Specificity of the redox photoreaction of dimers of thiadicarbocyanine dyes

Abstract

The primary steps of the redox reaction of dimers of the thiadicarbocyanine dye and its 5,5′-dichloro derivative in aqueous solutions were studied in the presence of 4-nitroacetophenone, ascorbic acid, or hydroquinone. In water the dye molecules (anion, M⁻) mainly exist as dimers M₂ ²⁻. The laser pulse irradiation (10 ns, 532 nm) results in the population of the lowest triplet level M₂ ²⁻, whose depletion occurs due to both intersystem crossing to the ground state and photoinduced transition to the highest triplet state of the dimer followed by photoionization. Photoionization at low intensities of a laser pulse proceeds via the one-quantum mechanism going to the two-quantum mechanism with an increase in the laser pulse intensity. The photooxidation of the dimer in the lowest triplet state with 4-nitroacetophenone results in the formation of unstable radical anion M₂ ^−· that spontaneously dissociates to monomer M⁻ and radical M^· of the dye. In the presence of electron donors (ascorbic acid, hydroquinone), the dimers in the triplet state are not photoreduced, but the electron donors reduce M₂ ^−· and M^· to the dye dimer and monomer, respectively.