The key role of acceptor moieties on the structural and the electronic properties of thermally activated delayed fluorescence emitters in excited states: A computational study

The insight into understanding the effect of acceptor moieties with phenoxazine (PXD) donor and bridging phenyl (Ph) spacer on structural and electronic properties of excited state was theoretically investigated through density functional theory simulations. Di-phenyltriazine (DPhTRZ), di-phenyl pyrimidine (DPhPyM), and dipyridyl pyrimidine (DPyPyM) were chosen as acceptor moieties for thermally activated delayed fluorescence (TADF) emitters. It was found that planar structure between phenyl spacer and DPhTRZ/DPyPyM was dominantly determined by intramolecular H-bonds at inner side of acceptor moiety. Depending on the acceptor moieties, the adiabatic excitation energy in singlet and triplet state is shifted to higher energies in order: DPyPyM (blue) > DPhPyM (greenish-blue) > DPhTRZ (green). In the perspective of spin conversion, the exact spin flip barrier, defined as total energy barrier for triplet-to-singlet transition, is in order of DPhTRZ < DPyPyM < DPhPyM. The calculated spin-orbit coupling matrix element with DPyPyM is relatively larger than that with DPhTRZ and DPhPyM. The calculated reverse intersystem crossing rate for triplet conversion is the largest with DPhTRZ and the smallest with DPhPyM. The comprehensive analyses conclusively suggest that DPyPyM acceptor moiety can be utilized for blue TADF emitters with superb structural rigidity, large spin conversion rate and low spin flip barrier.