Open questions on the photophysics of thermally activated delayed fluorescence

Singlet-Triplet Inversion

The inversion of singlet and triplet states is a rare phenomenon, where, in opposition to Hund's first rule, singlet electronic states are stabilized relative to their triplet counterparts. The recent discovery of organic molecules exhibiting this inversion presents exciting new technological opportunities, such as addressing stability issues in organic light-emitting diodes (OLEDs). In this review, we describe fundamental molecular properties that can yield singlet-triplet inversion, generally ascribed to a phenomenon known as dynamic spin polarization. We discuss the systems in which singlet-triplet inversion was theoretically proposed, experimentally verified, and first implemented in an OLED device. We highlight key insights from the extensive computational work being carried out to understand the intricacies of these systems. Finally, we consider the outlook for future inverted singlet-triplet (IST) emitters.

Can ΔSCF and ROKS DFT-Based Methods Predict the Inversion of the Singlet-Triplet Gap in Organic Molecules?

Inverted singlet-triplet gap systems (INVEST) have emerged as an intriguing class of materials with potential applications as emitters in Organic Light Emitting Diodes (OLEDs). Indeed, this type of material exhibits a negative singlet-triplet energy gap (ΔEST), i.e., an inversion of the lowest singlet (S1) and triplet (T1) excited states, that goes against Hund's rule. In this study, the ΔEST of a set of 15 INVEST molecules has been computed within the framework of Restricted Open-Shell Kohn-Sham (ROKS) and Delta Self-Consistent Field (ΔSCF) methods and the results were benchmarked against wavefunction-based calculations performed at the EOM-CCSD, NEVPT2, and SCS-CC2 levels. We find that ROKS always (and wrongly) predicts a positive ΔEST with global hybrid, meta-GGA, and long-range corrected functionals and that this is almost functional-independent. We also show that the only way to obtain an inverted gap was to resort to double hybrid functionals. In contrast, using the above-mentioned functionals, ΔSCF usually gives a negative ΔEST, although the results are largely functional-dependent. Overall, applying a ΔSCF method based on the PBE0 functional provides the lowest MSD and MAD with respect to the EOM-CCSD results. We further show that the singlet-triplet inversion is driven by different degrees of orbital relaxation in the singlet versus triplet state and that this is well captured by ΔSCF calculations. As a matter of fact, this orbital relaxation in ΔSCF somehow mimics the involvement of double and higher-order excitations in EOM-CCSD, which leads to a difference in spatial localization of the α and β spins, and thus introduces (local) spin polarization effects sourcing the negative ΔEST. However, care should be taken when using the ΔSCF method to screen materials with potential INVEST behavior in view of their limited quantitative correlation with reference EOM-CCSD results on the molecular data basis used here.

Fluorescence lifetime imaging in drug delivery research

Once an exotic add-on to fluorescence microscopy for life science research, fluorescence lifetime imaging (FLIm) has become a powerful and increasingly utilised technique owing to its self-calibration nature, which affords superior quantification over conventional steady-state fluorescence imaging. This review focuses on the state-of-the-art implementation of FLIm related to the formulation, release, dosage, and mechanism of action of drugs aimed for innovative diagnostics and therapy. Quantitative measurements using FLIm have appeared instrumental for encapsulated drug delivery design, pharmacokinetics and pharmacodynamics, pathological investigations, early disease diagnosis, and evaluation of therapeutic efficacy. Attention is paid to the latest advances in lifetime-engineered nanomaterials and practical instrumentation, which begin to show preclinical and clinical translation potential beyond in vitro samples of cells and tissues. Finally, major challenges that need to be overcome in order to facilitate future perspectives are discussed.

Synthesis and characterization of machine learning designed TADF molecules

In this study, we present a novel approach to the development of thermally activated delayed fluorescence (TADF) molecules with potentials for organic light-emitting diode (OLED) applications, leveraging machine learning (ML) algorithms to guide the materials design process. Recognizing the imperative for high-efficiency, low-cost emissive materials, we integrated ML driven models with experimental characterization to expedite the discovery of TADF compounds. Initially, a database of ML-designed TADF molecules was employed, with samples being approved to possess optimized photophysical properties. The proposed molecules were synthesized using palladium-catalyzed coupling reactions. Subsequent characterization of these molecules utilized a suite of analytical methods, including nuclear magnetic resonance (NMR) spectroscopy, photoluminescence (PL) spectroscopy, and transient PL decay etc., to confirm their structural integrity and evaluate their performance metrics. The photophysical analysis revealed notable emission efficiencies and significant delayed fluorescence characteristics in solution phases, underscoring the potential of ML-designed TADF molecules. Theoretical validations, through quantum chemical calculations, corroborated the experimental findings, demonstrating the predictive power of our ML models. This interdisciplinary approach not only accelerates the pace of TADF molecule development but also provides a scalable framework for future material innovation especially in the OLED research field.

Achieving Tunable Monomeric TADF and Aggregated RTP via Molecular Stacking

Organic emitters with both thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) have attracted widespread interest for their intriguing luminescent properties. Herein, a series of triphenylamine-substituted isoquinoline derivatives possessing monomeric TADF and aggregated RTP properties are reported. As the molecules exhibited various forms of π-π and charge transfer (CT) stacking with different intensities, inter/intramolecular CT can be meticulously modulated to achieve tunable TADF-RTP. Aggregated phosphorescence originates from intermolecular CT initiated by CT dimers, whereas monomeric TADF is facilitated by intramolecular CT enhanced by π-π dimers. Leveraging the properties of these molecules, luminescent materials with tunable TADF-RTP properties in multistates are obtained by molecular substitution position alignment, dealing with different solvents, grinding, adjusting concentration, changing polymer matrix, photoactivation, and heat treatment. This work is critical for a deeper understanding of construction and regulation of the TADF-RTP dual-channel emission, enabling the development of advanced optoelectronic devices with tailored emission properties.

Singlet-Triplet Inversion in Triangular Boron Carbon Nitrides

The discovery of singlet-triplet (ST) inversion in some π-conjugated triangle-shaped boron carbon nitrides is a remarkable breakthrough that defies Hund's first rule. Deeply rooted in strong electron-electron interactions, ST inversion has garnered significant interest due to its potential to revolutionize triplet harvesting in organic LEDs. Using the well-established Pariser-Parr-Pople model for correlated electrons in π-conjugated systems, we employ a combination of CISDT and restricted active space configuration interaction calculations to investigate the photophysics of several triangular boron carbon nitrides. Our findings reveal that ST inversion in these systems is primarily driven by a network of alternating electron-donor and electron-acceptor groups in the molecular rim, rather than by the triangular molecular structure itself.

Steric and Electronic Influence of Excited-State Decay in Cu(I) MLCT Chromophores

ConspectusFor the past 11 years, a dedicated effort in our research group focused on fundamentally advancing the photophysical properties of cuprous bis-phenanthroline-based metal-to-ligand charge transfer (MLCT) excited states. We rationalized that, by gaining control over the numerous factors limiting the more widespread use of CuI MLCT photosensitizers, they would be readily adopted in numerous light-activated applications given the earth-abundance of copper and the extensive library of 1,10-phenanthrolines developed over the last century. Significant progress has been achieved by recognizing valuable structure-property concepts developed by other researchers in tandem with detailed ultrafast and conventional time-scale investigations, in-silico-inspired molecular designs to predict spectroscopic properties, and applying novel synthetic methodologies. Ultimately, we achieved a plateau in exerting cooperative steric influence to control CuI MLCT excited state decay. This led to combining sterics with π-conjugation and/or inductive electronic effects to further exert control over molecular photophysical properties. The lessons gleaned from our studies of homoleptic complexes were recently extended to heteroleptic bis(phenanthrolines) featuring enhanced visible light absorption properties and long-lived room-temperature photoluminescence. This Account navigates the reader through our intellectual journey of decision-making, molecular and experimental design, and data interpretation in parallel with appropriate background information related to the quantitative characterization of molecular photophysics using CuI MLCT chromophores as prototypical examples.Initially, CuI MLCT excited states, their energetics, and relevant structural conformation changes implicated in their photophysical decay processes are described. This is followed by a discussion of the literature that motivated our research in this area. This led to our first molecular design in 2013, achieving a 7-fold increase in excited state lifetime relative to the current state-of-the-art. The lifetime and photophysical property enhancement resulted from using 2,9-branched alkyl groups in conjunction with flanking 3,8-methyl substituents, a strategy we adapted from the McMillin group, which was initially described in the late 1990s. Applications of this newly conceived chromophore are presented in solar hydrogen-producing photocatalysis, photochemical upconversion, and photosensitization of [4 + 4] anthracene dimerization of potential interest in thermal storage of solar energy in metastable intermediates. Ultrafast transient absorption and fluorescence upconversion spectroscopic characterization of this and related CuI molecules inform the resultant photophysical properties and vice versa, so the most comprehensive structure-property understanding becomes realized when these experimental tools are collectively utilized to investigate the same series of molecules. Computationally guided structural designs generated newly conceived molecules featuring visible light-harvesting and 2,9-cycloalkane substituted complexes. The latter eventually produced record-setting excited state lifetimes in molecules leveraging both cooperative steric influence and electronic inductive effects. Using photoluminescence data from structurally homologous CuI MLCT excited states collected over 44 years, an energy gap correlation successfully modeled the data spanning a 0.3 eV emission energy range. Finally, a new research direction is revealed detailing structure-photophysical property relationships in heteroleptic CuI phenanthroline chromophores that are photoluminescent at room temperature.

Conformational Control of Donor-Acceptor Molecules Using Non-covalent Interactions

Controlling the architecture of organic molecules is an important aspect in tuning the functional properties of components in organic electronics. For purely organic thermally activated delayed fluorescence (TADF) molecules, design is focused upon orthogonality orientated donor and acceptor units. In these systems, the rotational dynamics around the donor and acceptor bond has been shown to be critical for activating TADF; however, too much conformational freedom can increase the non-radiative rate, leading to a large energy dispersion of the emitting states and conformers, which do not exhibit TADF. To date, control of the motion around the D-A bond has focused upon steric hindrance. In this work, we computationally investigate eight proposed donor-acceptor molecules, exhibiting a B-N bond between the donor and acceptor. We compare the effect of steric hindrance and noncovalent interactions, achieved using oxygen (sulfur) boron heteroatom interactions, in exerting fine conformational control of the excited state dynamics. This work reveals the potential for judiciously chosen noncovalent interactions to strongly influence the functional properties of TADF emitters, including the accessible conformers and the energy dispersion associated with the charge transfer states.

Confinement of Sustainable Carbon Dots Results in Long Afterglow Emitters and Photocatalyst for Radical Photopolymerization

Sustainable carbon dots based on cellulose, particularly carboxymethyl cellulose carbon dots (CMCCDs), were confined in an inorganic network resulting in CMCCDs@SiO2. This resulted in a material exhibiting long afterglow covering a time frame of several seconds also under air. Temperature-dependent emission spectra gave information on thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) while photocurrent experiments provided a deeper understanding of charge availability in the dark period, and therefore, its availability on the photocatalyst surface. The photo-ATRP initiator, ethyl α-bromophenylacetate (EBPA), quenched the emission from the millisecond to the nanosecond time frame indicating participation of the triplet state in photoinduced electron transfer (PET). Both free radical and controlled radical polymerization based on photo-ATRP protocol worked successfully. Metal-free photo-ATRP resulted in chain extendable macroinitiators based on a reductive mechanism with either MMA or in combination with styrene. Addition of 9 ppm Cu2+ resulted in Mw/Mn of 1.4 while an increase to 72 ppm improved uniformity of the polymers; that is Mw/Mn=1.03. Complementary experiments with kerria laca carbon dots confined materials, namely KCDs@SiO2, provided similar results. Deposition of Cu2+ (9 ppm) on the photocatalyst surface explains better uniformity of the polymers formed in the ATRP protocol.

Modulating the Photophysical Properties of Twisted Donor-Acceptor-Donor π-Conjugated Molecules: Effect of Heteroatoms, Molecular Conformation, and Molecular Topology

ConspectusModulating the photophysical properties of organic emitters through molecular design is a fundamental endeavor in materials science. A critical aspect of this process is the control of the excited-state energy, which is essential for the development of triplet exciton-harvesting organic emitters, such as those with thermally activated delayed fluorescence and room-temperature phosphorescence. These emitters are pivotal for developing highly efficient organic light-emitting diodes and bioimaging probes. A particularly promising class of these emitters consists of twisted donor-acceptor organic π-conjugated scaffolds. These structures facilitate a spatial separation of the frontier molecular orbitals, which is crucial for achieving a narrow singlet-triplet energy gap. This narrow gap is necessary to overcome the endothermic reverse intersystem crossing process, enhancing the efficiency of thermally activated delayed fluorescence. To precisely modulate the photophysical properties of these emitting materials, it is essential to understand the electronic structures of new donor-acceptor scaffolds, especially those influenced by heteroatoms, as well as their conformations and topologies. This understanding not only improves the efficiency of these emitters but also expands their potential applications in advance technologies.In 2014, the Takeda group made a significant breakthrough by discovering a novel method for synthesizing U-shaped diazaacenes (dibenzo[a,j]phenazine) through an oxidative skeletal rearrangement of 1,1'-binaphthalene-2,2'-diamines. This class of compounds is typically challenging to synthesize using conventional organic reactions. The resulting unique geometric and electronic structure of U-shaped diazaacenes opened new possibilities for photophysical applications. Leveraging the U-shaped structure, photoluminescent properties, and high electron affinity, we developed twisted donor-acceptor-donor compounds. These compounds exhibit efficient thermally activated delayed fluorescence, stimuli-responsive luminochromism, heavy atom-free room-temperature phosphorescence, and anion-responsive red shifts. These innovative emitters have demonstrated significant potential in various practical applications, including organic light-emitting diode devices and advanced sensing systems.In this Account, I summarize our achievements in modulating the photofunctions of dibenzo[a,j]phenazine-cored twisted donor-acceptor-donor compounds by controlling excited-state singlet-triplet energy gaps through conformational regulation. Our comprehensive studies revealed the significant impact of heteroatoms, molecular conformations, and topologies on the photophysics of these compounds. These findings highlight the importance of molecular engineering in tailoring the photophysical properties of organic donor-acceptor π-conjugated materials for specific applications. Our research has demonstrated that incorporating heteroatoms into the molecular framework effectively tunes the electronic properties and, consequently, the photophysical behavior of the compounds. Understanding the influence of heteroatoms, conformational dynamics, and molecular topology on excited-state behavior will open new avenues for next-generation optoelectronic devices and biological technologies. These advancements include ultra-low-power displays, photonic communication, and super-resolution biomedical imaging. Ultimately, our work highlights the potential of strategic molecular design in driving innovation across various fields, paving the way for the development of cutting-edge technologies that leverage the unique properties of organic emitters.

Predicting Emission Wavelengths in Benzobisoxazole-Based OLEDs with Gradient Boosted Ensemble Models

We demonstrate the use of gradient-boosted ensemble models that accurately predict emission wavelengths in benzobis[1,2-d:4,5-d']oxazole (BBO) based fluorescent emitters. We have curated a database of 50 molecules from previously published data by the Jeffries-EL group using density functional theory (DFT) computed ground and excited state features. We consider two machine learning (ML) models based on (i) whole cruciform molecules and (ii) their constituent fragment molecules. Both ML models provide accurate predictions with root-mean-square errors between 30 and 36 nm, competitive with state-of-the-art deep learning models trained on orders of magnitude more molecules, and this accuracy holds even when tested on four new BBO emitters unseen by the models. We also provide an interpretable feature importance analysis and discuss the relevant relationships between DFT and changes in predicted emission wavelength.

Photophysics and photochemistry of thermally activated delayed fluorescence emitters based on the multiple resonance effect: transient optical and electron paramagnetic resonance studies

The photochemistry of two representative thermally activated delayed fluorescence (TADF) emitters based on the multiple resonance effect (MRE) (DABNA-1 and DtBuCzB) was studied. No significant TADF was observed in fluid solution, although the compounds have a long-lived triplet state (ca. 30 μs). We found that these planar boron molecules bind with Lewis bases, e.g., 4-dimethylaminopyridine (DMAP) or an N-heterocyclic carbene (NHC). A new blue-shifted absorption band centered at 368 nm was observed for DtBuCzB upon formation of the adduct; however, the fluorescence of the adduct is the same as that of the free DtBuCzB. We propose that photo-dissociation occurs for the DtBuCzB-DMAP adduct, which is confirmed by femtosecond transient absorption spectra, implying that fluorescence originates from DtBuCzB produced by photo-dissociation; the subsequent in situ re-binding was observed with nanosecdon transient absorption spectroscopy. No photo-dissociation was observed for the NHC adduct. Time-resolved electron paramagnetic resonance (TREPR) spectra show that the triplet states of DABNA-1 and DtBuCzB have similar zero field splitting (ZFS) parameters (D = 1450 MHz). Theoretical studies show that the slow ISC is due to small SOC and weak Herzberg-Teller coupling, although the S1/T1 energy gap is small (0.14 eV), which rationalizes the lack of TADF.

The photochemistry and photophysics of benzoyl-carbazole

Benzoyl-carbazole and its derivatives are considered a platform for exploring processes such as room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF). They have also been reported to exhibit dual emission, but there is a great spectral variability in the relative intensity of the emission bands reported in different studies. To better understand the fundamental photophysical properties, we set to explore BCz and its perfluorinated derivative F5BCz using spectroscopy and quantum chemical simulations. We find that the reported dual fluorescence in solution and in films results from a photochemical process (photo-Fries rearrangement), producing carbazole among other products, explaining the variation in the reported emission spectra. In addition, BCz exhibits solvent dependent TADF, which is explained by the stabilization of the charge transfer S1 state in polar solvents. F5BCz undergoes an efficient photochemical process (Mallory reaction) from its single state to produce highly fluorescent product c-F5BCz, in 40% isolated yield. This photoreactivity also proceeds in films under ambient conditions, which have significant implications on the applications of BCz-based materials for optoelectronic applications.

Shedding light on thermally-activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) is a hot research topic in view of its impressive applications in a wide variety of fields from organic LEDs to photodynamic therapy and metal-free photocatalysis. TADF is a rare and fragile phenomenon that requires a delicate equilibrium between tiny singlet-triplet gaps, sizable spin-orbit couplings, conformational flexibility and a balanced contribution of charge transfer and local excited states. To make the picture more complex, this precarious equilibrium is non-trivially affected by the interaction of the TADF dye with its local environment. The concurrent optimization of the dye and of the embedding medium is therefore of paramount importance to boost practical applications of TADF. Towards this aim, refined theoretical and computational approaches must be cleverly exploited, paying attention to the reliability of adopted approximations. In this perspective, we will address some of the most important issues in the field. Specifically, we will critically review theoretical and computational approaches to TADF rates, highlighting the limits of widespread approaches. Environmental effects on the TADF photophysics are discussed in detail, focusing on the major role played by dielectric and conformational disorder in liquid solutions and amorphous matrices.

Enhanced inverted singlet-triplet gaps in azaphenalenes and non-alternant hydrocarbons

Inverted singlet-triplet gaps may lead to novel molecular emitters if a rational design approach can be achieved. We uncover a substituent strategy that enables tuning of the gap and succeed in inducing inversion in near-gapless molecules. Based on known inverted-gap emitters, we design substituted analogs with even more negative singlet-triplet gaps than in the parent systems. The inversion is lost if the reverse substituent-strategy is used. We thus demonstrate a definite set of conceptual design rules for inverted gap molecules.

Spin-Vibronic Intersystem Crossing and Molecular Packing Effects in Heavy Atom Free Organic Phosphor

We present a detailed investigation into the excited state properties of a planar D3h symmetric azatriangulenetrione, HTANGO, which has received significant interest due to its high solid-state phosphorescence quantum yield and therefore potential as an organic room temperature phosphorescent (ORTP) dye. Using a model linear vibronic coupling Hamiltonian in combination with quantum dynamics simulations, we observe that intersystem crossing (ISC) in HTANGO occurs with a rate of ∼1010 s-1, comparable to benzophenone, an archetypal molecule for fast ISC in heavy metal free molecules. Our simulations demonstrate that the mechanism for fast ISC is associated with the high density of excited triplet states which lie in close proximity to the lowest singlet states, offering multiple channels into the triplet manifold facilitating rapid population transfer. Finally, to rationalize the solid-state emission properties, we use quantum chemistry to investigate the excited state surfaces of the HTANGO dimer, highlighting the influence and importance of the rotational alignment between the two HTANGO molecules in the solid state and how this contributes to high phosphorescence quantum yield.

Shining Light on Inverted Singlet-Triplet Emitters

The inversion of the lowest singlet and triplet excited states, observed in several triangle-shaped organic molecules containing conjugated carbon and nitrogen atoms, is an astonishing result that implies the breakdown of Hund's rule. The phenomenon attracted interest for its potential toward triplet harvesting in organic LEDs. On a more fundamental vein, the singlet-triplet (ST) inversion sheds new light on the role of electron correlations in the excited-state landscape of π-conjugated molecules. Relying on the celebrated Pariser-Parr-Pople model, the simplest model for correlated electrons in π-conjugated systems, we demonstrate that the ST inversion does not require triangle-shaped molecules nor any specific molecular symmetry. Indeed, the ST inversion does not require strictly non-overlapping HOMO and LUMO orbitals but rather a small gap and a small exchange integral between the frontier orbitals.

Achieving the Reverse Intersystem Crossing in Chalcone Based Donor-Acceptor System through Down-Conversion of Triplet Exciton

In recent years, understanding the mechanism of thermally activated delayed fluorescence (TADF) has become the primary choice for designing high-efficiency, low-cost, metal-free organic light emitting diodes (OLEDs). Herein, we propose a strategically designed chalcone based donor-acceptor system, where intensification of delayed fluorescence with decrease in temperature (300 K to 100 K) is observed; the theoretical investigations of electronic states and orbital characters uncovered a new cold rISC pathway in donor-acceptor system, where rISC occurs through the down-conversation of higher triplet exciton (from T3 ) to lowest singlet state (S1 ), having negative energy splitting, thus no thermal energy is required. The comprehensive research described herein might open-up new avenues in donor-acceptor system over the conventional up-convention of triplet exciton and demonstrates that not necessarily all delayed fluorescence are thermally activated (TADF).

Double-bond delocalization in non-alternant hydrocarbons induces inverted singlet-triplet gaps

Molecules where the first excited singlet state is lower in energy than the first excited triplet state have the potential to revolutionize OLEDs. This inverted singlet-triplet gap violates Hund's rule and currently there are only a few molecules which are known to have this property. Here, we screen the complete set of non-alternant hydrocarbons consisting of 5-, 6-, 7-membered rings fused into two-, three- and four-ring polycyclic systems. We identify several molecules where the symmetry of the ground-state structure is broken due to bond-length alternation. Through symmetry-constrained optimizations we identify several molecular cores where the singlet-triplet gap is inverted when the structure is in a higher symmetry, pentalene being a known example. We uncover a strategy to stabilize the molecular cores into their higher-symmetry structures with electron donors or acceptors. We design several substituted pentalenes, s-indacenes, and indeno[1,2,3-ef]heptalenes with inverted gaps, among which there are several synthetically known examples. In contrast to known inverted gap emitters, we identify the double-bond delocalized structure of their conjugated cores as the necessary condition to achieve the inverted gap. This strategy enables chemical tuning and paves the way for the rational design of polycyclic hydrocarbons with inverted singlet-triplet gaps. These molecules are prospective emitters if their properties can be optimized for use in OLEDs.

Spin-orbit charge-transfer intersystem crossing in heavy-atom-free orthogonal covalent boron-dipyrromethene heterodimers

Boron-dipyrromethene (BODIPY) derivatives are prospective organic-based triplet photosensitizers. Since the triplet generation yield of the parent BODIPY is low, heavy atoms are widely used to improve the triplet yield. However, the dimerization of BODIPYs can also significantly improve their ability to produce triplets. Through a comparative study of the triplet formation dynamics of two heavy-atom-free orthogonal covalent BODIPY heterodimers that differ in their dihedral angles, we have demonstrated that the mechanism of spin-orbit charge-transfer intersystem crossing (SOCT-ISC) promotes the triplet generation of BODIPY heterodimers in solution. Different from the general understanding of SOCT-ISC, the heterodimer with a smaller dihedral angle and low structural rigidity showed better triplet generation due to (a) the stronger inter-chromophoric interaction in the heterodimer, which promoted the formation of a solvent-stabilized charge-transfer (CT) state, (b) the more favorable energy level alignment with sizeable spin-orbit coupling strength, and (c) the balance between the stabilized singlet CT state and limited direct charge recombination to the ground state in a weakly polar solvent. The complete spectral characterization of the triplet formation dynamics clarified the SOCT-ISC mechanism and important factors affecting the triplet generation in BODIPY heterodimers.

Identification of Unknown Inverted Singlet-Triplet Cores by High-Throughput Virtual Screening

Molecules where the energy of the lowest excited singlet state is found below the energy of the lowest triplet state (inverted singlet-triplet molecules) are extremely rare. It is particularly challenging to discover new ones through virtual screening because the required wavefunction-based methods are expensive and unsuitable for high-throughput calculations. Here, we devised a virtual screening approach where the molecules to be considered with advanced methods are pre-selected with increasingly more sophisticated filters that include the evaluation of the HOMO-LUMO exchange integral and approximate CASSCF calculations. A final set of 7 candidates (0.05% of the initial 15 000) were verified to possess inversion between singlet and triplet states with state-of-the-art multireference methods (MS-CASPT2). One of them is deemed of particular interest because it is unrelated to other proposals made in the literature.

Mind the GAP: quantifying the breakdown of the linear vibronic coupling Hamiltonian

Excited state dynamics play a critical role across a broad range of scientific fields. Importantly, the highly non-equilibrium nature of the states generated by photoexcitation means that excited state simulations should usually include an accurate description of the coupled electronic-nuclear motion, which often requires solving the time-dependent Schrödinger equation (TDSE). One of the biggest challenges for these simulations is the requirement to calculate the PES over which the nuclei evolve. An effective approach for addressing this challenge is to use the approximate linear vibronic coupling (LVC) Hamiltonian, which enables a model potential to be parameterised using relatively few quantum chemistry calculations. However, this approach is only valid provided there are no large amplitude motions in the excited state dynamics. In this paper we introduce and deploy a metric, the global anharmonicity parameter (GAP), which can be used to assess the accuracy of an LVC potential. Following its derivation, we illustrate its utility by applying it to three molecules exhibiting different rigidity in their excited states.

Tunable Photoinduced Charge Transfer at the Interface between Benzoselenadiazole-Based MOF Linkers and Thermally Activated Delayed Fluorescence Chromophore

Structural modifications to molecular systems that lead to the control of photon emission processes at the interfaces between photoactive materials play a key role in the development of fluorescence sensors, X-ray imaging scintillators, and organic light-emitting diodes (OLEDs). In this work, two donor-acceptor systems were used to explore and reveal the effects of slight changes in chemical structure on interfacial excited-state transfer processes. A thermally activated delayed fluorescence (TADF) molecule was chosen as the molecular acceptor. Meanwhile, two benzoselenadiazole-core MOF linker precursors, Ac-SDZ and SDZ, with the presence and absence of a C≡C bridge, respectively, were carefully chosen as energy and/or electron-donor moieties. We found that the SDZ -TADF donor-acceptor system exhibited efficient energy transfer, as evidenced by steady-state and time-resolved laser spectroscopy. Furthermore, our results demonstrated that the Ac-SDZ-TADF system exhibited both interfacial energy and electron transfer processes. Femtosecond-mid-IR (fs-mid-IR) transient absorption measurements revealed that the electron transfer process takes place on the picosecond timescale. Time-dependent density functional theory (TD-DFT) calculations confirmed that photoinduced electron transfer occurred in this system and demonstrated that it takes place from C≡C in Ac-SDZ to the central unit of the TADF molecule. This work provides a straightforward way to modulate and tune excited-state energy/charge transfer processes at donor-acceptor interfaces.

TADF Invariant of Host Polarity and Ultralong Fluorescence Lifetimes in a Donor-Acceptor Emitter Featuring a Hybrid Sulfone-Triarylboron Acceptor

10H-Dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B)-a high triplet (T₁ =3.05 eV) strongly electron-accepting boracycle was successfully utilised in thermally activated delayed fluorescence (TADF) emitters PXZ-Dipp-SO2B and CZ-Dipp-SO2B. We demonstrate the near-complete separation of highest occupied and lowest unoccupied molecular orbitals leading to a low oscillator strength of the S₁ →S₀ CT transition, resulting in very long ca. 83 ns and 400 ns prompt fluorescence lifetimes for CZ-Dipp-SO2B and PXZ-Dipp-SO2B, respectively, but retaining near unity photoluminescence quantum yield. OLEDs using CZ-Dipp-SO2B as the luminescent dopant display high external quantum efficiency (EQE) of 23.3 % and maximum luminance of 18600 cd m^-2 with low efficiency roll off at high brightness. For CZ-Dipp-SO2B, reverse intersystem crossing (rISC) is mediated through the vibronic coupling of two charge transfer (CT) states, without involving the triplet local excited state (³ LE), resulting in remarkable rISC rate invariance to environmental polarity and polarisability whilst giving high organic light-emitting diode (OLED) efficiency. This new form of rISC allows stable OLED performance to be achieved in different host environments.

Symmetry-Induced Singlet-Triplet Inversions in Non-Alternant Hydrocarbons

Molecules with inversion of the singlet and triplet excited-state energies are highly promising for the development of organic light-emitting diodes (OLEDs). To date, azaphenalenes are the only class of molecules where these inversions have been identified. Here, we screen a curated database of organic crystal structures to identify existing compounds for violations of Hund's rule in the lowest excited states. We identify two further classes with this behavior. The first, a class of zwitterions, has limited relevance to molecular emitters as the singlet-triplet inversions occur in the third excited singlet state. The second class consists of two D_2h -symmetry non-alternant hydrocarbons, a fused azulene dimer and a bicalicene, whose lowest excited singlet states violate Hund's rule. Due to the connectivity of the polycyclic structure, they achieve this symmetry through aromatic stabilization. These hydrocarbons show promise as the next generation of building blocks for OLED emitters.

Substituent Effect on Vibrationally Resolved Absorption Spectra and Exciton Dynamics of Dipyrrolonaphthyridinedione Aggregates

Dipyrrolonaphthyridinedione (DPND) thin films exhibit interesting photophysical properties and singlet fission (SF) processes. A recent experimental work found that the alkyl substitution in the DPND skeleton has the remarkable influence on the characteristics of electronic absorption spectra and SF rates. Here, we theoretically elucidate the microscopic mechanism of the substituent effect on the optical properties and exciton dynamics of materials by combining the electronic structure calculations and the quantum dynamics simulations. The results show that the alkyl substituent has a minor effect on the single molecular properties but dramatically changes those of DPND aggregates via varying the intermolecular interactions. The aggregates of DPND with and without alkyl side chains exhibit the more likely characters of H-type aggregations. In the former (DPND6), the weak degree of mixing of intramolecular localized excited (LE) states and intermolecular charge transfer (CT) states makes the low-energy absorption band possess the predominant optical absorption, while in the latter (DPND), the CT and LE states are close in energy, together with their strong interaction, resulting in the substantial state-mixing, so that its two low-energy absorption bands have nearly equal oscillator strengths and a wide energy spacing of more than 0.5 eV. The simulation of exciton dynamics elucidates that the photoinitiated states in both aggregates cannot generate the free charge carrier because of the lack of enough driving forces. However, the population exchanges between LE and CT states in DPND aggregates are much faster than in DPND6 aggregates, indicating the different SF behaviors, consistent with the experimental observation.

Thermally Activated Delayed Fluorescence: Polarity, Rigidity, and Disorder in Condensed Phases

We present a detailed and comprehensive picture of the photophysics of thermally activated delayed fluorescence (TADF). The approach relies on a few-state model, parametrized ab initio on a prototypical TADF dye, that explicitly accounts for the nonadiabatic coupling between electrons and vibrational and conformational motion, crucial to properly address (reverse) intersystem crossing rates. The Onsager model is exploited to account for the medium polarity and polarizability, with careful consideration of the different time scales of relevant degrees of freedom. TADF photophysics is then quantitatively addressed in a coherent and exhaustive approach that accurately reproduces the complex temporal evolution of emission spectra in liquid solvents as well as in solid organic matrices. The different rigidity of the two environments is responsible for the appearance in matrices of important inhomogeneous broadening phenomena that are ascribed to the intertwined contribution from (quasi)static conformational and dielectric disorder.

Virtual Screening for Organic Solar Cells and Light Emitting Diodes

The field of organic semiconductors is multifaceted and the potentially suitable molecular compounds are very diverse. Representative examples include discotic liquid crystals, dye-sensitized solar cells, conjugated polymers, and graphene-based low-dimensional materials. This huge variety not only represents enormous challenges for synthesis but also for theory, which aims at a comprehensive understanding and structuring of the plethora of possible compounds. Eventually computational methods should point to new, better materials, which have not yet been synthesized. In this perspective, it is shown that the answer to this question rests upon the delicate balance between computational efficiency and accuracy of the methods used in the virtual screening. To illustrate the fundamentals of virtual screening, chemical design of non-fullerene acceptors, thermally activated delayed fluorescence emitters, and nanographenes are discussed.

Efficient Adversarial Generation of Thermally Activated Delayed Fluorescence Molecules

Adversarial generative models are becoming an essential tool in molecular design and discovery due to their efficiency in exploring the desired chemical space with the assistance of deep learning. In this article, we introduce an integrated framework by combining the modules of algorithmic synthesis, deep prediction, adversarial generation, and fine screening for the purpose of effective design of the thermally activated delayed fluorescence (TADF) molecules that can be used in the organic light-emitting diode devices. The retrosynthetic rules are employed to algorithmically synthesize the D-A complex based on the empirically defined donor and acceptor moieties, which is followed by the high-throughput labeling and prediction with the deep neural network. The new D-A molecules are subsequently generated via the adversarial autoencoder, with the excited-state property distributions perfectly matching those of the original samples. Fine screening of the generated molecules, including the spin-orbital coupling calculation and the excited-state optimization, is eventually implemented to select the qualified TADF candidates within the novel chemical space. Further investigation shows that the created structures fully mimic the original D-A samples by maintaining a significant charge transfer characteristic, a minimal adiabatic singlet-triplet gap, and a moderate spin-orbital coupling that are desirable for the delayed fluorescence.

Quantum simulations of thermally activated delayed fluorescence in an all-organic emitter

We investigate the prototypical NAI-DMAC thermally activated delayed fluorescence (TADF) emitter in the gas phase- and high-packing fraction limits at finite temperature, by combining first principles molecular dynamics with a quantum thermostat to account for nuclear quantum effects (NQE). We find a weak dependence of the singlet-triplet energy gap (ΔE_ST) on temperature in both the solid and the molecule, and a substantial effect of packing. While the ΔE_ST vanishes in the perfect crystal, it is of the order of ∼0.3 eV in the molecule, with fluctuations ranging from 0.1 to 0.4 eV at 300 K. The transition probability between the HOMOs and LUMOs has a stronger dependence on temperature than the singlet-triplet gap, with a desirable effect for thermally activated fluorescence; such temperature effect is weaker in the condensed phase than in the molecule. Our results on ΔE_ST and oscillator strengths, together with our estimates of direct and reverse intersystem crossing rates, show that optimization of packing and geometrical conformation is critical to increase the efficiency of TADF compounds. Our findings highlight the importance of considering thermal fluctuations and NQE to obtain robust predictions of the electronic properties of NAI-DMAC.