Photoreduction of thiazine dye with trimethylamine

الملخص EN

Thionine, being a typical representative of thiazine dyes, was selected for investigations of photochemical reactions with organic reductants such as trimethylamine in 50% aqueous methanol.

The photochemical reduction was carried out in a specially designed apparatus provided with a double-walled temperature-controlled reaction cell.

Transmitted light was observed in terms of electrical signals through calibrated galvanometers.

Acidities of solutions were controlled by acetate buffers and determined by means of the spectrophotometric method in terms of Hammett acidity function {H0).

It was observed that quantum yield (0) is a function of acidity (H0) as well as concentration of reductant (AH2).

The quantum yield was found to be independent of the concentration of thionine dye.

The results have been interpreted in terms of a proposed mechanism.

The ratios of certain rate constants for the proposed mechanism have been evaluated.

It was concluded that the protonated triplet excited state of thionine (Th.H 2t) makes a major contribution to the determination of quantum yield ().

The complex (Th.H2t.

AH2) is the rate controlling species and it may undergo either a electron transfer process or a free radical transfer process.

It was also found that the quantum yield is controlled by two equilibria: (i) triplet excited state of thionine with proton and protonated triplet state of thionine; and (ii) protonated triplet excited state of thionine with reductant and associated complex of thionine with reductant (Th.H 2t.

AH2).