Reverse intersystem crossing from upper triplet levels to excited singlet: a ‘hot excition’ path for organic light-emitting diodes

Since researches on the fate of highly excited triplet states demonstrated the existence of reverse intersystem crossing (RISC) from upper triplet levels to singlet manifold in naphthalene, quinoline, isoquinoline, etc. in the 1960s, this unique photophysical process was then found and identified in some other aromatic materials. However, the early investigations mainly focus on exploring the mechanism of this photophysical process; no incorporation of specific application was implemented. Until recently, our group innovatively used this ‘sleeping’ photophysical process to enhance the efficiency of fluorescent organic light-emitting diodes by simultaneously harvesting singlet and triplet excitons. Efforts are devoted to developing materials with high photoluminescence efficiency and effective RISC through appropriate molecular design in a series of donor–acceptor material systems. The experimental and theoretical results indicate that these materials exhibit hybridized local and charge-transfer excited state, which achieve a combination of the high radiation from local excited state and the high T m → S n ( m ≥2, n ≥1) conversion along charge-transfer excited state. As expected, the devices exhibited favourable external quantum efficiency and low roll-off, and especially an exciton utilization efficiency exceeding the limit of 25%. Considering the significant progress made in organic light-emitting diodes with this photophysical process, we review the relevant mechanism and material systems, as well as our design principle in materials and device application.