Laser photolysis of fluorone dyes in a chitosan matrix

Kinetics of laser-induced photobleaching of fluorone dyes (fluorescein, dibromofluorescein, eosin Y, erythrosin B, Rose Bengal) is studied in a chitosan matrix. For all dyes the bleaching kinetics at the intensities of laser radiation 0.7 — 11.9 W cm^-2 demonstrates quasi-monomolecular behaviour. The results are analysed using a kinetic model, based on the four-level (S₀, S₁, T₁, T_n) scheme of the dye with chemically active triplet states taken into account. It is shown that the rate constants of the chemical reaction involving higher triplet states in the dyes studied amount to (3.9 — 18.6) × 10⁶ s^-1 and exceed the analogous values for the reaction involving the first lower triplet states by nine orders of magnitude. The rate of reaction involving the first triplet states appeared to be higher by one — two orders of magnitude than that in the case of higher triplet states involved because of low population of the latter. The possible mechanism of dye bleaching with participation of chitosan that consists in reduction of the dye to the leuco form by transfer of hydrogen from the chitosan matrix is discussed.