Single-crystal To Single-crystal Research Articles

Halogen-Driven Single-Crystal to Single-Crystal Transformation Engineering the Cluster-based Spin Crossover Frameworks.

Cluster-based spin crossover (SCO) frameworks are a new class of smart metal-organic frameworks (MOFs) with diverse structures and topologies and unique bistable physicochemical properties. Here, we report a cluster-based SCO framework [Fe3{Ag4(CN)6(H2O)}2(TPBA)3](ClO4)2·7DMF (1) with an extremely rare 3,4,6-T108 topology, in which the tripodal [Ag{Ag(CN)2}3(H2O)]2- clusters axially link the Fe2+ ions to form 2D→3D n-fold Borromean entangled networks. Under the guidance of reticular chemistry, the post-synthetic modification (PSM) from 1 with 3,4,6-T108 topology to [Fe3{Ag8X8(CN)6}(TPBA)3] (2_X, X = Cl, Br, I) with urk topology is firstly achieved via single-crystal to single-crystal (SCSC) transformation. Moreover, the successive SCSC transformations from 2_Cl to 2_Br and then to 2_I are realized for the first time. Their SCO behaviors are also modified by halogen-driven stepwise cluster transformations. Hence, these findings provide new strategies for the development of cluster-based SCO MOFs towards the smart functional porous materials.

Halogen‐Driven Single‐Crystal to Single‐Crystal Transformation Engineering the Cluster‐based Spin Crossover Frameworks

Cluster‐based spin crossover (SCO) frameworks are a new class of smart metal‐organic frameworks (MOFs) with diverse structures and topologies and unique bistable physicochemical properties. Here, we report a cluster‐based SCO framework [Fe3{Ag4(CN)6(H2O)}2(TPBA)3](ClO4)2·7DMF (1) with an extremely rare 3,4,6‐T108 topology, in which the tripodal [Ag{Ag(CN)2}3(H2O)]2− clusters axially link the Fe2+ ions to form 2D→3D n‐fold Borromean entangled networks. Under the guidance of reticular chemistry, the post‐synthetic modification (PSM) from 1 with 3,4,6‐T108 topology to [Fe3{Ag8X8(CN)6}(TPBA)3] (2_X, X = Cl, Br, I) with urk topology is firstly achieved via single‐crystal to single‐crystal (SCSC) transformation. Moreover, the successive SCSC transformations from 2_Cl to 2_Br and then to 2_I are realized for the first time. Their SCO behaviors are also modified by halogen‐driven stepwise cluster transformations. Hence, these findings provide new strategies for the development of cluster‐based SCO MOFs towards the smart functional porous materials.

Photodimerization-Triggered Photopolymerization of Triene Coordination Polymers Enables Macroscopic Photomechanical Movements.

Controlling the packing of olefinic molecules in crystals is essential for triggering solid-state [2 + 2] photocycloaddition reactions and the synthesis of photocontrolled smart materials. Herein, we report the stepwise photodimerization-triggered photopolymerization of two triene coordination polymers (CPs), {[Zn(2-BBA)2(tpeb)]·0.5CH3CN}n (1, 2-HBBA = 2-bromobenzoic acid, tpeb = 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene) and {[Zn(3-BBA)2(tpeb)]·CH3CN)}n (2, 3-HBBA = 3-bromobenzoic acid). Upon irradiation with 420 nm light, each pair of closely packed and parallel olefinic bonds in 1 undergoes a [2 + 2] cycloaddition reaction, which connects two adjacent Z-shaped chains into a ladder-like coordination chain [Zn(2-BBA)2(bpbdpvpcb)0.5]n (1a, bpbdpvpcb = 1,3-bis(4-pyridyl)-2,4-bis(3,5-di(2-(4-pyridyl)vinyl)phenyl]cyclobutene) through single-crystal to single-crystal (SCSC) transformation. After photodimerization from 1 to 1a has occurred, the olefinic bonds that were initially distant are brought in close enough proximity to meet the requirements for a subsequent [2 + 2] cycloaddition reaction. Upon further light irradiation, the neighboring bpbdpvpcb ligands in 1a experience a SCSC photopolymerization based on [2 + 2] photocycloaddition and transform into poly-3b,4,5,5a,8b,9,10a-octahydro-4,5,9,10-tetrapyridyl-2,7-di(2-(4-pyridyl)vinyl)dicyclobuta[e,l]-pyren (poly-otpdpvdcbp). 2 showed similar structural changes under UV light illumination. Under light exposure, single crystals of 1 and 2 with different morphologies exhibit bending, cracking, and jumping photomechanical motions. The composite film (1-PVA) engineered by dispersing crystalline particles of 1 in poly(vinyl alcohol) (PVA) displays interesting light-wavelength-dependent photomechanical motions and can perform photodriven swimming on a liquid surface. This work provides a useful and promising approach to enable photodimerization of those photoinactive olefin pairs embedded in CPs and opens a new route to synthesize organic polymers by using olefinic CP platforms.

Single Crystal to Single Crystal Transformation of Porous Materials Based on Zinc Nodes and Mercaptobenzoic Acid.

Porous coordination polymers (PCPs) are an excellent class of porous crystalline materials with tunable properties and intriguing potential applications spanning multiple disciplines. In this work, we report the synthesis and characterization of a PCP (HI-103) based on 4,4'-dithiodibenzoic acid ligand and zinc nitrate with two DMF molecules residing in the porous network. The stability of the porous network was analyzed by heating the compound at 60.0 °C for two days, and the structural analysis revealed a new PCP (HI-104) was formed with one of the DMF molecules, indicating a single-crystal to single-crystal (SCSC) transformation. The solvent molecules were completely removed by extensive drying (HI-103-dry), and the integrity of the porous network was verified by powder X-ray diffraction (PXRD) and thermogravimetric analysis. The reversibility of SCSC transformation was confirmed by treating HI-103-dry with DMF molecules, resulting in HI-103 after five days. The adsorption studies of HI-103-dry with other solvents revealed that SCSC transformation was not observed for DMA and DEA, but some structural changes were observed in the presence of DMSO. The adsorption studies performed in the presence of an equimolar mixture of DMF, DMA, and DMA indicated that HI-103-dry could selectively adsorb DMF molecules from the analogous mixture.

Photoinitiated Single-Crystal to Single-Crystal Redox Transformations of Titanium-Oxo Clusters.

Titanium-oxo clusters can undergo photochemical reactions under UV light, resulting in the reduction of the titanium-oxo core and oxidation of surface ligands. This is an important step in photocatalytic processes in light-absorbing Ti/O-based clusters, metal-organic frameworks, and (nano)material surfaces; however, studying the direct outcome of this photochemical process is challenging due to the fragility of the immediate photoproducts. In this report, titanium-oxo clusters [TiO(OiPr)(L)]n (n = 4, L = O2PPh2, or n = 6, L = O2CCH2tBu) undergo a two-electron photoredox reaction in the single-crystal state via an irreversible single-crystal to single-crystal (SC-SC) transformation initiated by a UV laser. The process is monitored by single crystal X-ray diffraction revealing the photoreduction of the cluster with coproduction of an (oxidized) acetone ligand, which is retained in the structure as a ligand to Ti(3+). The results demonstrate that photochemistry of inorganic molecules can be studied in the single crystal phase, allowing characterization of photoproducts which are unstable in the solution phase.

Synergistic Induction of Solvent and Ligand-Substitution in Single-Crystal to Single-Crystal Transformations toward a MOF with Photocatalytic Dye Degradation.

External-stimuli responsiveness as found in natural organisms and smart materials is attractive for functional materials scientists who attempt to design and imitate fascinating behavior into their materials. Herein, we report a couple of new solvent-responsive isostructural two-dimensional cationic metal-organic frameworks (MOFs) of Mn(II) (1a) and Zn(II) (2a) that undergo unprecedented single-crystal to single-crystal (SCSC) transformation toward the corresponding isostructural three-dimensional MOFs of Mn(II) (1b) and Zn(II) (2b). The 2D MOFs 1a and 2a have been effortlessly and rapidly synthesized via the microwave-heating technique. The SCSC transformations are synergistically induced by solvent and ligand-substitution reactions and able to be triggered by water, methanol, ethanol, and n-propanol. Time-dependent SCSC transformations were studied by in situ X-ray diffraction. Investigations on photodegradation of methyl orange showed that Zn-MOF 2b has higher efficiency than Mn-MOF 1b under UV-C irradiation at 300 min, 94.27%, and 21.91%, respectively. The influence of charge on the dye molecules, heterogeneity of the catalysis, and •OH radical-scavenging test was studied. First-principles computations suggest that the high photocatalytic activity of 2b may be attributed to its suitable band-edge position for redox reactions.

Single-Crystal Transformation Engineering the Spin Change of Metal-Organic Frameworks via Cluster Deconstruction.

Spin crossover (SCO) materials with new architectures will expand and enrich the research in the SCO field. Here, we report two metal-organic frameworks (MOFs) containing tetradentate organic ligands and hexatopic linkers [Ag8X8(CN)6]6- (X = Br and I), which represents the first SCO MOF with clusters as building blocks. The silver halide cluster can be further removed after reacting with LiTCNQ. Such post-synthetic modification (PSM) is realized via single-crystal to single-crystal (SCSC) transformation from urk to nbo topology. Accordingly, the spin state and fluorescence properties are greatly modified by cluster deconstruction. These achievements will provide new ideas for the design of new SCO systems and the development of PSM methods.

Single‐Crystal Transformation Engineering the Spin Change of Metal–Organic Frameworks via Cluster Deconstruction

AbstractSpin crossover (SCO) materials with new architectures will expand and enrich the research in the SCO field. Here, we report two metal–organic frameworks (MOFs) containing tetradentate organic ligands and hexatopic linkers [Ag8X8(CN)6]6− (X=Br and I), which represents the first SCO MOF with clusters as building blocks. The silver halide cluster can be further removed after reacting with lithium tetracyanoquinodimethan (LiTCNQ). Such post‐synthetic modification (PSM) is realized via single‐crystal to single‐crystal (SCSC) transformation from urk to nbo topology. Accordingly, the spin state and fluorescence properties are greatly modified by cluster deconstruction. Therefore, these achievements will provide new ideas for the design of new SCO systems and the development of PSM methods.

Multidirectional Solvent-Induced Structural Transformation in Designing a Series of Polycatenated Cobalt(II) Coordination Polymers: Impact on Carbon Dioxide and Hydrogen Uptake.

Coordination polymers with external stimuli-responsive structural transformation acquired paramount importance in the advanced material research field due to their eye-catching application to deal with the existing challenging issue, and Co(II) metal complex with d7 electronic configuration is a renowned candidate for kinetic accountability and has the potentiality of structural transformation. Bearing these factors in mind, here, a Co(II) congener of a previously reported high hydrogen-adsorbing Cu(II)-based coordination polymer (CP), {[Cu(4-bpe)(2-ntp)]}n [where 2-ntp2- = 2-nitroterephthalate and 4-bpe = 1,2-bis-(4-pyridyl)ethane], has been synthesized to study the metal change impact on hydrogen adsorption and solvent-induced structural transformation with their impact on hydrogen uptake. This modified framework has a 2D + 2D → 3D inclined polycatenated framework as comparable to our previously published Cu(II) framework. Here, on the variation of different solvents, the labile Co(II)-containing framework exhibits a structural change through single-crystal to single-crystal (SC-SC) structural transformation and results in three new framework structures. All four frameworks are structurally characterized by elemental analysis, IR, PXRD, TGA, and single-crystal X-ray diffraction. The desolvated parent framework with exposed metal centers exhibits excellent results of H2 adsorption of 1.3 wt % (145 cc/g) at 77 K and pressure of 1 bar with structural sustainability and CO2 uptake of 130 cc/g at 195 K and 1 bar. For the other three solvent-mediated structural derivatives, H2 and CO2 adsorption have been studied, and the results are correlated with their structure.

Photoresponsive Magnetic Relaxation Behavior of a Highly Stable π···π Interaction-Stacked Framework Based on a DyIII Single-Ion Magnet

Endowing lanthanide single-ion magnets (Ln-SIMs) with high stability and stimulus responsiveness is of significance for the practical applications of these novel magnetic materials, but still of challenge at present. Herein, we report on a highly stable and photoresponsive Ln-SIM system achieved by designing and synthesizing a π···π interaction-stacked framework based on DyIII-SIM (πOF-1). Its molecular formula is [Dy(thqa)] (H3thqa = tris[(8-hydroxyquinoline-2-yl)methylene]amino). Each of its C3-symmetric molecules consists of a thqa3– ligand and a DyIII ion, and it is linked to three neighbors by intermolecular π···π interactions, forming a supermolecular two-dimensional (2D) honeycombed topology. Such unique structure endows πOF-1 with not only air and thermal stability but also excellent chemical stability in different solvents, even in alkali solutions, including a 20 M NaOH aqueous solution. Dual magnetic relaxation behavior was observed in πOF-1. Magnetic dilution revealed that it is related to π···π-mediated intermolecular magnetic interactions. More significantly, both the static magnetic susceptibility and the dual-relaxation behavior of πOF-1 show response to photoirradiation, which could be ascribed to the photo-triggered intermolecular π → π* charge transfer (CT). So far as we know, πOF-1 could be the first Ln-SIM system possessing both a high stability and photoresponsiveness, and also the first photoresponsive Ln-SIM supermolecular system that does not rely on the single-crystal to single-crystal (SCSC) structural transformation. Its successful design and synthesis suggests that the introduction of π···π interactions can be a new strategy to fabricate stable and/or photoresponsive Ln-SIMs.

Single-Crystal to Single-Crystal Transformations: Stepwise CO2 Insertions into Bridging Hydrides of [(NHC)CuH]2 Complexes.

Mechanistic studies of substrate insertion into dimeric [(NHC)CuH]2 (NHC=N-heterocyclic carbene) complexes with two bridging hydrides have been shown to require dimer dissociation to generate transient, highly reactive (NHC)Cu-H monomers in solution. Using single-crystal to single-crystal (SC-SC) transformations, we discovered a new pathway of stepwise insertion of CO2 into [(NHC)CuH]2 without complete dissociation of the dimer. The first CO2 insertion into dimeric [(IPr*OMe)CuH]2 (IPr*OMe=N,N'-bis(2,6-bis(diphenylmethyl)-4-methoxy-phenyl)imidazole-2-ylidene) produced a dicopper formate hydride [(IPr*OMe)Cu]2 (μ-1,3-O2 CH)(μ-H). A second CO2 insertion produced a dicopper bis(formate), [(IPr*OMe)Cu]2 (μ-1,3-O2 CH)(μ-1,1-O2 CH), containing two different bonding modes of the bridging formate. These dicopper formate complexes are inaccessible from solution reactions since the dicopper core cleanly ruptures to monomeric complexes when dissolved in a solvent.

Single‐Crystal to Single‐Crystal Transformations: Stepwise CO2 Insertions into Bridging Hydrides of [(NHC)CuH]2 Complexes

AbstractMechanistic studies of substrate insertion into dimeric [(NHC)CuH]2 (NHC=N‐heterocyclic carbene) complexes with two bridging hydrides have been shown to require dimer dissociation to generate transient, highly reactive (NHC)Cu−H monomers in solution. Using single‐crystal to single‐crystal (SC‐SC) transformations, we discovered a new pathway of stepwise insertion of CO2 into [(NHC)CuH]2 without complete dissociation of the dimer. The first CO2 insertion into dimeric [(IPr*OMe)CuH]2 (IPr*OMe=N,N′‐bis(2,6‐bis(diphenylmethyl)‐4‐methoxy‐phenyl)imidazole‐2‐ylidene) produced a dicopper formate hydride [(IPr*OMe)Cu]2(μ‐1,3‐O2CH)(μ‐H). A second CO2 insertion produced a dicopper bis(formate), [(IPr*OMe)Cu]2(μ‐1,3‐O2CH)(μ‐1,1‐O2CH), containing two different bonding modes of the bridging formate. These dicopper formate complexes are inaccessible from solution reactions since the dicopper core cleanly ruptures to monomeric complexes when dissolved in a solvent.

Water-Induced Single-Crystal to Single-Crystal Transformation of Ionic Hydrogen-Bonded Organic Frameworks with Enhanced Proton Conductivity.

Two ionic hydrogen-bonded organic frameworks (iHOF-10, iHOF-11) were prepared using 1,1'-diamino-4,4'-bipyridine diiodide (Dbpy ⋅ 2I) and tetrakis(4-sulfophenyl)ethylene (H4 TPE). With increasing RH and temperature, water molecules induce single crystal to single crystal (SCSC) transformation of iHOF-10, resulting in the formation of iHOF-11. At 90 °C, 98 % RH, the proton conductivity of iHOF-11 (7.03×10-3 S cm-1 ) is 2.09 times higher than iHOF-10 (3.37×10-3 S cm-1 ). At 50 °C, 98 % RH, iHOF-11 (9.49×10-4 S cm-1 ) is 19.06 times higher than iHOF-10 (4.98×10-5 S cm-1 ). The proton conductivity shows water molecules enter the crystal and induce crystal transformation and reorganization of the hydrogen bonding structure, thus increasing the proton conductivity and stability.

Iron(II) Complexes of 2,6-Di[4-(ethylcarboxy)pyrazol-1-yl]pyridine with Reversible Guest-Modulated Spin-Crossover Behavior.

Three solvatomorphs of the iron(II) complex of 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (bpCOOEt2p) of formulas [Fe(bpCOOEt2p)2](ClO4)2·1.5MeNO2 (1), [Fe(bpCOOEt2p)2](ClO4)2·MeNO2 (2), and [Fe(bpCOOEt2p)2](ClO4)2·2MeNO2 (3) have been prepared and characterized. They show interesting spin-crossover (SCO) properties ranging from partial to complete thermal spin transitions and a light-induced excited spin-state trapping (LIESST) effect. In solvatomorph 2, a robust structure is formed with channels that enable the entrance or removal of solvent molecules by vapor diffusion without losing the crystallinity. Thus, solvent-exchanged samples [Fe(bpCOOEt2p)2](ClO4)2·MeNO2 (2·MeNO 2 ), [Fe(bpCOOEt2p)2](ClO4)2·MeCN (2·MeCN), [Fe(bpCOOEt2p)2](ClO4)2·0.5Me2CO (2·Me 2 CO), and [Fe(bpCOOEt2p)2](ClO4)2·MeCOOH (2·MeCOOH) were prepared by vapor diffusion of the solvents in a crystal of the compound previously heated to 400 K in a single-crystal to single-crystal (SCSC) fashion. Interestingly, this causes a change of spin state with a stabilization of the low-spin state in 2·Me 2 CO and the high-spin state in 2·MeCN. Therefore, the SCO properties of 2 can be tuned in a reversible way by exposure to different solvents.

Structural Transformable Coulomb Lattice of n-Type Semiconductors for Guest Sorption

In recent years, highly designable organic porous materials have attracted considerable attention in the development of new types of molecular adsorption-desorption materials. The adsorption-desorption process also changes the electronic structure via the existence of guest molecules. Therefore, it is possible to change the physical property during the guest adsorption-desorption cycle using an appropriate chemical design of the host crystal lattice. As the development of n-type organic semiconductors has been limited, we focused on designing an n-type organic semiconductor material to control the host crystal lattice, electronic dimensionality, chemical stability, and high electron mobility using an ionic naphthalenediimide (NDI) derivative. Low symmetrical dianionic bis(benzene-m-sulfonate)-naphthalenediimide (m-BSNDI2-) forms various types of single-crystal (M+)2(m-BSNDI2-)·n(guest) with a combination of M+ = Na+, K+, Rb+, and guest = H2O, CH3OH. Four crystals of (K+)2(m-BSNDI2-)·n(H2O), (K+)2(m-BSNDI2-)·n(CH3OH), α-(K+)2(m-BSNDI2-), and β-(K+)2(m-BSNDI2-) were transformable using the guest adsorption-desorption cycle. Two kinds of single-crystal (K+)2(m-BSNDI2-)·n(CH3OH) with n = 0 and 2.0 showed a single-crystal to single-crystal (SCSC) transformation through CH3OH desorption. On the contrary, five kinds of single crystals with n = 0, 3.0, 3.3, 4.75, and 5.5 were identified in the single-crystal X-ray structural analyses of (K+)2(m-BSNDI2-)·n(H2O). Systematic change of the ionic radii in (M+)2(m-BSNDI2-) modified the crystal lattice flexibility for the guest adsorption-desorption cycles.

Single Crystal to Single Crystal Transformation of CuII Complexes Induced by Dehydrating and Hydrating of Ligands with Chroma Rewritable Behaviors.

In this work, the single crystal to single crystal (SCSC) transformations in three mononuclear copper complexes [CuL22]Cl2·2H2O (1), [CuL12Cl2] (2), and [CuL22]Cl2·4H2O (3) (L1 = di-2-pyridyl ketone, L2 = di(pyridin-2-yl)methanediol) are realized by the irreversible dehydration and hydration reaction of L1 and L2. Dark purple crystal 1 is obtained by self-assembly of L1 and CuCl2·2H2O in solvothermal reactions, in which the carbonyl group of L1 undergoes a hydration addition reaction to form L2. On heating, 1 transforms to 2 by dehydrating water accompanied by the change of the color and coordination octahedron of CuII ions. In a saturated water vapor environment, 2 can absorb six water molecules and transform to 3 with the same color and coordination environment with 1 but different lattice water. The SCSC process from 2 to 3 is reversible: 3 can transform back to 2 on heating like that of 1. Chroma rewritable behaviors in the structural transformation of the complexes make them visually identifiable temperature or water probes.

Single-Crystalline Hydrogen-Bonded Crosslinked Organic Frameworks and Their Dynamic Guest Sorption

ConspectusPorous organic framework materials constructed by periodically aligned molecular entities offer chemically tailored microenvironments to absorb molecules and ions for various applications. Fundamentally understanding the microenvironments of these porous organic materials─from pore size, shape, and dynamics to potential substrate binding sites─is critical for the rational design of porous organic materials. The solid-state structures of these porous organic materials, such as covalent organic frameworks (COFs), can provide unambiguous atomic-level structural details. However, it remains challenging to synthesize these materials as single crystals that can be fully characterized by single-crystal X-ray diffraction (SCXRD) or rotational electron diffraction (RED) analysis. In addition, the balance of single crystallinity, permanent porosity, and good chemical stability requires delicate control of the assembly of the molecular building blocks and covalent crosslinking during synthesis. In this Account, we discuss the development of hydrogen-bonded crosslinked organic frameworks (HCOFs) possessing balanced single crystallinity and high chemical stability. HCOFs are obtained through covalently crosslinking molecular crystals that are preorganized via hydrogen bonding. Due to the dual hydrogen-bonded network and covalent crosslinking, HCOFs can deform upon guest adsorption by breaking the hydrogen bonds and subsequently restore their original form through the desorption of guests by re-establishing the hydrogen-bonded networks. Thus, HCOFs can dynamically adjust their pore sizes according to the framework–substrate interactions. In the discussion, we link HCOFs with COFs and single-crystalline 2D polymers by comparing their synthetic approaches to accessing high crystallinity. The method to synthesize HCOFs allows for the employment of various flexible building blocks and linking motifs that are largely avoided in the current design regimes of COFs and 2D polymers. We also draw the connections between HCOFs and hydrogen-bonded organic frameworks (HOFs) by highlighting their shared design principles for constructing hydrogen-bonding networks with large voids. Compared to their hydrogen-bonded precursor crystals, reinforcing the hydrogen-bonded networks with covalent linkages endows HCOFs with enhanced chemical and structural stability. In addition, we emphasize that the structure elucidation of HCOFs often requires combined SCXRD analysis and experimental evidence, with the methods and challenges thoroughly discussed. The details are presented in the following sequence: (1) synthesizing single-crystalline COFs by matching the polycondensation rate to the nucleation rate and their subsequent analyses by SCXRD/RED; (2) obtaining single-crystalline polymers and networks through topochemical reactions; (3) constructing HOFs with designed voids using highly directional hydrogen bonding building blocks; and (4) developing HCOFs via monomer crystal engineering followed by single-crystal to single-crystal (SCSC) synthesis and studying their unique dynamic guest sorption behaviors. We hope this Account will inspire researchers to expand the synthetic methods for advancing HCOFs with detailed solid-state structures, as well as designing porous organic framework materials with dynamic sorption capabilities to enhance their performance for applications in molecular storage, separation, catalysis, etc.

Photoreactive Crystals Exhibiting [2 + 2] Photocycloaddition Reaction and Dynamic Effects.

ConspectusConducting a reaction in the solid state eliminates the usage of solvents. If such reactions are conducted in a single-crystal to single-crystal (SCSC) fashion, then structural characterization by single-crystal X-ray crystallography (SCXRD) techniques provides unequivocal structural details. Although topochemical principles govern, getting single crystals at the end of a SCSC reaction purely depends on the experimental skills of the researchers. SCSC reactions are common among solid-state [2 + 2] cycloaddition reactions (hereafter "photoreaction") after the classical work of Schmidt and co-workers in 1960s. Synthons and tectons in the crystal engineering box can be exploited to bring the functional groups into the required alignment and packing to achieve the desired chemical reactivities and physical properties, respectively. Bringing a pair of alkenes closer together in the organic molecules provides an effective starting point to achieve the goal of crystal engineering.Further, understanding and controlling photoreactivity in the solid state provide a gateway to designing new advanced materials, for example, making cycloreversible optical storage materials, photosalient and photomechanical materials, highly crystalline or even single-crystalline organic polymers, covalent organic framework structures, and organic polymers incorporated inside metal-organic frameworks (MOFs). Photoreactions often proceed in a SCSC manner due to the limited movements of the closely disposed reactive functional groups in the crystals. Thus, these photoreactions yield not only quantitative photoproducts but also regio- and stereospecificity, which are otherwise inaccessible by solution syntheses.The traditional definition of crystals being hard, rigid, and brittle is no longer valid ever since the mechanically responsive crystals were discovered. These dynamic crystals undergo various movements like curling, jumping, hopping, popping, splitting, and wiggling, when exposed to light (called "photosalient effect") or heat (called "thermosalient" effect). These crystals generate new methods of transforming light and heat energy into mechanical work. Recently, photosalient behavior during the [2 + 2] cycloaddition reaction under UV light has been frequently observed. With the emergence of the field of "crystal adaptronics", dynamic photoreactive crystals have emerged as smart actuating materials.This Account aims to provide an overview of the development in this area, since it has garnered much attention among solid state chemists. While presenting selected examples of important strategies, we try to illustrate the intentions and concepts behind the methods developed, which will help in a rational approach for the fabrication of advanced solid state materials. Apart from topochemical transformations, the important roles played by weak interactions, guest solvents, and mechanical grinding have been highlighted in several classes of compounds to show structural transformations that defy the expected outcomes. Overall, the progress of [2 + 2] cycloaddition reaction in solid state materials has been discussed from UV induced structural transformations to the development of smart actuating materials.

Photoinduced Single-Crystal to Single-Crystal Transformation via Conformational Change with Turn-On Fluorescence

Single-crystal to single-crystal (SCSC) transformation by phototrigger is presently attracting immense research interest in many fields such as sensors, actuators, artificial muscles, soft robotics, and energy harvesting. However, the only success of rare examples has been achieved by SCSC transformation so far, involving a change in molecular composition based on photocyclization reactions. Here, we show photoinduced SCSC transformation of 3-methylene-2-(quinolin-8-yl) isoindolin-1-one (MQIO) derivatives from monoclinic to triclinic phase via conformational change, just as “small input and big output”. UV light induces a molecular conformation change of MQIO-H with rotation (11.3°) of the carbon–nitrogen single bond. The conformational changes of MQIO-H lead to the slides of molecular packing in crystals, finally resulting in the SCSC transformation from the monoclinic to the triclinic phase. In addition, the SCSC transformation was accompanied by turn-on fluorescence and shape changes of single crystal. The shape changes of single crystals can be clearly observed by UV light triggering under an optical microscope. This work provides a strategy to achieve SCSC transformation via photoinduction.

Topochemical [2 + 2] Cycloaddition in a Two-Dimensional Metal-Organic Framework via SCSC Transformation Impacts Halogen···Halogen Interactions.

A photoactive two-dimensional metal-organic framework (2D MOF) [Zn(4-spy)(DCTP)]n (1) [where 4-spy = 4-styrylpyridine and H2DCTP = 2,5-dichloroterephthalic acid] undergoes photochemical [2 + 2] cycloaddition on UV irradiation to obtain three-dimensional (3D) MOF [Zn(rctt-4-ppcb)(DCTP)]n (2) [rctt-4-ppcb = 1,3-bis(4'-pyridyl)-2,4-bis(phenyl)cyclobutane] in a single-crystal to single-crystal (SCSC) manner. This structural transformation leads to stronger halogen···halogen interaction that is well-corroborated by density functional theory (DFT) calculations.