Deuterium effect on the quenching of aromatic hydrocarbon triplet excited states by oxygen

Abstract

By using ultraviolet light flash excitation, the triplet (phosphorescence) decay time τ of a series of protonated (p) and deuterated (d) aromatic hydrocarbons dissolved in a fluffy freeze-dried polystyrene matrix was studied as a function of oxygen pressure [O2] at 298°K. The quenching constants γ were calculated from linear plots of τ−1 as a function of [O2] and the ratios of γ(p)/γ(d) are 1.2±0.1 for anthracene and 1,2-benzanthracene, 1.9±0.2 for pyrene, 1.4±0.2 for chrysene, and 2.5±0.4 for naphthalene. These results are interpreted in terms of Franck-Condon factors and the energy transfer quenching model proposed by Kawaoka, Khan, and Kearns in which the 1Δg and Σg+1 excited states of oxygen are generated, and in which the remaining energy is dissipated by the vibrational modes of the quenched aromatic molecules. In pyrene and in naphtalene, the 1Δg pathway appears to be more important than in chrysene and dominates over the Σg+1 mechanism. No conclusions can be reached about the preferred mode of energy transfer to oxygen in anthracene and 1,2-benzanthracene.