Up to 100% Formation Ratio of Doublet Exciton in Deep-Red Organic Light-Emitting Diodes Based on Neutral π‑Radical

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

Doublet–Triplet Energy Transfer-Dominated Photon Upconversion

Stable luminescent π-radicals with doublet emission have aroused a growing interest for functional molecular materials. We have demonstrated a neutral π-radical dye (4-<i>N</i>-carbazolyl-2,6-dichlorophenyl)­bis­(2,4,6-trichlorophenyl)-methyl (TTM-1Cz) with remarkable doublet emission, which could be used as triplet sensitizer to initiate the photophysical process of triplet–triplet annihilation photon upconversion (TTA-UC). Dexter-like excited doublet-triplet energy transfer (DTET) was confirmed by theoretical calculation. With the same sensitizer, a mixed solution of TTM-1Cz and aromatic emitters could upconvert red light (λ = 635 nm) to blue or cyan light. An anti-Stokes energy shift as large as 0.92 eV was observed from red to blue light upconversion. This finding of DTET phenomena offers a new kind of triplet sensitizer for TTA-UC

Novel Luminescent Benzimidazole-Substituent Tris(2,4,6-trichlorophenyl)methyl Radicals: Photophysics, Stability, and Highly Efficient Red-Orange Electroluminescence

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 (Φ_F) 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%