Excellent and reversible mechanofluorochromism in donor–acceptor π-systems based on bisarylic methanone derivatives

Abstract

Mechanofluorochromic (MFC) materials characterized by high-contrast fluorescent colors and/or high intensities have rarely been reported. This can be attributed to the complex mechanism and molecular design associated with MFC phenomenon. Herein, the route followed for the preparation of two D–A type bisarylic methanone fluorescent molecules CAR-BZ-POZ and CAR-BZ-PTZ, has been presented. Results obtained from systematic photophysical experiments reveal that CAR-BZ-POZ and CAR-BZ-PTZ are characterized by twisted intramolecular charge-transfer (TICT) states, highly distorted molecular conformations, good solid-state emission properties, and high-contrast reversible mechanofluorochromism. The as-prepared CAR-BZ-POZ and CAR-BZ-PTZ solids emitted intense yellow-green fluorescence (531 and 550 nm, respectively), and the solid-state luminescence efficiencies recorded were 32.54% and 24.92%, respectively. When the samples were ground, orange-red fluorescence was emitted (602 and 594 nm, for CAR-BZ-POZ and CAR-BZ-PTZ, respectively). Results obtained by conducting mechanistic studies suggest that the MFC phenomenon can be attributed to the phase transition (from crystalline to amorphous phase) of the molecules. The red-shift in the PL spectral profiles, achieved following the process of grinding, could be attributed to the extension of the molecular conjugation length and the planar intramolecular charge transfer (PICT) properties.