A Highly Fluorescent cis-Stiff-Stilbene

Because of the ultrafast C=C bond torsion and electrocyclic reaction in the excited state, cis-stilbenes are generally nonfluorescent (Φf <10−4) in common solvents at ambient temperature. The known examples of highly fluorescent cis-stilbenes are limited to those with the C=C configuration structurally locked. Herein we report a novel cis-stiff-stilbene c-2 a that displays a high fluorescence quantum efficiency up to 0.47 in acetonitrile (MeCN) without sacrificing the photoisomerization ability. Through comparison with the corresponding aminostilbene c-1 and positional isomer c-2 b, the fluorescence turn-on for c-2 a takes root in two structural origins: namely, a rigid stilbene backbone that suppresses the cis→dihydrophenanthrene (DHP) photocyclization and a non-twisted meta-N,N-dimethylamino (DMA) substituent that slows down the cis→trans photoisomerization. The corresponding trans isomers t-1, t-2 a, and t-2 b are also investigated to gain insights into the meta-DMA effect in stiff-stilbene.