Twisted Intramolecular Charge Transfer State for Long-Wavelength Thermally Activated Delayed 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-PXZ-substituted 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

Dual Intramolecular Charge-Transfer Fluorescence Derived from a Phenothiazine-Triphenyltriazine Derivative

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. Temperature-dependence of transient PL spectra of a 2 wt % PTZ-TRZ-doped film in 3,3′-bis­(<i>N</i>-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

Post-Modification of Pyrrolopyrrole Aza-BODIPY toward High Near-Infrared Fluorescence Brightness

Pyrrolopyrrole aza-BODIPYs (PPABs), dimeric aza-BODIPY analogues, exhibit intense absorption and fluorescence in the visible and near-infrared (NIR) regions. Here, we developed a facile postmodification by palladium-catalyzed coupling reactions to synthesize a series of donor–acceptor–donor (D-A-D) PPABs. Despite the possible fluorescence quenching dictated by the energy-gap low, D-A-D PPABs exhibit high-fluorescence brightness in the NIR region, implying their potential use as a bright NIR emitter

Enhancing Triplet–Triplet Upconversion Efficiency and Operational Lifetime in Blue Organic Light-Emitting Diodes by Utilizing Thermally Activated Delayed Fluorescence Materials

In the process of triplet–triplet upconversion (TTU), a bright excited singlet can be generated because of the collision of two dark excited triplets. In particular, the efficiency of TTU is crucial for achieving a high exciton production yield in blue fluorescence organic light-emitting diodes (OLEDs) beyond the theoretical limit. While the theoretical upper limit of TTU contribution yield is expected to be 60%, blue OLEDs with the maximum TTU contribution are still scarce. Herein, we present a proof of concept for realizing the maximum TTU contribution yield in blue OLEDs, achieved through the doping of thermally activated delayed fluorescence (TADF) molecules in the carrier recombination zone. The bipolar carrier transport ability of TADF materials enables direct carrier recombination on the molecules, resulting in the expansion of the recombination zone. Although the external electroluminescence quantum efficiency of OLEDs is slightly lower than that of conventional TTU-OLEDs due to the low photoluminescence quantum yield of the doped layer, the TTU efficiency approaches the upper limit. Furthermore, the operational device lifetime of OLEDs employing TADF molecules increased by five times compared to the conventional ones, highlighting the expansion of the recombination zone as a crucial factor for enhancing overall OLED performance in TTU-OLEDs

Highly Efficient Thermally Activated Delayed Fluorescence from an Excited-State Intramolecular Proton Transfer System

Thermally activated delayed fluorescence (TADF) materials have shown great potential for highly efficient organic light-emitting diodes (OLEDs). While the current molecular design of TADF materials primarily focuses on combining donor and acceptor units, we present a novel system based on the use of excited-state intramolecular proton transfer (ESIPT) to achieve efficient TADF without relying on the well-established donor–acceptor scheme. In an appropriately designed acridone-based compound with intramolecular hydrogen bonding, ESIPT leads to separation of the highest occupied and lowest unoccupied molecular orbitals, resulting in TADF emission with a photoluminescence quantum yield of nearly 60%. High external electroluminescence quantum efficiencies of up to 14% in OLEDs using this emitter prove that efficient triplet harvesting is possible with ESIPT-based TADF materials. This work will expand and accelerate the development of a wide variety of TADF materials for high performance OLEDs

Utilization of Multi-Heterodonors in Thermally Activated Delayed Fluorescence Molecules and Their High Performance Bluish-Green Organic Light-Emitting Diodes

We report a series of pentacarbazolyl-benzonitrile derivatives such as 2,4,6-tri­(9H-carbazol-9-yl)-3,5-bis­(3,6-di­(pyridin-3-yl)-9H-carbazol-9-yl)­benzonitrile (mPyBN), 3,5-bis­(3,6-bis­(4-(trifluoromethyl)­phenyl)-9H-carbazol-9-yl)-2,4,6-tri­(9H-carbazol-9-yl)­benzonitrile (pCF3BN), 2,4,6-tri­(9H-carbazol-9-yl)-3-(3,6-di­(pyridin-3-yl)-9H-carbazol-9-yl)-5-(3,6-diphenyl-9H-carbazol-9-yl)­benzonitrile (PyPhBN), 3-(3,6-bis­(4-(trifluoromethyl)­phenyl)-9H-carbazol-9-yl)-2,4,6-tri­(9H-carbazol-9-yl)-5-(3,6-di­(pyridin-3-yl)-9H-carbazol-9-yl)­benzonitrile (PyCF3BN), and 3-(3,6-bis­(4-(trifluoromethyl)­phenyl)-9H-carbazol-9-yl)-2,6-di­(9H-carbazol-9-yl)-5-(3,6-di­(pyridin-3-yl)-9H-carbazol-9-yl)-4-(9H-pyrido­[3,4-b]­indol-9-yl)­benzonitrile (CbPyCF3BN) in which some of the carbazoles are substituted with modified 3,5-diphenyl carbazoles, exhibiting thermally activated delayed fluorescence (TADF) properties. These emitters comprised two, three, and four different types of donors, capable of bluish-green emission of around 480 nm with relatively high photoluminescence quantum yields over 90% in solution. Emitters, namely, PyPhBN, PyCF3BN, and CbPyCF3BN, composed of three and four different types of donors endowed a rather short delayed lifetime (τd) of 4.25, 5.01, and 3.65 μs in their film state, respectively. Bluish-green organic light-emitting diodes based on PyPhBN, PyCF3BN, and CbPyCF3BN exhibit a high external quantum efficiency of 20.6, 19.5, and 19.6%, respectively, with unsurpassed efficiency roll-off behavior. These results indicate that the TADF properties of multidonor type molecules can be manipulated by controlling the types and number of electron donor units

DataSheet1_Multiple resonance type thermally activated delayed fluorescence by dibenzo [1,4] azaborine derivatives.docx

We studied the photophysical and electroluminescent (EL) characteristics of a series of azaborine derivatives having a pair of boron and nitrogen aimed at the multi-resonance (MR) effect. The computational study with the STEOM-DLPNO-CCSD method clarified that the combination of a BN ring-fusion and a terminal carbazole enhanced the MR effect and spin-orbit coupling matrix element (SOCME), simultaneously. Also, we clarified that the second triplet excited state (T2) plays an important role in efficient MR-based thermally activated delayed fluorescence (TADF). Furthermore, we obtained a blue–violet OLED with an external EL quantum efficiency (EQE) of 9.1%, implying the presence of a pronounced nonradiative decay path from the lowest triplet excited state (T1).</p