Photochemistry of Benzothiazole Models of Pheomelanin

The photochemistry of several 4‐hydroxy‐ and 4‐methoxybenzothiazoles has been investigated by laser flash photolysis. In aqueous solutions of pH3–12, the 4‐hydroxybenzothiazole chromophore undergoes monophotonic photoionization to afford e ‐ aq with quantum yields on the order of 0.06; no evidence for triplet species was obtained. The spectra and stability of the resultant free radicals were determined using pulse radiolysis. In contrast, triplet transients with life‐times on the order of 8 mUs are readily observable upon irradiation of the 4‐methoxybenzothiazole analog. Triplet sensitization experiments with the water‐soluble carotenoid crocetin were employed to obtain the triplet extinction coefficients and subsequently the triplet quantum yields. The significance of these differences in photochemical behavior is discussed in relationship to the photochemistry and photobiology of the epidermal melanin pigment pheomelanin.