Primary photoprocesses in quinones and dyes I. Spectroscopic detection of intermediates

Abstract

The flash photolysis of many quinones in solution produces a number of transient intermediates, each with its characteristic absorption spectrum. A preliminary study of these showed that the appearance of intermediates is clearly related to the efficiency of the quinone as a photosensitizer. A detailed study of the transient spectra of duroquinone as a function of solvent and of pH enabled an unequivocal assignment to be made to all the principal absorptions. The main intermediates are (1) The semiquinone radical with first absorption maximum at 410 mμ which appears in all solvents containing abstractable hydrogen at pH 7 and below. (2) The semiquinone radical ion with first absorption maximum at 430 mμ which appears only at pH 7 and above. (3) The triplet state which is only observed strongly in the viscous solvent liquid paraffin and has a relatively short life. Analogous spectra of similar intermediates are reported for benzoquinone, toluquinone, xyloquinone, naphthaquinone and anthraquinones.