Optical and Electrochemical Properties of Indotricarbocyanine Dyes for Photodynamic Therapy

The effect of the molecular structure of indotricarbocyanine dyes on their luminescence spectral and redox properties was studied. The introduction of a chlorine-substituted bridge into the polymethine chain leads to a significant hypsochromic shift of the absorption and fluorescence spectra, whereas change in the structure of substituents at the indolenine terminal groups only has a weak effect on the spectra. The electrochemical properties of the polymethine dyes were studied by cyclic voltammetry. The oxidation and reduction potentials of the dyes were determined, and the corresponding energies of the highest occupied and lowest unoccupied molecular orbitals were calculated. Reduction of the molecules of the dye is an irreversible process, whereas the reversibility of the oxidation process depends on the structure of the polymethine chain and the nature of the counterion. Introduction of a chlorine–orthophenylene bridge into the polymethine chain leads to a significant increase in the reversibility of the oxidation reaction of the dyes on account of increase in the stability of the dication-radicals. It was shown that oxidation of the bromide ions occurs at lower potentials compared with oxidation of the dyes with a substituted polymethine chain, which probably leads to decrease in the reversibility of electrochemical oxidation of the dyes when bromide ions are present in solution.