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 %.

The Importance of High-Frequency Modes in the Prediction of RISC Rates for TADF Molecules

The reverse intersystem crossing (RISC) rate determines the efficiency of dyes displaying thermally activated delayed fluorescence (TADF). Such a rate can be predicted at the full quantum level by considering all the vibrational normal modes or adopting an approximated methodology which relies on single classical modes. We evaluated the importance of considering all of the vibrational degrees of freedom in computations for targeting the design of novel emissive materials from first principles. We computed the RISC rate for 17 molecules of interest for TADF by comparing a full quantum mechanics treatment based on Fermi's golden rule with Marcus-based semiclassical approaches. The results are quantitatively and sometimes qualitatively different in the two approaches, especially when the reorganization energy is small, a common occurrence for molecules exhibiting TADF. The importance of high-frequency modes varies across the set of molecules considered, suggesting that their evaluation should become part of the molecular design process.

Transforming delayed fluorescence into blue-shifted phosphorescence in aminoboranes via oxygen-to-sulfur substitution on the donor amine

Thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) materials are found in diverse applications, from optoelectronic devices to time-gated bioimaging. Recently, aminoboranes with donor-acceptor structures have been identified as promising candidates due to their inherent capacity to harvest the triplet excitons by their unique orbital configurations (El Sayed rule). This work reports the delayed luminescence behaviors of two aminoboranes, BNO and BNS, featuring phenoxazine (PXZ) or phenothiazine (PTZ) donors coupled with a dixylylborane acceptor. BNO exhibits efficient TADF emission in aggregates, thin films, and solid states. In contrast, BNS shows delayed fluorescence (DF) in aggregate states and RTP in solid and thin-film states. Notably, BNS shows a rare blue-shifted phosphorescence relative to its prompt fluorescence, which has not been reported for aminoboranes. Photoluminescence studies and computational calculations reveal that ISC and rISC processes in these systems involve higher triplet states. The unprecedented blue-shifted phosphorescence in BNS is attributed to perturbations in energy levels, which are driven by unique quasi-axial and quasi-equatorial conformations and the stronger spin-orbit coupling of heavier S over O.

Tuning the Electronic and Optical Properties of Phenoxaborin Based Thermally Activated Delayed Fluorescent Materials: A DFT Study

The electronic and optoelectronic properties of molecules constituted by benzene as linker, phenoxaborin as acceptor coupled with different types of donor moieties are investigated using the density functional theoretical method. The energy gap between the first excited singlet and triplet states (ΔEST) of the designed molecules (1-9) is found to be less than 0.5 eV suggesting them as ideal candidates for thermally activated delayed fluorescence (TADF) emitters. The analysis of frontier molecular orbitals of the molecules revealed a minimum spatial overlap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in favor of the small values of ΔEST. Among the molecules studied, the one in which dihydrophenazine acts as the donor has the lowest value of ΔEST. All designed molecules are good electron transporters. The non-linear optical properties of the molecules are also examined.

Oxygen-doped Carbon Nitrides with Visible Room-temperature Phosphorescence and Invisible Thermal-Stimuli-Responsive Ultraviolet Delayed Fluorescence for Security Applications

Multi-mode emissive materials with stimuli-responsive producing invisible signals are very attractive for advanced security applications, but development of such materials remains highly challenging. In this work, oxygen-doped carbon nitrides (O-CNs) are prepared via microwave-assisted heating of urea, which exhibit ultraviolet (UV) solid-state fluorescence (SSFL), visible room temperature phosphorescence (RTP) and thermal-stimuli production of invisible UV delayed fluorescence (DF) properties. Further studies confirmed that the SSFL and RTP could be attributed to the introduction of oxygen functional group (e. g., C=O) in the skeleton of O-CNs, thus minimizing the aggregation caused quenching effect, facilitating intersystem crossing, and stabilizing the excited triplet states. The specific thermal-stimuli production of UV DF is deemed to be the relatively large energy gap between ground and excited singlet states as well as an effective triplet-triplet annihilation. Notably, the emission maximum of UV DF locates at ~310 nm with an ultra-narrow full width at half maximum (FWHM) down to 19 nm, so it is completely invisible to the naked eyes, but detectable by a UV camera. To employ the unique characteristics of O-CNs, security protection strategies with superior concealment by virtue of the thermal-stimuli quenching visible RTP and meanwhile producing invisible UV DF are demonstrated.

Anomalous Pressure-Induced Blue-Shifted Emission of Ionic Copper-Iodine Clusters: The Competitive Effect between Cuprophilic Interactions and Through-Space Interactions

Developing ionic copper-iodine clusters with multiple emitting is crucial for enriching lighting and display materials with various colors. However, the luminescent properties of traditional ionic copper-iodine clusters are often closely associated with low-energy cluster-centered triplet emission, which will redshift further as the Cu⋅⋅⋅Cu bond length decreases. This article utilizes a pressure-treated strategy to achieve an anomalous pressure-induced blue-shifted luminescence phenomenon in ionic Cu4I6(4-dimethylamino-1-ethylpyridinium)2 crystals for the first time, which is based on dominant through-space charge-transfer (TSCT). Herein, we reveal that the more advantageous through-space interactions in the competition between cuprophilic interactions and through-space interactions can lead to a blue-shifted luminescence. High-pressure angle-dispersive X-ray diffraction and high-pressure infrared experiments show that the enhanced through-space interactions mainly originate from forming new intermolecular C-H⋅⋅⋅I hydrogen bonds and the enhancement of van der Waals interactions between organic cations and anionic clusters. Theoretical calculations and experimental studies of excited-state dynamics confirm that the blue-shifted emission is due to the increased energy gap between the excited triplet and ground states caused by the electron delocalization under stronger through-space interactions. This work deepens previous understanding and provides a new avenue to design and synthetic ionic copper-iodine clusters with high-energy TSCT emission.

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.

Robust Sandwich-Structured Thermally Activated Delayed Fluorescence Molecules Utilizing 11,12-Dihydroindolo[2,3-a]carbazole as Bridge

Constructing folded molecular structures is emerging as a promising strategy to develop efficient thermally activated delayed fluorescence (TADF) materials. Most folded TADF materials have V-shaped configurations formed by donors and acceptors linked on carbazole or fluorene bridges. In this work, a facile molecular design strategy is proposed for exploring sandwich-structured molecules, and a series of novel and robust TADF materials with regular U-shaped sandwich conformations are constructed by using 11,12-dihydroindolo[2,3-a]carbazole as bridge, xanthone as acceptor, and dibenzothiophene, dibenzofuran, 9-phenylcarbazole and indolo[3,2,1-JK]carbazole as donors. They hold outstanding thermal stability with ultrahigh decomposition temperatures (556-563 °C), and exhibit fast delayed fluorescence and excellent photoluminescence quantum efficiencies (86 %-97 %). The regular and close stacking of acceptor and donors results in rigidified molecular structures with efficient through-space interaction, which are conducive to suppressing intramolecular motion and reducing reorganized excited-state energy. The organic light-emitting diodes (OLEDs) using them as emitters exhibit excellent electroluminescence performances, with maximum external quantum efficiencies of up to 30.6 %, which is a leading value for the OLEDs based on folded TADF emitters. These results demonstrate the proposed strategy of employing 11,12-dihydroindolo[2,3-a]carbazole as bridge for planar donors and acceptors to construct efficient folded TADF materials is applicable.

Innovative N-Acridine Thiosemicarbazones and Their Zn(II) Complexes Transmetallate with Cu(II): Redox Activity and Suppression of Detrimental Oxy-Myoglobin Oxidation

The coordination chemistry and electrochemistry of novel N-acridine thiosemicarbazones (NATs) were investigated along with their redox activity, antiproliferative efficacy, transmetalation, and dissociation properties. The ability of NAT Fe(III) complexes to inhibit detrimental oxy-myoglobin (oxy-Mb) oxidation was also examined. The NATs act as tridentate ligands with a 2:1 L/Zn(II) complex crystal structure, revealing a distorted octahedral geometry, where both ligands bind Zn(II) in a meridional conformation. The NAT Fe(III) complexes exhibited fully reversible one-electron FeIII/II couples with more positive potentials than the Fe(III) complexes of a related clinically trialed thiosemicarbazone (e.g., [Fe(DpC)2]+) due to the electron-donating capacity of acridine. Surprisingly, the NAT-Zn(II) complexes showed generally greater or similar antiproliferative activity than their ligands, Cu(II), or Fe(III) complexes. This may be explained by (1) formation of a highly lipophilic Zn(II) complex that acts as a chaperone to promote cellular uptake and (2) the capacity of the Zn(II) complex to dissociate or undergo transmetalation to the redox-active Cu(II) complex. Of the NAT-Fe(III) complexes, [Fe(AOBP)2]+ demonstrated a significant (p < 0.0001) improvement in preventing oxy-Mb oxidation than the Fe(III) complex of the clinically trialed thiosemicarbazone, DpC. This article advances our understanding of NAT coordination chemistry, electrochemistry, and the intriguing biological activity of their complexes.

Remote C─H Bond Activation via Enantioselective Carbopalladation and 1,4-Pd Migration Cascade Process

Carbopalladation-initiated cascade reaction involving 1,4-Pd migration is a straightforward and powerful approach to activate remote C─H bond, forging versatile fused polycyclic compounds containing fluorene fragment which are highly valuable synthetic targets. However, its asymmetric variants pose considerable challenges and have not been explored. Here the first asymmetric palladium-catalyzed tandem carbopalladation is reported, 1,4-Pd migration reaction of ortho-iodophenol-derived allyl ether under mild conditions, allowing the transformation of a wide range of substrates in good to excellent enantioselectivities, and providing a facile and straight forward access to tetracyclic dihydroindeno[1,2,3-de]chromene bearing a chiral fluorene skeleton. A good functional group tolerance, high stereoselectivity, as well as the good chiroptical properties (high fluorescence quantum yields, circular dichroism) of the products make this approach highly attractive. Moreover, density functional theory (DFT) calculations indicate that the protonation of five-membered palladacycle intermediate is more favorable rather than its direct reductive elimination process.

Donor-only substituted benzene achieves thermally activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) is a promising mechanism for harvesting triplet excitons in organic light-emitting diodes (OLEDs). The donor-acceptor (D-A) design is the most conventional strategy for developing efficient TADF emitters. A subsequently emerged approach, known as the multiple resonance (MR) effect, also employs electron-donating and electron-withdrawing functional groups. Thus, developing TADF materials has traditionally relied on ingenuity in selecting and combining two functional units. Here, we have realized a TADF molecule by utilizing only a carbazole donor moiety. This molecule is an unusual example in the family of TADF materials and offers better insight into the electronic structures in the excited states for luminescent materials.

Synthesis, Photophysical and Electrochemical Properties of Spiro-Phosphonium Compounds

A series of spiro-phosphonium compounds have been synthesized by copper-mediated coupling reaction of phosphacyclic compounds with alkynes. Their photophysical properties are tuned by varying substituents and exhibit different luminescent colors from blue to green, and finally, yellow. The fluorescence quantum efficiency of diethyl spiro-xanthenebenzophosphole 3aa in solid and liquid states reached 31% and 76%, respectively. Diphenyl spiro-xanthenebenzophosphole 3ad displayed relatively low cytotoxicity toward lung cancer cells A549 and was able to effectively penetrate the cell membrane and maintain strong staining. Moreover, density functional theory (DFT) and time-dependent DFT calculations have been performed to explore the origin of their photophysical properties.

Targeting lysosomes by design: novel N-acridine thiosemicarbazones that enable direct detection of intracellular drug localization and overcome P-glycoprotein (Pgp)-mediated resistance

Innovative N-acridine thiosemicarbazones (NATs) were designed along with their iron(iii), copper(ii), and zinc(ii) complexes. Lysosomal targeting was promoted by specifically incorporating the lysosomotropic Pgp substrate, acridine, into the thiosemicarbazone scaffold to maintain the tridentate N, N, S-donor system. The acridine moiety enables a significant advance in thiosemicarbazone design, since: (1) it enables tracking of the drugs by confocal microscopy using its inherent fluorescence; (2) it is lysosomotropic enabling lysosomal targeting; and (3) as acridine is a P-glycoprotein (Pgp) substrate, it facilitates lysosomal targeting, resulting in the drug overcoming Pgp-mediated resistance. These new N-acridine analogues are novel, and this is the first time that acridine has been specifically added to the thiosemicarbazone framework to achieve the three important properties above. These new agents displayed markedly greater anti-proliferative activity against resistant Pgp-expressing cells than very low Pgp-expressing cells. The anti-proliferative activity of NATs against multiple Pgp-positive cancer cell-types (colon, lung, and cervical carcinoma) was abrogated by the third generation Pgp inhibitor, Elacridar, and also Pgp siRNA that down-regulated Pgp. Confocal microscopy demonstrated that low Pgp in KB31 (-Pgp) cells resulted in acridine's proclivity for DNA intercalation promoting NAT nuclear-targeting. In contrast, high Pgp in KBV1 (+Pgp) cells led to NAT lysosomal sequestration, preventing its nuclear localisation. High Pgp expression in KBV1 (+Pgp) cells resulted in co-localization of NATs with the lysosomal marker, LysoTracker™, that was significantly (p < 0.001) greater than the positive control, the di-2-pyridylketone-4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC) Zn(ii) complex, [Zn(DpC)2]. Incorporation of acridine into the thiosemicarbazone scaffold led to Pgp-mediated transport into lysosomes to overcome Pgp-resistance.

Benchmarking Charge-Transfer Excited States in TADF Emitters: ΔDFT Outperforms TD-DFT for Emission Energies

Charge-transfer (CT) excited states are crucial to organic light-emitting diodes (OLEDs), particularly to those based on thermally activated delayed fluorescence (TADF). However, accurately modeling CT states remains challenging, even with modern implementations of (time-dependent) density functional theory [(TD-)DFT], especially in a dielectric environment. To identify shortcomings and improve the methodology, we previously established the STGABS27 benchmark set with highly accurate experimental references for the adiabatic energy gap between the lowest singlet and triplet excited states (ΔEST). Here, we diversify this set to the STGABS27-EMS benchmark by including experimental emission energies (Eem) and use this new set to (re)-evaluate various DFT-based approaches. Surprisingly, these tests demonstrate that a state-specific (un)restricted open-shell Kohn-Sham (U/ROKS) DFT coupled with a polarizable continuum model for perturbative state-specific nonequilibrium solvation (ptSS-PCM) provides exceptional accuracy for predicting Eem over a wide range of density functionals. In contrast, the main workhorse of the field, Tamm-Dancoff-approximated TD-DFT (TDA-DFT) paired with the same ptSS-PCM, is distinctly less accurate and strongly functional-dependent. More importantly, while TDA-DFT requires the choice of two very different density functionals for good performance on either ΔEST or Eem, the time-independent U/ROKS/PCM approaches deliver excellent accuracy for both quantities with a wide variety of functionals.

π-Conjugated Nanohoops: A New Generation of Curved Materials for Organic Electronics

Nanohoops, cyclic association of π-conjugated systems to form a hoop-shaped molecule, have been widely developed in the last 15 years. Beyond the synthetic challenge, the strong interest towards these molecules arises from their radially oriented π-orbitals, which provide singular properties to these fascinating structures. Thanks to their particular cylindrical arrangement, this new generation of curved molecules have been already used in many applications such as host-guest complexation, biosensing, bioimaging, solid-state emission and catalysis. However, their potential in organic electronics has only started to be explored. From the first incorporation as an emitter in a fluorescent organic light emitting diode (OLED), to the recent first incorporation as a host in phosphorescent OLEDs or as charge transporter in organic field-effect transistors and in organic photovoltaics, this field has shown important breakthroughs in recent years. These findings have revealed that curved materials can play a key role in the future and can even be more efficient than their linear counterparts. This can have important repercussions for the future of electronics. Time has now come to overview the different nanohoops used to date in electronic devices in order to stimulate the future molecular designs of functional materials based on these macrocycles.

Uncovering the Aggregation-Induced Emission Mechanisms of Phenoxazine and Phenothiazine Groups

Molecules with both aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) properties are potential organic light-emitting diode materials; however, the AIE and TADF mechanisms are still debatable. In this work, four molecules incorporating carbazole (Cz), phenoxazine (PXZ), and phenothiazine (PTZ) as donor groups to the diphenylsulfone acceptor were investigated. The experiment results indicate that a molecule containing Cz exhibits solely TADF properties, whereas molecules containing PXZ and PTZ demonstrate both TADF and AIE characteristics. As for DPS-PTZ, the result indicates that the thin-film environment restricts molecular twisting, consequently reducing nonradiative decay, thereby attributing to the AIE property by density functional theory and molecular dynamics simulation. As for DPS-PXZ, the result suggests that the restricted access to a conical intersection in a singlet excited via an expansion in the C-S-C angle is the pivotal factor for the AIE characteristic. The C-S-C angle twist of DPS-PXZ is impeded in the aggregate state and resulted in luminescence. Understanding the mechanisms serves as a valuable guide for the development of new AIE systems, enabling their application in various practical domains.

Achievement of efficient thermally activated delayed fluorescence materials based on 1,8-naphthalimide derivatives exhibiting piezochromic and thermochromic luminescence

In this study, we developed a D-A type imide derivative based on 1,8-naphthalimide, NI-mPCz, which exhibited outstanding thermally activated delayed fluorescence (TADF) properties. Additionally, it demonstrates characteristics of piezochromic and thermochromic luminescence. The thermochromic luminescence observed is attributed to crystalline transformations occurring during the heating process, as evidenced by differential scanning calorimetry (DSC) and microscopic examinations. Moreover, the good compatibility of NI-mPCz with HeLa cells and its excellent imaging performance indicate its potential for application in the field of biological imaging. These results provide valuable insights for the design and development of new organic electronic and bioimaging materials with high-efficiency TADF characteristics.

HLCT-Type Acceptor Molecule-Based Exciplex System for Highly Efficient Solution-Processable OLEDs with Suppressed Efficiency Roll-Offs

Exciplex systems are promising candidates for thermally activated delayed fluorescence (TADF) molecules because of the small energy difference between the lowest singlet and triplet excited states (ΔEST). However, realizing high-efficiency and low-external-quantum-efficiency (EQE) roll-off in solution-processed organic light-emitting diodes (OLEDs) using an exciplex system remains a formidable challenge. In this study, two (HLCT)-type isomers with a spiro skeleton, 2-tBuspoCz-TRZ and 10-tBuspoCz-TRZ, are designed and synthesized as acceptors of exciplexes, where tert-butylspirofluorene indole is regarded as a donor and the triazine unit as an acceptor. Green exciplex emissions are observed for the 2-tBuspoCz-TRZ:TAPC and 10-tBuspoCz-TRZ:TAPC exciplexes, indicating distinct TADF characteristics with a very small ΔEST of 35 ± 5 meV. By using the TADF exciplex system based on the HLCT acceptor as an emitter, solution-processable OLEDs achieve a maximum external quantum efficiency (EQEmax) of 20.8%. Furthermore, a high EQEmax > 25% with a very low-efficiency roll-off (≈3.5% at 1000 cd m-2) is obtained for solution-processable phosphorescent devices using HLCT-based exciplexes as the host matrix of phosphors. This study paves the way for a novel strategy for designing acceptor exciplex molecules for effective TADF molecules and host matrices in solution-processable OLEDs.

Decoupling excitons from high-frequency vibrations in organic molecules

The coupling of excitons in π-conjugated molecules to high-frequency vibrational modes, particularly carbon-carbon stretch modes (1,000-1,600 cm-1) has been thought to be unavoidable1,2. These high-frequency modes accelerate non-radiative losses and limit the performance of light-emitting diodes, fluorescent biomarkers and photovoltaic devices. Here, by combining broadband impulsive vibrational spectroscopy, first-principles modelling and synthetic chemistry, we explore exciton-vibration coupling in a range of π-conjugated molecules. We uncover two design rules that decouple excitons from high-frequency vibrations. First, when the exciton wavefunction has a substantial charge-transfer character with spatially disjoint electron and hole densities, we find that high-frequency modes can be localized to either the donor or acceptor moiety, so that they do not significantly perturb the exciton energy or its spatial distribution. Second, it is possible to select materials such that the participating molecular orbitals have a symmetry-imposed non-bonding character and are, thus, decoupled from the high-frequency vibrational modes that modulate the π-bond order. We exemplify both these design rules by creating a series of spin radical systems that have very efficient near-infrared emission (680-800 nm) from charge-transfer excitons. We show that these systems have substantial coupling to vibrational modes only below 250 cm-1, frequencies that are too low to allow fast non-radiative decay. This enables non-radiative decay rates to be suppressed by nearly two orders of magnitude in comparison to π-conjugated molecules with similar bandgaps. Our results show that losses due to coupling to high-frequency modes need not be a fundamental property of these systems.

Tuning of the Singlet-Triplet Energy Gap of Donor-Linker-Acceptor Based Thermally Activated Delayed Fluorescent Emitters

Thermally activated delayed fluorescent emitters based on carbazole donor, benzonitrile acceptor with the linkers biphenyl, bipyridine and naphthalene are investigated using the density functional theoretical method. The molecule in which bipyridine acts as the linker with the least ΔEST is further selected for the designing of a series of D-L-A framework TADF molecules. Remarkably, the ΔEST is decreased successively by attaching the additional cyano groups at the acceptor site which is further reduced when the electron donating methoxy groups are attached at the donor site. To know the effect of substituents on ΔEST, the acceptor moiety of the D-L-A framework is modified with -F, -Cl and -CF3 substituents. The studies showed a relatively less decrement in the value of ΔEST compared to the cyano substituted molecules. However, ΔEST significantly reduced further on attaching methoxy groups at the donor site.

Organic Optoelectronic Materials: A Rising Star of Bioimaging and Phototherapy

Recently, many organic optoelectronic materials (OOMs), especially those used in organic light-emitting diodes (OLEDs), organic solar cells (OSCs), and organic field-effect transistors (OFETs), are explored for biomedical applications including imaging and photoexcited therapies. In this review, recently developed OOMs for fluorescence imaging, photoacoustic imaging, photothermal therapy, and photodynamic therapy, are summarized. Relationships between their molecular structures, nanoaggregation structures, photophysical mechanisms, and properties for various biomedical applications are discussed. Mainly four kinds of OOMs are covered: thermally activated delayed fluorescence materials in OLEDs, conjugated small molecules and polymers in OSCs, and charge-transfer complexes in OFETs. Based on the OOMs unique optical properties, including excitation light wavelength and exciton dynamics, they are respectively exploited for suitable biomedical applications. This review is intended to serve as a bridge between researchers in the area of organic optoelectronic devices and those in the area of biomedical applications. Moreover, it provides guidance for selecting or modifying OOMs for high-performance biomedical uses. Current challenges and future perspectives of OOMs are also discussed with the hope of inspiring further development of OOMs for efficient biomedical applications.

Deep Learning-Based Chemical Similarity for Accelerated Organic Light-Emitting Diode Materials Discovery

Thermally activated delayed fluorescence (TADF) material has attracted great attention as a promising metal-free organic light-emitting diode material with a high theoretical efficiency. To accelerate the discovery of novel TADF materials, computer-aided material design strategies have been developed. However, they have clear limitations due to the accessibility of only a few computationally tractable properties. Here, we propose TADF-likeness, a quantitative score to evaluate the TADF potential of molecules based on a data-driven concept of chemical similarity to existing TADF molecules. We used a deep autoencoder to characterize the common features of existing TADF molecules with common chemical descriptors. The score was highly correlated with the four essential electronic properties of TADF molecules and had a high success rate in large-scale virtual screening of millions of molecules to identify promising candidates at almost no cost, validating its feasibility for accelerating TADF discovery. The concept of TADF-likeness can be extended to other fields of materials discovery.

An effective method in modulating thermally activated delayed fluorescence (TADF) emitters from green to blue emission: the role of the phenyl ring

Developing efficient blue emitters with high performance and low cost is crucial for the further development of organic light-emitting diodes (OLEDs). Based on the two experimentally reported green thermally activated delayed fluorescence (TADF) emitters, which are thioxanthone derivatives consisting of carbazole as an electron donor and 9H-thioxanthen-9-one-S,S-dioxide (SOXO) as an electron acceptor with donor-acceptor (D-A) or donor-acceptor-donor (D-A-D) structures, two new blue TADF emitters are designed by simply inserting a phenyl ring between D and A units. The TADF processes of the four thioxanthone derivatives are studied systematically through first-principles calculations. The role of the introduced phenyl ring in the excited state properties of the designed molecules is explored by analyzing the changes in molecular geometries, frontier molecular orbital distributions, the lowest singlet-triplet energy splitting (ΔEST), the spin orbit coupling (SOC) constants, the radiative decay rates (kr) and the nonradiative decay rates (knr), as well as the intersystem crossing rates (kISC) and reverse intersystem crossing rates (kRISC). The results show that when incorporating phenyl units into the D-A and D-A-D structures, both high kr and enhanced kRISC are achieved in Cz-Ph-SOXO and DCz-DPh-SOXO, demonstrating that incorporating the phenyl unit in D-A and D-A-D structures is an efficient way for developing new SOXO-based TADF molecules. It is worth noting that the kRISC values for Cz-Ph-SOXO and DCz-DPh-SOXO are significantly increased with respect to those of the experimental molecules. The present results would provide helpful guidelines for developing new SOXO-based TADF molecules experimentally.

High-Performance Ultraviolet Organic Light-Emitting Diodes Enabled by Double Boron-Oxygen-Embedded Benzo[m]tetraphene Emitters

Ultraviolet organic light-emitting diodes (UV OLEDs) have attracted increasing attention because of their promising applications in healthcare, industry, and agriculture; however, their development has been hindered by the shortage of robust UV emitters. Herein, we embedded double boron-oxygen units into nonlinear polycyclic aromatic hydrocarbons (BO-PAHs) to regulate their molecular configurations and excited-state properties, enabling novel bent BO-biphenyl (BO-bPh) and helical BO-naphthyl (BO-Nap) emitters with hybridized local and charge-transfer (HLCT) characteristics. They could be facilely synthesized in gram-scale amounts via a highly efficient two-step route. BO-bPh and BO-Nap showed strong UV and violet-blue photoluminescence in toluene with full width at half-maximum values of 25 and 37 nm, along with quantum efficiencies of 98 and 99%, respectively. A BO-bPh-based OLED showed high color purity UV electroluminescence peaking at 394 nm with Commission Internationale de l'Eclairage (CIE) coordinates of (0.166, 0.021). Moreover, the device demonstrated a record-high maximum external quantum efficiency (EQE) of 11.3%, achieved by successful hot exciton utilization. This work demonstrates the promising potential of double BO-PAHs as robust emitters for future UV OLEDs.

Styrylpyrimidine chromophores with bulky electron-donating substituents: experimental and theoretical investigation

Styrylpyrimidines with bulky 9,9-dimethylacridan, phenoxazine and phenothiazine electron-donating fragments were designed. Thermally activated delayed fluorescence (TADF) properties were expected for these structures. These chromophores exhibit peculiar emission properties. For 9,9-dimethylacridan and phenoxazine derivatives, a single emission highly sensitive to the polarity is observed in solution whereas for phenothiazine derivative a dual emission is observed in solution and is attributed to the coexistence of quasi-axial (Qax) and quasi-equatorial (Qeq) conformers. This study intends to understand through theoretical and experimental works, why the studied chromophores do not exhibit TADF properties, contrary to what was expected. The absence of phosphorescence both at room temperature and 77 K tends to indicate the impossibility to harvest triplet states in these systems. Wave-function based calculations show that for both conformers of the three chromophores the S1-T1 splitting is significantly larger than 0.2 eV. The second triplet state T2 of Qeq conformers is found very close in energy to the singlet S1 state, but S1 and T2 states possess similar charge transfer characters. This prevents efficient spin-orbit coupling between the states, which is consistent with the absence of TADF.

Exploration of violet-to-blue thermally activated delayed fluorescence emitters based on "CH/N" and "H/CN" substitutions at diphenylsulphone acceptor. A DFT study

The violet-to-blue thermally activated delayed fluorescence (TADF) emitters were created employing several substituents based on 5,5-dimethyl-5,10-dihydropyrido [2,3-b][1,8] naphthyridine-diphenylsulphone (DMDHPN-DPS) called 1a via "CH/N" and "H/CN" substitutions at the diphenylsulphone acceptor (DPS) moiety. The parent compound 1a was selected from our former work after extensive research employing "CH/N" substitution on Dimethyl-acridine (DMAC) donor moiety. There is a little overlap amid the highest occupied molecular orbitals (HOMOs) and lowest un-occupied molecular orbitals (LUMOs) due to the distribution of HOMOs and LUMOs primarily on the DMDHPN donor and the DPS acceptor moieties, respectively. It resulted in a narrower energy gap (∆E ST) between the lowest singlet (S1) and triplet (T1) excited state. In nearly all derivatives, the steric hindrance results in a larger torsional angle (85°-98°) between the plane of the DMDHPN and the DPS moieties. The predicted ΔE ST values of the compounds with "H/CN" substitution were lower than those of the comparable "CH/N" substituents, demonstrating the superiority of the reversible inter-system crossing (RISC) from the T1 → S1 state. All derivatives have emission wavelengths (λ em) in the range of 357-449 nm. The LUMO → HOMO transition energies in the S1 states are lowered by the presence of -CN groups or -N = atoms at the ortho or meta sites of a DPS acceptor unit, causing the λ em values to red-shift. Furthermore, the λ em showed a greater red-shift as there were more-CN groups or -N = atoms. Three of the derivatives named 1b, 1g, and 1h, emit violet (394 nm, 399 nm, and 398 nm, respectively), while two others, 1f and 1i, emit blue shade (449 nm each) with reasonable emission intensity peak demonstrating that these derivatives are effective violet-to-blue TADF nominees. The lower ΔE ST value for derivative 1i (0.01 eV) with λ em values of 449 nm make this molecule the finest choice for blue TADF emitter amongst all the studied derivatives. We believe our research might lead to the development of more proficient blue TADF-OLEDs in the future.

Simultaneously enhancing the planarity and electron-donating capability of donors for through-space charge transfer TADF towards deep-red emission

Through-space charge transfer (TSCT) has been proven effective for designing thermally activated delayed fluorescence (TADF) emitters due to the separation of the frontier molecular orbitals. Although tuning of the interaction between the donor and acceptor by controlling the conformation is known to be crucial for the photophysical properties of TSCT excited states, it remains a challenge to realize efficient red and deep-red emissions. Herein, we designed two TSCT molecules, namely TPXZ-QX and TPXZ-2QX, by using oxygen-bridged triphenylamine (TPXZ) as the electron donor with enhanced planarity and electron-donating capability. With a face-to-face orientation of the donor and acceptor segments and close π-π contacts, the new emitters have strong intramolecular noncovalent donor-acceptor interactions. The emissions of TPXZ-QX and TPXZ-2QX in doped thin films lie in the red (λmax = 632 nm) to deep-red (λmax = 665 nm) region. The photoluminescence quantum yields are 41% and 32% for TPXZ-QX and TPXZ-2QX, respectively. Organic light-emitting diodes (OLEDs) based on TPXZ-QX and TPXZ-2QX show external quantum efficiencies (EQEs) of up to 13.8% and 11.4%, respectively. This work indicates that the modulation of TSCT excited states based on strong intramolecular cofacial π-stacking interactions is a viable choice for the development of high-efficiency long-wavelength TADF emitters.

The Rise and Current Status of Polaritonic Photochemistry and Photophysics

The interaction between molecular electronic transitions and electromagnetic fields can be enlarged to the point where distinct hybrid light-matter states, polaritons, emerge. The photonic contribution to these states results in increased complexity as well as an opening to modify the photophysics and photochemistry beyond what normally can be seen in organic molecules. It is today evident that polaritons offer opportunities for molecular photochemistry and photophysics, which has caused an ever-rising interest in the field. Focusing on the experimental landmarks, this review takes its reader from the advent of the field of polaritonic chemistry, over the split into polariton chemistry and photochemistry, to present day status within polaritonic photochemistry and photophysics. To introduce the field, the review starts with a general description of light-matter interactions, how to enhance these, and what characterizes the coupling strength. Then the photochemistry and photophysics of strongly coupled systems using Fabry-Perot and plasmonic cavities are described. This is followed by a description of room-temperature Bose-Einstein condensation/polariton lasing in polaritonic systems. The review ends with a discussion on the benefits, limitations, and future developments of strong exciton-photon coupling using organic molecules.

Highly Twisted Thermally Activated Delayed Fluorescence (TADF) Molecules and Their Applications in Organic Light-Emitting Diodes (OLEDs)

Thermally activated delayed fluorescence (TADF) materials have attracted great potential in the field of organic light-emitting diodes (OLEDs). Among thousands of TADF materials, highly twisted TADF emitters have become a hotspot in recent years. Compared with traditional TADF materials, highly twisted TADF emitters tend to show multi-channel charge-transfer characters and form rigid molecular structures. This is advantageous for TADF materials, as non-radiative decay processes can be suppressed to facilitate efficient exciton utilization. Accordingly, OLEDs with excellent device performances have also been reported. In this Review, we have summarized recent progress in highly twisted TADF materials and related devices, and give an overview of the molecular design strategies, photophysical studies, and the performances of OLED devices. In addition, the challenges and perspectives of highly twisted TADF molecules and the related OLEDs are also discussed.

Donor, Acceptor, and Molecular Charge Transfer Emission All in One Molecule

The molecular photophysics in the thermally activated delayed fluorescence (TADF) spiro-acridine-anthracenone compound, ACRSA, is dominated by the rigid orthogonal spirocarbon bridging bond between the donor and acceptor. This critically decouples the donor and acceptor units, yielding photophysics, which includes (dual) phosphorescence and the molecular charge transfer (CT) states giving rise to TADF, that are dependent upon the excitation wavelength. The molecular singlet CT state can be directly excited, and we propose that supposed "spiro-conjugation" between acridine and anthracenone is more accurately an example of intramolecular through-space charge transfer. In addition, we show that the lowest local and CT triplet states are highly dependent upon spontaneous polarization of the environment, leading to energy reorganization of the triplet states, with the CT triplet becoming lowest in energy, profoundly affecting phosphorescence and TADF, as evident by a (thermally controlled) competition between reverse intersystem crossing and reverse internal conversion, i.e., dual delayed fluorescence (DF) mechanisms.

The Rational Design of Reducing Organophotoredox Catalysts Unlocks Proton-Coupled Electron-Transfer and Atom Transfer Radical Polymerization Mechanisms

Photocatalysis has become a prominent tool in the arsenal of organic chemists to develop and (re)imagine transformations. However, only a handful of versatile organic photocatalysts (PCs) are available, hampering the discovery of new reactivities. Here, we report the design and complete physicochemical characterization of 9-aryl dihydroacridines (9ADA) and 12-aryl dihydrobenzoacridines (12ADBA) as strong reducing organic PCs. Punctual structural variations modulate their molecular orbital distributions and unlock locally or charge-transfer (CT) excited states. The PCs presenting a locally excited state showed better performances in photoredox defunctionalization processes (yields up to 92%), whereas the PCs featuring a CT excited state produced promising results in atom transfer radical polymerization under visible light (up to 1.21 Đ, and 98% I*). Unlike all the PC classes reported so far, 9ADA and 12ADBA feature a free NH group that enables a catalytic multisite proton-coupled electron transfer (MS-PCET) mechanism. This manifold allows the reduction of redox-inert substrates including aryl, alkyl halides, azides, phosphate and ammonium salts (E_red up to -2.83 vs SCE) under single-photon excitation. We anticipate that these new PCs will open new mechanistic manifolds in the field of photocatalysis by allowing access to previously inaccessible radical intermediates under one-photon excitation.

Donor-Acceptor Type of Fused-Ring Thermally Activated Delayed Fluorescence Compounds Constructed through an Oxygen-Containing Six-Membered Ring

Nowadays, thermally activated delayed fluorescence (TADF) compounds with a fused-ring core skeleton are getting increasing research interest because of their use in high-performance organic light-emitting diodes (OLEDs). In this study, TADF compounds featuring a D-A-type fused-ring core skeleton are developed. The challenging compatibility of a planarized D-A arrangement and the TADF property is achieved through linking the D and A moieties with two oxygen atoms within a six-membered ring. Compared with a single-oxygen analogue possessing a flexible skeleton and a twisted D-A arrangement, these fused-ring compounds with higher skeleton rigidity show higher photoluminescence quantum yields and narrower emission spectra in toluene and in doped thin films. Their electroluminescent devices achieve high external quantum efficiencies (up to 19.4%), suggesting the potential of rarely achieved D-A-type fused-ring TADF systems to serve as high-performance emitters of OLEDs.

Cyclometalated Spirobifluorene Imidazolylidene Platinum(II) Complexes with Predominant 3LC Emissive Character and High Photoluminescence Quantum Yields

Two novel bidentate C^C*_spiro cyclometalated platinum(II) complexes comprising a spiro-conjugated bifluorene ligand and different β-diketonate auxiliary ligands are synthesized and characterized. Their preparation employs a robust and elaborate synthetic protocol commencing with an N-heterocyclic carbene precursor. Structural characterization by means of NMR techniques and solid-state structures validate the proposed and herein presented molecular scaffolds. Photophysical studies, including laser flash photolysis methods, reveal an almost exclusively ligand-centered triplet state, governed by the C^C*_spiro-NHC ligand. The high triplet energies and the long triplet lifetimes in the order of 30 μs in solution make the complexes good candidates for light-emitting diode-driven photocatalysis, as initial energy transfer experiments reveal. In-depth time-dependent density functional theory investigations are in excellent accordance with our spectroscopic findings. The title compounds are highly emissive in the bluish-green color region with quantum yields of up to 87% in solid-state measurements.

Synthesis of Tritylones via Cascade Reaction of Arynes with 5-Ethoxyoxazoles

A cascade reaction involving arynes and 5-ethoxyoxazoles has been developed toward the synthesis of 9-alkyl/aryl tritylones. 5-Ethoxyoxazoles undergo a [4 + 2] cycloaddition reaction with arynes followed by retro-[4 + 2] cycloaddition, a second intermolecular [4 + 2] cycloaddition reaction, and hydrolytic ring cleavage to generate substituted tritylones in good yields. The conversion of tritylone products to a series of spirocyclic anthrone derivatives has been demonstrated. The reaction is expeditious, exhibits wide scope, and employs readily available starting materials.

Spiroconjugated Tetraaminospirenes as Donors in Color-Tunable Charge-Transfer Emitters with Donor-Acceptor Structure

Charge-transfer emitters are attractive due to their color tunability and potentially high photoluminescence quantum yields (PLQYs). We herein present tetraaminospirenes as donor moieties, which, in combination with a variety of acceptors, furnished 12 charge-transfer emitters with a range of emission colors and PLQYs of up to 99 %. The spatial separation of their frontier molecular orbitals was obtained through careful structural design, and two DA structures were confirmed by X-ray crystallography. A range of photophysical measurements supported by DFT calculations shed light on the optoelectronic properties of this new family of spiro-NN-donor-acceptor dyes.

Turning conventional non-TADF units into high-lying reverse intersystem crossing TADF emitters: different symmetric D–A–D-type modified donor units

DFT and TD-DFT calculations were performed to turn conventional non-TADF units into high-lying reverse intersystem crossing D–A–D-type TADF emitters.

Through-Space Charge-Transfer Emitters Developed by Fixing the Acceptor for High-Efficiency Thermally Activated Delayed Fluorescence

Through-space charge-transfer (TSCT) emitters have been extensively explored for thermally activated delayed fluorescence (TADF), but arranging various donors and acceptors into rigid cofacial conformations for various efficient TSCT TADF emitters has remained challenging. Here, we report a "fixing acceptor" design to reach various efficient TSCT TADF emitters. By chemically fixing the acceptor (benzophenone) with a rigid spiro-structure and cofacially aligning various donors with the fixed acceptor, a series of efficient TSCT TADF emitters have been developed. Single-crystal structures and theoretical calculations have verified closely packed cofacial donor/acceptor conformations and favorable TSCT in the emitters. In doped films, the emitters afford sky blue to yellow TADF emission, with high photoluminescence efficiencies up to 0.92 and reverse intersystem crossing rates up to 1.0 × 10⁶ s^-1. Organic light-emitting diodes using the emitters afford sky blue to yellow electroluminescence with high external quantum efficiencies up to 20.9%. The work opens a new avenue toward a wide variety of efficient TSCT TADF emitters.

Direct Observation of Ultrafast Access to a Solvent-Independent Singlet-Triplet Equilibrium State in Acridone Solutions

Acridone and its derivatives have potential application as emitters for highly efficient blue organic light-emitting diodes (OLEDs). In this paper, we demonstrated ultrafast access of a solvent-independent singlet-triplet equilibrium state in acridone solutions by using femtosecond time-resolved spectroscopy. Our spectral data show that due to highly effective forward and reverse intersystem crossing (both k_ISC and k_rISC over 10¹⁰ s^-1), a singlet-triplet equilibrium state is always populated in acridone in all solvents studied. However, the lifetimes of the equilibrium state varied a lot in different solvent environments and the final decay pathway of this state can switch between high quantum yield fluorescence emission and further internal conversion to the lowest triplet state. These findings provide direct experimental evidence to understand the distinct photophysical behaviors of acridone and also provide guidance for further design of acridone and its derivatives as blue OLED emitters.

Incorporation of Fluorene and Its Heterocyclic Spiro Derivatives To Realize High-Performance and Stable Sky-Blue-Emitting Arylgold(III) Complexes

A series of arylgold(III) complexes of tridentate diphenylpyridine ligand incorporated with fluorene and its heterocyclic spiro derivatives, spiro[fluorene-9,9'-xanthene] and spiro[acridine-9,9'-fluorene], as auxiliary ligands has been prepared. This class of complexes exhibits high decomposition temperatures of up to 387 °C, excellent film morphologies in solid-state thin films with a root-mean-square roughness smaller than 0.20 nm, as well as high photoluminescence quantum yields of up to 0.72 in solid-state thin films. Solution-processed organic light-emitting devices (OLEDs) fabricated from this series of complexes as dopants show intense electroluminescence in the sky-blue region with maximum external quantum efficiencies of 10.0%. Taking advantage of their high thermal stability, vacuum-deposited OLEDs have also been fabricated and satisfactory operational lifetimes of ∼300 h have been recorded.

Controlling the thermally activated delayed fluorescence of axially chiral organic emitters and their racemate for information encryption

A pair of axially chiral organic enantiomers were facilely prepared through a one-pot sequential synthesis. They exhibit circularly polarized luminescence activities and have thermally activated delayed fluorescence (TADF) and aggregation-induced emission enhancement properties. Meanwhile, these two enantiomers present remarkable and reversible thermochromism in the crystalline state, enabling dual-colour TADF switching between orange and red. However, when they form cocrystals, the resulting racemate shows opposite thermochromic behaviors. These intriguing results probably emanate from their different optical activities, leading to distinct molecular packing modes and molecular conformation variations. Moreover, information encryption based on thermochromism of organic enantiomers and their racemate has been presented for the first time. This work may expand the application scope of chiral organic luminogens and pave a new way to construct intelligent luminescent systems.

A pair of axially chiral organic enantiomers with circularly polarized thermally activated delayed fluorescence and aggregation-induced emission enhancement properties show opposite thermochromic behaviors to their racemate.