Pre-resonance Raman and IR absorption spectroscopy of imidazophenazine and its derivatives: Experimental and ab initio study

The pre-resonance Raman and IR absorption spectroscopy techniques have been applied to study the influence of substitutes in the imidazole ring of imidazo-[4,5-d]-phenazine (F1) on its vibrational structure. The following F1 derivatives in the KBr pellets have been investigated: 2-methylimidazo-[4,5-d]-phenazine (F2), 2-trifluoridemethylimidazo-[4,5-d]-phenazine (F3), and 1,2,3-triazole-[4,5-d]-phenazine (F4). The experimental Raman and IR data were analyzed using the harmonic frequencies, the IR intensities, and the pre-resonance Raman activities calculated at the DFT(B3LYP)/6-31++G** level of theory. We found a good agreement between the observed and the calculated spectra for all the normal modes except the ones attributed to the vibrations of the NH group. These latter modes are the most affected by the strong intermolecular interactions that occur in the solid samples. The bending ring vibrations dominate in the spectra of all compounds studied. The vibrational structure of the F1 derivatives is similar to that of the parent compound, though for each compound, characteristic bands also appear in the spectrum. Among the F1 derivatives, the vibrational spectrum of the F4 compound where the C atom of the imidazole ring is replaced by the N atom, changes the most from the spectrum of the parent compound. The nature of the six lowest electronic transitions to the S₁-S₆ states in the F1-F4 compounds and the forms of the π, π^* and n orbitals involved in the transitions were also determined. This revealed that the first singlet excited state in all studied compounds corresponds to a π → π^* transition.