Resonance Raman Spectroscopy and Quantum-Chemical Calculations of Push−Pull Molecules:  4-Hydroxy-4‘-nitroazobenzene and Its Anion

The deprotonation of the push−pull molecule 4-hydroxy-4‘-nitroazobenzene leads to a substantial variation in the charge distribution over the donor and acceptor moieties in the D-π-azo-π-A system. The extra charge stabilizes the excited state, leading to a drastic red shift of ca. 100 nm in the λmax of the electronic transition and consequently causes significant changes in the resonance Raman enhancement profiles. In the neutral species the chromophore involves several modes, as ν(CN), ν(NN), and νs(NO2), while in the anion the selective enhancement of the νs(NO2) and ν(CO-) modes indicates a greater geometric variation of the NO2 and CO- moieties in the resonant excited electronic state. The interpretation of the electronic transitions and the vibrational assignment are supported by quantum-mechanical calculations, allowing a consistent analysis of the enhancement patterns observed in the resonance Raman spectra.