Regulating photosalient behavior in dynamic metal-organic crystals

Manipulating Dynamic Light-Driven Solid-Liquid Transition and Static Reversible Photochromism by an Organic Cocrystal Strategy

Developing of molecular crystalline materials with light-induced multiple dynamic deformation in space dimension and photochromism on time scales has attracted much attention for its potential applications in actuators, sensoring and information storage. Nevertheless, organic crystals capable of both photoinduced dynamic effects and static color change are rare, particularly for multi-component cocrystal system. In this study, we first report the construction of charge transfer cocrystal allows their light-induced solid-to-liquid transition and photochromic behaviors to be controlled by trans-stilbene (TSB) as an electron donor and 3,4,5,6-tetrafluorophthalonitrile (TFP) as an electron acceptor. In this case, the dynamic photo-responsive solid-to-liquid phase transition is due to the photoisomerization of TSB under UV light irradiation, while the accumulation of melted droplets during solid-state photochemical process causes mechanical deformation of TSB-TFP cocrystals. The subsequent reversible photochromic behavior is attributed to the emergence of free radicals through a photo-induced electron transfer. Moreover, TSB-TFP microcrystals present typical excitation wavelength dependent emission (EWDE) fluorescence by surfactant-mediated method. This work realizes the dynamic-static photochemical cascade processes in response to UV light irradiation in an organic cocrystal system, providing the effective method for a new type of smart photo-responsive materials.

Salts of trans-3-Styryl Pyridine: The Effect of N-Quaternization on Solid-State Photoreactivity, and Photochemical Crystal Melting

Investigating solid-state photoreactivity, driven by crystal packing, has been a major enduring research theme in Crystal Engineering. Trans-3-styryl pyridine (3-StPy), an unsymmetric olefin, is photo-stable. However, when converted to a series of salts, they exhibited solid-state photoreactivity under UV irradiation. Crystal structures of 3-StPy, its protonated salts, namely, (3-StPyH)(HSO4) ⋅ H2O (1), (3-StPyH)(CF3CO2) (2), and (3-StPyH)(p-Tol-SO3) (3), and methylated salt (3-StPyMe)I (4) were determined by X-ray diffraction. 3-StPy molecules were found not to align in the parallel arrangement required to undergo solid-state [2+2] photocycloaddition reaction; however, upon protonation and methylation, the unsymmetric 3-StPyH+ and [3-StPyMe]+ cations aligned in a head-to-tail parallel arrangement, predominantly governed by the cation⋅⋅⋅π interactions. Various structural features, e. g., the patterns of hydrogen bonding, etc. have been addressed, and established by Hirshfeld surface analysis. The salt with p-tolyl sulfonate anion (3) with Z'>1 represents a rare crystal class. It was also noted that the crystals of 1 and 2 melted into a liquid state upon photodimerization reaction. UV-vis absorption and fluorescence properties have been explored. The electronic structures of 3-StPy, 3-StPyH+, [3-StPyMe]+, and the dimer [3,3'-MPyPhCB]2+ cations have been elucidated by DFT calculations, and the effect of N-quaternization on crystal structures and photophysical properties has been discussed.

Light-Driven Adaptive Molecular Crystals Activated by [2+2] and [4+4] Cycloadditions

Photomechanical crystals act as light-driven material-machines that can convert the energy carried by photons into kinetic energy via shape deformation or displacement, and this capability holds a paramount significance for the development of photoactuated devices. This transformation is usually attributed to anisotropic expansion or contraction of the unit cell engendered by light-induced structural modifications that lead to accumulation and release of stress that generates a momentum, resulting in readily observable mechanical effects. Among the available photochemical processes, the photoinduced [2+2] and [4+4] reactions are known for their robustness, predictability, amenability to control with molecular and supramolecular engineering approaches, and efficiency that has already been elevated to a proof-of-concept smart devices based on organic crystals. This review article presents a summary of the recent research progress on photomechanical properties of organic and metal-organic crystals where the mechanical effects are based on [2+2] and [4+4] cycloaddition reactions. It consolidates the current understating of the chemical strategies and structure-property correlations, and highlights the advantages and drawbacks of this class of adaptive crystals within the broader field of crystal adaptronics.

Precise Photochemical Post-Processing of Molecular Crystals

Molecular crystals carry a great potential as new soft smart materials, with a plethora of recent examples overcoming the major obstacle of mechanical flexibility, and this research direction holds enormous potential to revolutionize optics, electronics, medicine, and space exploration. However, shaping organic crystals into desired shapes and sizes remains a major practical challenge due to the lack of control over the crystallization process, and the difficulties in mechanical post-processing without introduction of defects that are usually imparted by their soft nature. Here we present an innovative approach that employs photochemical processing for precise and nondestructive cutting of a molecular crystal. Our proposed method uses light to post-process crystals of the desired size and shape, similar to using light to cut other materials. This reaction induces strain, ensuring sharp cleavage without the need for melting or other processes. We further demonstrate the potential of this approach by producing crystals of arbitrary size, which can be used as controllable optical waveguides. Among other potential applications, this method can be used to prepare dynamic crystals, particularly those with aspect ratios crucial for mechanical deformation, such as flexible electronics, soft robotics, and sensing.

Polymorph driven diversification of photosalient responses in a zinc(II) coordination complex

A novel crystallographic form of a Zn(II) coordination complex [Zn(4-ohbz)2(4-nvp)2] (1-Form-III) (H4-ohbz = 4-hydroxybenzoic acid and 4-nvp = (E)-4-(1-naphthylvinyl)pyridine), undergoes a solid-state photochemical [2+2] cycloaddition reaction accompanied by a moderate photosalient effect, whereby single-crystals show cracking and splitting. This UV-induced cycloaddition accompanies a single-crystal to single-crystal transformation, allowing for continuous monitoring of the unit cell parameters. The new polymorph represents an intermediate form of the two previously reported crystallographic forms of [Zn(4-ohbz)2(4-nvp)2], and provides novel insight into moderating the magnitude of photosalient responses across polymorphic materials.

Photomechanical properties in metal-organic crystals

The emergence of materials that can effectively convert photon energy (light) into motion (mechanical work) and change their shapes on command is of great interest for their potential in the fabrication of devices (powered by light) that will revolutionize the technologies of optical actuators, smart medical devices, soft robotics, artificial muscles and flexible electronics. Recently, metal-organic crystals have emerged as desirable smart hybrid materials that can hop, split and jump. Thus, their incorporation into polymer host objects can control movement from molecules to millimetres, opening up a new world of light-switching smart materials. This feature article briefly summarizes the recent part of the fast-growing literature on photomechanical properties in metal-organic crystals, such as coordination compounds, coordination polymers (CPs), and metal-organic frameworks (MOFs). The article highlights the contributions of our group along with others in this area and aims to provide a consolidated idea of the engineering strategies and structure-property relationships of these hybrid materials for such rare phenomena with diverse potential applications.

Construction of olefinic coordination polymer single crystal platforms: precise organic synthesis, in situ exploration of reaction mechanisms and beyond

Olefin [2+2] photocycloaddition reactions based on coordination-bond templates provide numerous advantages for the selective synthesis of cyclobutane compounds. This review outlines the recent advances in the design and construction of single crystal platforms of olefinic coordination polymers for precise organic synthesis, in situ exploration of reaction mechanisms, and possible developments as comprehensively as possible. Numerous examples are presented to illustrate how the arrangements of the olefin pairs influence the solid-state photoreactivity and examine the types of cyclobutane products. Furthermore, the photocycloaddition reaction mechanisms are investigated by combining advanced techniques such as single crystal X-ray diffraction, powder X-ray diffraction, nuclear magnetic resonance, infrared spectroscopy, fluorescence spectroscopy, laser scanning confocal microscopy and theoretical calculations. Finally, potential applications resulting from promising physicochemical properties before and after photoreactions are discussed, and existing challenges and possible solutions are also proposed.

Photoinduced Puffing with Large Volume Expansion and Photomechanical Motions induced by Topochemical [4+4] Reactions in Molecular Crystal Solvates

In this work, we report the first example of two crystal solvates of an anthracene-benzhydrazide based molecule (Ant) that display very distinct photo-responsive behaviour when 365 or 405 nm or visible light is illuminated. For the first time, the crystal hydrate that has water molecule in the lattice (hereafter named as Ant-H2O) display fascinating puffing behavior with large volume expansion upto 50 % accompanied with surface modulation when illuminated with 405 nm light, a phenomenon very much similar to the rice or popcorn puffing by thermal treatment. Utilizing the properties of photoconverted Ant-H2O crystals, we have demonstrated their application in photoinduced enhanced liquid absorption using various liquids/solutions. The other crystal solvate having DMF in the crystal lattice (hereafter named as Ant-DMF) responds to 405 nm light by bending, twisting, chopping, jumping or splitting etc. The chopping of Ant-DMF crystal was also observed under ambient/white light but at a slower rate compared to 405 nm light. Single crystal X-ray diffraction study reveals that the photoinduced puffing and photomechanical effects of these materials are rooted to the topochemical [4+4] cycloaddition reaction between the anthracene moieties that facilitate molecular packing change assisted by the reconfiguration of intermolecular non-covalent interactions involving lattice trapped solvent molecules.

Hybrid and composite materials of organic crystals

Organic molecular crystals have historically been viewed as delicate and fragile materials. However, recent studies have revealed that many organic crystals, especially those with high aspect ratios, can display significant flexibility, elasticity, and shape adaptability. The discovery of mechanical compliance in organic crystals has recently enabled their integration with responsive polymers and other components to create novel hybrid and composite materials. These hybrids exhibit unique structure-property relationships and synergistic effects that not only combine, but occasionally also enhance the advantages of the constituent crystals and polymers. Such organic crystal composites rapidly emerge as a promising new class of materials for diverse applications in optics, electronics, sensing, soft robotics, and beyond. While specific, mostly practical challenges remain regarding scalability and manufacturability, being endowed with both structurally ordered and disordered components, the crystal-polymer composite materials set a hitherto unexplored yet very promising platform for the next-generation adaptive devices. This Perspective provides an in-depth analysis of the state-of-the-art in design strategies, dynamic properties and applications of hybrid and composite materials centered on organic crystals. It addresses the current challenges and provides a future outlook on this emerging class of multifunctional, stimuli-responsive, and mechanically robust class of materials.

A dual-functional 2D coordination polymer exhibiting photomechanical and electrically conductive behaviours

A photoactive two-dimensional coordination polymer (2D CP) [Zn2(4-spy)2(bdc)2]n (1) [4-spy = 4-styrylpyridine and H2bdc = 1,4-benzendicarboxylic acid] undergoes a photochemical [2 + 2] cycloaddition reaction upon UV irradiation. Interestingly, the crystals of 1 show different photomechanical effects, such as jumping, swelling, and splitting, during UV irradiation. In addition, the CP was employed for conductivity measurements before and after UV irradiation via current density-voltage characteristics and impedance spectroscopy, which suggest that they are semiconducting in nature and can be used as Schottky diodes. Thus, this work demonstrates the potential dual applications of a 2D CP based on photosalient and conductivity properties.

Photomechanical effects based on a one-dimensional Zn coordination polymer crystal driven by [4 + 4] cycloaddition

Photomechanical crystals are promising candidates for photo actuators due to their ability to convert light energy into mechanical energy. We synthesized a coordination polymer crystal that can undergo [4 + 4] cycloaddition reactions with mechanical motion. The inclusion of {[ZnL2(4,4'-bipy)(CH3OH)2]}∞ in a polymer membrane significantly magnified the actuation behavior.

Construction of Photoreactive Chiral Metal-Organic Frameworks and Their [2 + 2] Photocycloaddition Reactions

Chiral metal-organic frameworks (CMOFs) and solid-state [2 + 2] photocyclization have been explored as independent areas in crystal engineering. We herein report the photoreactive CMOFs that undergo a [2 + 2] photocycloaddition reaction for the first time. Through the incorporation of a dipyridyl olefin ligand, 1,4-bis[2-(4-pyridyl)ethenyl]benzene, and d-camphoric acid or l-camphoric acid, we constructed a pair of homochiral Zn(II) CMOFs (d-1 or l-1) with a two-dimensional sql topology via a two-step procedure to avoid racemization. Both d-1 and l-1 were photoinert due to the large olefin bond separation. The removal of the solvent molecules between layers enabled them (d-1a and l-1a) to undergo [2 + 2] cycloaddition reactions; d-1a is more reactive (70%) than l-1a (20%) probably due to proper desolvation-induced rearrangement. The photoluminescence properties are also discussed. This work presents a new perspective on photoreactive homochiral network materials with diverse topologies and applications.