Twisted intramolecular charge-transfer state of trans-3-(N,N-Dimethylamino)-4'-cyanostilbene: The C-C bond twisting

Whether a twisted intramolecular charge-transfer (TICT) state is formed is an important issue for understanding the fluorescence properties of a push-pull organic dye. Here we report a position effect of the donor substituent on the TICT state formation of aminostilbenes: namely, trans-3-(N,N-dimethylamino)-4' -cyanostilbene (mDCS) behaves differently from its TICT-free para isomer DCS and forms a TICT state in acetonitrile (MeCN). The TICT state of mDCS also differs from that of many previously reported aminostilbenes by twisting a C-C bond instead of a C-N bond. In addition, among the ring-bridged model compounds mDCS-N, mDCS-C1, and mDCS-C2, we observed an unusual electronic effect of the ring-bridging in mDCS-C2 that mitigates the impact of the TICT state on the fluorescence properties. Both the C-C bond twisting in mDCS and the ring-bridging electronic effect in mDCS-C2 provide new insights into the TICT chemistry of aminostilbenes.