Regulating the Nature of Triplet Excited States of Thermally Activated Delayed Fluorescence Emitters

Quantifying Design Principles for Light-Emitting Materials with Inverted Singlet-Triplet Energy Gaps

Molecular engineering of organic emitter molecules with inverted singlet-triplet energy gaps (INVEST) has emerged as a powerful approach for enhancing fluorescence efficiency through triplet harvesting. In these unique materials, the first excited singlet state (S1) lies below the lowest triplet state (T1), enabling efficient reverse intersystem crossing. Previous computational studies have focused on accurately calculating the inverted energy gap and establishing qualitative structure-property relationships. Here, we present quantitative relationships that link the molecular structure to the S1-T1 energy gap, ΔEST, by introducing a benchmark set of 15 heptazine-based INVEST molecules (HEPTA-INVEST15). We identify a strong linear correlation (R2 > 0.94) between ΔEST and both the degree of intramolecular charge transfer and the deviation from a single-excitation character, as quantified by %R1 values and transition density matrix norms. These trends persist across our expanded set of 44 mono-, di-, and tri-substituted heptazines (HEPTA-INVEST44), underscoring the generality of our findings. Notably, strongly electron-donating groups, such as -NH2, minimize the magnitude of inverted gaps in mono-substituted heptazines yet produce the most negative ΔEST in certain tri-substituted derivatives, a result arising from competing resonance effects and excited-state aromaticity. Although ΔEST shows no clear correlation with Hammett parameters, our results reveal that physically meaningful, computable descriptors offer a mechanistic foundation for the future data-driven design of INVEST emitters. These findings pave the way for machine-learning approaches that connect the molecular structure to ΔEST without requiring high-level excited-state calculations.

A triply linked propellane-nanoring hybrid serving as a good host

A novel propellane-nanoring hybrid, TPPTI-[9]CMP, was synthesized by triply combining [9]cyclo-meta-phenylene ([9]CMP) with triperyleno[3,3,3]propellane triimides (TPPTI). This structure features two [9]CMP subunits, which necessarily fill the voids of TPPTI and promote assembly of the hybrid to form a porous superstructure held together by attractive dispersion between the [9]CMP subunits of neighboring molecules. In the structure, three large spatial cavities are formed, which allow efficient binding of up to three C60 within a single hybrid. Transient absorption spectroscopy revealed that TPPTI-[9]CMP and C60 interact with each other to form a stable complex and produce long-lived triplet states. Notably, the hybrid can adsorb ethane (C2H6) with very excellent selectivity over ethylene (C2H4), leading to a highly selective C2H6/C2H4 separation.

Fused Dual-Donor Design for Accelerating Reverse Intersystem Crossing Rates of Spatially Folded Through-Space Charge Transfer Emitters

The development of through-space charge transfer (TSCT)-thermally activated delayed fluorescence (TADF) material is defective in relatively low reverse intersystem crossing (RISC) rates (commonly <5 × 105 s-1). Herein, we fuse two 3,6-dimethyl-8H-indolo[3,2,1-de]acridine (IAc) donor units to obtain large planar donors (m-bIAc and p-bIAc) for forming spatially folded A-D|D-A configured TSCT emitters (DCT-1 and DCT-2). The configuration of highly parallel and large-plane intramolecular multiple π-stacking has been achieved. The symmetrical multi-channel charge transfer networks of emitters induce multiple energetically proximal excited states within a small energy range (<0.12 eV) at the lowest excited state, creating additional configuration interaction and spin-orbit coupling channels to accelerate the RISC process. This molecular configuration yields enhanced RISC rates of 6.19 × 105 s-1 for DCT-1 and 1.05 × 106 s-1 for DCT-2. Solution-processed organic light-emitting diodes employing these emitters achieve maximum external quantum efficiencies of 18.9% (DCT-1, 474 nm sky-blue emission) and 23.9% (DCT-2, 498 nm green emission), with attenuated efficiency roll-offs of DCT-2 (12% at 1000 cd m-2). This work provides a critical pathway for manipulating dense excited states to address the bottleneck of the RISC rates while maintaining structural rigidity, promoting further advancement of TSCT-TADF materials.

Developing Red and Near-Infrared Delayed Fluorescence Emission in Nitrogen-Substituted Donor-Acceptor Polycyclic Hydrocarbon OLED Emitters: A Theoretical Study

Nitrogen substitutions have shown a great impact for the development of thermally activated delayed fluorescence (TADF)-based organic light-emitting diode (OLED) materials. In particular, much focus has been devoted to nitrogen-substituted polycyclic aromatic hydrocarbons (PAHs) for TADF emitters. In this context, we provide here a molecular design approach for symmetric nitrogen substitutions in fused benzene ring PAHs based on the dibenzo[a,c]picene (DBP) molecule. We designed possible donor-acceptor (D-A) compounds with dimethylcarbazole (DMCz) and dimethyldiphenylamine (DMDPA) donors and studied the structure and photophysics of the designed D-A compounds. The twisted and extended D-A-type PAH emitters demonstrate red and near-infrared (NIR) TADF emission. Nitrogen substitutions lead to significant LUMO stabilization and reduced HOMO-LUMO energy gaps as well. Additionally, we computed significantly smaller singlet-triplet energy splittings (ΔEST) in comparison to non-nitrogen-substituted compounds. The investigated ortho-linked D-A compounds show relatively large donor-acceptor twisting separation and small ΔEST compared to their para-linked counterparts. For higher number nitrogen (4N)-substituted emitters, we predict small adiabatic ΔEST (ΔESTadia) in the range 0.01-0.13 eV, and with the tert-butylated donors, we even obtained ΔESTadia values as small as 0.007 eV. Computed spin-orbit coupling (SOC) for the T1 triplet state on the order of 0.12-2.28 cm-1 suggests significant repopulation of singlet charge transfer (1CT) excitons from the triplet CT and locally excited (3CT+LE) states. Importantly, the small ΔESTadia and large SOC values induce a reverse intersystem crossing (RISC) rate as high as 1 × 106 s-1, which will cause red and NIR delayed fluorescence in the 4N-substituted D-A emitters. Notably, we predict red TADF emission for the para-linked compound B4 at 670 nm and the ortho-linked compound D4 at 713 nm and delayed NIR emission at 987 and 1217 nm for the ortho-linked compounds D3 and E3, respectively.

Recent Advances in Thermally Activated Delayed Fluorescence-Based Organic Afterglow Materials

Thermally activated delayed fluorescence (TADF)-based materials are attracting widespread attention for different applications owing to their ability of harvesting both singlet and triplet excitons without noble metals in their structures. As compared to the conventional fluorescence and room-temperature phosphorescence pathways, TADF originates from the reverse intersystem crossing process from the excited triplet state (T1) to the singlet state (S1). Therefore, TADF emitters enabling activated and long lifetime T1 excitons are potential candidates for generating long-lived afterglow emission, an effect that can still be observed for a while by the naked eye after the removal of the excitation light source. Recently, TADF-based organic afterglow materials featuring high photoluminescence quantum yields and long lifetimes above 100 ms under ambient conditions, have emerged for advanced information security, high-contrast biological imaging, optoelectronic devices, and intelligent sensors, whereas the related systematic review is still lacking. Herein, the recent progress in TADF-based organic afterglow materials is summarized and an overview of the photophysical mechanism, design strategies, and the performances for relevant applications is given. In addition, the challenge and perspective of this area are given at the end of the review.

Enhancing Reverse Intersystem Crossing in TSCT-TADF Emitters: Heavy Atom Modulation of Multiresonance Acceptors

With the rapid development of thermally activated delayed fluorescence (TADF) materials, achieving efficient reverse intersystem crossing (RISC) to mitigate triplet-triplet annihilation has emerged as a prominent research focus. This study investigates five derivative molecules, featuring varied bridging atoms/groups (O, S, Se, -CH2-), designed from the reported TADF molecule AC-BO with through-space charge transfer (TSCT) properties. Utilizing time-dependent density functional theory coupled with a PCM solution model, their excited state behaviors were simulated in a toluene environment. Interestingly, it was observed that RISC in AC-BO and one derivative, AC-BCO, occurs predominantly via the T2 state rather than the typical T1 state (3LEB, where B denotes the fluorene bridge), distinguishing it from conventional TSCT-TADF compounds, where RISC typically involves transitions between the 3CT and 1CT states. This distinctive mode is attributed to reduced spin-orbit coupling (SOC) between 1CT and 3LEB, with T2 representing a significant contributor to the RISC process through its 3CT character. Introduction of heavy atoms enhances the electron-withdrawing ability of the acceptor unit, leading to the T1 transitions exhibiting 3MRCT characteristics and increased SOC, thereby favoring RISC via 3MRCT to 1CT transitions. This study not only deepens our understanding of transition mechanisms in TSCT-TADF compounds but also provides crucial insights into the molecular design and regulation of excited triplet states.

Development of Automatic Method for Glucose Detection Based on Platinum Octaethylporphyrin Sol-Gel Film with Long-Term Stability

In this study, an approach has been proposed in response to the urgent need for a sensitive and stable method for glucose detection at low concentrations. Platinum octaethylporphyrin (PtOEP) was chosen as the probe and embedded into the matrix material to yield a glucose-sensing film, i.e., Pt/TE-MTS, through a sol-gel process. The optical parameter (OP) was defined as the ratio of phosphorescence in the absence and presence of glucose, and the relationship between OP and glucose concentration (GC) was established in a theoretical way based on the Stern-Volmer equation and further obtained by photoluminescence measurement. OP exhibited a linear relationship with GC in a range of 0-720 μM. The time required by the photoluminescence of the film to reach equilibrium was measured to ensure the completion of the reaction, and it was found that the equilibrium time decreased as the GC increased. The photobleaching behavior and stabilization of the film were monitored, and the result showed that the film exhibited excellent resistance to photobleaching and was quite stable in an aqueous solution. Additionally, a LabVIEW-based GC-detection system was developed to achieve the practical application of the sensing film. In summary, the Pt/TE-MTS film exhibited high sensitivity in detecting the GC with excellent reproducibility, which is of high value in applications.

Individually Tunable Energy Levels of Oligomers Based on N-B←N Units

Opto-electronic properties and device performance of organic semiconductors are mainly determined by energy levels of their frontier molecular orbitals, e.g. lowest unoccupied molecular orbital (ELUMO) and highest occupied molecular orbital (EHOMO) in the ground state, first singlet state (ES1) and first triplet state (ET1) in the excited state. These energy levels are always intricately intertwined. Herein, we report a series of monodisperse oligomers based on double B←N bridged bipyridine (BNBP) units. With the increasing number of repeating units, the oligomers exhibit gradually downshifted ELUMO and nearly unchanged EHOMO due to the different distribution of the frontier molecular orbitals of the oligomers. Moreover, the oligomers exhibit gradually decreasing ES1 and nearly unchanged ET1 because of the different contributions of the charge transfer component in the excited state. This work provides new insight into energy level tuning of organic semiconductors, which is important for high-performance organic opto-electronic devices.

Constructing Organic Phosphorescent Scintillators with Enhanced Triplet Exciton Utilization Through Multi-Mode Radioluminescence for Efficient X-Ray Imaging

The development of organic phosphorescent scintillators with high exciton utilization efficiency has attracted significant attention but remains a difficult challenge because of the inherent spin-forbidden feature of X-ray-induced triplet excitons. Herein, a design strategy is proposed to develop organic phosphorescent scintillators through thermally activated exciton release to convert stabilized spin-forbidden triplet excitons to spin-allowed singlet excitons, which enables singlet exciton-dominated multi-mode emission simultaneously from the lowest singlet, triplet, and stabilized triplet states. The resultant scintillators demonstrate a maximum photoluminescence efficiency of 65.8% and a minimum X-ray radiation detection limit of 110 nGy s-1; this allows efficient radiography imaging with a spatial resolution of ≈10.0 lp mm-1. This study advances the fundamental understanding of exciton dynamics under X-ray excitation, significantly broadening the practical use of phosphorescent materials for safety-critical industries and medical diagnostics.

Acceptor Copolymerized Axially Chiral Conjugated Polymers with TADF Properties for Efficient Circularly Polarized Electroluminescence

Chiral conjugated polymer has promoted the development of the efficient circularly polarized electroluminescence (CPEL) device, nevertheless, it remains a challenge to develop chiral polymers with high electroluminescence performance. Herein, by the acceptor copolymerization of axially chiral biphenyl emitting skeleton and benzophenone, a pair of axially chiral conjugated polymers namely R-PAC and S-PAC are synthesized. The target polymers exhibit obvious thermally activated delayed fluorescence (TADF) activities with high photoluminescence quantum yields of 81%. Moreover, the chiral polymers display significant circularly polarized luminescence features, with luminescence dissymmetry factor (|glum|) of nearly 3 × 10-3. By using the chiral polymers as emitters, the corresponding circularly polarized organic light-emitting diodes (CP-OLEDs) exhibit efficient CPEL signals with electroluminescence dissymmetry factor |gEL| of 3.4 × 10-3 and high maximum external quantum efficiency (EQEmax) of 17.8%. Notably, considering both EQEmax and |gEL| comprehensively, the device performance of R-PAC and S-PAC is the best among all the reported CP-OLEDs with chiral conjugated polymers as emitters. This work provides a facile approach to constructing chiral conjugated TADF polymers and discloses the potential of axially chiral conjugated luminescent skeletons in architecting high-performance CP-OLEDs.

Enhancing Reverse Intersystem Crossing in Triptycene-TADF Emitters: Theoretical Insights into Reorganization Energy and Heavy Atom Effects

Thermally activated delayed fluorescence (TADF) emitters based on the triptycene skeleton demonstrate exceptional performance, superior stability, and low efficiency roll-off. Understanding the interplay between the luminescent properties of triptycene-TADF molecules and their assembly environments, along with their excited-state characteristics, necessitates a comprehensive theoretical exploration. Herein, we predict the photophysical properties of triptycene-TADF molecules in a thin film environment using the quantum mechanics/molecular mechanics method and quantify their substantial dependency on the heavy atom effects and reorganization energies using the Marcus-Levich theory. Our calculated photophysical properties for two recently reported molecules closely align with experimental values. We design three novel triptycene-TADF molecules by incorporating chalcogen elements (O, S, and Se) to modify the acceptor units. These newly designed molecules exhibit reduced reorganization energies and enhanced reverse intersystem crossing (RISC) rates. The heavy atom effect amplifies spin-orbit coupling, thereby facilitating the RISC process, particularly at a remarkably high rate of ∼109 s-1.

Transforming Thermally Activated Delayed Fluorescence to Room-Temperature Phosphorescence through Modulation of the Donor in Charge-Transfer Cocrystals

A cocrystallization strategy is used through incorporation of 1,2,4,5-tetracyanobenzene (TCNB) as an acceptor with halogen-substituent thioxanthone (TX) derivatives as donors. The resulting cocrystals TT-R (R = H, F, Cl, Br, or I) transform the thermally activated delayed fluorescence emission in the TT-H, TT-F, and TT-Cl cocrystals to room-temperature phosphorescence in the TT-Br and TT-I cocrystals. Definite crystal packing structures demonstrate a 1:1 alternative donor-acceptor stacking in the TT-H cocrystal, a 2:1 alternative donor-acceptor stacking in the TT-F and TT-Cl cocrystals, and a separate stacking of donor and acceptor in the TT-Br and TT-I cocrystals. A transformation law can be revealed that with an increase in atomic number from H, F, Cl, Br, to I, the cocrystals show the structural transformation of the number of aggregated TX-R molecules from monomers to dimers and finally to multimers. This work will facilitate an understanding of the effect of halogen substituents on the crystal packing structure and luminescence properties in the cocrystals.

Perspectives on development of optoelectronic materials in artificial intelligence age

Optoelectronic devices, such as light-emitting diodes, have been demonstrated as one of the most demanded forthcoming display and lighting technologies because of their low cost, low power consumption, high brightness, and high contrast. The improvement of device performance relies on advances in precisely designing novelty functional materials, including light-emitting materials, hosts, hole/electron transport materials, and yet which is a time-consuming, laborious and resource-intensive task. Recently, machine learning (ML) has shown great prospects to accelerate material discovery and property enhancement. This review will summarize the workflow of ML in optoelectronic materials discovery, including data collection, feature engineering, model selection, model evaluation and model application. We highlight multiple recent applications of machine-learned potentials in various optoelectronic functional materials, ranging from semiconductor quantum dots (QDs) or perovskite QDs, organic molecules to carbon-based nanomaterials. We furthermore discuss the current challenges to fully realize the potential of ML-assisted materials design for optoelectronics applications. It is anticipated that this review will provide critical insights to inspire new exciting discoveries on ML-guided of high-performance optoelectronic devices with a combined effort from different disciplines.

Heavy Atom Effect in Halogenated mCP and Its Influence on the Efficiency of the Thermally Activated Delayed Fluorescence of Dopant Molecules

In this study, we explore the impact of halogen functionalization on the photophysical properties of the commonly used organic light-emitting diode (OLED) host material, 1,3-bis(N-carbazolyl)benzene (mCP). Derivatives with different numbers and types of halogen substituents on mCP were synthesized. By measuring steady-state and transient photoluminescence at 6 K, we study the impact of the type, number, and position of the halogens on the intersystem crossing and phosphorescence rates of the compounds. In particular, the functionalization of mCP with 5 bromine atoms results in a significant increase of the intersystem crossing rate by a factor of 300 to a value of (1.5 ± 0.1) × 1010 s-1, and the phosphorescence rate increases by 2 orders of magnitude. We find that the singlet radiative decay rate is not significantly modified in any of the studied compounds. In the second part of the paper, we describe the influence of these compounds on the reverse intersystem crossing of the 7,10-bis(4-(diphenylamino)phenyl)-2,3-dicyanopyrazino-phenanthrene (TPA-DCPP), a TADF guest, via the external heavy atom effect. Their use results in an increase of the reverse intersystem crossing (RISC) rate from (8.1 ± 0.8) × 103 s-1 for mCP to (2.7 ± 0.1) × 104 s-1 for mCP with 5 bromine atoms. The effect is even more pronounced for the mCP analogue containing a single iodine atom, which gives a RISC rate of (3.3 ± 0.1) × 104 s-1. Time-dependent DFT calculations reveal the importance of the use of long-range corrected functionals to predict the effect of halogenation on the optical properties of the mCP, and the relativistic approximation (ZORA) is used to provide insight into the strength of the spin-orbit coupling matrix element between the lowest-lying excited singlet and triplet states in the different mCP compounds.

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

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