Resonant Raman spectra of molecules with diradical character: multiconfigurational wavefunction investigation of neutral viologens.

The resonant Raman and UV/vis absorption spectra of two diradicaloïd compounds, methyl viologen and phenylene-extended viologen in their neutral state, have been simulated using multiconfigurational wavefunction methods. For methyl viologen, a good agreement with experiment is evidenced for the UV/vis absorption vibronic structure, provided dynamic correlation is accounted for to get the vibrational frequencies and normal modes. To some extent, the agreement with experiment is also good for the RR spectrum and the differences have been attributed to the presence in the experimental spectrum of surface-enhanced effects due to adsorption on the electrodes. As a result of inserting a phenylene group between the pyridinium units, the simulations have demonstrated that (i) in the UV/vis absorption spectrum, the relative intensity of the second band with respect to the 0-0 band increases, (ii) additional strong bands are observed in the RR spectrum, and (iii) the RR excitation profiles of the phenylene-extended viologen present less structure than in the case of methyl viologen where the relative mode intensities can strongly depend on the incident light wavelength. These differences are signatures of the extension of the effective conjugation length as well as of the increase in diradical character.