π-Delocalization and the Vibrational Spectroscopy of Conjugated Materials: Computational Insights on Raman Frequency Dispersion in Thiophene, Furan, and Pyrrole Oligomers
Link to Publication: ACS Publication
Contributing Authors: Paul Donohoo-Vallett, Art Bragg
Date Published: January 27, 2015
The symmetric C=C stretching frequency (νЯ) 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 νЯ. 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 νЯ 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 to 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 νЯ and the coupling of oscillators that increase νЯ. These findings are used to address how variations in inter- and intramolecular order impact the Raman spectroscopy of polythiophenes.