Phototransformation of cyanine dye with two chromophores. Effects of oxygen and dye concentration

Abstract

Phototransformation of a cyanine dye with two chromophores (biscyanine dye, BCD) subject to visible light in phosphate buffer was studied using optical absorption and fluorescence spectroscopic techniques. The effects of the dye concentration and molecular oxygen were demonstrated. The monoexponencial dependence of optical absorption of the BCD solution on the irradiation time was observed. The increase of the characteristic phototransformation time with the dye concentration is explained in terms of the dye aggregation. The explanation is confirmed by experiments in the presence of SDS micelles, where the concentration effect is absent, since at binding with SDS micelles BCD does not aggregate. The dye phototransformation rate increase in the presence of oxygen is explained in terms of singlet oxygen formation due to energy transfer from the dye excited triplet state to molecular oxygen. Singlet oxygen was detected, and its quantum yield was determined by an indirect chemical method. The effect of singlet oxygen on the BCD phototransformation was confirmed by reduction of its phototransformation rate in the presence of 2,5-dimethylfuran, a known singlet oxygen water soluble quencher. Singlet oxygen may destroy the π-conjugation of the BCD chromophores. A possible structure of the dye photoproduct is proposed based on the similarity of its spectral characteristics and those of monochromophoric cyanine dye and confirmed by quantum chemical modelling.