Anthraquinone-based intramolecular charge-transfer compounds: computational molecular design, thermally activated delayed fluorescence, and highly efficient red electroluminescence

Hypsochromic Shift of Multiple-Resonance-Induced Thermally Activated Delayed Fluorescence by Oxygen Atom Incorporation

Herein, we reported an ultrapure blue multiple-resonance-induced thermally activated delayed fluorescence (MR-TADF) material (ν-DABNA-O-Me) with a high photoluminescence quantum yield and a large rate constant for reverse intersystem crossing. Because of restricted π-conjugation of the HOMO rather than the LUMO induced by oxygen atom incorporation, ν-DABNA-O-Me shows a hypsochromic shift compared to the parent MR-TADF material (ν-DABNA). An organic light-emitting diode based on this material exhibits an emission at 465 nm, with a small full-width at half-maximum of 23 nm and Commission Internationale de l'Eclairage coordinates of (0.13, 0.10), and a high maximum external quantum efficiency of 29.5 %. Moreover, ν-DABNA-O-Me facilitates a drastically improved efficiency roll-off and a device lifetime compared to ν-DABNA, which demonstrates significant potential of the oxygen atom incorporation strategy.

Achieving High Efficiency at High Luminance in Fluorescent Organic Light-Emitting Diodes through Triplet-Triplet Fusion Based on Phenanthroimidazole-Benzothiadiazole Derivatives

Achieving high efficiency at high luminance is one of the most important prerequisites towards practical application of any kind of light-emitting diode (LED). Herein, we report highly emissive organic fluorescent molecules based on phenanthroimidazole-benzothiadiazole derivatives capable of maintaining high external quantum efficiency (EQE) at high luminance enabled by triplet-triplet fusion (TTF) in doped organic LEDs. The PIBzP-, PIBzPCN-, and PIBzTPA-based devices showed EQEs of 8.27, 9.15, and 8.64 %, respectively, at luminance of higher than 1000 cd m^-2 , with little efficiency roll-off.

Optimizing Charge Transfer and Out-Coupling of A Quasi-Planar Deep-Red TADF Emitter: towards Rec.2020 Gamut and External Quantum Efficiency beyond 30

Herein, we report a deep-red TADF emitter pCNQ-TPA, composed of quinoxaline-5,8-dicarbonitrile (pCNQ) acceptor and triphenylamine (TPA) donor. pCNQ-TPA supported its OLED with desired CIE coordinates of (0.69, 0.31) and the record maximum external quantum efficiency of 30.3 %, which is the best red TADF diode with Rec.2020 gamut for UHDTV. It is showed that through tuning pCNQ-TPA doping concentration, intra- and inter-molecular charge transfer are balanced to synchronously improve emission color saturation and TADF radiation, and remedy aggregation-induced quenching, rendering photoluminescence quantum yield (PLQY) reaching 90 % for deep-red emission peaked at ≈690 nm. Quasi-planar structure further endows pCNQ-TPA with an improved horizontal ratio of emitting dipole orientation, which increases light out-coupling ratio to 0.34 for achieving the state-of-the-art device efficiencies.

Efficient Intramolecular Charge-Transfer Fluorophores Based on Substituted Triphenylphosphine Donors

Triphenylphosphine (TPP)-based luminescent compounds are rarely investigated because of the low photoluminescence quantum yield (PLQY). Here, we demonstrate that introducing steric hindrance groups to the TPP moiety and separating the orbitals involved in the transition can drastically suppress the non-radiative decay induced by structural distortion of TPP in the excited state. High PLQY up to 0.89 as well as thermally activated delayed fluorescence are observed from the intramolecular charge-transfer (ICT) molecules with substituted TPP donors (sTPPs) in doped films. The red organic light-emitting diodes employing these emitters achieve comparable external quantum efficiencies to the control device containing a classical phosphorescent dye, revealing the great potential of the ICT emitters based on electrochemically stable sTPPs.

Non-toxic near-infrared light-emitting diodes

Harnessing cost-efficient printable semiconductor materials as near-infrared (NIR) emitters in light-emitting diodes (LEDs) is extremely attractive for sensing and diagnostics, telecommunications, and biomedical sciences. However, the most efficient NIR LEDs suitable for printable electronics rely on emissive materials containing precious transition metal ions (such as platinum), which have triggered concerns about their poor biocompatibility and sustainability. Here, we review and highlight the latest progress in NIR LEDs based on non-toxic and low-cost functional materials suitable for solution-processing deposition. Different approaches to achieve NIR emission from organic and hybrid materials are discussed, with particular focus on fluorescent and exciplex-forming host-guest systems, thermally activated delayed fluorescent molecules, aggregation-induced emission fluorophores, as well as lead-free perovskites. Alternative strategies leveraging photonic microcavity effects and surface plasmon resonances to enhance the emission of such materials in the NIR are also presented. Finally, an outlook for critical challenges and opportunities of non-toxic NIR LEDs is provided.

Efficient Red Electroluminescence From Phenanthro[9,10-d]imidazole-Naphtho[2,3-c][1,2,5]thiadiazole Donor-Acceptor Derivatives

Red emission is one of the three primary colors and is indispensable for full color displays. Fluorescent materials that can generate efficient red electroluminescence (EL) are limited and need to be developed. In this work, we report efficient red emitters based on phenanthro[9,10-d]imidazole-naphtho[2,3-c][1,2,5]thiadiazole donor-acceptor derivatives. The molecules, abbreviated as PINzP and PINzPCN, exhibited high photoluminescence quantum yield (PLQY) up to unity in doped films. They can also reach a relatively high PLQY of ∼30% in neat films. PINzP and PINzPCN were capable of generating efficient red EL in doped devices with a maximum external quantum efficiency (EQE) of 6.96% and 5.92%, respectively.

Management of Locally Excited States for Purine-based TADF Emitters: A Method to Reduce Device Efficiency Roll-Off

The programmed arylation of purine has been developed to construct a series of efficient thermally activated delayed fluorescent (TADF) materials. The corresponding organic light-emitting diodes (OLEDs) exhibit external quantum efficiency as high as 16.0% alongside small efficiency roll-off. Intriguingly, this work proves that the good management of localized states is an efficient way to reduce device efficiency roll-off and is crucial for the future design of high-performance OLEDs.

Room-Temperature Stable Noncovalent Charge-Transfer Dianion Biradical to Produce Singlet Oxygen by Visible or Near-Infrared Light Photoexcitation

Noncovalent interaction between small molecules can generate a charge-transfer (CT) state, achieving the effect of a conjugated large molecule as well as a transition-metal complex. Herein, we demonstrate a room-temperature stable dianion biradical conveniently produced by noncovalent intermolecular CT interaction between anthraquinone (AQ) and potassium tert-butoxide (KOtBu). Essentially, CT from KOtBu to AQ boosts absorption bands from the UV to visible and near-infrared (NIR) range, enabling AQ-KOtBu to have new absorption bands around 400, 550, and 900 nm. The absorption bands of AQ-KOtBu are dramatically enhanced after blue-to-green or NIR light excitation. Interestingly, both ground state AQ-KOtBu (C₍₁₎) and photoexcited AQ-KOtBu (C₍₂₎) are quenched by oxygen to produce singlet oxygen. Furthermore, C₍₁₎ can be photoactivated by purged nitrogen in solution, and C₍₂₎ can be regenerated after the photoexcitation and purged nitrogen in solution, which may serve as a photosensitizer under visible and NIR light excitation.

Efficient Red Thermally Activated Delayed Fluorescence Emitters Based on a Dibenzonitrile-Substituted Dipyrido[3,2-a:2',3'-c]phenazine Acceptor

How to construct efficient red-emitting thermally activated delayed fluorescence (TADF) materials is a challenging task in the field of organic light-emitting diodes (OLEDs). Herein, an electron acceptor moiety, 3,6-DCNB-DPPZ, with high rigidity and strong acceptor strength was designed by introducing two cyanobenzene groups into the 3,6-positions of a dipyrido[3,2-a:2′,3′-c]phenazine unit. A red-emitting compound, 3,6_R, has been designed and synthesized by combining the rigid acceptor unit with two triphenylamine donors. Due to high molecular rigidity and strong intramolecular charge transfer characteristic in donor–acceptor–donor skeleton, 3,6_R exhibited a red emission with a high photoluminescence quantum yield of 86% and distinct TADF nature with short delayed fluorescence lifetime of about 1 microsecond. Accordingly, the OLED using 3,6_R as the guest emitter gained a high external quantum efficiency of 12.0% in the red region with an electroluminescence peak of 619 nm and favorable Commission Internationale de l’Eclairage coordinates of (0.62, 0.38).

Recovering the Thermally Activated Delayed Fluorescence in Aggregation-Induced Emitters of Carborane

The aggregation-induced emission (AIE) behaviors of carborane-based hybrid emitters have been extensively reported, while their combinations with the thermally activated delayed fluorescence (TADF) are still scarce. We designed and synthesized three Janus carboranes (the chemical structures resemble the double-faced god, Janus) Cb-1/2/3 with different carbazole moieties. All of the Janus carboranes exhibited quenched emission in solution with Φ_PL (quantum efficiency of photoluminescence (PL)) lower than 0.01. The PL performance was improved by proceeding to the aggregates in THF/water (Φ_PL 0.17-0.35) and further improved in the crystals or solid with Φ_PL up to 0.99 for Cb-1, 0.85 for Cb-2, and 0.61 for Cb-3, which agreed with the AIE enhancement. Although the PL of solid Cb-1/2/3 showed non-TADF properties with lifetimes only at several nanoseconds, the crystallographic studies have shown a root cause of π···π stacking that quenched the TADF, and the theoretical calculations forecasted small singlet-triplet energy gaps (ΔE_S-T) therein. According to these findings, TADF was recovered in Cb-1/2/3 by doping into 1,3-bis(carbazol-9-yl)benzene (mCP). The 10 wt % doped films of Cb-1/2/3 have achieved a trade-off of Φ_PL (0.84 in Cb-3 and 0.83 in Cb-1) and delayed lifetime (up to 8 μs). The doped devices of organic light-emitting diodes incorporating Cb-1/2/3 achieved the highest external quantum efficiency at 10.1% and the maximized luminance of 5920 cd/m² at a driving voltage of 8 V.

High-Efficiency Red Electroluminescence Based on a Carbene-Cu(I)-Acridine Complex

How to develop efficient red-emitting organometallics of earth-abundant copper(I) is a formidable challenge in the field of organic light-emitting diodes (OLEDs) because Cu(I) complexes have weak spin-orbit coupling and a serious excited-state reorganization effect. Here, a red Cu(I) complex, MAC*-Cu-DPAC, was developed using a rigid 9,9-diphenyl-9,10-dihydroacridine donor ligand in a carbene-metal-amide motif. The Cu(I) complex achieved satisfactory red emission, a high photoluminescence quantum yield of up to 70%, and a sub-microsecond lifetime. Thanks to a linear geometry and the acceptor and donor ligands in a coplanar conformation, the complex exhibited a high horizontal dipole ratio of 77% in the host matrix, first demonstrated for coinage metal(I) complexes. The resulting OLEDs delivered high external quantum efficiencies of 21.1% at a maximum and 20.1% at 1000 nits, together with a red emission peak at ∼630 nm. These values represent the state-of-the-art performance for red-emitting OLEDs based on coinage metal complexes.

Triazolotriazine-based thermally activated delayed fluorescence materials for highly efficient fluorescent organic light-emitting diodes (TSF-OLEDs)

Thermally activated delayed fluorescence (TADF) sensitized fluorescent organic light-emitting diodes (TSF-OLEDs) have shown great potential for the realization of high efficiency with low efficiency roll-off and good color purity. However, the superior examples of TSF-OLEDs are still limited up to now. Herein, a trade-off strategy is presented for designing efficient TADF materials and achieving high-performance TSF-OLEDs via the construction of a new type of triazolotriazine (TAZTRZ) acceptor. The enhanced electron-withdrawing ability of TAZTRZ acceptor, fused by triazine (TRZ) and triazole (TAZ) together, enables TADF luminogens with small singlet-triplet energy gap (ΔE_ST) values. Meanwhile, the increased planarity from the TRZ-phenyl linkage (6:6 system) to the TAZ-phenyl linkage (5:6 system) can compensate the decrease of oscillator strength (f) while lowing ΔE_ST, thus achieving a trade-off between small ΔE_ST and high f. As a result, the related TSF-OLED achieved an extremely low turn-on voltage of 2.1 V, an outstanding maximum external quantum efficiency (EQE_max) of 23.7% with small efficiency roll-off (EQE₁₀₀₀ of 23.2%; EQE₅₀₀₀ of 20.6%) and an impressively high maximum power efficiency of 82.1 lm W^-1, which represents the state-of-the-art performance for yellow TSF-OLEDs.

Enhanced Upconversion of Triplet Excitons for Conjugated Polymeric Thermally Activated Delayed Fluorescence Emitters by Employing an Intramolecular Sensitization Strategy

Endowed by a thermally activated delayed fluorescence (TADF) sensitizer with a high constant rate of reverse intersystem crossing, the singlet excitons could be accumulated and then delivered to emitting states through favorable Förster resonance energy transfer, bypassing the inefficient intersystem transition processes of emitters. However, the conventional intermolecular sensitization strategies suffer from inherent aggregation-induced quenching and inevitable phase segregation of TADF sensitizers and emitters. In this context, we proposed a novel intramolecular sensitization strategy by covalently incorporating the TADF sensitizer into conjugated polymeric emitters. After rationally regulating the proportions of sensitizer and emitter units in polymers, the intramolecular sensitized conjugated TADF polymers with anticipated photophysical properties and stable device performance were obtained. A superior k_RISC value over 10⁶ s^-1 accompanied by a suppressed nonradiative transition of the triplet exciton could be gained; therefore, the photoluminescence quantum yield (PLQY) could reach nearly 90%. In accord with the superior PLQY values enhanced by our intramolecular sensitization strategy, the solution-processed organic light-emitting diodes (OLEDs) can achieve a maximum external quantum efficiency (EQE) value of 17.8% while still maintaining 16.0% at 1000 cd/m² with extremely low efficiency roll-off. These results convincingly manifest the significance of an intramolecular sensitization strategy for designing high-efficiency polymeric TADF emitters.

Harvesting triplet excitons for near-infrared electroluminescence via thermally activated delayed fluorescence channel

Near-infrared (NIR) emission is useful for numerous practical applications, such as communication, biomedical sensors, night vision, etc., which encourages researchers to develop materials and devices for the realization of efficient NIR organic light-emitting devices. Recently, the emerging organic thermally activated delayed fluorescence (TADF) emitters have attracted wide attention because of the full utilization of electron-generated excitons, which is crucial for achieving high device efficiency. Up to now, the TADF emitters have shown their potential in the deep red/NIR region. Considering the color purity and efficiency, however, the development of NIR TADF emitters still lags behind RGB TADF emitters, indicating that there is still much room to improve their performance. In this regard, this perspective mainly summarizes the past progress of molecular design on constructing TADF NIR emitters. We hope this perspective could provide a new vista in developing NIR materials and enlighten breakthroughs in both fundamental research and applications.

Face-to-Face Orientation of Quasiplanar Donor and Acceptor Enables Highly Efficient Intramolecular Exciplex Fluorescence

Intramolecular through-space charge-transfer (TSCT) excited states have been exploited for developing thermally activated delayed fluorescence (TADF) emitters, but the tuning of excited state dynamics by conformational engineering remains sparse. Designed here is a series of TSCT emitters with precisely controlled alignment of the donor and acceptor segments. With increasing intramolecular π-π interactions, the radiative decay rate of the lowest singlet excited state (S₁ ) progressively increased together with a suppression of nonradiative decay, leading to significantly enhanced photoluminescence quantum yields of up to 0.99 in doped thin films. A high-efficiency electroluminescence device, with a maximum external quantum efficiency (EQE) of 23.96 %, was achieved and maintains >20 % at a brightness of 1000 cd m^-2 . This work sheds light on the importance of conformation control for achieving high-efficiency intramolecular exciplex emitters.

Spiral Donor Design Strategy for Blue Thermally Activated Delayed Fluorescence Emitters

Thermally activated delayed fluorescence (TADF) emitters with a spiral donor show tremendous potential toward high-level efficient blue organic light-emitting diodes (OLEDs). However, the underlying design strategy of the spiral donor used for blue TADF emitters remains unclear. As a consequence, researchers often do "try and error" work in the development of new functional spiral donor fragments, making it slow and inefficient. Herein, we demonstrate that the energy level relationships between the spiral donor and the luminophore lead to a significant effect on the photoluminescent quantum yields (PLQYs) of the target materials. In addition, a method involving quantum chemistry simulations that can accurately predict the aforementioned energy level relationships by simulating the spin density distributions of the triplet excited states of the spiral donor and corresponding TADF emitters and the triplet excited natural transition orbitals of the TADF emitters is established. Moreover, it also revealed that the steric hindrance in this series of molecules can form a nearly unchanged singlet (S₁) state geometry, leading to a reduced nonradiative decay and high PLQY, while a moderated donor-acceptor (D-A) torsion in the triplet (T₁) state can induce a strong vibronic coupling between the charge-transfer triplet (³CT) state and the local triplet (³LE) state, achieving an effective reverse intersystem crossing (RISC) process. Furthermore, an electric-magnetic coupling is formed between the high-lying ³LE state and the charge-transfer singlet (¹CT) state, which may open another RISC channel. Remarkably, in company with the optimized molecular structure and energy alignment, the pivotal TADF emitter DspiroS-TRZ achieved 99.9% PLQY, an external quantum efficiency (EQE) of 38.4%, which is the highest among all blue TADF emitters reported to date.

TD-DFT and Experimental Methods for Unraveling the Energy Distribution of Charge-Transfer Triplet/Singlet States of a TADF Molecule in a Frozen Matrix

Reverse intersystem crossing (RISC) rate of a thermally activated delayed fluorescence (TADF) molecule is sensitive to the energy alignment of the singlet charge-transfer state (¹CT), triplet charge-transfer state (³CT), and locally excited triplet state (³LE). However, the energy distribution of the charge-transfer states originating from the conformational distribution of TADF molecules in a solid matrix inevitably generated during the preparation of a solid sample due to the rotatable donor-acceptor linkage is rarely considered. Moreover, the investigation of the energy distribution of the ³CT state is both theoretically and experimentally difficult due to the triplet instabilities of time-dependent density functional (TD-DFT) calculations and difficulties in phosphorescence measurements, respectively. As a result, the relationships between conformational distribution, configurations of excited state transition orbitals, and excited state energies/dynamics have not been clearly explained. In this work, we determined the energy distribution of CT states of the TADF emitter TPSA in frozen toluene at 77 K by the measurement of time-resolved spectra in the full time range (1 ns to 30 s) of emission including prompt fluorescence, TADF, ³CT phosphorescence, and ³LE phosphorescence. We obtained the energy band of CT states where ¹CT and ³CT states are distributed in the range of 2.85-3.00 and 2.64-2.96 eV, respectively. We tested various global hybrid and long-range corrected functionals for the TD-DFT calculation of ³CT energy of TPSA and found that only the M11 functional shows consistent results without triplet instability. We performed TD-DFT with the M11* functional optimized for a robust dihedral angle scan of ³CT states without triplet instability and reproduced the energy band structure obtained from the experiment. Through TD-DFT and experimental investigations, it is estimated that the dihedral angles of donor-acceptor (θ_D-A) and acceptor-linker (θ_A) of TPSA in frozen toluene lie within the range 70° ≤ θ_D-A ≤ 90° and 0° ≤ θ_A ≤ 30° respectively. Our results show that the dihedral angle distribution must be considered for further investigation of the photophysics of TADF molecules and the development of stable and efficient TADF emitters.

Highly Efficient Orange-Red Thermally Activated Delayed Fluorescence Compounds Comprising Dual Dicyano-Substituted Pyrazine/Quinoxaline Acceptors

The π-conjugation of molecules has a large influence on their excited state properties, especially for red thermally activated delayed fluorescence (TADF) materials. Two orange-red TADF compounds comprising dual dicyano-substituted pyrazine/quinoxaline acceptors have been designed and synthesized. TPA-2DCNQ (3,3'-((phenylazanediyl)bis(4,1-phenylene))bis(2-phenylquinoxaline-6,7-dicarbonitrile) with extended π-conjugated quinoxaline as the acceptor exhibits higher photoluminescence quantum yields (ca. 0.67-0.71) in doped films. A smaller energy splitting (ΔE_st ) between the first singlet excited state and triplet excited state is also achieved, indicating that extending the π-conjugation of the acceptor rationally is an effective approach to designing highly efficient long-wavelength TADF materials. Devices with TPA-2DCNQ as the emitter display maximum external quantum efficiencies (EQEs) of 12.6-14.0 %, which are more than twice those of devices containing TPA-2DCNPZ (6,6'-((phenylazanediyl)bis(4,1-phenylene))bis(5-phenylpyrazine-2,3-dicarbonitrile).

The inspiration and challenge for through-space charge transfer architecture: from thermally activated delayed fluorescence to non-linear optical properties

Organic molecules consisting of electron donor (D) and electron acceptor (A) subunits linked by π-conjugated bridges are promising building blocks for thermally activated delayed fluorescence (TADF) and non-linear optics (NLO) materials due to their intramolecular charge transfer (CT) processes in response to external stimuli. According to the electron interaction pattern, the CT process in D-π-A architectures can be divided into two categories, through-bond/-space charge transfer (TB/TSCT). To date, research into the TADF properties of TSCT characteristic molecules has since seen significant growth. In fact, TSCT characteristic materials show great advantages in such NLO responses. In this perspective, we first briefly introduced the basic principles of NLO and TADF effects. Successively, we discuss the influence of TBCT and TSCT patterns on NLO and TADF properties, especially for TSCT characteristic. In the final part, we address the diversity and potential advantages of TSCT characteristic molecules as high-performance NLO materials. With these, it is expected that the greater structural flexibility of spatial conjugation can bring more functionality to NLO materials in the future.

Excited-state symmetry breaking in 9,10-dicyanoanthracene-based quadrupolar molecules: the effect of donor-acceptor branch length

Excited-state symmetry breaking is investigated in a series of symmetric 9,10-dicyanoanthracenes linked to electron-donating groups on the 2 and 6 positions via different spacers, allowing for a tuning of the length of the donor–acceptor branches. The excited-state properties of these compounds are compared with their dipolar single-branch analogues. The changes in electronic structure upon their optical excitation are monitored by transient electronic spectroscopy in the visible and near-infrared regions as well as by transient vibrational spectroscopy in the mid-infrared. Our results reveal that, with the shortest branches, electronic excitation remains distributed almost symmetrically over the molecule even in polar environments. Upon increasing the donor–acceptor distance, excitation becomes unevenly distributed and, with the longest one, it fully localises on one branch in polar solvents. The influence of the branch length on the propensity of quadrupolar dyes to undergo excited-state symmetry breaking is rationalised in terms of the balance between interbranch coupling and solvation energy.

Excited-state symmetry breaking in quadrupolar molecules depends on the balance between inter-branch coupling and polar solvation energy.

Proton-controlled non-exponential photoluminescence in a pyridylamidine-substituted Re(I) complex

Chemical intuition and well-known design principles can typically be used to create ligand environments in transition metal complexes to deliberately tune reactivity for desired applications. However, intelligent ligand design does not always result in the expected outcomes. Herein we report the synthesis and characterization of a tricarbonyl rhenium (2,2'-bipyridine) 4-pyridylamidine, Re(4-Pam), complex with unexpected photophysical properties. Photoluminescence kinetics of Re(4-Pam) undergoes non-exponential decay, which can be deconvolved into two emission lifetimes. However, upon protonation of the amidine functionality of the 4-pyridylamidine to form Re(4-PamH), a single exponential decay is observed. To understand and rationalize these experimental observations, density functional theory (DFT) and time-dependent density functional theory (TDDFT) are employed. The symmetry or asymmetry of the protonated or deprotonated 4-pyridylamidine ligand, respectively, is the key factor in switching between one and two photoluminescence lifetimes. Specifically, rotation of the dihedral angle formed between the bipyridine and 4-Pam ligand leads to two rotamers of Re(4-Pam) with degenerate triplet- to ground-state transitions.

Ni-catalyzed cascade coupling reactions: synthesis and thermally-activated delayed fluorescence characterization of quinazolinone derivatives

A nickel-catalyzed cascade coupling of 2-(2-(arylcarbonyl)-4-oxoquinazolin-3(4H)-yl)acetonitrile and arylboronic acid for the synthesis of pyrazino-fused quinazolinones has been developed. The TADF effect of 3a in the solid-state was investigated.

Synthesis of novel π-extended D–A–D-type dipyrido[3,2-a:2′,3′-c]phenazine derivatives and their photosensitized singlet oxygen generation

Electron donor–acceptor–donor (D–A–D) π-conjugated molecules based on dipyrido[3,2-a:2′,3′-c]phenazine (dppz) were developed as photosensitizers for singlet oxygen generation.

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

Organic compounds that show Thermally Activated Delayed Fluorescence (TADF) have become wildly popular as next-generation emitters in organic light emitting diodes (OLEDs). Since 2016, a subset of these have found increasing use as photocatalysts. This review comprehensively highlights their potential by documenting the diversity of the reactions where an organic TADF photocatalyst can be used in lieu of a noble metal complex photocatalyst. Beyond the small number of TADF photocatalysts that have been used to date, the analysis conducted within this review reveals the wider potential of organic donor-acceptor TADF compounds as photocatalysts. A discussion of the benefits of compounds showing TADF for photocatalysis is presented, which paints a picture of a very promising future for organic photocatalyst development.

B(C6F5)3-catalyzed three-component tandem reaction to construct novel polycyclic quinone derivatives: synthesis of a carbonate salt chromogenic chemosensor

A series novel polycyclic quinone derivatives were constructed providing a carbonate salt chromogenic chemosensor.

Synthesis and photophysical properties of donor-substituted phenyl-phosphachromones as potential TADF materials

A series of novel arylamine-substituted phenyl-phosphachromones were constructed via post-functionalization.

Charge transfer induced by electronic state mixing in a symmetric X-Y-X-type multi-chromophore system

Charge transfer (CT) from electron donor (D) to acceptor (A) plays an important role in photoelectric or electrochemical devices and is a useful concept for a molecule with D and A well distinguishable. Here, we report our finding that even in a molecule with D and A not resolvable, CT can be induced by electronic state mixing (ESM) in a symmetric multi-chromophore system (MCS), namely 1,4-di(1-pyrenyl)benzene (Py-Benz-Py). Unlike Py and Py-Benz, Py-Benz-Py exhibits unique photophysical properties attributable to the reduction of the energy gap between two electronic states induced by ESM. The ESM for Py-Benz-Py is due to the extended π-conjugation owing to the further introduction of Py into Py-Benz, and consequently leads to the favorable intramolecular CT, followed by the planarization due to the twisting motion between Py and phenyl moieties. Time-resolved spectroscopic data demonstrate that the twisting process of the Py moiety in acetonitrile occurs with two unequal time constants, suggesting the localized CT state and the asynchronous twisting dynamics of two Py moieties unlike the delocalized CT state in nonpolar and low-polarity solvents leading to the synchronous twisting of two Py moieties. This means that the symmetry-breaking CT in MCSs can induce an asynchronous twisting motion. The results reported here support that a molecule without CT can be turned into another molecule with CT induced by ESM and demonstrate that the excited-state relaxation dynamics can be regulated through the ESM induced by introducing the substituents or changing the environmental factors such as solvent polarities.

Thermally Activated Delayed Fluorescence from d10 -Metal Carbene Complexes through Intermolecular Charge Transfer and Multicolor Emission with a Monomer-Dimer Equilibrium

A series of two-coordinate AuI and CuI complexes (3 a, 3 b and 5 a, 5 b) are reported as new organometallic thermally activated delayed fluorescence (TADF) emitters, which are based on the carbene-metal-carbazole model with a pyridine-fused 1,2,3-triazolylidene (PyTz) ligand. PyTz features low steric hindrance and a low-energy LUMO (LUMO=-1.47 eV) located over the π* orbitals of the whole ligand, which facilitates intermolecular charge transfer between a donor (carbazole) and an accepter (PyTz). These compounds exhibit efficient TADF with microsecond lifetimes. Temperature-dependent photoluminescence kinetics of 3 a supports a rather small energy gap between S1 and T1 (ΔES 1 - T 1 =60 meV). Further experiments reveal that there are dual-emission properties from a monomer-dimer equilibrium in solution, exhibiting single-component multicolor emission from blue to orange, including white-light emission.

Doubly Boron-Doped TADF Emitters Decorated with ortho-Donor Groups for Highly Efficient Green to Red OLEDs

Doubly boron-doped thermally activated delayed fluorescence (TADF) emitters based on a 9,10-diboraanthracene (DBA) acceptor decorated with ortho-donor groups (Cz2oDBA, 2; BuCz2oDBA, 3; DMAC2oDBA, 4) are prepared to realize high-efficiency green-to-red organic light-emitting diodes (OLEDs). X-ray diffraction analyses of 2 and 4 reveal the symmetrical and highly twisted ortho-donor-acceptor-donor (D-A-D) structure of the emitters. The twisted conformation leads to a very small energy splitting (ΔE_ST <0.08 eV) between the excited singlet and triplet states that gives rise to strong TADF, as supported by theoretical studies. Depending on the strength of the donor moieties, the emission color is fine-tuned in the visible region from green (2) to yellow (3) to red (4). Carbazole-containing 2 and 3 exhibit high photoluminescence quantum yields (PLQYs) approaching 100 %, whereas DMAC-substituted 4 is moderately emissive (PLQY=44 %) in a doped host film. Highly efficient green-to-red TADF-OLEDs are realized with the proposed ortho-D-A-D compounds as emitters. The green and yellow OLEDs incorporating Cz2oDBA (2) and BuCz2oDBA (3) emitters exhibit high external quantum efficiencies (EQEs) of 26.6 % and 21.6 %, respectively. In particular, the green device shows an excellent power efficiency above 100 lm W^-1 . A red OLED fabricated with a DMAC2oDBA (4) emitter exhibits a maximum EQE of 10.1 % with an electroluminescence peak at 615 nm.

Photoluminescent Behaviors of Thermally Activated Delayed Fluorescence Polymeric Emitters in Nanofibers

Anisotropic 1D nanostructures with high surface-area-to-volume ratio display the enhanced optoelectronic properties of light-emitting compounds compared to bulk or 2D systems. To study the effect of nanometer-constrained space on photoluminescent behavior of thermally activated delayed fluorescence (TADF) polymeric emitters, electrospinning technique is used to produce nanofibers of TADF emitters. Herein, two TADF polymer (P1 and P3) nanofibers with 90% polyacrylonitrile (PAN) are fabricated and their photophysical properties are studied and compared with their spin-coated film counterparts. The distinguishing polarized photoluminescencent properties of P1/PAN or P3/PAN electrospun nanofibers are obtained due to high orientation degree and superior molecular arrangement. Moreover, the better TADF properties in nanofibers can be observed comparing with their spin-coated films, including longer-lived excited states, higher photoluminescence quantum efficiency, lower internal conversion decay rate, and higher reverse intersystem crossing rate constant.

Solid-state effect on luminescent properties of thermally activated delayed fluorescence molecule with aggregation induced emission: A theoretical perspective

Solid-state effect plays an important role in defining the nature of excited states for thermally activated delayed fluorescence (TADF) molecules and further affects their luminescence properties. Theoretical investigation of photophysical properties with explicit consideration of intermolecular interactions in solid phase, is highly desired. In this work, the luminescent properties of new TADF molecule SBF-BP-DMAC with aggregation induced emission (AIE) feature are theoretically studied both in solution and solid phase. Solvent environment effect in Tetrahydrofuran (THF) is simulated by polarizable continuum model (PCM) and solid-state effect is considered by the combined quantum mechanics and molecular mechanics (QM/MM) method. By combing thermal vibration correlation function (TVCF) theory with first principles calculation, excited state energy consumption process is investigated. Our results show that the calculated prompt fluorescence efficiency, delayed fluorescence efficiency and total fluorescence efficiency in THF is 3.0%, 0.4‰ and 3.0% respectively, and corresponding value increases to 14.4%, 31.5% and 45.9% for molecule in solid phase, this verifies the AIE feature. To detect the inner mechanisms, the geometrical structures, Huang-Rhys (HR) factors and reorganization energies as well as excited state transition properties are analyzed. Decreased HR factor and reorganization energy are found in solid phase, this is caused by the restricted torsion motion of DMAC unit in rigid environment. Thus, non-radiative energy consumption process is suppressed and enhanced fluorescence efficiency is found in the solid phase. Moreover, the smaller energy gap between S₁ and T₁ in the solid state than that in THF, is more conducive for reverse intersystem crossing process and further improves the efficiency. This work provides reasonable explanation for the experimental measurements and reveals the inner perspectives for AIE and TADF mechanisms, which is advantageous to develop new non-doped OLEDs with advanced feature.

Twisted Intramolecular Charge Transfer State of a "Push-Pull" Emitter

The excited state Raman spectra of 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) in the locally-excited (LE) and the intramolecular charge transfer (ICT) states have been separately measured by time-resolved stimulated Raman spectroscopy. In a polar dimethylsulfoxide solution, the ultrafast ICT of DCM with a time constant of 1.0 ps was observed in addition to the vibrational relaxation in the ICT state of 4–7 ps. On the other hand, the energy of the ICT state of DCM becomes higher than that of the LE state in a less polar chloroform solution, where the initially-photoexcited ICT state with the LE state shows the ultrafast internal conversion to the LE state with a time constant of 300 fs. The excited-state Raman spectra of the LE and ICT state of DCM showed several major vibrational modes of DCM in the LE and ICT conformer states coexisting in the excited state. Comparing to the time-dependent density functional theory simulations and the experimental results of similar push-pull type molecules, a twisted geometry of the dimethylamino group is suggested for the structure of DCM in the S₁/ICT state.

Tumor-Activated and Metal-Organic Framework Assisted Self-Assembly of Organic Photosensitizers

Tumor accumulation and intratumoral singlet oxygen (¹O₂) generation efficiency of photosensitizers (PSs) are two essential factors that determine their photodynamic therapy (PDT) efficacies. How to maximize the PS performance at the tumor site is of great research interest. Herein, we report a metal-organic framework (ZIF-8, ZIF = zeolitic imidazolate framework) assisted in vivo self-assembly nanoplatform, ZIF-8-PMMA-S-S-mPEG, as an effective tool for organic PS payloads to achieve efficient PDT. Using an organic PS with aggregation-induced emission as an example, under intratumoral bioreduction, PS-loaded ZIF-8-PMMA-S-S-mPEG (PS@ZIF-8-PMMA-S-S-mPEG) was self-assembled into large ordered hydrophobic clusters, which greatly enhance tumor retention and accumulation of the PS. Moreover, hydrophobic ZIF-8 assemblies greatly isolate the loaded PSs from water and improve O₂ transport for the PSs to effectively produce ¹O₂ inside tumors under light irradiation. The organic PS is therefore endowed with optimal tumor accumulation and intratumoral ¹O₂ production, demonstrating the effectiveness of the developed self-assembly strategy in PDT application.

Solution-Processable Pure Green Thermally Activated Delayed Fluorescence Emitter Based on the Multiple Resonance Effect

Thermally activated delayed fluorescence (TADF) materials based on the multiple resonance (MR) effect are applied in organic light-emitting diodes (OLEDs), combining high color purity and efficiency. However, they are not fabricated via solution-processing, which is an economical approach toward the mass production of OLED displays. Here, a solution-processable MR-TADF material (OAB-ABP-1), with an extended π-skeleton and bulky substituents, is designed. OAB-ABP-1 is synthesized from commercially available starting materials via a four-step process involving one-shot double borylation. OAB-ABP-1 presents attractive photophysical properties, a narrow emission band, a high photoluminescence quantum yield, a small energy gap between S₁ and T₁ , and low activation energy for reverse intersystem crossing. These properties are attributed to the alternating localization of the highest occupied and lowest unoccupied molecular orbitals induced by the boron, nitrogen, and oxygen atoms. Furthermore, to facilitate charge recombination, two novel semiconducting polymers with similar ionization potentials to that of OAB-ABP-1 are synthesized for use as interlayer and emissive layer materials. A solution-processed OLED device is fabricated using OAB-ABP-1 and the aforementioned polymers; it exhibits pure green electroluminescence with a small full-width at half-maximum and a high external quantum efficiency with minimum efficiency roll-off.

Improving Brightness and Stability of Si-Rhodamine for Super-Resolution Imaging of Mitochondria in Living Cells

Photostable and bright organic dyes emitting in the near-infrared region are highly desirable for long-term dynamic bioimaging. Herein, we report a synthetic approach to build novel methoxy modified Si-rhodamine (SiRMO) dyes by introducing the methoxybenzene on the xanthene moiety. The brightness of SiRMO increased from 2300 M^-1 cm^-1 (SiRMO-0) to 49000 M^-1 cm^-1 (SiRMO-2) when the substituent 2,5-dimethoxybenzene was replaced with 2,6-dimethoxybenzene. Moreover, the stability of SiRMO-2 was significantly improved due to the steric hindrance protection of the two methoxy groups on the ninth carbon atom of the xanthene. After fast cellular uptake, the SiRMO dyes selectively stained the mitochondria with a low background in live cultured cells and primary neurons. The high brightness and stability of SiRMO-2 significantly improved the capability of monitoring mitochondria dynamic processes in living cells under super-resolution conditions. Moreover, with the fluorescence nanoscopy techniques, we observed the structure of mitochondrial cristae and mitochondria fission, fusion, and apoptosis with a high temporal resolution. Under two-photon illumination, SiRMO-2 showed also enhanced two-photon brightness and stability, which are important for imaging in thick tissue.