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.

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).

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 we showcase a host-guest strategy that allows for energy transfer between a chiral polymer host and a representative chiral TADF emitter. Such a mechanism results in large amplification of the circular polarization of the emitter. As such, this study presents a promising avenue to further boost the performance of CP-OLED devices, enabling their further development and eventual commercialization. This article is protected by copyright. All rights reserved.

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%.

A novel B,O,N-doped mesogen with narrowband MR-TADF emission.

Modification of an unsymmetric B,O,N-doped aromatic core with peripheral mesogenic units triggers self-assembly into a columnar hexagonal mesophase, which is stable between 22 and 144 °C. The columnar assembly is preserved in a glassy state below 22 °C. The B,O,N-doped mesogen displays narrowband sky-blue multiresonance thermally activated delayed fluorescence (MR-TADF) under diluted conditions and bright excimer emission in condensed phase. Our combined experimental and theoretical approach provides insight into the development of strongly aggregating liquid crystalline MR-TADF emitters.

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.

Lessons learnt in photocatalysis - the influence of solvent polarity and the photostability of the photocatalyst.

Herein, we show that there is significant variation in both the triplet energies and redox properties of photocatalysts as a function of solvent based on a study of eight PCs in four solvents of varying polarity. A range of photocatalytic electron and energy transfer reactions were investigated using a subset of the PCs. For the photoredox reactions, the yields are not correlated with solvent polarity. Instead, when the PC could promote the formation of the target product, we observed photodegradation for all PCs across all solvents, something that is rarely investigated in the literature. This, therefore, makes it difficult to ascertain whether the parent PC and/or the photodegraded product is responsible for the photochemistry, or indeed, whether photodegradation is actually detrimental to the reaction yield. Conversely, the PCs were found to be photostable for energy transfer reactions; however, yields were not correlated to the triplet energies of the PCs, highlighting that triplet energies alone are not a suitable descriptor to discriminate the performance between PCs in photoinduced energy transfer processes.

Metal-free photocatalytic cross-electrophile coupling enables C1 homologation and alkylation of carboxylic acids with aldehydes.

In contemporary drug discovery, enhancing the sp3-hybridized character of molecular structures is paramount, necessitating innovative synthetic methods. Herein, we introduce a deoxygenative cross-electrophile coupling technique that pairs easily accessible carboxylic acid-derived redox-active esters with aldehyde sulfonyl hydrazones, employing Eosin Y as an organophotocatalyst under visible light irradiation. This approach serves as a versatile, metal-free C(sp3)-C(sp3) cross-coupling platform. We demonstrate its synthetic value as a safer, broadly applicable C1 homologation of carboxylic acids, offering an alternative to the traditional Arndt-Eistert reaction. Additionally, our method provides direct access to cyclic and acyclic ß-arylethylamines using diverse aldehyde-derived sulfonyl hydrazones. Notably, the methodology proves to be compatible with the late-stage functionalization of peptides on solid-phase, streamlining the modification of intricate peptides without the need for exhaustive de-novo synthesis.

Thermally activated delayed fluorescence emitters showing wide-range near-infrared piezochromism and their use in deep-red OLEDs.

Organic small molecules exhibiting both thermally activated delayed fluorescence (TADF) and wide-ranging piezochromism (Δλ > 150 nm) in the near-infrared region have rarely been reported in the literature. We present three emitters MeTPA-BQ, tBuTPA-BQ and TPPA-BQ based on a hybrid acceptor, benzo[g]quinoxaline-5,10-dione, that emit via TADF, having photoluminescence quantum yields, ΦPL, of 39-42% at photoluminescence (PL) maxima, λPL, of 625-670 nm in 2 wt% doped films in 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP). Despite their similar chemical structures, the PL properties in the crystalline states of MeTPA-BQ (λem = 735 nm, ΦPL = 2%) and tBuTPA-BQ (λem = 657 nm, ΦPL = 11%) are significantly different. Further, compounds tBuTPA-BQ and TPPA-BQ showed a significant PL shift of ∼98 and ∼165 nm upon grinding of the crystalline samples, respectively. Deep-red organic light-emitting diodes with MeTPA-BQ and tBuTPA-BQ were also fabricated, which showed maximum external quantum efficiencies, EQEmax, of 10.1% (λEL = 650 nm) and 8.5% (λEL = 670 nm), respectively.

A fluorene-bridged double carbonyl/amine multiresonant thermally activated delayed fluorescence emitter for efficient green OLEDs.

Herein, we report a fluorene-bridged double carbonyl/amine-based MR TADF emitter DDiKTa-F, formed by locking the conformation of the previously reported compound DDiKTa. Using this strategy, DDiKTa-F exhibited narrower, brighter, and red-shifted emission. The OLEDs with DDiKTa-F emitted at 493 nm and showed an EQEmax of 15.3% with an efficiency roll-off of 35% at 100 cd m-2.

Heavy Atom Effect in Halogenated mCP and Its Influence on the Efficiency of the Thermally Activated Delayed Fluorescence of Dopant Molecules.

In this study, we explore the impact of halogen functionalization on the photophysical properties of the commonly used organic light-emitting diode (OLED) host material, 1,3-bis(N-carbazolyl)benzene (mCP). Derivatives with different numbers and types of halogen substituents on mCP were synthesized. By measuring steady-state and transient photoluminescence at 6 K, we study the impact of the type, number, and position of the halogens on the intersystem crossing and phosphorescence rates of the compounds. In particular, the functionalization of mCP with 5 bromine atoms results in a significant increase of the intersystem crossing rate by a factor of 300 to a value of (1.5 ± 0.1) × 1010 s-1, and the phosphorescence rate increases by 2 orders of magnitude. We find that the singlet radiative decay rate is not significantly modified in any of the studied compounds. In the second part of the paper, we describe the influence of these compounds on the reverse intersystem crossing of the 7,10-bis(4-(diphenylamino)phenyl)-2,3-dicyanopyrazino-phenanthrene (TPA-DCPP), a TADF guest, via the external heavy atom effect. Their use results in an increase of the reverse intersystem crossing (RISC) rate from (8.1 ± 0.8) × 103 s-1 for mCP to (2.7 ± 0.1) × 104 s-1 for mCP with 5 bromine atoms. The effect is even more pronounced for the mCP analogue containing a single iodine atom, which gives a RISC rate of (3.3 ± 0.1) × 104 s-1. Time-dependent DFT calculations reveal the importance of the use of long-range corrected functionals to predict the effect of halogenation on the optical properties of the mCP, and the relativistic approximation (ZORA) is used to provide insight into the strength of the spin-orbit coupling matrix element between the lowest-lying excited singlet and triplet states in the different mCP compounds.

The Blue Problem: OLED Stability and Degradation Mechanisms.

OLED technology has revolutionized the display industry and is promising for lighting. Despite its maturity, there remain outstanding device and materials challenges to address. Particularly, achieving stable and highly efficient blue OLEDs is still proving to be difficult; the vast array of degradation mechanisms at play, coupled with the precise balance of device parameters needed for blue high-performance OLEDs, creates a unique set of challenges in the quest for a suitably stable yet high-performance device. Here, we discuss recent progress in the understanding of device degradation pathways and provide an overview of possible strategies to increase device lifetimes without a significant efficiency trade-off. Only careful consideration of all variables that go into OLED development, from the choice of materials to a deep understanding of which degradation mechanisms need to be suppressed for the particular structure, can lead to a meaningful positive change toward commercializable blue devices.

Mechanophotocatalysis: A Generalizable Approach to Solvent-minimized Photocatalytic Reactions for Organic Synthesis.

This proof-of-concept study cements the viability and generality of mechanophotocatalysis, merging mechanochemistry and photocatalysis to enable solvent-minimized photocatalytic reactions. We demonstrate the transmutation of four archetypal solution-state photocatalysis reactions to a solvent-minimized environment driven by the combined actions of milling, light, and photocatalysts. The chlorosulfonylation of alkenes and the pinacol coupling of aldehydes and ketones were conducted under solvent-free conditions with competitive or superior efficiencies to their solution-state analogues. Furthermore, decarboxylative alkylations are shown to function efficiently under solvent-minimized conditions, while the photoinduced energy transfer promoted [2+2] cycloaddition of chalcone experiences a significant initial rate enhancement over its solution-state variant. This work serves as a platform for future discoveries in an underexplored field: validating that solvent-minimized photocatalysis is not only generalizable and competitive with solution-state photocatalysis, but can also offer valuable advantages.

Borylation-Reduction-Borylation for the Formation of 1,4-Azaborines.

Given the current interest in materials containing 1,4-azaborine units, the development of new routes to these structures is important. Carbonyl directed electrophilic borylation using BBr3 is a facile method for the ortho-borylation of N,N-diaryl-amide derivatives. Subsequent addition of Et3SiH results in carbonyl reduction and then formation of 1,4-azaborines that can be protected in situ using a Grignard reagent. Overall, borylation-reduction-borylation is a one-pot methodology to access 1,4-azaborines from simple precursors.

Room-Temperature Multiple Phosphorescence from Functionalized Corannulenes: Temperature Sensing and Afterglow Organic Light-Emitting Diode.

Corannulene-derived materials have been extensively explored in energy storage and solar cells, however, are rarely documented as emitters in light-emitting sensors and organic light-emitting diodes (OLEDs), due to low exciton utilization. Here, we report a family of multi-donor and acceptor (multi-D-A) motifs, TCzPhCor, TDMACPhCor, and TPXZPhCor, using corannulene as the acceptor and carbazole (Cz), 9,10-dihydro-9,10-dimethylacridine (DMAC), and phenoxazine (PXZ) as the donor, respectively. By decorating corannulene with different donors, multiple phosphorescence is realized. Theoretical and photophysical investigations reveal that TCzPhCor shows room-temperature phosphorescence (RTP) from the lowest-lying T1 ; however, for TDMACPhCor, dual RTP originating from a higher-lying T1 (T1 H ) and a lower-lying T1 (T1 L ) can be observed, while for TPXZPhCor, T1 H -dominated RTP occurs resulting from a stabilized high-energy T1 geometry. Benefiting from the high-temperature sensitivity of TPXZPhCor, high color-resolution temperature sensing is achieved. Besides, due to degenerate S1 and T1 H states of TPXZPhCor, the first corannulene-based solution-processed afterglow OLEDs is investigated. The afterglow OLED with TPXZPhCor shows a maximum external quantum efficiency (EQEmax ) and a luminance (Lmax ) of 3.3 % and 5167 cd m-2 , respectively, which is one of the most efficient afterglow RTP OLEDs reported to date.

Thermally activated delayed fluorescence and high-contrast mechanochromism of anthrone-based donor-acceptor systems.

The development of materials that emit in the deep-red to near-infrared region of the spectrum has attracted significant attention due to their potential as optical sensing and imaging reagents in biology. Herein, we report the synthesis and optoelectronic characterization of four anthraquinone-based emitters, T-tBuCz-AQ, T-MeOCz-AQ, C-tBuCz-AQ, and C-MeOCz-AQ, and two pyrazoloanthrone-based emitters, tBuCz-PA and DMAC-PA. Depending on the donor, these compounds emit in the spectral range between 640 and 750 nm in the neat film, while the emission of the 10 wt% doped films in poly(methyl methacrylate) (PMMA) is blue-shifted between 600 and 700 nm and has low photoluminescence quantum yields between 2.6% and 6.6%. Of these compounds, T-tBuCz-AQ, T-MeOCz-AQ, and C-tBuCz-AQ exhibited thermally activated delayed fluorescence (TADF) in 10 wt% doped films in PMMA, while the crystals of T-tBuCz-AQ also showed TADF. Compound tBuCz-PA showed a high-contrast and reversible photoluminescence (PL) response upon mechanical grinding and hexane fuming.

Brominated B1-Polycyclic Aromatic Hydrocarbons for the Synthesis of Deep-Red to Near-Infrared Delayed Fluorescence Emitters.

Bromo-functionalized B1-polycyclic aromatic hydrocarbons (PAHs) with LUMOs of less than -3.0 eV were synthesized and used in cross-couplings to form donor-acceptor materials. These materials spanned a range of S1 energies, with a number showing thermally activated delayed fluorescence and significant emission in the near-infrared region of the spectrum. These B1-PAHs represent a useful family of acceptors that can be readily synthesized and functionalized.

Materials for Electrochemiluminescence: TADF, Hydrogen-Bonding, and Aggregation- and Crystallization-Induced Emission Luminophores.

Electrochemiluminescence (ECL) is a rapidly growing discipline with many analytical applications from immunoassays to single-molecule detection. At the forefront of ECL research is materials chemistry, which looks at engineering new materials and compounds exhibiting enhanced ECL efficiencies compared to conventional fluorescent materials. In this review, we summarize recent molecular design strategies that lead to high efficiency ECL. In particular, we feature recent advances in the use of thermally activated delayed fluorescence (TADF) emitters to produce enhanced electrochemiluminescence. We also document how hydrogen bonding, aggregation, and crystallization can each be recruited in the design of materials showing enhanced electrochemiluminescence.