Evolution occurs due to the phenomenon known as natural selection, where organisms adapt and change to survive in their environment. Natural selection is based on the idea of “survival of the fittest”, where the most adaptive organisms will gradually change or evolve as the environmental changes via locomotion.1 Animal locomotion is seemingly autonomour e.g. walking, running, swimming, jumping, hopping, flying, soaring and gliding. In the case of plants, a variety of mechanisms are employed in order to achieve their fast movements, e.g. the fast closing trap (100 ms) of the venus flytrap2 or the opening of petals of the dogwood bunchberry’s flower (0.5 ms). Some plants are able to move their leaves very rapidly in response to mechanical stimuli,3 with many plants spreading their seeds or pollen by rapid movement. Cardamine hirsuta has seed pods which explode on touching. Some beans that twist as they dry out, putting tension on the seam, which at some point will split suddenly and violently flinging the seeds meters from the maternal plant.4
This locomotion can be mimicked and utilized in molecular crystals at the macroscopic level.5 As Mexican beans respond to heat, small crystals can also be made respond to light by jumping, swelling or bursting. In this regard, a number of organic and metal-organic crystals have been found to exhibit bending, twisting and swelling upon light illumination. Beyond this, some crystals may burst, scatter, jump, hop, curl, coil, swim, and at the extreme level fragment into pieces ̶ commonly known as the “photosalient” (PS) effect.6–8 Underlying the concept of such photomechanical is the accumulation of stress due to anisotropic expansion/contraction of unit cell volume via photoinduced structural changes, which eventually releases in the form of macroscopic mechanical motion of crystals.9,10 Out of various PS effects, structural change via photochemical [2 + 2] cycloaddition is one of the recent developments in the study of such phenomena.11
Among various photomechanical phenomena, the PS effect involves single crystals that violently explode and shatter into pieces.12,13 In terms of actuation; this is a single time event due to the disintegration of the crystals.14,15 Furthermore, there lies a grey area between the PS effect and single-crystal-to-single-crystal (SCSC) transformation. While SCSC transformations provide exact structural insights into the transformed structures,11 it is elusive in the case of PS effects as the single crystals are normally broken into small pieces.16 Predominantly the structural changes are analyzed from a partially dimerized structure or from the recrystallized product.17–19 However, the photoreaction may not be fully realised in the whole crystal due to a high absorbance of molecules in the crystal or potential side reactions. Achieving a clean SCSC transformation at the end of a violent PS reaction is a challenge. Herein, photocrystallography20,22 is an important concept that brings us closer to being able to watch solid-state processes occur in real time (either in a metastable or short-lived excited state) during the determination of a single crystal of a complex and thus we may have better insight into the photoconversion process of crystals.
Here, we report two iso-structural photoreactive metal-organic crystals of formulae[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} that undergo topochemical [2 + 2] cycloaddition under UV light as well as sunlight to generate a dimerized product of a 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') via SCSC process respectively. Interestingly, during this photoreaction, the Zn-complex 1 shows mechanical motion such as swelling, splitting, jumping and scattering. However, after the photomechanical effect is induced, PS crystals uniquely maintain their single crystallinity nature and allow for single crystal structural elucidation. This is a rare example of a metal-complex that exhibits a single crystal structure even after PS effect. The Cd(II)-based crystal 2 does not demonstrate this PS effect although it maintains itssingle crystal nature. In addition, we have obtained thick crystals of [Zn(4-ohbz)2(4-nvp)2] (p1) by maintaining the reaction mixture for a long time, which exists as asupramolecular isomer of 1. In p1 crystals, the 4-nvp ligands are not aligned and thus are found to be photoinert. Various experimental evidences have been attempted to establish structure-property relationship of PS effect in crystals. However, there is no example of metal-complex where PS effect is explained by experimental as well theoretical predication. Here, we use for the first time, density functional theory (DFT) to predict the mechanical properties of each crystal to understand the atomic-scale mechanisms and mechanical shifts that occur under irradiation.