Phosphorescence and its characteristics in pyridine vapor

The phosphorescence emission of pyridine-d₀ and -d₅ has been observed in the vapor phase by means of time-resolved spectroscopy. The results of the experiments, which are described in full detail in this paper, on the phosphorescence spectrum, the excitation spectrum, the phosphorescence decay and sensitization of biacetyl phosphorescence indicate that the emission concerned is the genuine phosphorescence of the pyridines. For pyridine-d₀ (-d₅), the wavelength of the phosphorescence maximum is 450 nm (440 nm), the phosphorescence quantum yield 1.5 × 10^?6) (1.7 × 10^?6) and the phosphorescence lifetime is 1.2 μs (2.1 μs), the values for pyridine-d₅ being given in parentheses. The phosphorescence characteristics of pyridine are compared with those of other related molecules. The nature of the phosphorescent triplet state of pyridine is discussed with particular regard to the exceptionally fast non-radiative decay from that state.