Delayed Fluorescence Emitters Research Articles

Recent progress in multi-resonance thermally activated delayed fluorescence emitters with an efficient reverse intersystem crossing process.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have become an active research topic at the forefront of organic light-emitting diodes (OLEDs) owing to their excellent photophysical properties such as high efficiency and narrow emission characteristics. However, MR-TADF materials always exhibit slow reverse intersystem crossing rates (kRISC) due to the large energy gap and small spin-orbit coupling values between singlet and triplet excited states. In order to optimize the RISC process, strategies such as heavy-atom-integration, metal perturbation, π-conjugation extension and peripheral decoration of donor/acceptor units have been proposed to construct efficient MR-TADF materials for high-performance OLEDs. This article provides an overview of the recent progress in MR-TADF emitters with an efficient RISC process, focusing on the structure-activity relationship between the molecular structure, optoelectronic feature, and OLED performance. Finally, the potential challenges and future prospects of MR-TADF materials are discussed to gain a more comprehensive understanding of the opportunities for efficient narrowband OLEDs.

Asymmetric multiple resonance thermally activated delayed fluorescence emitters for sky-blue and pure blue electroluminescence

Two asymmetric MR-TADF materials, Cz-SCz and BCz-SCz, exhibit sky-blue and pure blue emission with small FWHMs of 28 and 24 nm, respectively. OLEDs display EQEmax values of up to 20.5%, and that with BCz-SCz achieves pure blue emission with a CIEy coordinate below 0.15.

Triplet dynamics reveal loss pathways in multi-resonance thermally activated delayed fluorescence emitters.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials are of interest for light-emitting applications due to their narrow emission bandwidths and high photoluminescence quantum yields. Whilst there have been numerous examples of multi-resonance molecules exhibiting efficient TADF, the photophysics and mechanism of TADF in multi-resonance emitters have not been investigated to the same extent as the more conventional spatially separated donor-acceptor TADF materials, limiting the development of MR-TADF devices. Here we study the photophysics of a multi-resonance TADF material, OQAO(mes)2, using transient absorption spectroscopy to spectrally resolve the triplet population(s). We identify multiple triplet populations with distinct spectral contributions, and resolve the dynamics between them. Unlike conventional donor-acceptor TADF materials that have previously been studied, we find these triplet states are not formed in equilibrium, instead exhibiting a slow evolution from a high-energy triplet to a low-energy triplet. Delayed fluorescence predominantly reflects the lifetime of the high-energy triplet state, indicating that the formation of the low-energy triplet is a loss pathway for TADF. We also find that greater amounts of the low-energy triplet are formed in a higher dielectric environment, which leads to less delayed fluorescence. These triplet dynamics have significant implications for TADF in devices, as depending on the identity of the triplet formed by electrical excitation, there will either be a significant barrier to TADF, or a competing nonradiative decay pathway.

Heavy Atom Effect on the Luminescent Properties of Multi-resonance Thermally Activated Delayed Fluorescence Emitters: Theoretical Insights

Heavy Atom Effect on the Luminescent Properties of Multi-resonance Thermally Activated Delayed Fluorescence Emitters: Theoretical Insights

Ultrafast photophysics of an orange-red thermally activated delayed fluorescence emitter: the role of external structural restraint.

The application of thermally activated delay fluorescence (TADF) emitters in the orange-red regime usually suffers from the fast non-radiative decay of emissive singlet states (kSNR), leading to low emitting efficiency in corresponding organic light-emitting diode (OLED) devices. Although kSNR has been quantitatively described by energy gap law, how ultrafast molecular motions are associated with the kSNR of TADF emitters remains largely unknown, which limits the development of new strategies for improving the emitting efficiency of corresponding OLED devices. In this work, we employed two commercial TADF emitters (TDBA-Ac and PzTDBA) as a model system and attempted to clarify the relationship between ultrafast excited-state structural relaxation (ES-SR) and kSNR. Spectroscopic and theoretical investigations indicated that S1/S0 ES-SR is directly associated with promoting vibrational modes, which are considerably involved in electronic-vibrational coupling through the Huang-Rhys factor, while kSNR is largely affected by the reorganization energy of the promoting modes. By restraining S1/S0 ES-SR in doping films, the kSNR of TADF emitters can be greatly reduced, resulting in high emitting efficiency. Therefore, by establishing the connection among S1/S0 ES-SR, promoting modes and kSNR of TADF emitters, our work clarified the key role of external structural restraint for achieving high emitting efficiency in TADF-based OLED devices.

Double charge transfer processes enable a green multiple resonance-induced thermally activated delayed fluorescence emitter for an efficient narrowband OLED.

Benefitting from short-range charge transfer (SR-CT) and through-space charge transfer (TSCT) effects, an efficient green narrowband emitter, BNDCN, was developed. Owing to the synergistic effect of double CT processes, a BNDCN-based organic light-emitting diode showed a high external quantum efficiency of 32.3%.

Designing thermally activated delayed fluorescence emitters with through-space charge transfer: a theoretical study.

Recently, thermally activated delayed fluorescence (TADF) molecules with through-space charge transfer (TSCT) features have been widely applied in developing organic light-emitting diodes with high luminescence efficiencies. The performance of TSCT-TADF molecules depends highly on their molecular structures. Therefore, theoretical investigation plays a significant role in designing novel highly efficient TSCT-TADF molecules. Herein, we theoretically investigate two recently reported TSCT-TADF molecules, 1'-(2,12-di-t-butyl[1,4]benzoxaborinino[2,3,4-kl]phenoxaborinin-7-yl)-10-phenyl-10H-spiro[acridine-9,9'-fluorene] (AC-BO) and 1-(2,12-di-t-butyl[1,4]benzoxaborinino[2,3,4-kl]phenoxaborinin-7-yl)-9',9'-dimethyl-9'H-spiro [fluorene-9,5'-quinolino[3,2,1-de]acridine](QAC-BO). The calculated photophysical properties (e.g. excited state energy levels and luminescence properties) for these two compounds are in good agreement with experimental data. Based on the systematic analysis of structure-performance relationships, we design three novel TSCT-TADF molecules with high molecular rigidity and evident TSCT features, i.e., DQAC-DBO, DQAC-SBO, and DQAC-NBO. They exhibit deep-blue light emissions and fast reverse intersystem crossing rates (KRISCs). Our calculations demonstrate that the nearly coplanar orientation of the donor and acceptor is critical to achieve remarkable KRISCs and fluorescence efficiencies in TSCT-TADF molecules.

Modification of Thermally Activated Delayed Fluorescence Emitters Comprising Fluorinated Acridan-Quinazoline or Spiroacridan Moieties for Efficient Green OLEDs

Modification of Thermally Activated Delayed Fluorescence Emitters Comprising Fluorinated Acridan-Quinazoline or Spiroacridan Moieties for Efficient Green OLEDs

A molecular descriptor of a shallow potential energy surface for the ground state to achieve narrowband thermally activated delayed fluorescence emission.

Narrowband thermally activated delayed fluorescence (TADF) molecules have extensive applications in optoelectronics, biomedicine, and energy. The full-width at half-maximum (FWHM) holds significant importance in assessing the luminescence efficiency and color purity of TADF molecules. The goal is to achieve efficient and stable TADF emissions by regulating and optimizing the FWHM. However, a bridge from the basic physical parameters (such as geometric structure and reorganization energy) to the macroscopic properties (delayed fluorescence, efficiency, and color purity) is needed and it is highly necessary and urgent to explore the internal mechanisms that influence FWHM. Herein, first-principles calculations coupled with the thermal vibration correlation function (TVCF) theory were performed to study the energy consumption processes of the excited states for the three TADF molecules (2,3-POA, 2,3-DPA, and 2,3-CZ) with different donors; inner physical parameters affecting the FWHM were detected. By analyzing the basic geometric and electronic structures as well as the transition properties and reorganization energies, three main findings in modulating FWHM were obtained, namely a large local excitation (LE) proportion in the first singlet excited state is advantageous in reducing FWHM, a donor group with weak electron-donating ability is beneficial for achieving narrowband emission, and small reorganization energies for the ground state are favorable for reducing FWHM. Thus, wise molecular design strategies to achieve efficient narrowband TADF emission are theoretically proven and proposed. We hope that these results will promote an in-depth understanding of FWHM and accelerate the development of high color purity TADF emitters.

Facilitating intrinsic delayed fluorescence of conjugated emitters by inter-chromophore interaction.

Delayed fluorescence (DF) is a unique emitting phenomenon of great interest for important applications in organic optoelectronics. In general, DF requires well-separated frontier orbitals, inherently corresponding to charge transfer (CT)-type emitters. However, facilitating intrinsic DF for local excited (LE)-type conjugated emitters remains very challenging. Aiming to overcome this obstacle, we demonstrate a new molecular design strategy with a DF-inactive B,N-multiple resonance (MR) emitter as a model system. Without the necessity of doping with heavy atoms, we synthesized a co-facial dimer in which an excimer-like state (Sexc) was expected to facilitate efficient reverse intersystem crossing (RISC, T1 → Sexc) and intrinsic DF. Benefiting from greatly enhanced SOC and reduced ΔE ST, the proof-of-concept emitter Np-2CzB exhibited k RISC up to 6.5 × 105 s-1 and intrinsic DF with >35% contribution (Φ DF/Φ F) in dilute solution. Further investigation indicated that Sexc state formation relies on an optimized co-facial distance (d = ∼4.7 Å), strong inter-chromophore interaction (J coul > 450 cm-1) and a rigid structure (Γ S1→S0 < 350 cm-1). Although our strategy was demonstrated with a B,N-MR emitter, it can be applicable to many LE-type conjugated emitters without intrinsic DF. By triggering potential DF emission, many classic emitters might play a more important role in optoelectronics.

Excited-state dynamics of C3-symmetric heptazine-based thermally activated delayed-fluorescence emitters.

Heptazine-based materials have recently emerged as a promising motif for thermally activated delayed fluorescence, as their near-zero or negative singlet-triplet energy gaps enable extremely fast reverse intersystem crossing (rISC) rates. Another method for achieving a high rate of rISC is through the use of highly symmetric emitters, which benefit from energy-level degeneracies and a high density of states. Here, we investigate the effect of combining these two design strategies on the excited-state dynamics of C3-symmetric emitters containing heptazine cores. We find that in two of the four emitters studied, the S1 state has a high degree of locally excited (LE) character with density on the heptazine moiety, preventing excited-state localization and a loss of symmetry in the energy-minimized S1 geometry. Surprisingly, these symmetric molecules still suffer from a loss of density of triplet states below the S1 state. Overall, we find that maintaining C3 symmetry will not necessarily maintain density of states, but that heptazine-based materials with LE S1 states still benefit from maximized rISC rates via increased spin-orbit coupling with low-lying charge-transfer triplet states and exhibit advantageous photophysical properties, such as near-unity photoluminescence quantum yields and high colour purity.

Fully space-confined donor-acceptor interaction for highly efficient thermally activated delayed fluorescence emitters

The emerging through-space charge transfers (TSCT) emitters with thermally activated delayed fluorescence (TADF) have drawn increasing interest. However, the low luminescence efficiency and slow reverse intersystem crossing (RISC) always limit their device performances. A new molecular design strategy for TSCT-TADF emitters with a fully space-confined donor/acceptor conformation that integrates high luminescence efficiency and fast RISC is proposed herein. Two new TSCT-TADF emitters, TRZ-STFMe and TRZ-STFPh, are developed by introducing the methyl and phenyl substituents at the C2 of the rigid fluorene scaffold, respectively. The repulsive interaction between methyl and the acceptor induces a close and face-to-face donor–acceptor stacking to generate highly efficient TADF and suppress structural changes during the RISC process in TRZ-STFMe. In contrast, TRZ-STFPh shows relatively low luminescence efficiency and slow RISC because of its conformationally unstable phenyl substituent. With a high luminescence efficiency of 97 % and a significant RISC rate of 5.23 × 105 s−1, TRZ-STFMe demonstrates a high electroluminescence performance with a maximum external quantum efficiency (EQE) of 29.6 % and an EQE of 26.5 % at 1000 cd m−2.

Cost-Effective Approach for Modeling of Multiresonant Thermally Activated Delayed Fluorescence Emitters.

Multiresonant thermally activated delayed fluorescence (MR-TADF) emitters have recently attracted great interest for application in organic light-emitting diodes due to their remarkable electroluminescent efficiency and narrow emission spectra. It is therefore essential to establish computational methodologies that can accurately model the excited states of these materials at manageable computational costs. With regard to MR-TADF design and their associated photophysics, previous works have highlighted the importance of wave function-based methods, at much higher computational costs, over the traditional time-dependent density functional theory approach. Herein, we employ two independent techniques built on different quantum mechanical frameworks, highly correlated wave function-based STEOM-DLPNO-CCSD and range-separated double hybrid density functional, TD-B2PLYP, to investigate their performance in predicting the excited state energies in MR-TADF emitters. We demonstrate a remarkable mean absolute deviation (MAD) of ∼0.06 eV in predicting ΔEST compared to experimental measurements across a large pool of chemically diverse MR-TADF molecules. Furthermore, both methods yield superior MAD in estimating S1 and T1 energies over earlier reported SCS-CC2 computed values [J. Chem. Theory Comput. 2022, 18, 4903]. The short-range charge-transfer nature of low-lying excited states and narrow fwhm values, hallmarks of this class of emitters, are precisely captured by both approaches. Finally, we show the transferability and robustness of these methods in estimating rates of radiative and nonradiative events with adequate agreement against experimental measurements. Implementing these cost-effective computational approaches is poised to streamline the identification and evaluation of potential MR-TADF emitters, significantly reducing the reliance on costly laboratory synthesis and characterization processes.

An A-D-A-Type Thermally Activated Delayed Fluorescence Emitter with Intrinsic Yellow Emission Realizing Record-High Red/NIR OLEDs upon Modulating Intermolecular Aggregations.

Realizing efficient red/near-infrared (NIR) electroluminescence (EL) by precisely modulating molecular aggregations of thermally activated delayed fluorescence (TADF) emitters is an attractive pathway, yet the molecular designs are elusive. Here, a new approach is proposed to manage molecular aggregation via a mild-twist acceptor-donor-acceptor (A-D-A)-type molecular design. A proof-of-concept TADF molecule, QCN-PhSAC-QCN, is developed that furnishes a fast radiative rate and obvious aggregation-induced emission feature. Its emission bands can be facilely shifted from intrinsic yellow to the red/NIR region via fine-tuning doping levels and molecular aggregates while maintaining elegant photoluminescence quantum yields benefiting from suppressed exciton annihilation processes. As a result, a QCN-PhSAC-QCN-based organic light-emitting diode (OLED) exhibits a record-setting external quantum efficiency (EQE) of 39.1% at a doping ratio of 10wt.%, peaking at 620nm. Moreover, its nondoped NIR OLED affords a champion EQE of 14.3% at 711nm and retains outstanding EQEs of5.40% and 2.35% at current densities of 10 and 100 mA cm-2 , respectively, which are the highest values among all NIR-TADF OLEDs at similar density levels. This work validates the feasibility of such mild-twist A-D-A-type molecular design for precisely controlling molecular aggregation while maintaining high efficiency, thus providing a promising pathway for high-performance red/NIR TADF OLEDs.

Versatile Thermally Activated Delayed Fluorescence Emitters via Non‐conjugated Extension Strategy Enabling OLEDs with Efficiency Over 37%

AbstractIn recent years, thermally activated delayed fluorescence (TADF) materials with high horizontal dipole ratios (Θ//) have attracted enormous attention in constructing high‐efficiency organic light–emitting diodes (OLEDs). However, the common strategy to improve Θ// based on extending π‐conjugation tends to induce bathochromic emission shift, and emitters with Θ// exceeding 90% are rarely reported. Herein, a non‐conjugated extension strategy is developed to promote molecular horizontal dipole orientation. By simply modifying the spiro‐acridine with extra 9‐phenylcarbazole groups, two high‐efficiency and versatile TADF emitters 36DPCZSFAC‐TRZ and 36DPCZSFAC‐PYPM are constructed without bathochromic emission shift, and state‐of‐the‐art ultra‐high Θ// of 92% and 96% are achieved, respectively. More fascinatingly, due to the special spiro molecular structures, the TADF emitters hold distinguished merits of aggregation‐induced emission property, near unitary photoluminescence quantum yields, fast reverse intersystem crossing rates, and enhanced charge‐carrier transport property, which enable outstanding external quantum efficiency values of up to 22.1/20.9% and 37.4/34.2% in non‐doped/doped OLEDs for 36DPCZSFAC‐TRZ and 36DPCZSFAC‐PYPM, respectively, demonstrating their great potentials in the fabrication of high‐efficiency OLEDs.

Designing Promising Thermally Activated Delayed Fluorescence Emitters via Machine Learning-Assisted High-Throughput Virtual Screening

Designing Promising Thermally Activated Delayed Fluorescence Emitters via Machine Learning-Assisted High-Throughput Virtual Screening

Constructing Multiresonance Thermally Activated Delayed Fluorescence Emitters for Organic LEDs: A Computational Investigation.

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials have acquired substantial attention due to their high electroluminescence efficiency with narrow emission spectra. However, the existing MR-TADF emitters suffer from substantial efficiency roll-off due to insufficient rate constants of the reverse intersystem crossing (kRISC) process compared to the traditional TADF materials. Herein, we employ the DLPNO-STEOM-CCSD method, which is computationally less expensive than the wave function-based EOM-CCSD method, to evaluate the electronic and photophysical properties of MR-TADF materials accurately. The predicted singlet-triplet energy gap (ΔEST), one of the critical parameters governing the TADF efficiency, exhibits remarkable agreement with the experimental measurement, with a standard deviation value of 0.026 eV (obtained based on five experimentally synthesized MR-TADF systems). The proposed technique was utilized to determine the suitability of 15 newly designed MR-TADF emitters via their computed radiative and nonradiative rates, luminescence efficiencies, and exciton characteristics. Moreover, most conceived molecules exhibit blue emission with decent to strong oscillator strengths, making them potential candidates for practical light-emitting applications. The proposed computational route will undoubtedly accelerate the designing and prescreening of potential MR-TADF emitters before their expensive laboratory synthesis and characterization.

Quinoxaline derivatives as attractive electron-transporting materials.

This review article provides a comprehensive overview of recent advancements in electron transport materials derived from quinoxaline, along with their applications in various electronic devices. We focus on their utilization in organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), organic field-effect transistors (OFETs), organic-light emitting diodes (OLEDs) and other organic electronic technologies. Notably, the potential of quinoxaline derivatives as non-fullerene acceptors in OSCs, auxiliary acceptors and bridging materials in DSSCs, and n-type semiconductors in transistor devices is discussed in detail. Additionally, their significance as thermally activated delayed fluorescence emitters and chromophores for OLEDs, sensors and electrochromic devices is explored. The review emphasizes the remarkable characteristics and versatility of quinoxaline derivatives in electron transport applications. Furthermore, ongoing research efforts aimed at enhancing their performance and addressing key challenges in various applications are presented.

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 357-449nm. 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 (394nm, 399nm, and 398nm, respectively), while two others, 1f and 1i, emit blue shade (449nm 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 (0.01eV) with λ em values of 449nm 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.

Molecular design strategies for the highly efficient vacuum and solution-processable long wavelength thermally activated delayed fluorescence emitters

Simple device architecture-based solution-processed organic light-emitting diodes (OLEDs) are more advantageous over the multilayer vacuum-processed OLEDs for their commercial application. Highly efficient long wavelength thermally activated delayed fluorescent (TADF) emitters suitable for both vacuum and solution-processed OLEDs are rarely reported because of the stringent molecular design rules and the inherent limitation of the energy gap law. To resolve this issue, we designed and synthesized two TADF emitters namely, 5,8-bis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)-2,3-bis(4-(tert-butyl)phenyl)quinoxaline-6,7-dicarbonitrile (2tBuTPA-QxCN) and 2,3,5,8-tetrakis(4-(bis(4-(tert-butyl)phenyl)amino)phenyl)quinoxaline-6,7-dicarbonitrile (4tBuTPA-QxCN), for their use in both vacuum and solution-processed OLEDs. High external quantum efficiency (EQE) of 22 % and emission peak at 608 nm were demonstrated for the vacuum-deposited OLED fabricated with 4tBuTPA-QxCN TADF emitter as compared to the OLED fabricated with 2tBuTPA-QxCN TADF emitter exhibiting EQE value of 21.1 % and emission peak at 605 nm. In their solution-processed OLEDs, a high EQE of 15.6 % and pure-red emission peak at 630 nm was demonstrated for the 2tBuTPA-QxCN TADF emitter as compared to the EQE value of 11.6 % and emission peak at 628 nm for the 4tBuTPA-QxCN TADF emitter.