Polymorphism manipulating topo-photochemical reactions, photoactuation, and the mechanofluorochromism of benzimidazoleylaryl acrylonitriles

Abstract

Polymorphism arising from different conformations or packing arrangements in crystal lattices would manipulate physical properties, including morphology, electrical conductivity, emission, and nonlinear optical properties. However, the photochemistry tuned by polymorphism is seldom reported. In this work, photochemical reactive isomers of (E)-2-(1H-benzo[d]imidazol-2-yl)-3-(4-fluorophenyl)acrylonitrile (E-CN-BIFPh) and (Z)-3-(1H-benzo[d]imidazol-2-yl)-2-(4-fluorophenyl)acrylonitrile (Z-BIFPh-CN) in which a cyano group is located on different positions in conjugated skeletons were synthesized. Interestingly, E-CN-BIFPh crystallized into sheet-like structures emitting orange-red fluorescence (OR-phase) and block-like structures showing green fluorescence (G-phase). Besides, cuboid-like crystals with green fluorescence (G-phase) and needle-like crystals emitting blue light (B-phase) were formed from Z-BIFPh-CN. It is interesting that [2+2] cycloaddition took place only in the OR-phase crystal of E-CN-BIFPh and the B-phase crystal of Z-BIFPh-CN, which powered the molecular crystals to exhibit obvious photoactuation. In the case of the G-phase crystals of E-CN-BIFPh and Z-BIFPh-CN, the geometric parameters for “olefin pair” deviated from Schmidt's criteria, and hence, no photodimerization occurred. Notably, the OR-phase crystal of E-CN-BIFPh exhibited high-contrast mechanofluorochromism (MFC) with a red-shift of 115 nm in emission. The phase transition of the G-phase crystals of Z-BIFPh-CN upon heating at 105 °C led to a change in the emission color from green to blue, showing thermochromism. Such polymorphism crystallization-controlled multi-stimuli-responsive molecular crystals might be employed as elements in artificial muscles, smart microrobots, wearable systems and sensors.