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Fine tuning the Photophysics of Donor‐Acceptor (D‐A3) Thermally Activated Delayed Fluorescence Emitters Using Isomerisation

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  • Paloma Lays dos Santos
  • Daniel de Sa Pereira
  • Julien Eng
  • Jonathan S. Ward
  • Martin R. Bryce
  • Thomas J. Penfold
  • Andrew P. Monkman
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Article numbere202200248
<mark>Journal publication date</mark>28/02/2023
<mark>Journal</mark>CHEMPHOTOCHEM
Issue number2
Volume7
Number of pages7
Publication StatusPublished
Early online date15/12/22
<mark>Original language</mark>English

Abstract

Here two D–A 3 regioisomers, comprising three dibenzothiophene-S,S-dioxide acceptor units attached to a central triazatruxene core, are studied. Both molecules show thermally activated delayed fluorescence (TADF), however, the efficiency of the TADF mechanism is strongly affected by the D–A substitution position. The meta- substituted emitter (1 b) shows a slightly higher-lying singlet charge transfer state and a lower-lying triplet state than that observed in the para- substituted emitter (1 a), resulting in a larger singlet–triplet splitting (ΔE ST) of 0.28 eV compared to only 0.01 eV found in 1 a. As expected, this ΔE ST difference strongly impacts the reverse intersystem crossing (rISC) rates and the para- isomer 1 a exhibits a much faster delayed fluorescence emission. Calculations show that the triplet energy difference between the two isomers is due to steric hindrance variances along the donor–acceptor rotation axis in these molecules: as 1 b is less restricted, rotation of its acceptor unit leads to a lower T 1 energy, further away from the region of high density of states (the region where larger vibronic coupling is found, favouring rISC). Therefore, our results show how the substitution pattern has a marked effect on triplet state energies and character, verifying the key structural designs for highly efficient TADF materials.