighted full-matrix least-squares methods applied to F 2 . 65−67 Powder X-ray diffraction. PXRD patterns were collected using Bruker D8 Advance Eco diffractometer equipped with Lynxeye silicon strip detector, Cu sealed tube radiation source and a capillary stage at room temperature. Samples were ground to a powder using an agate mortar inside the glovebox and loaded into glass capillaries 0.3 or 0.5 mm diameter. The capillaries were broken in half inside the glovebox and the open end was sealed using silicon grease before they were moved to the PXRD instrument and mounted on the goniometer head using bee wax. The simulated PXRD patterns were obtained from the scXRD data using Mercury software. 68 The experimental PXRD pattern for 1 was subjected to background correction using the DIFFRAC algorithm implemented in the DIFFRAC.EVA V5 software. Periodic density functional theory (DFT) calculations. All periodic DFT computations were performed using CRYSTAL23 software. 69 Initial geometrical parameters for low-spin and high-spin state structures ( 1 and 2 , respectively) were adopted from the single-crystal X-ray diffraction data ( 1 - CCDC no. 2352258; 2 - CCDC no. 2352263). Hydrogen atoms were added according to chemical criteria in the positions of electron density maxima from the diffraction data. Geometry optimizations within Unrestricted Kohn-Sham (UKS) formalism were conducted using several hybrid and range-separated hybrid functionals (details in Supplementary Information) and pob-TZVP basis set. 70 , 71 Best fit to the experimental data was obtained for HISS middle-range corrected range-separated hybrid functional. 72 – 74 In each run, a 4 × 4 × 4 k -point mesh in the reciprocal space was generated in line with the Monkhorst–Pack methodology. 75 In all computations, tighter tolerances on the exchange and Coulomb integrals were used with the TOLINTEG set to 10, 10, 10, 10 and 20. 69 Numerical integration accuracy was provided using pruned XXLGRID comprising 99 radial and 1454 angular points in the region of chemical interest. The self-consistent field (SCF) convergence criterion was set to 10 –8 a.u. To achieve the correct spin states for the computed systems, the difference between the number of α and β electrons was fixed at all k -points during the initial 30 SCF cycles of the first optimization step. The same spin-locking protocol was used to reproduce the spin-state change. The pre-optimized structure of the low-spin state was fully re-optimized with the number of unpaired electrons per unit cell set as that expected for two high-spin Mo atoms (α-β = 6) during the initial 50 SCF cycles of the first optimization step. High-spin to low-spin transformation was replicated using an analogous procedure setting the number of unpaired electrons per unit cell as α-β = 2. CASSCF/CASPT2 calculations. Complete active space self-consistent field (CASSCF) and complete active space second-order perturbation theory (CASPT2) calculations of the excitation energies were done with the MOLCAS 8.2 code. 76 1 was represented as a [Mo(CN) 7 ] 4− unit embedded in 15 K + ions represented with model potentials and 203 optimized point charges to take into account the long-range Madelung potential. Relativistic corrections were accounted for with DKH Hamiltonian and dynamic electron correlation was calculated on the CASPT2 level. In case of 2 , the [Mo(CN) 6 ] 3− ·CN − moiety was embedded in 16 K + ions represented with model potentials and 162 optimized point charges that represent the Madelung potential in the [Mo(CN) 6 ] 3− ·CN − region. The molecular orbitals were expanded in the ANO-RCC all electron basis sets: Mo (6s, 5p, 4d, 1f), K (4s, 3p), C and N (3s, 2p, 1d), O (3s, 2p), H (2s). 77 The active space consists of 9 orbitals (5 Mo-4d, 2 CN-π, and 2 CN-π*) and 7 electrons. The ions used to embed the central unit are represented with the large core Hay and Wadt effective core potentials with a net charge of + 1. 78 CASPT2 correlates all the electrons except the deep core ones (C, N, O-1s, Mo-1s...3p). The standard IPEA = 0.25 zeroth-order Hamiltonian was used and an imaginary shift of 0.15 Eh was added to the denominators to avoid the appearance of intruder states. The Cholesky decomposition is used to speed-up the handling of the two-electron integrals with a threshold of 10 − 3 Eh. X-ray Absorption Spectroscopy (XAS) . XAS spectra at Mo L 2,3 -edges were obtained at the ID12 beamline (ESRF, The European Synchrotron). 79 The data were collected using total fluorescence yield detection mode and were subsequently corrected for reabsorption effects. Compound 2 was obtained in situ by irradiating 1 for 12 hours at 4 K, using 405 nm laser with a power of P = 200 mW. The X-ray Magnetic Circular Dichroism (XMCD) spectra of 2 were obtained as the difference between two consecutive XAS spectra recorded with opposite photon helicities and corrected for the incomplete circular polarization rate. To ensure the absence of experimental artefacts the measurements were systematically performed for both magnetic field directions. The normalized spectra were analyzed using the magneto-optical sum rules given for L 2 and L 3 absorption edges 56 to afford effective spin magnetic moment ( M S ,eff = -2 µ B ) and orbital magnetic moment ( M L = - µ B ): $$\:⟨{L}_{z}⟩=\frac{2⟨{n}_{h}⟩}{3}\bullet\:\frac{{I}_{{L}_{3}}^{XMCD}+{I}_{{L}_{2}}^{XMCD}}{{I}_{{L}_{3}}^{XAS}+{I}_{{L}_{2}}^{XAS}}$$ $$\:⟨{S}_{eff}⟩=⟨{S}_{z}⟩+\frac{7}{2}⟨{T}_{z}⟩=\frac{⟨{n}_{h}⟩}{2}\bullet\:\frac{{I}_{{L}_{3}}^{XMCD}-2{I}_{{L}_{2}}^{XMCD}}{{I}_{{L}_{3}}^{XAS}+{I}_{{L}_{2}}^{XAS}}$$ The precise sample temperature and the absolute magnitude of the magnetic moment ( M tot = M S + M L ) were determined by scaling the field and temperature dependent XMCD signal intensity to the results of magnetic measurements for 2 (Fig. 3 c), following the previously described method. 80 Infra-red (IR) spectroscopy. IR spectra were recorded using Nicolet iN10 MX FT-IR microscope in a transmission mode. A polycrystalline sample was spread onto the surface of a BaF 2 optical window and sealed under Ar atmosphere inside a Linkam THMS350V temperature-controlled stage. The temperature control was achieved with a flow of liquid nitrogen, and the irradiation experiment was conducted using a 405 nm laser diode with a power P = 7–8 mW·cm − 2 . UV-visible spectroscopy. UV-vis spectra for 1 and 3 were measured at room temperature in transmission mode using a Shimadzu UV-3600i Plus spectrophotometer. A solution of 1 was prepared by dissolving a mixture of K 4 [Mo III (CN) 7 ]·2H 2 O (4.7 mg; 0.01 mmol) and crypt-222 (19 mg, 0.05 mmol) in 20 mL of deoxygenated acetonitrile, and then filtered using a syringe equipped with a 0.22 µm pore size PTFE membrane to remove insoluble impurities. Both solutions were placed inside air-tight quartz cuvettes and measured immediately after preparation. Solid sample of 3 was mixed with paraffin oil between two quartz slides and measured with an integrating sphere. Electron paramagnetic resonance (EPR). To enable quantitative in situ irradiation of the EPR sample in the resonator cavity, powder of 1 was deposited as a thin layer on a double-sided adhesive tape glued onto a plastic holder, which was placed in a standard 4 mm X-band EPR tube (Wilmad). Continuous-wave EPR spectra were recorded on a Bruker ELEXSYS E580 spectrometer with a Bruker ER 4118X-MD5 resonator and an Oxford Instruments ER 4118CF helium flow cryostat, at a microwave frequency of 9.628 GHz, with microwave power of 40 µW, modulation amplitude of 0.5 mT and at 10 K. Optical reflectivity measurements. Reflectivity measurements have been performed with a home-built system, operating between 10 and 300 K (at 4 K·min − 1 ) and in the range of 400 to 1000 nm. A halogen-tungsten light source (Leica CLS 150 XD tungsten halogen source adjustable from 0.05 mW·cm − 2 to 1 W·cm − 2 ) was used as the light source for the high-sensitivity Hamamatsu 10083CA spectrometer. The measurements were calibrated with barium sulphate as the reference sample. As the samples are potentially very photosensitive, the light exposure time was minimized during the experiments keeping the samples in the dark except during the measurements when white light is shined on the sample surface ( P = 0.08 mW·cm − 2 ). For all excitation/de-excitation experiments performed at 10 K, the sample was initially placed at this temperature keeping the sample in the dark to avoid any excitation. Light-emitting diodes (LEDs) operating between 365 and 1050 nm (from Thorlabs) were used for excitation experiments. For the excitation/de-excitation experiments in optical reflectivity, the power and the time of the irradiation were systematically adapted to the optical properties of the material to obtain the fastest possible response and to minimize thermal heating effects. Magnetic and photomagnetic measurements. Magnetic measurements were performed using Quantum Design MPMS3 Evercool SQUID magnetometer in the magnetic fields up to 7 T. Polycrystalline sample of 1 (21.8 mg) was inserted into a polyethylene (PE) bag and sealed under an argon gas atmosphere inside the Inert PureLab HE glovebox (O 2 < 0.1 ppm and H 2 O < 0.5 ppm). The PE bag was attached to the quartz sample holder using Kapton tape. Polycrystalline sample of 3 (32.8 mg) was inserted into Delrin sample holder described elsewhere, 81 which was inserted into the plastic straw. The experimental magnetic data were corrected for the diamagnetism of the sample and the sample holder. Photomagnetic measurements were performed for samples (0.3-1.0 mg) placed between two layers of adhesive tape and inserted into the plastic straw. For the relaxation measurements, very small amount of sample (ca. 0.1 mg) was used to facilitate fast and complete conversion from 1 to 2 . The accurate sample mass and diamagnetic corrections were determined by comparison of the data collected in the dark on bulk samples. The irradiation was performed using laser diodes (405 nm – P = 5 mW·cm − 2 , 638 nm – P = 15–20 mW·cm − 2 ). Declarations Acknowledgements This work was financed by the European Union within the Horizon Europe Framework Programme, ERC Consolidator Grant LUX-INVENTA no. 101045004. Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. The authors would like to thank the European Synchrotron Radiation Facility for beamtime at the ID12 (proposal CH-6581 and CH-6239) and Diamond Light Source for beamtime at I19-2 instrument (proposal MT18908) and the staff of beamline I19 for assistance with crystal testing, data collection and photoirradiation setup. Access to the ESRF was financed by the Polish Ministry of Education and Science - decision no. 2021/WK/11. We also acknowledge the DUS grant from the Faculty of Chemistry under the Strategic Programme Excellence Initiative at the Jagiellonian University. The study was partly carried out using the research infrastructure cofounded by the European Union in the framework of the Smart Growth Operational Program, Measure 4.2; Grant No. POIR.04.02.00-00-D001/20, “ATOMIN 2.0 – ATOMic scale science for the INnovative economy". 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Supplementary Files Animation1.mp4 Animation showing the reversible light-induced photodissociation and photoassociation reactions within the single crystal of potassium heptacyanomolybdate(III) dihydrate Movie1.mp4 Photoirradiation of a single crystal of potassium heptacyanomolybdate(III) dihydrate at 8 K using 405 nm laser light compound1CCDC2352264checkcif.pdf Article File - compound 1' CCDC2352264_checkcif compound1CCDC2352257.txt Article File - compound 1 CCDC2352257 compound1relaxed638nmCCDC2352259checkcif.pdf Article File - compound 1 relaxed638nm CCDC2352259_checkcif compound1testCCDC2352258checkcif.pdf Article File - compound 1test CCDC2352258_checkcif compoundKcryptMoCN6100KCCDC2352265checkcif.pdf Article File - compound KcryptMoCN6_100K CCDC2352265_checkcif compound1relaxed200KCCDC2352262.txt Article File - compound 1' relaxed200K CCDC2352262 compound1testCCDC2352258.txt Article File - compound 1test CCDC2352258 compound1test2CCDC2352260checkcif.pdf Article File - compound 1test2 CCDC2352260_checkcif compound2CCDC2352261checkcif.pdf Article File - compound 2' CCDC2352261_checkcif compound2CCDC2352263checkcif.pdf Article File - compound 2 CCDC2352263_checkcif compound1relaxed200KCCDC2352262checkcif.pdf Article File - compound 1' relaxed200K CCDC2352262_checkcif compoundKcryptMoCN6293KCCDC2352266checkcif.pdf Article File - compound KcryptMoCN6_293K CCDC2352266_checkcif compound2CCDC2352261.txt Article File - compound 2' CCDC2352261 compound2CCDC2352263.txt Article File - compound 2 CCDC2352263 compoundKcryptMoCN6293KCCDC2352266.txt Article File - compound KcryptMoCN6_293K CCDC2352266 compound1test2CCDC2352260.txt Article File - compound 1test2 CCDC2352260 compound1relaxed638nmCCDC2352259.txt Article File - compound 1 relaxed638nm CCDC2352259 compoundKcryptMoCN6100KCCDC2352265.txt Article File - compound KcryptMoCN6_100K CCDC2352265 compound1CCDC2352264.txt Article File - compound 1' CCDC2352264 compound1CCDC2352257checkcif.pdf Article File - compound 1 CCDC2352257_checkcif SI.pdf Supplementary Information Cite Share Download PDF Status: Published Journal Publication published 29 Sep, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5611691","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":400613310,"identity":"5117d95a-50bb-4654-8fea-097234b359b9","order_by":0,"name":"Dawid Pinkowicz","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABC0lEQVRIiWNgGAWjYDCCA0DEw8DAA4QMDAkMDHJgIRCHj1gtxnAtbHi0gBVACAaGxAYYB5cWvuNnDA+8YbgjY85z+JjEg5q69A0HDzA+eNvGkIdLi+SZHIODcxie8Vj2tqVJJBxjy91w4ACz4dw2hmJcWgwO5G44zMNwmMfgPI/ZjcQGHpAWNmneNobENlxazr9F0SKRbnDgAPtvvFpuwGw52wPSYpAA1MLGjE+L5I33Hw7OMXjGY3DmWPqPhGMJhjMPHGyWnHNOAqdf+M6nJX94U3HH3uBM8mHDHzV18nw3Dh/88KbMJo8fhxZYICBxJA42gMgEvDogEQoD/A1gipCWUTAKRsEoGDkAAGSfZJRTBuXwAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0002-9958-3116","institution":"Jagiellonian University","correspondingAuthor":true,"prefix":"","firstName":"Dawid","middleName":"","lastName":"Pinkowicz","suffix":""},{"id":400613311,"identity":"c5b06d0e-1d72-46c7-9ae8-0d951f8a1ee0","order_by":1,"name":"Michał Magott","email":"","orcid":"https://orcid.org/0000-0002-4566-2636","institution":"Jagiellonian University","correspondingAuthor":false,"prefix":"","firstName":"Michał","middleName":"","lastName":"Magott","suffix":""},{"id":400613312,"identity":"ea8c45ae-b55c-4cbb-b863-9c72c1effb8d","order_by":2,"name":"Mirosław Arczyński","email":"","orcid":"","institution":"Jagiellonian University","correspondingAuthor":false,"prefix":"","firstName":"Mirosław","middleName":"","lastName":"Arczyński","suffix":""},{"id":400613313,"identity":"fef27a90-a718-4615-8279-31ad6bd619cf","order_by":3,"name":"Leszek Malec","email":"","orcid":"","institution":"Jagiellonian University","correspondingAuthor":false,"prefix":"","firstName":"Leszek","middleName":"","lastName":"Malec","suffix":""},{"id":400613314,"identity":"30e77f2f-6d67-49e7-a65b-05d716d8f93a","order_by":4,"name":"Michał Rams","email":"","orcid":"","institution":"Jagiellonian University","correspondingAuthor":false,"prefix":"","firstName":"Michał","middleName":"","lastName":"Rams","suffix":""},{"id":400613315,"identity":"6f5c8657-d6bf-46b7-8ba5-331eaf9c3ed0","order_by":5,"name":"Mathieu Rouzières","email":"","orcid":"https://orcid.org/0000-0003-3457-3133","institution":"1Univ. 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Bordeaux, CNRS, Centre de Recherche Paul Pascal, CRPP, UMR 5031","correspondingAuthor":false,"prefix":"","firstName":"Itziar","middleName":"","lastName":"Oyarzabal","suffix":""},{"id":400613319,"identity":"564e0e26-3198-44bd-89b0-ebe588b01ecc","order_by":9,"name":"Thomas Lohmiller","email":"","orcid":"https://orcid.org/0000-0003-0373-1506","institution":"Max-Planck-Institute for Chemical Energy Conversion, 45470 Mülheim a. d. Ruhr","correspondingAuthor":false,"prefix":"","firstName":"Thomas","middleName":"","lastName":"Lohmiller","suffix":""},{"id":400613320,"identity":"6bd8ad79-dd5a-4cba-a0ab-aa1dbcc44ca4","order_by":10,"name":"Alexander Schnegg","email":"","orcid":"https://orcid.org/0000-0002-2362-0638","institution":"Max Planck Institute for Chemical Energy Conversion","correspondingAuthor":false,"prefix":"","firstName":"Alexander","middleName":"","lastName":"Schnegg","suffix":""},{"id":400613321,"identity":"a5a0c645-85c6-455f-93bb-1d770ef43cc0","order_by":11,"name":"Coen de Graaf","email":"","orcid":"https://orcid.org/0000-0001-8114-6658","institution":"Universitat Rovira I Virgilli","correspondingAuthor":false,"prefix":"","firstName":"Coen","middleName":"","lastName":"de Graaf","suffix":""},{"id":400613322,"identity":"1bd3bff2-3850-4137-b656-4ca28d0d92c3","order_by":12,"name":"Corine Mathonière","email":"","orcid":"https://orcid.org/0000-0002-4774-1610","institution":"1Univ. Bordeaux, CNRS, Centre de Recherche Paul Pascal, CRPP, UMR 5031","correspondingAuthor":false,"prefix":"","firstName":"Corine","middleName":"","lastName":"Mathonière","suffix":""},{"id":400613323,"identity":"91c50e9a-4927-4a40-bb84-f7ed8435d835","order_by":13,"name":"Rodolphe CLERAC","email":"","orcid":"https://orcid.org/0000-0001-5429-7418","institution":"Univ. Bordeaux, CNRS, Centre de Recherche Paul Pascal, CRPP, UMR 5031","correspondingAuthor":false,"prefix":"","firstName":"Rodolphe","middleName":"","lastName":"CLERAC","suffix":""}],"badges":[],"createdAt":"2024-12-09 21:15:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5611691/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5611691/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-63523-x","type":"published","date":"2025-09-29T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":78880792,"identity":"c1b511cd-71fd-44b3-9937-fca28bf1556e","added_by":"auto","created_at":"2025-03-20 08:27:48","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":135198,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDetails of the scXRD photocrystallographic analysis of 1 and its photo-product, 2, at \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eT\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e = 30 K.\u003c/strong\u003e Molecular geometry of the [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4-\u003c/sup\u003e anion and selected interatomic distances in Å: \u003cstrong\u003ea\u003c/strong\u003e, before irradiation in \u003cstrong\u003e1\u003c/strong\u003e; \u003cstrong\u003eb\u003c/strong\u003e, after 405 nm irradiation in \u003cstrong\u003e2\u003c/strong\u003e; and \u003cstrong\u003ec\u003c/strong\u003e, after 638 nm irradiation (red interatomic distances) or thermal treatment (orange interatomic distances) in \u003cstrong\u003e1 \u003c/strong\u003eafter relaxation; Color code: Mo – yellow, O – red, N – blue, C – grey; Variation of (\u003cstrong\u003ed\u003c/strong\u003e) the Mo-C bond lengths and (\u003cstrong\u003ef\u003c/strong\u003e) unit cell parameters before irradiation (\u003cstrong\u003e1\u003c/strong\u003e or \u003cstrong\u003e1\u003c/strong\u003e' in black; symbol ’ indicates experiments with a second crystal), after 405 nm irradiation (\u003cstrong\u003e2 \u003c/strong\u003eor \u003cstrong\u003e2\u003c/strong\u003e' in blue) and (i) subsequent 638 nm irradiation (\u003cstrong\u003e1\u003c/strong\u003e in red) or (ii) 200-K thermal treatment (\u003cstrong\u003e1\u003c/strong\u003e' in orange). \u003cstrong\u003ee\u003c/strong\u003e, Overlay of the molecular structures of the [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4-\u003c/sup\u003e anion in \u003cstrong\u003e1\u003c/strong\u003e and \u003cstrong\u003e2\u003c/strong\u003e illustrating the photoinduced changes of the Mo coordination geometry.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/da6eb33c583b71d2b5056670.png"},{"id":78880799,"identity":"35356593-b357-487f-b9dd-76139bd6dcd9","added_by":"auto","created_at":"2025-03-20 08:27:49","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":125464,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSpectroscopic studies (IR, XAS/XMCD and EPR) of 1 and its photoproduct, 2. a\u003c/strong\u003e, Comparison of the IR spectra shown in the 1820-2320 cm\u003csup\u003e-1\u003c/sup\u003e range and recorded at 100 K before irradiation (\u003cstrong\u003e1\u003c/strong\u003e, black line), after 405-nm irradiation (\u003cstrong\u003e2\u003c/strong\u003e; blue line) and after thermal relaxation at 200 K (\u003cstrong\u003e1\u003c/strong\u003e; orange line).\u003cstrong\u003e b\u003c/strong\u003e, XAS spectra recorded at 4 K for \u003cstrong\u003e1\u003c/strong\u003e (before irradiation; grey line) and for \u003cstrong\u003e2\u003c/strong\u003e (after 405-nm irradiation; dark blue line), and XMCD signal for \u003cstrong\u003e2\u003c/strong\u003e (light blue line with blue filling). \u003cstrong\u003ec\u003c/strong\u003e, Scheme illustrating the change of the XAS intensity of the respective Mo 2\u003cem\u003ep → \u003c/em\u003e4\u003cem\u003ed\u003c/em\u003e transitions (at the pre-edge and edge) related to the electronic configurations of \u003cstrong\u003e1\u003c/strong\u003e (capped trigonal prism geometry) and \u003cstrong\u003e2 \u003c/strong\u003e(pseudo-octahedral geometry). \u003cstrong\u003ed\u003c/strong\u003e, X-band EPR spectra recorded at 10 K before irradiation (\u003cstrong\u003e1\u003c/strong\u003e, black line), after 405 nm irradiation (\u003cstrong\u003e2\u003c/strong\u003e, blue line; multiplied by a factor of 20) and after thermal relaxation (\u003cstrong\u003e1\u003c/strong\u003e, orange line). Asterisks mark the resonator background signals in the spectrum of \u003cstrong\u003e2\u003c/strong\u003e.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/836c0aba877881b952129173.png"},{"id":78880826,"identity":"16d098f4-a894-4e45-a6c5-7e18b8a3eb39","added_by":"auto","created_at":"2025-03-20 08:27:51","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":150993,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMagnetic and photomagnetic studies of\u003c/strong\u003e \u003cstrong\u003e1.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e, Time evolution of the \u003cem\u003ecT\u003c/em\u003e product of \u003cstrong\u003e1\u003c/strong\u003e at 0.1 T during 405-nm irradiation (purple symbols), converting \u003cstrong\u003e1\u003c/strong\u003e into \u003cstrong\u003e2\u003c/strong\u003e, and subsequent 638-nm irradiation (red symbols), restoring \u003cstrong\u003e1 \u003c/strong\u003efrom\u003cstrong\u003e 2 \u003c/strong\u003e(\u003cem\u003eT\u003c/em\u003e = 10 K; note that no correction was applied for the thermal heating due to light irradiation, with black symbols representing the \u003cem\u003ecT\u003c/em\u003e product in the absence of light). \u003cstrong\u003eb\u003c/strong\u003e, Temperature dependence of the molar magnetic susceptibility (shown as the \u003cem\u003ecT\u003c/em\u003e product; at 0.1 T) for \u003cstrong\u003e1\u003c/strong\u003e (before irradiation, black points), \u003cstrong\u003e2 \u003c/strong\u003e(after 405-nm irradiation, blue points) and \u003cstrong\u003e1\u003c/strong\u003e (after thermal relaxation of \u003cstrong\u003e2\u003c/strong\u003e, orange points). For comparison, the green points show \u003cem\u003ecT\u003c/em\u003e(\u003cem\u003eT\u003c/em\u003e) for the reference [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3-\u003c/sup\u003e product \u003cstrong\u003e3\u003c/strong\u003e. All curves were recorded at a 2 K·min\u003csup\u003e-1\u003c/sup\u003e heating rate. \u003cstrong\u003ec\u003c/strong\u003e, Magnetization (\u003cem\u003eM\u003c/em\u003e) versus magnetic field (\u003cem\u003eH\u003c/em\u003e) plots for \u003cstrong\u003e1\u003c/strong\u003e (before irradiation, square symbols) and \u003cstrong\u003e2\u003c/strong\u003e (after 405 nm irradiation, circle symbols) recorded between 2.0 and 6.0 K, compared to XMCD signal (diamond symbol) at the Mo \u003cem\u003eL\u003c/em\u003e\u003csub\u003e3 \u003c/sub\u003eedge at 2520 eV for \u003cstrong\u003e2\u003c/strong\u003e at 4 K. \u003cstrong\u003ed\u003c/strong\u003e, Time evolution of the \u003cem\u003ecT\u003c/em\u003e product (at 0.1 T) of \u003cstrong\u003e1\u003c/strong\u003e during 10 consecutive cycles of 405-nm irradiation at 10 K, each followed by thermal relaxation at 200 K, confirming reversible photo-induced transformation at the bulk level. \u003cstrong\u003ee\u003c/strong\u003e, Time evolution of the \u003cem\u003ecT\u003c/em\u003e product (at 0.1 T) recorded for \u003cstrong\u003e2\u003c/strong\u003e in the dark after 405-nm irradiation of \u003cstrong\u003e1\u003c/strong\u003e at 10 K and subsequent heating to target temperatures in the 132.5-147.5 K range (heating rate 15 K·min\u003csup\u003e‑1\u003c/sup\u003e). Each relaxation study was followed by heating to 200 K to ensure complete restoration of \u003cstrong\u003e1\u003c/strong\u003e. Solid lines represent fits to stretched exponential decay functions, providing relaxation times, \u003cem\u003eτ\u003c/em\u003e. \u003cstrong\u003ef\u003c/strong\u003e, Plot of the relaxation time, \u003cem\u003eτ\u003c/em\u003e, as a function of \u003cem\u003eT\u003c/em\u003e\u003csup\u003e‑1\u003c/sup\u003e (diamond symbol), with an Arrhenius fit (solid line) depicting the thermally activated relaxation process of \u003cstrong\u003e2 \u003c/strong\u003ereverting to \u003cstrong\u003e1\u003c/strong\u003e.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/6cac225b73733b2816fa22ee.png"},{"id":92473381,"identity":"ed8f23bc-cba0-43bc-bba6-b15cb34fbad6","added_by":"auto","created_at":"2025-09-30 07:05:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1603392,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/0002c3a1-0dd7-4d05-ad50-a1031b33efb1.pdf"},{"id":78880803,"identity":"c2e2f294-2dec-42cf-b33a-530064927783","added_by":"auto","created_at":"2025-03-20 08:27:49","extension":"mp4","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":3831384,"visible":true,"origin":"","legend":"\u003cp\u003eAnimation showing the reversible light-induced photodissociation and photoassociation reactions within the single crystal of potassium heptacyanomolybdate(III) dihydrate\u003c/p\u003e","description":"","filename":"Animation1.mp4","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/e6c5b2b8ba7629312497c14a.mp4"},{"id":78880801,"identity":"b6426c41-4958-4b83-bebc-d269c538be35","added_by":"auto","created_at":"2025-03-20 08:27:49","extension":"mp4","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":575433,"visible":true,"origin":"","legend":"\u003cp\u003ePhotoirradiation of a single crystal of potassium heptacyanomolybdate(III) dihydrate at 8 K using 405 nm laser light\u003c/p\u003e","description":"","filename":"Movie1.mp4","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/38fcc25d1e409f35aa42636a.mp4"},{"id":78880794,"identity":"974c987f-26ad-447f-875f-f9fdb00873d4","added_by":"auto","created_at":"2025-03-20 08:27:48","extension":"pdf","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":72312,"visible":true,"origin":"","legend":"\u003cp\u003eArticle File - 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compound KcryptMoCN6_293K CCDC2352266_checkcif\u003c/p\u003e","description":"","filename":"compoundKcryptMoCN6293KCCDC2352266checkcif.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/a651489fc1f6e011c336e8f4.pdf"},{"id":78880987,"identity":"bb9a043f-8008-4c83-9eb4-8e4f2c187d70","added_by":"auto","created_at":"2025-03-20 08:35:49","extension":"txt","order_by":15,"title":"","display":"","copyAsset":false,"role":"supplement","size":513675,"visible":true,"origin":"","legend":"\u003cp\u003eArticle File - compound 2' CCDC2352261\u003c/p\u003e","description":"","filename":"compound2CCDC2352261.txt","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/15e7992b6cc9e838376ec711.txt"},{"id":78880830,"identity":"32d02d07-d62a-46ab-8186-a85592199e4c","added_by":"auto","created_at":"2025-03-20 08:27:52","extension":"txt","order_by":16,"title":"","display":"","copyAsset":false,"role":"supplement","size":470509,"visible":true,"origin":"","legend":"\u003cp\u003eArticle File - compound 2 CCDC2352263\u003c/p\u003e","description":"","filename":"compound2CCDC2352263.txt","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/eb3d73b5618f1195546916db.txt"},{"id":78880816,"identity":"7a034cd4-c9cf-4cd2-9070-22373b5658f7","added_by":"auto","created_at":"2025-03-20 08:27:50","extension":"txt","order_by":17,"title":"","display":"","copyAsset":false,"role":"supplement","size":5268299,"visible":true,"origin":"","legend":"\u003cp\u003eArticle File - compound KcryptMoCN6_293K CCDC2352266\u003c/p\u003e","description":"","filename":"compoundKcryptMoCN6293KCCDC2352266.txt","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/1b22799af1c8b828d6a66f81.txt"},{"id":78880828,"identity":"fb41f838-b97e-4c26-8192-acd56d761e5b","added_by":"auto","created_at":"2025-03-20 08:27:51","extension":"txt","order_by":18,"title":"","display":"","copyAsset":false,"role":"supplement","size":503580,"visible":true,"origin":"","legend":"\u003cp\u003eArticle File - compound 1test2 CCDC2352260\u003c/p\u003e","description":"","filename":"compound1test2CCDC2352260.txt","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/d033c34fb13a348dae40585f.txt"},{"id":78881628,"identity":"b5ec96b2-2307-4843-bc71-dd4086a82913","added_by":"auto","created_at":"2025-03-20 08:43:51","extension":"txt","order_by":19,"title":"","display":"","copyAsset":false,"role":"supplement","size":464250,"visible":true,"origin":"","legend":"\u003cp\u003eArticle File - compound 1 relaxed638nm CCDC2352259\u003c/p\u003e","description":"","filename":"compound1relaxed638nmCCDC2352259.txt","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/7d355e9911834a52c0b52cf8.txt"},{"id":78880989,"identity":"b6c26c83-1f41-462b-bce9-6a32194ef85c","added_by":"auto","created_at":"2025-03-20 08:35:50","extension":"txt","order_by":20,"title":"","display":"","copyAsset":false,"role":"supplement","size":4749591,"visible":true,"origin":"","legend":"\u003cp\u003eArticle File - compound KcryptMoCN6_100K CCDC2352265\u003c/p\u003e","description":"","filename":"compoundKcryptMoCN6100KCCDC2352265.txt","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/e348ba6a3ded16721147a302.txt"},{"id":78880807,"identity":"7257aed9-52f7-4a14-9e6d-ae3bf4fe92c1","added_by":"auto","created_at":"2025-03-20 08:27:49","extension":"txt","order_by":21,"title":"","display":"","copyAsset":false,"role":"supplement","size":480645,"visible":true,"origin":"","legend":"\u003cp\u003eArticle File - compound 1' CCDC2352264\u003c/p\u003e","description":"","filename":"compound1CCDC2352264.txt","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/6e234f4f043c955c27c10abe.txt"},{"id":78880812,"identity":"3842acb4-d51d-48fb-9df7-11914435e39a","added_by":"auto","created_at":"2025-03-20 08:27:50","extension":"pdf","order_by":22,"title":"","display":"","copyAsset":false,"role":"supplement","size":72038,"visible":true,"origin":"","legend":"\u003cp\u003eArticle File - compound 1 CCDC2352257_checkcif\u003c/p\u003e","description":"","filename":"compound1CCDC2352257checkcif.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/57dd21bbeff45d2f786874c6.pdf"},{"id":78880800,"identity":"423b6f5b-a4cf-4f94-b592-c86b1262b3ce","added_by":"auto","created_at":"2025-03-20 08:27:49","extension":"pdf","order_by":23,"title":"","display":"","copyAsset":false,"role":"supplement","size":3108117,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SI.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5611691/v1/89db9a079d9c41209a43ae63.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Reversible single crystal photochemistry and spin state switching in a metal-cyanide complex","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe absorption of visible light induces substantial changes in the electronic structure of molecules, playing a crucial role in photo-activated processes such as photosynthesis, solar energy harvesting, and photocatalysis. In most cases, the lifetime of the photoexcited state is extremely short. However, certain systems can be thermally trapped in a metastable state for hours, days or even months before eventually relaxing back to the ground state. This phenomenon enables photoswitching between states with distinct physical and chemical properties. The potential applications of photoswitchable materials are vast, including light-responsive molecular junctions,\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e molecular valves and machines,\u003csup\u003e\u003cspan additionalcitationids=\"CR3\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e information storage and processing devices,\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e and solar energy harvesting.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e A well-known example of photoswitching is the photoisomerization of rhodopsin, the protein responsible for vision in humans and many animals, which continues to be an active area of research.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eMany current research efforts in the field of photoswitchable materials focus on light-induced isomerization of purely organic molecules.\u003csup\u003e\u003cspan additionalcitationids=\"CR12 CR13\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e While these systems dominate the field, they do not exhibit changes in magnetic properties upon light absorption with only a few exceptions.\u003csup\u003e\u003cspan additionalcitationids=\"CR16\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e To achieve the photomagnetic response, these organic photoswitchable molecules must be coupled with metal centers.\u003csup\u003e\u003cspan additionalcitationids=\"CR19 CR20 CR21 CR22\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e However, this approach has thus far only produced paramagnetic systems. In contrast, inorganic photomagnetic compounds typically rely on spin crossover (SCO) sites, which display light-induced excited spin state trapping (LIESST) originating at the metal center.\u003csup\u003e\u003cspan additionalcitationids=\"CR25 CR26\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e Unfortunately, the photoexcited state of SCO systems suffers from rapid thermal relaxation above 80 K, which limits their practical applications as photomagnetic switches at room temperature. Nevertheless, a careful analysis of the microscopic origins of their photomagnetism offers clues for designing high-temperature magnetic photoswitches. For example, in the [Fe\u003csup\u003eII\u003c/sup\u003e(LN\u003csub\u003e5\u003c/sub\u003e)(CN)\u003csub\u003e2\u003c/sub\u003e]\u0026middot;MeOH SCO complex, the formation and breaking of a coordination bond extends the lifetimes of its photoexcited state that can be observed up to 130 K.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Building on these insights, we propose a strategy towards high-temperature photomagnetic systems based on photochemistry \u0026ndash; specifically, the photodissociation of cyanometallates. While cyanide photodissociation has been extensively studied in aqueous solutions\u003csup\u003e\u003cspan additionalcitationids=\"CR30 CR31\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e and, to a lesser extent, in aprotic solvents or solids,\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e it has always been found to be irreversible. Herein, we demonstrate a fully reversible metal-cyanide photodissociation occurring in single crystals of potassium heptacyanomolybdate(III) dihydrate\u003csup\u003e\u003cspan additionalcitationids=\"CR36\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e K\u003csub\u003e4\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO (\u003cb\u003e1\u003c/b\u003e), and compare it to the photodissociation observed in its solution. The single crystals of \u003cb\u003e1\u003c/b\u003e undergo a complete metal-cyanide bond photodissociation upon absorption of violet light, resulting in the formation of a hexacoordinate complex, K\u003csub\u003e4\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u0026middot;CN\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO (\u003cb\u003e2\u003c/b\u003e). Remarkably, compound \u003cb\u003e1\u003c/b\u003e is fully recovered through red light absorption by \u003cb\u003e2\u003c/b\u003e, representing an unprecedented solid-state ligand photoassociation reaction. These transformations are non-destructive and maintain the crystallinity of \u003cb\u003e1\u003c/b\u003e throughout the process. If the unique properties of [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e could be extended to its bimetallic assemblies,\u003csup\u003e\u003cspan additionalcitationids=\"CR36 CR37 CR38 CR39 CR40\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e this would enable the light control of the long-range magnetic ordering,\u003csup\u003e\u003cspan additionalcitationids=\"CR39 CR40\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e or slow relaxation of the magnetization\u003csup\u003e\u003cspan additionalcitationids=\"CR43\" citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e in these compounds.\u003c/p\u003e \u003cp\u003eThe photochemical switching behavior of \u003cb\u003e1\u003c/b\u003e was thoroughly characterized using a combination of techniques, including single-crystal X-ray diffraction (scXRD), infrared (IR) and electron paramagnetic resonance (EPR) spectroscopies, optical reflectivity studies and X-ray absorption spectroscopy (XAS), complemented by magnetization measurements and X-ray magnetic circular dichroism (XMCD). For comparison, the product of the irreversible photodissociation of \u003cb\u003e1\u003c/b\u003e in acetonitrile (MeCN) solution, [K(crypt-222)]\u003csub\u003e3\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u0026middot;2MeCN (\u003cb\u003e3\u003c/b\u003e), was also isolated and fully characterized.\u003c/p\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSynthesis, solid state photochemistry and crystallography\u003c/h2\u003e \u003cp\u003e \u003cb\u003e1\u003c/b\u003e was obtained by reacting MoCl\u003csub\u003e3\u003c/sub\u003e(THF)\u003csub\u003e3\u003c/sub\u003e with KCN in deoxygenated H\u003csub\u003e2\u003c/sub\u003eO, followed by crystallization from a H\u003csub\u003e2\u003c/sub\u003eO/MeOH mixture, similar to the method reported by Young.\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e The identity and purity of \u003cb\u003e1\u003c/b\u003e were confirmed through scXRD, PXRD (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e), IR spectroscopy (Fig. S2) and elemental analysis (see Methods). The crystal structure of \u003cb\u003e1\u003c/b\u003e was investigated by using synchrotron radiation (Diamond Light Source, I19) at 30 K. The experiments were conducted on two single crystals, both before and after 405 nm irradiation, followed either by 638 nm irradiation or by heating to 200 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e with crystallographic details). Before irradiation, the [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e anion adopts a capped trigonal prism (CTPR-7) geometry with seven cyanide ligands coordinated to the Mo\u003csup\u003eIII\u003c/sup\u003e center via carbon atoms (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea and Table S2). Structural analysis after 20 minutes of 405 nm irradiation reveals major structural changes in \u003cb\u003e1\u003c/b\u003e. The most striking feature is the \u0026lsquo;disappearance\u0026rsquo; of one cyanide ligand from the Mo coordination sphere. As a consequence, the Mo coordination number decreases from seven to six, forming K\u003csub\u003e4\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u0026middot;CN\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO (\u003cb\u003e2\u003c/b\u003e; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb and \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee, Animation 1). In this photoinduced structure, the remaining six cyanide ligands are arranged at the vertices of a distorted octahedron. The displaced C7N7 cyanide is located 4.195(10) \u0026Aring; from the Mo center, compared to 2.143(9) \u0026Aring; before irradiation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed), and is now trapped between two water molecules.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNotably, all other Mo-C bonds in the octahedral complex \u003cb\u003e2\u003c/b\u003e with an average length of 2.172(6) \u0026Aring;, become more uniform and slightly longer than 2.153(8) \u0026Aring; observed in the pristine \u003cb\u003e1\u003c/b\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed).\u003c/p\u003e \u003cp\u003eIndeed, the bond lengths in \u003cb\u003e2\u003c/b\u003e are very similar to those found in Li\u003csub\u003e3\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u0026middot;6DMF reported by Beauvais and Long.\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e The lattice volume changes from 759.04(13) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e for \u003cb\u003e1\u003c/b\u003e to 819.08(15) \u0026Aring;\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e for \u003cb\u003e2\u003c/b\u003e, representing a 7.9% expansion \u0026mdash; the largest volume increase in response to light irradiation ever reported (black symbols in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef and Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). This is also reflected in the change of the crystal size upon 405 nm irradiation at 8 K (Movie 1). Despite the large displacement of the C7N7 cyanide upon photodissociation to form \u003cb\u003e2\u003c/b\u003e, the original crystal structure is fully restored either through 638 nm irradiation or by heating the crystal to 200 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec and Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Both restored structures closely resemble the original \u003cb\u003e1\u003c/b\u003e in terms of molecular features: bond lengths, supramolecular arrangement and unit cell parameters.\u003c/p\u003e \u003cp\u003eThe observed photo-induced changes suggest that the photodissociation of [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e leads to the formation of [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e and a free CN\u003csup\u003e\u0026minus;\u003c/sup\u003e anion trapped between H\u003csub\u003e2\u003c/sub\u003eO molecules. This process appears to occur without any change in the Mo oxidation state, as indicated by minimal variation in Mo-C bond lengths (Fig. S3). The C7-N7 bond length of the photodissociated cyanide in \u003cb\u003e2\u003c/b\u003e measures 1.19(1) \u0026Aring;, which is typical for a 'free' cyanide, as seen in anhydrous KCN or NaCN.\u003csup\u003e\u003cspan additionalcitationids=\"CR47\" citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e This value is only slightly longer than the average C\u0026ndash;N bond length observed for cyanide ligands coordinated to Mo in \u003cb\u003e1\u003c/b\u003e (1.16(1) \u0026Aring;) or in other cyanometallates.\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSolution photochemistry\u003c/h3\u003e\n\u003cp\u003eSeveral transition metal cyanides are known to undergo photolysis in solution.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e However, compound \u003cb\u003e1\u003c/b\u003e has never been studied in this context, despite reports suggesting that heptacyanomolybdate(III) anion may 'transform' into hexacyanomolybdate(III).\u003csup\u003e\u003cspan additionalcitationids=\"CR52\" citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e Encouraged by the quantitative and reversible photochemical reactivity of \u003cb\u003e1\u003c/b\u003e in the solid-state, we decided to test its behavior in a non-aqueous solution. Since \u003cb\u003e1\u003c/b\u003e is insoluble in organic solvents, [2.2.2]cryptand (crypt-222)\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e was added to dissolve it in anhydrous acetonitrile. The resulting yellow solution was irradiated with either violet or white light (Fig. S4), and in both cases, bleaching occured within minutes (Fig. S5). This is consistent with the photobleaching of a single crystal of \u003cb\u003e1\u003c/b\u003e observed at 8 K (Movie 1). Colorless plate crystals of [K(crypt-222)]\u003csub\u003e3\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u0026middot;2CH\u003csub\u003e3\u003c/sub\u003eCN (\u003cb\u003e3\u003c/b\u003e) were isolated as the sole Mo-based product from this solution (Table S3 and Figs. S6 and S7) while the by-product [K(crypt-222)]CN remains in solution. The isolated compound showed no electronic transitions in the visible range of the UV-vis spectrum, either in the solid state or in MeCN solution (Fig. S8), supporting that bleaching results from the light-induced dissociation of a metal-cyanide bond. The scXRD analysis of \u003cb\u003e3\u003c/b\u003e revealed a nearly identical geometry of the [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e anion to that observed in \u003cb\u003e2\u003c/b\u003e (see Supplementary Information for detailed discussion). Thus, the photochemical reactivity of [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e in aprotic solution mirrors the solid state behavior, though it is irreversible in solution.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eSpectroscopic characterization\u003c/h3\u003e\n\u003cp\u003eOptical reflectivity measurements were conducted on \u003cb\u003e1\u003c/b\u003e in the solid state, revealing broad bands that are nearly temperature independent (Figs. S9a-b). Exposure to specific wavelengths within the 365\u0026ndash;1050 nm range at 10 K produced an increase in reflectivity below 800 nm, particularly under 365, 385 and 405 nm irradiations (Figs. S9c-f), which again is in line with the bleaching of the green crystals of \u003cb\u003e1\u003c/b\u003e upon 405 nm irradiation producing nearly colorless \u003cb\u003e2\u003c/b\u003e (Movie 1).\u003c/p\u003e \u003cp\u003eTo obtain the spectrum of the photoexcited state, which persists up to 150 K (Fig. S9f), \u003cb\u003e1\u003c/b\u003e was irradiated for 2 hours at 405 nm \u0026mdash; the same wavelength used in photocrystallography (Fig. S9e). Above 150 K, the characteristic bands of the photoexcited state begin to fade, disappearing entirely above 160 K as the original spectrum of \u003cb\u003e1\u003c/b\u003e is restored (Fig. S9e-f). Additionally, \u003cb\u003e1\u003c/b\u003e and its photo-induced state \u003cb\u003e2\u003c/b\u003e can be reversibly cycled using 385 and 660 nm irradiations over multiple cycles (Fig. S10).\u003c/p\u003e \u003cp\u003eThe reversible photodissociation of the metal-cyanide bond was also confirmed by IR spectroscopy (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) in a polycrystalline sample of \u003cb\u003e1\u003c/b\u003e. The C\u0026thinsp;\u0026equiv;\u0026thinsp;N stretching vibrations give strong IR absorption bands, highly sensitive to the metal center's electronic configuration and the complex's geometry and symmetry. At 100 K, the \u003cem\u003eν\u003c/em\u003e(C\u0026thinsp;\u0026equiv;\u0026thinsp;N) band for \u003cb\u003e1\u003c/b\u003e appears as two peaks: a main one at 2069 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and a weaker component at 2105 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, consistent with the literature values.\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e Following 405 nm irradiation, the band maximum shifts to 2104 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with a shoulder at 2073 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The main peak position in \u003cb\u003e2\u003c/b\u003e aligns well with the cyanide stretching observed in Li\u003csub\u003e3\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u0026middot;6DMF, reported at 2115 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e,\u003csup\u003e45\u003c/sup\u003e with a small difference highlighting the influence of the second coordination sphere of [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e. Further support comes from the IR spectrum of \u003cb\u003e3\u003c/b\u003e, which shows a single, narrow \u003cem\u003eν\u003c/em\u003e(C\u0026thinsp;\u0026equiv;\u0026thinsp;N) band at 2088 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig. S11). It is worth noting that heating \u003cb\u003e2\u003c/b\u003e to 200 K fully restores the initial spectrum typical for \u003cb\u003e1\u003c/b\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003eXAS measurements at 4 K before and after irradiation support also the photo-induced formation of [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). In the initial Mo \u003cem\u003eL\u003c/em\u003e\u003csub\u003e2,3\u003c/sub\u003e-edge spectra of \u003cb\u003e1\u003c/b\u003e, prominent 'white line' resonances arise from 2\u003cem\u003ep\u0026rarr;\u003c/em\u003e4\u003cem\u003ed\u003c/em\u003e excitations, with fine structures reflecting ligand field splitting of the 4\u003cem\u003ed\u003c/em\u003e states. Following 405 nm irradiation, the two primary signals weaken, while lower-energy peaks intensify. This aligns with the capped trigonal prism geometry of the [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e anion, characterized by a single electron hole in the [(4\u003cem\u003ed\u003c/em\u003e\u003csub\u003ex2\u0026thinsp;\u0026minus;\u0026thinsp;y2\u003c/sub\u003e)(4\u003cem\u003ed\u003c/em\u003e\u003csub\u003exz\u003c/sub\u003e)]\u003csup\u003e3\u003c/sup\u003e orbital pair,\u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e leading to a lower intensity of the 2\u003cem\u003ep\u003c/em\u003e\u0026rarr;[(4\u003cem\u003ed\u003c/em\u003e\u003csub\u003ex2y2\u003c/sub\u003e)(4\u003cem\u003ed\u003c/em\u003e\u003csub\u003exz\u003c/sub\u003e)] transition (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec). In contrast, the nearly octahedral geometry of the photo-induced [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e corresponds to the \u003cem\u003et\u003c/em\u003e\u003csub\u003e2g\u003c/sub\u003e\u003csup\u003e3\u003c/sup\u003e\u003cem\u003ee\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e\u003csup\u003e0\u003c/sup\u003e configuration with three holes in the \u003cem\u003et\u003c/em\u003e\u003csub\u003e2g\u003c/sub\u003e orbitals. This results in a higher intensity of the low energy pre-edge region at the expense of the main resonances, mirroring the comparison between the XAS spectra of \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e (Fig. S12). The integrated white-line intensities for \u003cb\u003e1\u003c/b\u003e, \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e are nearly identical, indicating consistent 4\u003cem\u003ed\u003c/em\u003e occupancy across these compounds. Further insights into the unoccupied Mo 4\u003cem\u003ed\u003c/em\u003e orbitals were provided by the XMCD measurements, showing that the dichroic signals for \u003cb\u003e2\u003c/b\u003e are predominantly concentrated at the low energy peaks for both Mo \u003cem\u003eL\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e and \u003cem\u003eL\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e edges (light blue line in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eApplication of the magneto-optical sum rules\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e, \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e to the XMCD spectra reveals contributions from both spin magnetic moment (\u003cem\u003eM\u003c/em\u003e\u003csub\u003eS\u003c/sub\u003e) and orbital magnetic moment (\u003cem\u003eM\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e) in the magnetization of the photo-product \u003cb\u003e2\u003c/b\u003e. The calculated values of \u003cem\u003eM\u003c/em\u003e\u003csub\u003eS\u003c/sub\u003e = 2.86 \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003eB\u003c/sub\u003e and \u003cem\u003eM\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e = -0.16 \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003eB\u003c/sub\u003e agree well with the those expected for high-spin hexacyanomolybdate(III), where the \u003cem\u003et\u003c/em\u003e\u003csub\u003e2g\u003c/sub\u003e\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e\u003cem\u003ee\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e\u003csup\u003e0\u003c/sup\u003e electron configuration implies a minimal orbital contribution.\u003c/p\u003e \u003cp\u003eThe solid-state EPR spectrum of powdered \u003cb\u003e1\u003c/b\u003e is consistent with the findings of Rossman et al.\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e showing an \u003cem\u003eS\u003c/em\u003e = \u0026frac12; system with slight \u003cem\u003eg\u003c/em\u003e-factor anisotropy (\u003cem\u003eg\u003c/em\u003e\u003csub\u003ez\u003c/sub\u003e = 2.108, \u003cem\u003eg\u003c/em\u003e\u003csub\u003exy\u003c/sub\u003e = 1.964, black line in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). Upon 405-nm irradiation at 10 K, the \u003cem\u003eS\u003c/em\u003e = \u0026frac12; signal gradually decreases and vanishes within tens of minutes, while a new broad resonance appears around 160 mT, corresponding to \u003cem\u003eg\u003c/em\u003e\u0026rsquo; \u0026asymp; 4.3 (blue line in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). This EPR signal is attributed to the anticipated \u003cem\u003eS\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3/2 state of \u003cb\u003e2\u003c/b\u003e, as inferred from the XMCD study. Following thermal relaxation at room temperature, the initial \u003cem\u003eS\u003c/em\u003e = \u0026frac12; spectrum of \u003cb\u003e1\u003c/b\u003e is recovered, while the 160 mT signal vanishes (orange line in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed), further confirming the reversible transformation observed in crystallographic, optical reflectivity, and IR studies.\u003c/p\u003e\n\u003ch3\u003eComputational studies\u003c/h3\u003e\n\u003cp\u003eTo investigate the photoswitching mechanism, \u003cem\u003eab initio\u003c/em\u003e calculations (CASSCF/CASPT2) of the electronic transitions in \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e were conducted. For \u003cb\u003e1\u003c/b\u003e, the light-driven transformation to \u003cb\u003e2\u003c/b\u003e likely involves either a spin-allowed transition calculated at 361.5 nm or spin-forbidden ones at 373.4 and 460.9 nm (Table S5). However, in the solid state, the absorption bands are broad and heavily overlap, allowing efficient photoconversion to \u003cb\u003e2\u003c/b\u003e experimentally at 365, 385 and 405 nm, as observed in optical reflectivity studies (Fig. S9c).\u003c/p\u003e \u003cp\u003eIn contrast, the calculated spin-allowed transitions for \u003cb\u003e2\u003c/b\u003e fall within the deep UV region, while the \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e\u003cem\u003eA\u003c/em\u003e\u003csub\u003e2g\u003c/sub\u003e\u003cem\u003e\u0026rarr;\u003c/em\u003e\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e\u003cem\u003eT\u003c/em\u003e\u003csub\u003e2g\u003c/sub\u003e transitions appear at 587.6 and 604.8 nm, matching the de-excitation wavelengths (Table S6 and Fig. S10a). These spin-forbidden transitions gain intensity due to a slight distortion in the octahedral geometry of [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e observed in \u003cb\u003e2\u003c/b\u003e, potentially enabling the light-induced reverse metal-cyanide bond association process.\u003c/p\u003e \u003cp\u003ePeriodic DFT geometry optimizations accurately reproduce changes in the metric parameters of the crystal structures and the molecular geometry of the complex associated with a spin-state change at the Mo\u003csup\u003eIII\u003c/sup\u003e center. The \u003cb\u003e1\u003c/b\u003e\u0026rarr;\u003cb\u003e2\u003c/b\u003e transformation was reproduced by re-optimizing the low-spin state structure of \u003cb\u003e1\u003c/b\u003e, fixing the number of unpaired electrons per Mo to 3 within the first optimization step, as inferred for the high-spin state structure of \u003cb\u003e2\u003c/b\u003e from EPR and XMCD (see Methods and Supplementary Information for details). The reverse \u003cb\u003e2\u003c/b\u003e\u0026rarr;\u003cb\u003e1\u003c/b\u003e transformation was similarly modeled by re-optimizing the structure of \u003cb\u003e2\u003c/b\u003e for one unpaired electron per Mo. Figs. S13-S16 and Table S7-S8 compare the experimental and optimized geometries of the [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e and [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e anions in their respective crystal structures.\u003c/p\u003e\n\u003ch3\u003ePhotoswitching of the magnetization\u003c/h3\u003e\n\u003cp\u003eThe magnetic and \u003cem\u003ein situ\u003c/em\u003e photomagnetic studies of \u003cb\u003e1\u003c/b\u003e were conducted using variable temperature/magnetic field magnetometry with light delivered to the sample chamber through an optical fiber. The temperature dependence of the \u003cem\u003eχT\u003c/em\u003e product (product of the molar magnetic susceptibility,\u003cem\u003e χ\u003c/em\u003e, and temperature, \u003cem\u003eT\u003c/em\u003e) measured at 0.1 T for pristine \u003cb\u003e1\u003c/b\u003e shows behavior typical of an \u003cem\u003eS\u003c/em\u003e = \u0026frac12;, \u003cem\u003eg\u003c/em\u003e\u0026thinsp;=\u0026thinsp;2.0 paramagnetic system (Fig. S17a), in agreement with the EPR results (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). Upon cooling below 50 K, the \u003cem\u003eχT\u003c/em\u003e(\u003cem\u003eT\u003c/em\u003e) data deviates from the constant value of 0.36 cm\u003csup\u003e3\u003c/sup\u003e\u0026middot;K\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, ultimately decreasing to 0.03 cm\u003csup\u003e3\u003c/sup\u003e\u0026middot;K\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 1.8 K, indicating supramolecular antiferromagnetic interactions between \u003cem\u003eS\u003c/em\u003e = \u0026frac12; Mo\u003csup\u003eIII\u003c/sup\u003e spins. Below 3.5 K, these couplings lead to an antiferromagnetically ordered state, as evidenced by the \u003cem\u003eM\u003c/em\u003e(\u003cem\u003eH\u003c/em\u003e) data (\u003cem\u003eM\u003c/em\u003e - molar magnetization, \u003cem\u003eH\u003c/em\u003e - magnetic field strength; Fig. S17b) and corroborated by low-temperature heat capacity measurements, discussed in detail in the Supplementary Information (Figs. S17-S19).\u003c/p\u003e \u003cp\u003ePhotomagnetic experiments (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) conducted on \u003cb\u003e1\u003c/b\u003e at 10 K show an increase in \u003cem\u003eχT\u003c/em\u003e from 0.30 to 1.79 cm\u003csup\u003e3\u003c/sup\u003e\u0026middot;K\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e upon 405-nm irradiation (ca. 5 mW\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, 2 h, violet and black symbols in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The \u003cem\u003eχT\u003c/em\u003e(\u003cem\u003eT\u003c/em\u003e) profile recorded in the dark, immediately after irradiation, reveals a plateau around 1.84 cm\u003csup\u003e3\u003c/sup\u003e\u0026middot;K\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the 20\u0026ndash;100 K range (blue symbols in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) close to 1.875 cm\u003csup\u003e3\u003c/sup\u003e\u0026middot;K\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e expected for a magnetically isotropic \u003cem\u003eS\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3/2 state (with \u003cem\u003eg\u003c/em\u003e\u0026thinsp;=\u0026thinsp;2). This aligns with the 1.82 cm\u003csup\u003e3\u003c/sup\u003e\u0026middot;K\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e value observed for \u003cb\u003e3\u003c/b\u003e (green symbols in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb; for details see Supplementary Information, Figs. S20-21). Additionally, the \u003cem\u003eM\u003c/em\u003e(\u003cem\u003eH\u003c/em\u003e) dependence for \u003cb\u003e2\u003c/b\u003e shows a magnetization close to 3.0 \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003eB\u003c/sub\u003e at 7 T, indicating a high-spin \u003cem\u003eS\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3/2 configuration (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), in good agreement with XMCD results and the magnetic data for \u003cb\u003e3\u003c/b\u003e (Fig. S21). Collectively, these photomagnetic experiments at 10 K suggest thermal trapping of the \u003cem\u003eS\u003c/em\u003e\u0026thinsp;=\u0026thinsp;3/2 metastable state, analogous to spin-crossover systems, confirming the formation of light-induced high-spin hexacyanomolybdate(III) complex. As the temperature rises (at 2 K\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), the photoinduced state relaxes to the original \u003cem\u003eS\u003c/em\u003e = \u0026frac12; ground state of the [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e complex around 150 K, indicated by a sharp drop in the \u003cem\u003eχT\u003c/em\u003e product (blue symbol in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). Notably, the relaxation temperature of 150 K for \u003cb\u003e2\u003c/b\u003e surpasses that of previously reported spin-crossover systems,\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u003c/sup\u003e suggesting an unusually long lifetime of the photoinduced state in this molybdenum cyanide complex. To quantify the characteristic time of the thermally induced \u003cb\u003e2\u003c/b\u003e\u0026rarr;\u003cb\u003e1\u003c/b\u003e relaxation, time-dependent magnetization studies were performed at several temperatures in the 132.5-147.5 K range (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee). The temperature dependence of the estimated relaxation time (Table S9) follows an Arrhenius behavior with an activation energy of 4770(90) K (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef). The relaxation behavior of \u003cb\u003e2\u003c/b\u003e is comparable with that reported for the record-holding photomagnetic molecular Prussian blue analogue {Co\u003csub\u003e4\u003c/sub\u003eFe\u003csub\u003e4\u003c/sub\u003e}.\u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u003c/sup\u003e This result indicates that considerable thermal energy is needed for the complex to overcome the energetic barrier between the high-spin and low-spin states. This aligns with the substantial structural reconfiguration upon the \u003cb\u003e2\u003c/b\u003e\u0026rarr;\u003cb\u003e1\u003c/b\u003e relaxation \u0026mdash; specifically, the displacement of the C7N7 cyanide group by about 2 \u0026Aring; back towards the Mo center. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed, the robustness and reversibility of the 405 nm irradiation at 10 K (\u003cb\u003e1\u003c/b\u003e\u0026rarr;\u003cb\u003e2\u003c/b\u003e) and thermal relaxation (\u003cb\u003e2\u003c/b\u003e\u0026rarr;\u003cb\u003e1\u003c/b\u003e) processes was confirmed across multiple cycles by magnetization measurements. Moreover, similar behavior is observed at the record-high temperature of 100 K over at least four consecutive cycles of 405 nm irradiation and thermal relaxation (Fig. S22) with only slight radiation damage to the sample.\u003c/p\u003e \u003cp\u003eImportantly, the substantial energy barrier of the thermally activated metal-cyanide association (\u003cb\u003e2\u003c/b\u003e\u0026rarr;\u003cb\u003e1\u003c/b\u003e) does not prevent this process from happening under a 638 nm irradiation at low temperatures as demonstrated by scXRD at 30 K (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) or optical reflectivity at 10 K (Fig. S10). Photomagnetic experiments at 10 K further support this observation (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea): following the initial 405 nm irradiation of \u003cb\u003e1\u003c/b\u003e at 10 K, which photoconverts \u003cb\u003e1\u003c/b\u003e into \u003cb\u003e2\u003c/b\u003e, a subsequent 638 nm irradiation fully restores the magnetic susceptibility of \u003cb\u003e1\u003c/b\u003e (red symbols in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea).\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eA single crystal of the inorganic compound K\u003csub\u003e4\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO demonstrates a reversible photochemical reaction, enabling the molybdenum center to switch between the 7-coordinate low-spin and 6-coordinate high-spin geometries. Exposure to 405 nm light causes dissociation of one of the seven metal-cyanide bonds, resulting in a 2 \u0026Aring; displacement from the Mo center, while subsequent irradiation at 638 nm reverses this effect, restoring the original structure. This bidirectional photoswitching is remarkably robust and non-destructive, yielding colossal structural, electronic, and magnetic changes. Interestingly, photodissociation in solution is irreversible, permitting isolation of the [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e6\u003c/sub\u003e]\u0026sup3;⁻ complex.\u003c/p\u003e \u003cp\u003eK\u003csub\u003e4\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO stands out as a unique example of bidirectional photoswitching in an inorganic coordination compound in the crystalline phase, with its high performance highlighted by three key factors: (i) photo-induced ligand dissociation achieves 100% efficiency, (ii) the photodissociation is fully reversible with light of a different wavelength resulting in photo-association, and (iii) the record-high relaxation temperature allows photoswitching at temperatures exceeding 100 K. These exceptional photochemical properties set the stage for designing new photomagnetic materials that could switch between paramagnetic and ferromagnetic (or ferrimagnetic) states at room temperature. By leveraging similar mechanisms in coordination polymers and materials based on the [Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e building-block, or its analogs, this approach holds promise for advanced high-temperature photomagnetic applications.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eSyntheses\u003c/h2\u003e \u003cp\u003eMoCl\u003csub\u003e3\u003c/sub\u003e(THF)\u003csub\u003e3\u003c/sub\u003e precursor was synthesized under an argon gas atmosphere inside the Inert PureLab HE glovebox (O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.1 ppm and H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;\u0026lt;\u0026thinsp;0.5 ppm), in a direct, two-step reduction of MoCl\u003csub\u003e5\u003c/sub\u003e according to a literature method.\u003csup\u003e\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e All other reagents were commercially available and used without further purification. The distilled water used in the synthesis of \u003cb\u003e1\u003c/b\u003e was deoxygenated by refluxing under an argon atmosphere for at least 12 hours. Deoxygenated methanol, acetonitrile and diethyl ether were prepared by passing HPLC grade solvents through the Inert PureSolv EN7 solvent purification system.\u003c/p\u003e \u003cp\u003e \u003cb\u003eK\u003c/b\u003e \u003csub\u003e \u003cb\u003e4\u003c/b\u003e \u003c/sub\u003e \u003cb\u003e[Mo\u003c/b\u003e \u003csup\u003e \u003cb\u003eIII\u003c/b\u003e \u003c/sup\u003e \u003cb\u003e(CN)\u003c/b\u003e \u003csub\u003e \u003cb\u003e7\u003c/b\u003e \u003c/sub\u003e \u003cb\u003e]\u0026middot;2H\u003c/b\u003e \u003csub\u003e \u003cb\u003e2\u003c/b\u003e \u003c/sub\u003e \u003cb\u003eO\u003c/b\u003e (\u003cb\u003e1\u003c/b\u003e) was synthesized using a modified procedure based on established literature methods.\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e Inside the oxygen-free LABCONCO glovebox (O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.1%), a Schlenk flask was charged with MoCl\u003csub\u003e3\u003c/sub\u003e(THF)\u003csub\u003e3\u003c/sub\u003e (1.7 g, 4.1 mmol), KCN (3.0 g, 46.0 mmol; Sigma-Aldrich, \u0026ge;\u0026thinsp;96%) and 20 mL of deoxygenated water. The flask was sealed with a glass stopper, removed from the glovebox and connected to the Schlenk line. The mixture was subjected to vigorous magnetic stirring and slowly evacuated until it began to boil gently, at which point the connection to the Schlenk line was sealed. The flask was wrapped in an aluminum foil to prevent light exposure and heated to 50\u0026deg;C for 24 hours. Then the reaction mixture was transferred to an oxygen-free glovebox and vacuum-filtered through a sintered-glass funnel (G4) to remove any insoluble impurities. The filtrate was treated with 13 mL of deoxygenated methanol (MeOH), sealed in the glass jar and kept at\u003c/p\u003e \u003cp\u003e4\u0026deg;C for more than 24 hours. After that time, the precipitation of dark-green crystals was observed, which were collected by vacuum filtration inside oxygen-free glovebox and washed with MeOH:H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;=\u0026thinsp;7:3, MeOH:H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;=\u0026thinsp;9:1 and pure MeOH, then dried in vacuo for 15 minutes. The final product (1.5 g) was obtained in 78% yield, based on MoCl\u003csub\u003e3\u003c/sub\u003e(THF)\u003csub\u003e3\u003c/sub\u003e. Anal. Calcd. (found) for C\u003csub\u003e7\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eK\u003csub\u003e4\u003c/sub\u003eMoN\u003csub\u003e7\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, C: 17.87% (18.01%), H: 0.857% (0.873%), N: 20.84% (20.94%).\u003c/p\u003e \u003cp\u003e \u003cb\u003e[K(crypt-222)]\u003c/b\u003e \u003csub\u003e \u003cb\u003e3\u003c/b\u003e \u003c/sub\u003e \u003cb\u003e[Mo\u003c/b\u003e \u003csup\u003e \u003cb\u003eIII\u003c/b\u003e \u003c/sup\u003e \u003cb\u003e(CN)\u003c/b\u003e \u003csub\u003e \u003cb\u003e6\u003c/b\u003e \u003c/sub\u003e \u003cb\u003e]\u0026middot;2MeCN\u003c/b\u003e (\u003cb\u003e3\u003c/b\u003e) was synthesized via a photochemical route, similar to that utilized in photochemical preparation of [K(crypt-222)]\u003csub\u003e3\u003c/sub\u003e[Mo\u003csup\u003eIV\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u0026middot;3MeCN and [K(crypt-222)]\u003csub\u003e3\u003c/sub\u003e[W\u003csup\u003eIV\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u0026middot;4MeCN.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u003c/sup\u003e In the Inert PureLab HE glovebox (O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.1 ppm and H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;\u0026lt;\u0026thinsp;0.5 ppm), K\u003csub\u003e4\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO (0.24 g, 0.51 mmol) was placed in a vial filled with 7 mL of deoxygenated CH\u003csub\u003e3\u003c/sub\u003eCN in the presence of [2.2.2]cryptand (crypt-222; 0.9 g, 2.39 mmol; Merck, \u0026ge;\u0026thinsp;99%). The yellow suspension was irradiated using white light (Photonic LED-Light-Source F3000) until it turned into a nearly colorless solution. Then it was subjected to slow diffusion of diethyl ether vapor for two days, resulting in the precipitation of colorless plate crystals, which were collected by vacuum filtration. The purity of the product varies from batch to batch; at elevated ambient temperatures, increased evaporation of diethyl ether can lead to the precipitation of a by-product, [K(crypt-222)]CN. In this case, the product can be purified by dissolving it in 4 mL of CH\u003csub\u003e3\u003c/sub\u003eCN and then performing repeated diffusion of diethyl ether vapors. The final product (0.7 g) is obtained in 87% yield based on K\u003csub\u003e4\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO. The purity of the compound was checked by PXRD, with the experimental pattern (Fig. S7) matching perfectly the simulated one from the scXRD structural model obtained at 293 K (CCDC 2352266).\u003c/p\u003e \u003cp\u003e \u003cb\u003eStructure determination and refinement.\u003c/b\u003e Single-crystal X-ray diffraction experiments for the 1st crystal of pristine \u003cb\u003e1\u003c/b\u003e (CCDC 2352257), after 405 nm irradiation \u003cb\u003e2\u003c/b\u003e (CCDC 2352263), after 638 nm irradiation \u003cb\u003e1\u003c/b\u003e (2352259), for the 2nd crystal of pristine \u003cb\u003e1\u0026rsquo;\u003c/b\u003e (CCDC 2352264), after 405 nm irradiation \u003cb\u003e2\u0026rsquo;\u003c/b\u003e (CCDC 2352261) and after thermal relaxation \u003cb\u003e1\u0026rsquo;\u003c/b\u003e (CCDC 2352262) were all performed at 30 K using synchrotron X-ray radiation (Diamond Light Source, United Kingdom) at the I19 beamline (EH2) equipped with Newport 4-circle κ-diffractometer, monochromatic X-ray synchrotron radiation (λ\u0026thinsp;=\u0026thinsp;0.6889 \u0026Aring;), Pilatus 300K detector and an N-Helix Oxford Cryosystems gas cooler.\u003csup\u003e\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e \u003cem\u003eIn situ\u003c/em\u003e irradiation experiments were conducted with 405 nm (50 mW\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) and 638 nm (100 mW \u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) laser diodes (Thorlabs). Absorption corrections, data reduction and unit cell refinement were performed using Xia2 and CrysAlisPRO software (Rigaku Oxford Diffraction, 2019).\u003csup\u003e\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e\u003c/sup\u003e Introduction of hydrogen atoms in the structures of \u003cb\u003e2\u003c/b\u003e and \u003cb\u003e2\u0026rsquo;\u003c/b\u003e led to the instability of the refinement; therefore, to maintain consistency, we decided to remove hydrogen atoms from the corresponding structural models.\u003c/p\u003e \u003cp\u003eDiffraction experiments for \u003cb\u003e3\u003c/b\u003e were performed at 100 (CCDC 2352265) and 293 K (CCDC 2352266) using Bruker D8 Quest Eco diffractometer (Mo Kα radiation, Triumph\u0026reg; monochromator). Absorption corrections, data reduction and unit cell refinements were performed using SADABS and SAINT programs included in the Apex3 suite. All the structures were solved using direct methods and refined anisotropically using weighted full-matrix least-squares methods applied to \u003cem\u003eF\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e.\u003csup\u003e65\u0026minus;67\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003cb\u003ePowder X-ray diffraction.\u003c/b\u003e PXRD patterns were collected using Bruker D8 Advance Eco diffractometer equipped with Lynxeye silicon strip detector, Cu sealed tube radiation source and a capillary stage at room temperature. Samples were ground to a powder using an agate mortar inside the glovebox and loaded into glass capillaries 0.3 or 0.5 mm diameter. The capillaries were broken in half inside the glovebox and the open end was sealed using silicon grease before they were moved to the PXRD instrument and mounted on the goniometer head using bee wax. The simulated PXRD patterns were obtained from the scXRD data using Mercury software.\u003csup\u003e\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e\u003c/sup\u003e The experimental PXRD pattern for \u003cb\u003e1\u003c/b\u003e was subjected to background correction using the DIFFRAC algorithm implemented in the DIFFRAC.EVA V5 software.\u003c/p\u003e \u003cp\u003e \u003cb\u003ePeriodic density functional theory (DFT) calculations.\u003c/b\u003e All periodic DFT computations were performed using CRYSTAL23 software.\u003csup\u003e\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e\u003c/sup\u003e Initial geometrical parameters for low-spin and high-spin state structures (\u003cb\u003e1\u003c/b\u003e and \u003cb\u003e2\u003c/b\u003e, respectively) were adopted from the single-crystal X-ray diffraction data (\u003cb\u003e1\u003c/b\u003e - CCDC no. 2352258; \u003cb\u003e2\u003c/b\u003e - CCDC no. 2352263). Hydrogen atoms were added according to chemical criteria in the positions of electron density maxima from the diffraction data. Geometry optimizations within Unrestricted Kohn-Sham (UKS) formalism were conducted using several hybrid and range-separated hybrid functionals (details in Supplementary Information) and pob-TZVP basis set.\u003csup\u003e\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e, \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e\u003c/sup\u003e Best fit to the experimental data was obtained for HISS middle-range corrected range-separated hybrid functional.\u003csup\u003e\u003cspan additionalcitationids=\"CR73\" citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e\u003c/sup\u003e In each run, a 4 \u0026times; 4 \u0026times; 4 \u003cem\u003ek\u003c/em\u003e-point mesh in the reciprocal space was generated in line with the Monkhorst\u0026ndash;Pack methodology.\u003csup\u003e\u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e75\u003c/span\u003e\u003c/sup\u003e In all computations, tighter tolerances on the exchange and Coulomb integrals were used with the TOLINTEG set to 10, 10, 10, 10 and 20.\u003csup\u003e\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e\u003c/sup\u003e Numerical integration accuracy was provided using pruned XXLGRID comprising 99 radial and 1454 angular points in the region of chemical interest. The self-consistent field (SCF) convergence criterion was set to 10\u003csup\u003e\u0026ndash;8\u003c/sup\u003e a.u. To achieve the correct spin states for the computed systems, the difference between the number of α and β electrons was fixed at all \u003cem\u003ek\u003c/em\u003e-points during the initial 30 SCF cycles of the first optimization step. The same spin-locking protocol was used to reproduce the spin-state change. The pre-optimized structure of the low-spin state was fully re-optimized with the number of unpaired electrons per unit cell set as that expected for two high-spin Mo atoms (α-β\u0026thinsp;=\u0026thinsp;6) during the initial 50 SCF cycles of the first optimization step. High-spin to low-spin transformation was replicated using an analogous procedure setting the number of unpaired electrons per unit cell as α-β\u0026thinsp;=\u0026thinsp;2.\u003c/p\u003e \u003cp\u003e \u003cb\u003eCASSCF/CASPT2 calculations.\u003c/b\u003e Complete active space self-consistent field (CASSCF) and complete active space second-order perturbation theory (CASPT2) calculations of the excitation energies were done with the MOLCAS 8.2 code.\u003csup\u003e\u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e76\u003c/span\u003e\u003c/sup\u003e \u003cb\u003e1\u003c/b\u003e was represented as a [Mo(CN)\u003csub\u003e7\u003c/sub\u003e]\u003csup\u003e4\u0026minus;\u003c/sup\u003e unit embedded in 15 K\u003csup\u003e+\u003c/sup\u003e ions represented with model potentials and 203 optimized point charges to take into account the long-range Madelung potential. Relativistic corrections were accounted for with DKH Hamiltonian and dynamic electron correlation was calculated on the CASPT2 level. In case of \u003cb\u003e2\u003c/b\u003e, the [Mo(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e\u0026middot;CN\u003csup\u003e\u0026minus;\u003c/sup\u003e moiety was embedded in 16 K\u003csup\u003e+\u003c/sup\u003e ions represented with model potentials and 162 optimized point charges that represent the Madelung potential in the [Mo(CN)\u003csub\u003e6\u003c/sub\u003e]\u003csup\u003e3\u0026minus;\u003c/sup\u003e\u0026middot;CN\u003csup\u003e\u0026minus;\u003c/sup\u003e region. The molecular orbitals were expanded in the ANO-RCC all electron basis sets: Mo (6s, 5p, 4d, 1f), K (4s, 3p), C and N (3s, 2p, 1d), O (3s, 2p), H (2s).\u003csup\u003e\u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e77\u003c/span\u003e\u003c/sup\u003e The active space consists of 9 orbitals (5 Mo-4d, 2 CN-π, and 2 CN-π*) and 7 electrons. The ions used to embed the central unit are represented with the large core Hay and Wadt effective core potentials with a net charge of +\u0026thinsp;1.\u003csup\u003e78\u003c/sup\u003e CASPT2 correlates all the electrons except the deep core ones (C, N, O-1s, Mo-1s...3p). The standard IPEA\u0026thinsp;=\u0026thinsp;0.25 zeroth-order Hamiltonian was used and an imaginary shift of 0.15 Eh was added to the denominators to avoid the appearance of intruder states. The Cholesky decomposition is used to speed-up the handling of the two-electron integrals with a threshold of 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e Eh.\u003c/p\u003e \u003cp\u003e \u003cb\u003eX-ray Absorption Spectroscopy (XAS)\u003c/b\u003e. XAS spectra at Mo \u003cem\u003eL\u003c/em\u003e\u003csub\u003e2,3\u003c/sub\u003e-edges were obtained at the ID12 beamline (ESRF, The European Synchrotron).\u003csup\u003e\u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e79\u003c/span\u003e\u003c/sup\u003e The data were collected using total fluorescence yield detection mode and were subsequently corrected for reabsorption effects. Compound \u003cb\u003e2\u003c/b\u003e was obtained \u003cem\u003ein situ\u003c/em\u003e by irradiating \u003cb\u003e1\u003c/b\u003e for 12 hours at 4 K, using 405 nm laser with a power of \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;200 mW. The X-ray Magnetic Circular Dichroism (XMCD) spectra of \u003cb\u003e2\u003c/b\u003e were obtained as the difference between two consecutive XAS spectra recorded with opposite photon helicities and corrected for the incomplete circular polarization rate. To ensure the absence of experimental artefacts the measurements were systematically performed for both magnetic field directions. The normalized spectra were analyzed using the magneto-optical sum rules given for \u003cem\u003eL\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e and \u003cem\u003eL\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e absorption edges\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e to afford effective spin magnetic moment (\u003cem\u003eM\u003c/em\u003e\u003csub\u003e\u003cem\u003eS\u003c/em\u003e,eff\u003c/sub\u003e = -2\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003eS\u003c/em\u003e\u003csub\u003eeff\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;\u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003eB\u003c/sub\u003e) and orbital magnetic moment (\u003cem\u003eM\u003c/em\u003e\u003csub\u003e\u003cem\u003eL\u003c/em\u003e\u003c/sub\u003e = -\u0026lt;\u003cem\u003eL\u003c/em\u003e\u003csub\u003ez\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;\u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003eB\u003c/sub\u003e):\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\u0026lang;{L}_{z}\u0026rang;=\\frac{2\u0026lang;{n}_{h}\u0026rang;}{3}\\bullet\\:\\frac{{I}_{{L}_{3}}^{XMCD}+{I}_{{L}_{2}}^{XMCD}}{{I}_{{L}_{3}}^{XAS}+{I}_{{L}_{2}}^{XAS}}$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:\u0026lang;{S}_{eff}\u0026rang;=\u0026lang;{S}_{z}\u0026rang;+\\frac{7}{2}\u0026lang;{T}_{z}\u0026rang;=\\frac{\u0026lang;{n}_{h}\u0026rang;}{2}\\bullet\\:\\frac{{I}_{{L}_{3}}^{XMCD}-2{I}_{{L}_{2}}^{XMCD}}{{I}_{{L}_{3}}^{XAS}+{I}_{{L}_{2}}^{XAS}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe precise sample temperature and the absolute magnitude of the magnetic moment (\u003cem\u003eM\u003c/em\u003e\u003csub\u003etot\u003c/sub\u003e = \u003cem\u003eM\u003c/em\u003e\u003csub\u003eS\u003c/sub\u003e + \u003cem\u003eM\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e) were determined by scaling the field and temperature dependent XMCD signal intensity to the results of magnetic measurements for \u003cb\u003e2\u003c/b\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), following the previously described method.\u003csup\u003e\u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e80\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003cb\u003eInfra-red (IR) spectroscopy.\u003c/b\u003e IR spectra were recorded using Nicolet iN10 MX FT-IR microscope in a transmission mode. A polycrystalline sample was spread onto the surface of a BaF\u003csub\u003e2\u003c/sub\u003e optical window and sealed under Ar atmosphere inside a Linkam THMS350V temperature-controlled stage. The temperature control was achieved with a flow of liquid nitrogen, and the irradiation experiment was conducted using a 405 nm laser diode with a power \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;7\u0026ndash;8 mW\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cb\u003eUV-visible spectroscopy.\u003c/b\u003e UV-vis spectra for \u003cb\u003e1\u003c/b\u003e and \u003cb\u003e3\u003c/b\u003e were measured at room temperature in transmission mode using a Shimadzu UV-3600i Plus spectrophotometer. A solution of \u003cb\u003e1\u003c/b\u003e was prepared by dissolving a mixture of K\u003csub\u003e4\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO (4.7 mg; 0.01 mmol) and crypt-222 (19 mg, 0.05 mmol) in 20 mL of deoxygenated acetonitrile, and then filtered using a syringe equipped with a 0.22 \u0026micro;m pore size PTFE membrane to remove insoluble impurities. Both solutions were placed inside air-tight quartz cuvettes and measured immediately after preparation. Solid sample of \u003cb\u003e3\u003c/b\u003e was mixed with paraffin oil between two quartz slides and measured with an integrating sphere.\u003c/p\u003e \u003cp\u003e \u003cb\u003eElectron paramagnetic resonance (EPR).\u003c/b\u003e To enable quantitative \u003cem\u003ein situ\u003c/em\u003e irradiation of the EPR sample in the resonator cavity, powder of \u003cb\u003e1\u003c/b\u003e was deposited as a thin layer on a double-sided adhesive tape glued onto a plastic holder, which was placed in a standard 4 mm X-band EPR tube (Wilmad). Continuous-wave EPR spectra were recorded on a Bruker ELEXSYS E580 spectrometer with a Bruker ER 4118X-MD5 resonator and an Oxford Instruments ER 4118CF helium flow cryostat, at a microwave frequency of 9.628 GHz, with microwave power of 40 \u0026micro;W, modulation amplitude of 0.5 mT and at 10 K.\u003c/p\u003e \u003cp\u003e \u003cb\u003eOptical reflectivity measurements.\u003c/b\u003e Reflectivity measurements have been performed with a home-built system, operating between 10 and 300 K (at 4 K\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and in the range of 400 to 1000 nm. A halogen-tungsten light source (Leica CLS 150 XD tungsten halogen source adjustable from 0.05 mW\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e to 1 W\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) was used as the light source for the high-sensitivity Hamamatsu 10083CA spectrometer. The measurements were calibrated with barium sulphate as the reference sample. As the samples are potentially very photosensitive, the light exposure time was minimized during the experiments keeping the samples in the dark except during the measurements when white light is shined on the sample surface (\u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.08 mW\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e). For all excitation/de-excitation experiments performed at 10 K, the sample was initially placed at this temperature keeping the sample in the dark to avoid any excitation. Light-emitting diodes (LEDs) operating between 365 and 1050 nm (from Thorlabs) were used for excitation experiments. For the excitation/de-excitation experiments in optical reflectivity, the power and the time of the irradiation were systematically adapted to the optical properties of the material to obtain the fastest possible response and to minimize thermal heating effects.\u003c/p\u003e \u003cp\u003e \u003cb\u003eMagnetic and photomagnetic measurements.\u003c/b\u003e Magnetic measurements were performed using Quantum Design MPMS3 Evercool SQUID magnetometer in the magnetic fields up to 7 T. Polycrystalline sample of \u003cb\u003e1\u003c/b\u003e (21.8 mg) was inserted into a polyethylene (PE) bag and sealed under an argon gas atmosphere inside the Inert PureLab HE glovebox (O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.1 ppm and H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;\u0026lt;\u0026thinsp;0.5 ppm). The PE bag was attached to the quartz sample holder using Kapton tape. Polycrystalline sample of \u003cb\u003e3\u003c/b\u003e (32.8 mg) was inserted into Delrin sample holder described elsewhere,\u003csup\u003e\u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e81\u003c/span\u003e\u003c/sup\u003e which was inserted into the plastic straw. The experimental magnetic data were corrected for the diamagnetism of the sample and the sample holder. Photomagnetic measurements were performed for samples (0.3-1.0 mg) placed between two layers of adhesive tape and inserted into the plastic straw. For the relaxation measurements, very small amount of sample (ca. 0.1 mg) was used to facilitate fast and complete conversion from \u003cb\u003e1\u003c/b\u003e to \u003cb\u003e2\u003c/b\u003e. The accurate sample mass and diamagnetic corrections were determined by comparison of the data collected in the dark on bulk samples. The irradiation was performed using laser diodes (405 nm \u0026ndash; \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;5 mW\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, 638 nm \u0026ndash; \u003cem\u003eP\u003c/em\u003e\u0026thinsp;=\u0026thinsp;15\u0026ndash;20 mW\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis work was financed by the European Union within the Horizon Europe Framework Programme, ERC Consolidator Grant LUX-INVENTA no. 101045004. Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them. The authors would like to thank the European Synchrotron Radiation Facility for beamtime at the ID12 (proposal CH-6581 and CH-6239) and Diamond Light Source for beamtime at I19-2 instrument (proposal MT18908) and the staff of beamline I19 for assistance with crystal testing, data collection and photoirradiation setup. Access to the ESRF was financed by the Polish Ministry of Education and Science - decision no. 2021/WK/11. We also acknowledge the DUS grant from the Faculty of Chemistry under the Strategic Programme Excellence Initiative at the Jagiellonian University. The study was partly carried out using the research infrastructure cofounded by the European Union in the framework of the Smart Growth Operational Program, Measure 4.2; Grant No. POIR.04.02.00-00-D001/20, \u0026ldquo;ATOMIN 2.0 \u0026ndash; ATOMic scale science for the INnovative economy\". Polish high-performance computing infrastructure PLGrid (HPC Centers: ACK Cyfronet AGH) is acknowledged for providing computer facilities and support within computational grants nos. PLG/2024/017534 and PLG/2023/016609. R.C., C. M. and M.R. thank the University of Bordeaux, the R\u0026eacute;gion Nouvelle Aquitaine, Quantum Matter Bordeaux (QMBx), and the Association Fran\u0026ccedil;aise de Magn\u0026eacute;tisme Mol\u0026eacute;culaire for support and funding. D.P., M.M., M.A., L.M., R.C., C. M. and M.R. acknowledge the continuous support of the Centre National de la Recherche Scientifique (CNRS) in particular via the International Emergent Actions program in 2023 (Poland/France IEA-302).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eJia C et al (2016) Covalently bonded single-molecule junctions with stable and reversible photoswitched conductivity. 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J Am Chem Soc 141:19067\u0026ndash;19077\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5611691/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5611691/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eManipulating the physical properties of solid matter using only photons is a major challenge in materials science. In this study, we present the photochemistry occurring in a single crystal of a simple cyanide complex, K\u003csub\u003e4\u003c/sub\u003e[Mo\u003csup\u003eIII\u003c/sup\u003e(CN)\u003csub\u003e7\u003c/sub\u003e]\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO. Upon exposure to visible light at different wavelengths, a reversible breaking and reformation of dative bonds is triggered, resulting in a photoswitching of the Mo\u003csup\u003eIII\u003c/sup\u003e coordination geometry between 6- and 7-coordinate. This transformation, in turn, induces a spin state change. The observed solid-state photochemical reactivity is robust, quantitative and occurs at a record-high temperature. It paves the way for the development of new photo-switchable high-temperature magnets and nanomagnets.\u003c/p\u003e","manuscriptTitle":"Reversible single crystal photochemistry and spin state switching in a metal-cyanide complex","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-03-20 08:27:39","doi":"10.21203/rs.3.rs-5611691/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"bd4fcca9-986d-4d13-89e9-0744e729e65d","owner":[],"postedDate":"March 20th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":42701422,"name":"Physical sciences/Materials science"},{"id":42701423,"name":"Physical sciences/Chemistry/Coordination chemistry"},{"id":42701424,"name":"Physical sciences/Chemistry/Inorganic chemistry/Solid-state chemistry"},{"id":42701425,"name":"Physical sciences/Chemistry/Photochemistry"}],"tags":[],"updatedAt":"2025-09-30T07:05:15+00:00","versionOfRecord":{"articleIdentity":"rs-5611691","link":"https://doi.org/10.1038/s41467-025-63523-x","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-09-29 04:00:00","publishedOnDateReadable":"September 29th, 2025"},"versionCreatedAt":"2025-03-20 08:27:39","video":"","vorDoi":"10.1038/s41467-025-63523-x","vorDoiUrl":"https://doi.org/10.1038/s41467-025-63523-x","workflowStages":[]},"version":"v1","identity":"rs-5611691","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5611691","identity":"rs-5611691","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}