Substituent effect on photophysical properties, crystal structures and mechanochromism of D-π-A phenothiazine derivatives

Abstract

This study aimed to investigate the effect of substituents on the photophysical properties of four 10-ethyl-3-styryl-phenothiazine derivatives with CH3O (PVBMA), COOMe (PVBM), CHO (PVBA), or NO2 (NSP) moieties in solution and in crystal state. The responses of the derivatives to mechanical force stimuli were also investigated. Results suggested that the absorption and emission wavelengths of the compounds in solutions were strongly dependent on the electron-withdrawing (EW) abilities of the substituent groups. Electrochemical investigations indicated that the derivatives had identical HOMO energy levels, and the substituents only influenced the LUMO energy levels of the derivatives. The crystals of the derivatives emitted blue, green, yellow, and orange-red fluorescence. The introduction of a strong EW group resulted in the emission of wavelengths with increased length. Single-crystal structures illustrated that PVBM and PVBA crystals adopted antiparallel π-stacking, whereas NSP adopted a T-type arrangement with weak π-stacking. All four compounds could change fluorescence color under mechanical force stimuli. PVBA presented the largest spectral shift of 46 nm and PVBM exhibited the smallest shift of 27 nm after grinding. Notably, PVBM and PVBA could spontaneously recover their fluorescence within 1 h at room temperature. Meanwhile, PVBMA may retain mechanically written text for at least 1 day.