Spectroscopic investigation of dipyridinium dications and their associated radical monocations
A range of compounds related to the diquaternised salts of 2,2'-dtpyridine and 4,4'-dipyridine was studied in the present investigation. The effect of the change in the redox-state of the compounds was determined by vibrational spectroscopic and molecular orbital techniques. The investigation of 2,7-dimethylthieno[2,3-c:5,4-c'] dipyridinium diiodide monohydrate was undertaken by X-ray diffraction, vibrational spectroscopic and molecular-orbital techniques. These investigations showed that the thiophenic portion of the dication exhibited partial aromaticity. On formation of the radical monocation, a pattern of bond length changes consistent with the formation of a quinoidal structure was predicted by molecular-orbital calculations. This suggests that the electron delocalisation occurs over the entire molecule. The resonance Raman spectrum of the cation monoradical exhibits bands which may be assigned to the thiophenic portion, which suggests that this portion is associated with the chromophore. IR spectroscopic investigations into the electrogenerated radical of N,N'-diheptyl 4,4' - pyridinium, which has been investigated as the coloured component of electrochromic displays, were performed. The reflectance IR spectra of the radical monocations were obtained through bulk-sampling potential subtraction (PS) and surface-selective polarisation modulation (PM) techniques. The comparison between PS and PM spectra of the radical generated as a solid film in an aqueous medium, with the PS spectrum of the radical generated in acetonitrile revealed differences. These were interpreted in terms of an equilibrium between monomeric and aggregated forms of the radical. Similar results were obtained for the related compounds, methyl viologen and its dicyano substituted salt. The simplicity in the pattern of bands in the PS IR spectra of the monomeric radicals of methyl- and dicyanomethyl viologens was investigated by calculating the IR spectra of the dications and cation radicals of these compounds by molecular orbital techniques. The intense spectral features could be interpreted in terms of a charge-oscillation of the radical electron across the pyridine rings. The dipole moment of such modes is large and leads to the enhancement of these modes. The effect of various counterions associated with heptyl viologen was investigated by Raman and IR techniques. The pattern of bands in the Raman spectrum exhibited little variation with differing the counterions. The PS IR spectra exhibited disappearance features associated with the counterion in contrast to the PM spectra where no such bands were observed. PS IR spectra were also obtained by synchronising data acquisition with potential sweeps. The intensity of disappearance features in the PS IR spectra attributable to the CH modes varied at a similar rats to bands assigned to the radical aggregate. This suggested that a reorientation of the dipyridine ring and the quaternising alkyl chain probably occurs on reduction to form the radical. The lack of reversibility associated with the degradation of viologen films was investigated by PS IR techniques. The effect is probably associated with changes in the composition of the solid rather than a change in the concentrations of the monomeric and aggregated forms of the viologen radical. Dialkyl bridged diquaternary compounds of 2,2'-dipyridine were investigated by IR and theoretical techniques. The observation that the calculated dihedral angles for the dications and cation radicals were similar allowed the trends in the electronic absorption maxima (ultra-violet) and electrode potentials to be interpreted. Selective enhancements of bands in the PS IR spectra of the radicals suggest that a charge-oscillation mechanism may also be operative in these compounds. The utility of a combination of structural, theoretical and vibrational spectroscopic studies is demonstrated in these studies.