Electric and steric hindrance effects of substituents on luminescence and force-stimuli response of cyanovinyl phenothiazine derivatives

Abstract

Six cyanovinyl phenothiazine derivatives bearing substituents with different sizes, and electron-donating and -withdrawing abilities were designed and synthesized. The effect of substituents on photophysical properties in solution and solid states were systematically investigated. The results suggested that electron-donating groups almost did not influence absorption and emission properties in solutions because the distributions of molecular HOMOs and LUMOs were independent of electron-donating groups. In contrast, trifluoromethyl (CF3) and cyano as electron-withdrawing units induced bathochromic shifts in both absorption and fluorescence spectra owing to the decrease in LUMO energy levels. Furthermore, it was found that the photophysical properties in crystal states strongly depended on the terminal substituent. Larger tert-butyl (t-butyl), methoxy and CF3 groups resulted in smaller spectral shifts in relative to smaller groups after crystallization. Single crystal structures suggested both CF3 and CN units lead a poor π-stacking, but charge transfer existed in PCVCN crystals. J-type dimers were found in PCVH and PCVN crystals. More interestingly, t-butyl, CF3 and methoxy moieties induced obvious color conversion and larger spectral shifts of 76 nm, 66 nm and 55 nm, respectively, under force stimuli. On the other hand, the compounds with terminal hydrogen atom, naphthyl and cyano group possessed shifts of 46 nm, 34 nm and 23 nm.