ELECTROCHEMISTRY AND ELECTRONIC-SPECTRA OF CYANINE DYE RADICALS IN ACETONITRILE

The stability of the radicals formed during the one-electron oxidation and reduction of cationic cyanine dyes depends strongly on the type and extent of substitution in the polymethine chain of the dye. Radical persistence is greatly improved by alkyl substitution at the methine carbons to electronically stabilize the radical and/or sterically inhibit radical-radical coupling reactions. Within a family of related dyes, radical dication half-lives range from 10(-3)to 10(4) s in acetonitrile. Electronic absorption spectra that were recorded for 25 electrochemically generated cyanine radicals indicate the main absorption band of the radical to be hypsochromically shifted from that of the parent dye molecule. Most cyanine radical dications also exhibit weak absorption in the near-infrared region. The spectral results are in agreement with INDO molecular orbital calculations.