FOURIER-TRANSFORM LUMINESCENCE SPECTROSCOPY OF SOLVATED SINGLET OXYGEN

The emission peaks and solvent-induced spectral shifts of the singlet oxygen transitions to the ground and first vibrational level of the ground triplet state of oxygen have been determined in a large number of solvents. The spectra were measured with a substantially higher accuracy than previously reported, by use of an FT spectrometer and a filtered white-light source. The results were interpreted in terms of the formation of van der Waals (vdW) complexes of singlet oxygen with four or six solvent molecules and existing theory satisfactorily accounts for the observed shifts in a number of solvents. For perfluorocarbons, no vdW complex is formed and it is suggested that weak long-range repulsive forces dominate, leading to small blue shifts in the emission peaks. The theory as used takes no account of specific interactions such as charge transfer, which could explain why the shifts predicted for the dispersion forces are smaller than the observed shifts in solvents of low ionisation potential. In solvents such as benzene, pyridine, toluene, hexachlorobutadiene, tetrachloroethene and iodopentafluorobenzene, charge-transfer interactions between all the solvent molecules and singlet oxygen are apparently important and lead to a further stabilisation of the complex. The bandwidths of the transitions did not correlate particularly well with the radiative rates, but did show a good correlation with the solvent spectral shifts. A number of other subtle solvent perturbations were observed. The ratio of the intensities of the (0-0) to (0-1) transitions showed a solvent dependence, as did the vibrational spacing of the ground triplet state of oxygen. These results indicate the existence of weak ground-state complexes and a relaxation of the poential-energy surfaces describing the complex states.