Dual Channel Emissions of Kasha and Anti-Kasha from a Single Radical Molecule.

Stable open-shell luminescent radicals have recently attracted much attention due to their unique luminescence properties. However, a radical molecule with both Kasha and anti-Kasha doublet emission properties has not been reported. Herein, we have successfully synthesized a stable chlorine-substituted Chichibabin's hydrocarbon, TTM-TTM, along with its mono-radical counterpart, TTM-HTTM. The emission of TTM-TTM follows Kasha's rule in the near infrared region. However, TTM-HTTM shows dual channel doublet emissions of Kasha and anti-Kasha. Remarkably, these two types of emission compete dynamically in both solution and condensed states. Our findings provide valuable insights into the rational design and discovery of stable radicals that possess distinctive luminescent properties, thus broadening the horizons of luminescent materials research.

Luminescent Crystalline Carbon- and Nitrogen-Centered Organic Radicals Based on N-Heterocyclic Carbene-Triphenylamine Hybrids.

Developing luminescent radicals with tunable emission is a challenging task due to the limitation of alternative skeletons. Herein, a series of carbene-triphenylamine hybrids were prepared by the direct C2-arylation of N-heterocyclic carbenes with 4-bromo-N,N-bis(4-methoxyphenyl)aniline. These hybrids showed multiple redox-active properties and could be converted to carbon-centered luminescent radicals with blue-to-cyan emissions (λmax : 436-486 nm) or nitrogen-centered luminescent radicals with orange emissions (λmax : 590-623 nm) through chemical reduction or oxidation, respectively. The radical species were characterized by electron paramagnetic resonance spectroscopy, ultraviolet-visible spectroscopy, and single-crystal X-ray diffractometry analysis. Notably, the corresponding nitrogen-centered radicals exhibited good stability in atmospheric air, and their thermal decomposition temperatures were determined to be above 200 °C. In addition, spectral and theoretical calculations indicate that all radicals exhibit anti-Kasha emissions.

Delayed Doublet Emission in a Cerium(III) Complex.

Doublet emission from open-shell molecules has demonstrated its research and application value in recent years. However, understandings of the photoluminescence mechanism of open-shell molecules are far less than that of closed-shell molecules, leading to challenges in molecular design of efficient doublet emission systems. Here we report a cerium(III) 4-(9H-carbozol-9-yl)phenyl-tris(pyrazolyl)borate complex Ce(CzPhTp)3 with a new luminescence mechanism of delayed doublet emission, which also represents the first example with metal-centered delayed photoluminescence. The energy gap between the doublet and triplet excited states of Ce(CzPhTp)3 is reduced by the management of the inner and outer coordination spheres, thereby promoting efficient energy transfer between the two excited states and activating the delayed emission. The photoluminescence mechanism discovered may provide a new way for the design of efficient doublet emission and bring insights into rational molecular design and energy level regulation in open-shell molecules.

Unusual Quartet-Doublet Phosphorescence from the Phosphaethynyl Radical, CP.

We report the first detection of phosphorescence from the phosphaethynyl radical. This rare instance of quartet-doublet emission, studied here in solid argon, is presumably promoted by efficient intersystem crossing from the originally photoexcited doublet (B2 Σ+ ) to the adjacent second quartet state, 14 Δ. Vibronic progressions were traced for the a4 Σ+ -X2 Σ+ and a4 Σ+ -A2 Пi systems from their origins up to (v'=0)→(v''=5) and (v'=0)→(v''=2) bands, respectively. The measured phosphorescence lifetime is 108 ± 3 ms.

A Pure-Red Doublet Emission with 90 % Quantum Yield: Stable, Colorless, Iodinated Triphenylmethane Solid.

Red luminescence is found in off-white tris(iodoperchlorophenyl)methane (3I-PTMH ) crystals which is characterized by a high photoluminescence quantum yield (PLQY 91 %) and color purity (CIE coordinates 0.66, 0.34). The emission originates from the doublet excited state of the neutral radical 3I-PTMR , which is spontaneously formed and becomes embedded in the 3I-PTMH matrix. The radical defect can also be deliberately introduced into 3I-PTMH crystals which maintain a high PLQY with up to 4 % radical concentration. The immobilized iodinated radical demonstrates excellent photostability (estimated half-life >1 year under continuous irradiation) and intriguing luminescent lifetime (69 ns). TD-DFT calculations demonstrate that electron-donating iodine atoms accelerate the radiative transition while the rigid halogen-bonded matrix suppresses the nonradiative decay.

Fast Transfer of Triplet to Doublet Excitons from Organometallic Host to Organic Radical Semiconductors.

Spin triplet exciton formation sets limits on technologies using organic semiconductors that are confined to singlet-triplet photophysics. In contrast, excitations in the spin doublet manifold in organic radical semiconductors can show efficient luminescence. Here the dynamics of the spin allowed process of intermolecular energy transfer from triplet to doublet excitons are explored. A carbene-metal-amide (CMA-CF3) is employed as a model triplet donor host, since following photoexcitation it undergoes extremely fast intersystem crossing to generate a population of triplet excitons within 4 ps. This enables a foundational study for tracking energy transfer from triplets to a model radical semiconductor, TTM-3PCz. Over 74% of all radical luminescence originates from the triplet channel in this system under photoexcitation. It is found that intermolecular triplet-to-doublet energy transfer can occur directly and rapidly, with 12% of triplet excitons transferring already on sub-ns timescales. This enhanced triplet harvesting mechanism is utilized in efficient near-infrared organic light-emitting diodes, which can be extended to other opto-electronic and -spintronic technologies by radical-based spin control in molecular semiconductors.

Efficient rare earth cerium(III) complex with nanosecond d-f emission for blue organic light-emitting diodes.

In the field of RGB diodes, development of a blue organic light-emitting diode (OLED) is a challenge because of the lack of an emitter which simultaneously has a short excited state lifetime and a high theoretical external quantum efficiency (EQE). We demonstrate herein a blue emissive rare earth cerium(III) complex Ce-2 showing a high photoluminescence quantum yield of 95% and a short excited state lifetime of 52.0 ns in doped film, which is considerably faster than that achieved in typical efficient phosphorescence or thermally activated delayed fluorescence emitters (typical lifetimes >1 µs). The corresponding OLED shows a maximum EQE up to 20.8% and a still high EQE of 18.2% at 1000 cd m-2, as well as an operation lifetime 70 times longer than that of a classic phosphorescence OLED. The excellent performance indicates that cerium(III) complex could be a candidate for efficient and stable blue OLEDs because of its spin- and parity-allowed d-f transition from the Ce3+ ion.