An Elastic Organic Crystal Enables Macroscopic Photoinduced Crystal Elongation

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.

Spatiotemporal Control of Photoisomerization Dynamics via Domino Barriers for Programmatically Responsive Heterostructures

Controlling the photoisomerization reaction at the micro-/nanoscale is important for the realization of high-end photonic components. Unfortunately, spatiotemporal manipulation of the photoisomerization dynamics still faces a significant challenge. Here, we propose an effective strategy to control the photoisomerization reaction spatiotemporally through introducing a steric-hindrance effect by the aid of alloy engineering. The external guest molecules behave like domino barriers and efficiently regulate the photoisomerization dynamics. Moreover, the flexible assembly of the organic heterostructures with different steric-hindrance degrees enabled us to spatiotemporally modulate the photoisomerization dynamics in 1D, 2D, and even annular morphologies. Interestingly, the photoisomerization reaction exhibits anisotropic change characteristics in 2D microcrystals. Our work provides deep insight into the modulation of the photoisomerization reaction and would promote the development of smart responsive barcodes with improved security level toward advanced anti-counterfeiting applications.

Photoinduced On and Off Polymeric Room Temperature Phosphorescence Based On Polycyclic Aromatic Hydrocarbon Isomers

As family members of polycyclic aromatic hydrocarbons, compound anthracene (Ant) and phenanthrene (Phe) as isomers are widely used in organic optical materials and electronic materials. But their photochemical and physical properties are very different. In this work, the room temperature phosphorescence (RTP) properties of PVA-B-Ant and PVA-B-Phe are discussed carefully which are prepared by B-O click reaction through polyvinyl alcohol (PVA) with 9-anthraceneboronic acid (B-Ant) and 9-phenanthrenylboronic acid (B-Phe), respectively. PVA-B-Phe 1 % film exhibits excellent fluorescence (FL) emission at 374 nm and RTP emission at 523 nm with green afterglow and around 1.9 s phosphorescence lifetime. However, PVA-B-Ant 1 % film only shows strong blue FL emission at 414 nm, and the emission intensity decreases seriously with the extension of irradiation time. Experimental and theoretical calculations results suggest that the photodimer of Ant which is formed in PVA matrix under the UV light irradiation would be competitive with the process of RTP emission. This work demonstrates that the RTP properties of organic molecules might be probably affected by the photostability of the organic phosphor under UV irradiation.

Photomechanical Crystals as Light-Activated Organic Soft Microrobots

In the field of materials science, dynamic molecular crystals have attracted significant attention as a novel class of energy-transducing materials. However, their development into becoming fully functional actuators remains somewhat limited. This study focuses on one family of dynamic crystalline materials and delves into exploring the efficiency of conversion of light energy to mechanical work. A simple setup is designed to determine a set of performance indices of anthracene-based crystals as an exemplary class of dynamic molecular crystals. The ability of these crystals to reversibly bend due to dimerization is realistically assessed from the perspective of the envisaged soft robotics applications, where wireless photomechanical grippers manipulate and assemble microscopic objects driven and controlled by light instead of lines and motors. The approach described here not only guides the quantification of responsive molecular crystals' actuation potential but also aims to attract an interdisciplinary interest to further develop this class of materials into controllable all-organic actuating elements to be used in microrobotics for engineering or biomedicine.

Gas Release as an Efficient Strategy to Tune Mechanical Properties and Thermoresponsiveness of Dynamic Molecular Crystals

Dynamic molecular crystals combining multiple and finely tunable functionalities are attracting and an increasing attention due to their potential applications in a broad range of fields as efficient energy transducers and stimuli-responsive materials. In this context, a multicomponent organic salt, piperazinium trifluoroacetate (PZTFA), endowed with an unusual multidimensional responsive landscape is reported. Crystals of the salt undergo smooth plastic deformation under mechanical stress and thermo-induced jumping. Furthermore, via controlled crystal bending and release of trifluoroacetic acid from the lattice, which is anticipated from the design of the material, both the mechanical response and the thermoresponsive behavior are efficiently tuned while partially preserving the crystallinity of the system. In particular, mechanical deformation hampers guest release and hence the macroscopic jumping effect, while trifluoroacetic acid release stiffens the crystals. These complex adaptive responses establish a new crystal engineering strategy to gain further control over dynamic organic crystals.

Trimodal operation of a robust smart organic crystal

We describe a dynamic crystalline material that integrates mechanical, thermal, and light modes of operation, with unusual robustness and resilience and a variety of both slow and fast kinematic effects that occur on very different time scales. In the mechanical mode of operation, crystals of this material are amenable to elastic deformation, and they can be reversibly morphed and even closed into a loop, sustaining strains of up to about 2.6%. Upon release of the external force, the crystals resume their original shape without any sign of damage, demonstrating outstanding elasticity. Application of torque results in plastic twisting for several rotations without damage, and the twisted crystal can still be bent elastically. The thermal mode of operation relies on switching the lattice at least several dozen times. The migration of the phase boundaries depends on the crystal habit. It can be precisely controlled by temperature, and it is accompanied by both slow and fast motions, including shear deformation and leaping. Parallel boundaries result in a thermomechanical effect, while non-parallel boundaries result in a thermosalient effect. Finally, the photochemical mode of operation is driven by isomerization and can be thermally reverted. The structure of the crystal can also be switched photochemically, and the generation of a bilayer induces rapid bending upon exposure to ultraviolet light, an effect that further diversifies the mechanical response of the material. The small structural changes, low-energy and weak intramolecular hydrogen bonds, and shear deformation, which could dissipate part of the elastic energy, are considered to be the decisive factors for the conservation of the long-range order and the extraordinary diversity in the response of this, and potentially many other dynamic crystalline materials.

Bending, Twisting, and Propulsion of Photoreactive Crystals by Controlled Gas Release

The rapid release of gas by a chemical reaction to generate momentum is one of the most fundamental ways to elicit motion that could be used to sustain and control the motility of objects. We report that hollow crystals of a three-dimensional supramolecular metal complex that releases gas by photolysis can propel themselves or other objects and advance in space when suspended in mother solution. In needle-like regular crystals, the reaction occurs mainly on the surface and results in the formation of cracks that evolve due to internal pressure; the expansion on the cracked surface of the crystal results in bending, twisting, or coiling of the crystal. In hollow crystals, gas accumulates inside their cavities and emanates preferentially from the recess at the crystal terminus, propelling the crystals to undergo directional photomechanical motion through the mother solution. The motility of the object which can be controlled externally to perform work delineates the concept of "crystal microbots", realized by photoreactive organic crystals capable of prolonged directional motion for actuation or delivery. Within the prospects, we envisage the development of a plethora of light-weight, efficient, autonomously operating robots based on organic crystals with high work capacity where motion over large distances can be attained due to the large volume of latent gas generated from a small volume of the crystalline solid.

One-Pot Synthesis of Bis(arylamino)pentiptycenes by TiCl4-DABCO Assisted Reductive Amination of Pentiptycene Quinone

The previously eight-step synthesis of bis(arylamino)pentiptycenes (2) from pentiptycene quinone (1) can now be achieved in a single step with 18-90% yields through TiCl4-DABCO assisted reductive amination with anilines. Both the dual amination of 1 and the in situ reduction of quinone diimines are unprecedented. The π system of 2 can be further expanded, including the formation of bis(diarylamino)pentiptycenes. This work also provides mechanistic insights into the challenges encountered in the dual reductive amination of 1 with other amines.

Photoactuators Based on Plastically Flexible α-Cyanostilbene Molecular Crystals Driven by the Solid-State [2+2] Cycloaddition Reaction

Organic photomechanical molecular crystals are promising candidates for photoactuators, which have potential applications as smart materials in various fields. However, it is still challenging to fabricate photomechanical molecular crystals with flexibility because most of the molecular crystals are brittle and the mechanism of flexible crystals remains controversial. Here, a plastically flexible α-cyanostilbene crystal has been synthesized that can undergo solid-state [2+2] cycloaddition reaction under violet or UV irradiation and exhibits excellent photomechanical bending properties. A hook-shaped crystal can lift 0.7 mg object upward by 1.5 cm, which proves its potential for application as photoactuators. When complex with the agarose polymer, the molecules will be in the form of macroscopic crystals, which can drive the composite films to exhibit excellent photomechanical bending performance. Upon irradiation with UV light, the composite film can quickly lift 18.0 mg object upward by 0.3 cm. The results of this work may facilitate the application of macroscale crystals as photoactuators.

Bifurcated Polymorphic Transition and Thermochromic Fluorescence of a Molecular Crystal Involving Three-Dimensional Supramolecular Gear Rotation

The ability of molecules to move and rearrange in the solid state accounts for the polymorphic transition and stimuli-responsive properties of molecular crystals. However, how the crystal structure determines the molecular motion ability remains poorly understood. Here, we report that a three-dimensional (3D) supramolecular gear network in the green-emissive polymorph 1G of a dialkylamino-substituted anthracene-pentiptycene π-system (1) enables an unusual bifurcated polymorphic transition into a yellow-emissive polymorph (1Y) and a new green-emissive polymorph (1G*) via 3D correlated supramolecular rotation. The 90° forward correlated rotation causes the molecular conformation between the octyl and the anthracene units to change from syn to anti, the ladder-like supramolecular columns to constrict, and the gear network to disengage. This cooperative molecular motion is marked by the gradual formation of an intermediate state (1I) across the entire crystal from 170 to 230 °C, which then undergoes bifurcated (forward or backward rotation) and irreversible transitions to form polymorphs 1Y and 1G* at 230-235 °C. Notably, 1G* is similar to 1G but lacks gear engagement, preventing its transformation into 1Y. Nevertheless, 1G can be restored by grinding 1Y or 1G* or fuming with dichloromethane (DCM) vapor. This work illustrates the correlation between the crystal structure and solid-state molecular motion behavior and demonstrates how a 3D molecular gear system efficiently transmits thermal energy to drive the polymorphic transition and induce fluorochromism through significant conformational and packing changes.

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.

Negative to positive axial thermal expansion switching of an organic crystal: contribution to multistep photoactuation

Materials displaying negative thermal expansion (NTE), in contrast to typical materials with positive thermal expansion (PTE), are attractive for both fundamental research and practical applications, including the development of composites with near-zero thermal expansion. A recent data mining study revealed that approximately 34% of organic crystals may present NTE, indicating that NTE in organic crystals is much more common than generally believed. However, organic crystals that switch from NTE to PTE or vice versa have rarely been reported. Here, we report the crystal of N-3,5-di-tert-butylsalicylide-3-nitroaniline in the enol form (enol-1) as the first organic crystal in which the axial thermal expansion changes from negative to positive at around room temperature. When heated, the crystal shrinks along the a-axis below 30 °C and then it expands above 30 °C. Geometric calculations revealed that below 30 °C, the decrease in the tilt angle of the molecule exceeds the increase in the interplanar distance, causing NTE, whereas above 30 °C, the increase in the interplanar distance outweighs the decrease in the tilt angle, resulting in PTE. By combining photoisomerisation and the NTE-PTE switching induced by the photothermal effect, multistep crystal photoactuation was achieved. Moreover, actuation switching of the same crystal sample by changing atmosphere temperature was realised by utilising the NTE-PTE change. Such NTE-PTE switching without a thermal phase transition provides not only new insight into organic crystals but also a new strategy for designing crystal actuators.

Light-driven flagella-like motion of coordination compound single crystals

Single crystals of coordination complexes that show mechanical motion under the influence of external stimuli are of great interest due to their applications in photoactuators, sensors and probes. The solid-state [2+2] cycloaddition reaction has been one of the most prominent chemical reactions for photoresponsive materials in recent years. However, a relatively limited number of compounds have been reported, and most of these compounds have only shown destructive photosalient effects. Here, we report two photoreactive Zn(II) metal complexes with a thiophene-based photoreactive linker, 2tpy (4-(2-(thiophen-2-yl)vinyl)pyridine). In addition, under photoirradiation these complexes showed flagella-like bending, first towards and subsequently away from the excitation light source. This is the first report of metal-complexes and the solid-state [2+2] cycloaddition reaction that presents flagella-like motion in single crystals.