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The role of excited-state character, structural relaxation, and symmetry breaking in enabling delayed fluorescence activity in push-pull chromophores

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Thermally activated delayed fluorescence (TADF) is a current promising route for generating highly efficient light-emitting devices. However, the design process of new chromophores is hampered by the complicated underlying photophysics. In this work, four closely related donor-π-acceptor-π-donor systems are investigated, two of which were synthesised previously, with the aim of elucidating their varying effectiveness for TADF. We outline that the frontier orbitals are insufficient for discriminating between the molecules. Subsequently, a detailed analysis of the excited states at a correlated ab initio level highlights the presence of a number of closely spaced singlet and triplet states of varying character. Results from five density functionals are compared against this reference revealing dramatic changes in, both, excited state energies and wavefunctions following variations in the amount of Hartree-Fock exchange included. Excited-state minima are optimised in solution showing the crucial role of structural variations and symmetry breaking for producing a strongly emissive S1 state. The adiabatic singlet-triplet gaps thus obtained depend strongly on the range separation parameter used in the hybrid density functional calculations. More generally, this work highlights intricate differences present between singlet and triplet excited state wavefunctions and the challenges in describing them accurately.

History

School

  • Science

Department

  • Chemistry

Published in

Physical Chemistry Chemical Physics

Volume

23

Issue

46

Pages

26135-26150

Publisher

Royal Society of Chemistry (RSC)

Version

VoR (Version of Record)

Rights holder

© The Authors

Publisher statement

This is an Open Access Article. It is published by Royal Society of Chemistry under the Creative Commons Attribution-Non Commercial 4.0 International Licence (CC BY-NC). Full details of this licence are available at: https://creativecommons.org/licenses/by-nc/4.0/

Acceptance date

05/11/2021

Publication date

2021-11-08

Copyright date

2021

ISSN

1463-9076

eISSN

1463-9084

Language

en

Depositor

Dr Felix Plasser . Deposit date: 15 November 2021