Beyond Radical Emitters: Hot-Exciton Electroluminescence From Singlet Diradicaloids.

Overcoming the 25% spin-statistical limit of fluorescent emitters remains a central challenge in organic electroluminescence. Although radical emitters efficiently harvest doublet excitons in organic light-emitting diodes, their optoelectronic performance can be affected by limited charge-carrier mobility and efficiency roll-off. Here we introduce trityl-based singlet diradicaloids with small diradical character as a new class of open-shell emitters and demonstrate their operation in organic light-emitting transistors (OLETs) using the polychlorinated Thiele hydrocarbon (TTH) as emitter. Comparison with previously reported radical-based devices reveals spin utilization around or exceeding 0.38 in TTH-based OLETs. Time-resolved electroluminescence, fs and ns transient absorption measurements, and quantum-chemical calculations demonstrate that this enhancement does not originate from triplet-triplet annihilation but from hot-exciton mechanism enabled by the diradical electronic structure, where the T2 state efficiently repopulates the emissive singlet via reverse intersystem crossing (RISC). Unlike conventional hot-exciton emitters based on donor-acceptor architectures, this system demonstrates hot-exciton behavior in a singlet diradicaloid framework. Reduced charge trapping further highlights singlet diradicaloids with small diradical character as promising platforms for efficient and stable electroluminescent devices.