Radiationless deactivation of singlet oxygen (1Δg) sensitized by 9-acetylanthracene in liquid and supercritical ethane: local density augmentation in the vicinity of the singlet oxygen and sensitizer molecules

The lifetime of singlet oxygen (1Δg) sensitized by apparently non-fluorescent 9-acetylanthracene (ACA) was determined at pressures up to about 35 MPa in supercritical ethane (SCF ethane; 35, 45 and 55 °C) as well as in liquid ethane (25 °C) from the phosphorescence decay measurements. For all the systems examined, the decay rate constant (τΔ−1) increased significantly at the lower pressure region and monotonically as pressure increased further. It was also found that the pressure dependence of the bimolecular rate constant for quenching, kD, is similar to that of τΔ−1. The apparent activation volume for kD varied from −120 to −32 cm3 mol−1 at the lower pressure region and from −24 to −15 cm3 mol−1 at the higher pressure region. In liquid solution over the pressure range examined, the pressure dependence of kD was interpreted well by a mechanism of the encounter complex formed between the singlet oxygen and the solvent molecules, together with that of the radial distribution function at contact with hard spheres, whereas it was not for the SCF systems in the low pressure region. In SCF solution, the deviation from this model was attributed to the contribution of the local density augmentation around the solute molecule. The degree of the local density augmentation evaluated is compared with that determined from the peak shift in absorption spectra of ACA measured in this work.