Fluorescence Quantum Yields of trans-Stilbene-d0 and -d2 in n-Hexane and n-Tetradecane. Medium and Deuterium Isotope Effects on Decay Processes

Article abstract of DOI:10.1021/j100113a009

Fluorescence quantum yields of trans-stilbene-d0, t-d0, and α,α'-d2, t-d2 in n-hexane and in n-tetradecane are reported as a function of temperature.Together with literature lifetimes these are used to define radiative, k_f, and radiationless, k_tp, decay rate constants of trans-stilbene.The index of refraction dependence of k_f is given by k_f = k_f⁰n^x, where x = 1.65 +/- 0.08 and k_f⁰ = 3.75 * 10⁸s^-1, the known radiative rate constant for jet-cooled isolated trans-stilbene in the gas-phase.Inclusion of the n-dependence of k_f gives identical k_tp's from fluorescence lifetime and quantum yield data.This eliminates small but opposite systematic errors in derived activation parameters for torsional relaxation that were previously based on fluorescence quantum yields or lifetimes, separately, by assuming a constant k_f.Substitution of vinyl hydrogens with deuteria leads to a 50-60percent increase in the fluorescence quantum yields.The dependence of k_tpd0/k_tpd2 on T indicates that deuteriation increases ΔH_tp by as much as 0.19 +/- 0.11 kcal/mol in n-hexane and that a more significant increase of 0.32 +/- 0.10 kcal/mol applies in n-tetradecane.