Photochemical properties of meso-substituted thiacarbocyanine dyes in solutions and in complexes with DNA

The processes of cis-trans photoisomerization and thermal back isomerization as well as the effect of DNA on the spectral and kinetic characteristics of the triplet state of a number of meso-substituted thiacarbocyanine dyes: 3,3'-diethyl-9-methoxythiacarbocyanine iodide (K1), 3,3',9-triethylthiacarbocyanine iodide (K2), 3,3'-diethyl-9-methylthiacarbocyanine iodide (M), and 3,3'-diethyl-9-chlorothiacarbocyanine perchlorate (K4), were studied by the flash photolysis method. Upon flash photoexcitation, the processes of trans-cis and cis-trans photoisomerization were observed for dye K1; the data on the structure of the absorption bands of the photoisomers were obtained. Complexation with DNA leads to an increase in the quantum yield of the triplet state of the dyes, which is explained by growing rigidity of the bound molecules. In the presence of DNA, triplet state deactivation follows the two-exponential law, thus showing that the dyes form complexes of two different types. The processes of quenching of the dye triplet state by oxygen were studied in solutions and in complexes with DNA. The rate constants for oxygen quenching of the triplet state of the dyes in complexes with DNA were found to be much lower than the values expected for the diffusion-controlled reactions (with allowance for the spin statistical factor, k(qO2) < 1/9k(dif)), which is explained by the steric factor of the complexation.