Regulating photosalient behavior in dynamic metal-organic crystals

Abstract

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.

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. Hydrogen atoms are not shown for clarity. e Solid-state photoluminescence of 1 and 1′ (Excitation wavelength 350 nm with slit 3 nm).

Fig. 2. Crystal structure of the synthesized compounds.

a A perspective view of the compound 2. b 1D CP 2′ formed via [2 + 2] photodimerization. Hydrogen atoms are not shown for clarity. c Solid-state photoluminescence of 2 and 2′ (Excitation wavelength 350 nm with slit 3 nm).

Fig. 3. Illustration of PS effect.

a FESEM images of crystal 1 before UV irradiation. b FESEM images of crystal 1 after popping under UV irradiation. c Optical microscopic image of needle crystal 1. d Optical microscopic image of block-shaped crystal 1. e Optical microscopic image of plate-like crystal p1.

Fig. 4. Theoretical prediction.

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

Fig. 5. Structural prediction.

Crystal structures of (a) 1. b i1. c 1′. d 2. Color code: Zn (silver); Cd (pink); H (white); N (blue); C (brown); O (red).

Fig. 6. 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.