Fluorescence of Trans^-Cyano^4-dimethylaminostilbene; No Evidence for a TICT State

The fluorescence properties of trans-4-cyano-4'-dimethylaminostilbene (trans-1) and a rigid analogue, in which the dimethylaminophenyl group is replaced by an indoline ring, trans-1(4-cyanophenyl)-2-(5-[2,3-dihydro]N-methylindolyl)ethylene (trans-2) were studied in 20 solvents. The quantum yields of fluorescence (tf>f) and of trans-»eis photoisomerization (<2>t-c) as well as the fluorescence maximum (vf) and the bandwidth (zlvf) were examined. The activation energies (range: 3.5-7.8 kcal/mol) obtained from <2>f and <£t_c measurements are nearly the same for 1 and 2 in a given solvent. 0 f , vf and Jv f as a function of solvent polarity and temperature show similar trends for 1 and 2. No dual emission was observed for either of the cyanostilbenes. These findings indicate that a twisted intramolecular charge transfer (TICT) state is not involved in the relaxation process of the first excited trans singlet state ('t*). The main deactivation pathway of 't* competing with fluorescence is trans eis photoisomerization by twisting about the C = C double bond in the first excited singlet state for both.