Controlling dancing crystals

This study highlights the tuning of mechanical motion in Zn(II)/Cd(II) based coordination complexes, and presents the first molecular mechanism for photosalience based on DFT-calculated elastic stiffness tensors.
Controlling dancing crystals
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Light induced actuation materials that effectively convert external controllable stimuli into mechanical energy are of great interest towards addressing the global energy  issue e.g. harnessing solar energy to power light-driven actuators and mechanical devices. It has been known since 1834 that some crystals containing photoresponsive molecules can burst uncontrollably when exposed to the sun. If this photomechanical response could instead be controlled, it could be used to make tiny crystals hop, skip and jump on command without even touching them, like Mexican jumping beans responding to heat- a phenomenon commonly known as the “photosalient” (PS) effect. The reason behind such rare effect is the accumulation of stress due to anisotropic expansion/contraction of unit cell volume during photoinduced structural changes, which eventually releases in the form of macroscopic mechanical motion of crystals. However, the development of anisotropic photoactuation systems in microscopic crystals that can convert photonic energy into mechanical energy at the macroscopic scale still remains an imperative challenge. Out of various PS effects, structural change via photochemical [2+2] cycloaddition is one of the most commonly utilized chemical tools in the study of such phenomena. However, in the case of PS effects, single crystals are normally broken into small pieces and therefore it is a challenging task to develop system that undergoes single-crystal-to-single-crystal (SCSC) transformation. Herein we have utilized scaffolding-like metal-organic crystals based on Zn(II)/Cd(II) ions to give us that control. We have designed two iso-structural photoreactive 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} 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') (Fig. 1) and one-dimensional coordination polymer (1D CP) [Cd(4-ohbz)2(rctt-4-pncb)] (2') via a 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, these 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.

Fig. 1 Crystal structure of the synthesized compounds. a A perspective view of the compound 1. b Intermediate  compound i1 via partial photdimerization. c dimerized compound 1¢. d Supramolecular isomer p1.

Interestingly, on irradiation with UV light for 4 s, Zn crystals underwent [2+2] cycloaddition in a single-crystal to single-crystal (SCSC) manner accompanied by violent jumping, hopping, split, curl and broken into pieces (video 1). The mechanical motion was also realized by field emission scanning electron microscopy (FESEM) images of the crystals before and after photoirradiation. The reason behind the photosalient effect of these crystals can be assigned to the strain created by sudden contraction during the [2+2] cycloaddition reaction. The closure look at the cell parameters reveal that the cell volume is also decreased by 227 Å3 (3%) from 1 to 1', compared to 150 Å3 (2%) from 2 to 2' which is in line with the PS effect, wherein substantive amount of changes in unit cell parameters (nanoscale property) occur and the change in cell volume results in extra stress which is released in the form of mechanical motion (macroscopic property). However, isotypical Cd crystals did not show such behaviour, providing us with a negative control to underpin the fundamental mechanistic information on such photoreactions in confinement.

 Video 1. Illustration of PS effect.  

The anisotropic mechanical properties of PS crystals were investigated via DFT calculations which suggested after 5 minutes of irradiation, crystal i1 is predicted to be stiffer than crystal 1 (Fig. 2). The UV light seemingly strengthening some intermolecular interactions (i.e., during cycloaddition) resulting in an increased Young’s Modulus of 10.0 GPa. The low shear stiffness along the c axis disappears, with c66 more than doubling to 2.0 GPa and matching the predicted stiffness along the a axis. However, the bulk modulus is predicted to decrease slightly to 10.7 GPa due to decreases in the c11, c44 and c55values. This quantitatively validates that irradiation causes mechanical restriction in some directions, while facilitating movement in others- a good way to induce spontaneous breakage or movement.

Fig. 2 DFT-predicted trends in elastic stiffness constants during UV-induced photosalience.

 We have also performed nanoindentation experiments to correlate the DFT predicted mechanical behaviours which provides a concrete foundation to verify the mechano-structure-property relationship. Characteristic load−displacement (P−h) curves and scanning probe microscopy (SPM) images of the nanoindentation impression of crystals 1 and 2 have been obtained (Fig. 3), and the elastic modulus (E) and hardness (H) obtained by standard Oliver−Pharr (O−P) method were in line with the DFT-predicted values.    

Fig. 3 Experimental nanoindentation benchmarks of SC mechanical properties. a The P−h curve of compound 1. b 3D mapped surface of 1. c 2D SPM image of crystal 1. d The P−h curve of compound 2. e 3D mapped surface of 2. f 2D SPM image of crystal 2.

 The study presented here will open up this nascent field and pave the way for researchers to control the dynamic motion and thereby develop new soft robotics with real-world applications. This property generates extremely rapid energy transduction which could be utilized in diverse field including efficient solar energy harvesting, in medicine, in microfluidics for diagnostic purposes, and in other applications that call for controlled motion of dynamic systems. In future, micro-solar panels can be made that turn themselves to face the sun, just like flowers do, or laser-directed micromachines to travel through the human body. The possibilities are endless!

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