Ultrafast charge-transfer-induced spin transition in cobalt-tungstate molecular photomagnets

Ultrafast charge-transfer-induced spin transition in cobalt-tungstate molecular photomagnets | 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 Ultrafast charge-transfer-induced spin transition in cobalt-tungstate molecular photomagnets Shin-ichi Ohkoshi, Kazuki Nakamura, Koji Nakabayashi, Laurent Guérin, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5901007/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 06 Jun, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract In photoinduced phase transition materials, where both charge transfer and spin transition occur, there has been a long debate on which of the two processes are leading the phase transition. Herein, we present experimental evidence supporting an optically charge-transfer-induced spin transition ( CTIST ) process, as demonstrated through femtosecond optical spectroscopy in two-dimensional cyanido-bridged cobalt-tungstate photomagnets. Optical and magnetic investigations reveal that the optical excitation of the ground low-temperature (LT) Co III LS -W IV state drives a photoinduced phase transition towards the Co II HS -W V state, similar to the high temperature (HT) state. Ultrafast spectroscopy further indicates that this optical excitation of the intermetallic W-to-Co charge-transfer band produces a transient photoexcited (PE) Co II LS -W V state, which decays within 130 fs through a spin transition towards the Co II HS -W V state. Here we show that the CTIST dynamics corresponds to the Co III LS -W IV (LT) → Co II LS -W V (PE) → Co II HS -W V (HT) sequence. The present work sheds a new light on understanding optical dynamics underlying the photoinduced phase transitions. Physical sciences/Chemistry/Physical chemistry Physical sciences/Chemistry/Inorganic chemistry charge transfer spin transition cyanido-bridged assemblies photoinduced phase transition photomagnetism ultrafast spectroscopy Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The rational design of molecular materials 1–5 aims to develop and control their physical properties for specific applications, posing a substantial challenge in material science, chemistry, and physics. Phase transitions in molecular materials, which involve changes in various physical properties, 6–8 can be regulated by external stimuli of chemical (solvent, pH) 9–11 and physical (temperature, current, pressure, and light) nature. 12–15 Photoinduced phase transitions (PIPT) represent a promising avenue for switching physical properties by altering electronic and structural degrees of freedom under photoirradiation. 16,17 Ultrafast time-resolved optical techniques allow to gain substantial knowledge in the understanding of the photoinduced processes at play in PIPTs, enabling the study of electronic and structural dynamics 18–21 at the molecular scale and cooperative transformations at the macroscopic scale. 22–24 This research has facilitated advancements in applications, such as photonic actuators, memory devices, and other photonic technologies. Cyanido-bridged heterometallic assemblies are promising molecular materials because of their ability to exhibit electronic changes driven by coupled charge transfer (CT) and/or spin transition (ST), which can be triggered by temperature or light. 25–28 These transitions enable functional switching of magnetic, optical, and thermodynamical properties. 29–32 Among the heterometallic assemblies, cyanido-bridged cobalt-tungstate assemblies show phase transitions with a thermal hysteresis loop and photomagnetism originated from an optical transition between the low-spin Co III LS -W IV low-temperature (LT) state and the high-spin Co II HS -W V high-temperature (HT) state (Fig. 1 a). 33–35 These electronic transformations are further associated with distinct ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectra in the LT and HT states, due to an important change of electronic transitions derived from the metal-to-metal CT (MMCT), corresponding to a charge transfer from Co III -W IV to Co II -W V or vice versa. Their optical and magnetic properties and phase transitions can be tunable by varying the counter ions, solvents, and organic ligands. For instance, the cyanido-bridged cobalt-tungstate assembly, (H 5 O 2 + )[Co(4-bromopyridine) 2 {W(CN) 8 }] (CoW) exhibits a stable LT state across a wide temperature range, extending beyond room temperature (RT) (Fig. 1 b). 36 In contrast, the partially-Cs-substituted cobalt-tungstate assembly, Cs + 0.1 (H 5 O 2 + ) 0.9 [Co(4-bromopyridine) 2.3 {W(CN) 8 }] (CsCoW), exhibits a LT–HT thermal phase transition with LT–HT bistability at RT along with an 8% volume expansion (Fig. 1 c). 37 The partial substitution indeed destabilizes the hydrogen-bond network between the layers, resulting in a more flexible lattice that stabilizes the higher-volume high spin HT state at RT. In PIPT materials involving both CT and/or ST, there has been a longstanding debate spanning approximately 30 years on which of the two processes is leading the phase transition: Charge-transfer-induced Spin transition ( CTIST ), or vice versa (Spin transition-induced Charge transfer: STICT)? Recently, an optically STICT process was reported in a CoFe Prussian blue analogue. 38 In contrast, density functional theory (DFT) calculations for cyanido-bridged cobalt-tungstate assemblies indicate that optical excitation of the MMCT band in the LT state corresponds to electron transfer from W to Co. 36 In the present work, we examine the ultrafast photoinduced dynamics in CsCoW at RT by comparing with the ultrafast spectroscopic changes observed in CoW. Sub-picosecond (ps) and 10’s ps dynamics studies demonstrate the photoinduced dynamics at molecular and lattice scales. Our results provide experimental evidence for CTIST in both CsCoW and CoW. This process occurs on sub-ps molecular dynamics, while a slower thermoelastic conversion is observed only in CsCoW, near the transition temperature and under high laser fluence, occurring on the 10’s ps timescale. Results Photoinduced phase transition at low temperature The PIPT of CsCoW at low temperatures was investigated using UV-vis-NIR absorption spectroscopy and a SQUID magnetometer. In the LT state, CsCoW exhibits an MMCT band from Co III -W IV to Co II -W V , centered at 760 nm (Fig. 1 d). The absorption band is comparable to the MMCT band of CoW, where the valence band's top consists of d z 2 orbitals of W IV and p orbitals of nitrogen (N) atoms. In contrast, the conduction band's bottom comprises d z 2 orbitals of Co III and sp orbitals of N atoms, as indicated by DFT calculations. 36 Therefore, a continuous-wave (cw) diode laser operating at 785 nm was used as the photoirradiation source. Figure 1 d illustrates the UV-vis-NIR spectra before and after photoirradiation at 4 K. Under photoirradiation, the absorption band around 760 nm, which is characteristic of the ground LT state, disappears, and the sample color changes from blue to red (Fig. 1 d inset) as a new band emerges at 530 nm, which is a distinctive feature of the photoinduced (PI) state and corresponds to the MMCT band from Co II -W V to Co III -W IV . This color change resembles the thermochromism observed during the LT to HT phase transition. 37 The MMCT band in the PI state is slightly red-shifted compared to the band at 495 nm in the HT state, which is attributed to an increase in the ligand field strength at the Co site owing to the volume contraction at low temperatures (Supplementary Fig. 4a). Upon warming, the 530 nm peak of the PI state disappears while the 760 nm peak intensifies around 70 K, signifying thermal relaxation from the PI state to the LT state (Supplementary Fig. 4b). Additionally, reverse-PIPT was studied to confirm the photoreversibility from the PI state to the LT state using a cw diode laser at 532 nm, which aligns with the MMCT band of the PI state at 4 K. During irradiation of the PI state, the 530 nm peak decreases, and the 760 nm peak reappears (Supplementary Fig. 4c), directly evidencing the reverse-PIPT from the PI state to the LT state. These findings demonstrate that CsCoW undergoes a reversible-PIPT at LTs, exhibiting characteristics similar to those of CoW. 36 We conducted photomagnetic measurements at low temperatures, under the same irradiation conditions as the UV-vis-NIR measurements. Figure 1 e shows the field cooling magnetization (FCM) curves under 20 Oe. The LT state before photoirradiation is paramagnetic. Under a 785 nm irradiation in the LT state at 3 K, the magnetization increases due to the ferromagnetic nature of the PI state, characterized by a Curie temperature ( T C ) of 27 K. Since T C is identical to that reported for CoW, we conclude that the superexchange interaction between Co and W in CsCoW and CoW has similar strength, as both share a similar cyanido-bridged layer structure. 36 The PI state returns to the initial paramagnetic LT state upon thermal treatment at 100 K, which is consistent with the UV-vis-NIR spectra. The magnetization versus magnetic field ( M–H ) plots at 2 K in the PI state show a coercive field of 7500 Oe, substantially larger than the 2000 Oe observed in CoW (Supplementary Fig. 5c). The saturated magnetization under 50 kOe was 3.1 µ B , aligning with the calculated value of 3.2 µ B , considering the ferromagnetic interaction between Co II ( g Co = 13/3, S Co = 1/2 (Kramers doublet)) and W V ( g W = 2.0, S W = 1/2) in the PI state. Remarkably, the PI state persisted for at least 1 day after the light was turned off at 3 K. The reverse-PIPT of the PI state was investigated using a 532-nm cw diode laser. As shown in Fig. 1 e, the magnetization of the PI state at 3 K decreased to the initial value. Both photothermal and photoreversible PIPTs were cycled at least three times without any signs of degradation (Supplementary Figs. 5e and 5f). The FCM and M – H curves also returned to the magnetization of the paramagnetic LT state. These measurements confirmed the photoreversible switching between the LT paramagnetic Co III LS -W IV and PI ferromagnetic Co II HS -W V states, which is similar to the HT state. Above 100 K, the lifetime of the PI state was too short to be observed using conventional techniques. General consideration of time-resolved optical spectroscopy Femtosecond time-resolved optical spectroscopy was conducted to investigate the ultrafast dynamics underlying the PIPT through CT and ST processes in cyanido-bridged cobalt-tungstate assemblies. The visible region (500–740 nm) 39 was probed, where the drastic optical density change in CsCoW is the characteristic of the electronic state switching. Measurements were performed above 100 K, where the short-lived PI state enables pump-probe measurements at a 1 kHz repetition rate. The sample was photoexcited with an approximate 65 fs laser pulse at 850 nm (8 mJ cm − 2 ). Two-time ranges of optical density changes (ΔOD( t )) were analyzed to investigate distinct dynamics: ultrafast photoinduced dynamics (from − 0.5 to 2.0 ps) and consecutive thermoelastic dynamics (from − 50 to 800 ps). The dynamical responses in CsCoW and CoW were compared at RT, where the LT state of CoW and the LT state branch of the thermal hysteresis loop in CsCoW were photoexcited. Additionally, the temperature-dependent photoresponse of CsCoW was examined using a cryogenic system, with measurements conducted at 230 and 100 K, i.e., temperatures near and far from the thermal hysteresis loop, respectively. Time-resolved optical spectroscopy in sub-ps timescale Figure 2 (top) shows the ΔOD( t ) on the ps timescale, measured at RT following femtosecond laser excitation for CoW and CsCoW. In both cases, a decrease in OD was observed at 700 nm, corresponding to the strong bleaching of the LT Co III LS -W IV state. Additionally, a short transient increase in OD was evident around 600 nm for both compounds. The time trace ΔOD( t ) at 600 nm remains positive in CsCoW, which can be attributed to the formation of the PI Co II HS -W V state. The ΔOD( t ) data measured at 230 and 100 K for CsCoW resemble the data at RT for CsCoW and CoW, respectively, showing a transient OD increase near 600 nm and an OD decrease near 700 nm, accompanied by coherent oscillations (Supplementary Fig. 6). For both compounds, similar coherent oscillations are evident in the time traces, at the same wavelength. In order to extract the characteristic timescales of the photoinduced dynamics, the OD changes were analyzed, resulting from the depopulation of the LT Co III LS -W IV state toward an initially electronic photoexcited (PE) Co II LS -W V state, identified by the intermediate transient peak around t = 0. Population fitting of ΔOD( t ) at 600 and 700 nm was performed using an exponential decay model for the transient PE state, which subsequently populates the PI Co II HS -W V state (Figs. 2 c and 2 d, Supplementary Note 6). The fitting results indicate that the transient PE state decays within approximately 130 fs into the PI state, which is similar to the HT state. This timescale is typical for photoinduced dynamics by ST, as reported in different spin-crossover materials exhibiting light-induced excited spin state trapping (LIESST effect) induced by metal-to-ligand CT photoexcitation. 40–43 Supplementary Fig. 7 shows the oscillating component of the ΔOD( t ) signal after 300 fs, obtained as a residual difference between the experimental data and the fitted exponential functions (Supplementary Note 6). A fast Fourier transform (FFT) of this signal revealed the frequencies of these oscillating components. The wavelength vs wavenumber vs FFT intensity maps are similar for CoW and CsCoW (Supplementary Figs. 8 and 9). Two common vibrational modes were identified for CsCoW, regardless of temperature: a fast oscillation with an approximate value of 259 fs period (approximately 130 cm − 1 ) and a slower oscillation with an approximate value of 580 fs period (approximately 58 cm − 1 ) (Supplementary Table 2). The spectral weight of the slower oscillation is more prominent at longer wavelengths. The oscillating signals at 600 and 700 nm, fitted using only these two nodes, closely match the experimental data (Figs. 2 c and 2 d insets, Supplementary Figs. 10 and 11). At RT, ΔOD( t ) remains positive near 600 nm after the transient peak, directly indicating the formation of the PI Co II HS -W V state in CsCoW. However, this OD increase is not clearly observed for CoW at RT or CsCoW at 100 K, which is likely due to the strong bleaching of the ground LT state absorption (Supplementary Fig. 14). Consideration of phonon modes To understand the origin of the oscillations, we performed DFT calculations that included phonon modes using the Gaussian 16 package. 44 The coordination environments of the Co site ([Co(4-bromopyridine) 2 (NC) 4 ] n− ) were evaluated for various electronic states: trivalent low-spin (Co III LS ), divalent low-spin (Co II LS ), and divalent high-spin (Co II HS ) (Supplementary Note 7). Although these calculations were simplified by focusing solely on the Co environment, the phonon mode frequencies closely aligned with experimental values obtained from IR and THz spectra (Supplementary Fig. 12). The frequencies of similar phonon modes increased in the order Co III LS < Co II LS < Co II HS (Supplementary Table 3). Phonon modes arising from cyanide ligands were observed below 500 and around 2100 cm − 1 . Meanwhile, modes originating from 4-bromopyridine ligands, including stretching and bending vibrations of the pyridine ring and C-H bonds, were observed in the 600–1600 cm − 1 range and around 3200 cm − 1 , respectively. Unique breathing modes were identified around 130 cm − 1 in Co II LS and Co II HS , based on the optimization of Co II LS . These modes involve the stretching of organic and cyanide ligands. This type of mode is known to be strongly coupled with STs as they act as the reaction coordinate. Consequently, these modes strongly affect metal-centered optical transitions. This type of coherent activation of breathing modes has been reported in other spin-crossover materials during the LIESST process. 40–43 DFT calculations also revealed a phonon mode at approximately 58 cm − 1 , corresponding to a global torsion of the cyanido-bridged layer. Such torsion modes have been previously reported in other Prussian blue analogues (Co–Fe and Mn–Fe systems). 45–47 The ST leads to a local expansion of the cyanido-bridged network on a timescale too short for lattice expansion, activating torsion modes to accommodate these local distortions of the cyanide (Fig. 3 c). Charge-transfer-induced spin transition in cyanido-bridged cobalt-tungstate assemblies Based on femtosecond optical spectroscopy and DFT calculations, we propose that the MMCT process involves a ground LT Co III LS -W IV and a PE Co II LS -W V states (Fig. 3 a). Figure 3 b shows a schematic of the CTIST dynamics for both CsCoW and CoW, which share similar layered structures. The ultrafast photoinduced dynamics in cyanido-bridged cobalt-tungstate assemblies occur in two steps. First, optical excitation of the ground LT Co III LS -W IV state produces a PE Co II LS -W V state, characterized by a transient OD peak around 600 nm. This PE state is presumed to be stabilized by Jahn–Teller distortion and partial elongation of the average Co–N bonds, which can drive the ST. In the second step, by causing further elongation of the Co–N bond, the PE Co II LS -W V state decays within 130 fs into the lower-lying PI Co II HS -W V state. As discussed for the LIESST effect, the PE state functions as a mediator, with structural dynamics and electronic reorganization being entangled. 48 The timescale of this ST (approximately 130 fs) corresponds to half the period of the breathing mode associated with stretching of the C-N bonds and organic and cyanide ligands (approximately 259 fs) (Fig. 3 b). This timescale is consistent with those reported for other Prussian blue analoguess and ST materials. 40–43, 45–47 On the 130 fs timescale of Co–N bond elongation trapping of the PE state, the crystalline lattice has no time to expand sufficiently, necessitating distortions to accommodate local lattice expansion. The ultrafast Co–N bond elongation launches consequently torsion modes of the W––Co––W cyanido-bridge, calculated at approximately 65 cm − 1 and consistent with the coherent oscillations observed at 58 cm − 1 (Fig. 3 c). These sub-ps structural dynamics localized at the molecular scale are associated with the structural trapping of the PE state. Time-resolved optical spectroscopy on 10’s ps timescale We performed optical measurements on a longer ps timescale for the two compounds to monitor slower consecutive dynamics (Fig. 4 ). In all cases, the ΔOD( t ) map around 700 nm reveals a rapid decay of the MMCT band, indicating ground-state bleaching immediately following photoexcitation. For CsCoW, an OD increase of around 600 nm was observed within 100 ps, characteristic of PI Co II HS -W V state formation near the thermal phase transition temperatures (at RT and 230 K) under high laser fluence. The maximum of this transient absorption band gradually shifts toward shorter wavelengths and decays within 800 ps (Fig. 4 a, Supplementary Figs. 13a and 13b). This shift can be attributed to lattice relaxation affecting the Co–N bonds, influencing the MMCT between Co II HS and W V owing to the larger e g -t 2g energy gap in the Co ligand field compared to the original HT state. The dynamics exhibit strong temperature dependence. At low temperature (100 K) in CsCoW, ΔOD( t ) is dominated by the pronounced decrease of the MMCT band of the LT state, which masks the PI state band (Supplementary Figs. 14 and 15, Supplementary Tables 4 and 5). The OD change map at 100 K closely resembles CoW at RT (Fig. 4 a), where no thermal phase transition occurs. Notably, slower conversion is observed at 230 K and RT only under high fluence. These features of the slower conversion are characteristic of the so-called thermoelastic conversion, as recently reported for spin-crossover materials. 49–51 Thermoelastic conversion We next investigated the fluence dependence of the photoresponse in CsCoW and CoW at RT to examine the thermoelastic conversion (Figs. 4 d and 4 e, Supplementary Figs. 16 and 17). In CsCoW, ΔOD( t ) around 600 nm is positive and increases with higher pump fluence. In contrast, for CoW, ΔOD( t ) around 600 nm is negative and decreases consistently with fluence. The OD changes at 580 and 700 nm, measured 100 ps after photoexcitation, are shown in Figs. 4 d and 4 e. The nonlinear increase in ΔOD( t ) for CsCoW indicates a cooperative process driving the PI state, while the linear decrease in ΔOD( t ) for CoW reflects a local process dominated by the bleaching of the LT MMCT band. These observations strongly indicate that the dynamics in CsCoW within 100 ps correspond to thermoelastic conversion, as in the case of spin-crossover materials. Remarkably, when the pump fluence exceeded 13 mJ cm − 2 for CsCoW, no OD change was detected during the time-resolved optical spectroscopy. This result indicates persistent PIPT at RT, transitioning from the LT to PI (HT) branches of the hysteresis. Overall, the dynamics in the cyanido-bridged cobalt-tungstate assemblies correspond to the thermoelastic conversion from the LT to PI states, driven by two photoinduced phenomena (Fig. 4 f). First, local molecular photoswitching occurs within 130 fs, involving the volume change by STs that generate internal pressure, stabilizing the higher-volume PI state. Second, the lattice warming caused by the laser heating facilitates the thermal conversion from the LT to PI states. Thus, our data align with the physics of thermoelastic processes, where elastic lattice expansion, driven by the local molecular PIPT favors the thermal population of the high-entropy PI state. 49–51 When the system is far from a thermal phase transition, as is the case for CsCoW at 100 K and CoW at RT, the thermoelastic conversion is weak. However, near the thermal phase transition, as it is for CsCoW at 230 K and RT, the laser pump fluence induces substantial volume strain and temperature jumps, resulting in robust thermoelastic conversion. As highlighted earlier, CsCoW enhances the HT state stability by destabilizing the hydrogen-bond network in the crystal structure through partial Cs + ion substitution. This substitution in CsCoW allows shifting the thermal phase transition closer to RT by controlling the relationship between the effect on the intermolecular interactions and cooperativity mediated by the cyanido-bridged networks, enabling thermoelastic conversion in the PIPT out-of-equilibrium dynamics in the vicinity of RT. Discussion In conclusion, we investigated the photoinduced CTIST process in cyanido-bridged cobalt-tungstate assemblies by comparing CsCoW and CoW materials. Optical and magnetic measurements at low temperatures for CsCoW confirmed reversible PIPT in CsCoW, accompanied by a dramatic optical color change between blue in the LT state and red in the PI state resembling the HT state, and photomagnetic behavior. The photomagnetic properties of the PI state in CsCoW exhibited photoinduced magnetization, with a Curie temperature of 27 K and a coercive field of 7500 Oe at 2 K. We also studied the out-of-equilibrium photoinduced dynamics of CoW and CsCoW using femtosecond optical spectroscopy. The dynamics comprise a molecular sub-ps process and a macroscopic ps thermoelastic conversion. The first step involves, at the molecular level, the PE state resulting from MMCT from Co III LS -W IV to Co II LS -W V , followed by ST within 130 fs to the PI Co II HS -W V state. This process is characterized by the coherent activation of the breathing mode of the CoN 6 core (approximately 130 cm − 1 ). These initial CTIST dynamics were observed in both CoW and CsCoW across all temperatures, as the two compounds exhibit similar molecular and electronic structures around the photoactive Co and W sites. This local photoswitching process drives a subsequent thermoelastic step, where molecular expansion within the lattice, coupled with laser heating, balances the relative stabilities of the LT and PI (HT) states. Thermoelastic conversion is consequently observed in the vicinity of the thermal phase transition and at high fluences in CsCoW, consistent with theoretical predictions for other bistable molecular materials. 49–51 Overall, the present work underlines the direct evidence of photoinduced CTIST in cyanido-bridged cobalt-tungstate assemblies directly and highlights the critical role of Cs + ion substitution, which is not only responsible for shifting the thermal transition towards RT but also allows thermoelastic conversion in PIPT. Methods Syntheses Crystalline powders of CsCoW and CoW was synthesized following established protocols in the literature. 36,37 Characterizations induced CHN analysis, powder X-ray diffraction, IR spectroscopy, UV-vis-NIR spectroscopy, and magnetic measurements (Supplementary Note 1). Elemental analysis results were as follows: CsCoW: Calculated: C, 27.2%; H, 1.6%; N, 16.8%. Found: C, 27.0%; H, 1.8%; N, 16.5%, for CoW: Calculated: C, 26.9%; H, 1.6%; N, 17.4%. Found: C, 26.8%; H, 1.8%; N, 17.3%. Measurements Time-resolved pump-probe experiments were conducted at the Institut de Physique de Rennes using a shot-to-shot optical setup described in Supplementary Note 2. The pump light was set to 850 nm, targeting the tail of the W IV -to-Co III MMCT band to maximize the penetration depth. The probe light was generated using a sapphire plate, producing an intense polarized pulse spanning a spectral range from visible to NIR (450–800 nm). 39 A 750-nm long-pass filter was inserted into the probe path to block the pump beam, allowing only the probe supercontinuum to be monitored. Samples were dispersed in paraffin oil and sandwiched between thin glass plates. A reference sample was prepared by sandwiching paraffin oil between identical glass plates. Sample temperature was controlled using an Oxford Instruments Cryojet with an N 2 cryojet. Data were analyzed using iterative fitting of multiple exponential functions convoluted with a Gaussian instrument response function with a full-width at half maximum of 65 fs. For sub-ps timescales, contributions from the substrate (glass and liquid paraffin) were subtracted from transient two-dimensional maps, isolating the photoinduced signal from the sample (see Supplementary Note 2). Calculation DFT calculations were conducted to elucidate the origin of the oscillations observed in the sub-ps time delay measurements. 44 Since the crystal structures were too complex for phonon mode calculations, we focused on the Co site unit, Co(4-bromopyridine) 2 (NC) 4 , in three distinct electronic states: Co III LS , Co II LS , and Co II HS . Geometry optimization and vibrational frequency calculations were performed for all states using the B3LYP 6-31G + (2df,2p) functional. A restricted spin approach was applied for Co III LS , while an unrestricted spin approach was used for Co II LS and Co II HS , with computations carried out in Gaussian16. The vibrational frequencies obtained are detailed in Supplementary Note 7. Declarations Data availability The supplementary information provides all data generated in this study, including characterizations, experimental configurations, raw data treatment, data fitting for time-resolved optical measurements, and DFT calculations. Relevant data are also available upon request from the authors. Acknowledgments This work was supported in part by a Grant-in-Aid for Scientific Research (A) from JSPS KAKENHI (Grant Number 20H00369), Advanced Technologies for Carbon-Neutral (ALCA)-Next from JST (JPMJAN23 A2), CNRS-University of Tokyo“Excellence Science” Joint Research Program, Second CNRS –University of Tokyo PhD Joint Program. The Cryogenic Research Center, The University of Tokyo, the Center for NanoLithography & Analysis, The University of Tokyo, and Quantum Leap Flagship Program (Q-LEAP, Grant Number JPMXS0118068681) by MEXT are also acknowledged for support. K. Nakamura was supported by the World-Leading Innovative Graduate Study Program for Materials Research, Information, and Technology (MERIT-WINGS) and the Fellowship for Integrated Materials Science and Career Development. K. Nakabayashi acknowledges the Iketani Science and Technology Foundation (Grant Number 0351111-A). The authors gratefully acknowledge the Agence Nationale de la Recherche for financial support under grant ANR-19-CE30-0004 ELECTROPHONE, ANR-19-CE29-0018 MULTICROSS. E.C. thanks the University of Rennes, the Fondation Rennes 1, and Region Bretagne (Boost'ERC) for funding. Author Contributions K. Nakamura, L. G., K. Nakabayashi, E. C., and S. O. conceived the project. K. Nakamura synthesized and characterized samples and performed photoinduced UV-vis-NIR absorption measurements and photomagnetic measurements for CsCoW and theoretical calculations. K. Nakamura, G. P., and L.G. conducted the transient absorption measurements. K. Nakamura, G. P., L. G., and M. H. analyzed the data. All authors discussed the experimental and theoretical results. K. Nakamura, L. G., K. Nakabayashi, and E. C. wrote the manuscript, with contributions from all authors. Competing interests The authors declare no competing financial interest. Additional information Supplementary information The online version contains supplementary material available at https://doi.org/. Correspondence and requests for materials should be addressed to Eric Collet and Shin-ichi Ohkoshi. 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Phys Rev B 108:014306 Enachescu C et al (2017) Theoretical approach for elastically driven cooperative switching of spin-crossover compounds impacted by an ultrashort laser pulse. Phys Rev B 95:224107 Additional Declarations There is NO Competing Interest. Supplementary Files Supplementaryvideo.mp4 Supplementary movie 1 SI250125.pdf Supplementary information Cite Share Download PDF Status: Published Journal Publication published 06 Jun, 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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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-5901007","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":411691044,"identity":"95b1145b-34c0-4e4a-82c8-678ea733eb8f","order_by":0,"name":"Shin-ichi Ohkoshi","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0001-9359-5928","institution":"The University of Tokyo","correspondingAuthor":true,"prefix":"","firstName":"Shin-ichi","middleName":"","lastName":"Ohkoshi","suffix":""},{"id":411691045,"identity":"f3988d8b-9042-44ab-b097-875b828a56e4","order_by":1,"name":"Kazuki Nakamura","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Kazuki","middleName":"","lastName":"Nakamura","suffix":""},{"id":411691046,"identity":"37c5a544-8fd9-413a-8a70-d75f4b31a381","order_by":2,"name":"Koji Nakabayashi","email":"","orcid":"","institution":"University of Tokyo","correspondingAuthor":false,"prefix":"","firstName":"Koji","middleName":"","lastName":"Nakabayashi","suffix":""},{"id":411691047,"identity":"5ebcbfab-47ec-4b87-96de-3001a043d89b","order_by":3,"name":"Laurent Guérin","email":"","orcid":"https://orcid.org/0000-0002-0509-8444","institution":"CNRS, IRL DYNACOM","correspondingAuthor":false,"prefix":"","firstName":"Laurent","middleName":"","lastName":"Guérin","suffix":""},{"id":411691048,"identity":"c5f10294-f3af-4497-a0f2-80ae9902cedb","order_by":4,"name":"Gaël Privault","email":"","orcid":"","institution":"University of Rennes","correspondingAuthor":false,"prefix":"","firstName":"Gaël","middleName":"","lastName":"Privault","suffix":""},{"id":411691049,"identity":"4622bfb2-f3f1-4285-a2b0-a62f8c1be0a0","order_by":5,"name":"Marius Herve","email":"","orcid":"https://orcid.org/0000-0003-2291-6469","institution":"Institut de Physique de Rennes","correspondingAuthor":false,"prefix":"","firstName":"Marius","middleName":"","lastName":"Herve","suffix":""},{"id":411691050,"identity":"d91e9159-5f0b-41b7-a9de-1f259341e225","order_by":6,"name":"Eric Collet","email":"","orcid":"https://orcid.org/0000-0003-0810-7411","institution":"Univ Rennes","correspondingAuthor":false,"prefix":"","firstName":"Eric","middleName":"","lastName":"Collet","suffix":""}],"badges":[],"createdAt":"2025-01-25 10:20:13","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5901007/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5901007/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-60401-4","type":"published","date":"2025-06-06T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":75702954,"identity":"be757807-7c45-419d-8ab2-d9c710a5e822","added_by":"auto","created_at":"2025-02-07 09:44:50","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":33507289,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMagnetic and optical properties of cyanido-bridged cobalt-tungstate assemblies.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e A schematic representation of the cyanido-bridged cobalt-tungstate moiety, illustrating low-spin Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e(\u003cem\u003eS\u003c/em\u003e = 0) W\u003csup\u003eIV\u003c/sup\u003e(\u003cem\u003eS\u003c/em\u003e = 0) low temperature state (LT, blue) state and high-spin Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e(\u003cem\u003eS\u003c/em\u003e = 3/2)-W\u003csup\u003eV\u003c/sup\u003e(\u003cem\u003eS\u003c/em\u003e = 1/2) high temperature (HT) or photoinduced (PI) states (red). The spin transition on the Co site in the HT or PI states reduces the \u003cem\u003et\u003c/em\u003e\u003csub\u003e2g\u003c/sub\u003e-\u003cem\u003ee\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e splitting, accompanied by elongation of the Co–N bonds (orange arrows). \u003cstrong\u003eb, c\u003c/strong\u003e The crystal structures and \u003cem\u003eχ\u003c/em\u003e\u003csub\u003eM\u003c/sub\u003e\u003cem\u003eT\u003c/em\u003e vs \u003cem\u003eT\u003c/em\u003e plots for CoW and CsCoW, respectively, reproduced with permission from references 36 and 37 (RSC). CoW remains in the LT state,\u003csup\u003e36\u003c/sup\u003e whereas CsCoW undergoes a thermal transition between HT (red) and LT (blue) states.\u003csup\u003e37\u003c/sup\u003e The crystal structures depict cyanido-bridged Co-W layers and interlayer ions (Cs\u003csup\u003e+\u003c/sup\u003e: green, H\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e: pink). \u003cstrong\u003ed\u003c/strong\u003e UV-vis-NIR spectra of CsCoW were measured at 4 K before irradiation (black line), after irradiation (red line; 785 nm, 160 mW cm\u003csup\u003e−2\u003c/sup\u003e), and after thermal treatment at 100 K (dotted black line). The inset shows images of the drastic color change before (bottom, LT state) and after irradiation (top, PI state, red). \u003cstrong\u003ee\u003c/strong\u003e Photomagnetism behavior of CsCoW. Field-cooled magnetization curves under 20 Oe show the paramagnetic LT state (black circles). Photoinduced magnetization is observed at 785 nm (240 mW cm\u003csup\u003e−2\u003c/sup\u003e, 3 K; red), which thermally relaxes to the LT state above 100 K (open black circles). Photoexcitation at 532 nm (145 mW cm\u003csup\u003e−2\u003c/sup\u003e, 3 K) switches the system to the paramagnetic LT state (blue).\u003c/p\u003e","description":"","filename":"Fig.1.png","url":"https://assets-eu.researchsquare.com/files/rs-5901007/v1/10c23fe037fbe7edfae044c8.png"},{"id":75701946,"identity":"a12e4366-914b-4452-b9c6-29759b3d17fe","added_by":"auto","created_at":"2025-02-07 09:36:50","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":37460473,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTime-resolved optical spectroscopy on sub-ps timescale.\u003c/strong\u003e \u003cstrong\u003ea, b\u003c/strong\u003e Time delay ΔmOD (ΔOD ×10\u003csup\u003e−3\u003c/sup\u003e) maps measured at room temperature for CsCoW and CoW. \u003cstrong\u003ec, d\u003c/strong\u003e Time traces of ΔmOD at 600 nm (±15 nm, red open circles) and 700 nm (±15 nm, blue open circles) for CsCoW and CoW, respectively. The fit at 700 nm (blue) represents the exponential dynamics toward the PI state, resembling the HT state. The fit at 600 nm (red) includes contributions from the PI state (orange) and an intermediate photoexcited (PE) state (green). The inset black line shows the oscillation fitting function. (Supplementary Note 6)\u003c/p\u003e","description":"","filename":"Fig.2.png","url":"https://assets-eu.researchsquare.com/files/rs-5901007/v1/669e4ed3f21a31b7fdc22ae1.png"},{"id":75701938,"identity":"a8cd4cb1-7fba-4ca5-b589-d17778b9e5bf","added_by":"auto","created_at":"2025-02-07 09:36:50","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":11425771,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic representation of the potential energy curves and phonon modes.\u003c/strong\u003e \u003cstrong\u003ea \u003c/strong\u003eCharge density maps of the valence (left) and conduction (right) bands in the LT state, reproduced from reference 36 with permission from RSC. Optical excitation (\u003cem\u003ehν\u003c/em\u003e) of the ground LT Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eIV\u003c/sup\u003e state leads to the photoexcited (PE) Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state by promoting one electron transfer from W\u003csup\u003eIV\u003c/sup\u003e (d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e) to Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e (d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e) orbitals. \u003cstrong\u003eb\u003c/strong\u003e The PE state decays within 130 fs towards the PI Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state through the spin transition with the activation of the CoN\u003csub\u003e6\u003c/sub\u003e breathing mode. \u003cstrong\u003ec\u003c/strong\u003e Schematic representation of the phonon modes. The breathing mode (left) involves Co–N bond elongation (red arrows), while the torsion modes (right) represent distortions of Co–NC–W bridges (black arrows). During structural relaxation, Co–N bonds of the excited Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e site elongate (approximately 130 cm\u003csup\u003e−1\u003c/sup\u003e phonon mode), causing lattice distortions and activation of the lattice torsion mode (approximately 58 cm\u003csup\u003e−1\u003c/sup\u003e). The modes are shown in the supplementary movie.\u003c/p\u003e","description":"","filename":"Fig.3.png","url":"https://assets-eu.researchsquare.com/files/rs-5901007/v1/730b2a1b76ffd75f0838c86e.png"},{"id":75701948,"identity":"2f0913e6-54f4-4edc-8b1e-c161c54c465f","added_by":"auto","created_at":"2025-02-07 09:36:50","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":37166490,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTime-resolved optical spectroscopy on 10’ps timescale. a\u003c/strong\u003e Time delay ΔmOD (ΔOD ×10\u003csup\u003e−3\u003c/sup\u003e) maps for CsCoW at room temperature, 230, and 100 K and CoW at room temperature (8 mJ cm\u003csup\u003e−2\u003c/sup\u003e). \u003cstrong\u003eb, c\u003c/strong\u003e Corresponding time traces at 580 nm (±20 nm) and 700 nm (±20 nm), respectively. \u003cstrong\u003ed, e\u003c/strong\u003e Fluence dependence of ΔmOD at 580 nm (±20 nm) and 700 nm (±20 nm) measured 100 ps (±20 ps) after photoexcitation at room temperature for CsCoW and CoW. \u003cstrong\u003ef\u003c/strong\u003e Schematic representation of the photoinduced dynamics in cyanido-bridged cobalt-tungstate assemblies. Photoexcitation of the LT state (blue) induces the local charge-transfer-induced spin transition (\u003cstrong\u003eCTIST\u003c/strong\u003e) within 130 fs to the PI state (red). At high fluence and elevated temperatures, thermoelastic conversion occurs in CsCoW.\u003c/p\u003e","description":"","filename":"Fig.4.png","url":"https://assets-eu.researchsquare.com/files/rs-5901007/v1/b51a07c97b79926520be3a55.png"},{"id":75704723,"identity":"f7b6139b-977d-4647-ba27-7f4fe6c11423","added_by":"auto","created_at":"2025-02-07 10:01:57","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":112183806,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5901007/v1/c3c9ef2f-405f-49d5-9df4-fd1d803fb9f8.pdf"},{"id":75701940,"identity":"3517fdcc-c052-44be-8e8f-3d437acb761b","added_by":"auto","created_at":"2025-02-07 09:36:50","extension":"mp4","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2525758,"visible":true,"origin":"","legend":"Supplementary movie 1","description":"","filename":"Supplementaryvideo.mp4","url":"https://assets-eu.researchsquare.com/files/rs-5901007/v1/0086096fc6f34a26b4035cfc.mp4"},{"id":75701942,"identity":"33026cbd-2397-4d7b-b533-52d962048d93","added_by":"auto","created_at":"2025-02-07 09:36:50","extension":"pdf","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":5042185,"visible":true,"origin":"","legend":"Supplementary information","description":"","filename":"SI250125.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5901007/v1/ec2c9247ba8598010409e5e8.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Ultrafast charge-transfer-induced spin transition in cobalt-tungstate molecular photomagnets","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe rational design of molecular materials\u003csup\u003e1\u0026ndash;5\u003c/sup\u003e aims to develop and control their physical properties for specific applications, posing a substantial challenge in material science, chemistry, and physics. Phase transitions in molecular materials, which involve changes in various physical properties,\u003csup\u003e6\u0026ndash;8\u003c/sup\u003e can be regulated by external stimuli of chemical (solvent, pH)\u003csup\u003e9\u0026ndash;11\u003c/sup\u003e and physical (temperature, current, pressure, and light) nature.\u003csup\u003e12\u0026ndash;15\u003c/sup\u003e Photoinduced phase transitions (PIPT) represent a promising avenue for switching physical properties by altering electronic and structural degrees of freedom under photoirradiation.\u003csup\u003e16,17\u003c/sup\u003e Ultrafast time-resolved optical techniques allow to gain substantial knowledge in the understanding of the photoinduced processes at play in PIPTs, enabling the study of electronic and structural dynamics\u003csup\u003e18\u0026ndash;21\u003c/sup\u003e at the molecular scale and cooperative transformations at the macroscopic scale.\u003csup\u003e22\u0026ndash;24\u003c/sup\u003e This research has facilitated advancements in applications, such as photonic actuators, memory devices, and other photonic technologies.\u003c/p\u003e \u003cp\u003eCyanido-bridged heterometallic assemblies are promising molecular materials because of their ability to exhibit electronic changes driven by coupled charge transfer (CT) and/or spin transition (ST), which can be triggered by temperature or light.\u003csup\u003e25\u0026ndash;28\u003c/sup\u003e These transitions enable functional switching of magnetic, optical, and thermodynamical properties.\u003csup\u003e29\u0026ndash;32\u003c/sup\u003e Among the heterometallic assemblies, cyanido-bridged cobalt-tungstate assemblies show phase transitions with a thermal hysteresis loop and photomagnetism originated from an optical transition between the low-spin Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eIV\u003c/sup\u003e low-temperature (LT) state and the high-spin Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e high-temperature (HT) state (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003ea).\u003csup\u003e33\u0026ndash;35\u003c/sup\u003e These electronic transformations are further associated with distinct ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectra in the LT and HT states, due to an important change of electronic transitions derived from the metal-to-metal CT (MMCT), corresponding to a charge transfer from Co\u003csup\u003eIII\u003c/sup\u003e-W\u003csup\u003eIV\u003c/sup\u003e to Co\u003csup\u003eII\u003c/sup\u003e-W\u003csup\u003eV\u003c/sup\u003e or vice versa. Their optical and magnetic properties and phase transitions can be tunable by varying the counter ions, solvents, and organic ligands. For instance, the cyanido-bridged cobalt-tungstate assembly, (H\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e)[Co(4-bromopyridine)\u003csub\u003e2\u003c/sub\u003e{W(CN)\u003csub\u003e8\u003c/sub\u003e}] (CoW) exhibits a stable LT state across a wide temperature range, extending beyond room temperature (RT) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003eb).\u003csup\u003e36\u003c/sup\u003e In contrast, the partially-Cs-substituted cobalt-tungstate assembly, Cs\u003csup\u003e+\u003c/sup\u003e\u0026thinsp;\u003csub\u003e0.1\u003c/sub\u003e(H\u003csub\u003e5\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e)\u003csub\u003e0.9\u003c/sub\u003e[Co(4-bromopyridine)\u003csub\u003e2.3\u003c/sub\u003e{W(CN)\u003csub\u003e8\u003c/sub\u003e}] (CsCoW), exhibits a LT\u0026ndash;HT thermal phase transition with LT\u0026ndash;HT bistability at RT along with an 8% volume expansion (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003ec).\u003csup\u003e37\u003c/sup\u003e The partial substitution indeed destabilizes the hydrogen-bond network between the layers, resulting in a more flexible lattice that stabilizes the higher-volume high spin HT state at RT.\u003c/p\u003e \u003cp\u003eIn PIPT materials involving both CT and/or ST, there has been a longstanding debate spanning approximately 30 years on which of the two processes is leading the phase transition: Charge-transfer-induced Spin transition (\u003cb\u003eCTIST\u003c/b\u003e), or vice versa (Spin transition-induced Charge transfer: STICT)? Recently, an optically STICT process was reported in a CoFe Prussian blue analogue.\u003csup\u003e38\u003c/sup\u003e In contrast, density functional theory (DFT) calculations for cyanido-bridged cobalt-tungstate assemblies indicate that optical excitation of the MMCT band in the LT state corresponds to electron transfer from W to Co.\u003csup\u003e36\u003c/sup\u003e In the present work, we examine the ultrafast photoinduced dynamics in CsCoW at RT by comparing with the ultrafast spectroscopic changes observed in CoW. Sub-picosecond (ps) and 10\u0026rsquo;s ps dynamics studies demonstrate the photoinduced dynamics at molecular and lattice scales. Our results provide experimental evidence for \u003cb\u003eCTIST\u003c/b\u003e in both CsCoW and CoW. This process occurs on sub-ps molecular dynamics, while a slower thermoelastic conversion is observed only in CsCoW, near the transition temperature and under high laser fluence, occurring on the 10\u0026rsquo;s ps timescale.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003ePhotoinduced phase transition at low temperature\u003c/h2\u003e \u003cp\u003eThe PIPT of CsCoW at low temperatures was investigated using UV-vis-NIR absorption spectroscopy and a SQUID magnetometer. In the LT state, CsCoW exhibits an MMCT band from Co\u003csup\u003eIII\u003c/sup\u003e-W\u003csup\u003eIV\u003c/sup\u003e to Co\u003csup\u003eII\u003c/sup\u003e-W\u003csup\u003eV\u003c/sup\u003e, centered at 760 nm (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). The absorption band is comparable to the MMCT band of CoW, where the valence band's top consists of d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbitals of W\u003csup\u003eIV\u003c/sup\u003e and p orbitals of nitrogen (N) atoms. In contrast, the conduction band's bottom comprises d\u003csub\u003ez\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e orbitals of Co\u003csup\u003eIII\u003c/sup\u003e and sp orbitals of N atoms, as indicated by DFT calculations.\u003csup\u003e36\u003c/sup\u003e Therefore, a continuous-wave (cw) diode laser operating at 785 nm was used as the photoirradiation source. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003ed illustrates the UV-vis-NIR spectra before and after photoirradiation at 4 K. Under photoirradiation, the absorption band around 760 nm, which is characteristic of the ground LT state, disappears, and the sample color changes from blue to red (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003ed inset) as a new band emerges at 530 nm, which is a distinctive feature of the photoinduced (PI) state and corresponds to the MMCT band from Co\u003csup\u003eII\u003c/sup\u003e-W\u003csup\u003eV\u003c/sup\u003e to Co\u003csup\u003eIII\u003c/sup\u003e-W\u003csup\u003eIV\u003c/sup\u003e. This color change resembles the thermochromism observed during the LT to HT phase transition.\u003csup\u003e37\u003c/sup\u003e The MMCT band in the PI state is slightly red-shifted compared to the band at 495 nm in the HT state, which is attributed to an increase in the ligand field strength at the Co site owing to the volume contraction at low temperatures (Supplementary Fig.\u0026nbsp;4a). Upon warming, the 530 nm peak of the PI state disappears while the 760 nm peak intensifies around 70 K, signifying thermal relaxation from the PI state to the LT state (Supplementary Fig.\u0026nbsp;4b). Additionally, reverse-PIPT was studied to confirm the photoreversibility from the PI state to the LT state using a cw diode laser at 532 nm, which aligns with the MMCT band of the PI state at 4 K. During irradiation of the PI state, the 530 nm peak decreases, and the 760 nm peak reappears (Supplementary Fig.\u0026nbsp;4c), directly evidencing the reverse-PIPT from the PI state to the LT state. These findings demonstrate that CsCoW undergoes a reversible-PIPT at LTs, exhibiting characteristics similar to those of CoW.\u003csup\u003e36\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eWe conducted photomagnetic measurements at low temperatures, under the same irradiation conditions as the UV-vis-NIR measurements. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003ee shows the field cooling magnetization (FCM) curves under 20 Oe. The LT state before photoirradiation is paramagnetic. Under a 785 nm irradiation in the LT state at 3 K, the magnetization increases due to the ferromagnetic nature of the PI state, characterized by a Curie temperature (\u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e) of 27 K. Since \u003cem\u003eT\u003c/em\u003e\u003csub\u003eC\u003c/sub\u003e is identical to that reported for CoW, we conclude that the superexchange interaction between Co and W in CsCoW and CoW has similar strength, as both share a similar cyanido-bridged layer structure.\u003csup\u003e36\u003c/sup\u003e The PI state returns to the initial paramagnetic LT state upon thermal treatment at 100 K, which is consistent with the UV-vis-NIR spectra. The magnetization versus magnetic field (\u003cem\u003eM\u0026ndash;H\u003c/em\u003e) plots at 2 K in the PI state show a coercive field of 7500 Oe, substantially larger than the 2000 Oe observed in CoW (Supplementary Fig.\u0026nbsp;5c). The saturated magnetization under 50 kOe was 3.1 \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003eB\u003c/sub\u003e, aligning with the calculated value of 3.2 \u003cem\u003e\u0026micro;\u003c/em\u003e\u003csub\u003eB\u003c/sub\u003e, considering the ferromagnetic interaction between Co\u003csup\u003eII\u003c/sup\u003e (\u003cem\u003eg\u003c/em\u003e\u003csub\u003eCo\u003c/sub\u003e = 13/3, \u003cem\u003eS\u003c/em\u003e\u003csub\u003eCo\u003c/sub\u003e = 1/2 (Kramers doublet)) and W\u003csup\u003eV\u003c/sup\u003e (\u003cem\u003eg\u003c/em\u003e\u003csub\u003eW\u003c/sub\u003e = 2.0, \u003cem\u003eS\u003c/em\u003e\u003csub\u003eW\u003c/sub\u003e = 1/2) in the PI state. Remarkably, the PI state persisted for at least 1 day after the light was turned off at 3 K. The reverse-PIPT of the PI state was investigated using a 532-nm cw diode laser. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e1\u003c/span\u003ee, the magnetization of the PI state at 3 K decreased to the initial value. Both photothermal and photoreversible PIPTs were cycled at least three times without any signs of degradation (Supplementary Figs.\u0026nbsp;5e and 5f). The FCM and \u003cem\u003eM\u003c/em\u003e\u0026ndash;\u003cem\u003eH\u003c/em\u003e curves also returned to the magnetization of the paramagnetic LT state. These measurements confirmed the photoreversible switching between the LT paramagnetic Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eIV\u003c/sup\u003e and PI ferromagnetic Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e states, which is similar to the HT state. Above 100 K, the lifetime of the PI state was too short to be observed using conventional techniques.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eGeneral consideration of time-resolved optical spectroscopy\u003c/h3\u003e\n\u003cp\u003eFemtosecond time-resolved optical spectroscopy was conducted to investigate the ultrafast dynamics underlying the PIPT through CT and ST processes in cyanido-bridged cobalt-tungstate assemblies. The visible region (500\u0026ndash;740 nm)\u003csup\u003e39\u003c/sup\u003e was probed, where the drastic optical density change in CsCoW is the characteristic of the electronic state switching. Measurements were performed above 100 K, where the short-lived PI state enables pump-probe measurements at a 1 kHz repetition rate. The sample was photoexcited with an approximate 65 fs laser pulse at 850 nm (8 mJ cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e). Two-time ranges of optical density changes (ΔOD(\u003cem\u003et\u003c/em\u003e)) were analyzed to investigate distinct dynamics: ultrafast photoinduced dynamics (from \u0026minus;\u0026thinsp;0.5 to 2.0 ps) and consecutive thermoelastic dynamics (from \u0026minus;\u0026thinsp;50 to 800 ps). The dynamical responses in CsCoW and CoW were compared at RT, where the LT state of CoW and the LT state branch of the thermal hysteresis loop in CsCoW were photoexcited. Additionally, the temperature-dependent photoresponse of CsCoW was examined using a cryogenic system, with measurements conducted at 230 and 100 K, i.e., temperatures near and far from the thermal hysteresis loop, respectively.\u003c/p\u003e\n\u003ch3\u003eTime-resolved optical spectroscopy in sub-ps timescale\u003c/h3\u003e\n\u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e2\u003c/span\u003e (top) shows the ΔOD(\u003cem\u003et\u003c/em\u003e) on the ps timescale, measured at RT following femtosecond laser excitation for CoW and CsCoW. In both cases, a decrease in OD was observed at 700 nm, corresponding to the strong bleaching of the LT Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eIV\u003c/sup\u003e state. Additionally, a short transient increase in OD was evident around 600 nm for both compounds. The time trace ΔOD(\u003cem\u003et\u003c/em\u003e) at 600 nm remains positive in CsCoW, which can be attributed to the formation of the PI Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state. The ΔOD(\u003cem\u003et\u003c/em\u003e) data measured at 230 and 100 K for CsCoW resemble the data at RT for CsCoW and CoW, respectively, showing a transient OD increase near 600 nm and an OD decrease near 700 nm, accompanied by coherent oscillations (Supplementary Fig.\u0026nbsp;6). For both compounds, similar coherent oscillations are evident in the time traces, at the same wavelength. In order to extract the characteristic timescales of the photoinduced dynamics, the OD changes were analyzed, resulting from the depopulation of the LT Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eIV\u003c/sup\u003e state toward an initially electronic photoexcited (PE) Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state, identified by the intermediate transient peak around \u003cem\u003et\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0. Population fitting of ΔOD(\u003cem\u003et\u003c/em\u003e) at 600 and 700 nm was performed using an exponential decay model for the transient PE state, which subsequently populates the PI Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state (Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e2\u003c/span\u003ed, Supplementary Note 6). The fitting results indicate that the transient PE state decays within approximately 130 fs into the PI state, which is similar to the HT state. This timescale is typical for photoinduced dynamics by ST, as reported in different spin-crossover materials exhibiting light-induced excited spin state trapping (LIESST effect) induced by metal-to-ligand CT photoexcitation.\u003csup\u003e40\u0026ndash;43\u003c/sup\u003e Supplementary Fig.\u0026nbsp;7 shows the oscillating component of the ΔOD(\u003cem\u003et\u003c/em\u003e) signal after 300 fs, obtained as a residual difference between the experimental data and the fitted exponential functions (Supplementary Note 6). A fast Fourier transform (FFT) of this signal revealed the frequencies of these oscillating components. The wavelength \u003cem\u003evs\u003c/em\u003e wavenumber vs FFT intensity maps are similar for CoW and CsCoW (Supplementary Figs.\u0026nbsp;8 and 9). Two common vibrational modes were identified for CsCoW, regardless of temperature: a fast oscillation with an approximate value of 259 fs period (approximately 130 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and a slower oscillation with an approximate value of 580 fs period (approximately 58 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) (Supplementary Table\u0026nbsp;2). The spectral weight of the slower oscillation is more prominent at longer wavelengths. The oscillating signals at 600 and 700 nm, fitted using only these two nodes, closely match the experimental data (Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e2\u003c/span\u003ed insets, Supplementary Figs.\u0026nbsp;10 and 11). At RT, ΔOD(\u003cem\u003et\u003c/em\u003e) remains positive near 600 nm after the transient peak, directly indicating the formation of the PI Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state in CsCoW. However, this OD increase is not clearly observed for CoW at RT or CsCoW at 100 K, which is likely due to the strong bleaching of the ground LT state absorption (Supplementary Fig.\u0026nbsp;14).\u003c/p\u003e\n\u003ch3\u003eConsideration of phonon modes\u003c/h3\u003e\n\u003cp\u003eTo understand the origin of the oscillations, we performed DFT calculations that included phonon modes using the Gaussian 16 package.\u003csup\u003e44\u003c/sup\u003e The coordination environments of the Co site ([Co(4-bromopyridine)\u003csub\u003e2\u003c/sub\u003e(NC)\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003en\u0026minus;\u003c/sup\u003e) were evaluated for various electronic states: trivalent low-spin (Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e), divalent low-spin (Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e), and divalent high-spin (Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e) (Supplementary Note 7). Although these calculations were simplified by focusing solely on the Co environment, the phonon mode frequencies closely aligned with experimental values obtained from IR and THz spectra (Supplementary Fig.\u0026nbsp;12). The frequencies of similar phonon modes increased in the order Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e \u0026lt; Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e \u0026lt; Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e (Supplementary Table\u0026nbsp;3). Phonon modes arising from cyanide ligands were observed below 500 and around 2100 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Meanwhile, modes originating from 4-bromopyridine ligands, including stretching and bending vibrations of the pyridine ring and C-H bonds, were observed in the 600\u0026ndash;1600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e range and around 3200 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively. Unique breathing modes were identified around 130 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e and Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e, based on the optimization of Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e. These modes involve the stretching of organic and cyanide ligands. This type of mode is known to be strongly coupled with STs as they act as the reaction coordinate. Consequently, these modes strongly affect metal-centered optical transitions. This type of coherent activation of breathing modes has been reported in other spin-crossover materials during the LIESST process.\u003csup\u003e40\u0026ndash;43\u003c/sup\u003e DFT calculations also revealed a phonon mode at approximately 58 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to a global torsion of the cyanido-bridged layer. Such torsion modes have been previously reported in other Prussian blue analogues (Co\u0026ndash;Fe and Mn\u0026ndash;Fe systems).\u003csup\u003e45\u0026ndash;47\u003c/sup\u003e The ST leads to a local expansion of the cyanido-bridged network on a timescale too short for lattice expansion, activating torsion modes to accommodate these local distortions of the cyanide (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e3\u003c/span\u003ec).\u003c/p\u003e\n\u003ch3\u003eCharge-transfer-induced spin transition in cyanido-bridged cobalt-tungstate assemblies\u003c/h3\u003e\n\u003cp\u003eBased on femtosecond optical spectroscopy and DFT calculations, we propose that the MMCT process involves a ground LT Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eIV\u003c/sup\u003e and a PE Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e states (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e3\u003c/span\u003eb shows a schematic of the \u003cb\u003eCTIST\u003c/b\u003e dynamics for both CsCoW and CoW, which share similar layered structures. The ultrafast photoinduced dynamics in cyanido-bridged cobalt-tungstate assemblies occur in two steps. First, optical excitation of the ground LT Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eIV\u003c/sup\u003e state produces a PE Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state, characterized by a transient OD peak around 600 nm. This PE state is presumed to be stabilized by Jahn\u0026ndash;Teller distortion and partial elongation of the average Co\u0026ndash;N bonds, which can drive the ST. In the second step, by causing further elongation of the Co\u0026ndash;N bond, the PE Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state decays within 130 fs into the lower-lying PI Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state. As discussed for the LIESST effect, the PE state functions as a mediator, with structural dynamics and electronic reorganization being entangled.\u003csup\u003e48\u003c/sup\u003e The timescale of this ST (approximately 130 fs) corresponds to half the period of the breathing mode associated with stretching of the C-N bonds and organic and cyanide ligands (approximately 259 fs) (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). This timescale is consistent with those reported for other Prussian blue analoguess and ST materials.\u003csup\u003e40\u0026ndash;43, 45\u0026ndash;47\u003c/sup\u003e On the 130 fs timescale of Co\u0026ndash;N bond elongation trapping of the PE state, the crystalline lattice has no time to expand sufficiently, necessitating distortions to accommodate local lattice expansion. The ultrafast Co\u0026ndash;N bond elongation launches consequently torsion modes of the W\u0026ndash;\u0026ndash;Co\u0026ndash;\u0026ndash;W cyanido-bridge, calculated at approximately 65 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and consistent with the coherent oscillations observed at 58 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). These sub-ps structural dynamics localized at the molecular scale are associated with the structural trapping of the PE state.\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eTime-resolved optical spectroscopy on 10\u0026rsquo;s ps timescale\u003c/h2\u003e \u003cp\u003eWe performed optical measurements on a longer ps timescale for the two compounds to monitor slower consecutive dynamics (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In all cases, the ΔOD(\u003cem\u003et\u003c/em\u003e) map around 700 nm reveals a rapid decay of the MMCT band, indicating ground-state bleaching immediately following photoexcitation. For CsCoW, an OD increase of around 600 nm was observed within 100 ps, characteristic of PI Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state formation near the thermal phase transition temperatures (at RT and 230 K) under high laser fluence. The maximum of this transient absorption band gradually shifts toward shorter wavelengths and decays within 800 ps (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, Supplementary Figs.\u0026nbsp;13a and 13b). This shift can be attributed to lattice relaxation affecting the Co\u0026ndash;N bonds, influencing the MMCT between Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e and W\u003csup\u003eV\u003c/sup\u003e owing to the larger e\u003csub\u003eg\u003c/sub\u003e-t\u003csub\u003e2g\u003c/sub\u003e energy gap in the Co ligand field compared to the original HT state. The dynamics exhibit strong temperature dependence. At low temperature (100 K) in CsCoW, ΔOD(\u003cem\u003et\u003c/em\u003e) is dominated by the pronounced decrease of the MMCT band of the LT state, which masks the PI state band (Supplementary Figs.\u0026nbsp;14 and 15, Supplementary Tables\u0026nbsp;4 and 5). The OD change map at 100 K closely resembles CoW at RT (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), where no thermal phase transition occurs. Notably, slower conversion is observed at 230 K and RT only under high fluence. These features of the slower conversion are characteristic of the so-called thermoelastic conversion, as recently reported for spin-crossover materials.\u003csup\u003e49\u0026ndash;51\u003c/sup\u003e\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eThermoelastic conversion\u003c/h3\u003e\n\u003cp\u003eWe next investigated the fluence dependence of the photoresponse in CsCoW and CoW at RT to examine the thermoelastic conversion (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee, Supplementary Figs.\u0026nbsp;16 and 17). In CsCoW, ΔOD(\u003cem\u003et\u003c/em\u003e) around 600 nm is positive and increases with higher pump fluence. In contrast, for CoW, ΔOD(\u003cem\u003et\u003c/em\u003e) around 600 nm is negative and decreases consistently with fluence. The OD changes at 580 and 700 nm, measured 100 ps after photoexcitation, are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee. The nonlinear increase in ΔOD(\u003cem\u003et\u003c/em\u003e) for CsCoW indicates a cooperative process driving the PI state, while the linear decrease in ΔOD(\u003cem\u003et\u003c/em\u003e) for CoW reflects a local process dominated by the bleaching of the LT MMCT band. These observations strongly indicate that the dynamics in CsCoW within 100 ps correspond to thermoelastic conversion, as in the case of spin-crossover materials. Remarkably, when the pump fluence exceeded 13 mJ cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e for CsCoW, no OD change was detected during the time-resolved optical spectroscopy. This result indicates persistent PIPT at RT, transitioning from the LT to PI (HT) branches of the hysteresis.\u003c/p\u003e \u003cp\u003eOverall, the dynamics in the cyanido-bridged cobalt-tungstate assemblies correspond to the thermoelastic conversion from the LT to PI states, driven by two photoinduced phenomena (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef). First, local molecular photoswitching occurs within 130 fs, involving the volume change by STs that generate internal pressure, stabilizing the higher-volume PI state. Second, the lattice warming caused by the laser heating facilitates the thermal conversion from the LT to PI states. Thus, our data align with the physics of thermoelastic processes, where elastic lattice expansion, driven by the local molecular PIPT favors the thermal population of the high-entropy PI state.\u003csup\u003e49\u0026ndash;51\u003c/sup\u003e When the system is far from a thermal phase transition, as is the case for CsCoW at 100 K and CoW at RT, the thermoelastic conversion is weak. However, near the thermal phase transition, as it is for CsCoW at 230 K and RT, the laser pump fluence induces substantial volume strain and temperature jumps, resulting in robust thermoelastic conversion. As highlighted earlier, CsCoW enhances the HT state stability by destabilizing the hydrogen-bond network in the crystal structure through partial Cs\u003csup\u003e+\u003c/sup\u003e ion substitution. This substitution in CsCoW allows shifting the thermal phase transition closer to RT by controlling the relationship between the effect on the intermolecular interactions and cooperativity mediated by the cyanido-bridged networks, enabling thermoelastic conversion in the PIPT out-of-equilibrium dynamics in the vicinity of RT.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn conclusion, we investigated the photoinduced \u003cb\u003eCTIST\u003c/b\u003e process in cyanido-bridged cobalt-tungstate assemblies by comparing CsCoW and CoW materials. Optical and magnetic measurements at low temperatures for CsCoW confirmed reversible PIPT in CsCoW, accompanied by a dramatic optical color change between blue in the LT state and red in the PI state resembling the HT state, and photomagnetic behavior. The photomagnetic properties of the PI state in CsCoW exhibited photoinduced magnetization, with a Curie temperature of 27 K and a coercive field of 7500 Oe at 2 K.\u003c/p\u003e \u003cp\u003eWe also studied the out-of-equilibrium photoinduced dynamics of CoW and CsCoW using femtosecond optical spectroscopy. The dynamics comprise a molecular sub-ps process and a macroscopic ps thermoelastic conversion. The first step involves, at the molecular level, the PE state resulting from MMCT from Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eIV\u003c/sup\u003e to Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e, followed by ST within 130 fs to the PI Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state. This process is characterized by the coherent activation of the breathing mode of the CoN\u003csub\u003e6\u003c/sub\u003e core (approximately 130 cm\u003csup\u003e− 1\u003c/sup\u003e). These initial \u003cb\u003eCTIST\u003c/b\u003e dynamics were observed in both CoW and CsCoW across all temperatures, as the two compounds exhibit similar molecular and electronic structures around the photoactive Co and W sites. This local photoswitching process drives a subsequent thermoelastic step, where molecular expansion within the lattice, coupled with laser heating, balances the relative stabilities of the LT and PI (HT) states. Thermoelastic conversion is consequently observed in the vicinity of the thermal phase transition and at high fluences in CsCoW, consistent with theoretical predictions for other bistable molecular materials.\u003csup\u003e49–51\u003c/sup\u003e Overall, the present work underlines the direct evidence of photoinduced \u003cb\u003eCTIST\u003c/b\u003e in cyanido-bridged cobalt-tungstate assemblies directly and highlights the critical role of Cs\u003csup\u003e+\u003c/sup\u003e ion substitution, which is not only responsible for shifting the thermal transition towards RT but also allows thermoelastic conversion in PIPT.\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003c/div\u003e \u003c/div\u003e "},{"header":"Methods","content":"\u003ch2\u003eSyntheses\u003c/h2\u003e\u003cp\u003eCrystalline powders of CsCoW and CoW was synthesized following established protocols in the literature.\u003csup\u003e36,37\u003c/sup\u003e Characterizations induced CHN analysis, powder X-ray diffraction, IR spectroscopy, UV-vis-NIR spectroscopy, and magnetic measurements (Supplementary Note 1). Elemental analysis results were as follows: CsCoW: Calculated: C, 27.2%; H, 1.6%; N, 16.8%. Found: C, 27.0%; H, 1.8%; N, 16.5%, for CoW: Calculated: C, 26.9%; H, 1.6%; N, 17.4%. Found: C, 26.8%; H, 1.8%; N, 17.3%.\u003c/p\u003e\u003ch2\u003eMeasurements\u003c/h2\u003e\u003cp\u003eTime-resolved pump-probe experiments were conducted at the Institut de Physique de Rennes using a shot-to-shot optical setup described in Supplementary Note 2. The pump light was set to 850 nm, targeting the tail of the W\u003csup\u003eIV\u003c/sup\u003e-to-Co\u003csup\u003eIII\u003c/sup\u003e MMCT band to maximize the penetration depth. The probe light was generated using a sapphire plate, producing an intense polarized pulse spanning a spectral range from visible to NIR (450–800 nm).\u003csup\u003e39\u003c/sup\u003e A 750-nm long-pass filter was inserted into the probe path to block the pump beam, allowing only the probe supercontinuum to be monitored. Samples were dispersed in paraffin oil and sandwiched between thin glass plates. A reference sample was prepared by sandwiching paraffin oil between identical glass plates. Sample temperature was controlled using an Oxford Instruments Cryojet with an N\u003csub\u003e2\u003c/sub\u003e cryojet. Data were analyzed using iterative fitting of multiple exponential functions convoluted with a Gaussian instrument response function with a full-width at half maximum of 65 fs. For sub-ps timescales, contributions from the substrate (glass and liquid paraffin) were subtracted from transient two-dimensional maps, isolating the photoinduced signal from the sample (see Supplementary Note 2).\u003c/p\u003e\u003ch2\u003eCalculation\u003c/h2\u003e\u003cp\u003eDFT calculations were conducted to elucidate the origin of the oscillations observed in the sub-ps time delay measurements.\u003csup\u003e44\u003c/sup\u003e Since the crystal structures were too complex for phonon mode calculations, we focused on the Co site unit, Co(4-bromopyridine)\u003csub\u003e2\u003c/sub\u003e(NC)\u003csub\u003e4\u003c/sub\u003e, in three distinct electronic states: Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e, Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e, and Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e. Geometry optimization and vibrational frequency calculations were performed for all states using the B3LYP 6-31G\u003csup\u003e+\u003c/sup\u003e(2df,2p) functional. A restricted spin approach was applied for Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e, while an unrestricted spin approach was used for Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e and Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e, with computations carried out in Gaussian16. The vibrational frequencies obtained are detailed in Supplementary Note 7.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe supplementary information provides all data generated in this study, including characterizations, experimental configurations, raw data treatment, data fitting for time-resolved optical measurements, and DFT calculations. Relevant data are also available upon request from the authors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported in part by a Grant-in-Aid for Scientific Research (A) from JSPS KAKENHI (Grant Number 20H00369), Advanced Technologies for Carbon-Neutral (ALCA)-Next from JST (JPMJAN23 A2), CNRS-University of Tokyo\u0026ldquo;Excellence Science\u0026rdquo; Joint Research Program, Second CNRS \u0026ndash;University of Tokyo PhD Joint Program. The Cryogenic Research Center, The University of Tokyo, the Center for NanoLithography \u0026amp; Analysis, The University of Tokyo, and Quantum Leap Flagship Program (Q-LEAP, Grant Number JPMXS0118068681) by MEXT are also acknowledged for support. K. Nakamura was supported by the World-Leading Innovative Graduate Study Program for Materials Research, Information, and Technology (MERIT-WINGS) and the Fellowship for Integrated Materials Science and Career Development. K. Nakabayashi acknowledges the Iketani Science and Technology Foundation (Grant Number 0351111-A). The authors gratefully acknowledge the Agence Nationale de la Recherche for financial support under grant ANR-19-CE30-0004 ELECTROPHONE, ANR-19-CE29-0018 MULTICROSS. E.C. thanks the University of Rennes, the Fondation Rennes 1, and Region Bretagne (Boost\u0026apos;ERC) for funding.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eK. Nakamura, L. G., K. Nakabayashi, E. C., and S. O. conceived the project. K. Nakamura synthesized and characterized samples and performed photoinduced UV-vis-NIR absorption measurements and photomagnetic measurements for \u003cstrong\u003eCsCoW\u0026nbsp;\u003c/strong\u003eand theoretical calculations. K. Nakamura, G. P., and L.G. conducted the transient absorption measurements. K. Nakamura, G. P., L. G., and M. H. analyzed the data. All authors discussed the experimental and theoretical results. K. Nakamura, L. G., K. Nakabayashi, and E. C. wrote the manuscript, with contributions from all authors.\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information\u0026nbsp;\u003c/strong\u003eThe online version contains supplementary material available at\u003c/p\u003e\n\u003cp\u003ehttps://doi.org/.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorrespondence\u0026nbsp;\u003c/strong\u003eand requests for materials should be addressed to Eric Collet and Shin-ichi Ohkoshi.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReprints and permission information\u0026nbsp;\u003c/strong\u003eis available at http://www.nature.com/reprints\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePublisher\u003c/strong\u003e\u003cstrong\u003e\u0026rsquo;\u003c/strong\u003e\u003cstrong\u003es note\u0026nbsp;\u003c/strong\u003eSpringer Nature remains neutral with regard to jurisdictional claims in published\u003c/p\u003e\n\u003cp\u003emaps and institutional affiliations.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eEspallargas GM, Coronado E (2018) Magnetic functionalities in MOFs: from the framework to the pore. 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Phys Rev B 95:224107\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":"charge transfer, spin transition, cyanido-bridged assemblies, photoinduced phase transition, photomagnetism, ultrafast spectroscopy","lastPublishedDoi":"10.21203/rs.3.rs-5901007/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5901007/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn photoinduced phase transition materials, where both charge transfer and spin transition occur, there has been a long debate on which of the two processes are leading the phase transition. Herein, we present experimental evidence supporting an optically \u003cem\u003echarge-transfer-induced spin transition\u003c/em\u003e (\u003cb\u003eCTIST\u003c/b\u003e) process, as demonstrated through femtosecond optical spectroscopy in two-dimensional cyanido-bridged cobalt-tungstate photomagnets. Optical and magnetic investigations reveal that the optical excitation of the ground low-temperature (LT) Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eIV\u003c/sup\u003e state drives a photoinduced phase transition towards the Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state, similar to the high temperature (HT) state. Ultrafast spectroscopy further indicates that this optical excitation of the intermetallic W-to-Co charge-transfer band produces a transient photoexcited (PE) Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state, which decays within 130 fs through a spin transition towards the Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e state. Here we show that the \u003cb\u003eCTIST\u003c/b\u003e dynamics corresponds to the Co\u003csup\u003eIII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eIV\u003c/sup\u003e (LT) \u0026rarr; Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eLS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e (PE) \u0026rarr; Co\u003csup\u003eII\u003c/sup\u003e\u003csub\u003eHS\u003c/sub\u003e-W\u003csup\u003eV\u003c/sup\u003e (HT) sequence. The present work sheds a new light on understanding optical dynamics underlying the photoinduced phase transitions.\u003c/p\u003e","manuscriptTitle":"Ultrafast charge-transfer-induced spin transition in cobalt-tungstate molecular photomagnets","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-07 09:36:45","doi":"10.21203/rs.3.rs-5901007/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":"fdd54f39-5c54-46b0-84aa-ee4b3224f647","owner":[],"postedDate":"February 7th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":43900288,"name":"Physical sciences/Chemistry/Physical chemistry"},{"id":43900289,"name":"Physical sciences/Chemistry/Inorganic chemistry"}],"tags":[],"updatedAt":"2025-06-07T07:07:32+00:00","versionOfRecord":{"articleIdentity":"rs-5901007","link":"https://doi.org/10.1038/s41467-025-60401-4","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-06-06 04:00:00","publishedOnDateReadable":"June 6th, 2025"},"versionCreatedAt":"2025-02-07 09:36:45","video":"","vorDoi":"10.1038/s41467-025-60401-4","vorDoiUrl":"https://doi.org/10.1038/s41467-025-60401-4","workflowStages":[]},"version":"v1","identity":"rs-5901007","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5901007","identity":"rs-5901007","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}