Molecular Design Strategy of Thermally Activated Delayed Fluorescent Emitters Using CN-Substituted Imidazopyrazine as a New Electron-Accepting Unit

Thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) have attracted enormous attention recently due to their capability to replace conventional phosphorescent organic light-emitting diodes for practical applications. In this work, a newly designed CN-substituted imidazopyrazine moiety was utilized as an electron-accepting unit in a TADF emitter. Two TADF emitters, 8-(3-cyano-4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (Ac-CNImPyr) and 8-(3-cyano-4-(10H-phenoxazin-10-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (PXZ-CNImPyr), were developed based on the CN-substituted imidazopyrazine acceptor combined with acridine and phenoxazine donor, respectively. A CN-substituted phenyl spacer was introduced between the donor and acceptor for a sufficiently small singlet-triplet energy gap (ΔE_ST) and molecular orbital management. Small ΔE_ST of 0.07 eV was achieved for the phenoxazine donor-based PXZ-CNImPyr emitter. As a result, an organic light-emitting diode based on the PXZ-CNImPyr emitter exhibited a high external quantum efficiency of up to 12.7 %, which surpassed the EQE limit of common fluorescent emitters. Hence, the CN-modified imidazopyrazine unit can be introduced as a new acceptor for further modifications to develop efficient TADF-based OLEDs.