Prediction of excitation energies for conjugated polymers using time-dependent density functional theory

Excitation energies of light-emitting conjugated polymers have been investigated with time-dependent density functional theory (TDDFT) within the adiabatic approximation. Our calculations show that the accuracy of the calculated TDDFT excitation energies largely depends on the dihedral angles obtained by the ground-state DFT geometry optimization. We find that, when the DFT torsional dihedral angles are close to experimental estimates, the TDDFT excitation energies agree well with experiments. This trend is observed based on calculations of eight different polymeric systems considered here. We further show that while hybrid density functionals can respect the thumb rule of ET≈2ES/3, where ES is the singlet-singlet excitation energy and ET the singlet-triplet excitation energy, nonhybrid functionals do not.