DIMERIZATION REACTIONS OF CYANINE RADICAL DICATIONS

Radical dications formed during the one-electron oxidation of representative cationic dicarbocyanine (pentamethine) dyes that lack alkyl substitution on the even-numbered carbon atoms of the methine chain were found to undergo irreversible dimerization in methanol or acetonitrile solution. Deprotonation of the resultant UV-absorbing dimer gives a dicationic bis-dye whose spectral properties are similar to those of the parent dye. These bis-dyes are susceptible to further oxidation via a reversible two-electron mechanism to yield a highly unsaturated, cross-conjugated, tetracationic species. The chemistry of radical dications derived from the one-electron oxidation of carbocyanine (trimethine) dyes was found to depend on the nature of the dye heterocycle as well as the degree of alkyl substitution in the methine chain. Some thiacarbocyanine radical dications irreversibly dimerized and gave products analogous to those observed for dicarbocyanines. A persistent radical dication was obtained by the one-electron oxidation of an indolocarbocyanine and a chain-substituted thiacarbocyanine dye. © 1990, American Chemical Society. All rights reserved.