Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes

Temperature dependency of energy shift of excitonic states in a donor-acceptor type TADF molecule

In recent years, thermally activated delayed fluorescence (TADF) has attracted intense attention owing to its straightforward application to high-efficiency organic light-emitting diodes. Further, to develop high-performance TADF materials, many researchers have designed novel molecules that have a small energy gap between the lowest excited singlet and triplet states ( Δ E S T ), and detailed analysis suggests a significant contribution of higher-lying excited states for spin flipping processes. In this study, we demonstrate a peculiar thermal behaviour of emission decay of a donor-acceptor type TADF molecule, TMCz-BO, which seems like thermal deactivation of delayed fluorescence that can be explained without a negative Δ E S T by comprehensive kinetic analysis across various temperatures and solvents. While the activation energy has previously been treated as being temperature-independent, we stress that it should be a dynamic parameter affected by changing the solvent-solute interaction with the environmental temperature, especially in the case of a small energy gap.

The role of branching in the ultrafast dynamics and two-photon absorption of two pyrimidine push-pull molecules

The dynamics and two-photon absorption (2PA) properties of two pyrimidine chromophores are studied using femtosecond time-resolved fluorescence and two-photon excited fluorescence techniques. The pyrimidine is used as an electron withdrawing group and is substituted at the C2 position with a phenylacridan fragment, while diphenylaministyryl donor moieties are appended at positions C4/6 to afford the pseudo-dipolar and pseudo-quadrupolar molecules 1 and 2, respectively. Chromophore 2 shows more efficient fluorescence emission, while 1 exhibits larger Stokes shifts. Their decay pathways are discussed through an emission from a Franck-Condon charge transfer (FC-CT) and a relaxed charge transfer (R-CT) state. Ultrafast dynamics in tetrahydrofuran show population of the R-CT state for 1 that is faster than solvation, while for 2, due to its pseudo-quadrupolar nature, R-CT population is slower and occurs from the solvated FC-CT state. Finally, molecule 2 shows better 2PA properties with cross sections reaching 560 GM at 820 nm.

Unsymmetrical S-annulated perylene diester imide, stabilizing room temperature columnar phase as a dopant for greenish-yellow OLEDs with an outstanding EQE of 6.9

The quest for highly luminescent, cost-effective, and metal-free materials is driving innovation in organic light-emitting diode (OLED) technology. Herein, we unveil a novel unsymmetrical bay S-incorporated perylene diester imide (PEI-SST), stabilizing a room temperature columnar hexagonal phase over a wide thermal range, that was prepared by microwave-assisted synthesis. PEI-SST demonstrates a remarkable photoluminescence quantum yield of 83%. As a dopant in a CBP-based host-guest OLED, at 1.0 wt%, PEI-SST delivered a promising greenish-yellow electroluminescence with an exceptional EQEmax of 6.9% and a brightness of 2529 cd m-2.

Substituted acridones: simple deep blue HIGHrISC emitters in an aprotic environment

N-Methylacridones (NMAs) substituted at positions 2 and 7 with +M groups (fluorine and methoxy) were synthesized and characterized by steady-state and time-resolved spectroscopy. Solutions of the NMA derivatives in an aprotic solvent (tetrahydrofuran) emit in the deep blue region of the visible spectrum with radiative rate constants larger than 5.4 × 107 s-1 and fluorescence quantum yields up to 0.84. Sensitization experiments employing 1,4-dichlorobenzene give evidence for HIGHrISC behavior of the NMAs, that is, reverse intersystem crossing (rISC) from a higher triplet state Tn≥2 occurs. The spectroscopic results, which are corroborated by quantum chemical calculations, render these derivatives very promising for applications in organic light emitting diodes (OLEDs).

Simultaneous Multi-Resonant Thermally Activated Delayed Fluorescence and Room Temperature Phosphorescence from Biluminescent Nitrogen-Containing Indolocarbazoles

Organic biluminescence, the simultaneous emission from both the singlet and triplet excited state manifolds, is a rare and incompletely understood emission process. However, biluminescent compounds have wide-reaching applications, such as in sensing, anti-counterfeiting, and optoelectronics, owing to the complex interplay of excited states having distinct spectral profiles and lifetimes. Herein, the biluminescence of a family of polycyclic aromatic heterocycles known as nitrogen-containing indolocarbazoles (NICz) is described. As 1 wt.% doped films in polymethylmethacrylate (PMMA), these compounds exhibit dual fluorescence/room temperature phosphorescence (RTP) with λPL in the near-UV (≈375 nm) and green (≈500 nm), respectively, and remarkably long phosphorescence lifetimes extending into the multi-second regime. This RTP is shown to persist even at doping concentrations as low as 0.1 wt.%. Additionally, two of the emitters exhibit multi-resonant thermally activated delayed fluorescence (MR-TADF)/RTP biluminescence, which, to the best of knowledge, would be the first examples of such behavior. Finally, insight is provided into the dependence of these competing emission pathways on the temperature and concentration, with supporting wavefunction-based computations.

Amphoteric tetrazole-substituted eleven-ring-fused acene derivatives with multiple fluorescent protonation states

We synthesized novel tetrazole-substituted diacenaphthoanthracenediimides 2 by azide cycloaddition to the corresponding cyano-substituted precursors. Reversible protonation/deprotonation of the tetrazole moieties provides distinct fluorescent species with photoluminescence quantum yields of 12-34%. The facile deprotonation of 2 enables its processing in non-halogenated solvents (alcohol).

Pyrimidine-Based Four-Coordinate O^N^O Boron Complexes: Synthesis, Photophysical and Theoretical Studies, and TADF-Based OLED Devices

This article describes the synthesis, along with the full photophysical and computational characterization, of a series of push-pull boron complexes comprising a sterically hindered donor connected to a pyrimidine-based O^N^O boron chelate. The dimethylacridan-functionalized fluorophore appears to be the most emissive in the series, both in toluene solution and in the solid state (as a powder), displaying thermally activated delayed fluorescence (TADF) emission in degassed media due to a small singlet-triplet energy gap. This result is in line with a previously reported pyridine analogue, which also exhibits delayed emission. Incorporation of this TADF compound into an organic light-emitting diode led to the observation of intense electroluminescence, with EQEmax values reaching 9.7% at a 5 wt% doping concentration.

Blue-to-Green Fine-Tunable Narrowband Delayed Fluorescence in Peripherally Dendritic Modified Bis-Indolocarbazoles

Highly efficient narrowband organic luminophores offer immense potentials for organic light-emitting diodes (OLEDs) with high color purity of electroluminescence (EL). Appropriately controlling their structural relaxation and vibronic couplings between the excited and ground states enables the desired narrowband emissions. Herein, based on a design concept featuring dendritic modifications, we demonstrate the exquisite color tuning of narrowband emissions from deep blue to green in non-boron ubiquitous molecular systems. By attaching two or four arylamine-based dendritic units to the periphery of the indolo[3,2,1-jk]indolo[1',2',3':1,7]indolo[3,2-b]carbazole (BICz) core, BICz-1.5G, BICz-2G, and BICz-2GII were developed as deep-blue, sky-blue, and green narrowband emitters, respectively. Comprehensive photophysical and computational studies revealed that the dendritic modifications do not impair the inherent narrowband emission ability of BICz while enhancing its thermally activated delayed fluorescence properties. OLEDs incorporating BICz-1.5G and BICz-2GII demonstrated high-efficiency, high-color-purity blue and green EL, with maximum external quantum efficiencies as high as 24.0% and 28.5%, respectively, and CIE coordinates of (0.13, 0.10) and (0.21, 0.68). Our findings will contribute to expanding the search space for narrowband organic luminophores.

Core-Substituted Pyromellitic Diimides: A Versatile Molecular Scaffold for Tunable Triplet Emission

Arylene diimides represent a versatile class of n-type organic semiconductors, widely recognized for tunable photophysical properties, making them highly relevant across various optoelectronic applications. While their fluorescence can be finely modulated through core substitution, triplet-state emission has received comparatively little attention. This is particularly surprising given the growing field of ambient-organic triplet harvesting materials, such as thermally activated delayed fluorescence and phosphorescent systems, which would greatly benefit from structural modifications to the π-conjugated backbone and core substitution of arylene diimides to achieve the desired properties. Realizing tunable triplet states within a family of molecules is crucial for advancing organic triplet-based materials for applications in lighting, photocatalysis, and beyond. In this context, we present an unprecedented study demonstrating tunable triplet emission in pyromellitic diimides, the smallest member of the arylene diimide family, with an accessible emissive triplet state due to a narrow singlet-triplet energy gap. Herein, we report the synthesis of a series of core-substituted pyromellitic diimides (cPmDIs) using diverse synthetic strategies. Core substitution not only induces a wide spectrum of fluorescence colors but, notably, enables a wide-range phosphorescence spanning across the visible spectrum, depending on the core substituent. This article details the synthesis and photophysical and electrochemical characterization of a library of cPmDIs, supported by theory. Furthermore, we demonstrate the potential of this molecular design in achieving ambient-orange phosphorescence, as exemplified by the thiophenyl-cPmDI derivative, which exhibits triplet emission in the crystalline and film states by minimizing vibrational dissipation. In this regard, we envision that the present study represents a significant step toward the predictive structure-property design of ambient-organic phosphors and triplet harvesting materials.

Equalized Dual Emissions from Copper Complexes via Multichannel Balanced Intersystem Crossing: Toward 100% Quantum Efficiencies

Luminescent materials have important applications in biology, medicine, catalysis, energy, information technology, and so on. However, to suppress quenching and improve efficiencies, how to balance singlet and triplet radiations for efficient dual emission composed of equalized thermally activated delayed fluorescence (TADF) and phosphorescence remains a formidable challenge. Here, we report that based on a rigid skeleton of triphosphine CuI complex, modification with carbazole donors results in high-lying ligand-centered charge transfer states, which provide additional channels for accurately optimizing the singlet-triplet ratios. We achieve equalized dual emissions containing 53% TADF and 47% phosphorescence, ∼100% photo- and electro-luminescence quantum efficiencies and record-high external quantum efficiencies of ∼30% for pure-yellow organic light-emitting diodes. Photophysical and exciton kinetics analyses indicate the incorporation of high-lying ligand-centered triplet states into dual-emissive electroluminescence is based on energy-level matching with the first triplet states of host matrixes.

Efficient donor-σ-acceptor emitters with strengthened intramolecular charge-transfer and their use for high-efficiency organic light-emitting diodes

Thermally-activated delayed fluorescence (TADF) materials have emerged as next-generation emitters for organic light-emitting diodes (OLEDs). The donor-σ-acceptor molecule is a promising paradigm for developing TADF, but its radiative decay rate (kr,s) and photoluminescent efficiency (ФPL) require large improvements, due to weak intramolecular charge-transfer (CT). Here, efficient donor-σ-acceptor emitters (1-3) with strengthened intramolecular CT are developed by directly linking the donor and acceptor with a short alkyl chain. 9,9-dimethyl-9,10-dihydroacridine and 2,4,6-triphenyl-1,3,5-triazine are employed as the donor and acceptor, respectively, and -CH2- (for 1), -CH2CH2- (for 2) and -CH2CH2CH2- (for 3) are employed as the σ-linkers. The chemical structures of 1-3 have been verified by X-ray crystallography. In dilute solution and lightly doped films, emitters 1-3 show considerably strong intramolecular CT, due to the σ-π hyperconjugation between the donor/acceptor and the alkyl σ-linker. In the 20 wt.% doped films, emitters 1-3 show green-blue TADF with combined intra- and inter-molecular CT, with high ФPLkr,s and reverse intersystem crossing rates up to 0.91, 8.5 × 106 s-1 and 2.6 × 106 s-1, respectively. OLEDs based on emitters 1-3 show green-blue emission with high external quantum efficiencies (EQEs) over 20 %. A hyperfluorescent OLED with emitter 3 as the sensitizer and a typical multiple resonance emitter (DtBuCzB) as the terminal emitter shows narrowband blue-green emission with a high EQE of 28.1 %.

Cascade Radical Cyclization of Propargylamines for Functionalized 3-Arylthioquinoline Formation

A novel and efficient strategy for the direct synthesis of 3-arylthioquinoline derivatives via radical induced tandem cyclization of propargylamines with diaryl disulfides was developed. This protocol undergoes a cascade sulfuration/ cyclization/ oxidation/ aromatization pathway to afford the desired products in a broad substrate scope using readily available starting materials under mild conditions. Based on this strategy, we further modified 3-arylquinolines to obtain two novel deep blue fluorescent molecules, QLSCz and QLSTCz, with good optical properties through two-step synthesis by oxidation and electron donor modification.

A self-aggregated thermally activated delayed fluorescence nanoprobe for HClO imaging and activatable photodynamic therapy

Hypochlorous acid (HClO/ClO-) is a common ROS that exhibits elevated activity levels in cancer cells. In this study, an ClO--triggered TADF probe, PTZ-MNI, was designed based on a naphthalimide core. PTZ-MNI self-assemble in aqueous environments, exhibiting significantly enhanced fluorescence that demonstrated typical aggregation-induced delayed fluorescence (AIDF) characteristics. The probe not only showed high sensitivity to ClO- but also exhibited remarkable selectivity over other reactive oxygen species and disturbance. PTZ-MNI displayed TADF characteristic, including sensitivity to oxygen in toluene, insensitivity to oxygen in aggregated states that maintain long fluorescence lifetimes, a vertical conformation, and a minimal ΔEST of 0.01 eV. Cell imaging studies showed the probe could trace ClO- by red to green fluorescence in HeLa cell. The colocalization analysis indicated its excellent lysosome-targeting specificity. In addition, PTZ-MNI-O, the compound after oxidation, exhibited effective ROS generation ability and significant PDT effect after irradiation. This work provides guidance for the rational design of responsive TADF luminescent materials used in cell imaging and activatable-PDT.

Day-Long Organic Persistent Luminescence in Flexible Polymeric Materials

The progress in organic afterglow materials has drawn significant attention due to their extensive applications in fields such as optoelectronics, anti-counterfeiting, and bioimaging. Nonetheless, a general limitation of organic afterglow materials is their short emission lifetimes, typically spanning from milliseconds to seconds, which creates a substantial challenge in developing day-long organic afterglow (DOA). In this study, a DOA system is demonstrated through the incorporation of electron donor/acceptor exciplexes. Polyethylene naphthalate is used for both the electron acceptor and charge storage units, coupling with a spirobifluorene-carbazole derivative as the electron donor, providing effective charge separations under UV-light and sunlight excitation. The resulting DOA polymers demonstrate an exceptional bluish-green afterglow that endures for over 28 h under ambient conditions, setting a new record for the longest afterglow duration in polymeric materials. Moreover, the DOA-doped polymers, as both films and fibers, exhibit outstanding flexibility and transparency, making them highly suitable for flexible technologies and wearable devices.

A dark-state-dominated photochemical upconversion afterglow via triplet energy transfer relay

Photochemical afterglow materials have drawn considerable attention due to their attractive luminescent properties and great application potential. Considering the classical photochemical afterglow materials always exhibit poor luminescence, it is urgent to gain fundamental understanding of the main limiting factors. Here, we identified the existence of a dark-state triplet in the photochemical process, and an overwhelming percentage of ~98.5% was revealed for this non-emissive triplet state. Guided by these observations, we proposed to activate an unprecedented triplet energy transfer relay to simultaneously harness the singlet and triplet energy. Consequently, an upconverted afterglow material was constructed with amazing luminescence performance albeit its moderate fluorescence emission property. The generality of this strategy was evidenced by the adaptation to similar emitters with varied emission wavelengths. The optimized afterglow performance enabled time-gated upconversion bioimaging under ultralow-power excitation. This study not only reveals the energy transfer pathways for photochemical afterglow but also paves the way for rational design of bright upconverted materials with ultralong lifetime.

Copper(I) Halide Complex Featuring Blue Thermally Activated Delayed Fluorescence and Aggregate Induced Emission for Efficient X-ray Scintillation and Imaging

Developing solution-processable and stable scintillators with high light yields, low detection limits and high imaging resolutions holds great significance for flexible X-ray imaging. However, attaining an optimal equilibrium among X-ray absorption capacity, exciton utilization efficiency, and decay lifetime of scintillators remains a substantial challenge. Here, a new Cu(I) halide complex was synthesized in a mild condition. It exhibits intense blue thermally activated delayed fluorescence (TADF), remarkable aggregation-induced emission (AIE) characteristic, as well as good water-oxygen stability and photochemical stability. Notably, the complex shows excellent radiation resistance and efficient radioluminescence (RL) with an ultra-low detection limit of 42.5 nGyairs-1. This superior scintillation performance can be attributed to the synergistic effect of effective X-ray absorption by the heavy Cu2I2 core, the high radiation-induced exciton utilization rate in TADF process, and the remarkable suppression of non-radiative transitions by the restriction of intramolecular motions in solid state. Furthermore, the favourable solution processability of the complex facilitates the successful fabrication of a flexible film, achieving high-quality X-ray imaging with a resolution of 17.3 lp mm-1. This work highlights the potential of hybrid Cu(I) halides with AIE-TADF effects for high-energy radiation detection and imaging, opening up new avenues for the exploration of cost-effective and high-performance scintillators.

Enhanced Delayed Fluorescence in Nonlocal Metasurfaces: The Role of Electronic Strong Coupling

Strong light-matter coupling has garnered significant attention for its potential to optimize optoelectronic responses. In this study, we designed open cavities featuring nonlocal metasurfaces composed of aluminum nanoparticle arrays. The surface lattice resonances in these metasurfaces exhibit electronic strong coupling with the boron difluoride curcuminoid derivative, which is known for its highly efficient thermally activated delayed fluorescence in the near-infrared. Our results show that delayed fluorescence induced by triplet-triplet annihilation can be enhanced by a factor of 2.0-2.6 in metasurfaces that are either tuned or detuned to the molecular electronic transition. We demonstrate that delayed fluorescence enhancements in these systems primarily stem from increased absorption in the organic layer caused by the nanoparticle array, while strong coupling has negligible effects on reverse intersystem crossing rates, aligning with previous studies. We support these findings with finite-difference-time-domain simulations. This study elucidates how light-matter interactions affect delayed fluorescence, highlighting the potential applications in optoelectronic devices.

Peripheral Modification Strategy of Heavy Atom for High-Performance Multi-Resonance Thermally Activated Delayed Fluorescence Emitters

Organic light-emitting materials with multi-resonance thermally activated delayed fluorescence (MR-TADF) have shown great potential for realizing highly efficient narrowband organic light-emitting diodes (OLEDs). However, the heavy efficiency roll-off caused by the slow reverse intersystem crossing (RISC) process remains a challenging issue for the further practical application of MR-TADF materials. Here, we develop two TADF emitters, BNDBT and BNDBF, in which the dibenzothiophene and dibenzofuran substituents are attached at the bottom of the B/N frameworks. They all exhibit the similar high photoluminescence quantum yields of 90 % and 87 %. The sulfur-containing material BNDBT exhibits a RISC rate (kRISC) of 6.02×104 s-1, which is three-folded higher than BNDBF (2.09×104 s-1) without heavy atom. The corresponding green OLED based on BNDBT exhibits an improved external quantum efficiency of 35.5 % and lower efficiency roll-offs at high brightnesses of 100 cd m-2 and 1000 cd m-2, respectively. In addition, the BNDBT-based OLED maintains high color purity without causing a sharp increase in FWHM as compared with that of BNDBF. This work indicates that introducing the heavy atom at the bottom of the B/N skeleton is an effective strategy to enhance kRISC while maintaining narrow FWHM, thereby achieving high-performance MR-TADF emitters.

Modulation of delayed fluorescence pathways via rational molecular engineering

One of the key challenges in developing efficient organic light-emitting diodes (OLEDs) is overcoming the loss channel of triplet excitons. A common approach to mitigate these losses to enhance the external quantum efficiency of OLEDs is employing emitter molecules optimized for thermally activated delayed fluorescence (TADF) or triplet-triplet annihilation (TTA). However, achieving both in the solid state from the same organic chromophore poses a formidable challenge due to energetic and structural requirements needing to be met simultaneously. Here, we demonstrate TADF and TTA in donor-acceptor phthalimide derivatives by employing triphenylamine (TPA) or phenyl carbazole (PhCz) as a donor. Thin films of the TPA-substituted phthalimides doped in the poly(methyl methacrylate) matrix exhibit TADF emission from the singlet charge-transfer (CT) state. On the contrary, PhCz-substituted emitters display dominant TTA-induced delayed fluorescence in the neat film due to long-range molecular ordering that facilitates efficient triplet diffusion. The present study provides insight into how dual TADF-TTA delayed fluorescence can be realized in thin films of molecular semiconductors via rational molecular design.

Deep-Blue OLEDs with BT. 2020 Blue Gamut, External Quantum Efficiency Approaching 40

The hyperfluorescence (HF) technology holds great promise for the development of high-quality organic light-emitting diodes (OLEDs) for their excellent color purity, high efficiency, and low-efficiency roll-off. Sensitizer plays a crucial role in the performance of HF devices. However, designing sensitizers with simultaneous high photoluminescence quantum yield (PLQY), rapid radiative decay (kr), and fast reverse intersystem crossing rate (kRISC) poses a great challenge, particularly for the thermally activated delayed fluorescence (TADF) sensitizers targeting deep-blue HF device. Herein, by introducing a boron-containing multi-resonance-type acceptor into the multi-tert-butyl-carbazole encapsulated benzene molecular skeleton, two TADF emitters featuring hybridized multi-channel charge-transfer pathways, including short-range multi-resonance, weakened through-bond, and compact face-to-face through-space charge-transfer. Benefiting from the rational molecular design, the proof-of-concept sensitizers exhibit simultaneous rapid kr of 5.3 × 107 s-1, fast kRISC up to 5.9 × 105 s-1, a PQLY of near-unity, as well as ideal deep-blue emission in both solution and film. Consequently, the corresponding deep-blue HF devices not only achieve chromaticity coordinates that fully comply with the latest BT. 2020 standards, but also showcase record-high maximum external quantum efficiencies nearing 40%, along with suppressed efficiency roll-off.

Intramolecular charge transfer assisted multi-resonance thermally activated delayed fluorescence emitters for high-performance solution-processed narrowband OLEDs

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have been actively employed in high-resolution solution-processed organic light emitting diodes (OLEDs) due to their excellent color purity. Nonetheless, they are always confronted with intrinsic slow spin flip of triplet excitons, impeding the electroluminescence properties, especially in non-sensitized OLEDs. Herein, we constructed intramolecular charge transfer (ICT) assisted MR-TADF emitters by grafting donor-acceptor-type moieties with a meta- or para-substitution as a pendant on an organoboron multi-resonance core. The newly designed MR-TADF emitters not only maintain short range charge transfer characteristics in emissive states without sacrificing color purity but the accelerated spin flips facilitated by the ICT process at a high-lying state are also confirmed by ultrafast spectroscopy and theoretical calculation, achieving over a 10-fold increase in the reverse intersystem crossing rate compared with unsubstituted counterpart emitters. In sensitizer-free solution-processed OLEDs, a cutting-edge external quantum efficiency of 27.8% can be achieved together with reduced efficiency roll-offs and an attractive full width at half maximum of 29 nm, representing a breakthrough in efficiency for solution-processed MR-TADF based narrowband OLEDs.

Functionalization of Clar's Goblet Diradical with Heteroatoms: Tuning the Excited-State Energies to Promote Triplet-to-Singlet Conversion

The ground-state spin multiplicity as well as the energy difference between the lowest-energy spin-singlet (S1) and spin-triplet (T1) excited states of topologically frustrated organic (diradical) molecules can be tuned by doping with a pair of heteroatoms (N or B atoms). We have thus systematically studied here a set of Clar's Goblet derivatives upon a controlled substitution at different C sites, to alter the electronic structure of the molecules and disclose the positions at which: (i) the ground-state multiplicity becomes a closed-shell singlet and (ii) the energy difference between S1 and T1 is considerably small (i.e., below 0.1-0.2 eV to induce a triplet exciton recovery upon thermal effects). This electronic structure outcome is driven by strong correlation effects; therefore, we have here applied a variety of single-reference [TD-DFT, CIS(D), SCS-CC2] and multireference [CASSCF, NEVPT2, RAS-srDFT] methods. For TD-DFT, we have covered global hybrid (PBE0, M06-2X), range-separated hybrid (ωB97X), and double-hybrid (PBE-QIDH, SOS1-PBE-QIDH, and PBE0-2) functionals to ascertain whether the results were highly dependent on the functional choice. Overall, we found that the heterosubstitution strategy could largely modify the electronic and optical properties of the pristine diradical system, with these organic forms thus constituting a new set of compounds with further optoelectronic applications.

Static and Dynamic Studies of Excitation in a Fullerene-Anthracene Complex

We investigate the static and dynamic aspects of an anthracene-fullerene complex. Our detailed investigation includes (a) systematic scrutiny of the complex in its stable state through the examination of its chemical reactivity parameters and absorption spectra and (b) exploration of its dynamic behavior in excited states at a femtosecond time scale. To achieve this, we employ a combination of ab initio molecular dynamics (AIMD) and time-dependent density functional theory (TDDFT) to study not only the system's behavior in the excited state but also the temporal evolution of chemical reactivity parameters when it moves in a particular excited state. It shows greater reactivity in the excited state as compared to that in the ground state. Interestingly, our findings reveal that the complex can even switch between excited states during its movement in certain trajectories. Accordingly, we conduct an extensive examination of the interactions among coupling components, which coincides with the occurrence of trajectory surface hopping.

Carbazolylpyridine (cp)-based tetradentate platinum(II) complexes containing fused 6/5/6 metallocycles

A series of carbazolylpyridine (cp)-based 6/5/6 Pt(II) complexes featuring tetradentate ligands with nitrogen or oxygen atoms as bridging groups was designed and synthesized, and the bridging nitrogen atoms were derived from acridinyl (Ac), azaaceridine (AAc) and carbazole (Cz). Systematic experimental and theoretical studies reveal that the ligand structures have a significant effect on the electrochemical, photophysical and excited state properties of these complexes. Their oxidation processes mainly occur on the carbazole-Pt moieties, whereas the reduction processes typically occur on the electron-deficient pyridine (Py) moieties. Time-dependent density functional theory (TD-DFT) and natural transition orbital (NTO) calculations reveal that the cp-based Pt(II) complexes have a metal-to-ligand charge transfer (3MLCT) state mixed with ligand-centered (3LC) and intra-ligand charge-transfer (3ILCT) characteristics. Pt(cp-1) shows strong red luminescence with a dominant peak at 611 nm and an excited-state lifetime of 10.7 μs in dichloromethane at room temperature, 602 nm and 10.9 μs in toluene, and 602 nm and 8.2 μs in PMMA films. It also exhibits high photoluminescence quantum efficiencies of 85%, 84% and 60% in dichloromethane, toluene and PMMA, respectively. These studies indicate the potential application of the cp-based Pt(II) complexes as phosphorescent emitters in the field of organic light-emitting diodes (OLEDs).

Synthesis of Thiazolo[5,4-d]thiazoles in an Eco-Friendly L-Proline-Ethylene Glycol Mixture

The hazardousness of solvents used in synthetic organic chemistry is well established. In this context, it is relevant to search for safer and greener alternatives. Within the last decades, deep eutectic solvents have been considered as possible and promising alternatives. Consequently, this study aims at using deep eutectic solvents to synthesize an emerging class of heteroaromatic compounds named thiazolo[5,4-d]thiazoles, for which interest is growing in the field of organics, electronics, and biology. To address this challenge, we developed a straightforward synthetic protocol consisting of condensing dithiooxamide and aromatic aldehyde in deep eutectic solvents to yield the desired thiazolo[5,4-d]thiazole without further purification. The first hit was obtained with the well-known L-proline:glycerol (1:2) mixture at 130 °C. However, dithiooxamide is degraded under these conditions, leading to the formation of impurities that may arise from the consequent amount of reactive L-proline. Reaction conditions were optimized by modifying the deep eutectic solvent nature and proportions, applying various temperatures, changing the activation and heating source, or adding auxiliary oxidants. As a consequence, eight thiazolo[5,4-d]thiazoles were synthesized in equal or better yields (20 to 75%) than the reported procedure under safe and eco-friendly conditions in a mixture of L-proline and ethylene glycol (1:50) with sodium metabisulfite at 130 °C for one hour.

Enabling High-Performance Multi-Resonant TADF Emitters via Intramolecular Hydrogen Bond

Hydrogen bonds (HBs), prevalent strong interactions in organic compounds, can effectively constrain single bond rotation, leading to rigid planar configurations. This rigidity enhances emission efficiency and narrows the emission spectrum of luminescent materials. Recent advances have leveraged HBs to advance high-performance donor-acceptor thermally activated delayed fluorescence (TADF) materials. However, their application in multi-resonance (MR) TADF emitters remains limited. We herein developed MR-TADF emitters incorporating intramolecular hydrogen bonds (IHBs) with pyrimidine as the HB acceptor. The rigid planar conformation induced by IHBs significantly improved photoluminescence quantum yield, extended emission wavelength, reduced full-width at half-maximum, and decreased non-radiative decay rates for BN-2Pm compared to BN-5Pm. Devices based on BN-2Pm achieved a maximum external quantum efficiency of 36.5 %, a current efficiency of 102.3 cd A-1, a power efficiency of 84.6 lm W-1, and Commission Internationale de l'Éclairage (CIE) coordinates of (0.18, 0.71). In contrast, BN-5Pm exhibited lower values: 14.6 %, 36.8 cd A-1, 27.6 lm W-1, and (0.12, 0.57), respectively.

Rational Molecular Design for Balanced Locally Excited and Charge- Transfer Nature for Two-Photon Absorption Phenomenon and Highly Efficient TADF-Based OLEDs

The pursuit of highly efficient thermally activated delayed fluorescence (TADF) emitters with two-photon absorption (2PA) character is hampered by the concurrent achievement of a small singlet-triplet energy gap (ΔEST) and high photoluminescence quantum yield (ΦPL). Here, by introducing a terephthalonitrile unit into a sterically crowded donor-π-donor structure, inducing a hybrid electronic excitation character, we designed unique TADF emitters possessing 2PA ability. This rational molecular design was achieved through a main π-conjugated donor-acceptor-donor backbone in line with locally excited feature renders a large oscillator strength and transition dipole moment, maintaining a high 2PA cross-section value. The ancillary N-donor-acceptor-donor with charge transfer character highly balances the TADF phenomenon by minimizing ΔEST. A near-unity ΦPL value with a large radiative decay rate over an order of magnitude higher than the intersystem crossing rate and a high horizontal orientation ratio of 0.95 were simultaneously attained for TPCz2NP. The organic light-emitting diodes fabricated with this material exhibit a high maximum external quantum efficiency of 25.4 % with an elevated 2PA cross-section (σ2) value up to 143 GM at 850 nm. These findings offer a venue for designing high-performance TADF emitters with exceptional performance inclusive of 2PA properties, expanding for future functional material design.

Excited-State Engineering of Chalcogen-Bridged Chiral Molecules for Efficient OLEDs with Diverse Luminescence Mechanisms

The exploration of circularly polarized luminescence is important for advancing display and lighting technologies. Herein, by utilizing isomeric molecular engineering, a novel series of chiral molecules are designed to exploit both thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) mechanisms for efficient luminescence. The cooperation of a small singlet-triplet energy gap, moderate spin-orbital coupling (SOC), and large oscillator strength enables efficient TADF emission, with photoluminescence quantum yields exceeding 90 %. By altering the symmetry of molecular structures, it is demonstrated that the intrinsic electronic SOC and vibrational SOC effects can be greatly enhanced to facilitate RTP emission. Notably, through modulating simultaneous TADF and RTP emissions, single-molecule white emission is successfully achieved. Accordingly, the TADF-based organic light-emitting diode (OLED) achieves a maximum external quantum efficiency up to 30 %, representing exceptional performance of non-aromatic amine-based emitters. Furthermore, the first single-molecule white OLED based on TADF and RTP dual-emissive chiral material is developed, establishing a benchmark for the development of advanced display and lighting technologies.

Dual Thermally Activated Delayed Fluorescence from a Single Carbazole Derivative with Multiple Charge Transfer Pathways

Two carbazole derivatives BPA-BNC and 4BPABNC are designed and synthesized to explore the probable dualemission behavior and mechanisms. BPA-BNC contains two arylamino donors and one B-N multi-resonance (MR) segment at 3,6,9-positions of a single carbazole, while 4BPABNC has two extraarylamine donors at 1,8-positions in addition to the above substituent groups. Two compounds in polar solvents show a clear dual emission peaked at near 490 and 540 nmwith high photoluminescence quantum yields of up to 98 %, short delayed lifetimesand extremely small single-triplet splitting energies. The dual emissions originate fromthe MR and through-bond charge transfer transitions, which are supported by theoretical calculation and experimental data. The solution-processed devices based on BPA-BNC and 4BPABNC also exhibit dual emissions and achieve the maximum external quantum efficiencies(EQEs) of 13.1 %and 12.7 %, whereas the model MR molecule provides an EQE of 7.3 % under the same device architecture.

Carbonyl-nitrogen multi-resonance emitters for efficient OLEDs with high color purity

Multi-resonance (MR) materials hold an intriguing feature of narrow emission spectra and have attracted considerable attention in the manufacture of high-definition organic light-emitting diodes (OLEDs). However, the majority of MR materials are composed by a boron-nitrogen skeleton, which is unfavorable for expanding the scope of luminescent materials with narrow emission spectra to meet various application demands. In this work, we wish to report a new carbonyl-nitrogen (C = O/N) skeleton of 5,12-dihydroquinolino[2,3-b]acridine-7,14-dione (QA), and three tailored C = O/N MR molecules are synthesized and fully characterized by crystallography, thermal measurement, cyclic voltammetry, steady-state and transient spectroscopy and theoretical calculation. They show efficient green emissions with narrow full width at half maximum (FWHM) of about 27 nm and high photoluminescence quantum yields of up to 93% in doped films. Efficient hyperfluorescence OLEDs are fabricated using these materials as emitters, providing pure green lights with electroluminescence peaks at 526‒538 nm, narrow FWHMs of 29‒33 nm, excellent external quantum efficiencies of up to 29.48% and small efficiency roll-offs. These results reveal that QA could be a potential skeleton for exploring efficient C = O/N MR molecules.

High-Performance Solution-Processable Organic Light-Emitting Diode Based on a Narrowband Near-Ultraviolet Emitter and a Hot Exciton Strategy

Achieving high efficiency narrowband near-ultraviolet (NUV) emitters in organic light emitting diode (OLED) is still a formidable challenge. Herein, a proof-of-concept hybridized local and charge transfer (HLCT) molecule, named ICz-BO, is prepared and characterized, in which both multiresonant (MR) skeletons are integrated via conjugation connection. A slightly distorted structure and weak intramolecular charge transfer (CT) interaction between two MR subunits lead to a high-lying reverse intersystem crossing (h-RISC) channel of T6→S1, also evidenced by both experimental and calculated results. Impressively, the ICz-BO emitter exhibits outstanding narrowband NUV emission at 404 nm with a full-width at half maximum of 28 nm in toluene solution. The solution processable OLED shows an excellent device performance with the recorded maximum external quantum efficiency of 12.01 %, concomitant with an extremely low y-axis Commission Internationale de l'Éclairage (CIEy) value of 0.031. To the best of our knowledge, this is the highest efficiency reported for the HLCT-based NUV-OLEDs to date. This research proves that the MR skeleton plays a positive effect on the narrowband hot exciton emitter, which provides an alternative paradigm for developing high-efficiency NUV emitters.

A Multiresonant Thermally Activated Delayed Fluorescent Dendrimer with Intramolecular Energy Transfer: Application for Efficient Host-Free Green Solution-Processed Organic Light-emitting Diodes

The development of narrowband emissive, bright, and stable solution-processed organic light-emitting diodes (SP-OLEDs) remains a challenge. Here, a strategy is presented that merges within a single emitter a TADF sensitizer responsible for exciton harvesting and an MR-TADF motif that provides bright and narrowband emission. This emitter design also shows strong resistance to aggregate formation and aggregation-cause quenching. It is based on a known MR-TADF emitter DtBuCzB with a donor-acceptor TADF moiety consisting of either tert-butylcarbazole donors (tBuCzCO2HDCzB) or second-generation carbazole-based donor dendrons (2GtBuCzCO2HDCzB) and a benzoate acceptor. The TADF moiety acts as an exciton harvesting antenna and transfers these excitons via Förster resonance energy transfer to the MR-TADF emissive core. The SP-OLEDs with 2GtBuCzCO2HDCzB and tBuCzCO2HDCzB thus show very high maximum external quantum efficiencies (EQEmax of 27.9 and 22.0%) and minimal efficiency roll-off out to 5000 cd m-2.

Modulation of Donor in Purely Organic Triplet Harvesting AIE-TADF Photosensitizer for Image-guided Photodynamic Therapy

Image-guided photodynamic therapy is acknowledged as one of the most demonstrative therapeutic modalities for cancer treatment because of its high precision, non-invasiveness, and improved imaging ability. A series of purely organic photosensitizers denoted as BTMCz, BTMPTZ, and BTMPXZ, have been designed and synthesized and are found to exhibit both thermally activated delayed fluorescence and aggregation-induced emission simultaneously. Experimental and theoretical studies are combined to reveal that modulation of the donor of the photosensitizer enables distinct thermally activated delayed fluorescence via a second-order spin-orbit perturbation mechanism involving lowest singlet charge-transfer and higher-lying triplet locally excited states, respectively. Further, different donor strengths and unique aggregations (H-, J- and X-type packings) greatly influence their color-tunable up-converted luminescence and endow them with superb dispersibility in water. The confocal microscopy-based cellular uptake study confirms the successful internalization of the nano-probes, while BTMCz enables the generation of reactive oxygen species (singlet oxygen) under white-light irradiation, enabling the efficient killing of cancer cells.

Deciphering the Photophysical Properties of Nonplanar Heterocyclic Compounds in Different Polarity Environments

Nonplanar (butterfly-shaped) phenothiazine (PTZ) and its derivative's (M-PTZ) photophysical and spectral properties have been tuned by varying the solvents and their polarity and investigated employing spectroscopic techniques such as UV-Vis, steady-state and time-resolved fluorescence, and TDDFT calculations. The UV-Vis absorption studies and TDDFT calculations reveal two distinct bands for both compounds: a strong π-π* transition at shorter wavelengths and a weaker n-π* transition, which displays a little bathochromic shift in polar solvents. The detailed emission studies reveal that such dual emission is a result of the photoinduced excited-state conjugation enhancement (ESCE) process. The band at a shorter wavelength corresponds to the locally excited (LE) state, while the longer wavelength band arises from the planarized excited state resulting from ESCE. With the increase in solvent polarity, the LE band is less affected, whereas strong positive solvatochromism is observed for the ESCE band. As the solvent polarity increases, the ESCE band demonstrates significant positive solvatochromism, while emission intensity decreases with higher solvent polarity, suggesting stabilization of the excited state. The biexponential decay of fluorescence lifetimes further corroborates the dual emission behavior of PTZ and M-PTZ. PTZ exhibits a higher photoluminescence quantum yield (PLQY) than that observed for M-PTZ, and the solvent viscosity influences the PLQY, indicating that nonradiative decay is activated during the planarization of the excited state, also known as excited-state conjugation enhancement. Furthermore, the (time-dependent) density functional theory (TD) DFT calculations performed to understand the geometrical parameters and the electronic transitions of these model molecules further corroborate experimental findings. These findings underscore the significant influence of solvent polarity and molecular structure on the dual emission and excited-state dynamics of PTZ and M-PTZ, which eventually hold substantial implications for advanced photophysical applications.

Design Rule of Tetradentate Ligand-Based Pt(II) Complex for Efficient Singlet Exciton Harvesting in Fluorescent Organic Light-Emitting Diodes

Controlling intermolecular interactions, such as triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA), is crucial for achieving high quantum efficiency in organic light-emitting diodes (OLEDs) by suppressing exciton loss. This study investigates the molecular design of tetradentate Pt(II) complexes used for singlet exciton harvesting in fluorescent OLEDs to elucidate the relationship between the chemical structure of the ligands and exciton quenching mechanisms. It was discovered that the bulkiness of substituents is pivotal for maximizing quantum efficiency in these devices. An exciton dynamics study conducted during device operation quantitatively analyzed the contribution of substituents to the OLED operation mechanism, demonstrating that complexes with bulky 2,6-diisopropylphenyl and tert-butyl substituents enhance singlet exciton harvesting by suppressing TTA and TPA, thereby facilitating Förster energy transfer.

Integration of Motion and Stillness: A Paradigm Shift in Constructing Nearly Planar NIR-II AIEgen with Ultrahigh Molar Absorptivity and Photothermal Effect for Multimodal Phototheranostics

The two contradictory entities in nature often follow the principle of unity of opposites, leading to optimal overall performance. Particularly, aggregation-induced emission luminogens (AIEgens) with donor-acceptor (D-A) structures exhibit tunable optical properties and versatile functionalities, offering significant potential to revolutionize cancer treatment. However, trapped by low molar absorptivity (ε) owing to the distorted configurations, the ceilings of their photon-harvesting capability and the corresponding phototheranostic performance still fall short. Therefore, a research paradigm from twisted configuration to near-planar structure featuring a high ε is urgently needed for AIEgens development. Herein, by introducing the strategy of "motion and stillness" into a highly planar A-D-A skeleton, we successfully developed a near-infrared (NIR)-II AIEgen of Y5-2BO-2BTF, which boasts an impressive ε of 1.06 × 105 M-1 cm-1 and a photothermal conversion efficiency (PCE) of 77.8%. The modification of steric hindrance on the benzene ring in the acceptor unit of the aggregation-caused quenching counterpart Y5-2BO, to a meta-CF3-substituted naphthyl, leads to reversely staggered packing and various intermolecular noncovalent conformational locks in Y5-2BO-2BTF ("stillness"). Furthermore, the -CF3 moiety acted as a flexible motion unit with an ultralow energy barrier, significantly facilitating the photothermal process in loose Y5-2BO-2BTF aggregates ("motion"). Accordingly, Y5-2BO-2BTF nanoparticles enabled tumor eradication and pulmonary metastasis inhibition through NIR-II fluorescence-photoacoustic-photothermal imaging-navigated type I photodynamic-photothermal therapy. This work provides the first evidence that the highly planar conformation with a reversely staggered stacking arrangement could serve as a novel molecular design direction for AIEgens, shedding new light on constructing superior phototheranostic agents for bioimaging and cancer therapy.

De(sulfon)amidative Cyclization: The Synthesis of Dibenzolactams and Dibenzosultams for Organic Light Emitting Diode Materials

This study addresses a challenge in organic synthetic chemistry: the direct cleavage of amide bonds, which is typically hampered by the thermodynamic stability of the C(Ar)-C(acyl) bond. Previous methods often rely on "CO" extrusion-jointing transition metal-catalyzed process and require activated tertiary amides, limiting their applicability due to incompatibility with reactive functional groups such as halogens. Herein, we report a transition metal-free approach for the deamidative cyclization of biaryl diamides via a radical process, yielding dibenzolactam derivatives. Along this line, we have developed the desulfonamidative cyclization of biaryl disulfonamides to produce dibenzosultams through direct nucleophilic aromatic substitution, demonstrating high selectivity for unsymmetrical structures. Additionally, unsymmetrical sulfamoyl biaryl amides, containing both amide and sulfonamide functionalities, can selectively undergo desulfonamidative coupling with the amide to form dibenzolactams, which offers a complementary synthetic pathway to unsymmetric dibenzolactams. These protocols exhibit excellent compatibility with reactive functional groups, including halogens, providing an innovative synthetic toolbox for the development of thermally activated delayed fluorescence (TADF) materials used in organic light emitting diodes (OLEDs). DMAC-PDO, incorporating a dibenzolactam as the acceptor unit, serves as an efficient blue TADF emitter with a maximum external quantum efficiency (EQEmax) of 23.4 %.

Thermally Activated Delayed Fluorescence in B,N-Substituted Tetracene Derivatives: A Theoretical Pathway to Enhanced OLED Materials

Polycyclic aromatic hydrocarbons (PAHs) exhibit intriguing characteristics that position them as promising candidates for advancements in organic semiconductor technologies. Notably, tetracene finds substantial utility in Electronics due to its application in organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). The strategic introduction of an isoelectronic boron-nitrogen (B,N) pair to replace a carbon-carbon pair in acenes introduces changes in the electronic structure, allowing for the controlled modulation of diradical characteristics. Consequently, this B,N substitution enables precise adjustments in chemical, optical, and electronic attributes. In this work, we undertook a systematic exploration of thermally activated delayed fluorescence (TADF) phenomena within a set of 77 B,N-substituted derivatives of tetracene. The primary objective was to identify and select prospective molecules for the fabrication of OLEDs. Employing multiconfigurational methods of computational quantum chemistry, we conducted an extensive investigation to unravel the potential candidates. As a result, we identified molecules that might exhibit the sought-after TADF behavior. Descriptors such as excitation energies, harmonic oscillator model of aromaticity (HOMA) and fractional occupation number weighted density (FOD) were assessed and indicated five candidates with stability comparable to that of pristine tetracene. This research not only contributes to a deeper understanding of the influence of B,N substitution on acene derivatives but also opens doors for the development of organic electronics by harnessing the properties of these selected molecules.

External electric field induced emission behavior for ESIPT-based 2-(benzo[d]thiazol-2-yl)-4-(pyren-1-yl)phenol towards near-infrared region

Organic light-emitting diodes (OLEDs) for low energy transfer and double emission, but the current methods for regulating ESIPT processes are mostly solvent and substituent effects. Here, utilizing the density theory functional (DFT) and time-dependent density functional theory (TD-DFT) methods, the ESIPT process controlled by an external electric field (EEF) is proposed, and the changes in photophysical properties of 2-(benzo[d]thiazol-2-yl)-4-(pyren-1-yl)phenol (PyHBT) are investigated. Structural parameter variations and IR vibrational spectra measure the prerequisite for the ESIPT process, namely, intramolecular hydrogen bond (IHB) strength, and the scanned potential energy curves (PECs) demonstrate that the ESIPT process of PyHBT is harder to execute as the positive EEF increases, and the opposite is true for the negative EEF. The absorption and fluorescence spectra show shifts under the distinct EEFs, and even the emission wavelength reaches the short-wave near-infrared (SW-NIR) region (780-1100 nm), such as 815.2 nm for a positive EEF of + 30 × 10-4 a.u. in the keto form. Additionally, the fluorescence intensity of PyHBT is strongly influenced by the positive EEF, especially in the enol form, and the investigation of the mechanism by hole-electron analysis demonstrates that under the positive EEF, the twisted intramolecular charge transfer (TICT) process is induced, which triggers the weakening of the fluorescence intensity. In summary, our work not only complements the theoretical approach to modulate the ESIPT process, but also reveals that the photophysical properties of materials affected by the external electric field are even expected to reach the NIR region.

Star-shaped multifunctional organic emitters based on N-(2-cyanophenyl) carbazole frameworks: Effects of steric hindrance fluorene and heavy-atom bromine

To investigate the effects of steric hindrance fluorene and heavy-atom bromine on the general optoelectronic properties of star-shaped organic emitters based on 9-(2-cyanophenyl) carbazole (OCzPhCN) frameworks, heavy element of bromine and steric hindrance fluorene were introduced into OCzPhCN to produce four derivatives of 2-(3-bromo-9H-carbazol-9-yl)benzonitrile (BrCzPhCN), 2-(3-bromo-6-(9-(4-ethoxyphenyl)-9H-fluoren-9-yl)-9H-carbazol-9-yl)benzonitrile (BrFCzPhCN), 2-(3-(9-(4-ethoxyphenyl)-9H-fluoren-9-yl)-9H-carbazol-9-yl)benzonitrile (FCzPhCN) and 2-(3,6-bis(9-(4-ethoxyphenyl)-9H-fluoren-9-yl)-9H-carbazol-9-yl)benzonitrile (2FCzPhCN). The fluorene units obviously improve the thermal stability of the obtained compounds, and 2FCzPhCN has the highest thermal stability with 5 % mass heat loss temperature reaching 447 °C. In different polar solvents, the absorption peaks wavelength of OCzPhCN, FCzPhCN and 2FCzPhCN are basically unchanged, and the redshifted emission peaks are positively correlated with solvent polarity. The photoluminescence quantum yields (PLQYs) of OCzPhCN, BrCzPhCN, FCzPhCN, BrFCzPhCN and 2FCzPhCN powders were 20.17 %, 5.43 %, 30.75 %, 3.27 % and 23.56 %. The fluorescence and phosphorescent quantum efficiencies of OCzPhCN, BrCzPhCN, FCzPhCN, BrFCzPhCN and 2FCzPhCN powders are 9.76 % and 10.41 %, 1.2 % and 3.23 %, 28.45 % and 2.3 %, 3.27 % and 0 %, 23.56 % and 0 %. OCzPhCN, BrCzPhCN and FCzPhCN powders show obvious room temperature phosphorescent emission, and the phosphorescent emission lifetime of OCzPhCN, BrCzPhCN and FCzPhCN powders at 561 nm, 576 nm and 568 nm are 193.17 ms, 18.65 ms and 7.25 ms. Compared with OCzPhCN, the introduction of bromine decreases the PLQY and the phosphorescent lifetime of BrCzPhCN powder, while the fluorescence quantum efficiency of the compound FCzPhCN powder has been improved. The corresponding single-triplet energy splitting (ΔEST) of OCzPhCN, FCzPhCN and 2FCzPhCN in solutions are 0.49 eV, 0.63 eV and 0.63 eV, and the corresponding ΔEST values of OCzPhCN, BrCzPhCN FCzPhCN powders are 1.19 eV, 0.74 eV and 0.55 eV. The steric hindrance fluorene units result in smaller and stabilized ΔEST in the solid powder states, and the same situation is opposite in the unimolecular solutions. The maximum external quantum efficiency of organic light-emitting diode based on 10,10'-(4,4'-sulfonylbis (4,1-phenylene)) bis (9,9-dimethyl-9,10-dihydroacridine) hosted by OCzPhCN reaches 12.7 %, and the external quantum efficiency at 100 cd/m2 rolls down to 11 %. OCzPhCN is the best emitters in terms of room temperature phosphorescent emission and host applications.

Exploration of red and deep red Thermally activated delayed fluorescence molecules constructed via intramolecular locking strategy

Red and deep red (DR) organic light-emitting diodes (OLEDs) have garnered increasing attention due to their widespread applications in display technology and lighting devices. However, most red OLEDs exhibit low luminescence efficiency, severely limiting their practical applications. To address this challenge, we theoretically design four novel TADF molecules with red and DR luminescence using intramolecular locking strategies building upon the experimental findings of DCN-DLB and DCN-DSP, and their crystal structures are predicted with the lower energy and higher packing density. The photophysical properties and luminescence mechanism of six molecules in toluene and crystal are clarified using the first principles calculation and thermal vibration correlation function (TVCF) method. The proposed design strategy is anticipated to offer several advantages: enhanced electron-donating capabilities, more rigid structures, longer emission wavelengths and higher luminescence efficiency. Specifically, we introduce oxygen atoms and nitrogen atoms as intramolecular locks, and the newly developed DCN-DBF and DCN-PHC have redshifted emission, narrow singlet-triplet energy gap (ΔEST), fast reverse intersystem crossing rate and enhanced photoluminescence quantum yield (PLQY). Notably, DCN-DBF achieves both long wavelength emission and high efficiency, with emission peaks at 598 nm and 587 nm corresponding to PLQY of 52.13 % and 43.42 % in toluene and crystal, respectively. Our work not only elucidates the relationship between molecular structures and photophysical properties, but also proposes feasible intramolecular locking design strategies and four promising red and DR TADF molecules, which could provide a valuable reference for the design of more efficient red and DR TADF emitters.

Organic Circularly Polarized Room-Temperature Phosphorescence: Strategies, Applications and Challenges

Organic circularly polarized luminescence (CPL) plays crucial roles in chemistry and biology for the potential in chiral recognition, asymmetric catalysis, 3D displays, and biological probes. The long-lived luminescence, large Stokes shift, and unique chiroptical properties make organic circularly polarized room-temperature phosphorescence (CPP) a new research hotspot in recent years. Nevertheless, achieving high-performance organic CPP is still challenging due to the sensitivity and complexity of integrating triplet excitons and polarization within organic materials. This review summarizes the latest advances in organic CPP, ranging from design strategies and photophysical properties to underlying luminescence mechanisms and potential applications. Specifically, the design strategies for generating CPP are systemically categorized and discussed according to the interactions between chiral units and chromophores. The applications of organic CPP in organic light-emitting diodes, sensing, chiral recognition, afterglow displays, and information encryption are also illustrated. In addition, we present the current challenges and perspectives on developing organic CPP. We expect this review to provide some instructive design principles to fabricate high-performance organic CPP materials, offering an in-depth understanding of the luminescence mechanism and paving the way toward diverse practical applications.

Sulfur-locked multiple resonance emitters for high performance orange-red/deep-red OLEDs

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials are preferred for their high efficiency and high colour purity in organic light-emitting diodes (OLEDs). However, the design strategies of MR-TADF emitters in the red region are very limited. Herein, we propose a concept for a paradigm shift in orange-red/deep-red MR emitters by linking the outer phenyl groups in a classical MR framework through intramolecular sulfur (S) locks. Endowed with the planar architectural feature of the MR mother core, the proof-of-concept S-embedded emitters S-BN and 2S-BN also exhibit considerable flatness, which proves critical in avoiding the direct establishment of potent charge transfer states and inhibiting the non-radiative decay process. The emission maxima of S-BN and 2S-BN are 594 nm and 671 nm, respectively, and both have a high photoluminescence quantum yield of ~100%, a rapid radiative decay rate of around 107 s-1, and a remarkably high reverse intersystem crossing rates of about 105 s-1. Notably, maximum external quantum efficiencies of 39.9% (S-BN, orange-red) and 29.3% (2S-BN, deep-red) were also achieved in typical planar OLED structures with ameliorated efficiency roll-offs.

Ag(i) emitters with ultrafast spin-flip dynamics for high-efficiency electroluminescence

Carbene-metal-amide (CMA) complexes are appealing emitters for organic light-emitting diodes (OLEDs). However, little is known about silver(i)-CMA complexes, particularly electroluminescent ones. Here we report a series of Ag(i)-CMA complexes prepared using benzothiophene-fused carbazole derivatives as amide ligands. These complexes emit via thermally activated delayed fluorescence (TADF), together with high photoluminescence quantum yields of up to 72% in thin films. By strengthening the π-donating ability of the amide ligands, ultrashort emission lifetimes of down to 144 ns in thin films and 11 ns in solution (with a radiative rate constant of ∼107 s-1) are realized, among the shortest lifetimes for TADF emitters. Key to this unique feature is the ultrafast spin-flip dynamics consisting of forward and reverse intersystem crossing rates of up to ∼109 s-1 and ∼108 s-1, respectively, verified by the transient absorption spectroscopic study. The resulting solution-processed OLEDs based on the optimal complex afford record external quantum efficiencies of 16.2% at maximum and 13.4% at 1000 nits, representing the state-of-the-art performance for Ag(i) emitters. This work presents an effective approach for the development of short-lived TADF materials for high-efficiency OLEDs.

Metal complex-based TADF: design, characterization, and lighting devices

The development of novel, efficient and cost-effective emitters for solid-state lighting devices (SSLDs) is ubiquitous to meet the increasingly demanding needs of advanced lighting technologies. In this context, the emergence of thermally activated delayed fluorescence (TADF) materials has stunned the photonics community. In particular, inorganic TADF material-based compounds can be ad hoc engineered by chemical modification of the coordinated ligands and the type of metal centre, allowing control of their ultimate photo-/electroluminescence properties, while providing a viable emitter platform for enhancing the efficiency of state-of-the-art organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs). By presenting an overview of the state of the art of all metal complex-based TADF compounds, this review aims to provide a comprehensive, authoritative and critical reference for their design, characterization and device application, highlighting the advantages and drawbacks for the chemical, photonic and optoelectronic communities involved in this interdisciplinary research field.

Calcination-Controlled Synthesis of Carbon Dots@MgF2 Composites with Yellow, White, and Ultraviolet-Blue Thermally Activated Delayed Fluorescence for Multilevel Information Encryption

It is attractive but challenging to develop carbon dot (CD) based materials with tunable thermally activated delayed fluorescence (TADF), especially in the long wavelength region. Here, by simply calcinating the mixture of m-phenylenediamine and MgF2 at 300-500 °C, a series of CDs@MgF2 composites exhibiting yellow, white, and ultraviolet-blue TADF with high photoluminescence quantum yields of up to 37.6% are prepared. Photophysical studies reveal that the yellow TADF with long lifetimes of 810-1106 ms originates from the surface emission centers of CDs, while the ultraviolet-blue TADF with short lifetimes of 266-379 ms is related to the carbon core emission centers. The combination of yellow and ultraviolet-blue TADF in a single material triggers dynamic afterglow with time-dependent colors from white to yellow. The MgF2 matrix offers multiple confinement of CDs through a rigid network, strong space constraint, and robust covalent/hydrogen bonding, thus preventing the triplet excitations from non-radiative transitions. The electron-withdrawing fluorine atoms induce substantial spin-orbit coupling and reduce the singlet-triplet energy gap, consequently facilitating the reverse intersystem crossing to enhance TADF efficiency. Importantly, the CDs@MgF2 composites possess outstanding optical stability against water, organic solvents, strong acids, bases, and oxidants. The fascinating TADF features enable the successful demonstration of multilevel information encryption using CDs@MgF2.

Regulating Optoelectronic and Thermoelectric Properties of Organic Semiconductors by Heavy Atom Effects

Heavy atom effects can be used to enhance intermolecular interaction, regulate quinoidal resonance properties, increase bandwidths, and tune diradical characters, which have significant impacts on organic optoelectronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), etc. Meanwhile, the introduction of heavy atoms is shown to promote charge transfer, enhance air stability, and improve device performances in the field of organic thermoelectrics (OTEs). Thus, heavy atom effects are receiving more and more attention. However, regulating heavy atoms in organic semiconductors is still meeting great challenges. For example, heavy atoms will lead to solubility and stability issues (tellurium substitution) and lack of versatile design strategy and effective synthetic methods to be incorporated into organic semiconductors, which limit their application in electronic devices. Therefore, this work timely summarizes the unique functionalities of heavy atom effects, and up-to-date progress in organic electronics including OFETs, OPVs, OLEDs, and OTEs, while the structure-performance relationships between molecular designs and electronic devices are clearly elucidated. Furthermore, this review systematically analyzes the remaining challenges in regulating heavy atoms within organic semiconductors, and design strategies toward efficient and stable organic semiconductors by the introduction of novel heavy atoms regulation are proposed.

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

High triplet energy host material with a 1,3,5-oxadiazine core from a one-step interrupted Fischer indolization

Energy-efficient and deep-blue organic light-emitting diode (OLED) with long operating stability remains a key challenge to enable a disruptive change in OLED display and lighting technology. Part of the challenge is associated with a very narrow choice of the robust host materials having over 3 eV triplet energy level to facilitate efficient deep-blue emission and deliver excellent performance in the OLED device. Here we show the molecular design of new 1,3,5-oxadiazines (NON)-host materials with high triplet energy over 3.2 eV, enabling deep-blue OLED devices with a peak external quantum efficiency of 21%. A series of NON-host materials are prepared by the condensation of substituted arylhydrazines and cyclohexylcarbaldehyde in a 2:3 ratio. This straightforward "one-pot" procedure enables the formation of indoline-containing derivatives with three fused heterocyclic rings and two stereogenic centres. All materials emit UV-fluorescence in the range of 315-338 nm while possessing highly desirable characteristics for application in deep-blue OLED devices: good thermal stability, a wide energy gap (3.9 eV), a high triplet energy level of (3.3 eV), and excellent volatility during sublimation.

Synthesis and characterization of machine learning designed TADF molecules

In this study, we present a novel approach to the development of thermally activated delayed fluorescence (TADF) molecules with potentials for organic light-emitting diode (OLED) applications, leveraging machine learning (ML) algorithms to guide the materials design process. Recognizing the imperative for high-efficiency, low-cost emissive materials, we integrated ML driven models with experimental characterization to expedite the discovery of TADF compounds. Initially, a database of ML-designed TADF molecules was employed, with samples being approved to possess optimized photophysical properties. The proposed molecules were synthesized using palladium-catalyzed coupling reactions. Subsequent characterization of these molecules utilized a suite of analytical methods, including nuclear magnetic resonance (NMR) spectroscopy, photoluminescence (PL) spectroscopy, and transient PL decay etc., to confirm their structural integrity and evaluate their performance metrics. The photophysical analysis revealed notable emission efficiencies and significant delayed fluorescence characteristics in solution phases, underscoring the potential of ML-designed TADF molecules. Theoretical validations, through quantum chemical calculations, corroborated the experimental findings, demonstrating the predictive power of our ML models. This interdisciplinary approach not only accelerates the pace of TADF molecule development but also provides a scalable framework for future material innovation especially in the OLED research field.