FLUORESCENCE QUANTUM YIELDS OF TRANS-STILBENE-D(0) AND TRANS-STILBENE-D(2) IN N-HEXANE AND N-TETRADECANE - MEDIUM AND DEUTERIUM-ISOTOPE EFFECTS ON DECAY PROCESSES

Fluorescence quantum yields of trans-stilbene-d0, t-d0, and -alpha,alpha'-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(f)0n(x), where x = 1.65 +/- 0.08 and k(f)0 = 3.75 x 10(8) s-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 50-60% increase in the fluorescence quantum yields. The dependence of k(tpd0)/k(tpd2) on T indicates that deuteration increases DELTAH(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.