Fluorescence Properties of meta-Amino Substituted Stiff Stilbenes

Because of the high efficiency in photoisomerization, stiff stilbene derivatives have been employed as photoswitches and sterically restricted substrates. As a result of efficient photoisomerization, stiff stilbenes are low fluorescent chromophores. Regarding the significant meta-amino effect on fluorescence enhancement of trans stilbene, we are interested in the fluorescence properties of stiff stilbene containing N, N-dimethylamino substituent at the meta positions. In this context, meta-amino stiff stilbenes are investigated. To have a full understanding of positional and cis/trans isomers, we designed four molecules, t-mASS1、c-mASS1、t-mASS2 and c-mASS2, for comparison. According to the crystal structures of t-mASS1 and t-mASS2, the N, N-dimethylamino group is planar in the former but twisted in the latter compound, leading to distinct meta amino effect. The fluorescence quantum yields (Φf) in n-hexane are 0.73 and 0.01 for t-mASS1 and t-mASS2, respectively. In addition, the fluorescence is lower for the cis than the trans isomers, partly due to the photocyclization ability. The Φf values of c-mASS1 and c-mASS2 in n-hexane are 0.21 and 0.01, respectively. An unusually high Φf (0.47) was observed for c-mASS1 in acetonitrile.