Aryl-Substituted Acridine Donor Derivatives Modulate the Transition Dipole Moment Orientation and Exciton Harvesting Properties of Donor-Acceptor TADF Emitters.

Thermally activated delayed fluorescence (TADF) compounds are highly attractive as sensitizing and emitting materials for organic light-emitting diodes (OLEDs). The efficiency of the OLED depends on multiple parameters, most of which rely on the properties of the emitter including those that govern the internal quantum and outcoupling efficiencies. Herein, we investigate a series of aryl substituted acridine donor derivatives of the donor-acceptor TADF emitter DMAC-TRZ, with the objective of correlating their properties, such as triplet harvesting efficiency and transition dipole moment orientation, with their corresponding device efficiency. The decoration of the DMAC donor with substituted aryl groups not only modifies the molecular weight and length of the emitter but also affects the emission color and the capacity for the emitters to efficiently harvest triplet excitons. The presence of electron-withdrawing 4-cyanophenyl and 4-trifluoromethylphenyl groups in, respectively, CNPh-DMAC-TRZ and CF3Ph-DMAC-TRZ, blue-shifts the emission spectrum but slows down the reverse intersystem crossing rate constant (k RISC), while the opposite occurs in the presence of electron-donating groups in t BuPh-DMAC-TRZ and OMePh-DMAC-TRZ (red-shifted emission spectrum and faster k RISC). In contrast to our expectations, the OLED performance of the five DMAC-TRZ derivatives does not scale with their degree of horizontal emitter orientation but follows the k RISC rates. This, in turn, demonstrates that triplet harvesting (and not horizontal emitter orientation) is the dominant effect for device efficiency using this family of emitters. Nonetheless, highly efficient OLEDs were fabricated with t BuPh-DMAC-TRZ and OMePh-DMAC-TRZ as emitters, with improved EQEmax (∼28%) compared to the reference DMAC-TRZ devices.

Helically chiral multiresonant thermally activated delayed fluorescent emitters and their use in hyperfluorescent organic light-emitting diodes.

Chiral multiresonant thermally activated delayed fluorescence (MR-TADF) materials show great potential as emitters in circularly polarized (CP) organic light-emitting diodes (CP-OLEDs) owing to their bright and narrowband CP emission. Here, two new chiral MR-TADF emitters tBuPh-BN and DPA-tBuPh-BN possessing intrinsically helical chirality have been synthesized and studied. The large steric interactions between the tert-butylphenyl groups not only induce the helical chirality but also provide a notable configurational stability to the enantiomers. Racemic mixtures of tBuPh-BN and DPA-tBuPh-BN show narrowband emission at 490 and 477 nm with full-width at half maximum (FWHM) of 25 and 28 nm and photoluminescence quantum yields, Φ PL, of 85 and 54% in toluene. The separated enantiomers of tBuPh-BN and DPA-tBuPh-BN show symmetric circularly polarized luminescence (CPL) with respective dissymmetry factors |g PL| values of 1.5 × 10-3 and 0.9 × 10-3. The hyperfluorescence organic light-emitting diodes (HF-OLEDs) with tBuPh-BN and DPA-tBuPh-BN acting as terminal emitters and 2,3,4,5,6-penta-(9H-carbazol-9-yl)benzonitrile (5CzBN) as their assistant dopant exhibited, respectively, maximum external quantum efficiencies (EQEmax) of 20.9 and 15.9% at 492 and 480 nm with FWHM of 34 and 38 nm. This work demonstrates a strategy for developing intrinsically helically chiral MR-TADF emitters possessing significant configurational stability, which can be used in HF-OLEDs.

Molecular asymmetry and rigidification as strategies to activate and enhance thermally activated delayed fluorescence in deep-blue MR-TADF emitters.

Two novel deep-blue multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters, 1B-CzCrs and 2B-CzCrs, containing a fused carbazole unit were synthesized. The carbazole contributed to the emergence of TADF in these small molecules. Particularly, organic light-emitting diodes with 1B-CzCrs doped in the mCP host achieve a maximum external quantum efficiency of 12.8% at CIE coordinates of (0.146, 0.062).

A Mononuclear [PtCl(tpy)]PF6 Complex for a Yellow Emitting Solution-Processable Organic Light Emitting Diode.

We report square planar mononuclear Pt(II)-complexes of terpyridines in the form of [PtCl(L1/L2)]PF6 as phosphorescent emitters (where L1 = 4-(3-pyridine)2,2':6',2''-terpyridine and L2 = 4'-(3-pyridinyl)-4,4''-di(tert-butyl)-2,2':6'2''-terpyridine). Complex 2 showed emission at 534 nm in the DCM solution with photoluminescence quantum efficiency (ΦPL) = 14%, while in the mCBP host (5-wt % doped), the emission shifted to 584 nm with ΦPL = 37.8% and a phosphorescence lifetime (τphos) of 37.8 µs. Complex 2 in mCBP was used to fabricate a solution-processed phosphorescent organic light-emitting diode (PhOLED) which showed maximum external quantum efficiency (EQEmax) = 7.4% with yellow emission at λEL = 570 nm and exhibited a low efficiency roll-off with an EQE drop to 7.0% at 1000 cd/m2.

A temperature sensor with a wide spectral range based on a dual-emissive TADF dendrimer system.

Dual emission from thermally activated delayed fluorescence (TADF) emitters is often difficult to observe, especially in solution, limited by Kasha's rule. Two TADF dendrimers containing N-doped polycyclic aromatic hydrocarbons as acceptors are designed and synthesized. Compound 2GCzBPN, having a strongly twisted geometry, exhibits TADF, while 2GCzBPPZ, possessing a less twisted geometry, shows dual emission associated with the monomer and aggregate that is TADF. The demonstration reveals that 2GCzBPPZ can serve as a temperature sensor with excellent temperature sensitivity and remarkably wide emission color response in solution. By embedding 2GCzBPPZ in paraffin we demonstrate a spatial-temperature sensor that shows a noticeable emission shift from yellow to green and ultimately to blue as the temperature increases from 20 to 200 °C. We finally demonstrate the utility of these TADF dendrimers in solution-processed organic light-emitting diodes.

Design, Synthesis, and Photophysical Characterization of Biocompatible Thermally Activated Delayed Fluorescent Carbazole-Coumarins for Sensing Applications.

A series of fluorescent carbazole-coumarins exhibiting good photoluminescence quantum yields and thermally activated delayed fluorescence (TADF) properties have been designed and synthetized using computer-aided density functional theory calculations. The TADF characteristics of the carbazole-coumarins were systematically explored both in solution and in the solid state, utilizing poly(methyl methacrylate) (PMMA) as a matrix. The study revealed that the introduction of carbazole units onto the coumarin benzene ring led to compounds with thermally induced reverse intersystem crossing and delayed fluorescence. The study further demonstrated the potential utility of these compounds in practical applications by incorporating them into a Cmr-PMMA-based sensor for molecular oxygen detection. The resulting sensor exhibited promising performance, highlighting the adaptability and efficacy of the synthesized TADF-carbazole-coumarin compounds for reversible molecular oxygen sensing.

Enhancing Circularly Polarized Electroluminescence through Energy Transfer within a Chiral Polymer Host.

Organic light-emitting diodes (OLEDs) that are able to emit high levels of circularly polarized (CP) light hold significant promise in numerous future technologies. Such devices require chiral emissive materials to enable CP electroluminescence. However, the vast majority of current OLED emitter classes, including the state-of-the-art triplet-harvesting thermally activated delayed fluorescence (TADF) materials, produce very low levels of CP electroluminescence. Here a host-guest strategy that allows for energy transfer between a chiral polymer host and a representative chiral TADF emitter is showcased. Such a mechanism results in a large amplification of the circular polarization of the emitter. As such, this study presents a promising avenue to further boost the performance of circularly polarized organic light-emitting diode devices, enabling their further development and eventual commercialization.

α-Phenylthioaldehydes for the effective generation of acyl azolium and azolium enolate intermediates.

α-Phenylthioaldehydes are readily prepared using a simple multi-step procedure and herein are introduced as a new precursor for the NHC-catalysed generation of acyl azolium and azolium enolate intermediates that are of widespread synthetic interest and utility. Treatment of α-phenylthioaldehydes with an NHC precatalyst and base produces an efficient redox rearrangement via a Breslow intermediate, elimination of thiophenolate, and subsequent rebound addition to the generated acyl azolium to give the corresponding thiol ester. In the presence of an external alcohol, competition between redox rearrangement and redox esterification can be controlled through judicious choice of the N-aryl substituent within the NHC precatalyst and the base used in the reaction. With NEt3 as base, NHCs bearing electron-withdrawing (N-C6F5 or N-C6H2Cl3) substituents favour redox rearrangement, while triazolium precatalysts with electron-rich N-aryl substituents (N-Ph, N-Mes) result in preferential redox esterification. Using DBU, redox esterification is preferred due to transesterification of the initially formed thiol ester product. Additionally, α-phenylthioaldehyde-derived azolium enolates have been used in enantioselective formal [4 + 2]-cycloaddition reactions to access dihydropyridinone heterocycles with high enantioselectivity (up to >95 : 5 dr, 98 : 2 er).

A Boron, Nitrogen, and Oxygen Doped π-Extended Helical Pure Blue Multiresonant Thermally Activated Delayed Fluorescent Emitter for Organic Light Emitting Diodes That Shows Fast kRISC Without the Use of Heavy Atoms.

Narrowband emissive multiresonant thermally activated delayed fluorescence (MR-TADF) emitters are a promising solution to achieve the current industry-targeted color standard, Rec. BT.2020-2, for blue color without using optical filters, aiming for high-efficiency organic light-emitting diodes (OLEDs). However, their long triplet lifetimes, largely affected by their slow reverse intersystem crossing rates, adversely affect device stability. In this study, a helical MR-TADF emitter (f-DOABNA) is designed and synthesized. Owing to its π-delocalized structure, f-DOABNA possesses a small singlet-triplet gap, ΔEST, and displays simultaneously an exceptionally faster reverse intersystem crossing rate constant, kRISC, of up to 2 × 106 s-1 and a very high photoluminescence quantum yield, ΦPL, of over 90% in both solution and doped films. The OLED with f-DOABNA as the emitter achieved a narrow deep-blue emission at 445 nm (full width at half-maximum of 24 nm) associated with Commission Internationale de l'Éclairage (CIE) coordinates of (0.150, 0.041), and showed a high maximum external quantum efficiency, EQEmax, of ≈20%.

Organic Donor-Acceptor Thermally Activated Delayed Fluorescence Photocatalysts in the Photoinduced Dehalogenation of Aryl Halides.

We report a family of donor-acceptor thermally activated delayed fluorescent (TADF) compounds based on derivatives of DMAC-TRZ, that are strongly photoreducing. Both Eox and thus E*ox could be tuned via substitution of the DMAC donor with a Hammett series of p-substituted phenyl moieties while Ered remained effectively constant. These compounds were assessed in the photoinduced dehalogenation of aryl halides, and analogues bearing electron withdrawing groups were found to produce the highest yields. Substrates of up to Ered=-2.72 V could be dehalogenated at low PC loading (1 mol %) and under air, conditions much milder than previously reported for this reaction. Spectroscopic and chemical studies demonstrate that all PCs, including literature reference PCs, photodegrade, and that it is these photodegradation products that are responsible for the reactivity.