Dual-state emission and mechanofluorochromic properties of coumarin derivatives containing α-cyanostilbene

Four novel coumarin derivatives (CM-H, CM-F, CM-CH3 and CM-CF3) functionalized by alpha-cyanostilbene unit were designed and synthesized. The molecular spatial structure and photophysical performances were system-atically studied and discussed. The fluorescent spectra indicate that these coumarin derivatives display dual-state emission (DSE) characteristic. In dilute organic solutions, strong fluorescent emissions originated from intra-molecular charge transfer (ICT) process, and in aggregated states, the bright fluorescent emissions are attributed to the collective effects of restriction of intramolecular motions and J-aggregation stacking. Meanwhile, these coumarin derivatives all exhibited mechanofluorochromic (MFC) property, in particular, compound CM-CF3 exhibit the reversible conversion between yellow and red emission. The XRD and SEM experimental results confirm that the MFC mechanism is based on the conversion of degree of crystallinity. Our study provides a reasonable approach for designing DSE materials with the MFC property.