Spatial extension of excitons in triphenylene based polymers given by range-separated functionals

TL;DR

This study uses TDDFT with various functionals to analyze exciton localization and luminescence in triphenylene polymers, revealing limitations of current models and potential for improved predictions of triplet emission energies.

Contribution

It demonstrates the application of long-range corrected TDDFT to accurately predict phosphorescence energies and exciton localization in triphenylene-based polymers, highlighting its advantages over traditional functionals.

Findings

Long-range corrected functional predicts phosphorescence energy accurately.

Triplet state localization differs from experimental expectations.

Long-range corrected TDDFT shows promise for better energy and localization predictions.

Abstract

Motivated by an experiment in which the singlet-triplet gap in triphenylene based copolymers was effectively tuned, we used time dependent density functional theory (TDDFT) to reproduce the main results. By means of conventional and long-range corrected exchange correlation functionals, the luminescence energies and the exciton localization were calculated for a triphenylene homopolymer and several different copolymers. The phosphorescence energy of the pure triphenylene chain is predicted accurately by means of the optimally tuned long-range corrected LC-PBE functional and slightly less accurate by the global hybrid B3LYP. However, the experimentally observed fixed phosphorescence energy could not be reproduced because the localization pattern is different to the expectations: Instead of localizing on the triphenylene moiety - which is present in all types of polymers - the triplet state localizes on the different bridging units in the TDDFT calculations. This leads to different triplet emission energies for each type of polymer. Yet, there are clear indications that long-range corrected TDDFT has the potential to predict the triplet emission energies as well as the localization behavior more accurate than conventional local or semi-local functionals.