Photoinduced Nonlinear Contraction Behavior in Metal–Organic Frameworks

Abstract

The photoinduced dynamic behavior of flexible materials has received considerable attention for potential applications, such as in data storage or as smart optical devices and molecular mechanical actuators. Until now, precisely controlling expansion and contraction with light has remained a challenge. Unraveling the detailed mechanisms of photoinduced structural transformations remains a critical step necessary to understand the molecular architecture necessary for the design of sensitive photomechanical actuators. Herein, a two‐dimensional flexible metal–organic framework [Zn2(bdc)2(3‐CH3‐spy)2]⋅H2O (Zn2‐1; H2bdc=1,4‐benzenedicaboxylic acid; 3‐CH3‐spy=3‐methylstyrylpyridine) with a positive volumetric thermal expansion coefficient of +78.78×10−6 K−1 is reported. Upon light irradiation at different wavelengths, the MOF underwent a [2+2] cycloaddition, which afforded a family of isomeric, three‐dimensional MOFs (Zn2‐2 n, n=a–d) in a single‐crystal‐to‐single‐crystal (SCSC) manner. An unprecedented phenomenon, that is, photoinduced nonlinear contraction (PINC), was observed during this conversion. The PINC is caused by conformational changes in the 3‐CH3‐spy and bdc2− ligands, the bending of metal–ligand bonds, and the local distortion of the paddle‐wheel SBUs. The formation of a “wrinkle morphology” on the crystal surface after the photoreaction was observed by AFM. This PINC behavior can broaden the studies on materials expansion and offer a photodriven approach for the future design of supersensitive photomechanical actuators.

Sunlight‐Induced In Situ Isomerization of Both Ligands in a Mixed‐Ligand Coordination Polymer: From Photosalient to Photoinert Crystals

“…[4] However, mechanical effects accompanied by photodimerization in CPs are still scarce in literature. Of CPs, this observation has been found in few cases for onedimensional (1D) CPs [29][30][31][32] reported by Vittal and our groups and only two reports for two-dimensional (2D) CPs [33][34] depicted by Lang group.…”

Photoresponsive porous materials

“…Furthermore, when incorporated in COFs or MOFs in the linker backbones, stilbenes, owning to the spatial proximity of olefinic bonds, tend to undergo photocycloaddition, with high selectivity dictated by the molecular arrangement in the network, [83][84][85] which is often accompanied by contraction of the unit cell. [86][87][88] Structurally related so-called stiff-stilbenes, in which the molecular architecture features two five-or sixmembered rings attached to the double bond favours the efficient E/Z isomerization pathway (Figure 6b). [89][90][91] Overcrowded-alkene based molecular motors constitute a distinct class of molecular machines derived from stilbene photoswitches (Figure 6c).…”

Protective Coating with Crystalline Shells to Fabricate Dual-Stimuli Responsive Actuators

“…Mechanically responsive smart materials that can convert chemical energy into mechanical movement have been attracting considerable interest in a wide range of advanced applications such as robotics, actuators, and sensors. [1][2][3][4][5][6][7][8] This energy transformation can be achieved by the reversible deformation of materials (e.g., contraction, expansion, rotation, bending, twisting, and curling) under external stimuli such as electrical fields, temperature, light, pH, vapor, and pressure. [9][10][11][12] At present, most mechanically responsive materials are based on polymeric materials, including organic polymers, hydrogels, and liquid crystal elastomers, [13][14][15][16][17] while relatively less focus has been put on molecular crystals that possess precisely determined structures with ordered molecular packing.…”