π-Delocalization and the vibrational spectroscopy of conjugated materials: computational insights on Raman frequency dispersion in thiophene, furan, and pyrrole oligomers.

The symmetric C═C stretching frequency (ν(Я)) of conjugated polymers and oligomers is a sensitive spectroscopic reporter of molecular structure and material morphologies; however, thorough understanding of how structure affects this frequency is lacking because computational investigations of this relationship have been undertaken with limited approaches. We present a comprehensive computational investigation of the structure-dependent Raman spectroscopy of oligothiophenes, oligofurans, and oligopyrroles in their ground electronic states using density functional theory. We assessed how various functionals (BLYP, B3LYP, BHLYP, and CAM-B3LYP) impact predictions of length-dependent trends in ν(Я). The amount of Hartree-Fock exchange in a functional is critical for accurately treating π-delocalization and polarizability and hence the structure-dependent Raman behavior. BLYP and B3LYP fail to accurately predict trends in ν(Я) with oligomer length because they over-represent delocalization; in contrast, the range-corrected CAM-B3LYP functional produces the same trends observed experimentally for oligomers in solution and in the solid phase. Through comparisons with a simple mechanical model, we demonstrate that the length- and conformation-dependent spectroscopy of oligothiophenes results from a delicate balance between delocalization-induced softening of ν(Я) and the coupling of oscillators that increase ν(Я). These findings are used to address how variations in inter- and intramolecular order impact the Raman spectroscopy of polythiophenes.