Can time-dependent density functional theory predict the excitation energies of conjugated polymers?

TL;DR

This study evaluates the ability of time-dependent density functional theory (TDDFT) to predict excitation energies of conjugated polymers, emphasizing the importance of accurate ground-state geometries and functional choice.

Contribution

It demonstrates that TDDFT predictions depend heavily on ground-state geometry accuracy and highlights the limitations of semilocal functionals for triplet excitations.

Findings

TDDFT excitation energies align with experiments when geometries are accurate.

Hybrid functionals correctly predict singlet-triplet energy relationships.

Semilocal functionals are inadequate for triplet excitation predictions.

Abstract

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.