LAUSR

Reverse intersystem crossing (RISC) unexpectedly dominates the interconversion of polaron pairs in the traditional fluorescent material 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB), with a large energy gap (\ensuremath{\Delta}${E}_{\mathrm{ST}}$) between singlet and triplet excitons, by employing the magnetoelectroluminescence detection technique. Contrary to RISC among excitons with small \ensuremath{\Delta}${E}_{\mathrm{ST}}$ from conventional thermally activated delayed fluorescence, these RISC processes are increased at a high current density and low temperature, which is ascribed to an enhanced direct charge-trapping effect and prolonged lifetime of the triplet-polaron pair, respectively. More intriguingly, the conversion of RISC to ISC is observed by reducing the electron-hole distance of polaron pairs via increasing the concentration of DCJTB dopant. Furthermore, the host must have a high triplet exciton energy to prevent energy back-transfer of the triplet exciton from the guest to the host and ensure efficient occurrence of the RISC process. The RISC-dominated device has a markedly higher external quantum efficiency than that of the ISC-governed one. This work not only deepens the microscopic understanding of the spin-pair states of red-light-emitting DCJTB-doped devices, but also provides a useful method to utilize a conventional fluorescent dopant for designing high-efficiency organic light-emitting diodes.