Imidazole-based solid-state fluorescence switch: Stimuli-responsive emission, mechanochromism and acidochromism

Stimuli modulated fluorescence properties of imidazole-based molecular probe (E)-2-(5,5-dimethyl-3-(4-(1,4,5triphenyl-1H-imidazole-2-yl)styryl)cyclohex-2-en-1ylidene)malononitrile (Ph-ISO) in the solid state is presented. Not only did the probe display aggregation induced emission (AIE) activity with intense solid-state fluorescence emission, but also exhibited several repetitive cycles of reversible mechanochromism as well as acidochromism. The solid-state emission is ascribed to the intermolecular interactions in the highly twisted conformation of Ph-ISO via multiple C-H-N and C-H-pi interactions as confirmed by the single crystal X-ray analysis. The applied mechanical stress in the form of grinding results in the transformation of crystalline state to the amorphous state with a red shifted emission band attributed to attaining more planar conformation vs twisted conformation, with extended molecular conjugation. While reversible mechanochromism makes the probe suitable for rewritable papers, the switchable acidochromism is useful for the on-site monitoring of pH differences in biological and environmental media.

Automated summary

This study presents the stimuli modulated fluorescence properties of an imidazole-based molecular probe in the solid state. The probe displayed aggregation induced emission activity and intense solid-state fluorescence emission, as well as repetitive cycles of reversible mechanochromism and acidochromism. The solid-state emission was attributed to intermolecular interactions in the highly twisted conformation of the probe.