Electrochemical investigations of a covalently bonded viologen cyanine dye-system

Abstract

Abstract The electrochemical properties of a new class of cyanine dyes with and without a covalently linked viologen group, 2-[1-ethyl-1,2-dihydro-quinolinyliden-(2)-methyl]-1-ethylquinolinium chloride ( 1 ), 1-[3-[2-[(1-ethyl-2(1H)-quinolinylidene) methyl]quinolinium-1-yl]pentyl]-1′-methyl-4,4′-bipyridinium trichloride ( 2 ) and 1,1′-(4,4′-bipyridinium-1,1′-diyl-5,1-pentandiyl)bis[2-[(1-ethyl-2(1H)-quinolinylidene) methyl]chinolinium]tetra chloride ( 3 ) have been investigated both in dilute solutions (monomers) and in organized assemblies (J-aggregates). Fully reversible redox reactions of the viologen group were observed in acetonitrile while consecutive reactions (protonation) were found in aqueous solutions, resulting in the irreversibility of the second reduction step. Reduction potentials of the viologen unit in 2 and 3 were comparable with those of methyl viologen (MV 2+ ). In acetonitrile and at low concentrations in water, the electrochemistry (redox potentials and reversibility) of the viologen units was not affected by the covalently bonded cyanine dye group. When J-aggregates are formed at sufficiently high concentrations, different behavior was found for MV 2+ incorporated in aggregates of 1 and for the viologen units in 2 and 3 . While MV 2+ and 3 showed a decrease in the rate constant of the heterogeneous electron transfer, aggregates of 2 did not. Furthermore, adsorption of the cyanine dye was observed in some cases, leading to the passivation of the electrode.