The fluorescence yield ΦF, triplet yield ΦT, and fluorescence lifetime τF for pyrene-h10 and pyrene-d10 have been measured as a function of temperature. The phosphorescence yield ΦP and phosphorescence lifetime τPP have also been determined. At -196° ΦF ∼ 0.8, ΦT ∼ 0.2, and (Φ; + ΦT) ∼ 1 for both pyrene-h10 and pyrene-d10. However, in pyrene-d10 the value of ΦT increases with increasing temperature, indicating that the rate constant for intersystem crossing has a temperature-dependent part. At room temperature (ΦF + ΦT) is less than unity for both pyrene-h10 and pyrene-d10, indicating that internal conversion from the lowest excited singlet directly into the ground state is occurring. This process is totally temperature dependent and is negligible at -196°. The variation of fluorescence lifetime and triplet yield with temperature for pyrene-h10 is attributable in toto to this temperature-sensitive internal conversion; in pyrene-d10 both internal conversion and intersystem crossing are temperature dependent and contribute to the observed variations.
