THE TEMPERATURE AND MEDIUM DEPENDENCIES OF CIS-STILBENE FLUORESCENCE - THE ENERGETICS FOR TWISTING IN THE LOWEST EXCITED SINGLET-STATE

Fluorescence quantum yields of cis-stilbene-d0 and -d2 were measured as a function of temperature in n-hexane and n-tetradecane. cis-Stilbene fluorescence quantum yields were decomposed into emission contributions from 1c*,phi(fc)c, and adiabatically formed 1t*,phi(ft)c, by application of principal-component analysis. Rate constants, k(cp) for the 1c* --> 1p* torsional process, calculated from phi(fc)c give Arrhenius activation parameters E(cp)a = 0.33 +/- 0.08 and 1.90 +/- 0.08 kcal/mol and ln A = 27.80 +/- 0.13 and 30.09 +/- 0.12 for n-hexane (c-do and c-d2) and n-tetradecane (c-d0), respectively. Deuteration of the olefinic positions has no effect on k(cp) in n-hexane, but diminishes k(cp) by approximately 10% in n-tetradecane. Applying the medium-enhanced barrier model on these k(cp)'s and on literature k(cp)'s in methylcyclohexane/methylcyclopentane gives E(cp)a = -(1.07 +/- 0.05) + (0.89 +/- 0.01)E(eta)s kcal/mol indicating a negative intrinsic barrier, E(c)0 = -1.07 +/- 0.05 kcal/mol, and a nearly full imposition of the activation energy for viscous flow, E(eta)s, to the 1c* --> 1p* process, independent of deuterium substitution at the olefinic positions. The phi(ft)c values are remarkably T independent in both solvents. Assuming that adiabatic formation of 1t* takes the 1c* --> 1p* --> 1t* pathway, these results indicate that 1p* and 1t* represent essentially isoenergetic regions in the potential energy surface of the lowest excited stilbene singlet state, thus providing the first experimental estimate of the energy of the perpendicular singlet state, 1p*.