Triplet harvesting is a main challenge in organic light-emitting devices (OLEDs), because the radiative decay of the triplet is spin-forbidden. Here, we propose a new kind of OLED, in which an organic open-shell molecule, (4-N-carbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl (TTM-1Cz) radical, is used as an emitter, to circumvent the transition problem of triplet. For TTM-1Cz, there is only one unpaired electron in the highest singly occupied molecular orbital (SOMO). When this electron is excited to the lowest singly unoccupied molecular orbital (SUMO), the SOMO is empty. Thus, transition back of the excited electron to the SOMO is totally spin-allowed. Spectral analysis showed that electroluminescence of the OLED originated from the electron transition between SUMO and SOMO. The magneto-electroluminescence measurements revealed that the spin configuration of the excited state of TTM-1Cz is a doublet. Our results pave a new way to obtain 100% internal quantum efficiency of OLEDs.

In a neutral π-radical-based organic light-emitting diode (OLED), although the emission comes from the doublet excitons and their transition to the ground state is spin-allowed, the upper limit of internal quantum efficiency (IQE) is not clear, 50% or 100%? In this work, the deep-red OLEDs based on a neutral π-radical were fabricated. Up to 100% doublet exciton formation ratio was obtained through rational designing device structure and host-guest doping system. This indicates the IQE of neutral π-radical-based OLEDs will reach 100% if the nonradiative pathways of radicals can be suppressed. The maximum external quantum efficiency of the optimized device is as high as 4.3%, which is among the highest values of deep-red/near-infrared OLEDs with nonphosphorescent materials as emitters. Our results also indicate that using partially reduced radical mixture as emitter may be a way to solve aggregation-caused quenching in radical-based OLEDs.

Two D-A-type molecules, 4-N-[4-(9-phenylcarbazole)]-3,5-bis(4-diphenylamine)phenyl-4H-1,2,4-triazole and 4,4'-(9-(4-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-9H-carbazole-3,6-diyl) bis-(N,N-diphenylaniline), are designed and synthesized. Organic lightemitting diodes based on them exhibit deep-blue emission and the singlet formation ratios are higher than the simple spin-statistics of 25%. A triplet-polaroninteraction-induced upconversion from triplet to singlet through a one-electron transfer mechanism is proposed, and is proven by magnetocurrent measurement and quantum-chemistry computation.

Luminescent radicals have various applications because they simultaneously possess optoelectronic, electronic, and magnetic properties. Despite the development of some luminescent tris­(2,4,6-trichlorophenyl)­methyl (TTM)-based radicals, all the substituents directly attached to the TTM skeleton are electron-donating groups. Herein, the electron-withdrawing group is first attached to a p carbon of the parent TTM radical, and two novel stable open-shell adducts based on the benzimidazole unit with red-orange emission are obtained. Their photophysical properties, photochemical stabilities, and electroluminescent performances are fully investigated. Because of the introduction of the benzimidazole unit, the intramolecular charge transfer property of D–A type molecules is suppressed to a large extent, and the delocalization of the sole electron is strengthened. Both radicals exhibit largely improved photostability compared to that of the TTM core. High PL quantum yields (ΦF) of 0.39 and 0.36 in doped films are achieved, which are among the highest values for luminescent radicals. Extremely high-voltage-durable characteristic is demonstrated in the organic light-emitting diodes utilizing them as emitters. One device has a maximal external quantum efficiency that even exceeds the classical theoretical upper limit of 5%.