UV-VIS-NIR AND RAMAN SPECTROELECTROCHEMICAL STUDIES ON VIOLOGEN CATION RADICALS - EVIDENCE FOR THE PRESENCE OF VARIOUS TYPES OF AGGREGATE SPECIES

The spectral changes of 1-e(-) reduced species of unsymmetric viologen s, 1-methyl-1'-alkyl-4,4-bipyridinium (C1CnV2+: n=1, 8, 9, 12, 14, 16, 18) and 2-e(-) reduced species of bisviologens linked by polymethylen e chains (C1V2+CnV2+C1: n=3, 5, 8, 10) have been investigated spectroe lectrochemically in the region from 350 to 1500 nm. Subtracting the co ntribution of the monomeric viologen cation radical from the spectra o f reduced viologens, we obtained spectra of aggregates of viologen cat ion radicals. Three different spectral types were observed: Type I, th e usual dimer spectrum of viologen cation radicals exhibiting structur eless bands around 880 (band I), 530 (band II) and 360 nm (band III); Type II, red-shifts of the bands I and II of the Type I spectrum to as far as 1120 and 540 nm respectively; Type III, band positions similar to those of the Type I spectrum, but band II exhibiting a shoulder at around 510nm and the bandwidth of bandIII much narrower than that of the corresponding band in the Type I spectrum. The Type I spectrum is shown with C1CnV.+ when n is less than 8. The Type II spectrum is obse rved with C1CnV.+ when n greater than or equal to 9 and (C1VCnVC1)-C-. +-C-.+ when n greater than or equal to 8. Addition of cetyltrimethylam monium bromide (CTAB) to the solutions showing the Type II spectrum ca used a spectral change towards Type I. The Type III spectrum is shown with C4C4V.+ in the presence of gamma-cyclodextrin and (C1VC3VC1)-C-.-C-.+. The Raman spectra of viologen cation radicals showing Type I an d II spectra exhibit doublet features at 1507/1527 and 1334/1351 cm(-1 ), whereas singlet bands are observed for solutions showing the Type I II spectrum. We attributed the Type I spectrum to face-to-face viologe n radical dimers, the Type II spectrum to higher aggregates of viologe n radicals, and the Type III spectrum to a viologen radical dimer with oblique stacking of the viologen rings.