Crystal packing and mechanofluorochrmism of distyrylanthracene derivatives modulated by hydrogen bonds

Abstract

Two distyrylanthracene derivatives with ester and carboxyl moieties (AnDMB and AnDBA) were synthesized to investigate the influence of terminal group on their photophysical, crystal stacking and mechanofluorochromic (MFC) properties. Spectral and theory calculation results indicate that two compounds have similar absorption because their similar energy levels although they have different terminal groups. Both compounds exhibited crystallization-induced emission enhancement (CIEE). AnDMB crystals are rod-like and emitted strong blue-green fluorescence because of weak π-stacking. The blocky AnDBA crystals with DMAC (N, N-dimethylacetamide) molecules have a red-shifted emission. Moreover, instead of forming a hydrogen bond dimer between the carboxyl groups, a strong hydrogen bond is formed between the carbonyl group of DMAC and the carboxyl group. Crystal structure indicates that enhanced π-stacking should be responsible for its longer wavelength emission. Moreover, the crystals of two compounds could change their emission colors under force stimuli, but AnDMB exhibited higher color contrast than that of AnDBA. A shift of 40 nm might be observed when the AnDMB crystals were ground, while an emission spectral shift of 18 nm emerged after AnDBA crystals were ground. These results indicated that hydrogen bond did not only regulate molecular stacking and fluorescence properties in crystals, but also adjusted force-stimuli responsiveness.