Photodegradation mechanisms in laser dyes: A laser irradiated ESR study

The photodegradation mechanisms in xanthene dye-laser solutions are investigated at 77 K by the direct detection of a decrease in the concentration of dye molecules in the lowest excited triplet state (T1state) and of free radicals produced after degradation using an ESR technique under UV (3511 + 3638 Å) or 5145-Å laser irradiations. It is shown that the ESR spectral shape of the radicals induced by an UV laser beam is quite different from that of those induced by a 5145-Å beam. For those radicals, the irradiation time and power dependences, the effects of dye and solvent molecular structures and the role of the T1state by the addition of cyclooctatetraene (COT) quenchers are examined in some detail. As a result the two following laser-induced photochemical reactions are proposed: in UV laser- or flashlamp-pumped dye lasers, the C-H bond rupture of a solvent molecule due to the energy transfer of a dye molecule in a higher excited triplet state, which is produced by a absorption of a dye molecule in the T1state, results in a radical and a leuco compound of the dye. On the other hand, in the case of the 5145-Å laser excitation, a partially reversible change of a dye molecule in the T1state produced by only a one-photon absorption results in another radical which depends on the chromophoric structure of the dye.