Pyrene-Based Emitter with a Small Energy Gap Between High-Energy Triplet and Singlet State for High-Performance Blue Organic Light-Emitting Diodes

Reasonable energy level alignment of molecules is conducive to constructing high-performance blue emitter. Pyrene is a common building unit in blue emitters with a big energy gap between S1 and T1 (Delta ES1-T1) and small Delta ES1-Tn (n = 2-3). By introducing donor and acceptor groups in pyrene, the high-lying triplet excitons are utilized through reverse intersystem crossing (RISC) channel in obtained blue "hot exciton" emitter CPPCN. Herein, through changing the donor linking type to ortho-substituted, the obtained derivatives o-CPPCN exhibited deeper blue emission (lambda max = 448 nm) and a higher krisc (3.1x108 s-1). The non-doped device based on o-CPPCN demonstrates a maximum external quantum efficiency (EQEmax) of 10.3% with a CIEy of 0.11, along with an exciton utilization of 55%. Photophysical measurements indicate the performance difference between the CPPCN and o-CPPCN originates from the energy level alignment difference. Compared with CPPCN (0.26 eV), the Delta ES1-T2 of o-CPPCN has been reduced to 0.10 eV, proving the smaller gap between T2 and S1 state can promote the RISC process. Hence, o-CPPCN can be employed as the host to prepare "hot exciton" sensitized blue OLED due to its wide band gap and high krisc. The corresponding EQEmax of the doped device can be further improved to 11.8%. Reasonable energy level alignment of molecules is conducive to constructing high-performance blue emitter. The photophysical properties difference of two pyrene-based "hot exciton" materials CPPCN and o-CPPCN indicate that the smaller gap between high-energy triplet state and the singlet state can promote the reverse intersystem crossing process, which is in favor of realizing high efficiency and color purity simultaneously. image