Extraordinary anisotropic thermal expansion in photosalient crystals

Releasing a bound molecular spring with light: a visible light-triggered photosalient effect tied to polymorphism

Here we present a study on the solid state properties of trans tetra-ortho-bromo azobenzene (4Br-Azo). Two distinct crystal polymorphs were identified: the α-phase and β-phase. Notably, only the β-phase exhibited an extraordinary photosalient effect (jumping/breaking) upon exposure to a wide range of visible light. Powder X-ray diffraction and Raman spectroscopy revealed that the β-phase is metastable and can transition to the α-phase when subjected to specific stimuli like heat and light. Furthermore, single crystal X-ray diffraction and density functional theory calculations highlighted the significance of a highly strained conformer in the β-phase, showing that the metastability of the phase potentially arises from relieving this strain. This metastability leads to a light induced phase transition, which appears to be the cause of the photosalient effect in these crystals. Interestingly the polymorphism at the core of 4Br-Azo's dynamic behavior is based on different arrangements of halogen based intermolecular interactions. It is possible that continued study on combining visible light capturing chromophores with halogen interaction-based polymorphism will lead to the discovery of even more visible light controlled dynamic crystal materials.

Solid-State [2+2] Photoreaction of Isostructural Cd(II) Metal Complexes and Solid-State Fluorescence

A green method to synthesize cyclobutane derivatives has been developed over the past three decades in the form of solid-state [2+2] photochemical reactions. These solid-state reactions also play a major role in the structural transformation of hybrid materials. In this regard, crystal engineering has played a major role in designing photoreactive molecular systems. Here, we report three novel binuclear Cd(II) complexes with the molecular formula [Cd2(4spy)4L4], where 4spy = 4-styryl pyridine and L = p-toluate (1); 4-fluorobenzoate (2); and 3-fluorobenzoate (3). Although three different benzoates are used, all three complexes are isostructural, as corroborated through SCXRD experiments. Structural analysis also helped in identifying two potential photoreactions. These are both intra- and intermolecular in nature and are driven by the head-to-head (HH) and head-to-tail (HT) alignment of 4spy linkers within these metal complexes. 1H NMR spectroscopy studies showed evidence of a quantitative head-to-head photoreaction in all these three complexes, and SCXRD analysis of the recrystallization of the photoproducts also provided confirmation. TGA studies of these photoreactive complexes showed an increase in the thermal stability of the complexes due to the solid-state photoreaction. Photoluminescence studies of these complexes have been conducted, showing a blue shift in emission spectra across all three cases after the photoreaction.

Regulating photosalient behavior in dynamic metal-organic crystals

Dynamic photoactuating crystals have become a sensation due to their potential applications in developing smart medical devices, molecular machines, artificial muscles, flexible electronics actuators, probes and microrobots. Here we report the synthesis of two iso-structural metal-organic crystals, [Zn(4-ohbz)₂(4-nvp)₂] (1) and [Cd(4-ohbz)₂(4-nvp)₂] (2) {H4-ohbz = 4-hydroxy benzoic acid; 4-nvp = 4-(1-naphthylvinyl)pyridine} which undergo topochemical [2 + 2] cycloaddition under UV irradiation as well as sunlight to generate a dimerized product of discrete metal-complex [Zn(4-ohbz)₂(rctt-4-pncb)] {rctt-4-pncb = 1,3-bis(4'-pyridyl)-2,4-bis(naphthyl)cyclobutane} (1') and one-dimensional coordination polymer (1D CP) [Cd(4-ohbz)₂(rctt-4-pncb)] (2') respectively, in a single-crystal-to-single-crystal (SCSC) process. The Zn-based compound demonstrates photosalient behaviour, wherein crystals show jumping, splitting, rolling, and swelling upon UV irradiation. However, the Cd-based crystals do not show such behaviour maintaining the initial supramolecular packing and space group. Thus the photomechanical behaviour can be induced by choosing a suitable metal ion. The above findings are thoroughly validated by quantitative density functional theory (DFT) calculations which show that the Zn-based crystal shifts towards an orthorhombic structure to resolve the anisotropic UV-induced mechanical strain. Furthermore, the mechano-structure-property relationship has been established by complimentary nanoindentation measurements, which are in-line with the DFT-predicted single crystal values.

Organic Crystals with Response to Multiple Stimuli: Mechanical Bending, Acid-Induced Bending and Heating-Induced Jumping

Multiple stimuli-responsive molecular crystals are attracting extensive attentions due to their potential as smart materials, such as molecular machines, actuators, and sensors. However, the task of giving a single crystal multiple stimuli-responsive properties remains extremely challenging. Herein, we found two polymorphs (Form O and Form R) of a Schiff base compound, which could respond to multiple stimuli (external force, acid, heat). Form O and Form R have different elastic deformability, which can be attributed to the differences in the molecular conformation, structural packing and intermolecular interactions. Moreover, both polymorphs exhibit reversible bending driven by volatile acid vapor, which we hypothesize is caused by reversible protonation reaction of imines with formic acid. In addition, jumping can be triggered by heating due to the significant anisotropic expansion. The integration of reversible bending and jumping into one single crystal expands the application scope of stimuli-responsive crystalline materials.

Photomechanical Motions in Organoboron-Based Phosphorescent Molecular Crystals Driven by a Crystal-State [2 + 2] Cycloaddition Reaction

Photoluminescent molecular crystals integrated with the ability to transform light energy into macroscopic mechanical motions are a promising choice of materials for both actuating and photonic devices. However, such dynamic photomechanical effects, based on molecular organoboron compounds as well as phosphorescent crystalline materials, are not yet known. Here we present an intriguing example of photomechanical molecular single crystals of a newly synthesized organoboron containing Lewis acid-base molecular adduct (BN1, substituted triphenylboroxine and 1,2-di(4-pyridyl)ethylene) having a capsule shape molecular geometry. The single crystals of BN1 under UV light exhibit controllable rapid bending-shape recovery, delamination, violent splitting-jumping, and expanding features. The detailed structural investigation by single-crystal X-ray diffraction and ¹H NMR spectroscopy reveals that the photosalient behavior of the BN1 single crystals is driven by a crystal-to-crystal [2 + 2] cycloaddition reaction, supported by four donor-acceptor type B←N bonds. The instant photomechanical reaction in the BN1 crystals occurs under UV on account of sudden release of stress associated with the strained molecular geometry, significant solid-state molecular movements (supramolecular change), and cleavage of half intermolecular B←N linkages to result in a complete photodimerized single-crystalline product via the existence of two other intermediate photoproducts. In addition, the BN1 crystals display short-lived room temperature phosphorescence, and the photodynamic events are accompanied by the enhancement of their phosphorescence intensity to yield the photoproduct. Interestingly, the molecular crystals of the final photoproduct polymerize at ambient conditions when recrystallized from the solution forming a 2D supramolecular crystalline polymer stabilized by the retention of all B←N coordination modes.

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.

Tunable photosalient behaviours within coordination polymers via functional molecular prearrangements

Four Cd(II)/diene coordination polymers (CPs) with similar 1D chain motifs exhibit different photosalient (PS) behaviours in response to UV light. The [2+2] photoreaction between the CC groups within these CPs results in diverse PS behaviours of their crystals with different CC pair arrangements. The interesting PS behaviours of these CPs can be applied in design and fabrication of advanced photoactuating materials.

Regioisomers of singly bridged calix[6]crown-6 and their heavy alkali metal complexes: a molecular baseball glove for caesium(I)

We report the formation of heavy alkali metal complexes of bicyclic host molecules including the caesium(I) complex that catches the central metal ion with the deep pocket of the host similar to a baseball glove. For this, three regioisomers of singly bridged calix[6]crown-6 [1,2-bridged (H4 L 1,2), 1,3-bridged (H4 L 1,3) and 1,4-bridged (H4 L 1,4)] have been synthesized by alkyl-ation of calix[6]arene with penta-ethyl-ene glycol di-tosyl-ate in the presence of M 2CO3 (M = Na, K, Rb and Cs). The larger the cation size of the metal carbonate, the higher the yield of the H4 L 1,4 isomer, indicating the size-based template effect. A combination of H4 L 1,2 and RbOH allowed isolation of the mononuclear rubidium(I) complex (1) in which the metal center is six-coordinated in a loose fashion, the remaining two oxygen donors in the crown loop and two phenols in the calix rim are uncoordinated. Notably, the complexation of H4 L 1,2 with CsOH yielded the mononuclear caesium(I) complex (2), in which all possible ten binding sites on the deep and good-fit pocket participate in coordination via high cooperativity between the crown loop and calix rim, similar to a baseball glove. In dipolar organic solution, the caesium(I) complex 2 remains intact. H4 L 1,4 afforded a dicesium(I) complex (3) and adjacent complexes are linked by intermolecular cation-π interactions, giving rise to a pseudo one-dimensional coordination polymer. These results provide insight into the metal carbonate-dependent synthesis of calix[6]crowns and the influence of regioisomers on caesium(I) complexation.

Exploding and weeping ceramics

The systematic tuning of crystal lattice parameters to achieve improved kinematic compatibility between different phases is a broadly effective strategy for improving the reversibility, and lowering the hysteresis, of solid-solid phase transformations^1-11. (Kinematic compatibility refers to the fitting together of the phases.) Here we present an apparently paradoxical example in which tuning to near perfect kinematic compatibility results in an unusually high degree of irreversibility. Specifically, when cooling the kinematically compatible ceramic (Zr/Hf)O₂(YNb)O₄ through its tetragonal-to-monoclinic phase transformation, the polycrystal slowly and steadily falls apart at its grain boundaries (a process we term weeping) or even explosively disintegrates. If instead we tune the lattice parameters to satisfy a stronger 'equidistance' condition (which additionally takes into account sample shape), the resulting material exhibits reversible behaviour with low hysteresis. These results show that a diversity of behaviours-from reversible at one extreme to explosive at the other-is possible in a chemically homogeneous ceramic system by manipulating conditions of compatibility in unexpected ways. These concepts could prove critical in the current search for a shape-memory oxide ceramic^9-12.

Mechanical Motion in Crystals Triggered by Solid State Photochemical [2+2] Cycloaddition Reaction

Some special crystals respond to light by jumping, scattering or bursting just like popping of popcorn kernels on a hot surface. This rare phenomenon is called the photosalient (PS) effect. Molecular level control over the arrangement of light-responsive molecules in microscopic crystals for macroscale deformation or mechanical motion offers the possibility of using light to control smart material structures across the length scales. Photochemical [2+2] cycloaddition has recently emerged as a promising route to obtain photoswitchable structures and a wide variety of frameworks, but such reaction in crystals leading to macroscopic mechanical motion is relatively less explored. Study of chemistry of such novel soft crystals for the generation of smart materials is an imperative task. This minireview highlights recent advances in solid-state [2+2] cycloaddition in crystals to induce macroscale mechanical motion and thereby transduction of light into kinetic energy.

(9-Isocyanoanthracene)gold(I) Complexes Exhibiting Two Modes of Crystal Jumps by Different Structure Change Mechanisms

The first examples of single crystals exhibiting salient effects by different structure change mechanisms are reported. The crystals of newly prepared aryl(9-isocyanoanthracene)gold(I) complexes jump in response to two different external stimuli: ultraviolet (UV) irradiation and cooling. The photosalient effect is triggered by photodimerization reaction of the anthracene moieties under photoirradiation. By contrast, the thermosalient effect is caused by anisotropic thermal contraction upon cooling without a chemical structure change. By taking advantage of the multiple-jump feature, we also show sequential jumps of crystals by cooling and then UV irradiation for demonstration of the programmed motion of molecular crystals.

Construction of 2D Interdigitated Polyrotaxane Layers and their Transformation to a 3D Polyrotaxane by a Photocycloaddition Reaction between Wheels

Following the pioneering work of Sauvage and Stoddart on rotaxanes, construction of higher dimensional polyrotaxanes in metal-organic frameworks (MOFs) via a modified protocol is challenging. We present the formation of a two-dimensional (2D) polyrotaxane and its conversion to a three-dimensional (3D) polyrotaxane MOF via a photoreaction between interdigitated "olefin wheels". For this purpose, a 2-fold entangled 2D MOF [Pb₂(bpp)(sdc)₂] (1), showing a 2D + 2D → 2D polyrotaxane motif, has been synthesized from the solvothermal reaction of lead(II) nitrate, 3,3'-stilbenedicarboxylic acid (H₂sdc) containing an olefin group, and 1,4-bis(4-pyridyl)piperazine (bpp). The single-crystal X-ray diffraction analysis of 1 revealed that the adjacent entangled 2D layers are interdigitated, with the separation of 3.72 Å between C═C bond pairs in adjacent layers satisfying Schmidt's criteria for the occurrence of a [2 + 2] photocycloaddition reaction. Irradiation of the single crystals of 1 under UV light resulted in formation of a 3D polyrotaxane, [Pb₂(bpp)(rctt-tccb)]_n (2), due to a [2 + 2] photocycloaddition reaction between two wheels via a single-crystal to single-crystal transformation. The photocycloaddition and partial thermal cleavage reaction between 1 and 2 were confirmed by ¹H NMR and powder X-ray diffraction (PXRD) in solution and the solid state, respectively. The present approach could contribute to the understanding of the construction of higher dimensional polyrotaxanes which are not accessible by the traditional routes.

Thermal expansion properties of organic crystals: a CSD study

The thermal expansion properties of crystalline organic compounds are investigated by data mining of the Cambridge Structural Database (CSD). The mean volumetric thermal expansion coefficient is 168.8 × 10-6 K-1 and the mean uniaxial thermal expansion coefficient is 71.4 × 10-6 K-1, based on 745 and 1129 different observations, respectively. Normal and anomalous coefficients can be identified using these values and the associated standard deviations. The anisotropy of the thermal expansion is also evaluated and found to have a very broad distribution. 4719 different structures, comprising 4093 different molecular compounds and 626 additional polymorphs have been analyzed on their thermal expansion properties. Approximately 34% of these structures may have at least one orthogonal axis with negative thermal expansion, much more than generally believed. Moreover 127 structures have been identified which could have negative volumetric thermal expansion. Experimental validation using a robust protocol with data collected at more than 2 different temperatures is required to validate these cases.

Crystal engineering in IUCrJ: from 'the' crystal structure to 'a' crystal structure

What is 'structure' in the context of a molecular solid?

Reversible Thermosalience in a One-Dimensional Coordination Polymer Preceded by Anisotropic Thermal Expansion and the Shape Memory Effect

Thermally responsive crystals hold great potential for their use as actuating materials by acting as energy transducers to convert heat energy to mechanical work. Control over defined phase transition temperature with rapid reconfiguration is of great advantage for actuation. The thermosalient (TS) effect is a rarely observed phenomenon in coordination polymers (CPs), let alone the reversibility of thermosalience in CPs. Herein, we report the reversible TS effect in a one-dimensional CP due to the martensitic phase transition during both heating and cooling cycles. The TS effect was preceded by anisotropic thermal expansion showing high expansion coefficients. In addition, the nonmolecular crystals show reversible contraction and recovery during multiple heating-cooling cycles due to the self-restorative shape memory effect. The reversible actuation of the CP could be repeated for 20 heating-cooling cycles in differential scanning calorimetry experiments, suggesting its great potential as a multicyclic actuator. Such thermal responsive behavior is unique in metal-organic materials.

Dynamic effects in crystalline coordination polymers

Dynamic macroscopic behaviour of single crystals of coordination polymers when subjected to light, heat, and mechanical force.

Positive thermal expansion facilitates the formation of argentophilic forces following an order–disorder phase transition

Variable-temperature studies of the 0D silver-based complex [Ag₂(CF₃SO₃)₂(4-SB)₄] reveal formation of argentophilic forces upon warming due to a phase transition.

Synthesis, characterization and thermal behaviour of solid phases in the quasi-ternary system Mg(SCN)2-H2O-THF

Mg(SCN)2·4H2O can be converted into previously unknown compounds Mg(SCN)2·(4 - x) H2O·xTHF with x = 0, 2 and 4 by multiple recrystallization in tetrahydrofuran (THF). The phases were characterized by infrared spectroscopy (IR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and their crystal structures were solved from X-ray powder diffraction (XRPD) data. In the crystal structures isolated Mg(NCS)2(H2O)4-x(THF)x units form layered motifs. The thermal behavior of Mg(SCN)2·4H2O and Mg(SCN)2·4THF was investigated by temperature dependent in situ XRPD, where Mg(SCN)2·4THF was found to acquire a room temperature (α-form) and high temperature modification (β-form). The phase transformation is associated with an order-disorder transition of the THF molecules and with a reversion of the stacking order of the layered motifs. Further heating eventually leads to the formation of Mg(SCN)2·2THF. There thiocyanate related sulfur atoms fill the voids in the coordination sphere of magnesium, which leads to the formation of one dimensional electroneutral ∞[Mg(NCS)2/2(SCN)2/2(THF)2] chains. All investigated Mg(SCN)2·(4 - x) H2O·xTHF phases exhibit a remarkable anisotropic thermal expansion, and Mg(SCN)2·4H2O and Mg(SCN)2·2THF were found to show both positive and negative thermal expansion coefficients.

Single-Crystal-to-Single-Crystal [2 + 2] Photocycloaddition Reaction in a Photosalient One-Dimensional Coordination Polymer of Pb(II)

In a remarkable example, we report a one-dimensional coordination polymer (CP) of Pb(II) showing photosalient (PS) properties triggered by [2 + 2] cycloaddition of olefinic ligands, which is seldom observed in CPs. Macroscopic rod-shaped crystals show various photomechanical effects such as jumping, splitting, rolling, and breaking upon UV illumination. In this rare example, we could determine the solid-state structure of the 100% dimerized product and three intermediate structures, even after the shattering of crystals into small pieces. Detailed mechanistic investigation from the single-crystal data indicates that the strain generated in the unit cell due to anisotropic expansion played a bigger role for the PS effects. Nucleated growth of the photoproduct crystal created different domains inside the single crystal, which multiplied the already developed stress leading to the photomechanical movements. This example falls in the gray area of a clean single-crystal-to-single-crystal (SCSC) transformation and violent PS effect. Such photochemical behavior has never been reported before.