Multi-stimuli responsive organic polymorphic crystals: anisotropic elasticity and plasticity, mechanochromism and photomechanical motions

Abstract

The development of organic crystals responsive to multiple external stimuli (mechanical force, light and heat) as potential smart materials is always compelling but challenging. Herein, we reported three polymorphs of (Z)-3-(4-formylphenyl)-2-(4-methoxyphenyl)acrylonitrile which exhibit distinct multi-stimuli responsive behaviours. It was found that Form I could realize the integration of reversibly elastic and irreversibly plastic bending depending on crystal faces and twisting deformations by utilizing different modes of external forces. This unique property can be attributed to the compatibility of the interlocked crisscross packing and slippage plane. Owing to the sandwich structure of π stacking in Form II, it could undergo crystal-to-crystal transformation to Form III via grinding, accompanied by evident fluorescence switching from yellow-green to blue and a decrease of emission intensity. Moreover, the photochromism and photomechanical motions of bending, rolling, jumping and splitting could be triggered by [2+2] cycloaddition for Form II under UV illumination. Elaborate crystallographic and spectroscopic analyses were conducted to reveal the factors dominating their mechanical and luminescent properties. It was suggested that it was the synergic effect of molecular conformation, intermolecular interactions and packing mode that determines the luminescent properties of different polymorphs.