MECHANISM OF PHOTOSENSITIZED REDUCTION OF TETRAZOLIUM BLUE .1. HOMOGENEOUS SOLVENT - ETHANOL

The electron transfer from L-ascorbic acid to tetrazolium blue, photos ensitized by the first triplet state of chlorophyll a, was studied in a homogeneous medium (ethanol) by steady state (continuous irradiation ) and time-resolved (conventional flash photolysis) photochemical tech niques. The complete photoreduction of tetrazolium blue (TB2+) in this medium leads to the consecutive formation of two products: TBH+ and T BH2. TBH+, which is in an intermediate reduction state, can be detecte d spectroscopically (absorption maximum, approximately 530 nm) but is not easily isolated as it undergoes further reduction to form TBH2. TB H2 is the completely reduced product, which can be detected (absorptio n maximum, approximately 580 nm) after long reaction times. The yield of one or both of these products depends mainly on the components of t he system and their relative concentrations (especially the donor to a cceptor ratio), on the,presence and concentration of oxygen and on the duration of the experiment (time of irradiation), which determines th e ratio of the two electron acceptors of the chlorophyll triplet, TB2 and TBH+. A mechanistic model is proposed to rationalize the results obtained in both steady state and transient conditions for the differe nt types of system analysed. This model considers mainly the processes of the chlorophyll trip]et unimolecular decay, the triplet quenching by other triplet and ground state chlorophyll molecules and by the two electron accepters TB2+ and TBH+ and the subsequent recombination rea ctions of the resulting radicals.