Molecular Design Strategy for Orange-red Thermally Activated Delayed Fluorescence Emitters in OLEDs

Strategies for Enabling RGB Emission in Fused Carbazole Derivatives

The development of organic light-emitting diodes (OLEDs) has witnessed remarkable progress in material design and device architecture. Recent advancements, particularly in the fourth generation of OLEDs, have introduced groundbreaking innovations such as hyperfluorescence and multiresonance (MR) thermally activated delayed fluorescence (MRTADF) emitters. Carbazole has emerged as a versatile scaffold, playing a pivotal role in conventional fluorescence, TADF, roomtemperature phosphorescence (RTP), and MRTADF systems. In recent years, fused carbazole derivatives have gained significant attention as both emitting and host materials in OLEDs. The fusion of carbazole units enhances molecular rigidity and extends the πconjugation, enabling precise tuning of optoelectronic properties across a wide color gamut, including blue, green, orange, yellow, and red emissions. This review systematically explores the application of various fused carbazole systems such as indolocarbazole, thienocarbazole, furocarbazole, indenocarbazole, triazatruxene, acridinecarbazole, chromenocarbazole, pyrenocarbazole, helicene carbazole, and carbazolefused boron/carbonyl MRTADF emitters in OLEDs. The discussion is organized into three sections based on their application in blue, green, and red OLEDs, providing a comprehensive understanding of structure-property relationships. Additionally, other color-emitting OLEDs are discussed where relevant, offering a holistic perspective on the potential of fused carbazole derivatives in next-generation OLED technologies.

A theoretical investigation of heavy atom and oxidation effects in MR-TADF emitters for OLEDs: a combined DFT, double hybrid DFT, CCSD, and QM/MM approaches

The emerging multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with organoboron and nitrogen cores highlight their significance in OLEDs. However, their efficiency is challenged by slower rate constants in the reverse intersystem crossing (kRISC) process compared to conventional TADF emitters. The study entails an in-depth analysis focused on gaining a better understanding of the photophysical properties of MR-TADF emitters. Using DFT and TD-DFT analyses, 48 MR-TADF molecules are studied, incorporating heavy atoms such as sulfur and selenium, and their subsequent oxidation, and peripheral donors such as carbazole (Cz), tert-butyl-carbazole (tCz), diphenylacridine (DPAC), and dimethylacridine (DMAC) into organo boron and nitrogen-embedded systems. Moreover, the QM/MM approach was utilized to examine the excited state properties in the crystal phase. A comprehensive assessment of this molecular framework reveals that integrating heavy atoms and donors into MR-TADF molecules results in significant enhancements in ΔEST, larger SOC, and higher-order radiative (108 s-1) rates, leading to faster kISC (∼108 s-1) and kRISC (∼106 s-1) rates. Based on key criteria, eight potential molecules were selected and their excited-state properties were precisely analyzed using double-hybrid density functionals including B2PLYP and PBE0-2, along with highly correlated wave function STEOM-DLPNO-CCSD.

Fluorescence Properties of Novel Multiresonant Indolocarbazole Derivatives for Deep-Blue OLEDs from Multiscale Computer Modelling

Multiresonant fluorophores are a novel class of organic luminophores with a narrow emission spectrum. They can yield organic light-emitting devices, e.g., OLEDs, with high colour purity. In this study, we applied DFT and multiscale modelling to predict the electronic and optical properties of several novel derivatives of indolocarbazole pSFIAc, which had recently shown a high potential in deep-blue OLEDs. We found that the addition of phenyls to a certain position of the pSFIAc core can considerably increase the fluorescent rate, leaving other properties (HOMO, LUMO, lowest excited singlet and lowest triplet states' energies) virtually unaffected. This can improve the efficiency and stability of deep-blue organic light-emitting devices; the suggested phenyl-substituted indolocarbazoles have been shown to be compatible with two popular anthracene-based hosts. On the contrary, the addition of phenyls to another positions of the core is detrimental for optoelectronic properties. QM/MM and QM/EFP calculations yielded negligible inhomogeneous broadening of the emission spectrum of the studied luminophores when embedded as dopants in anthracene-based hosts, predicting high colour purity of the corresponding devices. On the basis of the obtained results, we selected one novel multiresonant indolocarbazole derivative that is most promising for organic light-emitting devices. We anticipate the revealed structure-property relationships will facilitate the rational design of efficient materials for organic (opto)electronics.

Highly and Deep Red-Luminescent Bisphenylamine-Appended Benzocarcogendiazole Fluorophores

Benzocarcogendiazole units have been frequently utilized for optoelectronics such as organic solar cells because of their robustness, rigidity, and band-gap tunability based on their strong electron-withdrawing properties. Focusing on the luminescent characteristics, these molecules have been utilized to demonstrate highly sensitive chromisms because of the potential of charge transfer. Here, we demonstrate deep red-emissions in bis(4-tert-butylphenyl)amine-appended benzocarcogendiazole-based donor-acceptor-donor (D-A-D) fluorophores, namely 1 and 2. Because benzocarcogendiazole and bis(4-tert-butylphenyl)amine serve as strong electron acceptor and donor, respectively, strong intramolecular charge transfer (ICT) enables long wavelength of photoluminescence (PL) even in the small molecular weight. Although photoluminescence (PL) in long wavelength tends to exhibit quite low absolute PL quantum efficiency (ΦPL), the values of solutions 1 and 2 are quite high (up to 50 %). According to X-ray crystallographic characterizations and DFT calculations, these high ΦPL values are attributable to the segregated π-planes of benzocarcogendiazole units, which is induced by the bulky substituents of bis(4-tert-butylphenyl)amines.

A nine-ring fused terrylene diimide exhibits switching between red TADF and near-IR room temperature phosphorescence

Herein, we report the first example of a terrylene diimide derivative that switches emission between thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) in the red region. By design, the molecule TDI-cDBT boasts a symmetrical, consecutively fused nine-ring motif with a kite-like structure. The rigid core formed by the annulated dibenzothiophene moiety favoured efficient intersystem crossing and yielded a narrow-band emission with a full-width half maxima (FWHM) of 0.09 eV, along with high colour purity. A small ΔE S1-T1 of 0.04 eV facilitated thermally activated delayed fluorescence, enhancing the quantum yield to 88% in the red region. Additionally, it also prefers a direct triplet emission from the aggregated state. The room temperature phosphorescence observed from the aggregates has a longer emission lifetime of 1.8 ms, which is further prolonged to 8 ms at 77 K in the NIR region. Thus, the current strategy is successful in not only reducing ΔE S1-T1 to favour TADF but also serves as a novel platform that can switch emission from TADF to RTP depending upon the concentration.

Rational Design of Highly Efficient Orange-Red/Red Thermally Activated Delayed Fluorescence Emitters with Submicrosecond Emission Lifetimes

The development of orange-red/red thermally activated delayed fluorescence (TADF) materials with both high emission efficiencies and short lifetimes is highly desirable for electroluminescence (EL) applications, but remains a formidable challenge owing to the strict molecular design principles. Herein, two new orange-red/red TADF emitters, namely AC-PCNCF3 and TAC-PCNCF3, composed of pyridine-3,5-dicarbonitrile-derived electron-acceptor (PCNCF3) and acridine electron-donors (AC/TAC) are developed. These emitters in doped films exhibit excellent photophysical properties, including high photoluminescence quantum yields of up to 0.91, tiny singlet-triplet energy gaps of 0.01 eV, and ultrashort TADF lifetimes of less than 1 µs. The TADF-organic light-emitting diodes employing the AC-PCNCF3 as emitter achieve orange-red and red EL with high external quantum efficiencies of up to 25.0% and nearly 20% at doping concentrations of 5 and 40 wt%, respectively, both accompanied by well-suppressed efficiency roll-offs. This work provides an efficient molecular design strategy for developing high-performance red TADF materials.

Reverse Designing the Wavelength-Specific Thermally Activation Delayed Fluorescent Molecules Using a Genetic Algorithm Coupled with Cheap QM Methods

Genetic algorithm (GA) optimization coupled with the semiempirical intermediate neglect of differential overlap (INDO)/CIS method is presented to inversely design the red thermally activation delayed fluorescent (TADF) molecules. According to the predefined donor-acceptor (DA) library to build an AD_n-type TADF candidate, we utilized the chemical notation language SMILES code to generate a TADF molecule and apply the RDKit program to produce the initial 3D molecular structure. A combined fitness function is proposed to evaluate the performance of the functional-lead TADF molecule. The fitness function includes three key parameters, i.e., the emission wavelength, the energy gap (ΔE_ST) between the lowest singlet (S₁)- and triplet (T₁)-excited states, and the oscillator strengths for electron transition from S₀ and S₁. A cheap QM method, i.e., INDO/CIS, on the basis of an xTB-optimized molecular geometry is applied to quickly calculate the fitness function. Finally, the GA approach is utilized to globally search for the wavelength-specific TADF molecules under our predefined DA library, and the optimum 630 nm red and 660 nm deep red TADF molecules are inversely designed according to the evolution of molecular fitness functions.

Realizing High-Efficiency Orange-Red Thermally Activated Delayed Fluorescence Materials through the Construction of Intramolecular Noncovalent Interactions

The development of highly efficient orange and red thermally activated delayed fluorescence (TADF) materials for constructing full-color and white organic light-emitting diodes (OLEDs) remains insufficient because of the formidable challenges in molecular design, such as the severe radiationless decay and the intrinsic trade-off between the efficiencies of radiative decay and reverse intersystem crossing (RISC). Herein, we design two high-efficiency orange and orange-red TADF molecules by constructing intermolecular noncovalent interactions. This strategy could not only ensure high emission efficiency via suppression of the nonradiative relaxation and enhancement of the radiative transition but also create intermediate triplet excited states to ensure the RISC process. Both emitters exhibit typical TADF characteristics, with a fast radiative rate and a low nonradiative rate. Photoluminescence quantum yields (PLQYs) of the orange (TPA-PT) and orange-red (DMAC-PT) materials reach up to 94 and 87%, respectively. Benefiting from the excellent photophysical properties and stability, OLEDs based on these TADF emitters realize orange to orange-red electroluminescence with high external quantum efficiencies reaching 26.2%. The current study demonstrates that the introduction of intermolecular noncovalent interactions is a feasible strategy for designing highly efficient orange to red TADF materials.

Forthcoming hyperfluorescence display technology: relevant factors to achieve high-performance stable organic light emitting diodes

Over the decade, there have been developments in purely organic thermally activated delayed fluorescent (TADF) materials for organic light-emitting diodes (OLEDs). However, achieving narrow full width at half maximum (FWHM) and high external quantum efficiency (EQE) is crucial for real display industries. To overcome these hurdles, hyperfluorescence (HF) technology was proposed for next-generation OLEDs. In this technology, the TADF material was considered a sensitizing host, the so-called TADF sensitized host (TSH), for use of triplet excitons via the reverse intersystem crossing (RISC) pathway. Since most of the TADF materials show bipolar characteristics, electrically generated singlet and triplet exciton energies can be transported to the final fluorescent emitter (FE) through Förster resonance energy transfer (FRET) rather than Dexter energy transfer (DET). This mechanism is possible from the S₁ state of the TSH to the S₁ state of the final fluorescent dopant (FD) as a long-range energy transfer. Considering this, some reports are available based on hyperfluorescence OLEDs, but the detailed analysis for highly efficient and stable devices for commercialization was unclear. So herein, we reviewed the relevant factors based on recent advancements to build a highly efficient and stable hyperfluorescence system. The factors include an energy transfer mechanism based on spectral overlapping, TSH requirements, electroluminescence study based on exciplex and polarity system, shielding effect, DET suppression, and FD orientation. Furthermore, the outlook and future positives with new directions were discussed to build high-performance OLEDs.

Design of Thermally Activated Delayed Fluorescence Materials with High Intersystem Crossing Efficiencies by Machine Learning-Assisted Virtual Screening

Efficient intersystem crossing (ISC) and reverse ISC (RISC) processes are of vital significance for thermally activated delayed fluorescence (TADF) materials to achieve 100% internal quantum efficiency. However, it is challenging to rapidly predict the ISC/RISC rates of large amounts of TADF materials and screen promising candidates because of their flexible molecular design. Here, we perform virtual screening of 564 candidates constructed from 20 unique building blocks linking in D-A, D-π-A, and D-A-D (D') configurations using the established machine learning models of GBRT and RF-GBRT-KNN with the Pearson's correlation coefficients (r) of 0.89 and 0.82, respectively. Novel descriptors of ΔE_LL, Polar, and ΔE_TT for predicting ISC/RISC rates were proposed, and nine TADF molecules with the predicted ISC and RISC rates of >7 × 10⁷ and 2 × 10⁵ s^-1, respectively, were revealed. We provide an efficient approach to predicting ISC and RISC rates of TADF molecules on a large scale, elucidating important building blocks and architectures to design high-performance optoelectronic materials for experimental explorations.