No single DFT method can predict Raman cross-sections, frequencies and electronic absorption maxima of oligothiophenes

Abstract

Density functional theory (DFT) is a popular method for modeling the properties of organic photovoltaic materials. The ability of various functionals to predict the electronic absorption and Raman spectra of oligothiophenes is assessed. Time-Dependent DFT (TD-DFT) is commonly used for modeling electron absorption spectra but often gives spurious results. We show here that a linear relationship between the percentage of Hartree–Fock exchange (%HF) used by a density functional and the predicted excitation energy exists. This allows for tuning of hybrid functionals which can accurately predict the bandgap of four different oligothiophenes. For instance, 33% HF is required for a four unit oligothiophene which absorbs at 379nm (3.27eV) while 46% HF is required for a ten unit oligothiophene absorbing at 458nm (2.71eV). Raman spectroscopy is also used to understand conjugation in highly conjugated molecules. The Raman scattering cross-sections and frequencies of the same four oligothiophenes were measured and compared to those modeled by DFT. We show that the DFT models tuned to correctly the visible absorption bands overestimate the Raman cross-sections and underestimate their frequencies. These results should be taken into account when modeling future conjugated materials.