Singlet-triplet Energy Gap Research Articles

Exploring the Theoretical Foundations of Thermally Activated Delayed Fluorescence (TADF) Emission: A Comprehensive TD-DFT Study on Phenothiazine Systems.

This study conducts a thorough theoretical investigation of Thermally Activated Delayed Fluorescence (TADF) in phenothiazine-based systems, examining ten molecular configurations recognized experimentally as TADF-active. Employing Time-Dependent Density Functional Theory (TD-DFT), our analysis spans the investigation of singlet-triplet energy gaps (ΔEST), spin-orbit coupling, and excitation characteristics using Multiwfn. This approach not only validates the adherence to El Sayed's rule across these systems but also provides a detailed understanding of charge transfer dynamics, as visualized through heat maps. A significant aspect of our study is the exploration of different oxidation states of sulfur and site substitutions on phenothiazine. This systematic variation aims to identify additional TADF-active compounds, drawing parallels with properties characterizing other known TADF emitters. Our investigation into Reverse Intersystem Crossing (rISC) rates and the analysis of dihedral angles in relation to ΔEST values offer nuanced insights into the TADF behaviours of these molecules. By integrating rigorous computational analysis with practical implications, we provide a foundational understanding that enhances the design and optimization of phenothiazine-based materials for optoelectronic applications. This work not only advances our theoretical understanding of TADF in phenothiazine derivatives but also serves as a guide for experimentalists and industry professionals in the strategic design of new TADF materials.

Recent Progress in Phenoxazine-Based Thermally Activated Delayed Fluorescent Compounds and Their Full-Color Organic Light-Emitting Diodes.

Third-generation organic light-emitting diodes (OLEDs) based on metal-free thermally activated delayed fluorescent (TADF) materials have sparked tremendous interest in the last decade due to their nearly 100% exciton utilization efficiency, which can address the low-efficiency issue of the first-generation fluorescent emitters and the high-cost issue of the second-generation organometallic phosphorescent emitters. Construction of efficient and stable TADF-OLEDs requires utilizing TADF materials with a narrow singlet-triplet energy gap (ΔEST), high photoluminescence quantum yield (PLQY) and short TADF lifetime. A small ΔEST is necessary for an efficient reverse intersystem crossing (RISC) process, which can be achieved through theeffective spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). TADF emitters have been generally designed as intramolecular charge transfer (ICT) molecules with highly twisted donor-acceptor (D-A) molecular architectures. A wide variety of combinations of electron donors and acceptors have been explored. In this review, we shall focus on recent progress in organic TADF molecules incorporating strong electron-donor phenoxazine moiety and their application as emitting layer (EML) in OLEDs.

Introduction of 5-membered boron-oxygen-heterocyclic carbene and its Si and Ge homologs and the M–L bonding analysis in their complexes with coinage metals: A theoretical study

Replacing an oxygen atom with one of the boron atoms in a 5-membered B-Heterocyclic carbene and its Si and Ge congeners (BHEs; E = C, Si, Ge) creates Boron-Oxygen-Heterocyclic divalents (BOHEs) which may have potential uses as organometallic ligands. In addition to a survey on their singlet–triplet energy gap (ΔET-S) for the stability concerns, the nature of the E–M bonds in XBOHE–MCl complexes (M = CuI, AgI, AuI; E = C, Si, Ge; X = F, Cl, Br) is theoretically investigated using the natural bond orbital (NBO), energy decomposition analysis (EDA), and extended transition state natural orbital for chemical valence (ETS-NOCV) analysis. The outcomes show interesting stabilities of singlet/triplet configurations in carbenes, silylenes, and germylenes. Also, the results exhibit that the E–M bonds in the complexes have a higher electrostatic character.

Extended theoretical modeling of reverse intersystem crossing for thermally activated delayed fluorescence materials.

Thermally activated delayed fluorescence (TADF) materials and multi-resonant (MR) variants are promising organic emitters that can achieve an internal electroluminescence quantum efficiency of ~100%. The reverse intersystem crossing (RISC) is key for harnessing triplet energies for fluorescence. Theoretical modeling is thus crucial to estimate its rate constant (kRISC) for material development. Here, we present a comprehensive assessment of the theory for simulating the RISC of MR-TADF molecules within a perturbative excited-state dynamics framework. Our extended rate formula reveals the importance of the concerted effects of nonadiabatic spin-vibronic coupling and vibrationally induced spin-orbital couplings in reliably determining kRISC of MR-TADF molecules. The excited singlet-triplet energy gap is another factor influencing kRISC. We present a scheme for gap estimation using experimental Arrhenius plots of kRISC. Erroneous behavior caused by approximations in Marcus theory is elucidated by testing 121 MR-TADF molecules. Our extended modeling offers in-depth descriptions of kRISC.

Significant room-temperature phosphorescence enhancement induced by matrix complexes

Incorporating phosphors into polymer matrix is a relatively mature method to develop organic room-temperature phosphorescent (RTP) materials but still needs to be further explored because of limited investigations about the effect of different matrix on the RTP properties of the same guest molecule. Colorful RTP (from blue to red, with τafterglow up to 1736 ms) in carboxymethyl chitosan (CC) systems was achieved in this work. To improve the RTP performance, quaternary chitosan (QC) was introduced to build a binary matrix system, which extended the lifetime by 200 ms and increased the phosphorescence quantum yield to 3.51 times. Systematic characterizations and theoretical calculations indicated that the RTP materials obtained from binary matrix had higher ion group density and stronger dipole–dipole interactions, which could narrow the singlet–triplet energy gap and promote the triplet excitons generation. Compared to single matrix, binary matrix could also provide more crosslinked networks to better stabilize triplet excitons, thus synergistically enhancing the RTP. Finally, based on the good biocompatibility and antibacterial properties of chitosan and phosphorescent emission, the films were expected to be used for monitoring the integrity of the package and multiple antibacterial coating films.

Microemulsion-Assisted Self-Assembly of Indium Porphyrin Photosensitizers with Enhanced Photodynamic Therapy.

Designing and constructing supramolecular photosensitizer nanosystems with highly efficient photodynamic therapy (PDT) is vital in the nanomedical field. Despite recent advances in forming well-defined superstructures, the relationship between molecular arrangement in nanostructures and photodynamic properties has rarely been involved, which is crucial for developing stable photosensitizers for highly efficient PDT. In this work, through a microemulsion-assisted self-assembly approach, indium porphyrin (InTPP) was used to fabricate a series of morphology-controlled self-assemblies, including nanorods, nanospheres, nanoplates, and nanoparticles. They possessed structure-dependent 1O2 generation efficiency. Compared with the other three nanostructures, InTPP nanorods featuring strong π-π stacking, J-aggregation, and high crystallinity proved to be much more efficient at singlet oxygen (1O2) production. Also, theoretical modeling and photophysical experiments verified that the intermolecular π-π stacking in the nanorods could cause a decreased singlet-triplet energy gap (ΔEST) compared with the monomer. This played a key role in enhancing intersystem crossing and facilitating 1O2 generation. Both in vitro and in vivo experiments demonstrated that the InTPP nanorods could trigger cell apoptosis and tumor ablation upon laser irradiation (635 nm, 0.1 W/cm2) and exhibited negligible dark toxicity and high phototoxicity. Thus, the supramolecular self-assembly strategy provides an avenue for designing high-performance photosensitizer nanosystems for photodynamic therapy and beyond.

Electronic Properties of Graphene Nano-Parallelograms: A Thermally Assisted Occupation DFT Computational Study.

In this computational study, we investigate the electronic properties of zigzag graphene nano-parallelograms (GNPs), which are parallelogram-shaped graphene nanoribbons of various widths and lengths, using thermally assisted occupation density functional theory (TAO-DFT). Our calculations revealed a monotonic decrease in the singlet-triplet energy gap as the GNP length increased. The GNPs possessed singlet ground states for all the cases examined. With the increase of GNP length, the vertical ionization potential and fundamental gap decreased monotonically, while the vertical electron affinity increased monotonically. Some of the GNPs studied were found to possess fundamental gaps in the range of 1-3 eV, lying in the ideal region relevant to solar energy applications. Besides, as the GNP length increased, the symmetrized von Neumann entropy increased monotonically, denoting an increase in the degree of the multi-reference character associated with the ground state GNPs. The occupation numbers and real-space representation of active orbitals indicated that there was a transition from the nonradical nature of the shorter GNPs to the increasing polyradical nature of the longer GNPs. In addition, the edge/corner localization of the active orbitals was found for the wider and longer GNPs.

Syntheses and coordination of linear and macrocyclic pentapyrroles possessing a fused moiety: Confusion modulated structural diversity

Porphyrinoids have attracted wide interest because of their vital roles in biological processes and fascinating structural and functional characters. In this work, a doubly N-confused pentapyrrane precursor N2C–P5 has been synthesized with terminal pyrrole ring A and middle pyrrole C incorporated in the N-confused mode. On this basis, pentapyrrin 1 and sapphyrin 2 have been synthesized by oxidizing N2C–P5 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Both 1 and 2 possess a [5.5.5]-tricyclic fused ring involving pyrrole B and the neighboring confused pyrrole C. Meanwhile, the formation of 2 involves ring closure between the terminal rings A and E. Notably, 2 exists as a diradical showing a half-field signal in the electron paramagnetic resonance (EPR) spectrum and a singlet ground state with a negative singlet-triplet energy gap of −0.50 kcal/mol. Excellent thermal stability has been observed for 2, which may be related to its large π-conjugated framework involving the fused structure. On this basis, 1 and 2 were treated with Cu(acac)2 to afford Cu(II) complexes 3 and 4, respectively. As a result, an acetylacetonyl moiety is attached at the terminal pyrrole ring E to afford an NNNO coordination environment for complex 3, and the sapphyrin macrocycle of 2 has been cleaved between rings A and B to afford a fused pentapyrrin ligand, furnishing an NNNN coordination environment for complex 4, in which a pentafluorophenyl-ketal group generated during the ring cleavage reaction is attached to ring A, and it can be hydrolyzed to the corresponding pentafluorobenzoyl substituent to afford complex 5. Compared with free bases 1 and 2, complexes 3−5 show more intense low-energy near-infrared (NIR) absorption bands at ca. 791, 926 and 907 nm, respectively. This work provides an effective approach for the syntheses of linear and macrocyclic oligopyrroles with intriguing structures and properties by oxidizing oligopyrranes with precisely designed numbers and positions of N-confused pyrroles.

Difluorenoheteroles: topological control of π conjugation in diradicaloids and mixed-valence radical ions.

Two families of difluorenoheterole diradicaloids were synthesized, featuring isomeric ring systems with distinct conjugation topologies. The two types of difluorenoheteroles contain, respectively, a Chichibabin-like motif (CH) and a newly introduced heteroatom-linked triphenylmethyl dyad (TD-X). Combined experimental and theoretical investigations show that the TD-X systems have reduced quinoidal character but the interaction between formal spin centers is sufficiently strong to ensure a singlet ground state. The singlet-triplet energy gaps in the TD-X difluorenoheteroles are strongly affected by the heterocyclic ring, with values of -4.3 and -0.7 kcal mol-1 determined for the pyrrole- and thiophene-containing analogues, respectively. In cyclic voltammetry experiments, the TD-X systems show diminished energy gaps and superior reversibility in comparison with their CH counterparts. The radical anions and cations obtained from these diradicaloids show extremely red-shifted bands, occasionally with λ max > 3500 nm. Computational studies show that some of these ions adopt distonic structures and may be characterized as class-II mixed-valence species.

Computational design of boron-free triangular molecules with inverted singlet-triplet energy gap.

A novel, computationally designed, class of triangular-shape organic molecules with an inverted singlet-triplet (IST) energy gap is investigated with ab initio electronic structure methods. The considered molecular systems are cyclic oligomers and their common feature is electronic conjugation along the molecular rim. Vertical excitation energies from the electronic ground state to the lowest singlet and triplet excited states were computed, as well as vertical emission energies from these states to the ground state. The results underscore the significance of optimizing excited-state geometries to accurately describe the optoelectronic properties of IST molecules, in particular with respect to their application in OLEDs.

Machine-learning-assisted performance improvements for multi-resonance thermally activated delayed fluorescence molecules.

With favorable colour purity, multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules exhibit enormous potential in high-definition displays. Due to the relatively small chemical space of MR-TADF molecules, it is challenging to improve molecular performance through domain-specific expertise alone. To address this problem, we focused on optimizing the classic molecule, DABNA-1, using machine learning (ML). Molecular morphing operations were initially employed to generate the adjacent chemical space of DABNA-1. Subsequently, a machine learning model was trained with a limited database and used to predict the properties throughout the generated chemical space. It was confirmed that the top 100 molecules suggested by machine learning present excellent electronic structures, characterized by small reorganization energy and singlet-triplet energy gaps. Our results indicate that the improvement in electronic structures can be elucidated through the view of the molecular orbital (MO). The results also reveal that the top 5 molecules present weaker vibronic peaks of the emission spectrum, demonstrating higher colour purity when compared to DABNA-1. Notably, the M2 molecule presents a high RISC rate, indicating its promising future as a high-efficiency MR-TADF molecule. Our machine-learning-assisted approach facilitates the rapid optimization of classical molecules, addressing a crucial requirement within the organic optoelectronic materials community.

Is a small singlet–triplet energy gap a guarantee of TADF performance in MR-TADF compounds? Impact of the triplet manifold energy splitting

A design strategy for efficient MR-TADF emitters needs (i) avoiding aggregation by inducing steric hindrance at peripheral strategic positions, and (ii) combining lateral groups of higher donor character as compared to the central core.

Superstructures of copper nanoclusters as NIR TADF emitters: solvent-dependent optical and morphological modulation.

Herein, we report 2-mercaptopyridine-templated copper nanoclusters (CuNCs) which display near infra-red (NIR) emission, both in the solid and colloidal states. Interestingly, the NIR emission can be modulated to orange emission by preparing the CuNCs in a mixed solvent system of chloroform and methanol instead of water. The drastic change in the photo-physical properties of the CuNCs when prepared in two different solvent systems is accompanied by a unique morphological tuning. Further studies reveal that the strong NIR emission is the result of thermally activated delayed fluorescence (TADF) which is confirmed by the long excited state lifetime (∼4 μs at room temperature), time resolved emission spectroscopy (TRES) measurements, temperature-dependent photoluminescence studies, temperature-dependent lifetime studies, and excitation-transmittance dependent TRES intensity measurements. The CuNCs exhibit an exceptionally small singlet-triplet energy gap of 58.2 meV, indicating a highly efficient TADF in the system. Moreover, the solvent-dependent morphological tuning of the nanocluster superstructures rendering a drastic change in the photo-physical signatures is the consequence of different ΔE(S1-T1) values for the CuNCs in different solvent environments. Further findings corroborate that the electronic structure of the surface ligands can also help us to tune the ΔE(S1-T1) energy gap for these nanoclusters.

Engineering high-brightness and long-lived organic room-temperature phosphorescence via systematic molecular design.

We report a systematic molecular design in BF2bdk-based afterglow emitters with photoluminescence quantum yields up to 46.3% and lifetimes around 1 s. Suitable excited-state types, diverse excited state species, relatively small singlet-triplet energy gaps and strong dipole-dipole interactions are critical in determining the afterglow properties.

Heavy-atom-free triplet benzothiophene-fused BODIPY derivatives for lipid droplet-specific biomaging and photodynamic therapy.

The twist fusion of a benzothiophene group and the introduction of a 4-methyloxystyryl donor group to the BODIPY core resulted in large spin-orbit coupling values and smaller singlet-triplet energy gaps for the novel infrared absorbed photosensitizers named BSBDP. They show a high reactive oxygen species efficiency exceeding 69% and a fluorescence quantum yield of 23% and are successfully applied in imaging-guided photodynamic therapy in vitro and in vivo.

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.

Modulation of ΔEST and room temperature phosphorescence in carbazole derivatives.

A simple strategy to modulate the singlet-triplet energy gap in 3,6-diaryl-N-acetophenylcarbazole derivatives is developed. Different substituents significantly influenced ΔEST, which is correlated for the first time with the singlet-triplet state dipole moments. Phosphorescence at ambient conditions in powder form (τ is up to 248 μs) and ultra long lifetime (up to 2.2 s) at 77 K is observed.

Utilizing weakly donor-acceptor ternary π-conjugated architecture to achieve single-component white luminescence and stimulus-responsive room-temperature phosphorescence.

Purely organic room-temperature phosphorescence (RTP) has garnered substantial attention for its delayed emission, environmental sensitivity, and potential diverse applications. However, the quest for high-performance RTP materials has always been a challenge. In this study, we introduce novel weakly donor-acceptor (D-A) ternary π-conjugated architecture to construct an efficient RTP system. The strategy utilizes synergistic effects of the analogous El-Sayed rule, halogen-free heavy-atom effect, reduction of the singlet-triplet energy gap, and manipulation of flexible molecular conformation. A remarkable enhancement in the phosphorescence-to-fluorescence ratio was achieved, elevating from 0.4 in carbazole to 35.2 in DBTDBTCZ. Furthermore, the RTP system demonstrates single-component white luminescence, yielding warm and cool white colors. Intriguingly, we unveil the novel position-dependent heavy-atom effects, discerningly promoting intersystem crossing or phosphorescence decay. Benefiting from efficient RTP, multifunctional applications of real-time humidity monitoring, oxygen sensing, anti-counterfeiting labeling, and white lighting are demonstrated.

Ultralong afterglow of heavy-atom-free carbon dots with a phosphorescence lifetime of up to 3.7 s for encryption and fingerprinting description.

Metal-free room-temperature phosphorescent (RTP) materials with changeable colors have attracted great attention in anti-counterfeiting information encryption. Most ultralong-lifetime RTP (URTP) luminophores are traditionally obtained through heavy atom effects via enhancing the spin-orbit coupling efficiency. Here, we report the self-assembly of URTP carbon dots (CDs) using diphenylaminourea as the precursor through a thermal-evaporation assisted covalent-binding approach in the presence of boric acid (BA). The BA-functionalized diphenylaminourea-derived CDs (denoted as D-CDs1.5/BA composites) show a rigid network structure with B-C linkages connected to the surface of the CDs, which can effectively suppress the free vibration of CDs to promote intersystem crossover, finally resulting in an excellent URTP afterglow performance. They feature a low singlet-triplet energy gap and reduced nonradiative attenuation properties. As a result, the D-CDs1.5/BA composites exhibit a bifunctional fluorescence/phosphorescence performance with a high phosphorescence quantum efficiency (12.67%) and an ultra-long green afterglow phosphorescence lifetime of up to 3.66 s. A high-level information encryption and fingerprinting description based on the URTP D-CDs1.5/BA composites were then investigated. This work contributes to the feasible design and preparation of novel URTP CD materials with both ultra-long afterglow and a high phosphorescence efficiency, making them promising candidates for advanced anti-counterfeiting applications.

Detection of diheptacendiyl diradical intermediate in the cycloreversion of diheptacene to heptacene.

Cycloreversion of diheptacenes, the covalently bound dimers of heptacene, in the solid state produces heptacene. In addition, diheptacendiyl diradical can be detected by ESR spectroscopy. The diradical has a small singlet-triplet energy gap of -0.02 kJ mol-1 (-4.8 × 10-3 kcal mol-1) in favor of the singlet state and is persistent in solid heptacene.