Simulation of charge-transfer, UV-VIS and resonance Raman spectra of push-pull systems: a TDDFT and CASPT2 comparison

Time-Dependent Density-Functional Theory (TDDFT) and Extended Multi-State Complete Active Space Second-Order Perturbation Theory (XMS-CASPT2) methods, together with augmented correlation-consistent polarizable valence double-ζ (aug-cc-pVDZ) basis sets, were applied to simulate the vibronic and resonance Raman (RR) spectra of a push-pull model system, 4-nitroaniline (pNA) and its anion ([pNA]^-), within the Independent Mode Displaced Harmonic Oscillator (IMDHO) model. Both methods predict adequately well the vertical absorption spectra for both species and the well-known charge-transfer (CT; S11(ππ*)) excited state of pNA. Nevertheless, pNA and [pNA]^- absorption spectral band intensity and vibronic broadening are better reproduced at the XMS-CASPT2 level. RR spectra were also obtained using both methods, with a good agreement for both methods for pNA, for which the electronic wave functions are best described by a single state configuration. For the anion, for which the excited state presented a multiconfigurational nature, the TDDFT failed to predict the main intensification observed experimentally under resonance conditions. As to the resonance Raman excitation profile for the pNA species, the ν_S(NO₂) vibrational mode carries most of the intensity of the vibronic spectrum, but for [pNA]^- the contributions of main vibrational modes are more complex, being governed by different modes in different energies, with ring modes dominating at the maximum, as predicted by the XMS-CASPT2 method.