Lifetime broadening in the rotationally resolved electronic spectra of dibenzothiophene, 2,5-diphenylfuran, and 2,5-diphenyl-1,3,4-oxadiazole in the gas phase. Intersystem crossing dynamics in the statistical limit.

The fluorescence lifetime of the zero point vibrational level of the first excited electronic state of dibenzothiophene (DBT) has been determined to be 1.0 ns by analysis of its rotationally resolved S1 <-- S0 fluorescence excitation spectrum. The S1 lifetime of DBT is substantially shorter than those observed for fluorene (FLU), carbazole (CAR), and dibenzofuran (DBF), analogs of DBT in which the heavy sulfur atom is replaced by lighter ones. The electronic origin bands through the series CAR, FLU, DBF, and DBT exhibit a monotonic increase in Lorentzian broadening in their Voigt line shape profiles. Two other heterocyclic molecules manifest similar photophysical properties; 2,5-diphenylfuran and 2,5-diphenyl-1,3,4-oxadiazole. Lorentzian line shape broadenings of approximately 76 MHz were observed in the high-resolution spectra of their origin bands. Possible reasons for the short fluorescence lifetimes of these heterocycles are discussed.