Solvent effects on low-lying absorptions and vibrational spectra of thieno[3,4- b ]pyrazines: the role of unconventional C–H···N bonds

Difunctionalized thieno[3,4- b ]pyrazines are a class of fused-ring systems used as precursors for copolymers and since these molecules were first synthesized, there is a long discussion about their photophysics and vibrational properties. For 2,3-dimethylthieno[3,4- b ]pyrazine, different degrees of solvation were combined to determine how the environment tunes these molecular properties, which can allow future applications. The present results indicate that unconventional C–H···N hydrogen bonds have great influence in both UV–Vis and IR spectra. The low-energy region is composed by two close n – π * and π – π * absorptions, with the lowest excitation being associated with an n – π * line. This absorption is covered by an intense π – π * explaining the reason why n – π * absorptions were never observed for these polymeric compounds. Another effect of the weak C–H···N bond is to tune the highest vibrational frequencies corresponding to C–H stretching modes of the solute’s methyl groups providing strong and uncommon blueshifts of ~ 15 cm −1 and better stabilizing the solute in the liquid environment.