MECHANISM OF PHOTOSENSITIZED REDUCTION OF TETRAZOLIUM BLUE .2. HETEROGENEOUS SOLVENT - TRITON X-100 MICELLES

The electron transfer from sodium L-ascorbate to tetrazolium blue, pho tosensitized by the first triplet state of chlorophyll a, was studied in a heterogeneous medium (Triton X-100 micelles) by steady state (con tinuous irradiation) and time-resolved (conventional flash photolysis) photochemical techniques. As in the solvent ethanol (see part I of th is series), in Triton X-100 micelles the complete photoreduction of te trazolium blue (TB2+) leads to the consecutive formation of two produc ts: TBH+ (with an absorption maximum at 540 nm) which is in an interme diate reduction state and TBH2 (with an absorption maximum at 580 nm) which is the completely reduced product. The yield and ratio of these products depend mainly on the components of the system and their relat ive concentrations (especially the donor to acceptor ratio), the durat ion of the experiment (number of flashes performed of time or irradiat ion), and the micellar concentration, which determines the mean occupa tion number of the reactant species in the micelles and their distribu tion law in these aggregates. A mechanistic model is proposed to ratio nalize the combined steady state and transient results for the differe nt types of system analysed. This model considers mainly the processes of chlorophyll triplet unimolecular decay, quenching of the triplet b y oxidative electron transfer mechanisms resulting in radical species and the subsequent radical recombinations. The rate constants of some of these processes were evaluated using two different approaches: (1) a formalism based on bimolecular processes for the quenching reactions , which enabled the rate constants to be evaluated in terms of the tot al or analytical concentrations of the reagents; (2) a formalism based on unimolecular processes within the confined reaction medium of a mi celle, which enabled the rate constants to be evaluated in terms of th e effective concentrations or mean occupation numbers of the reagents in the micelles.