  Excitation energies of light-emitting organic conjugated polymers have been
investigated with time-dependent density functional theory (TDDFT) within the
adiabatic approximation for the dynamical exchange-correlation potential. Our
calculations show that the accuracy of the calculated TDDFT excitation energies
largely depends upon the accuracy of the dihedral angle obtained by the
geometry optimization on ground-state DFT methods. We find that, when the DFT
torsional dihedral angles between two adjacent phenyl rings are close to the
experimental dihedral angles, the TDDFT excitation energies agree fairly well
with experimental values. Further study shows that, while hybrid density
functionals can correctly respect the thumb rule between singlet-singlet and
singlet-triplet excitation energies, semilocal functionals do not, suggesting
inadequacy of the semilocal functionals in predicting triplet excitation
energies of conjugated polymers.
