Contribution of Diffusion to the Quenching by Oxygen of the Lowest Electronically Excited Singlet and Triplet States of Aromatic Molecules in Liquid Solution under High Pressure

The quenching by oxygen of the lowest electronically excited singlet (S1) and triplet (T1) states of five aromatic molecules in methylcyclohexane (MCH) at pressures up to 400 MPa was investigated. The apparent activation volume for the S1 state, Δ , at 0.1 MPa fell in the range from 14 to 16 cm3/mol, which is significantly smaller than Δ (25 cm3/mol) determined from the pressure dependence of the solvent viscosity, η, whereas that for the T1 state, Δ , at 0.1 MPa changed from +6.1 (anthracene) to −15.2 cm3/mol (triphenylene) and correlated approximately linearly with the triplet energy, ET. However, Δ at 350 MPa was positive and almost independent of ET (3−6 cm3/mol). The pressure dependence of was interpreted in the framework reported previously by us, and that of was attributed to the contribution of diffusion to the quenching in which the encounter complex pair with singlet, triplet, and quintet spin multiplicities formed between the oxygen and T1 state molecules is involved.