Efficient Deep-Blue Fluorescent OLEDs with a High Exciton Utilization Efficiency from a Fully Twisted Phenanthroimidazole–Anthracene Emitter

Abstract

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 (EQE_max) 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.