Nonradiative transitions in the first excited singlet state of perfluorocyclobutanone: Fluorescence decay times and fluorescence excitation spectroscopy

Fluorescence decay times (τ_F) of an ``isolated'' molecule of perfluorocyclobutanone (PFCB) in its first excited singlet state (S<sub>1</sub>) with little vibrational energy content and that of a vibrationally relaxed species are 110±4 nsec; the nonradiative lifetime is 112 nsec and the radiative lifetime is 5.4 μsec. These low energy PFCB (S<sub>1</sub>) species decay chiefly via the $S_1{\mathrm{\rightsquigarrow \; T}}_1$ intersystem crossing route. The value of τ<sub>F</sub> (λ<sub>ex</sub>=3577 Å) at ``low pressure'' is 49 nsec, indicating the probable existence of a new electronic relaxation channel below λ_ex=3800 Å. Fluorescence excitation spectroscopy at ``low pressure'' provides the intrinsic fluorescence quantum yield (φ_F) as a function of exciting wavelength, and it was found that φ_F begins to decrease below 3800 Å and it is reduced by a factor of 100 below 3480 Å. Collisional lengthening of the τ_F values at λ_ex=3577 Å indicates that ≤ 1 kcal/mol is lost per collision with a PFCB (S₀). From the overlap of the absorption and emission spectra, the 0–0 band of the first singlet n‐π* transition is estimated at 4100 Å.