Efficient Thermally Activated Delayed Fluorescence Research Articles

The photophysical properties of sulfone-based TADF emitters in relation to their structural properties.

In this work, thermally activated delayed fluorescence (TADF) of a series of emitters with sulfone-based acceptor moieties was studied by density functional theory (DFT) methods. Sulfone derivatives were shown to be high performing TADF emitters over recent years. When discussing the TADF efficiency, various properties, such as the singlet-triplet energy gap (ΔEST), spin-orbit coupling (SOC) and the nature of states, stand out due to their roles in reverse intersystem crossing (RISC). Here, we mainly focused on three important structural parameters that affect the intersystem crossing (ISC) and RISC pathways and their efficiencies. These three parameters are: (1) the effect of meta- and para-conjugation, (2) the effect of rigid acceptor moieties and (3) the effect of the phenyl bridge on photophysical properties.

A facile method to achieve red thermally activated delayed fluorescence emitters with EQE over 30% via molecular aspect ratio engineering

Bandgap narrowing via the incorporation of strong donor and/or acceptor units into the molecular structure is a well-known strategy for the design of the long-wavelength fluorophores. Due to the energy-gap law, achieving high emission quantum yield at long wavelength becomes a paradox. Therefore, the red fluorescence emitters with external quantum efficiency (EQE) over 30 % have rarely been reported in comparison with blue and green fluorophores. In this work, a series of the thermally activated delayed fluorescence (TADF) materials (PH-TPA, BZ-TPA and PO-TPA) featured with rigid skeleton demonstrated low non-radiative decay rates and high photoluminescent quantum efficiency. Importantly, the engineering of molecular aspect ratio of the TADF emitters led to high horizontal emitting dipole ratio, thus enhancing the device performance. As a result, organic light-emitting diodes based on PH-TPA, BZ-TPA and PO-TPA showed record-high EQEs of 33.0 % at 624 nm, 31.3 % at 634 nm and 34.9 % at 632 nm, respectively. This work provided a facile and yet effective method for the construction of efficient red TADFs via the integration of rigid emission core and the control of molecular aspect ratio.

Effects of Solvent Dielectric on Thermally Activated Delayed Fluorescence: A Predictive Computational Polarization Consistent Approach

We study computationally thermally activated delayed fluorescence (TADF) in donor-acceptor compounds. The relevant electronic excited states that are strongly affected by the dielectric environment are treated by a polarization consistent framework. The high fidelity potential energy surfaces are used following a quantum-mechanical Fermi's golden rule (FGR) picture to calculate rates of intersystem crossing (ISC) and reverse intersystem crossing (RISC). To demonstrate the potency of the approach, we consider isomers of benzonitrile functionalized tert-butyl-substituted dimethylacridine (DMAC-BN), which were recently found to perform well as TADF emitters. The calculated excited state energies that appear to reproduce well measured spectral trends with respect to the dielectric constant are used to parametrize ISC/RISC FGR rates. The calculated rates reproduce well measured rates, whereas semiclassical based rates are grossly underestimated. In particular, we find in agreement with the recent experimental study [Phys. Rev. Appl.2019, 12, 044021] that the ortho and meta isomers are significantly more effective as TADF emitters. The computational framework provides valuable insight at the molecular level into RISC rates and therefore can contribute to the design of materials of increased TADF efficiency.

Fine‐Tuning the Photophysics of Donor‐Acceptor (D‐A3) Thermally Activated Delayed Fluorescence Emitters Using Isomerisation

AbstractHere two D–A3 regioisomers, comprising three dibenzothiophene‐S,S‐dioxide acceptor units attached to a central triazatruxene core, are studied. Both molecules show thermally activated delayed fluorescence (TADF), however, the efficiency of the TADF mechanism is strongly affected by the D–A substitution position. The meta‐ substituted emitter (1 b) shows a slightly higher‐lying singlet charge transfer state and a lower‐lying triplet state than that observed in the para‐ substituted emitter (1 a), resulting in a larger singlet–triplet splitting (ΔEST) of 0.28 eV compared to only 0.01 eV found in 1 a. As expected, this ΔEST difference strongly impacts the reverse intersystem crossing (rISC) rates and the para‐ isomer 1 a exhibits a much faster delayed fluorescence emission. Calculations show that the triplet energy difference between the two isomers is due to steric hindrance variances along the donor–acceptor rotation axis in these molecules: as 1 b is less restricted, rotation of its acceptor unit leads to a lower T1 energy, further away from the region of high density of states (the region where larger vibronic coupling is found, favouring rISC). Therefore, our results show how the substitution pattern has a marked effect on triplet state energies and character, verifying the key structural designs for highly efficient TADF materials.

Tailoring Donor‐Acceptor Emitters to Minimise Localisation Induced Quenching of Thermally Activated Delayed Fluorescence

AbstractBy inverting the common structural motif of thermally activated delayed fluorescence (TADF) materials to a rigid donor core and multiple peripheral acceptors, it has been shown possible to achieve both a reverse intersystem crossing (rISC) rate 1×107 s−1 and a unity photoluminescence quantum yield [Dos Santos et al. Adv. Sci. 5 (2018) 1700989]. However, the rISC rate in this motif is quenched by localisation of the excited state electronic structure which causes in‐equivalence between the peripheral acceptors. In this paper, we explore a series of related molecular targets which seek to reduce the effect of localisation on the rISC rates. This includes structures that contain donors exhibiting three‐fold or four‐fold symmetry and different degrees of steric hindrance around the donor‐acceptor bond with the objective of using steric hindrance to exert finer conformational control of the excited state dynamics to enhance functional properties. We demonstrate that a triazatruxene central donor is most effective for TADF owing to the energetic position of the locally excited state, while cyano‐benzonitriles are the most effective acceptors for reducing the effect of localisation on the electronic structure. These also push the emission into the blue region of the spectrum, opening the possibilities for this to be a pathway to develop efficient blue TADF emitters.

Modified Intramolecular‐Lock Strategy Enables Efficient Thermally Activated Delayed Fluorescence Emitters for Non‐Doped OLEDs

The development of intramolecular-lock strategy is an appealing task for designing efficient thermally activated delayed fluorescence (TADF) molecules, but only limited examples have been reported so far. Herein we present a "medium ring"-lock strategy to develop TADF emitters for improving the efficiency of organic light-emitting diodes (OLEDs). The installation of an electron-deficient heptagonal diimide lock onto a highly rotatable biphenyl-based emitter not only enhances electron-withdrawing ability of acceptor that decreases singlet-triplet energy gap (ΔEST ), but also endows the skeleton with modest rigidity and flexibility that increases photoluminescence quantum yield (PLQY) in neat film. In particular, the integration of the diimide lock also leads to an increase in horizontal orientation factor (Θ// ) from 69 % to 83 %. Consequently, this modified intramolecular-lock strategy enables an efficient TADF emitter to assemble high-performance non-doped OLEDs with a high external quantum efficiency of 26.2 % and a power efficiency of 76.6 lm W-1 .

Modified Intramolecular‐Lock Strategy Enables Efficient Thermally Activated Delayed Fluorescence Emitters for Non‐Doped OLEDs

AbstractThe development of intramolecular‐lock strategy is an appealing task for designing efficient thermally activated delayed fluorescence (TADF) molecules, but only limited examples have been reported so far. Herein we present a “medium ring”‐lock strategy to develop TADF emitters for improving the efficiency of organic light‐emitting diodes (OLEDs). The installation of an electron‐deficient heptagonal diimide lock onto a highly rotatable biphenyl‐based emitter not only enhances electron‐withdrawing ability of acceptor that decreases singlet‐triplet energy gap (ΔEST), but also endows the skeleton with modest rigidity and flexibility that increases photoluminescence quantum yield (PLQY) in neat film. In particular, the integration of the diimide lock also leads to an increase in horizontal orientation factor (Θ//) from 69 % to 83 %. Consequently, this modified intramolecular‐lock strategy enables an efficient TADF emitter to assemble high‐performance non‐doped OLEDs with a high external quantum efficiency of 26.2 % and a power efficiency of 76.6 lm W−1.

Efficient TADF from carbon−carbon bonded donor−acceptor molecules based on boron-carbonyl hybrid acceptor

We present a new type of thermally activated delayed fluorescence (TADF) emitters, namely, PCzBCO and BuPCzBCO, composed of the donor (D) and acceptor (A) units linked by a C–C bond. The two emitters have N-aryl carbazole as a donor and a planarized boron-carbonyl (BCO) unit as an acceptor. The crystal structure of BuPCzBCO reveals a moderate twist between the D and A units with a dihedral angle of 49.2°. Although both emitters exhibit the prompt fluorescence only in solution, they exhibit distinct TADF characteristics in the rigid state, with the high photoluminescence quantum yields of 81% and 93% in an mCBP host, respectively. Importantly, fast reverse intersystem crossing (RISC) with the rate constants of ∼106 s−1 is obtained in their host films. Along with the small singlet−triplet energy gap below 50 meV, theoretical studies demonstrate that RISC is facilitated by the strong spin-orbit coupling between the charge-transfer singlet excited state (1CT, S1) and the BCO-centered, local triplet excited state (3LE, T2) with a 3nπ* character. With the PCzBCO and BuPCzBCO molecules as emitters, efficient greenish blue TADF-OLEDs with similar maximum external quantum efficiency of 16.2% are realized. These findings highlight the potential of C–C bonded D−A molecules as efficient TADF emitters.

Vibration‐Regulated Multi‐State Long‐Lived Emission from Star‐Shaped Molecules

AbstractHow to utilize molecular vibration to tune triplet‐involved emissions in multiple states is highly challenging. Here, star‐shaped triphenylamine derivatives have been employed as model systems to understand how molecular vibration affects thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) emissions in multiple states. Nonplanar, star‐shaped conformations allow molecules to generate appropriate vibrations in the solution state, facilitating vibronic coupling between their T1and T2states to generate effective TADF. More importantly, a relatively dispersed state can allow the molecules to efficiently vibrate in the solid state, and a crystalline environment further promotes a more efficient TADF. Lastly, by suppressing molecular vibration to inhibit the TADF, ultra‐long RTP was observed upon doping these molecules into polymers. These molecules can be used in information encryption and storage as well as bioimaging.

Vibration-Regulated Multi-State Long-Lived Emission from Star-Shaped Molecules.

How to utilize molecular vibration to tune triplet-involved emissions in multiple states is highly challenging. Here, star-shaped triphenylamine derivatives have been employed as model systems to understand how molecular vibration affects thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) emissions in multiple states. Nonplanar, star-shaped conformations allow molecules to generate appropriate vibrations in the solution state, facilitating vibronic coupling between their T1 and T2 states to generate effective TADF. More importantly, a relatively dispersed state can allow the molecules to efficiently vibrate in the solid state, and a crystalline environment further promotes a more efficient TADF. Lastly, by suppressing molecular vibration to inhibit the TADF, ultra-long RTP was observed upon doping these molecules into polymers. These molecules can be used in information encryption and storage as well as bioimaging.

Rational Molecular Design Enables Efficient Blue TADF−OLEDs with Flexible Graphene Substrate

AbstractObservation of thermally activated delayed fluorescence (TADF) in conjugated systems redefined the molecular design approach to realize highly efficient organic light emitting diodes (OLEDs) in the early 2010s. Enabling effective reverse intersystem crossing (RISC) by minimizing the difference between singlet and triplet excited state energies (ΔEST) is proven to be a widely applicable and fruitful approach, which results in remarkable external quantum efficiencies (EQE). The efficacy of RISC in these systems is mainly dictated by the first‐order mixing coefficient (λ), which is proportional to spin‐orbit coupling (HSO) and inversely proportional to ΔEST. While minimizing ΔEST has been the focus of the OLED community over the last decade, the effect of HSO in these systems is largely overlooked. Here, molecular systems with increased HSO are designed and synthesized by substituting selected heteroatoms of high‐performance TADF materials with heavy‐atom selenium. A new series of multicolor TADF materials with remarkable EQEs are achieved. One of these materials, SeDF‐B, results in pure blue emission with EQEs approaching 20%. Additionally, flexible graphene‐based electrodes are developed for OLEDs and revealed to have similar performance as standard indium tin oxide (ITO) in most cases. These devices are the first report of TADF based OLEDs that utilize graphene‐based anodes.

Theoretical perspective for substitution effect on luminescent properties of through space charge transfer-based thermally activated delayed fluorescence molecules

Recently, through space charge transfer (TSCT)-based thermally activated delayed fluorescence (TADF) molecules have shown advantages in achieving high efficiencies and tunable emissions. However, the relationships between basic molecular structures and luminescent properties are unclear. Theoretical investigations to reveal the substitution effects with different numbers and positions on excited-state properties are highly desired. Herein, by taking TSCT-based TADF molecules S-CNDF-S-tCz, S-CNDF-D-tCz and T-CNDF-T-tCz as skeletons, a series of promising TADF molecules are designed by adopting ortho, meta and para substitutions with different numbers and positions. Photophysical properties of total 16 molecules are theoretically studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods in chloroform combined with polarizable continuum model. Results indicate that molecules with ortho-substitution possess small geometric changes and short Donor-Acceptor distances which are induced by the intramolecular van der Waals interactions. Decreased non-radiative consumption and increased TSCT ratio and therefore excellent performance for them can be expected. For molecules with large substitution numbers, twist structures facilitate them to realize small adiabatic energy gaps between the lowest singlet excited state (S1) and the lowest triplet excited state (T1), this designing strategy is consistent with the TADF dendrimers. Thus, the relationships between molecular structures and luminescent properties are revealed and promising TSCT-based TADF molecules with high efficiencies are theoretically proposed. Our investigations provide theoretical perspectives for inner mechanisms of substitution effect, which could further afford meaningful guidance to design new efficient TSCT-based TADF molecules.

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes

Diabatic Decomposition Perspective on the Role of Charge Transfer and Local Excitations in Thermally Activated Delayed Fluorescence.

Thermally activated delayed fluorescence (TADF) is a phenomenon that relies on the upconversion of triplet excitons to singlet excitons by means of reverse intersystem crossing (rISC). It has been shown both experimentally and theoretically that the TADF mechanism depends on the interplay between charge transfer and local excitations. However, the difference between the diabatic and adiabatic character of the involved excited states is rarely discussed in the literature. Here we develop a diabatization procedure to implement a four-state model Hamiltonian to a set of TADF molecules. We provide physical interpretations of the Hamiltonian elements and show their dependence on the electronic state of the equilibrium geometry. We also demonstrate how vibrations affect the TADF efficiency by modifying the diabatic decomposition of the molecule. Finally, we provide a simple model that connects the diabatic Hamiltonian to the electronic properties relevant to TADF and show how this relationship translates into different optimization strategies for rISC, fluorescence, and overall TADF performance.

Suppressing triplet exciton quenching by regulating the triplet energy of crosslinkable hole transport materials for efficient solution-processed TADF OLEDs

Crosslinkable hole transport materials (HTMs) with high triplet energies would have a balance of carrier injection into the emitting material layer, suppressing the triplet exciton quenching and resulting in high-performance solution-processed organic light-emitting diode (OLED) devices. Two novel crosslinkable HTMs with different central units, N2, N8-di-p-tolyl-N2,N8-bis(4-vinylphenyl)dibenzo[b,d]thiophene-2,8-diamine (V-p-DBT) and N2,N8-di-p-tolyl-N2,N8-bis(4-vinylphenyl)dibenzo[b,d]furan-2,8-diamine (V-p-DBF), were designed and synthesized. The use of dibenzothiophene and dibenzofuran units increases the torsion angle compared with the commonly used N, N′-di-p-tolyl-N, N′-bis(4-vinylphenyl)-[1,1′-biphenyl]-4,4′-diamine (V-p-TPD), leading to high triplet energies of 2.57 and 2.64 eV, respectively. The triplet energies of V-p-DBT and V-p-DBF effectively suppress triplet exciton quenching. Furthermore, the crosslinked HTM layer showed excellent solvent-resistant abilities and high thermal stability. An outstanding maximum current efficiency (CEmax) of 79.94 cd A−1 and maximum external quantum efficiency (EQEmax) of 24.35% were obtained by V-p-DBF-based green thermally activated delayed fluorescent (TADF) OLEDs. This work provides a new molecular design strategy for achieving efficient solution-processed TADF OLEDs.

Molecular Engineering of Sulfur-Bridged Polycyclic Emitters Towards Tunable TADF and RTP Electroluminescence.

Highly efficient organic thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) emitters for organic light-emitting diodes (OLEDs) generally consist of a twisted donor-acceptor skeleton with aromatic amine donors. Herein, through introducing sulfur atoms into isomeric pentaphene and pentacene frameworks, we demonstrate a set of polycyclic luminophores exhibiting efficient TADF and RTP characters. The incorporation of sulfur atoms confirms a folded molecular plane, while intensifies singlet-triplet spin-orbit coupling. Further, the isomeric effect has a significant effect on the electronic structure of excited state, giving rise to the investigated compounds tunable luminescence mechanisms of TADF and RTP. With efficient triplet harvesting ability, maximum external quantum efficiencies up to 25.1 % and 8.7 % are achieved for the corresponding TADF and RTP OLEDs, verifying the great potential of sulfur-bridged frameworks for highly efficient devices.

Molecular Engineering of Sulfur‐Bridged Polycyclic Emitters Towards Tunable TADF and RTP Electroluminescence

AbstractHighly efficient organic thermally activated delayed fluorescence (TADF) and room‐temperature phosphorescence (RTP) emitters for organic light‐emitting diodes (OLEDs) generally consist of a twisted donor–acceptor skeleton with aromatic amine donors. Herein, through introducing sulfur atoms into isomeric pentaphene and pentacene frameworks, we demonstrate a set of polycyclic luminophores exhibiting efficient TADF and RTP characters. The incorporation of sulfur atoms confirms a folded molecular plane, while intensifies singlet–triplet spin‐orbit coupling. Further, the isomeric effect has a significant effect on the electronic structure of excited state, giving rise to the investigated compounds tunable luminescence mechanisms of TADF and RTP. With efficient triplet harvesting ability, maximum external quantum efficiencies up to 25.1 % and 8.7 % are achieved for the corresponding TADF and RTP OLEDs, verifying the great potential of sulfur‐bridged frameworks for highly efficient devices.

Highly Efficient Sky-Blue π-Stacked Thermally Activated Delayed Fluorescence Emitter with Multi-Stimulus Response Properties.

Organic materials with multi-stimulus response (MSR) properties have demonstrated many potential and practical applications. Herein, a π-stacked thermally activated delayed fluorescence (TADF) material with multi-stimulus response (MSR) properties, named SDMAC, was designed and synthesized using distorted 9,9-dimethyl-10-phenyl-9,10-dihydroacridine as a donor. SDMAC possesses a rigid π-stacked configuration with intramolecular through-space interactions and exhibits aggregation-induced emission enhancement (AIEE), solvatochromic, piezochromic, and circularly polarized luminescence (CPL) under different external stimuli. The rigid molecular structure and efficient TADF properties of SDMAC can be used in displays and lighting. Using SDMAC as an emitter, the maximum external quantum efficiency (EQE) of the fabricated organic light-emitting diodes (OLEDs) is as high as 28.4 %, which make them the most efficient CP-TADF OLEDs based on the through-space charge transfer strategy. The CP organic light-emitting diodes (CP-OLEDs) exhibit circularly polarized electroluminescence (CPEL) signals.

Highly Efficient Sky‐Blue π‐Stacked Thermally Activated Delayed Fluorescence Emitter with Multi‐Stimulus Response Properties

AbstractOrganic materials with multi‐stimulus response (MSR) properties have demonstrated many potential and practical applications. Herein, a π‐stacked thermally activated delayed fluorescence (TADF) material with multi‐stimulus response (MSR) properties, namedSDMAC, was designed and synthesized using distorted 9,9‐dimethyl‐10‐phenyl‐9,10‐dihydroacridine as a donor.SDMACpossesses a rigid π‐stacked configuration with intramolecular through‐space interactions and exhibits aggregation‐induced emission enhancement (AIEE), solvatochromic, piezochromic, and circularly polarized luminescence (CPL) under different external stimuli. The rigid molecular structure and efficient TADF properties ofSDMACcan be used in displays and lighting. UsingSDMACas an emitter, the maximum external quantum efficiency (EQE) of the fabricated organic light‐emitting diodes (OLEDs) is as high as 28.4 %, which make them the most efficient CP‐TADF OLEDs based on the through‐space charge transfer strategy. The CP organic light‐emitting diodes (CP‐OLEDs) exhibit circularly polarized electroluminescence (CPEL) signals.

(Invited) Structural Analysis of Organic Semiconducting Materials By Solid State NMR

Recently, our group have successfully developed several blue and green thermally activated delayed fluorescence (TADF) emitters showing high external quantum efficiencies in OLEDs through high-throughput screening based on quantum chemical calculations. Although a number of highly efficient TADF emitters have been reported, the study about their conformation and aggregated structure in an amorphous states, which is expected to be the origin of high device performance, is still limited. Thus we have carried out multiscale simulations and solid-state NMR (ssNMR) analysis of organic amorphous thin films. The ssNMR is one of the powerful technique to acquire the structural information of amorphous aggregated materials, which has been difficult by diffraction techniques due to the lack of long range order for amorphous state. In this presentation, we demonstrate ssNMR analysis of amorphous organic semiconducting materials.Reference(1) “Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy”, Suzuki, K.; Kubo, S.; Aussenac, F.; Engelke, F.; Fukushima, T.; Kaji, H. Angew. Chem. Int. Ed. 2017, 56, 14842.(2) “Efficient blue thermally activated delayed fluorescence emitters showing very fast reverse intersystem crossing”, Ren, Y., Wada, Y., Suzuki, K., Kusakabe, Y., Geldsetzer, J. and Kaji, H. Appl. Phys. Express, 2021, 14, 071003.(3) “Solution-processable thermally activated delayed fluorescence emitters for application in organic light emitting diodes”, Suzuki, K.; Adachi, C.; Kaji, H. J. Soc. Inf. Disp. 2017, 25, 480.(4) “Combined Inter- and Intramolecular Charge-Transfer Processes for Highly Efficient Fluorescent Organic Light-Emitting Diodes with Reduced Triplet Exciton Quenching”, Moon, C.-K.; Suzuki, K.; Shizu, K.; Adachi, C.; Kaji, H.; Kim, J.-J. Adv. Mater. 2017, 131, 6614.(5) “Triarylboron-based Fluorescent Organic Light-emitting Diodes with External Quantum Efficiencies Exceeding 20%”, Suzuki, K.; Kubo, S.; Shizu, K.; Fukushima, T.; Wakamiya, A.; Murata, Y.; Adachi, C.; Kaji, H.; Angew. Chem. Int. Ed. 2015, 54, 15231.(5) “Purely organic electroluminescent material realizing 100% conversion from electricity to light”, Kaji, H.; Suzuki, H.; Fukushima, T.; Shizu, K.; Suzuki, K.; Kubo, S.; Komino, T.; Oiwa, H.; Suzuki, F.; Wakamiya, A.; Murata, Y.; Adachi, C. Nat. Commun. 2015, 6, 8476.