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Flash photolysis experiments of the photoreduction of oxonine, thionine, and selenine showed that for all three dyes the rate constant of the electron transfer process between the reducing agent allylthiourea and the acid triplet form is considerably higher than that of the corresponding reaction with the basic form. The higher reactivity of the acid triplet form can be related to its higher redox potential which is anulable from the difference of the pK values of triplet and semichinone of the dye. The zero-field splitting parameters of ESR triplet spectra of the dyes, mentioned above, and their tetramethylsubstituted deri\-atives were determined. Introduction Studying the photoreduction of thionine TH+ (X = S, R = H) FISCHER et aP found that the reactivity of the acid triplet form 3TH 2++ towards the reducing agent allylthiourea (ATU) is considerably higher than that of the basic form 3TH+. :Meanwhile, similar results have been obtained by JoussoT-DuBIEN et al.2 for two other thiazine dyes azur A and methylene blue using ethylenediaminetetraacetic acid (EDTA) as reducing agent. The question arises whether the higher reactivity might be due to a change of the character of the lowest triplet state upon protonation (3TH+ having nn* character and 3TH 2++nn* character) or whether it may be correlated with some other property. * Institut fur Physikalische Chemie der Universitat Stuttgart, Germany ** Institut fur Physikalische Biochemie der Universitat Frankfurt/M., Germany 1 H. FISCHER, H. E. A. KRAMER, and A. MAUTE, Z. physik. Chem. Neue Folge 69 (1970) 113 and papers cited therein. 2 R. BONNEAU, PH. FORNIER DE VIOLET and J. JOUSSOT-DuBIEN, Photoehern. Photobiol. 19 (1974) 129 and papers cited therein. U

Photochemical Investigations and ESR Triplet Spectra of Oxazine, Thiazine and Seleneazine Dyes