High-performance organic light-emitting diodes based on intramolecular charge-transfer emission from donor-acceptor molecules: Significance of electron-donor strength and molecular geometry

Organic light-emitting diodes based on intramolecular-charge-transfer emission from two related donor-acceptor (D-A) molecules, 3,7-[bis(4-phenyl-2-quinolyl)]-10-methylphenothiazine (BPQ-MPT) and 3,6-[bis(4-phenyl-2-quinolyl)]-9-methylcarbazole (BPQ-MCZ), were found to have electroluminescence (EL) efficiencies and device brightnesses that differ by orders of magnitude. High brightness (> 40 000 cd m(-2)) and high efficiency (21.9 cd A(-1), 10.8 lm W-1, 5.78 % external quantum efficiency (EQE) at 1140 cd m(-2)) green EL was achieved from the BPQ-MPT emitter, which has its highest occupied molecular orbital (HOMO) level at 5.09 eV and a nonplanar geometry. In contrast, diodes with much lower brightness (2290 cd m-2) and efficiency (1.4 cd A(-1), 0.66 Im W-1, 1.7 % EQE at 405 cd m(-2)) were obtained from the BPQ-MCZ emitter, which has its HOMO level at 5.75 eV and exhibits a planar geometry. Compared to BPQ-MCZ, the higher-lying HOMO level of BPQ-MPT facilitates more efficient hole injection/transport and a higher charge-recombination rate, while its nonplanar geometry ensures diode color purity. White EL was observed from BPQ-MCZ diodes owing to a blue intramolecular charge-transfer emission and a yellow-orange intermolecular excimer emission, enabled by the planar molecular geometry. These results demonstrate that high-performance light-emitting devices can be achieved from intramolecular charge-transfer emission, while highlighting the critical roles of the electron-donor strength and the molecular geometry of D-A molecules.