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

The dideprotonation of 4‐(4‐nitrophenylazo)resorcinol generates an anionic 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 NO 2 moiety, shows a drastic red shift of ca. 200 nm in the λ max in the UV‐vis spectrum, leading to one of the lowest ICT energies observed (λ max = 630 nm in dimethyl sulfoxide (DMSO)) in this class of push‐pull molecular systems. Concomitantly, a threefold increase in the molar absorptivity (ε max ) 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(CN), ν(NN), ν(CC) and ν s (NO 2 ), whereas in the dianion, there is a selective enhancement of the NO 2 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.