Trans->Cis Photoisomerization, Fluorescence, and Relaxation Phenomena oxf trans-4-Nitro-4'-(dialkylamino)stilbenes and Analogues with a Nonrotatable Amino Group

Article abstract of DOI:10.1021/j100357a024

The steady-state fluorescence properties of 4-nitro-4'-aminostilbenes (4-nitro-4'-aminostilbene (1); 4-nitro-4'-(dimethylamino)stilbene (2); 4-nitro-4'-(diethylamino)stilbene (3); 1-(4-nitrophenyl)-2-(2,3-dihydro-N-R-indol-5-yl)ethylene, R = H (4) and R = CH3 (5) were studied in various solvents and as a function of temperature.The quantum yield of fluorescence for trans isomer (Φ_f), the Stokes shift, and the quantum yield of trans->cis photoisomerization (Φ_t->c) depend markedly on the nature of the solvent.When the polarity is increased the Stokes shift increases, Φ_t->c decreases strongly, and Φ_f at first rises and then also decreases strongly.The five trans isomers show similar effects of solvent polarity and, generally, only small differences for a given solvent.This shows that the reduction in Φ_t->c in solvents of moderate and higher polarity is not caused by deactivation of the first excited trans singlet state (¹t(excit)) via rotation of the dialkylamino group since in 4 and 5 it cannot rotate whereas in 1-3 it can.Activation energies in m-xylene, toluene, and dioxane, obtained from Φ_t->c (range: 7-13 kcal/mol) and from Φ_f measurements (3-4 kcal/mol), differ significantly.It is concluded that the substantial decrease of Φ_t->c on going from nonpolar to slightly polar solvents is due (i) to relaxation to the trans ground state of the transoid excited state (A(excit.)) that is not identical with the solvent-relaxed spectroscopic ¹t(excit.) state and (ii) to the presence of an energy barrier along the pathway of trans->cis photoisomerization (following the step ¹t(excit.) -> A(excit.)), which cannot be overcome in more polar solvents.