The intramolecular charge transfer in a donor-Π-acceptor dianion probed by resonance Raman spectroscopy and quantum chemical calculations

Abstract

The dideprotonation of 4-(4-nitrophenylazo)resorcinol generates ananionic species with substantial electronic Π delocalization. As compared to the parent neutral species, the anionic first excited electronic transition, characterized as an intramolecular charge transfer (ICT) from the CO- groups to the NO2 moiety, shows a drastic red shift of ca. 200 nm in the Λmax in the UV-vis spectrum, leading to one of the lowest ICT energies observed (Λmax = 630 nm in dimethyl sulfoxide (DMSO)) in this class of push-pull molecular systems. Concomitantly, a threefold increase in the molar absorptivity (εmax) in comparison to the neutral species is observed. The resonance Raman enhancement profiles reveal that in the neutral species the chromophore involves several modes, as ν(C-N), ν(N N), ν(C C) and νs(NO2), whereas in the dianion, there is a selective enhancement of the NO2 vibrational modes. The quantum chemical calculations of the electronic transitions and vibrational wavenumbers led to a consistent analysis of the enhancement patterns observed in the resonance Raman spectra. Copyright © 2009 John Wiley & Sons, Ltd.