Remote Heavy Atom Effect-Induced Efficient Ultralong Room-Temperature Phosphorescence of Biphenyl Imide

Despite its critical role in enhancing intersystem crossing (ISC) efficiency and promoting triplet exciton population, the heavy-atom effect (HAE) inevitably accelerates radiative decay, leading to a drastic reduction in the phosphorescence lifetime. This makes it particularly challenging to achieve highly efficient and long-lived room-temperature phosphorescence (RTP) under strong HAE conditions. In this study, we introduce halogenated aromatic groups as auxiliary units into a seven-membered biphenyl imide framework, leveraging a remote heavy-atom effect to facilitate ISC while preserving the slow radiative decay of triplet excitons. Through this rational molecular design, we obtain green phosphorescence with a balanced ultralong lifetime of 167.8–439.7 ms and a quantum yield of 0.8–10.8%. Single-crystal and photophysical analyses reveal that the remote heavy-atom effect─with distances exceeding 4.2 Å─plays a critical role in balancing the transition rates between triplet and singlet states. Modulating halogen bond distances to harness the remote heavy-atom effect offers a promising strategy for the development of high-performance organic phosphors.