Theoretical Study of Imidazole Derivatives for Blue Thermally Activated Delayed Fluorescence Emitter

Four novel thermally activated delayed fluorescence (TADF) materials with phenoxazine (PXZ) and 9,10-dihydro-9,9-dimethylacridine (ACR) as electron donors and 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PI), and 1,2-diphenyl-1H-benzo[d]imidazole (BI) as electron acceptors (PXZ-PI, ACR-PI, PXZ-BI, ACR-BI) were designed and theoretically investigated for blue OLED emitter. Using DFT and TDDFT calculations, we gained the electron distribution of HOMO and LUMO and the energy of the lowest singlet (S-1) and the lowest triplet (T-1) excited states. All of the materials have a small spatial overlap between HOMO and LUMO because of the relatively large dihedral angle between the phenyl ring and the acceptor moiety. In terms of the energy difference (Delta E-ST) between the S-1 state and the T-1 state, PXZ-PI, PXZ-BI, and ACR-BI showed the small Delta E-ST. However, ACR-PI showed the large Delta E-ST. Among the TADF materials, we showed that ACR-BI would have the best TADF properties in terms of small Delta E-ST and and blue OLED emitters.