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 viologens, l-methyl-1′-alkyl-4,4′-bipyridinium (C₁C_nV²⁺: n = 1, 8, 9, 12, 14, 16, 18) and 2-e^- reduced species of bisviologens linked by polymethylene chains (C₁V²⁺C_nV²⁺C₁: n = 3, 5, 8, 10) have been investigated spectroelectrochemically in the region from 350 to 1500 nm. Subtracting the contribution of the monomeric viologen cation radical from the spectra of reduced viologens, we obtained spectra of aggregates of viologen cation radicals. Three different spectral types were observed: Type I, the usual dimer spectrum of viologen cation radicals exhibiting structureless 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 510 nm and the bandwidth of band III much narrower than that of the corresponding band in the Type I spectrum. The Type I spectrum is shown with C₁C_nV^·+ when n is less than 8. The Type II spectrum is observed with C₁C_nV^·+ when n ≥ 9 and C₁V^·+C_nV^·+C ₁ when n ≥ 8. Addition of cetyltrimethylammonium bromide (CTAB) to the solutions showing the Type II spectrum caused a spectral change towards Type I. The Type III spectrum is shown with C₄C₄V^·+ in the presence of γ-cyclodextrin and C₁V^·+C₃V^·+C ₁. The Raman spectra of viologen cation radicals showing Type I and II spectra exhibit doublet features at 1507/1527 and 1334/1351 cm^-1, whereas singlet bands are observed for solutions showing the Type III spectrum. We attributed the Type I spectrum to face-to-face viologen radical dimers, the Type II spectrum to higher aggregates of viologen radicals, and the Type III spectrum to a viologen radical dimer with oblique stacking of the viologen rings.