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

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 (Delta E-ST) of 0.28 eV compared to only 0.01 eV found in 1 a. As expected, this Delta E-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.