A single-crystal platform for unveiling ultrafast and complex photochemical cascade reactions

A single-crystal platform for unveiling ultrafast and complex photochemical cascade reactions | 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 A single-crystal platform for unveiling ultrafast and complex photochemical cascade reactions Jian-Ping Lang, Qiaoqiao Zhang, Yun-Hu Deng, Yong Wang, Ming-Hao Du, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4993811/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Developing versatile crystalline platforms can significantly enhance the utility of time-resolved X-ray crystallography for observing diverse reaction mechanisms 1–6 . However, it is often limited by its inability to handle ultrafast and complex reactions. Here we propose a coordination polymer single-crystal platform that incorporates flexible cluster nodes and integrated reaction substrates. This advanced platform features enhanced diffraction capabilities and adaptability to substrate changes, enabling the observation of ultrafast, highly dynamic reactions involving multiple pathways and intermediates. By combining this platform with a cryo-assisted strategy, we investigate a complicated ultrafast photochemical cycloaddition reaction with five transient intermediates and three distinct routes, which are rationalized through theoretical calculations. Our findings underscore the feasibility of employing this enhanced single-crystal platform to unravel elusive reaction mechanisms, presenting a promising approach with broad applicability. Physical sciences/Chemistry/Inorganic chemistry/Solid-state chemistry Physical sciences/Chemistry/Photochemistry/Photocatalysis Physical sciences/Chemistry/Organic chemistry/Reaction mechanisms Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Understanding the mechanistic details of chemical reactions is key to the synthesis of new compounds and materials, the development of new processes, gaining insights into structure-function relationships 7-11 , and the development and optimisation of industrially relevant processes 12-16 . While spectroscopic methods based on laser pulses and free-electron lasers have been developed to identify transition states and elucidate associated mechanisms 17-22 , time-resolved single-crystal X-ray diffraction (TR-SCXRD) offers the advantage of providing precise structural information not otherwise available (Fig. 1a). However, its application requires retention of crystalline structures throughout the reaction process, thus limiting the types and scope of observable reactions. In 2009, Fujita et al. introduced a novel paradigm where the substrates were adsorbed onto the channels of a porous crystalline coordination polymer network 1 , allowing a short-lived hemiaminal intermediate to be captured (Fig. 1b). The development of this technique quickly garnered attention because it reduced the instrumentation and technical costs associated with studying reaction mechanisms and expanded the types of reactions that can be investigated. Subsequently, Sumby and Doonan et al. studied the carbonylation of MeX (X = I, Br) by metallating a Mn-MOF with a flexible bis-pyrazole chelating group on the skeleton (Fig. 1b), and enabled the study of chemically stable MOF having a poor ability to retain a crystalline state throughout the chemical reaction 2,3 . To date, similar approaches have enabled the mechanistic investigations on e.g. the palladium-mediated aromatic bromination and ozonolysis of olefins in aforementioned porous complexes 4,5 , and transformations of organic molecules within chiral pores 6 . Whilst the approaches employed to examine reaction mechanisms in the crystalline state are commonly tailored for specific reactions, the development of a general methodology, that has widespread application, would seem a worthwhile pursuit. The value of a new general approach may be assessed by determining its effectiveness in providing an understanding of complex and challenging reactions. Complexity may arise from numerous intermediates associated with multi-step mechanisms or perhaps reactions that occur via multiple pathways (Fig. 1c). Furthermore, reactions with large enthalpy changes or involving ultrafast processes provide challenges in measuring useful experimental data. Previously employed approaches using crystalline solids are not well-suited to the investigation of the types of complex and challenging reactions that are commonly encountered in both natural and artificial systems. To address the difficulties associated with complex and challenging reactions, a single-crystal platform is required that can withstand structural changes without compromising lattice integrity and diffraction capability. After evaluating various crystalline materials 23-30 , we identified a highly promising structure: a stacked one-dimensional (1D) coordination polymer chain composed of alternating dinuclear clusters and bipyridine ligands. The bipyridine ligands can be synthesized using commonly employed Heck or Suzuki procedures to append metal binding pyridyl groups to each end of the reaction substrate. The integration of the reaction substrate within bridging ligands mitigates issues of uneven distribution or improper orientation that can arise in pore diffusion methods. Furthermore, the stacking arrangement of the 1D chains avoids the problem of large solvent-filled pores which commonly leads to weak diffraction. In addition to these structural features, there are two structural aspects that bestow a degree of flexibility upon the crystal, allowing a relatively gentle transition from reactant to product that preserves lattice integrity. Firstly, the cluster nodes can switch between various coordination geometries and adapt to changes in the coordination orientation of the pyridine groups. Secondly, the chain structure ensures that the stress induced by structural transformations is not easily propagated in three dimensions, instead, weak secondary bonding interactions provide a buffer between 1D coordination polymers. These features are expected to enhance the single-crystal's ability to withstand the liberation of energy during the reaction (Fig. 1d). In this work, the bipyridyl bridging ligand 5-hydroxymethyl-1,3-bis(( E )-2-(pyridin-4‑yl)- vinyl)benzene (CH 2 OH-1,3-bpeb) was combined with Cd(II) ions and 3,5-dichlorobenzoic acid (3,5-HDCB) to yield a 1D coordination polymer [{Cd 4 (CH 2 OH-1,3-bpeb) 4 (3,5-DCB) 8 }·4H 2 O] n ( CP1 ) (Fig. 2a). CP1 has four crystallographically distinct sites that are each expected to undergo a [2 + 2] photocycloaddition reaction. When the crystals were irradiated with ultraviolet light at room-temperature, the reaction was violent, resulting in considerable lattice damage. In addition, the reaction was completed within a matter of seconds, representing the fastest recorded photocycloaddition process using a single-crystal platform (Supplementary Table 1) 31-35 . Examination of the reaction products by NMR revealed complete cyclization at four distinct sites but the sequential process may involve up to 14 intermediates and 75 reaction routes. Clearly, the elucidation of both the intermediates and reaction pathways represents a considerable challenge. By employing the single-crystal platform approach at low temperature, we have succeeded in using TR-SCXRD technology to capture 'snapshots' of the reaction, revealing five transient intermediates and three transformation routes within the cascade of reactions involving four pairs of C=C bonds. Theoretical calculations complement the experimental investigation. This work demonstrates that a cluster-based coordination polymer single-crystal platform can be used to tackle ultrafast and complex reaction mechanisms. It also illustrates that rational structural design has the potential to significantly enhance the versatility of single-crystal platform technology, addressing reaction mechanisms that conventional techniques cannot resolve. An X-ray analysis of CP1 reveals two chemically similar but crystallographically distinct chains. Each chain consists of pairs of closely separated Cd(II) centers which are coordinated by four 3,5-DCB ligands, two of which serve as bridging carboxylates between the two Cd(II) centers. This results in the four carboxylate groups being approximately coplanar with two Cd(II) centers. The binuclear unit Cd 2 (3,5-DBC) 4 is linked by two pairs of CH 2 OH-1,3-bpeb ligands to crystallographically equivalent units on either side. Two bridging CH 2 OH-1,3-bpeb ligands coordinate to each Cd(II) through mutually trans pyridyl groups approximately orthogonal to the carboxylate plane described above. Overall, a double chain structure is formed, as represented in Fig. 2b in red and blue. With the vinyl-pyridyl groups occupying the 3,5-positions on the phenyl ring, the bridging ligand is V-shaped, resulting in zigzag chains. Each Cd(II) in the two distinct Cd 2 (3,5-DBC) 4 units is coordinated by three carboxylate anions and two pyridyl groups. Whilst each Cd is coordinated by four oxygen atoms with Cd-O separations in the range 2.25-2.46 Å, one Cd(II) from each Cd 2 (3,5-DBC) 4 unit forms a weaker interaction with a fifth oxygen with a Cd-O separation of 2.6 Å resulting in a 7-coordinate Cd(II). Thus each Cd 2 (3,5-DBC) 4 unit contains a 6-coordinate and a 7-coordinate Cd(II). For each 6-coordinate Cd(II), there is an additional Cd···O contact beyond 2.8 Å. Although there are four crystallographically distinct CH 2 OH-1,3-bpeb ligands (two in each chain), they all adopt a similar configuration (ignoring the orientation of the OH group) which is different to that in Fig. 2a. In each CH 2 OH-1,3-bpeb ligand, one of the vinyl-pyridyl groups rotates around the vinyl C to phenyl ring bond by 180° resulting in the configuration (Fig. 2b). Pairs of crystallographically different CH 2 OH-1,3-bpeb ligands that link adjacent Cd 2 (3,5-DBC) 4 unit are closely separated and parallel to each other with an arrangement that places the olefinic bonds in relatively close proximity (Fig. 2b). With the asymmetric unit including four distinct CH 2 OH-1,3-bpeb ligands, there are eight crystallographically independent double bonds. Constrained by coordination of the CH 2 OH-1,3-bpeb ligands to pairs of Cd(II) centers in Cd 2 (3,5-DBC) 4 units, the maximum centroid separation between adjacent double bonds of neighboring diene molecules is 3.8 Å (Supplementary Fig. 5). The arrangement and separation of the double bonds in CP1 are expected to facilitate intermolecular cyclization reactions under appropriate illumination (Fig. 2c) 36 . For convenience, the positions of the eight double bonds are divided into four regions, namely, Regions 1 to 4, according to clockwise ordering (Fig. 2d). Irradiation of single crystals of CP1 at 298 K with ultraviolet light causes a photochemical process that results in violent fragmentation of single crystals as indicated by time lapse photographs (Fig. 3a and Supplementary Video 1). Crystals begin fragmenting within 0.5 s of irradiation, and the process continues until around 25 s after which no significant morphological changes occur. Compared to similar photocyclization reactions, the rapidity of this process is unprecedented. The products of this fragmentation process (Fig. 3b) were analyzed and Fig. 3c shows the 1 H NMR spectra of the samples after irradiation for different periods of time. As the photoreaction progresses, the intensity of the signals assigned to the H atoms of uncyclized CH 2 OH-1,3-bpeb, at 8.6 ppm and 4.6 ppm, decreases. The signals at 8.45 and 4.42 ppm, attributed to the mono-cyclized product of two CH 2 OH-1,3-bpeb molecules (Dimer-I), first increase and then decrease until they completely disappear after 25 s. The resonances at 8.40 and 4.18 ppm, identified as the dual-cyclized product of two CH 2 OH-1,3-bpeb molecules (Dimer-II), increase in intensity after first appearing at 0.5 s. After ca. 25 s, the 1 H NMR signals for CH 2 OH-1,3-bpeb and Dimer-I are no longer observable, and the signals associated with Dimer-II also cease to increase (Supplementary Figs. 6 and 7). This experiment suggests that the double bonds in all four regions undergo photodimerization. To our knowledge, this is the fastest reported [2 + 2] set of photoaddition reactions involving a crystalline platform (Supplementary Table 1). Understanding the underlying mechanism would have significant implications for the development of advanced light-driven devices and materials 37-43 and this relies on the identification of the reaction intermediates 44,45 . Given that the reaction involves the formation of four crystallographically distinct cyclobutane rings, the reaction pathway is complex even if the intermediates are simply considered to be the 14 states shown in Supplementary Fig. 8, associated with 75 possible pathways leading to the final product. Whilst violent fragmentation of the crystal occurs at 298 K, the reaction may be slowed down considerably by irradiating at low temperature. Irradiation of a crystal with UV light at 173 K led to no significant changes in the crystal's appearance (Fig. 3d). After exposure for 2 h, only slight cracking was apparent. NMR monitoring of the reaction progress was undertaken, as done for the experiment performed at 298 K. Whereas full conversion of the double bonds to cyclobutane rings was completed after 25 s at 298 K, the same conversion required 140 min at 173 K (Fig. 3e). As with the earlier analysis, the 1 H NMR signals of the uncyclized CH 2 OH-1,3-bpeb ligand decreased during irradiation whilst those associated with Dimer-I first increased and then decreased and those for Dimer-II grew over the irradiation period. The quality of the crystals even after 2 h of irradiation allowed a single-crystal structural determination which clearly established full conversion of the diene units in the four regions (Fig. 2d) to cyclobutane rings (Supplementary Fig. 9). Powder X-ray diffraction patterns (PXRD) for the samples irradiated at 298 K and 173 K revealed that the same crystalline product was obtained in each case and the data matched those simulated from its single-crystal data (Fig. 3f). The X-ray diffraction studies were fully consistent with the NMR analysis. The established stability of the crystalline platform upon irradiation at 173 K prompted a detailed X-ray analysis of the products obtained after different periods of irradiation at low temperature. In particular, the study focused on changes in each of the four regions of the asymmetric unit identified in Fig. 2d. We tested samples with an interval of 20 min and discovered seven crystal structures with distinct cyclization states (see Supplementary Figs. 9 and 10 and Supplementary Table 2). Site occupancy refinement provided information regarding the 'average' structure of the asymmetric unit in the crystal and thus refinements indicating sites partially occupied by cyclobutane units revealed the presence of one or more intermediate structures. Furthermore, understanding the true conversion pathway should consider the intermediates present in each crystal, specifically the cyclization states within the asymmetric units, rather than the apparent occupancy. Fig. 4a illustrates the potential cyclization states of intermediates observed in crystals at different reaction time, using a schematic representation. For the sample ( CP1-1 ) irradiated for 20 min, the occupancies of the unreacted olefin groups and cyclobutane unit in Region 1 were 54.4% and 45.6%, respectively, while the remaining three regions retained completely occupied by olefinic groups. After 40 min reaction time, Region 1 showed complete conversion to cyclobutane, everything else remaining intact. At longer reaction time, 100% photo-cyclization was achieved in Region 1, while diverse cyclization states in the remaining regions could not be attributed to a single sequential reaction process. This suggests that after complete cyclization occurred in Region 1, branching reaction pathways began to emerge. From data for CP1-3 collected after 60 min, complete cyclization in Region 1 was again apparent but only 21.2% in Region 2. The other two regions kept unchanged. Thus, two intermediates coexisted: Int1, corresponding to cyclization only in the first region, and Int2, corresponding to cyclization in both the first and second regions. Twenty minutes later, the conversion of cyclobutane units in the second region increased to 59.3%, and the third region began to show cyclobutane units with an occupancy of ca. 47.4% ( CP1-4 ). This disordered state is most likely due to uneven irradiation of the crystal, leading to sequential photo-cyclization at Regions 1-3 on the crystal surface. It should be acknowledged that this disorder may involve the coexistence of four intermediates: Int1, Int2, Int2' with cyclization in Regions 1 and 3, and Int3 with cyclization in Regions 1-3. The existence of Int2' and Int3 was confirmed by the direct observation of CP1-5 and CP1-6 after 100 min reaction. In CP1-5 , the olefinic groups in Regions 2 and 3 were fully converted to cyclobutane units, whereas no indication of cyclization in Region 4 was obtained. While CP1-6 showed complete conversion to cyclobutane units in Regions 1 and 3, but no cyclization in Regions 2 and 4, data for CP1-7 collected after 120 min indicated full conversion to cyclobutane units in Regions 1 and 3 but only of 46.8% in Regions 2 and 4. This crystal may contain Int2', Int3, Int3' with photocycloaddition in Regions 1, 2, and 4, and the final product with photocycloaddition at all four regions. After illumination for 140 min, all regions in the samples investigated showed complete conversion of the olefinic groups to cyclobutane units ( CP2 ). Combined the identification of five kinds of intermediates with the results of theoretical calculations (Extended Data Fig. 1), the transformation pathways were further narrowed down to the three pathways (Fig. 4b). They all involved a [2 + 2] photochemical addition in Region 1 and formation of intermediate Int1 in the initial step. Photochemical cycloaddition then proceeded in Region 2 or 3 resulting in the formation of intermediates Int2 or Int2', respectively. Int2 led to the final product CP2 by photochemical addition in Region 3 and then in Region 4 (Path 1). For Int2', the next step has two branches i) the photochemical addition reaction occurring first in Region 2, then Region 4 (Path 2); or ii) the photochemical addition reaction occurring first in Region 4, then in Region 2 (Path 3). In order to gain further insights into the reaction pathway and mechanism, odd-electron density (OED) distributions and spin-orbit coupling (SOC) effects for excited states were investigated using DFT calculations. For the initial structure, spin-orbit coupling calculations reveal a significant coupling effect between the S3 and T1 states, as well as between the S4 and T2 states of CP1 (Supplementary Fig. 11f and Supplementary Table 3). Odd-electron density distributions for both T1 and T2 states are mainly located on the alkene groups in Region 1 (Supplementary Fig. 11a, b), which is consistent with the experimentally determined site for the first cyclization process during the SCSC transformation (see CP1-1 and CP1-2 ). Subsequent calculations reveal that a series of triplet states for Int1 are accessible through SOC interactions (Supplementary Fig. 12f and Supplementary Table 4). The lowest energy singlet excited state, S1, is spin-orbit coupled with the T3 and T4 states, which concentrates the OED in Region 2 (Supplementary Fig. 12c, d). Furthermore, the lower triplet states, T1 and T2, with single electron density in Region 3 (Supplementary Fig. 12a, b) are also reachable by spin-orbit coupling with S3 and S4 states, or internal conversion (IC) from T3 and T4 states. These two conditions lead to the formation of Int2 and Int2', respectively, which is consistent with the experimental observation of two different cyclization sites ( CP1-3 and CP1-6 ) after CP1-2 . Spin-orbit coupling calculations suggest that there is no coupling between the lowest singlet excited state S1 and the triplet states within Int2. Instead, intersystem crossing (ISC) mainly occurs via spin-orbit coupling interactions involving S2, S3, and S5 with T1-T5 (Supplementary Fig. 13f and Supplementary Table 5). It is noteworthy that the lowest-energy triplet states T1 and T2, characterized by their OED in Region 3 (Supplementary Fig. 13a, b), exhibit similar energy levels. Conversely, the T3 and T4 states, with OED in Region 4, have higher energy levels (Supplementary Fig. 13c, d). Therefore, it is possible to move to a lower triplet state, T1 or T2, by IC events. This explains the predominant observation of the next cyclization event in Region 3 within the SCSC process ( CP1-4 and CP1-5 ). The S1 and S3 states of Int2' exhibit coupling effects with the T1, T3, and T5 triplet states (Supplementary Fig. 14f and Supplementary Table 6), which have concentrated OED in Region 2 (Supplementary Fig. 14a, c, and e). In contrast, the S2 state displays coupling effects with the T2 and T4 triplet states, characterized by OED in Region 4 (Supplementary Fig. 14b, d). The energy levels of the T1 and T2 triplet states are notably similar and markedly lower compared to those of the remaining triplet states. Thus, it is possible for Int2' to undergo photochemical cyclization in Region 2 or 4 through its T1 or T2 energy states, respectively. This provides an explanation for the observation of CP1-7 in the SCSC process. The energy change for each step has been evaluated using thermodynamic calculations (Extended Data Fig. 1, Supplementary Fig. 15 and Supplementary Table 7). The product with cyclization in Region 1 has both the lowest energy barrier and single point energy compared to any product cyclized in other Regions, which is consistent with Region 1 being where photocyclization occurs most readily. Spin-orbit coupling-time-dependent density functional theory (SOC-TDDFT) and OED calculations indicate that starting from Int1, there are two sites, Regions 2 and 3, where cyclization reactions may occur. From a thermodynamic perspective, it is also advantageous for reactions to occur at these two sites due to lower energy barriers. For Int2, the energy barrier for cyclization in Region 4 is higher. Therefore, for Int2, the formation of Int3 should be the preferred next step. This conclusion is also consistent with OED results and experimental observations. When Int2' undergoes photocyclization, the energy barrier difference between the cyclization sites in Regions 2 and 4 is minimal, supporting the simultaneous existence of two branching reaction pathways. The observation of such ultrafast and multi-path reactions is unprecedented in SCSC transformations 44-48 . Moreover, the single-crystal platform exhibited remarkable stability at low temperature, allowing considerable structural changes to occur with retention of the single-crystal character. To demonstrate the superior stability arising from the unique adaptive ability of the platform, the structural changes accompanying the reaction process are examined (Extended Data Fig. 2). From Extended Data Fig. 2a and Supplementary Table 8, we note that the separation between adjacent N atoms of parallel diene molecules increases by 5.4% to 16.34%, following the cyclization process. This is due to the formation of the cyclobutane ring which causes the pyridyl rings, which are close to parallel before cyclization, splaying apart from each other upon ring formation. The Cd···Cd distances in all Cd 2 (3,5-DBC) 4 cluster units increased by 5.6% as a consequence of the increased separation between the coordinating pyridyl groups. In the original structure, a single oxygen atom bridges each pair of Cd(II) centers. Due to the Cd(II) centers being forced apart by the pyridyl groups, this oxygen atom is unable to span the Cd(II) centers, which makes all Cd(II) centers be 6-coordinate in the final structure. The single-crystal stability exhibited at low temperature may be at least partially attributed to the ease with which the Cd 2 (3,5-DBC) 4 cluster units within the chain adapt to significant structural changes associated with the cyclization process (Extended Data Fig. 2b). In addition to the intrachain structural changes, some displacement of the chains is also noted. Whilst this is inevitable given the structural changes in the chains, the movement seems to proceed smoothly and in a manner that reduces internal strain caused by the significant intrachain changes. The result is that the single-crystal character is preserved at low temperature, thus allowing detailed investigations of the reaction pathway. In conclusion, we have resolved the mechanisms of ultrafast and multi-step [2 + 2] photochemical cycloaddition reactions involving multiple reaction pathways and multiple transient intermediates using a cryo-assisted coordination polymer single-crystal platform. This investigation has relied upon the robust nature of the single-crystal platform used which can tolerate significant structural rearrangements upon irradiation at low temperature with retention of its single-crystal character. The success of the cryo-assisted SCSC approach in elucidating this ultrafast and complex reaction is attributed to the flexibility of the Cd 2 (3,5-DBC) 4 unit and the gentle repositioning of the chains at low temperature which serves to reduce crystal strain arising from the photocyclization processes. The reaction mechanisms elucidated in this study involves severe challenges in speediness and process complexity, each of which was previously difficult to address with single-crystal platforms. This advancement marks a potentially significant expansion in the range of reactions that can be observed using single-crystal platforms for mechanistic studies. Consequently, this approach holds great promise for incorporating a number of previously elusive reaction mechanisms into the realm of investigation. Methods Materials and methods The diolefin ligand 5-hydroxymethyl-1,3-bis(( E )-2-(pyridin-4-yl)vinyl)benzene (CH 2 OH-1,3-bpeb) was synthesized by a palladium-catalyzed Heck reaction between (3,5-dibromophenyl)methanol and 4-vinylpyridine, according to the literature method 49 . Other chemicals and reagents were commercially purchased and used without further purification. Single-crystal X-ray diffraction (SCXRD) data were recorded on a Bruker D8 VENTURE diffractometer. Powder X-ray diffraction (PXRD) patterns were obtained by a PANalytical X'Pert PRO MPD system (PW3040/60) using Cu- Kα radiation (λ = 1.5406 Å) from 5º to 50º with a scanning step of 0.022º. Thermogravimetric (TG) analysis plot was obtained from a Mettler Toledo Star system under an N 2 atmosphere at a heating rate of 10 ºC min -1 . Nuclear magnetic resonance (NMR) spectra were recorded on BRUKER AVANCE III HD-400M and Varian UNITY plus-600M spectrometers at room temperature. The 1 H NMR spectrum at 0.5 s at 298 K and 1 H NMR spectra at 173 K were recorded at a Varian UNITY plus-600M spectrometer, and other 1 H NMR and 13 C NMR spectra were recorded at a BRUKER AVANCE III HD-400M spectrometer. Proton chemical shifts δ H = 7.26 ppm (CDCl 3 ) and δ H = 2.50 ppm (DMSO- d 6 ) are referenced for 1 H NMR spectra. Elemental analyses (C, H and N) were carried out using a PE 2400 II elemental analyzer. Mass spectrum (MS) was recorded on a Xevo G2-XS Tof mass spectrometer. The light irradiation experiments were conducted with a PLS-LED100C LED light source (ultraviolet light: λ = 365 nm; electric power: 80 W). The photomechanical motions of the crystals at 298 K and 173 K were respectively captured using a Zeiss SteREO Discovery. V8 microscope, and the photographs of the photomechanical motions of the crystals at 298 K were further edited though the Adobe Premiere Pro 2021 software, forming Supplementary Video 1. Synthesis of CH 2 OH-1,3-bpeb A mixture of (3,5-dibromophenyl)methanol (2.66 g, 10.0 mmol), 4-vinylpyridine (2.10 g, 20.0 mmol), potassium carbonate (2.76 g, 20.0 mol) and (PPh 3 ) 2 PdCl 2 (0.084 g, 0.012 mmol) was added into a 100 mL Schlenk tube. The tube was degassed under vacuum, and then backfilled with N 2 . Nextly, N , N -dimethylformamide (DMF, 10 mL) was added under the N 2 atmosphere, then the tube was sealed. The final reaction mixture in the tube was heated at 110 ºC for 24 hours. After cooling to room temperature, the black solid was put in H 2 O and extracted with CH 2 Cl 2 for three times. The organic phases were dried over anhydrous Na 2 SO 4 , and purified by column chromatography with CH 3 OH and CH 2 Cl 2 ( V 1 /V 2 = 1: 10, R f = 0.3) to obtain the light-yellow powder of CH 2 OH- 1,3-bpeb . Yield: 64.5%. 1 H NMR (400 MHz, CDCl 3 , ppm): δ 8.59 (d, J = 6.0 Hz, 4H), 7.60 (s, 1H), 7.52 (s, 2H), 7.38 (d, J = 6.0 Hz, 4H), 7.32 (d, J = 16.4 Hz, 2H), 7.09 (d, J = 16.4 Hz, 2H), 4.80 (s, 2H) (Supplementary Fig. 1); 13 C NMR (101 MHz, CDCl 3 , ppm): δ 150.3 144.6, 142.6, 137.1, 132.7, 126.9, 125.6, 125.1, 121.1, 64.8 (Supplementary Fig. 2); HRMS (ESI-TOF) m/z : [M+H] + Calcd for C 21 H 19 N 2 O 315.1492; found 315.1495 (Supplementary Fig. 3). Synthesis of [{Cd 4 (CH 2 OH-1,3-bpeb) 4 (3,5-DCB) 8 }·4H 2 O] n (CP1) The reactants CdSO 4 ∙8/3H 2 O (25.7 mg, 0.100 mmol), CH 2 OH-1,3-bpeb (7.85 mg, 0.025 mmol) and 3,5-dichlorobenzoic acid (3,5-HDCB) (19.1 mg, 0.100 mmol) were added into 2.25 mL of the mixed solvents DMF/CH 3 OH/H 2 O (1: 0.5: 7.5) and 20 µL of concentrated nitric acid, and their mixtures were sealed in a thick Pyrex tube and further homogeneously mixed by sonication for 10 min. The sealed reactant solution was then heated at 140 ºC for 3 h. After cooling to room temperature, light-yellow rod-like single crystals of CP1 were isolated after washing with H 2 O and drying at room temperature (Supplementary Figs. 4 and 5). Yield: 94.2% based on CH 2 OH-1,3-bpeb. Anal. Calcd for C 70 H 52 Cd 2 Cl 8 N 4 O 12 (%): C, 50.97; H, 3.18; N, 3.40; found: C, 50.96; H, 3,16; N, 3.41. 1 H NMR (400 MHz, DMSO- d 6 , ppm): δ 8.58 (d, J = 6.0 Hz, 8H), 7.86 (d, J = 2.0 Hz, 8H), 7.84 (s, 2H), 7.73 (t, J = 2.0 Hz, 4H), 7.61 – 7.57 (m, 16H), 7.33 (d, J = 16.8 Hz, 4H), 5.34 (s, 2H), 4.58 (d, J = 3.2 Hz, 4H) (Supplementary Fig. 6). Synthesis of CP2 Crystals of CP1 were dispersed on a glass plate and exposed to PLS-LED100C LED light source ( λ = 365 nm) for 25 seconds. A complete photocycloaddition reaction occurred which generated the photoproduct CP2 . Yield: 100% based on CP1 . 1 H NMR (400 MHz, DMSO- d 6 , ppm): δ 8.38 (dd, J = 13.2, 6.0 Hz, 8H), 7.85 (d, J = 2.0 Hz, 8H), 7.72 (t, J = 2.0 Hz, 4H), 7.33 (d, J = 6.0 Hz, 4H), 7.25 (d, J = 6.0 Hz, 4H), 7.11 (s, 2H), 6.71 (s, 2H), 6.46 (s, 2H), 4.83 (t, J = 5.6 Hz, 2H), 4.73 (dd, J = 14.8, 6.4 Hz, 4H), 4.50 (dd, J = 9.6, 6.4 Hz, 4H), 4.16 (d, J = 5.2 Hz, 4H) (Supplementary Fig. 7). Cryo-photochemical experiment The CP1 single crystals were irradiated using a cryo-assisted approach (173 K, ultraviolet light at 365 nm), and the distance between the single crystal sample and the light source was about 12 cm. They were taken out every 20 min to check the unit cell, and the proper crystals with relatively good diffractions were examined by SCXRD. At last, seven intermediates and one final product were captured (Supplementary Fig. 9). By ultraviolet irradiation at 173 K, CP1-1 , CP1-2 , CP1-3 , CP1-4 , CP1-5 , CP1-6 , CP1-7 and CP2 were obtained from the photocycloaddition of CP1 after 20 min, 40min, 60 min, 80 min, 100 min, 120 min, 140 min, respectively. Here, CP1-5 and CP1-6 were obtained after low-temperature photocycloaddition reaction at 100 min. X-ray data collection and structure determination The single-crystal-to-single-crystal (SCSC) transformations were monitored in situ by following a cryo-assisted approach. At 173 K, CP1-1 , CP1-2 , CP1-3 , CP1-4 , CP1-5 , CP1-6 , CP1-7 and CP2 photoproducts were successively acquired from the cycloaddition of CP1 using a LED lamp at 365 nm. The crystals of CP1 , CP1-1 , CP1-2 , CP1-3 , CP1-4 , CP1-5 , CP1-6 , CP1-7 , and CP2 were mounted on cryo-loop, and their single-crystal data were collected on a Bruker D8 VENTURE diffractometer with Mo-Kα radiation (λ = 0.71073 Å) and Ga-Kα radiation (λ = 1.34138 Å). All data were processed with Bruker APEX-III, and integrated with SAINT and a multi-scan absorption correction using SADABS was applied 50 . All structures were solved by direct methods using olex2.solve and refined by full-matrix least-squares methods against F 2 by SHELXL 51,52 . All non-hydrogen atoms were refined with anisotropic displacement parameters, and C-bound hydrogen atoms were refined isotropic on calculated positions using a riding model for all structures. For the seven CP photoproducts of CP1 , the poor quality of the diffraction data of the single crystals prevented a proper refinement of the disordered water molecules in the lattice. Thus, SQUEEZE command in the Platon program suite 53 was used to remove the lattice water molecules for these photoproducts. A summary of the crystallographic data for the compounds mentioned above is presented in Supplementary Table 2. Crystallographic data for the single crystal structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers 2359885 ( CP1 ), 2359886 ( CP1-1 ), 2359887 ( CP1-2 ), 2359888 ( CP1-3 ), 2359889 ( CP1-4 ), 2359890 ( CP1-5 ), 2359891 ( CP1-6 ), 2359892 ( CP1-7 ), 2359893 ( CP2 ). Theoretical calculations To gain good insight into internal mechanism of the cryogenically controlled solid-state photochemical reaction, all density functional theory (DFT) calculations were performed in ORCA Version 5.0.3 quantum computing software 54 . Based on the X-ray crystal structure, the smallest repeating unit in CP1 of this system was extracted as the initial structure for calculation. To save computational resources, the 1,3-dichlorobenzoic acid in CP1 was simplified to formic acid. To save computational time, the molecular structures of CP1 and its photocycloaddition products were optimized at BLYP-D3/def2-SVP level 55-58 . The relativistic effective core potential (RECP) for Cd was used in all calculations 59 . And, transition state theory was implemented to obtain transition state structures in photocycloaddition reaction. The coordinates of oxygen atoms in the carboxylate ligand were frozen during all the structure optimization for the consistency with keeping the experimental structure, where these oxygen atoms involving coordination bond changes are free in the structure optimization from CP1 to Int1 . Moreover, frequency calculations were also carried out to give the thermodynamic parameters of the compounds. Single point energy calculations for all compounds were implemented at M062X-D3/def2-TZVP level. Spin-orbit coupling-time-dependent density functional theory (SOC-TDDFT) was performed on ORCA quantum chemistry software at PBE0-D3/DKH-def2-TZV P level, and Gaussian 09 was used for time-dependent density functional theory (TDDFT) at PBE0-D3/def2-SVP level. Spin-orbit coupling (SOC) effect of singlet-triplet (S-T) excited states and odd electron density (OED) function of triplet excited states were analyzed by Multiwfn Version 3.8 (dev) soft 60-62 , and the pictures of molecular structures and OED were exported and rendered by VMD version 1.9.4a48 software 63 . Method references Wang, Y. et al. Tuning the configuration of the flexible metal-alkene-framework affords pure cycloisomers in solid state photodimerization. Chem. Commun. 57 , 1129-1132 (2021). Krause, L., Herbst-Irmer, R., Sheldrick, G. M. & Stalke, D. Comparison of silver and molybdenum microfocus X-ray sources for single-crystal structure determination. J. Appl. Crystallogr. 48 , 3-10 (2015). Sheldrick, G. M. Crystal structure refinement with SHELXL. Acta Crystallogr. Sect. C 71 , 3-8 (2015). Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Crystallogr. 42 , 339-341 (2009). Spek, A. Single-crystal structure validation with the program PLATON. J. Appl. Crystallogr. 36 , 7-13 (2003). Neese, F. The ORCA program system. WIREs Comput. Mol. Sci. 2 , 73-78 (2012). Grimme, S., Ehrlich, S. & Goerigk, L. Effect of the damping function in dispersion corrected density functional theory. J. Comput. Chem. 32 , 1456-1465 (2011). Grimme, S., Antony, J., Ehrlich, S. & Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 132 , 154104 (2010). Weigend, F. & Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: design and assessment of accuracy. Phys. Chem. Chem. Phys. 7 , 3297-3305 (2005). Weigend, F. Accurate coulomb-fitting basis sets for H to Rn. Phys. Chem. Chem. Phys. 8 , 1057-1065 (2006). Andrae, D., Häußermann, U., Dolg, M., Stoll, H. & Preuß, H. Energy-adjusted ab initio pseudopotentials for the second and third row transition elements. Theor. Chim. Acta 77 , 123-141 (1990). Takatsuka, K., Fueno, T. & Yamaguchi, K. Distribution of odd electrons in ground-state molecules. Theor. Chim. Acta 48 , 175-183 (1978). Nakano, M. et al. (Hyper)polarizability density analysis for open-shell molecular systems based on natural orbitals and occupation numbers. Theor. Chem. Acc. 130 , 711-724 (2011). Lu, T. & Chen, F. Multiwfn: a multifunctional wavefunction analyzer. J. Comput. Chem. 33 , 580-592 (2012). Humphrey, W., Dalke, A. & Schulten, K. J. J. o. m. g. VMD: visual molecular dynamics. J. Mol. Graphics 14 , 33-38 (1996). Declarations Data availability The data supporting the findings of this study are available within the paper and its Supplementary Information and from the Cambridge Crystallographic Data Centre (https://www.ccdc.cam.ac.uk/structures; crystallographic data are available free of charge under CCDC reference numbers CCDC 2359885 to 2359893). Acknowledgments The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 22271203), the Collaborative Innovation Centre of Suzhou Nano Science and Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Project of Scientific and Technologic Infrastructure of Suzhou (Grant No. SZS201905). Author contributions Conceptualization: M.H.D., J.P.L. Methodology: Q.Q.Z., Y.W., Y.H.D., Q.L., J.P.L. Investigation: Q.Q.Z., Y.W., Y.H.D., M.H.D., Q.L., J.P.L. Visualization: M.H.D., Q.Q.Z., Y.W., J.P.L. Funding acquisition: J.P.L. Project administration: J.P.L. Supervision: J.P.L. Writing – original draft: Q.Q.Z., Y.W., Y.H.D., M.H.D., Q.L., J.P.L. Writing – review & editing: B.F.A., P.B., M.H.D., Q.L., J.P.L. Competing interests The authors declare no competing interests. References Kawamichi, T., Haneda, T., Kawano, M. & Fujita, M. X-ray observation of a transient hemiaminal trapped in a porous network. Nature 461 , 633-635, (2009). Bloch, W. M. et al. Capturing snapshots of post-synthetic metallation chemistry in metal-organic frameworks. Nat. Chem. 6 , 906-912, (2014). Burgun, A. et al. Mapping-out catalytic processes in a metal-organic framework with single-crystal X-ray crystallography. Angew. Chem. Int. Ed. 56 , 8412-8416, (2017). Ikemoto, K., Inokuma, Y., Rissanen, K. & Fujita, M. X-ray snapshot observation of palladium-mediated aromatic bromination in a porous complex. J. Am. Chem. Soc. 136 , 6892-6895, (2014). Yoshioka, S., Inokuma, Y., Duplan, V., Dubey, R. & Fujita, M. X-ray structure analysis of ozonides by the crystalline sponge method. J. Am. Chem. Soc. 138 , 10140-10142, (2016). Saito, A., Sawada, T. & Fujita, M. X-ray crystallographic observation of chiral transformations within a metal-peptide pore. Angew. Chem. Int. Ed. 59 , 20367-20370, (2020). Kumar, A. et al. Solid-state reaction synthesis of nanoscale materials: strategies and applications. Chem. Rev. 122 , 12748-12863, (2022). Tzirakis, M. D. & Orfanopoulos, M. Radical reactions of fullerenes: from synthetic organic chemistry to materials science and biology. Chem. Rev. 113 , 5262-5321, (2013). de Oteyza, D. G. et al. Direct imaging of covalent bond structure in single-molecule chemical reactions. Science 340 , 1434-1437, (2013). Chen, J.-J. et al. Enantioconvergent Cu-catalysed N -alkylation of aliphatic amines. Nature 618 , 294-300, (2023). Tian, Y. et al. A general copper-catalysed enantioconvergent C(sp 3 )-S cross-coupling via biomimetic radical homolytic substitution. Nat. Chem. 16 , 466-475, (2024). Mo, F. et al. Diatomic catalysts for fenton and fenton-like reactions: a promising platform for designing/regulating reaction pathways. Chem. Sci. 14 , 7818-7827, (2023). Gao, X. et al. Urea catalytic oxidation for energy and environmental applications. Chem. Soc. Rev. 53 , 1552-1591, (2024). Roglans, A., Pla-Quintana, A. & Solà, M. Mechanistic studies of transition-metal-catalyzed [2 + 2 + 2] cycloaddition reactions. Chem. Rev. 121 , 1894-1979, (2021). Chizallet, C., Bouchy, C., Larmier, K. & Pirngruber, G. Molecular views on mechanisms of brønsted acid-catalyzed reactions in zeolites. Chem. Rev. 123 , 6107-6196, (2023). Chen, Z. et al. Toward understanding the formation mechanism and OER catalytic mechanism of hydroxides by in situ and operando techniques. Angew. Chem. Int. Ed. 62 , e202309293, (2023). Ráksi, F. et al. Ultrafast X-ray absorption probing of a chemical reaction. J. Chem. Phys. 104 , 6066-6069, (1996). Bressler, C. & Chergui, M. Ultrafast X-ray absorption spectroscopy. Chem. Rev. 104 , 1781-1812, (2004). Sellberg, J. A. et al. Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature. Nature 510 , 381-384, (2014). Rolles, D. Time-resolved experiments on gas-phase atoms and molecules with XUV and X-ray free-electron lasers. Adv. Phys.: X 8 , 2132182, (2023). Kuramochi, H. et al. Ultrafast Raman observation of the perpendicular intermediate phantom state of stilbene photoisomerization. Nat. Chem. 16 , 22-27, (2024). Maiuri, M., Garavelli, M. & Cerullo, G. Ultrafast spectroscopy: state of the art and open challenges. J. Am. Chem. Soc. 142 , 3-15, (2020). Wang, M.-F. et al. Coordination-driven stereospecific control strategy for pure cycloisomers in solid-state diene photocycloaddition. J. Am. Chem. Soc. 142 , 700-704, (2020). Claassens, I. E., Barbour, L. J. & Haynes, D. A. A multistimulus responsive porous coordination polymer: temperature-mediated control of solid-state [2 + 2] cycloaddition. J. Am. Chem. Soc. 141 , 11425-11429, (2019). Zheng, Y. et al. Flexible interlocked porous frameworks allow quantitative photoisomerization in a crystalline solid. Nat. Commun. 8 , 100, (2017). Liu, Y. et al. Weaving of organic threads into a crystalline covalent organic framework. Science 351 , 365-369, (2016). Hanikel, N. et al. Evolution of water structures in metal-organic frameworks for improved atmospheric water harvesting. Science 374 , 454-459, (2021). Gao, W.-X., Feng, H.-J., Guo, B.-B., Lu, Y. & Jin, G.-X. Coordination-directed construction of molecular links. Chem. Rev. 120 , 6288-6325, (2020). Chen, G., Huang, S., Ma, X., He, R. & Ouyang, G. Encapsulating and stabilizing enzymes using hydrogen-bonded organic frameworks. Nat. Protoc. 18 , 2032-2050, (2023). Lang, F., Pang, J. & Bu, X.-H. Stimuli-responsive coordination polymers toward next-generation smart materials and devices. eScience 4 , 100231, (2024). Guo, J. et al. Visible light-mediated intermolecular crossed [2 + 2] cycloadditions using a MOF-supported copper triplet photosensitizer. Nat. Catal. 7 , 307-320, (2024). Wang, J.-S. et al. Cage-confined photocatalysis for wide-scope unusually selective [2 + 2] cycloaddition through visible-light triplet sensitization. Nat. Commun. 11 , 4675, (2020). Gao, X., Friščić, T. & MacGillivray, L. R. Supramolecular construction of molecular ladders in the solid state. Angew. Chem. Int. Ed. 43 , 232-236, (2004). Wang, Z. et al. Polyladderane constructed from a gemini monomer through photoreaction. Angew. Chem. Int. Ed. 56 , 12155-12159, (2017). Zhang, Q., Wang, Y., Braunstein, P. & Lang, J.-P. Construction of olefinic coordination polymer single crystal platforms: precise organic synthesis, in situ exploration of reaction mechanisms and beyond. Chem. Soc. Rev. 53 , 5227-5263, (2024). Schmidt, G. M. J. Photodimerization in the solid state. Pure Appl. Chem. 27 , 647-678, (1971). Shi, Y.-X., Zhang, W.-H., Abrahams, B. F., Braunstein, P. & Lang, J.-P. Fabrication of photoactuators: macroscopic photomechanical responses of metal-organic frameworks to irradiation by UV light. Angew. Chem. Int. Ed. 58 , 9453-9458, (2019). Rath, B. B. & Vittal, J. J. Single-crystal-to-single-crystal [2 + 2] photocycloaddition reaction in a photosalient one-dimensional coordination polymer of Pb(II). J. Am. Chem. Soc. 142 , 20117-20123, (2020). Troyano, J., Carné-Sánchez, A. & Maspoch, D. Programmable self-assembling 3D architectures generated by patterning of swellable MOF-based composite films. Adv. Mater. 31 , 1808235, (2019). Yu, Q. et al. Fabrication of large single crystals for platinum-based linear polymers with controlled-release and photoactuator performance. Angew. Chem. Int. Ed. 58 , 18634-18640, (2019). Jiang, Y. et al. Substrate-bound diarylethene-based anisotropic metal-organic framework films as photoactuators with a directed response. Angew. Chem. Int. Ed. 62 , e202218052, (2023). Wu, Y., Dong, X., Kim, J.-K., Wang, C. & Sitti, M. Wireless soft millirobots for climbing three-dimensional surfaces in confined spaces. Sci. Adv. 8 , eabn3431, (2022). Hu, W., Lum, G. Z., Mastrangeli, M. & Sitti, M. Small-scale soft-bodied robot with multimodal locomotion. Nature 554 , 81-85, (2018). Yang, S.-Y. et al. Crystallographic snapshots of the interplay between reactive guest and host molecules in a porous coordination polymer: stereochemical coupling and feedback mechanism of three photoactive centers triggered by UV-induced isomerization, dimerization, and polymerization reactions. J. Am. Chem. Soc. 136 , 558-561, (2014). Wang, M.-F. et al. Controllable multiple-step configuration transformations in a thermal/photoinduced reaction. Nat. Commun. 13 , 2847, (2022). Ma, Z.-T., Yang, S.-Y., Zheng, L.-S. & Teo, B. K. First observation of unusual domino effect and triggering mechanism of sequential single-crystal-to-single-crystal photochemical reactions in a metal-organic framework with multiple photoreactive centers. Cryst. Growth Des. 19 , 3113-3119, (2019). Hu, F.-L., Wang, S.-L., Lang, J.-P. & Abrahams, B. F. In-situ X-ray diffraction snapshotting: determination of the kinetics of a photodimerization within a single crystal. Sci. Rep. 4 , 6815, (2014). Wang, M.-F. et al. In situ observation of a stepwise [2 + 2] photocycloaddition process using fluorescence spectroscopy. Nat. Commun. 14 , 7766, (2023). Additional Declarations There is NO Competing Interest. Supplementary Files AuxiliarySupplementaryMaterialCheckCIFreportsforCP16CP17CP2.pdf Additional Review Material (CheckCIF reports for CP1-6, CP1-7, CP2) AuxiliarySupplementaryMaterialCheckCIFreportsforCP1CP11CP12.pdf Additional Review Material (CheckCIF reports for CP1, CP1-1, CP1-2) AuxiliarySupplementaryMaterialCheckCIFreportsforCP13CP14CP15.pdf Additional Review Material (CheckCIF reports for CP1-3, CP1-4, CP1-5) SupplementaryInformation.pdf Supplementary Information SupplementaryVideo1.mp4 Supplementary Video 1 ExtendedDataFigure.doc Cite Share Download PDF Status: Posted 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. 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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-4993811","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":447578700,"identity":"60dc55c0-3080-4a66-8779-228ae8de9ffe","order_by":0,"name":"Jian-Ping Lang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2klEQVRIiWNgGAWjYBACxgbmBiBlA8QJQMxGlBZGkJY0ErSANAGJwyRoYZ7d2Pi54Nd5e4PjyQ8YPpQdZuCf3UDAjjkHm6Vn9t1O3HDmmQHjjHOHGSTuHCCgZUZigzRvz+0EgxsJBsy8bYcZDCQSCGpp/s3bc87e4Eb6B+a/RGppk+b5cYBxw40cA2ZGorTMOdhmzduQnDjzzJuCgz3n0nkkbhDQYji7+fBtnj929nzH0zc++FFmLcc/g5CWGSCr2iCcA0DMg189EMhLgMg/BNWNglEwCkbBSAYAssFJ2gki+Y8AAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0003-2942-7385","institution":"Soochow University","correspondingAuthor":true,"prefix":"","firstName":"Jian-Ping","middleName":"","lastName":"Lang","suffix":""},{"id":447578701,"identity":"c85c57af-b520-4f33-8abf-4d3fe798ba1f","order_by":1,"name":"Qiaoqiao Zhang","email":"","orcid":"","institution":"Soochow University","correspondingAuthor":false,"prefix":"","firstName":"Qiaoqiao","middleName":"","lastName":"Zhang","suffix":""},{"id":447578702,"identity":"a368ed0d-1336-4455-93bd-a7d3f9cdad91","order_by":2,"name":"Yun-Hu Deng","email":"","orcid":"https://orcid.org/0009-0005-8053-3265","institution":"Soochow University","correspondingAuthor":false,"prefix":"","firstName":"Yun-Hu","middleName":"","lastName":"Deng","suffix":""},{"id":447578703,"identity":"3cf4672d-0cc0-49de-84df-de542d67191c","order_by":3,"name":"Yong Wang","email":"","orcid":"","institution":"Soochow University","correspondingAuthor":false,"prefix":"","firstName":"Yong","middleName":"","lastName":"Wang","suffix":""},{"id":447578704,"identity":"0ac2f700-df75-4bb9-8480-e7c9845f3535","order_by":4,"name":"Ming-Hao Du","email":"","orcid":"","institution":"Soochow University","correspondingAuthor":false,"prefix":"","firstName":"Ming-Hao","middleName":"","lastName":"Du","suffix":""},{"id":447578705,"identity":"deb058fe-40b8-40fb-8bc6-da1454d7a73f","order_by":5,"name":"Qi Liu","email":"","orcid":"https://orcid.org/0000-0002-9377-8529","institution":"Soochow University","correspondingAuthor":false,"prefix":"","firstName":"Qi","middleName":"","lastName":"Liu","suffix":""},{"id":447578706,"identity":"7cfc3cc8-e9ff-48dd-b617-30b8ce7b19f9","order_by":6,"name":"Brendan F. Abrahams","email":"","orcid":"","institution":"University of Melbourne","correspondingAuthor":false,"prefix":"","firstName":"Brendan","middleName":"F.","lastName":"Abrahams","suffix":""},{"id":447578707,"identity":"af06a4be-9fe8-4666-bdc8-a138ed6ab3aa","order_by":7,"name":"Pierre Braunstein","email":"","orcid":"https://orcid.org/0000-0002-4377-604X","institution":"Université de Strasbourg","correspondingAuthor":false,"prefix":"","firstName":"Pierre","middleName":"","lastName":"Braunstein","suffix":""}],"badges":[],"createdAt":"2024-08-29 01:05:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4993811/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4993811/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":81368288,"identity":"84a1c6f4-3f41-48d3-9b43-3637dcf64de2","added_by":"auto","created_at":"2025-04-25 09:56:20","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1317652,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eIllustration of crystalline framework-based time-resolved X-ray crystallography principles, pioneering work, goals, challenges, and the enhanced platform proposed in this study. a\u003c/strong\u003e, The advent of time-resolved X-ray crystallography based on crystalline frameworks has enabled direct observation of substrate reactions. \u003cstrong\u003eb\u003c/strong\u003e, The development of porous coordination polymers and Mn-MOF platforms have realized some fundamental challenges. \u003cstrong\u003ec\u003c/strong\u003e, The ultimate goal of this technology is to develop versatile crystalline platforms that have wide-ranging applicability. Challenges remain in handling reactions that are highly exothermic, have rapid time scales, multiple steps, and complicated mechanisms. \u003cstrong\u003ed\u003c/strong\u003e, To improve the applicability of crystal platforms to a variety of complex reactions, a coordination polymer single-crystal platform is proposed which offers enhanced diffraction capability and preservation of crystallinity despite significant structural rearrangements. This platform successfully enables the investigation of a complex reaction that presents multiple challenges.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/920af9d32b829126d9e61ee8.png"},{"id":81368726,"identity":"c4e2e1cf-7d64-4f0a-9d0a-f60c7ed47108","added_by":"auto","created_at":"2025-04-25 10:04:20","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":882340,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSynthesis and structural analysis of CP1. a\u003c/strong\u003e, Representation of the synthetic reaction conditions used to generate \u003cstrong\u003eCP1\u003c/strong\u003e,the region highlighted in pale green represents the reactive diene unit connectedto twopyridyl groups. \u003cstrong\u003eb\u003c/strong\u003e, Structure of the two crystallographically distinct chains in \u003cstrong\u003eCP1\u003c/strong\u003e. \u003cstrong\u003ec\u003c/strong\u003e, Schematic representation of the Schmidt rule for a [2 + 2] photochemical addition reaction. \u003cstrong\u003ed\u003c/strong\u003e, Representation of the four active regions for photoaddition in the asymmetric unit and the separations between the C atoms of adjacent double bonds.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/5d9f63686d6d695de4d68686.png"},{"id":81368729,"identity":"abea9857-aa83-4ef3-a05e-1a28e7b5b9f5","added_by":"auto","created_at":"2025-04-25 10:04:21","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1837593,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAnalysis of the photochemical addition products of CP1. a\u003c/strong\u003e, \u003cstrong\u003ed\u003c/strong\u003e, Time-lapse photographs showing morphological changes of crystals under 365 nm light at 298 K (\u003cstrong\u003ea\u003c/strong\u003e) and 173 K (\u003cstrong\u003ed\u003c/strong\u003e). \u003cstrong\u003eb\u003c/strong\u003e, The products formed in photochemical addition reactions upon dissolution in DMSO-\u003cem\u003ed\u003c/em\u003e\u003csub\u003e\u003cem\u003e6\u003c/em\u003e\u003c/sub\u003e. \u003cstrong\u003ec\u003c/strong\u003e, \u003cstrong\u003ee\u003c/strong\u003e, Time-dependent \u003csup\u003e1\u003c/sup\u003eH NMR spectra recorded for the products of the photochemical reactions at 298 K (\u003cstrong\u003ec\u003c/strong\u003e) and 173 K (\u003cstrong\u003ee\u003c/strong\u003e). \u003cstrong\u003ef\u003c/strong\u003e, Comparison of PXRD patterns of final photochemical reaction products at 298 K and 173 K, alongside simulation patterns based on the \u003cstrong\u003eCP2\u003c/strong\u003e crystal structure.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/6620705763da149454c7580d.png"},{"id":81368297,"identity":"29286b5a-88ec-496a-99ae-b67f7f4b6b2b","added_by":"auto","created_at":"2025-04-25 09:56:20","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1722704,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eAnalysis of intermediates and reaction pathways for the photocycloaddition of CP1. a\u003c/strong\u003e, Crystal structures with disorder involved in \u003cstrong\u003eCP1\u003c/strong\u003e, \u003cstrong\u003eCP1-1\u003c/strong\u003e, \u003cstrong\u003eCP1-2\u003c/strong\u003e, \u003cstrong\u003eCP1-3\u003c/strong\u003e, \u003cstrong\u003eCP1-4\u003c/strong\u003e, \u003cstrong\u003eCP1-5\u003c/strong\u003e, \u003cstrong\u003eCP1-6\u003c/strong\u003e, \u003cstrong\u003eCP1-7\u003c/strong\u003e, \u003cstrong\u003eCP2\u003c/strong\u003e crystals and possible intermediates in these crystals (black spheres in the structures of CPs represent Cd atoms). \u003cstrong\u003eb\u003c/strong\u003e, Pathways inferred by intermediates observed in crystal structures and the Gibbs free energy change (kcal/mol) indicated by theoretical calculations.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/e3d22fc8565026b1aa413db4.png"},{"id":82807646,"identity":"e64954ca-4d4a-4a48-aa1d-3c25ac9432e4","added_by":"auto","created_at":"2025-05-15 12:49:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":7197423,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/c4508ed8-0bd2-4e40-82fe-bb25850b888b.pdf"},{"id":81368727,"identity":"4513d795-65a0-4aa1-a1a5-6712eae208ad","added_by":"auto","created_at":"2025-04-25 10:04:20","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":283189,"visible":true,"origin":"","legend":"\u003cp\u003eAdditional Review Material (CheckCIF reports for CP1-6, CP1-7, CP2)\u003c/p\u003e","description":"","filename":"AuxiliarySupplementaryMaterialCheckCIFreportsforCP16CP17CP2.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/855f7a9579dace76191dfb7b.pdf"},{"id":81368290,"identity":"d7d94a5f-6b72-4b1c-b91b-fe5d0db62b33","added_by":"auto","created_at":"2025-04-25 09:56:20","extension":"pdf","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":240941,"visible":true,"origin":"","legend":"\u003cp\u003eAdditional Review Material (CheckCIF reports for CP1, CP1-1, CP1-2)\u003c/p\u003e","description":"","filename":"AuxiliarySupplementaryMaterialCheckCIFreportsforCP1CP11CP12.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/1f36c015eba51bc8732edea8.pdf"},{"id":81368292,"identity":"dff0e26f-a318-4200-b03c-d4584aeeeb91","added_by":"auto","created_at":"2025-04-25 09:56:20","extension":"pdf","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":343568,"visible":true,"origin":"","legend":"\u003cp\u003eAdditional Review Material (CheckCIF reports for CP1-3, CP1-4, CP1-5)\u003c/p\u003e","description":"","filename":"AuxiliarySupplementaryMaterialCheckCIFreportsforCP13CP14CP15.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/a4186b0c3c45bb631cda0c7b.pdf"},{"id":81369686,"identity":"c9c13676-7f84-413c-8e45-1d4978047bdf","added_by":"auto","created_at":"2025-04-25 10:12:21","extension":"pdf","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":3376386,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Information\u003c/p\u003e","description":"","filename":"SupplementaryInformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/e3a9e40bc0df8d924e909146.pdf"},{"id":81368736,"identity":"f7420677-2e13-46b6-8185-a2f36ceaa1be","added_by":"auto","created_at":"2025-04-25 10:04:21","extension":"mp4","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":236899,"visible":true,"origin":"","legend":"Supplementary Video 1","description":"","filename":"SupplementaryVideo1.mp4","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/14fab2d8b6fcb4011a920db4.mp4"},{"id":81368299,"identity":"7988070d-4c74-4755-b9dd-45c6ce647c29","added_by":"auto","created_at":"2025-04-25 09:56:21","extension":"doc","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":1962496,"visible":true,"origin":"","legend":"","description":"","filename":"ExtendedDataFigure.doc","url":"https://assets-eu.researchsquare.com/files/rs-4993811/v1/2b9b7b58489d57f47a76c498.doc"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"A single-crystal platform for unveiling ultrafast and complex photochemical cascade reactions","fulltext":[{"header":"Introduction","content":"\u003cp\u003eUnderstanding the mechanistic details of chemical reactions is key to the synthesis of new compounds and materials, the development of new processes, gaining insights into structure-function relationships\u003csup\u003e7-11\u003c/sup\u003e, and the development and optimisation of industrially relevant processes\u003csup\u003e12-16\u003c/sup\u003e. While spectroscopic methods based on laser pulses and free-electron lasers have been developed to identify transition states and elucidate associated mechanisms\u003csup\u003e17-22\u003c/sup\u003e, time-resolved single-crystal X-ray diffraction (TR-SCXRD) offers the advantage of providing precise structural information not otherwise available (Fig. 1a). However, its application requires retention of crystalline structures throughout the reaction process, thus limiting the types and scope of observable reactions. In 2009, Fujita \u003cem\u003eet al.\u003c/em\u003e introduced a novel paradigm where the substrates were adsorbed onto the channels of a porous crystalline coordination polymer network\u003csup\u003e1\u003c/sup\u003e, allowing a short-lived hemiaminal intermediate to be captured (Fig. 1b). The development of this technique quickly garnered attention because it reduced the instrumentation and technical costs associated with studying reaction mechanisms and expanded the types of reactions that can be investigated. Subsequently, Sumby and Doonan et al. studied the carbonylation of MeX (X = I, Br) by metallating a Mn-MOF with a flexible bis-pyrazole chelating group on the skeleton (Fig. 1b), and enabled the study of chemically stable MOF having a poor ability to retain a crystalline state throughout the chemical reaction\u003csup\u003e2,3\u003c/sup\u003e. To date, similar approaches have enabled the mechanistic investigations on e.g. the palladium-mediated aromatic bromination and ozonolysis of olefins in aforementioned porous complexes\u003csup\u003e4,5\u003c/sup\u003e,\u0026nbsp;and transformations of organic molecules within chiral pores\u003csup\u003e6\u003c/sup\u003e. Whilst the approaches employed to examine reaction mechanisms in the crystalline state are commonly tailored for specific reactions, the development of a general methodology, that has widespread application, would seem a worthwhile pursuit. The value of a new general approach may be assessed by determining its effectiveness in providing an understanding of complex and challenging reactions. Complexity may arise from numerous intermediates associated with multi-step mechanisms or perhaps reactions that occur via multiple pathways (Fig. 1c). Furthermore, reactions with large enthalpy changes or involving ultrafast processes provide challenges in measuring useful experimental data. Previously employed approaches using crystalline solids are not well-suited to the investigation of the types of complex and challenging reactions that are commonly encountered in both natural and artificial systems.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo address the difficulties associated with complex and challenging reactions, a single-crystal platform is required that can withstand structural changes without compromising lattice integrity and diffraction capability. After evaluating various crystalline materials\u003csup\u003e23-30\u003c/sup\u003e, we identified a highly promising structure: a stacked one-dimensional (1D) coordination polymer chain composed of alternating dinuclear clusters and bipyridine ligands. The bipyridine ligands can be synthesized using commonly employed Heck or Suzuki procedures to append metal binding pyridyl groups to each end of the reaction substrate. The integration of the reaction substrate within bridging ligands mitigates issues of uneven distribution or improper orientation that can arise in pore diffusion methods. Furthermore, the stacking arrangement of the 1D chains avoids the problem of large solvent-filled pores which commonly leads to weak diffraction. In addition to these structural features, there are two structural aspects that bestow a degree of flexibility upon the crystal, allowing a relatively gentle transition from reactant to product that preserves lattice integrity. Firstly, the cluster nodes can switch between various coordination geometries and adapt to changes in the coordination orientation of the pyridine groups. Secondly, the chain structure ensures that the stress induced by structural transformations is not easily propagated in three dimensions, instead, weak secondary bonding interactions provide a buffer between 1D coordination polymers. These features are expected to enhance the single-crystal\u0026apos;s ability to withstand the liberation of energy during the reaction (Fig. 1d).\u003c/p\u003e\n\u003cp\u003eIn this work, the bipyridyl bridging ligand 5-hydroxymethyl-1,3-bis((\u003cem\u003eE\u003c/em\u003e)-2-(pyridin-4‑yl)- vinyl)benzene (CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb) was combined with Cd(II) ions and 3,5-dichlorobenzoic acid (3,5-HDCB) to yield a 1D coordination polymer [{Cd\u003csub\u003e4\u003c/sub\u003e(CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb)\u003csub\u003e4\u003c/sub\u003e(3,5-DCB)\u003csub\u003e8\u003c/sub\u003e}\u0026middot;4H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003en\u003c/sub\u003e (\u003cstrong\u003eCP1\u003c/strong\u003e) (Fig. 2a). \u003cstrong\u003eCP1\u003c/strong\u003e has four crystallographically distinct sites that are each expected to undergo a [2 + 2] photocycloaddition reaction. When the crystals were irradiated with ultraviolet light at room-temperature, the reaction was violent, resulting in considerable lattice damage. In addition, the reaction was completed within a matter of seconds, representing the fastest recorded photocycloaddition process using a single-crystal platform (Supplementary Table 1)\u003csup\u003e31-35\u003c/sup\u003e. Examination of the reaction products by NMR revealed complete cyclization at four distinct sites but the sequential process may involve up to 14 intermediates and 75 reaction routes. Clearly, the elucidation of both the intermediates and reaction pathways represents a considerable challenge. By employing the single-crystal platform approach at low temperature, we have succeeded in using TR-SCXRD technology to capture \u0026apos;snapshots\u0026apos; of the reaction, revealing five transient intermediates and three transformation routes within the cascade of reactions involving four pairs of C=C bonds. Theoretical calculations complement the experimental investigation. This work demonstrates that a cluster-based coordination polymer single-crystal platform can be used to tackle ultrafast and complex reaction mechanisms. It also illustrates that rational structural design has the potential to significantly enhance the versatility of single-crystal platform technology, addressing reaction mechanisms that conventional techniques cannot resolve.\u003c/p\u003e\n\u003cp\u003eAn X-ray analysis of \u003cstrong\u003eCP1\u003c/strong\u003e reveals two chemically similar but crystallographically distinct chains. Each chain consists of pairs of closely separated Cd(II) centers which are coordinated by four 3,5-DCB ligands, two of which serve as bridging carboxylates between the two Cd(II) centers. This results in the four carboxylate groups being approximately coplanar with two Cd(II) centers. The binuclear unit Cd\u003csub\u003e2\u003c/sub\u003e(3,5-DBC)\u003csub\u003e4\u003c/sub\u003e is linked by two pairs of CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb ligands to crystallographically equivalent units on either side. Two bridging CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb ligands coordinate to each Cd(II) through mutually trans pyridyl groups approximately orthogonal to the carboxylate plane described above. Overall, a double chain structure is formed, as represented in Fig. 2b in red and blue. With the vinyl-pyridyl groups occupying the 3,5-positions on the phenyl ring, the bridging ligand is V-shaped, resulting in zigzag chains. Each Cd(II) in the two distinct Cd\u003csub\u003e2\u003c/sub\u003e(3,5-DBC)\u003csub\u003e4\u003c/sub\u003e units is coordinated by three carboxylate anions and two pyridyl groups. Whilst each Cd is coordinated by four oxygen atoms with Cd-O separations in the range 2.25-2.46 \u0026Aring;, one Cd(II) from each Cd\u003csub\u003e2\u003c/sub\u003e(3,5-DBC)\u003csub\u003e4\u003c/sub\u003e unit forms a weaker interaction with a fifth oxygen with a Cd-O separation of 2.6 \u0026Aring; resulting in a 7-coordinate Cd(II). Thus each Cd\u003csub\u003e2\u003c/sub\u003e(3,5-DBC)\u003csub\u003e4\u003c/sub\u003e unit contains a 6-coordinate and a 7-coordinate Cd(II). For each 6-coordinate Cd(II), there is an additional Cd\u0026middot;\u0026middot;\u0026middot;O contact beyond 2.8 \u0026Aring;. Although there are four crystallographically distinct CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb ligands (two in each chain), they all adopt a similar configuration (ignoring the orientation of the OH group) which is different to that in Fig. 2a. In each CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb ligand, one of the vinyl-pyridyl groups rotates around the vinyl C to phenyl ring bond by 180\u0026deg; resulting in the configuration (Fig. 2b). Pairs of crystallographically different CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb ligands that link adjacent Cd\u003csub\u003e2\u003c/sub\u003e(3,5-DBC)\u003csub\u003e4\u003c/sub\u003e unit are closely separated and parallel to each other with an arrangement that places the olefinic bonds in relatively close proximity (Fig. 2b).\u003c/p\u003e\n\u003cp\u003eWith the asymmetric unit including four distinct CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb ligands, there are eight crystallographically independent double bonds. Constrained by coordination of the CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb ligands to pairs of Cd(II) centers in Cd\u003csub\u003e2\u003c/sub\u003e(3,5-DBC)\u003csub\u003e4\u0026nbsp;\u003c/sub\u003eunits, the maximum centroid separation between adjacent double bonds of neighboring diene molecules is 3.8 \u0026Aring; (Supplementary Fig. 5). The arrangement and separation of the double bonds in \u003cstrong\u003eCP1\u003c/strong\u003e are expected to facilitate intermolecular cyclization reactions under appropriate illumination (Fig. 2c)\u003csup\u003e36\u003c/sup\u003e. For convenience, the positions of the eight double bonds are divided into four regions, namely, Regions 1 to 4, according to clockwise ordering (Fig. 2d).\u003c/p\u003e\n\u003cp\u003eIrradiation of single crystals of \u003cstrong\u003eCP1\u003c/strong\u003e at 298 K with ultraviolet light causes a photochemical process that results in violent fragmentation of single crystals as indicated by time lapse photographs (Fig. 3a and Supplementary Video 1). Crystals begin fragmenting within 0.5 s of irradiation, and the process continues until around 25 s after which no significant morphological changes occur. Compared to similar photocyclization reactions, the rapidity of this process is unprecedented.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe products of this fragmentation process (Fig. 3b) were analyzed and Fig. 3c\u0026nbsp;shows the \u003csup\u003e1\u003c/sup\u003eH NMR spectra of the samples after irradiation for different periods of time. As the photoreaction progresses, the intensity of the signals assigned to the H atoms of uncyclized CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb, at 8.6 ppm and 4.6 ppm, decreases. The signals at 8.45 and 4.42 ppm, attributed to the mono-cyclized product of two CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb molecules (Dimer-I), first increase and then decrease until they completely disappear after 25 s. The resonances at 8.40 and 4.18 ppm, identified as the dual-cyclized product of two CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb molecules (Dimer-II), increase in intensity after first appearing at 0.5 s. After ca. 25 s, the \u003csup\u003e1\u003c/sup\u003eH NMR signals for CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb and Dimer-I are no longer observable, and the signals associated with Dimer-II also cease to increase (Supplementary Figs. 6 and 7). This experiment suggests that the double bonds in all four regions undergo photodimerization.\u003c/p\u003e\n\u003cp\u003eTo our knowledge, this is the fastest reported [2 + 2] set of photoaddition reactions involving a crystalline platform (Supplementary Table 1). Understanding the underlying mechanism would have significant implications for the development of advanced light-driven devices and materials\u003csup\u003e37-43\u003c/sup\u003e and this relies on the identification of the reaction intermediates\u003csup\u003e44,45\u003c/sup\u003e. Given that the reaction involves the formation of four crystallographically distinct cyclobutane rings, the reaction pathway is complex even if the intermediates are simply considered to be the 14 states shown in Supplementary Fig. 8, associated with 75 possible pathways leading to the final product.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWhilst violent fragmentation of the crystal occurs at 298 K, the reaction may be slowed down considerably by irradiating at low temperature. Irradiation of a crystal with UV light at 173 K led to no significant changes in the crystal\u0026apos;s appearance (Fig. 3d). After exposure for 2 h, only slight cracking was apparent. NMR monitoring of the reaction progress was undertaken, as done for the experiment performed at 298 K. Whereas full conversion of the double bonds to cyclobutane rings was completed after 25 s at 298 K, the same conversion required 140 min at 173 K (Fig. 3e). As with the earlier analysis, the \u003csup\u003e1\u003c/sup\u003eH NMR signals of the uncyclized CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb ligand decreased during irradiation whilst those associated with Dimer-I first increased and then decreased and those for Dimer-II grew over the irradiation period. The quality of the crystals even after 2 h of irradiation allowed a single-crystal structural determination which clearly established full conversion of the diene units in the four regions (Fig. 2d) to cyclobutane rings (Supplementary Fig. 9). Powder X-ray diffraction patterns (PXRD) for the samples irradiated at 298 K and 173 K revealed that the same crystalline product was obtained in each case and the data matched those simulated from its single-crystal data (Fig. 3f). The X-ray diffraction studies were fully consistent with the NMR analysis.\u003c/p\u003e\n\u003cp\u003eThe established stability of the crystalline platform upon irradiation at 173 K prompted a detailed X-ray analysis of the products obtained after different periods of irradiation at low temperature. In particular, the study focused on changes in each of the four regions of the asymmetric unit identified in Fig. 2d. We tested samples with an interval of 20 min and discovered seven crystal structures with distinct cyclization states (see Supplementary Figs. 9 and 10 and Supplementary Table 2).\u0026nbsp;Site occupancy refinement provided information regarding the \u0026apos;average\u0026apos; structure of the asymmetric unit in the crystal and thus refinements indicating sites partially occupied by cyclobutane units revealed the presence of one or more intermediate structures. Furthermore, understanding the true conversion pathway should consider the intermediates present in each crystal, specifically the cyclization states within the asymmetric units, rather than the apparent occupancy. Fig. 4a illustrates the potential cyclization states of intermediates observed in crystals at different reaction time, using a schematic representation. For the sample (\u003cstrong\u003eCP1-1\u003c/strong\u003e) irradiated for 20 min, the occupancies of the unreacted olefin groups and cyclobutane unit in Region 1 were 54.4% and 45.6%, respectively, while the remaining three regions retained completely occupied by olefinic groups. After 40 min reaction time, Region 1 showed complete conversion to cyclobutane, everything else remaining intact. At longer reaction time, 100% photo-cyclization was achieved in Region 1, while diverse cyclization states in the remaining regions could not be attributed to a single sequential reaction process. This suggests that after complete cyclization occurred in Region 1, branching reaction pathways began to emerge. From data for \u003cstrong\u003eCP1-3\u003c/strong\u003e collected after 60 min, complete cyclization in Region 1 was again apparent but only 21.2% in Region 2. The other two regions kept unchanged. Thus, two intermediates coexisted: Int1, corresponding to cyclization only in the first region, and Int2, corresponding to cyclization in both the first and second regions. Twenty minutes later, the conversion of cyclobutane units in the second region increased to 59.3%, and the third region began to show cyclobutane units with an occupancy of \u003cem\u003eca.\u003c/em\u003e 47.4% (\u003cstrong\u003eCP1-4\u003c/strong\u003e). This disordered state is most likely due to uneven irradiation of the crystal, leading to sequential photo-cyclization at Regions 1-3 on the crystal surface. It should be acknowledged that this disorder may involve the coexistence of four intermediates: Int1, Int2, Int2\u0026apos; with cyclization in Regions 1 and 3, and Int3 with cyclization in Regions 1-3. The existence of Int2\u0026apos; and Int3 was confirmed by the direct observation of \u003cstrong\u003eCP1-5\u003c/strong\u003e and \u003cstrong\u003eCP1-6\u003c/strong\u003e after 100 min reaction. In \u003cstrong\u003eCP1-5\u003c/strong\u003e, the olefinic groups in Regions 2 and 3 were fully converted to cyclobutane units, whereas no indication of cyclization in Region 4 was obtained.\u003c/p\u003e\n\u003cp\u003eWhile\u003cstrong\u003e\u0026nbsp;CP1-6\u003c/strong\u003e showed complete conversion to cyclobutane units in Regions 1 and 3, but no cyclization in Regions 2 and 4, data for \u003cstrong\u003eCP1-7\u003c/strong\u003e collected after 120 min indicated full conversion to cyclobutane units in Regions 1 and 3 but only of 46.8% in Regions 2 and 4. This crystal may contain Int2\u0026apos;, Int3, Int3\u0026apos; with photocycloaddition in Regions 1, 2, and 4, and the final product with photocycloaddition at all four regions. After illumination for 140 min, all regions in the samples investigated showed complete conversion of the olefinic groups to cyclobutane units (\u003cstrong\u003eCP2\u003c/strong\u003e). Combined the identification of five kinds of intermediates with the results of theoretical calculations (Extended Data Fig. 1), the transformation pathways were further narrowed down to the three pathways (Fig. 4b). They all involved a [2 + 2] photochemical addition in Region 1 and formation of intermediate Int1 in the initial step. Photochemical cycloaddition then proceeded in Region 2 or 3 resulting in the formation of intermediates Int2 or Int2\u0026apos;, respectively. Int2 led to the final product \u003cstrong\u003eCP2\u0026nbsp;\u003c/strong\u003eby photochemical addition in Region 3 and then in Region 4 (Path 1). For Int2\u0026apos;, the next step has two branches i) the photochemical addition reaction occurring first in Region 2, then Region 4 (Path 2); or ii) the photochemical addition reaction occurring first in Region 4, then in Region 2 (Path 3).\u003c/p\u003e\n\u003cp\u003eIn order to gain further insights into the reaction pathway and mechanism, odd-electron density (OED) distributions and spin-orbit coupling (SOC) effects for excited states were investigated using DFT calculations. For the initial structure, spin-orbit coupling calculations reveal a significant coupling effect between the S3 and T1 states, as well as between the S4 and T2 states of \u003cstrong\u003eCP1\u003c/strong\u003e (Supplementary\u0026nbsp;Fig.\u0026nbsp;11f\u0026nbsp;and Supplementary\u0026nbsp;Table 3). Odd-electron density distributions for both T1 and T2 states are mainly located on the alkene groups in Region 1 (Supplementary\u0026nbsp;Fig.\u0026nbsp;11a, b), which is consistent with the experimentally determined site for the first cyclization process during the SCSC transformation (see \u003cstrong\u003eCP1-1\u003c/strong\u003e and \u003cstrong\u003eCP1-2\u003c/strong\u003e). Subsequent calculations reveal that\u0026nbsp;a series of triplet states for Int1 are accessible through SOC interactions (Supplementary\u0026nbsp;Fig.\u0026nbsp;12f\u0026nbsp;and Supplementary\u0026nbsp;Table 4). The lowest energy\u0026nbsp;singlet excited state, S1, is spin-orbit coupled with the T3 and T4 states, which concentrates the OED in Region 2 (Supplementary\u0026nbsp;Fig.\u0026nbsp;12c, d). Furthermore, the lower triplet states, T1 and T2, with single electron density in Region 3 (Supplementary\u0026nbsp;Fig.\u0026nbsp;12a, b) are also reachable by spin-orbit coupling with S3 and S4 states, or internal conversion (IC) from T3 and T4 states. These two conditions lead to the formation of Int2 and Int2\u0026apos;, respectively, which is consistent with the experimental observation of two different cyclization sites (\u003cstrong\u003eCP1-3\u003c/strong\u003e and \u003cstrong\u003eCP1-6\u003c/strong\u003e) after \u003cstrong\u003eCP1-2\u003c/strong\u003e. Spin-orbit coupling calculations suggest that there is no coupling between the lowest singlet excited state S1 and the triplet states within Int2. Instead, intersystem crossing (ISC) mainly occurs via spin-orbit coupling interactions involving S2, S3, and S5 with T1-T5 (Supplementary\u0026nbsp;Fig.\u0026nbsp;13f\u0026nbsp;and Supplementary\u0026nbsp;Table 5). It is noteworthy that the lowest-energy triplet states T1 and T2, characterized by their OED in Region 3 (Supplementary\u0026nbsp;Fig.\u0026nbsp;13a, b), exhibit similar energy levels. Conversely, the T3 and T4 states, with OED in Region 4, have higher energy levels (Supplementary\u0026nbsp;Fig.\u0026nbsp;13c, d). Therefore, it is possible to move to a lower triplet state, T1 or T2, by IC events. This explains the predominant observation of the next cyclization event in Region 3 within the SCSC process (\u003cstrong\u003eCP1-4\u003c/strong\u003e and \u003cstrong\u003eCP1-5\u003c/strong\u003e). The S1 and S3 states\u0026nbsp;of\u0026nbsp;Int2\u0026apos;\u0026nbsp;exhibit coupling effects with the T1, T3, and T5 triplet states (Supplementary\u0026nbsp;Fig.\u0026nbsp;14f\u0026nbsp;and Supplementary\u0026nbsp;Table 6), which have concentrated OED in Region 2 (Supplementary\u0026nbsp;Fig.\u0026nbsp;14a, c,\u0026nbsp;and\u0026nbsp;e). In contrast, the S2 state displays coupling effects with the T2 and T4 triplet states, characterized by OED in Region 4 (Supplementary\u0026nbsp;Fig.\u0026nbsp;14b, d). The energy levels of the T1 and T2 triplet states are notably similar and markedly lower compared to those of the remaining triplet states. Thus, it is possible for\u0026nbsp;Int2\u0026apos; to undergo photochemical cyclization in Region 2 or 4 through its T1 or T2 energy states, respectively. This provides an explanation for the observation of \u003cstrong\u003eCP1-7\u003c/strong\u003e in the SCSC process.\u003c/p\u003e\n\u003cp\u003eThe energy change for each step has been evaluated using thermodynamic calculations (Extended Data Fig. 1, Supplementary Fig. 15 and Supplementary Table 7). The product with cyclization in Region 1 has both the lowest energy barrier and single point energy compared to any product cyclized in other Regions, which is consistent with Region 1 being where photocyclization occurs most readily. Spin-orbit coupling-time-dependent density functional theory (SOC-TDDFT) and OED calculations indicate that starting from Int1, there are two sites, Regions 2 and 3, where cyclization reactions may occur. From a thermodynamic perspective, it is also advantageous for reactions to occur at these two sites due to lower energy barriers. For Int2, the energy barrier for cyclization in Region 4 is higher. Therefore, for Int2, the formation of Int3 should be the preferred next step. This conclusion is also consistent with OED results and experimental observations. When Int2\u0026apos; undergoes photocyclization, the energy barrier difference between the cyclization sites in Regions 2 and 4 is minimal, supporting the simultaneous existence of two branching reaction pathways.\u003c/p\u003e\n\u003cp\u003eThe observation of such ultrafast and multi-path reactions is unprecedented in SCSC transformations\u003csup\u003e44-48\u003c/sup\u003e. Moreover, the single-crystal platform exhibited remarkable stability at low temperature, allowing considerable structural changes to occur with retention of the single-crystal character. To demonstrate the superior stability arising from the unique adaptive ability of the platform, the structural changes accompanying the reaction process are examined (Extended Data Fig. 2). From Extended Data Fig. 2a\u0026nbsp;and Supplementary Table 8, we note that the separation between adjacent N atoms of parallel diene molecules increases by 5.4% to 16.34%, following the cyclization process. This is due to the formation of the cyclobutane ring which causes the pyridyl rings, which are close to parallel before cyclization, splaying apart from each other upon ring formation. The Cd\u0026middot;\u0026middot;\u0026middot;Cd distances in all Cd\u003csub\u003e2\u003c/sub\u003e(3,5-DBC)\u003csub\u003e4\u003c/sub\u003e cluster units increased by 5.6% as a consequence of the increased separation between the coordinating pyridyl groups. In the original structure, a single oxygen atom bridges each pair of Cd(II) centers. Due to the Cd(II) centers being forced apart by the pyridyl groups, this oxygen atom is unable to span the Cd(II) centers, which makes all Cd(II) centers be 6-coordinate in the final structure. The single-crystal stability exhibited at low temperature may be at least partially attributed to the ease with which the Cd\u003csub\u003e2\u003c/sub\u003e(3,5-DBC)\u003csub\u003e4\u003c/sub\u003e cluster units within the chain adapt to significant structural changes associated with the cyclization process (Extended Data Fig. 2b). In addition to the intrachain structural changes, some displacement of the chains is also noted. Whilst this is inevitable given the structural changes in the chains, the movement seems to proceed smoothly and in a manner that reduces internal strain caused by the significant intrachain changes. The result is that the single-crystal character is preserved at low temperature, thus allowing detailed investigations of the reaction pathway.\u003c/p\u003e\n\u003cp\u003eIn conclusion, we have resolved the mechanisms of ultrafast and multi-step [2 + 2] photochemical cycloaddition reactions involving multiple reaction pathways and multiple transient intermediates using a cryo-assisted coordination polymer single-crystal platform. This investigation has relied upon the robust nature of the single-crystal platform used which can tolerate significant structural rearrangements upon irradiation at low temperature with retention of its single-crystal character. The success of the cryo-assisted SCSC approach in elucidating this ultrafast and complex reaction is attributed to the flexibility of the Cd\u003csub\u003e2\u003c/sub\u003e(3,5-DBC)\u003csub\u003e4\u003c/sub\u003e unit and the gentle repositioning of the chains at low temperature which serves to reduce crystal strain arising from the photocyclization processes. The reaction mechanisms elucidated in this study involves severe challenges in speediness and process complexity, each of which was previously difficult to address with single-crystal platforms. This advancement marks a potentially significant expansion in the range of reactions that can be observed using single-crystal platforms for mechanistic studies. Consequently, this approach holds great promise for incorporating a number of previously elusive reaction mechanisms into the realm of investigation.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eMaterials and methods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe\u0026nbsp;diolefin ligand 5-hydroxymethyl-1,3-bis((\u003cem\u003eE\u003c/em\u003e)-2-(pyridin-4-yl)vinyl)benzene (CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb) was synthesized by a palladium-catalyzed Heck reaction between (3,5-dibromophenyl)methanol and 4-vinylpyridine, according to the literature method\u003csup\u003e49\u003c/sup\u003e. Other chemicals and reagents were commercially purchased and used without further purification. Single-crystal X-ray diffraction (SCXRD) data were recorded on a Bruker D8 VENTURE diffractometer. Powder X-ray diffraction (PXRD) patterns were obtained by a PANalytical X\u0026apos;Pert PRO MPD system (PW3040/60) using Cu-\u003cem\u003eK\u0026alpha;\u003c/em\u003e radiation (\u0026lambda; = 1.5406 \u0026Aring;) from 5\u0026ordm; to 50\u0026ordm; with a scanning step of 0.022\u0026ordm;. Thermogravimetric (TG) analysis plot was obtained from a Mettler Toledo Star system under an N\u003csub\u003e2\u003c/sub\u003e atmosphere at a heating rate of 10 \u0026ordm;C min\u003csup\u003e-1\u003c/sup\u003e. Nuclear magnetic resonance (NMR) spectra were recorded on BRUKER AVANCE III HD-400M and Varian UNITY plus-600M spectrometers at room temperature. The \u003csup\u003e1\u003c/sup\u003eH NMR spectrum at 0.5 s at 298 K and \u003csup\u003e1\u003c/sup\u003eH NMR spectra at 173 K were recorded at a Varian UNITY plus-600M spectrometer, and other \u003csup\u003e1\u003c/sup\u003eH NMR and \u003csup\u003e13\u003c/sup\u003eC NMR spectra were recorded at a BRUKER AVANCE III HD-400M spectrometer. Proton chemical shifts \u0026delta; H = 7.26 ppm (CDCl\u003csub\u003e3\u003c/sub\u003e) and \u0026delta; H = 2.50 ppm (DMSO-\u003cem\u003ed\u003csub\u003e6\u003c/sub\u003e\u003c/em\u003e) are referenced for \u003csup\u003e1\u003c/sup\u003eH NMR spectra. Elemental analyses (C, H and N) were carried out using a PE 2400 II elemental analyzer. Mass spectrum (MS) was recorded on a Xevo G2-XS Tof mass spectrometer. The light irradiation experiments were conducted with a PLS-LED100C LED light source (ultraviolet light: \u0026lambda; = 365 nm; electric power: 80 W). The photomechanical motions of the crystals at 298 K and 173 K were respectively captured using a Zeiss SteREO Discovery. V8 microscope, and the photographs of the photomechanical motions of the crystals at 298 K were further edited though the Adobe Premiere Pro 2021 software, forming Supplementary Video 1.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSynthesis of CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA mixture of (3,5-dibromophenyl)methanol (2.66 g, 10.0 mmol), 4-vinylpyridine (2.10 g, 20.0 mmol), potassium carbonate (2.76 g, 20.0 mol) and (PPh\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e2\u003c/sub\u003e (0.084 g, 0.012 mmol) was added into a 100 mL Schlenk tube. The tube was degassed under vacuum, and then backfilled with N\u003csub\u003e2\u003c/sub\u003e. Nextly, \u003cem\u003eN\u003c/em\u003e,\u003cem\u003eN\u003c/em\u003e-dimethylformamide (DMF, 10 mL) was added under the N\u003csub\u003e2\u003c/sub\u003e atmosphere, then the tube was sealed. The final reaction mixture in the tube was heated at 110 \u0026ordm;C for 24 hours. After cooling to room temperature, the black solid was put in H\u003csub\u003e2\u003c/sub\u003eO and extracted with CH\u003csub\u003e2\u003c/sub\u003eCl\u003csub\u003e2\u003c/sub\u003e for three times. The organic phases were dried over anhydrous Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and purified by column chromatography with CH\u003csub\u003e3\u003c/sub\u003eOH and CH\u003csub\u003e2\u003c/sub\u003eCl\u003csub\u003e2\u003c/sub\u003e (\u003cem\u003eV\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u003cem\u003e/V\u003csub\u003e2\u003c/sub\u003e\u003c/em\u003e = 1: 10, \u003cem\u003eR\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e = 0.3) to obtain the light-yellow powder of CH\u003csub\u003e2\u003c/sub\u003eOH-\u003cstrong\u003e1,3-bpeb\u003c/strong\u003e. Yield: 64.5%.\u0026nbsp;\u003csup\u003e1\u003c/sup\u003eH NMR (400 MHz, CDCl\u003csub\u003e3\u003c/sub\u003e, ppm): \u0026delta; 8.59 (d, \u003cem\u003eJ\u003c/em\u003e = 6.0 Hz, 4H), 7.60 (s, 1H), 7.52 (s, 2H), 7.38 (d, \u003cem\u003eJ\u003c/em\u003e = 6.0 Hz, 4H), 7.32 (d, \u003cem\u003eJ\u003c/em\u003e = 16.4 Hz, 2H), 7.09 (d, \u003cem\u003eJ\u003c/em\u003e = 16.4 Hz, 2H), 4.80 (s, 2H) (Supplementary Fig. 1); \u003csup\u003e13\u003c/sup\u003eC NMR (101 MHz, CDCl\u003csub\u003e3\u003c/sub\u003e, ppm): \u003cem\u003e\u0026delta;\u0026nbsp;\u003c/em\u003e150.3 144.6, 142.6, 137.1, 132.7, 126.9, 125.6, 125.1, 121.1, 64.8 (Supplementary Fig. 2); HRMS (ESI-TOF) \u003cem\u003em/z\u003c/em\u003e: [M+H]\u003csup\u003e+\u003c/sup\u003e Calcd for C\u003csub\u003e21\u003c/sub\u003eH\u003csub\u003e19\u003c/sub\u003eN\u003csub\u003e2\u003c/sub\u003eO 315.1492; found 315.1495 (Supplementary Fig. 3).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSynthesis\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;of\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e[{Cd\u003csub\u003e4\u003c/sub\u003e(CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb)\u003csub\u003e4\u003c/sub\u003e(3,5-DCB)\u003csub\u003e8\u003c/sub\u003e}\u0026middot;4H\u003csub\u003e2\u003c/sub\u003eO]\u003csub\u003en\u003c/sub\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;(CP1)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe reactants\u0026nbsp;CdSO\u003csub\u003e4\u003c/sub\u003e∙8/3H\u003csub\u003e2\u003c/sub\u003eO (25.7 mg, 0.100 mmol),\u0026nbsp;CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb\u0026nbsp;(7.85 mg, 0.025 mmol) and 3,5-dichlorobenzoic acid (3,5-HDCB) (19.1 mg, 0.100 mmol) were added into 2.25 mL of the mixed solvents DMF/CH\u003csub\u003e3\u003c/sub\u003eOH/H\u003csub\u003e2\u003c/sub\u003eO (1: 0.5: 7.5) and 20 \u0026micro;L of concentrated nitric acid, and their mixtures were sealed in a thick Pyrex tube and further homogeneously mixed by sonication for 10 min. The sealed reactant solution was then heated at 140 \u0026ordm;C for 3 h. After cooling to\u0026nbsp;room\u0026nbsp;temperature, light-yellow rod-like\u0026nbsp;single crystals of\u0026nbsp;\u003cstrong\u003eCP1\u003c/strong\u003e were isolated after washing with H\u003csub\u003e2\u003c/sub\u003eO and drying at room\u0026nbsp;temperature (Supplementary\u0026nbsp;Figs.\u0026nbsp;4 and 5).\u0026nbsp;Yield: 94.2% based on\u0026nbsp;CH\u003csub\u003e2\u003c/sub\u003eOH-1,3-bpeb.\u0026nbsp;Anal. Calcd for C\u003csub\u003e70\u003c/sub\u003eH\u003csub\u003e52\u003c/sub\u003eCd\u003csub\u003e2\u003c/sub\u003eCl\u003csub\u003e8\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003eO\u003csub\u003e12\u003c/sub\u003e (%): C, 50.97; H, 3.18; N, 3.40; found: C, 50.96; H, 3,16; N, 3.41.\u0026nbsp;\u003csup\u003e1\u003c/sup\u003eH NMR (400 MHz, DMSO-\u003cem\u003ed\u003c/em\u003e\u003csub\u003e6\u003c/sub\u003e,\u003csub\u003e\u0026nbsp;\u003c/sub\u003eppm): \u0026delta; 8.58 (d, \u003cem\u003eJ\u003c/em\u003e = 6.0 Hz, 8H), 7.86 (d, \u003cem\u003eJ\u003c/em\u003e = 2.0 Hz, 8H), 7.84 (s, 2H), 7.73 (t, \u003cem\u003eJ\u003c/em\u003e = 2.0 Hz, 4H), 7.61 \u0026ndash; 7.57 (m, 16H), 7.33 (d, \u003cem\u003eJ\u003c/em\u003e = 16.8 Hz, 4H), 5.34 (s, 2H), 4.58 (d, \u003cem\u003eJ\u003c/em\u003e = 3.2 Hz, 4H) (Supplementary Fig. 6).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSynthesis\u0026nbsp;of\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eCP2\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCrystals of \u003cstrong\u003eCP1\u003c/strong\u003e were dispersed on a glass plate and exposed to PLS-LED100C LED light source (\u003cem\u003e\u0026lambda;\u003c/em\u003e = 365 nm) for 25 seconds. A complete photocycloaddition reaction occurred which generated the photoproduct \u003cstrong\u003eCP2\u003c/strong\u003e. Yield: 100% based on\u0026nbsp;\u003cstrong\u003eCP1\u003c/strong\u003e.\u0026nbsp;\u003csup\u003e1\u003c/sup\u003eH NMR (400 MHz, DMSO-\u003cem\u003ed\u003c/em\u003e\u003csub\u003e6\u003c/sub\u003e,\u003csub\u003e\u0026nbsp;\u003c/sub\u003eppm): \u0026delta; 8.38 (dd, \u003cem\u003eJ\u003c/em\u003e = 13.2, 6.0 Hz, 8H), 7.85 (d, \u003cem\u003eJ\u003c/em\u003e = 2.0 Hz, 8H), 7.72 (t, \u003cem\u003eJ\u003c/em\u003e = 2.0 Hz, 4H), 7.33 (d, \u003cem\u003eJ\u003c/em\u003e = 6.0 Hz, 4H), 7.25 (d, \u003cem\u003eJ\u003c/em\u003e = 6.0 Hz, 4H), 7.11 (s, 2H), 6.71 (s, 2H), 6.46 (s, 2H), 4.83 (t, \u003cem\u003eJ\u003c/em\u003e = 5.6 Hz, 2H), 4.73 (dd, \u003cem\u003eJ\u003c/em\u003e = 14.8, 6.4 Hz, 4H), 4.50 (dd, \u003cem\u003eJ\u003c/em\u003e = 9.6, 6.4 Hz, 4H), 4.16 (d, \u003cem\u003eJ\u003c/em\u003e = 5.2 Hz, 4H) (Supplementary Fig. 7).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCryo-photochemical experiment\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe \u003cstrong\u003eCP1\u003c/strong\u003e single crystals were irradiated using a cryo-assisted approach (173 K, ultraviolet light at 365 nm), and the distance between the single crystal sample and the light source was about 12 cm. They were taken out every 20 min to check the unit cell, and the proper crystals with relatively good diffractions were examined by SCXRD. At last, seven intermediates and one final product were captured (Supplementary Fig. 9). By ultraviolet irradiation at 173 K, \u003cstrong\u003eCP1-1\u003c/strong\u003e, \u003cstrong\u003eCP1-2\u003c/strong\u003e, \u003cstrong\u003eCP1-3\u003c/strong\u003e, \u003cstrong\u003eCP1-4\u003c/strong\u003e,\u003cstrong\u003e\u0026nbsp;CP1-5\u003c/strong\u003e, \u003cstrong\u003eCP1-6\u003c/strong\u003e, \u003cstrong\u003eCP1-7\u003c/strong\u003e and \u003cstrong\u003eCP2\u003c/strong\u003e were obtained from the photocycloaddition of \u003cstrong\u003eCP1\u003c/strong\u003e after 20 min, 40min, 60 min, 80 min, 100 min, 120 min, 140 min, respectively. Here, \u003cstrong\u003eCP1-5\u003c/strong\u003e and \u003cstrong\u003eCP1-6\u003c/strong\u003e were obtained after low-temperature photocycloaddition reaction at 100 min.\u003c/p\u003e\n\u003cp\u003eX-ray data collection and structure determination\u003c/p\u003e\n\u003cp\u003eThe single-crystal-to-single-crystal (SCSC) transformations were monitored in situ by following a cryo-assisted approach. At 173 K, \u003cstrong\u003eCP1-1\u003c/strong\u003e, \u003cstrong\u003eCP1-2\u003c/strong\u003e, \u003cstrong\u003eCP1-3\u003c/strong\u003e, \u003cstrong\u003eCP1-4\u003c/strong\u003e, \u003cstrong\u003eCP1-5\u003c/strong\u003e, \u003cstrong\u003eCP1-6\u003c/strong\u003e, \u003cstrong\u003eCP1-7\u003c/strong\u003e and \u003cstrong\u003eCP2\u003c/strong\u003e photoproducts were successively acquired from the cycloaddition of \u003cstrong\u003eCP1\u003c/strong\u003e using a LED lamp at 365 nm.\u0026nbsp;The crystals of \u003cstrong\u003eCP1\u003c/strong\u003e, \u003cstrong\u003eCP1-1\u003c/strong\u003e, \u003cstrong\u003eCP1-2\u003c/strong\u003e, \u003cstrong\u003eCP1-3\u003c/strong\u003e, \u003cstrong\u003eCP1-4\u003c/strong\u003e, \u003cstrong\u003eCP1-5\u003c/strong\u003e, \u003cstrong\u003eCP1-6\u003c/strong\u003e, \u003cstrong\u003eCP1-7\u003c/strong\u003e, and\u003cstrong\u003e\u0026nbsp;CP2\u003c/strong\u003e were mounted on cryo-loop, and their single-crystal data were collected on a Bruker D8 VENTURE diffractometer\u0026nbsp;with Mo-K\u0026alpha; radiation (\u0026lambda; = 0.71073 \u0026Aring;) and Ga-K\u0026alpha; radiation (\u0026lambda; = 1.34138 \u0026Aring;).\u0026nbsp;All data were processed with Bruker APEX-III, and integrated with SAINT and a multi-scan absorption correction using SADABS was applied\u003csup\u003e50\u003c/sup\u003e.\u003csup\u003e\u0026nbsp;\u003c/sup\u003eAll structures were solved by direct methods using olex2.solve and refined by full-matrix least-squares methods against \u003cem\u003eF\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e by SHELXL\u003csup\u003e51,52\u003c/sup\u003e. All non-hydrogen atoms were refined with anisotropic displacement parameters, and C-bound hydrogen atoms were refined isotropic on calculated positions using a riding model for all structures. For the seven CP photoproducts of \u003cstrong\u003eCP1\u003c/strong\u003e,\u0026nbsp;the poor quality of the diffraction data of the single crystals prevented a proper refinement of the disordered\u0026nbsp;water\u0026nbsp;molecules in the\u0026nbsp;lattice. Thus, SQUEEZE command in the Platon program suite\u003csup\u003e53\u003c/sup\u003e was used to remove the lattice water molecules for these photoproducts. A summary of the crystallographic data for the compounds mentioned above is presented in Supplementary Table 2. Crystallographic data for the single crystal structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers 2359885 (\u003cstrong\u003eCP1\u003c/strong\u003e), 2359886 (\u003cstrong\u003eCP1-1\u003c/strong\u003e), 2359887 (\u003cstrong\u003eCP1-2\u003c/strong\u003e), 2359888 (\u003cstrong\u003eCP1-3\u003c/strong\u003e), 2359889 (\u003cstrong\u003eCP1-4\u003c/strong\u003e), 2359890 (\u003cstrong\u003eCP1-5\u003c/strong\u003e), 2359891 (\u003cstrong\u003eCP1-6\u003c/strong\u003e), 2359892 (\u003cstrong\u003eCP1-7\u003c/strong\u003e), 2359893 (\u003cstrong\u003eCP2\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eTheoretical calculations\u003c/p\u003e\n\u003cp\u003eTo gain good insight into internal mechanism of the cryogenically controlled solid-state photochemical reaction, all density functional theory (DFT) calculations were performed in ORCA Version 5.0.3 quantum computing software\u003csup\u003e54\u003c/sup\u003e. Based on the X-ray crystal structure, the smallest repeating unit in \u003cstrong\u003eCP1\u003c/strong\u003e of this system was extracted as the initial structure for calculation. To save computational resources, the 1,3-dichlorobenzoic acid in \u003cstrong\u003eCP1\u003c/strong\u003e was simplified to formic acid. To save computational time, the molecular structures of \u003cstrong\u003eCP1\u003c/strong\u003e and its photocycloaddition products were optimized at BLYP-D3/def2-SVP level\u003csup\u003e55-58\u003c/sup\u003e. The relativistic effective core potential (RECP) for Cd was used in all calculations\u003csup\u003e59\u003c/sup\u003e.\u0026nbsp;And, transition state theory was implemented to obtain transition state structures in photocycloaddition reaction. The coordinates of oxygen atoms in the carboxylate ligand were frozen during all the structure optimization for the consistency with keeping the experimental structure, where these oxygen atoms involving coordination bond changes are free in the structure optimization from \u003cstrong\u003eCP1\u003c/strong\u003e to \u003cstrong\u003eInt1\u003c/strong\u003e. Moreover, frequency calculations were also carried out to give the thermodynamic parameters of the compounds. Single point energy calculations for all compounds were implemented at M062X-D3/def2-TZVP level. Spin-orbit coupling-time-dependent density functional theory (SOC-TDDFT) was performed on ORCA quantum chemistry software at PBE0-D3/DKH-def2-TZV P level, and Gaussian 09 was used for time-dependent density functional theory (TDDFT) at PBE0-D3/def2-SVP level. Spin-orbit coupling (SOC) effect of singlet-triplet (S-T) excited states and odd electron density (OED) function of triplet excited states were analyzed by Multiwfn Version 3.8 (dev) soft\u003csup\u003e60-62\u003c/sup\u003e, and the pictures of molecular structures and OED were exported and rendered by VMD version 1.9.4a48 software\u003csup\u003e63\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eMethod references\u003c/strong\u003e\u003c/p\u003e\n\u003col start=\"49\"\u003e\n\u003cli\u003eWang, Y. et al. Tuning the configuration of the flexible metal-alkene-framework affords pure cycloisomers in solid state photodimerization. \u003cem\u003eChem. Commun.\u003c/em\u003e\u003cstrong\u003e57\u003c/strong\u003e, 1129-1132 (2021).\u003c/li\u003e\n\u003cli\u003eKrause, L., Herbst-Irmer, R., Sheldrick, G. M. \u0026amp; Stalke, D. Comparison of silver and molybdenum microfocus X-ray sources for single-crystal structure determination. \u003cem\u003eJ. Appl. Crystallogr.\u003c/em\u003e\u003cstrong\u003e48\u003c/strong\u003e, 3-10 (2015).\u003c/li\u003e\n\u003cli\u003eSheldrick, G. M. Crystal structure refinement with SHELXL. \u003cem\u003eActa Crystallogr. Sect. C\u003c/em\u003e\u003cstrong\u003e71\u003c/strong\u003e, 3-8 (2015).\u003c/li\u003e\n\u003cli\u003eDolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. \u0026amp; Puschmann, H. OLEX2: a complete structure solution, refinement and analysis program. \u003cem\u003eJ. Appl. Crystallogr.\u003c/em\u003e\u003cstrong\u003e42\u003c/strong\u003e, 339-341 (2009).\u003c/li\u003e\n\u003cli\u003eSpek, A. Single-crystal structure validation with the program PLATON. \u003cem\u003eJ. Appl. Crystallogr.\u003c/em\u003e\u003cstrong\u003e36\u003c/strong\u003e, 7-13 (2003).\u003c/li\u003e\n\u003cli\u003eNeese, F. The ORCA program system. \u003cem\u003eWIREs Comput. Mol. Sci.\u003c/em\u003e\u003cstrong\u003e2\u003c/strong\u003e, 73-78 (2012).\u003c/li\u003e\n\u003cli\u003eGrimme, S., Ehrlich, S. \u0026amp; Goerigk, L. Effect of the damping function in dispersion corrected density functional theory. \u003cem\u003eJ. Comput. Chem.\u003c/em\u003e\u003cstrong\u003e32\u003c/strong\u003e, 1456-1465 (2011).\u003c/li\u003e\n\u003cli\u003eGrimme, S., Antony, J., Ehrlich, S. \u0026amp; Krieg, H. A consistent and accurate\u003cem\u003e ab\u003c/em\u003e\u003cem\u003einitio\u003c/em\u003e parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. \u003cem\u003eJ. Chem. Phys.\u003c/em\u003e\u003cstrong\u003e132\u003c/strong\u003e, 154104 (2010).\u003c/li\u003e\n\u003cli\u003eWeigend, F. \u0026amp; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: design and assessment of accuracy. \u003cem\u003ePhys. Chem. Chem. Phys.\u003c/em\u003e\u003cstrong\u003e7\u003c/strong\u003e, 3297-3305 (2005).\u003c/li\u003e\n\u003cli\u003eWeigend, F. Accurate coulomb-fitting basis sets for H to Rn. \u003cem\u003ePhys. Chem. Chem. Phys.\u003c/em\u003e\u003cstrong\u003e8\u003c/strong\u003e, 1057-1065 (2006).\u003c/li\u003e\n\u003cli\u003eAndrae, D., H\u0026auml;u\u0026szlig;ermann, U., Dolg, M., Stoll, H. \u0026amp; Preu\u0026szlig;, H. Energy-adjusted \u003cem\u003eab\u003c/em\u003e\u003cem\u003einitio \u003c/em\u003epseudopotentials for the second and third row transition elements. \u003cem\u003eTheor. Chim. Acta\u003c/em\u003e\u003cstrong\u003e77\u003c/strong\u003e, 123-141 (1990).\u003c/li\u003e\n\u003cli\u003eTakatsuka, K., Fueno, T. \u0026amp; Yamaguchi, K. Distribution of odd electrons in ground-state molecules. \u003cem\u003eTheor. Chim. Acta\u003c/em\u003e\u003cstrong\u003e48\u003c/strong\u003e, 175-183 (1978).\u003c/li\u003e\n\u003cli\u003eNakano, M. et al. (Hyper)polarizability density analysis for open-shell molecular systems based on natural orbitals and occupation numbers. \u003cem\u003eTheor. Chem. Acc.\u003c/em\u003e\u003cstrong\u003e130\u003c/strong\u003e, 711-724 (2011).\u003c/li\u003e\n\u003cli\u003eLu, T. \u0026amp; Chen, F. Multiwfn: a multifunctional wavefunction analyzer. \u003cem\u003eJ. Comput. Chem.\u003c/em\u003e\u003cstrong\u003e33\u003c/strong\u003e, 580-592 (2012).\u003c/li\u003e\n\u003cli\u003eHumphrey, W., Dalke, A. \u0026amp; Schulten, K. J. J. o. m. g. VMD: visual molecular dynamics. \u003cem\u003eJ. Mol. Graphics\u003c/em\u003e\u003cstrong\u003e14\u003c/strong\u003e, 33-38 (1996).\u003c/li\u003e\n\n\u003c/ol\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data supporting the findings of this study are available within the paper and its Supplementary Information and from the Cambridge Crystallographic Data Centre (https://www.ccdc.cam.ac.uk/structures; crystallographic data are available free of charge under CCDC reference numbers CCDC 2359885 to 2359893).\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eThe authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 22271203), the Collaborative Innovation Centre of Suzhou Nano Science and Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Project of Scientific and Technologic Infrastructure of Suzhou (Grant No. SZS201905).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e Conceptualization:\u0026nbsp;M.H.D., J.P.L. Methodology: Q.Q.Z., Y.W., Y.H.D., Q.L., J.P.L. Investigation: Q.Q.Z., Y.W., Y.H.D., M.H.D., Q.L., J.P.L. Visualization: M.H.D., Q.Q.Z., Y.W., J.P.L. Funding acquisition: J.P.L. Project administration: J.P.L. Supervision: J.P.L. Writing \u0026ndash; original draft: Q.Q.Z., Y.W., Y.H.D., M.H.D., Q.L., J.P.L. Writing \u0026ndash; review \u0026amp; editing: B.F.A., P.B., M.H.D., Q.L., J.P.L.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e The authors declare no competing interests.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eKawamichi, T., Haneda, T., Kawano, M. \u0026amp; Fujita, M. X-ray observation of a transient hemiaminal trapped in a porous network. \u003cem\u003eNature\u003c/em\u003e \u003cstrong\u003e461\u003c/strong\u003e, 633-635, (2009).\u003c/li\u003e\n\u003cli\u003eBloch, W. M. et al. Capturing snapshots of post-synthetic metallation chemistry in metal-organic frameworks. \u003cem\u003eNat. Chem.\u003c/em\u003e \u003cstrong\u003e6\u003c/strong\u003e, 906-912, (2014).\u003c/li\u003e\n\u003cli\u003eBurgun, A. et al. Mapping-out catalytic processes in a metal-organic framework with single-crystal X-ray crystallography. \u003cem\u003eAngew. Chem. Int. Ed.\u003c/em\u003e \u003cstrong\u003e56\u003c/strong\u003e, 8412-8416, (2017).\u003c/li\u003e\n\u003cli\u003eIkemoto, K., Inokuma, Y., Rissanen, K. \u0026amp; Fujita, M. X-ray snapshot observation of palladium-mediated aromatic bromination in a porous complex. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e136\u003c/strong\u003e, 6892-6895, (2014).\u003c/li\u003e\n\u003cli\u003eYoshioka, S., Inokuma, Y., Duplan, V., Dubey, R. \u0026amp; Fujita, M. X-ray structure analysis of ozonides by the crystalline sponge method. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e138\u003c/strong\u003e, 10140-10142, (2016).\u003c/li\u003e\n\u003cli\u003eSaito, A., Sawada, T. \u0026amp; Fujita, M. X-ray crystallographic observation of chiral transformations within a metal-peptide pore. \u003cem\u003eAngew. Chem. Int. Ed.\u003c/em\u003e \u003cstrong\u003e59\u003c/strong\u003e, 20367-20370, (2020).\u003c/li\u003e\n\u003cli\u003eKumar, A. et al. Solid-state reaction synthesis of nanoscale materials: strategies and applications. \u003cem\u003eChem. Rev.\u003c/em\u003e \u003cstrong\u003e122\u003c/strong\u003e, 12748-12863, (2022).\u003c/li\u003e\n\u003cli\u003eTzirakis, M. D. \u0026amp; Orfanopoulos, M. Radical reactions of fullerenes: from synthetic organic chemistry to materials science and biology. \u003cem\u003eChem. Rev.\u003c/em\u003e \u003cstrong\u003e113\u003c/strong\u003e, 5262-5321, (2013).\u003c/li\u003e\n\u003cli\u003ede Oteyza, D. G. et al. Direct imaging of covalent bond structure in single-molecule chemical reactions. \u003cem\u003eScience\u003c/em\u003e \u003cstrong\u003e340\u003c/strong\u003e, 1434-1437, (2013).\u003c/li\u003e\n\u003cli\u003eChen, J.-J. et al. Enantioconvergent Cu-catalysed \u003cem\u003eN\u003c/em\u003e-alkylation of aliphatic amines. \u003cem\u003eNature\u003c/em\u003e \u003cstrong\u003e618\u003c/strong\u003e, 294-300, (2023).\u003c/li\u003e\n\u003cli\u003eTian, Y. et al. A general copper-catalysed enantioconvergent C(sp\u003csup\u003e3\u003c/sup\u003e)-S cross-coupling via biomimetic radical homolytic substitution. \u003cem\u003eNat. Chem.\u003c/em\u003e \u003cstrong\u003e16\u003c/strong\u003e, 466-475, (2024).\u003c/li\u003e\n\u003cli\u003eMo, F. et al. Diatomic catalysts for fenton and fenton-like reactions: a promising platform for designing/regulating reaction pathways. \u003cem\u003eChem. Sci.\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 7818-7827, (2023).\u003c/li\u003e\n\u003cli\u003eGao, X. et al. Urea catalytic oxidation for energy and environmental applications. \u003cem\u003eChem. Soc. Rev.\u003c/em\u003e \u003cstrong\u003e53\u003c/strong\u003e, 1552-1591, (2024).\u003c/li\u003e\n\u003cli\u003eRoglans, A., Pla-Quintana, A. \u0026amp; Sol\u0026agrave;, M. Mechanistic studies of transition-metal-catalyzed [2 + 2 + 2] cycloaddition reactions. \u003cem\u003eChem. Rev.\u003c/em\u003e \u003cstrong\u003e121\u003c/strong\u003e, 1894-1979, (2021).\u003c/li\u003e\n\u003cli\u003eChizallet, C., Bouchy, C., Larmier, K. \u0026amp; Pirngruber, G. Molecular views on mechanisms of br\u0026oslash;nsted acid-catalyzed reactions in zeolites. \u003cem\u003eChem. Rev.\u003c/em\u003e \u003cstrong\u003e123\u003c/strong\u003e, 6107-6196, (2023).\u003c/li\u003e\n\u003cli\u003eChen, Z. et al. Toward understanding the formation mechanism and OER catalytic mechanism of hydroxides by in situ and operando techniques. \u003cem\u003eAngew. Chem. Int. Ed.\u003c/em\u003e \u003cstrong\u003e62\u003c/strong\u003e, e202309293, (2023).\u003c/li\u003e\n\u003cli\u003eR\u0026aacute;ksi, F. et al. Ultrafast X-ray absorption probing of a chemical reaction. \u003cem\u003eJ. Chem. Phys.\u003c/em\u003e \u003cstrong\u003e104\u003c/strong\u003e, 6066-6069, (1996).\u003c/li\u003e\n\u003cli\u003eBressler, C. \u0026amp; Chergui, M. Ultrafast X-ray absorption spectroscopy. \u003cem\u003eChem. Rev.\u003c/em\u003e \u003cstrong\u003e104\u003c/strong\u003e, 1781-1812, (2004).\u003c/li\u003e\n\u003cli\u003eSellberg, J. A. et al. Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature. \u003cem\u003eNature\u003c/em\u003e \u003cstrong\u003e510\u003c/strong\u003e, 381-384, (2014).\u003c/li\u003e\n\u003cli\u003eRolles, D. Time-resolved experiments on gas-phase atoms and molecules with XUV and X-ray free-electron lasers. \u003cem\u003eAdv. Phys.: X\u003c/em\u003e \u003cstrong\u003e8\u003c/strong\u003e, 2132182, (2023).\u003c/li\u003e\n\u003cli\u003eKuramochi, H. et al. Ultrafast Raman observation of the perpendicular intermediate phantom state of stilbene photoisomerization. \u003cem\u003eNat. Chem.\u003c/em\u003e \u003cstrong\u003e16\u003c/strong\u003e, 22-27, (2024).\u003c/li\u003e\n\u003cli\u003eMaiuri, M., Garavelli, M. \u0026amp; Cerullo, G. Ultrafast spectroscopy: state of the art and open challenges. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e142\u003c/strong\u003e, 3-15, (2020).\u003c/li\u003e\n\u003cli\u003eWang, M.-F. et al. Coordination-driven stereospecific control strategy for pure cycloisomers in solid-state diene photocycloaddition. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e142\u003c/strong\u003e, 700-704, (2020).\u003c/li\u003e\n\u003cli\u003eClaassens, I. E., Barbour, L. J. \u0026amp; Haynes, D. A. A multistimulus responsive porous coordination polymer: temperature-mediated control of solid-state [2 + 2] cycloaddition. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e141\u003c/strong\u003e, 11425-11429, (2019).\u003c/li\u003e\n\u003cli\u003eZheng, Y. et al. Flexible interlocked porous frameworks allow quantitative photoisomerization in a crystalline solid. \u003cem\u003eNat. Commun.\u003c/em\u003e \u003cstrong\u003e8\u003c/strong\u003e, 100, (2017).\u003c/li\u003e\n\u003cli\u003eLiu, Y. et al. Weaving of organic threads into a crystalline covalent organic framework. \u003cem\u003eScience\u003c/em\u003e \u003cstrong\u003e351\u003c/strong\u003e, 365-369, (2016).\u003c/li\u003e\n\u003cli\u003eHanikel, N. et al. Evolution of water structures in metal-organic frameworks for improved atmospheric water harvesting. \u003cem\u003eScience\u003c/em\u003e \u003cstrong\u003e374\u003c/strong\u003e, 454-459, (2021).\u003c/li\u003e\n\u003cli\u003eGao, W.-X., Feng, H.-J., Guo, B.-B., Lu, Y. \u0026amp; Jin, G.-X. Coordination-directed construction of molecular links. \u003cem\u003eChem. Rev.\u003c/em\u003e \u003cstrong\u003e120\u003c/strong\u003e, 6288-6325, (2020).\u003c/li\u003e\n\u003cli\u003eChen, G., Huang, S., Ma, X., He, R. \u0026amp; Ouyang, G. Encapsulating and stabilizing enzymes using hydrogen-bonded organic frameworks. \u003cem\u003eNat. Protoc.\u003c/em\u003e \u003cstrong\u003e18\u003c/strong\u003e, 2032-2050, (2023).\u003c/li\u003e\n\u003cli\u003eLang, F., Pang, J. \u0026amp; Bu, X.-H. Stimuli-responsive coordination polymers toward next-generation smart materials and devices. \u003cem\u003eeScience\u003c/em\u003e \u003cstrong\u003e4\u003c/strong\u003e, 100231, (2024).\u003c/li\u003e\n\u003cli\u003eGuo, J. et al. Visible light-mediated intermolecular crossed [2 + 2] cycloadditions using a MOF-supported copper triplet photosensitizer. \u003cem\u003eNat. Catal.\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, 307-320, (2024).\u003c/li\u003e\n\u003cli\u003eWang, J.-S. et al. Cage-confined photocatalysis for wide-scope unusually selective [2 + 2] cycloaddition through visible-light triplet sensitization. \u003cem\u003eNat. Commun.\u003c/em\u003e \u003cstrong\u003e11\u003c/strong\u003e, 4675, (2020).\u003c/li\u003e\n\u003cli\u003eGao, X., Fri\u0026scaron;čić, T. \u0026amp; MacGillivray, L. R. Supramolecular construction of molecular ladders in the solid state. \u003cem\u003eAngew. Chem. Int. Ed.\u003c/em\u003e \u003cstrong\u003e43\u003c/strong\u003e, 232-236, (2004).\u003c/li\u003e\n\u003cli\u003eWang, Z. et al. Polyladderane constructed from a gemini monomer through photoreaction. \u003cem\u003eAngew. Chem. Int. Ed.\u003c/em\u003e \u003cstrong\u003e56\u003c/strong\u003e, 12155-12159, (2017).\u003c/li\u003e\n\u003cli\u003eZhang, Q., Wang, Y., Braunstein, P. \u0026amp; Lang, J.-P. Construction of olefinic coordination polymer single crystal platforms: precise organic synthesis, in situ exploration of reaction mechanisms and beyond. \u003cem\u003eChem. Soc. Rev.\u003c/em\u003e \u003cstrong\u003e53\u003c/strong\u003e, 5227-5263, (2024).\u003c/li\u003e\n\u003cli\u003eSchmidt, G. M. J. Photodimerization in the solid state. \u003cem\u003ePure Appl. Chem.\u003c/em\u003e \u003cstrong\u003e27\u003c/strong\u003e, 647-678, (1971).\u003c/li\u003e\n\u003cli\u003eShi, Y.-X., Zhang, W.-H., Abrahams, B. F., Braunstein, P. \u0026amp; Lang, J.-P. Fabrication of photoactuators: macroscopic photomechanical responses of metal-organic frameworks to irradiation by UV light. \u003cem\u003eAngew. Chem. Int. Ed.\u003c/em\u003e \u003cstrong\u003e58\u003c/strong\u003e, 9453-9458, (2019).\u003c/li\u003e\n\u003cli\u003eRath, B. B. \u0026amp; Vittal, J. J. Single-crystal-to-single-crystal [2 + 2] photocycloaddition reaction in a photosalient one-dimensional coordination polymer of Pb(II). \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e142\u003c/strong\u003e, 20117-20123, (2020).\u003c/li\u003e\n\u003cli\u003eTroyano, J., Carn\u0026eacute;-S\u0026aacute;nchez, A. \u0026amp; Maspoch, D. Programmable self-assembling 3D architectures generated by patterning of swellable MOF-based composite films. \u003cem\u003eAdv. Mater.\u003c/em\u003e \u003cstrong\u003e31\u003c/strong\u003e, 1808235, (2019).\u003c/li\u003e\n\u003cli\u003eYu, Q. et al. Fabrication of large single crystals for platinum-based linear polymers with controlled-release and photoactuator performance. \u003cem\u003eAngew. Chem. Int. Ed.\u003c/em\u003e \u003cstrong\u003e58\u003c/strong\u003e, 18634-18640, (2019).\u003c/li\u003e\n\u003cli\u003eJiang, Y. et al. Substrate-bound diarylethene-based anisotropic metal-organic framework films as photoactuators with a directed response. \u003cem\u003eAngew. Chem. Int. Ed.\u003c/em\u003e \u003cstrong\u003e62\u003c/strong\u003e, e202218052, (2023).\u003c/li\u003e\n\u003cli\u003eWu, Y., Dong, X., Kim, J.-K., Wang, C. \u0026amp; Sitti, M. Wireless soft millirobots for climbing three-dimensional surfaces in confined spaces. \u003cem\u003eSci. Adv.\u003c/em\u003e \u003cstrong\u003e8\u003c/strong\u003e, eabn3431, (2022).\u003c/li\u003e\n\u003cli\u003eHu, W., Lum, G. Z., Mastrangeli, M. \u0026amp; Sitti, M. Small-scale soft-bodied robot with multimodal locomotion. \u003cem\u003eNature\u003c/em\u003e \u003cstrong\u003e554\u003c/strong\u003e, 81-85, (2018).\u003c/li\u003e\n\u003cli\u003eYang, S.-Y. et al. Crystallographic snapshots of the interplay between reactive guest and host molecules in a porous coordination polymer: stereochemical coupling and feedback mechanism of three photoactive centers triggered by UV-induced isomerization, dimerization, and polymerization reactions. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e136\u003c/strong\u003e, 558-561, (2014).\u003c/li\u003e\n\u003cli\u003eWang, M.-F. et al. Controllable multiple-step configuration transformations in a thermal/photoinduced reaction. \u003cem\u003eNat. Commun.\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 2847, (2022).\u003c/li\u003e\n\u003cli\u003eMa, Z.-T., Yang, S.-Y., Zheng, L.-S. \u0026amp; Teo, B. K. First observation of unusual domino effect and triggering mechanism of sequential single-crystal-to-single-crystal photochemical reactions in a metal-organic framework with multiple photoreactive centers. \u003cem\u003eCryst. Growth Des.\u003c/em\u003e \u003cstrong\u003e19\u003c/strong\u003e, 3113-3119, (2019).\u003c/li\u003e\n\u003cli\u003eHu, F.-L., Wang, S.-L., Lang, J.-P. \u0026amp; Abrahams, B. F. In-situ X-ray diffraction snapshotting: determination of the kinetics of a photodimerization within a single crystal. \u003cem\u003eSci. Rep.\u003c/em\u003e \u003cstrong\u003e4\u003c/strong\u003e, 6815, (2014).\u003c/li\u003e\n\u003cli\u003eWang, M.-F. et al. In situ observation of a stepwise [2 + 2] photocycloaddition process using fluorescence spectroscopy. \u003cem\u003eNat. Commun.\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 7766, (2023).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4993811/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4993811/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eDeveloping versatile crystalline platforms can significantly enhance the utility of time-resolved X-ray crystallography for observing diverse reaction mechanisms\u003c/strong\u003e \u003csup\u003e\u003cstrong\u003e1–6\u003c/strong\u003e\u003c/sup\u003e\u003csup\u003e \u003c/sup\u003e. \u003cstrong\u003eHowever, it is often limited by its inability to handle ultrafast and complex reactions. Here we propose a coordination polymer single-crystal platform that incorporates flexible cluster nodes and integrated reaction substrates. This advanced platform features enhanced diffraction capabilities and adaptability to substrate changes, enabling the observation of ultrafast, highly dynamic reactions involving multiple pathways and intermediates. By combining this platform with a cryo-assisted strategy, we investigate a complicated ultrafast photochemical cycloaddition reaction with five transient intermediates and three distinct routes, which are rationalized through theoretical calculations. Our findings underscore the feasibility of employing this enhanced single-crystal platform to unravel elusive reaction mechanisms, presenting a promising approach with broad applicability.\u003c/strong\u003e\u003c/p\u003e","manuscriptTitle":"A single-crystal platform for unveiling ultrafast and complex photochemical cascade reactions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-25 09:56:16","doi":"10.21203/rs.3.rs-4993811/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ad8faf62-e945-41c1-a4b5-0a7dc8144fb1","owner":[],"postedDate":"April 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":47641511,"name":"Physical sciences/Chemistry/Inorganic chemistry/Solid-state chemistry"},{"id":47641512,"name":"Physical sciences/Chemistry/Photochemistry/Photocatalysis"},{"id":47641513,"name":"Physical sciences/Chemistry/Organic chemistry/Reaction mechanisms"}],"tags":[],"updatedAt":"2025-05-15T12:41:04+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-25 09:56:16","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4993811","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4993811","identity":"rs-4993811","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}