A novel, efficient, deep-blue fluorescent emitter mPAC, with a meta-connected donor-acceptor structure containing phenanthroimidazole (PPI) as the donor and phenylcarbazole-substituted anthracene (An-CzP) as the acceptor, was designed and synthesized. The meta-linkage provided a highly twisted molecular conformation, which efficiently interrupts the intramolecular π-conjugation, resulting in a deep-blue emission. The optimized nondoped device based on mPAC displayed a deep-blue emission with a narrow full width at half-maximum of 56 nm and Commission Internationale de L'Eclairage coordinates of (0.16, 0.09). The maximum external quantum efficiency (EQEmax) is 6.76%, corresponding to a high exciton utilization efficiency (EUE) of 59.3-88.9%. Experimental results and theoretical analysis indicated that the high EUE is mainly ascribed to the reverse intersystem crossing (RISC) from T2 to S1, a "hot exciton" path in which the large T2-T1 energy gap (1.45 eV) and small T2-S1 energy difference (0.18 eV, T2 > S1) hamper the internal crossing from T2 to T1 and facilitate the RISC process. For the hot exciton path, the T2 state can be feasibly arranged to a high energy level, forming a thermal equilibrium with S1, even slightly higher than the deep-blue S1 to realize an exergonic RISC process, which is usually difficult for the thermally activated delayed fluorescence emitters.
Keywords: RISC; deep-blue fluorescent OLEDs; exciton utilization efficiency; fully twisted; hot exciton.