Laser-induced fluorescence and fluorescence lifetime of diphenylbenzobis(thiadiazole) in supersonic free jets

The technique of laser-induced fluorescence (LIF) spectroscopy in supersonic free jets was applied to diphenylbenzobis(thiadiazole) [Angew. Chem., Int. Ed. Engl. 33 (1994) 1977], in the excitation wavelength region of 500-595 nm. The position of the electronic origin was determined to be 589.81 ± 0.02 nm. The LIF excitation spectra indicate that the phenyl groups swing torsionally with large amplitude in the electronically excited state. The fluorescence lifetimes were measured at 11 major vibronic bands to vary between 42.0 and 43.4 ns, with no observable decrease for vibrational energies in the first excited singlet state up to 2605 cm^-1. The LIF spectrum and fluorescence lifetime of van der Waals complexes with xenon atoms were also measured. No heavy-atom effect on the fluorescence lifetimes was observed, and therefore the fluorescence quantum yields are expected to be unity even when the excited molecule had a high excess energy up to 2605 cm^-1.