Beyond Radical Emitters: Hot‐Exciton Electroluminescence From Singlet Diradicaloids

ABSTRACT

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 T ₂ 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.