Remarkable substitution influence on the mechanochromism of cyanostilbene derivatives

Abstract

Five twisted donor–π–acceptor cyanostilbene derivatives containing carbazole have been designed and synthesized with high yields. Their emission could be tuned from blue to orange by simply altering electron acceptors (donors). It was found that larger molecular dipole and the Stokes shifts of the donor–π–acceptor dyes could result in bathochromic shifts of absorption and emission. The density functional theory calculations further demonstrated that with the increase of the electron-donating or accepting abilities of the substituents, the energy gaps of the fluorophores gradually decreased, which elucidated the substituent effect of the organic fluorophores on their photophysical properties. Moreover, they exhibit typical intramolecular charge transfer (ICT), aggregation-induced emission (AIE), and crystallization-induced emission enhancement (CIEE) characteristics. Furthermore, the hydrogen- (2a), methyl- (2d) and chlorine-substituted (2e) derivatives exhibit remarkable reversible mechanochromic features (with emission wavelength changes up to 119 nm), while the methoxyl- (2b) and nitryl- substituted (2c) derivatives have no mechanochromic characteristics owing to the fact that they have strong crystallizability. The analysis of the X-ray crystal structure proved that the AIE-, CIEE-active and mechanochromic behavior was associated with the stator–rotor structures, twisted conformations and crystal packing modes. More importantly, the substitution effect of cyanostilbene derivatives would provide an effective way to obtain full-color optoelectronic materials with AIE and mechanochromic properties.