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 ]⋅H₂ O (Zn₂ -1; H₂ bdc=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 bdc^2- 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.

Keywords: flexible solid materials; metal-organic frameworks; photomechanical actuators; thermal expansion.