Spectroelectrochemical and ESR investigation of free radicals derived from indotricarbocyanine dyes for photodynamic therapy

Abstract

Cyclic voltammetry, UV–Vis absorption spectroscopy, and electron spin resonance are applied to study the properties of indotricarbocyanine dyes and their radicals formed during electrooxidation. We use electrochemistry to generate relatively stable free radicals of the dyes under controlled conditions. The dyes undergo electrooxidation during the chronoamperometric electrolysis at 1.1 V (versus saturated calomel electrode) yielding radical dications. The radicals exhibit an absorption band that is blue-shifted relative to the parent dye (561 nm versus 714 nm). In acetonitrile solutions, the radicals are characterized by a lifetime of ca. 20 min and a g factor of 2.002. The oxidation potential of the dyes is slightly greater than that of bromide ions allowing the radicals of the dyes to interact with bromide ions. The ability of the indotricarbocyanine dyes to sensitize generation of highly reactive bromine radicals might play an important role in their photodynamic activity.