Raman scattering in crystalline oligothiophenes: a comparison between density functional theory and bond polarizability model.

Raman intensity of intramolecular and lattice modes of crystalline R-bithiophene (R-2T) are investigated within density functional theory using a nonlinear response formalism. First, comparison between the calculated Raman spectrum and the experimental data allows the assignment of the main Raman lines over the whole frequency range. Then, a bond polarizability (BP) model, limited to first neighbors, is built. We show that, although the BP model cannot reproduce the changes of dielectric susceptibility under individual atomic displacements, it is accurate enough to reproduce the profile of the unpolarized nonresonant Raman spectrum of R-2T powder. Finally, the BP model, fitted on our first-principles results on R-2T, is applied with success to the R-quaterthiophene polymorph phases and R-sexithiophene, demonstrating on practical examples that first-principles and BP approaches are powerful complementary tools to calculate the nonresonant Raman spectrum of R-2T and make reasonable predictions on larger oligothiophenes.