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Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here, we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.

Fluorescence-based organic light-emitting diodes have continued to attract interest because of their long operational lifetimes, high colour purity of electroluminescence and potential to be manufactured at low cost in next-generation full-colour display and lighting applications. In fluorescent molecules, however, the exciton production efficiency is limited to 25% due to the deactivation of triplet excitons. Here we report fluorescence-based organic light-emitting diodes that realize external quantum efficiencies as high as 13.4-18% for blue, green, yellow and red emission, indicating that the exciton production efficiency reached nearly 100%. The high performance is enabled by utilization of thermally activated delayed fluorescence molecules as assistant dopants that permit efficient transfer of all electrically generated singlet and triplet excitons from the assistant dopants to the fluorescent emitters. Organic light-emitting diodes employing this exciton harvesting process provide freedom for the selection of emitters from a wide variety of conventional fluorescent molecules.

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Allergic asthma is caused by the aberrant expansion in the lung of T helper cells that produce type 2 (TH2) cytokines and is characterized by infiltration of eosinophils and bronchial hyperreactivity. This disease is often triggered by mast cells activated by immunoglobulin E (IgE)-mediated allergic challenge. Activated mast cells release various chemical mediators, including prostaglandin D2 (PGD2), whose role in allergic asthma has now been investigated by the generation of mice deficient in the PGD receptor (DP). Sensitization and aerosol challenge of the homozygous mutant (DP-/-) mice with ovalbumin (OVA) induced increases in the serum concentration of IgE similar to those in wild-type mice subjected to this model of asthma. However, the concentrations of TH2 cytokines and the extent of lymphocyte accumulation in the lung of OVA-challenged DP-/- mice were greatly reduced compared with those in wild-type animals. Moreover, DP-/- mice showed only marginal infiltration of eosinophils and failed to develop airway hyperreactivity. Thus, PGD2 functions as a mast cell-derived mediator to trigger asthmatic responses.

Efficient thermally activated delayed fluorescence (TADF) was developed in a material based on a phenoxazine (PXZ) electron donor unit and a 2,4,6-triphenyl-1,3,5-triazine (TRZ) electron acceptor unit. An organic light-emitting diode containing this novel TADF emitter layer was fabricated and exhibited a maximum external quantum efficiency of 12.5% with green emission.

A material containing a phenothiazine (PTZ) electron donor unit and 2,4,6-triphenyl-1,3,5-triazine (TRZ) electron acceptor unit, PTZ-TRZ, which exhibits thermally activated delayed fluorescence (TADF) was developed. Density functional theory calculations revealed the existence of two ground-state conformers with different energy gaps between the lowest singlet excited state and lowest triplet excited state (1.14 and 0.18 eV), which resulted from the distortion of PTZ, as confirmed by X-ray structure analysis. PTZ-TRZ in toluene solution showed two broad, structureless emissions, confirming the existence of two different excited states. From detailed analyses of the absorption and photoluminescence spectra, we determined that both emissions were intramolecular charge-transfer (ICT) fluorescence. Therefore, the excited-state conformers of PTZ-TRZ resulted in dual ICT fluorescence. Because previously reported dual fluorescence from single molecules involves locally excited and ICT fluorescence, the dual ICT fluorescence from PTZ-TRZ is novel. Temperaturedependence of transient PL spectra of a 2 wt % PTZ-TRZ-doped film in 3,3′-bis(N-carbazolyl)-1,1′-biphenyl measured by a streak camera revealed that the former and latter emissions were independent of and dependent on the film temperature, respectively. This confirms that the dual fluorescence involves TADF characteristics. An organic light-emitting diode containing PTZ-TRZ exhibited a maximum external quantum efficiency of 10.8 ± 0.5% with dual ICT fluorescence.

Emission wavelength tuning of thermally activated delayed fluorescence from green to orange in solid state films is demonstrated. Emission tuning occurs by stabilization of the intramolecular charge transfer state between a phenoxazine (PXZ) donor unit and 2,4,6-triphenyl-1,3,5-triazine (TRZ) acceptor unit separated by a large twist angle. The emission wavelengths of mono-, bis-, and tri-PXZsubstituted TRZ exhibit a gradual red shift while maintaining a small energy gap between the singlet and triplet excited states. An organic light-emitting diode containing a tri-PXZ-TRZ emitter exhibited a maximum external quantum efficiency of 13.3 ± 0.5% with yellow-orange emission. KEYWORDS: organic light-emitting diodes, thermally activated delayed fluorescence, emission wavelength tuning, intramolecular charge transfer ■ INTRODUCTIONSince our group reported the first observation of electroluminescence (EL) based on thermally activated delayed fluorescence (TADF) from a Sn 4+ −porphyrin complex, 1 the potential of TADF materials as emitters for organic lightemitting diodes (OLEDs) has been revealed. 2 A remarkable feature of TADF is up-conversion of excitons from the lowest triplet excited state (T 1 ) of a compound to its lowest singlet excited state (S 1 ), which strongly depends on the energy gap between them (ΔE S-T ). Up-conversion from T 1 to S 1 can be realized in molecules with donor−acceptor (D−A) moieties that induce intramolecular charge transfer (ICT). TADF materials are currently attracting considerable attention as third-generation OLEDs because of their high EL efficiency and lack of rare metals such as Ir and Pt. To replace the present OLEDs based on fluorescent and phosphorescent materials, methodology for RGB emission wavelength tuning is essential. The emission process of general fluorescent materials involves π−π* transitions via singlet excitons, and emission wavelength tuning is achieved by controlling the length of π-conjugation. Attaching substituents such as electron-donating or -withdrawing groups to fluorescent molecules is also an effective way to tune emission wavelength. 3 Conversely, the emission process of typical phosphorescent materials such as iridium complexes is based on triplet metal-to-ligand charge transfer ( 3 MLCT) transitions. Their emission wavelength has also been tuned by extending the conjugation of the ligand. 4 In the case of TADF materials, the emission process is categorized as ICT transitions via triplet excitons. To minimize ΔE S-T for efficient up-conversion, a TADF molecule needs its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to be effectively separated, which can be achieved using a twisted structure. Effective HOMO−LUMO separation induces the ICT transition from HOMO to LUMO. Regarding the molecular design of TADF materials, simple extension of the π-conjugated system results in a large ΔE S-T and weak ICT transition, which induces a locally excited state and dual fluorescence. 5 While efficient blue 2,6 and green 2,7 TAD...