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 a nd 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 inhi bit radical-radical coupling reactions. Within a family of related dye s, radical dication half-lives range from 10(-3)to 10(4) s in acetonit rile. Electronic absorption spectra that were recorded for 25 electroc hemically 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 absorp tion in the near-infrared region. The spectral results are in agreemen t with INDO molecular orbital calculations.