Resonance Raman studies of charge-transfer complexes of bipyridinium dications

Abstract

Resonance enhancement was exhibited by a range of bipyridinium—iodide charge-transfer complexes. The excitation profiles measured for each species, however, did not correspond closely with the visible absorption spectra. This was attributed to interference effects in the intensity measurements caused by the existence of a second, lower-wavelength absorption band in the visible spectrum. In one complex, enhancement of non-totally symmetric modes could be established, indicating that B term resonance enhancement was occurring as the result of vibronic coupling to another excited electronic state. The position of the Raman excitation profile maximum (and the higher-wavelength absorption maximum) was correlated with the half-wave potentials of the acceptors, showing the increase in wavelength (and therefore decrease in energy) of the charge-transfer transition with increasing acceptor ability. The effect of varying the extent of charge transfer with an acceptor was studied by Raman spectroscopy. Small frequency changes in the spectra could be correlated with differences in the extent of charge transfer.