Singlet-triplet Gap Research Articles

Computational Investigations of the Detailed Mechanism of Reverse Intersystem Crossing in Inverted Singlet-Triplet Gap Molecules.

Inverted singlet-triplet gap (INVEST) materials have promising photophysical properties for optoelectronic applications due to an inversion of their lowest singlet (S1) and triplet (T1) excited states. This results in an exothermic reverse intersystem crossing (rISC) process that potentially enhances triplet harvesting, compared to thermally activated delayed fluorescence (TADF) emitters with endothermic rISCs. However, the processes and phenomena that facilitate conversion between excited states for INVEST materials are underexplored. We investigate the complex potential energy surfaces (PESs) of the excited states of three heavily studied azaphenalene INVEST compounds, namely, cyclazine, pentazine, and heptazine using two state-of-the-art computational methodologies, namely, RMS-CASPT2 and SCS-ADC(2) methods. Our findings suggest that ISC and rISC processes take place directly between the S1 and T1 electronic states in all three compounds through a minimum-energy crossing point (MECP) with an activation energy barrier between 0.11 to 0.58 eV above the S1 state for ISC and between 0.06 and 0.36 eV above the T1 state for rISC. We predict that higher-lying triplet states are not populated, since the crossing point structures to these states are not energetically accessible. Furthermore, the conical intersection (CI) between the ground and S1 states is high in energy for all compounds (between 0.4 to 2.0 eV) which makes nonradiative decay back to the ground state a relatively slow process. We demonstrate that the spin-orbit coupling (SOC) driving the S1-T1 conversion is enhanced by vibronic coupling with higher-lying singlet and triplet states possessing vibrational modes of proper symmetry. We also rationalize that the experimentally observed anti-Kasha emission of cyclazine is due to the energetically inaccessible CI between the bright S2 and the dark S1 states, hindering internal conversion. Finally, we show that SCS-ADC(2) is able to qualitatively reproduce excited state features, but consistently overpredict relative energies of excited state structural minima compared to RMS-CASPT2. The identification of these excited state features elaborates design rules for new INVEST emitters with improved emission quantum yields.

Thiocarbonyl-Bridged N-Heterotriangulenes for Energy Efficient Triplet Photosensitization: A Theoretical Perspective.

Rigid metal-free organicmoleculesare of high demand for various triplet harvesting applications. Inefficient intersystem crossing (ISC) due to large singlet-triplet gap ([[EQUATION]]) and small spin-orbit coupling (SOC) between lowest excited singlet and triplet in these molecules often limits their efficiency. We computationally investigate excited-state electronic structures, fluorescence and ISC rates in several thiiocarbonyl-bridged N-heterotriangulene ([[EQUATION]]S-HTG) with systematically increased S-content ([[EQUATION]]0-3). All [[EQUATION]]S-HTGs are dynamically stable and also thermodynamically feasible to synthesize. Relative energies of several low-lying singlets ([[EQUATION]]) and triplets ([[EQUATION]]), and their excitation nature ([[EQUATION]] or [[EQUATION]]) and SOC for these [[EQUATION]]S-HTGs in dichloromethane are determined implementing time-dependent optimally-tuned range-separated hybrid. Low-energy optical peak displays gradual red-shift with increasing S-content due to relatively smaller electronic gap resulted from greater degree of orbital delocalization. Significantly large SOC due to different orbital-symmetry and heavy-atom effect produces remarkably high ISC rates ([[EQUATION]] ~1012 s-1) for enthalpically favoured [[EQUATION]]([[EQUATION]]) channel in these [[EQUATION]]S-HTGs, which outcompete radiative fluorescence rates (~108 s-1) even directly from higher lying optically bright [[EQUATION]] singlets (anti-Kasha's emission). Importantly, high energy triplet excitons of ~1.7 eV resulting from such significantly large ISC rates from non-fluorescent [[EQUATION]] make these thionated molecules ideal candidates for energy efficient triplet harvest including triplet-photosensitization.

Static and resonant properties of decorated square kagomé lattice KCu[formula omitted](TeO[formula omitted])(SO[formula omitted])[formula omitted]Cl

The magnetic subsystem of nabokoite, KCu7(TeO4)(SO4)5Cl, is constituted by copper ions forming a buckled square kagomé lattice decorated by quasi-isolated ions. This combination determines peculiar physical properties of this compound evidenced in electron spin resonance (ESR) spectroscopy, dielectric permittivity ɛ, magnetization M and specific heat Cp measurements. At lowering temperature, the magnetic susceptibility χ=M/H passes through a broad hump inherent for low-dimensional magnetic systems at about 150 K and a sharp peak at antiferromagnetic phase transition at TN=3.2 K. The Cp(T,H) curves demonstrate additional peak-like anomaly at Tpeak=5.7 K robust to magnetic field. The latter can be ascribed to low-lying singlet excitations filling the singlet-triplet gap in magnetic excitation spectrum of the square kagomé lattice (Richter et al., 2022). ESR spectroscopy provides indications that antiferromagnetic structure below TN is non-collinear. Separate issue is the observation of antiferroelectric-type behavior in ɛ at low temperatures, which tentatively reduces the symmetry and partially lifts frustration of magnetic interactions of decorating copper ions with buckled square kagomé lattice. These complex thermodynamic and resonant properties signal the presence of two weakly coupled magnetic subsystems in nabokoite, namely a spin-liquid in square kagomé lattice layers and an antiferromagnet represented by decorating ions.

A Boron, Nitrogen, and Oxygen Doped π-Extended Helical Pure Blue Multiresonant Thermally Activated Delayed Fluorescent Emitter for Organic Light Emitting Diodes That Shows Fast kRISC Without the Use of Heavy Atoms.

Narrowband emissive multiresonant thermally activated delayed fluorescence (MR-TADF) emitters are a promising solution to achieve the current industry targeted color standard, BT.2020, for blue color without using optical filters, aiming for high efficiency organic light-emitting diodes (OLEDs). However, their long triplet lifetimes, largely affected by their slow reverse intersystem crossing rates, adversely affect device stability. In this study, a helical MR-TADF emitter (f-DOABNA) has been designed and synthesized. Because of its π-delocalized structure, f-DOABNA possesses a small singlet-triplet gap, ΔEST, and displays simultaneously an exceptionally faster reverse intersystem crossing rate constant, kRISC, of up to 2 × 106 s-1 and a very high photoluminescence quantum yield, ΦPL, of over 90% in both solution and doped films. The OLED with f-DOABNA as the emitter achieved a narrow deep-blue emission at 445nm (full width at half-maximum of 24nm) associated with CIE coordinates of 0.150, 0.041, and showed a high maximum external quantum efficiency, EQEmax, of ∼20%. This article is protected by copyright. All rights reserved.

Two determinant distinguishable cluster.

A two reference determinant version of the distinguishable cluster with singles and doubles (DCSD) has been developed. We have implemented the two determinant distinguishable cluster (2D-DCSD) and the corresponding traditional 2D-CCSD method in a new open-source package written in Julia called ElemCo.jl. The methods were benchmarked on singlet and triplet excited states of valence and Rydberg character, as well as for singlet-triplet gaps of diradicals. It is demonstrated that the distinguishable cluster approximation improves the accuracy of 2D-CCSD.

Molecular Geometry Impact on Deep Learning Predictions of Inverted Singlet-Triplet Gaps.

We present a deep learning model able to predict excited singlet-triplet gaps with a mean absolute error (MAE) of ≈20 meV to obtain potential inverted singlet-triplet (IST) candidates. We exploit cutting-edge spherical message passing graph neural networks designed specifically for generating 3D graph representations in molecular learning. In a nutshell, the model takes as input a list of unsaturated heavy atom Cartesian coordinates and atomic numbers, producing singlet-triplet gaps as output. We exploited available large data collections to train the model on ≈40,000 heterogeneous density functional theory (DFT) geometries with available ADC(2)/cc-pVDZ singlet-triplet gaps. We ascertain the predictive power of the model from a quantitative perspective obtaining predictions on a test set of ≈14,000 molecules, whose geometries have been generated at DFT level (the same employed for the geometries in the training set), at GFN2-xTB level, and through Molecular Mechanics. We notice performance degradation upon switching to lower-quality geometries, with GFN2-xTB ones maintaining satisfactory results (MAE ≈ 50 meV on GFN2-xTB geometries, MAE ≈ 180 meV on generalized AMBER force field geometries), hinting at caution when dealing with specific chemical classes. Finally, we verify the performance of the model from the qualitative point of view, obtaining predictions on a different data set of ≈15,000 molecules already used to identify new IST molecules. We obtained predictions using both DFT and experimental X-ray geometries, with results on IST candidates similar to those provided by quantum chemical methods, with clear hints for the path toward improved performance.

Carbazole-Benzonitrile-Norbornadiene Conjugates for Photothermally Reversible Ambient Phosphorescence.

Organic photoswitches have attracted significant attention across various fields, such as sensing, bioimaging, photopharmacology, molecular machines, and solar energy storage. However, as a result of design complexities, achieving photothermally reversible ambient phosphorescence switching in the condensed state remains elusive. Herein, we explore the impact of norbornadiene (NBD)/quadricyclane (QC) substitution at position 5 of the benzonitrile acceptor covalently attached to the carbazole donor on photothermally reversible luminescence switching. Experimental investigations demonstrated that the CzN and TBCzN switches exhibited photothermally reversible fluorescence switching in solution. Moreover, in the condensed state, fluorescence and ambient phosphorescence switching were observed as a result of a low singlet-triplet (ΔEST) gap (CzN ⇆ CzQ, ΔESTCzN/CzQ = 0.05/0.28 eV; TBCzN ⇆ TBCzQ, ΔESTTBCzN/TBCzQ = 0.06/0.09 eV). Reversible ambient phosphorescence switching is primarily influenced by modulation of acceptor conjugation resulting from NBD ⇆ QC switching. This approach may provide important clues for the design of visible-light-absorbing photothermally reversible phosphorescent materials.

White Light Emission via Dual Thermally Activated Delayed Fluorescence from a Single-Component Phenothiazines-Diphenyl Quinoline Conjugate.

White light emission (WLE) via dual thermally activated delayed fluorescence (TADF) from a single-component-based organic system remains challenging as a result of the difficulty in design. Here, we introduce a conformational isomerization approach to achieve WLE from a twisted donor-acceptor (PTzQP1) that comprises two phenothiazines covalently attached to the 6,8-isomeric positions of 2,4-diphenyl quinoline via two C-N single bonds. Spectroscopic studies and quantum chemistry calculations revealed that PTzQP1 shows WLE via simultaneous blue TADF and orange TADF covering the visible range (420-800 nm) with a photoluminescence quantum yield of 45 ± 2% and Commission Internationale de l'Éclairage (CIE) coordinates of 0.30, 0.33. The dual TADF features with high rates of reverse intersystem crossing (kRISC1 = 1.38 × 107 ± 0.24 s-1 and kRISC2 = 5.04 × 106 ± 0.32 s-1) are realized as a result of the low singlet-triplet gaps (S1EQ-T1EQ = 0.04 eV and S1QA-T1QA = 0.05 eV) of the quasi-axial (QA) and quasi-equatorial (QE) conformers. This finding is expected to provide a new direction for designing high-energy-efficient WLE emitters.

Reaching High Accuracy for Energetic Properties at Second-Order Perturbation Cost by Merging Self-Consistency and Spin-Opposite Scaling.

Quantum chemical methods dealing with challenging systems while retaining low computational costs have attracted attention. In particular, many efforts have been devoted to developing new methods based on second-order perturbation that may be the simplest correlated method beyond Hartree-Fock. We have recently developed a self-consistent perturbation theory named one-body Møller-Plesset second-order perturbation theory (OBMP2) and shown that it can resolve issues caused by the noniterative nature of standard perturbation theory. In this work, we extend the method by introducing spin-opposite scaling to the double-excitation amplitudes, resulting in the O2BMP2 method. We assess the O2BMP2 performance on the triple-bond N2 dissociation, singlet-triplet gaps, and ionization potentials. O2BMP2 performs much better than standard MP2 and reaches the accuracy of coupled-cluster methods in all cases considered in this work.

An N-Heterocyclic Quinodimethane: A Strong Organic Lewis Base Exhibiting Diradical Reactivity.

We report the preparation of a new organic σ-donor with a C6H4-linker between an N-heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N-heterocyclic quinodimethane (NHQ). The aromatization of the C6H4-linker provides a decisive driving force for the reaction of the NHQ with an electrophile and renders the NHQ significantly more basic than analogous NHCs or N-heterocyclic olefins (NHOs), as shown by DFT computations and competition experiments. In solution, the NHQ undergoes an unprecedented dehydrogenative head-to-head dimerization by C-C coupling of the methylidene groups. DFT computations indicate that this reaction proceeds via an open-shell singlet pathway revealing the diradical character of the NHQ. The product of this dimerization can be described as conjugated N-heterocyclic bis-quinodimethane, which according to cyclic voltammetry is a strong organic reducing agent (E1/2=-1.71 V vs. Fc/Fc+) and exhibits a remarkable small singlet-triplet gap of ΔES→T=4.4 kcal mol-1.

Variational Pair-Density Functional Theory: Dealing with Strong Correlation at the Protein Scale.

Multiconfigurational pair-density functional theory (MC-PDFT) offers a promising solution to the challenges faced by traditional density functional theory (DFT) in addressing molecular systems containing transition metals, open-shells, or strong correlations in general. By utilizing both the density and on-top pair-density, MC-PDFT can make use of a more flexible multiconfigurational wave function to capture the necessary static correlation, while the pair-density functional also includes the effect of dynamic correlation. So far, MC-PDFT has been used after a multiconfigurational self-consistent field (MCSCF) step, using the orbitals and configuration interaction coefficients from the converged MCSCF wave function to compute PDFT energies and properties. Here, instead, we propose to perform a direct optimization of the wave function using the pair-density functionals, resulting in a variational formulation of MC-PDFT. We derive the expressions for the wave function gradient and illustrate their similarity to standard MCSCF equations. Furthermore, we illustrate the accuracy on a set of singlet-triplet gaps as well as dissociation curves. Our findings highlight one of MC-PDFT's standout features: a reduced dependency on the active space size compared to conventional multiconfigurational wave function methodologies. Additionally, we show that the computational cost of MC-PDFT is potentially lower than MCSCF and often on-par with standard Kohn-Sham DFT, which is demonstrated by performing a MC-PDFT calculation of the entire ferredoxin protein with 1447 atoms and nearly 12000 basis functions.

Conformational Effect of Catechol-Terephthalonitrile Emitters Leading to Ambient Violet Phosphorescence.

Organic ambient violet phosphorescent (AVP) materials are of great interest due to their involvement of high energy and longer-lived triplet excitons. Here, we show three fused ring functionalized donor-acceptor-donor (D-A-D/D-A-D') emitters (BPT1-BPT3), in which two catechol-based donors (3,4-dihydroxybenzophenone, catechol, or 3,5-ditert-butylcatechol) are covalently fused to the terephthalonitrile acceptor via four O-C single bonds. Spectroscopic analysis revealed that all the molecules show AVP (∼390-394 nm, τAVP = 73-101 μs) with phosphorescence quantum yields (ϕP) of 1.8-27.4% due to low singlet-triplet gaps (0.036-0.046 eV) and conformational effects. BPT3 with bulky tert-butyl groups increases AVP (ϕP = 27.4%). Quantum chemistry calculations reveal flat (F1) and twisted (F2) conformers (ground state) with a low energy difference (∼4-5 kcal/mol) for all molecules; the F1 conformer is responsible for efficient AVP, while weak blue thermally activated delayed fluorescence with longer-lived delayed components is realized from the F2 conformer. This approach may provide important clues for the design of high-energy organic phosphorescent materials.

Introduction of a Chiral Biphenanthrene-Diol Unit to Achieve Circularly Polarized Thermally Activated Delayed Fluorescence.

Circularly polarized luminescence (CPL) materials are promising candidates for future display technology. However, such highly efficient emitters suffer from the issues of difficult chiral separation and low photoluminescence quantum yield (PLQY). In this work, the chiral 4,4'-biphenanthrene-3,3'-diol (BIPOL) unit was introduced into a thermally activated delayed fluorescence (TADF) framework for the first time. We presented two series of enantiomers, R/S-o-DCzBPNCN and R/S-p-DCzBPNCN, and the synthesis of enantiopure BIPOL can be prepared via normal column chromatography. Notably, o-DCzBPNCN showed narrow singlet-triplet gap of 0.05 eV, efficient TADF, and high PLQYs of 82 % in doped films. In addition, R/S-o-DCzBPNCN exhibited high luminescence dissymmetry factor (gPL ) values of -1.94×10-2 /+1.91×10-2 in doped films. The strategy of BIPOL introduction offers a new approach to organic emitters with stereospecific synthesis, TADF, and chiroptical properties.

Extension of molecules with an inverted singlet-triplet gap with conjugated branches to alter the oscillator strength.

Molecules that violate Hund's rule and possess negative singlet-triplet gaps (ΔEST) have been actively studied for their potential usage in organic light emitting diodes without the need for thermal activation. However, the weak oscillator strength from the symmetry of such molecules has been recognized as their shortcoming for their application in optoelectronic devices. A group of molecules with a common structural motif involving the original molecule with an inverted gap having branches consisting of conjugated molecules of varied structures and extent of conjugation have been predicted to have desirable oscillator strength, but only few detailed and comprehensive studies regarding the form of excited states and the reason behind the improved oscillator strength have been carried out. We show in this work a series of analyses that suggest that the increase of oscillator strength is correlated with the nature of the excited state changing from a localized excitation to a delocalized excitation involving the central molecule and the branches. The resulting oscillator strength thus depends on the energetic matching of the branching molecule and the central molecule, rather than solely the oscillator strength of the central molecule. From the ΔEST inversion point of view, the static correlation with low-lying doubly excited configurations, the key mechanism behind the inversion in the localized excited state, weakens as the excited states delocalize. As a consequence, the dynamic correlation has a more decisive effect in determining the singlet-triplet gap.

Artificial design of organic emitters via a genetic algorithm enhanced by a deep neural network.

The design of molecules requires multi-objective optimizations in high-dimensional chemical space with often conflicting target properties. To navigate this space, classical workflows rely on the domain knowledge and creativity of human experts, which can be the bottleneck in high-throughput approaches. Herein, we present an artificial molecular design workflow relying on a genetic algorithm and a deep neural network to find a new family of organic emitters with inverted singlet-triplet gaps and appreciable fluorescence rates. We combine high-throughput virtual screening and inverse design infused with domain knowledge and artificial intelligence to accelerate molecular generation significantly. This enabled us to explore more than 800 000 potential emitter molecules and find more than 10 000 candidates estimated to have inverted singlet-triplet gaps (INVEST) and appreciable fluorescence rates, many of which likely emit blue light. This class of molecules has the potential to realize a new generation of organic light-emitting diodes.

Experimental and computational studies of the optical properties of 2,5,8-tris(phenylthiolato)heptazine with an inverted singlet-triplet gap.

Photophysical properties of the three-fold symmetric 2,5,8-tris(phenylthiolato)heptazine molecule (1) are studied from combined experimental and computational viewpoints. The intense blue photoemission of 1 in the solid state and in toluene solution is proposed to have a fluorescent origin on the basis of a relatively short emission lifetime and no detectable triplet decay. Calculations at correlated ab initio levels of theory also show that 1 has a large inverted singlet-triplet (IST) gap, a non-vanishing spin-orbit coupling matrix element between the first excited singlet and triplet states, and a fast intersystem crossing rate constant that leads to singlet population from the higher-lying triplet state. The IST gap implies that the first excited singlet state is the lowest excited one, agreeing with the measured fluorescent behaviour of 1. IST gaps are also obtained for the oxygen-containing (2) and selenium-containing (3) analogues of 1 at the ADC(2) level of theory, but not for the tellurium one (4). Calculations of the magnetically induced current density demonstrate that the heptazine core of 1 is globally non-aromatic due to the alternation of carbon and nitrogen atoms along its external rim.

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.

A Triplet/Singlet Ground-State Switch via the Steric Inhibition of Conjugation in 4,6-Bis(trifluoromethyl)-1,3-phenylene Bisnitroxide.

Ground triplet 4,6-bis(trifluoromethyl)-1,3-phenylene bis(tert-butyl nitroxide) (TF2PBN) reacted with [Y(hfac)3(H2O)2] (hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate), affording a doubly hydrogen-bonded adduct [Y(hfac)3(H2O)2(TF2PBN)]. The biradical was recovered from the adduct through recrystallization. Crystallographic analysis indicates that the torsion angles (|θ| ≤ 90°) between the benzene ring and nitroxide groups were 74.9 and 84.8° in the adduct, which are larger than those of the starting material TF2PBN. Steric congestion due to o-trifluoromethyl groups gives rise to the reduction of π-conjugation. Two hydrogen bonds enhance this deformation. Susceptometry of the adduct indicates a ground singlet with 2J/kB = -128(2) K, where 2J corresponds to the singlet-triplet gap. The observed magneto-structure relation is qualitatively consistent with Rajca's pioneering work. A density functional theory calculation at the UB3LYP/6-311+G(2d,p) level using the atomic coordinates determined provided a result of 2J/kB = -162.3 K for the adduct, whilst the corresponding calculation on intact TF2PBN provided +87.2 K. After a comparison among a few known compounds, the 2J vs. |θ| plot shows a negative slope with a critical torsion of 65(3)°. The ferro- and antiferromagnetic coupling contributions are balanced in TF2PBN, being responsible for ground-state interconversion by means of small structural perturbation like hydrogen bonds.

From Solution to Surface: Persistence of the Diradical Character of a Diindenoanthracene Derivative on a Metallic Substrate.

Organic diradicals are envisioned as elementary building blocks for designing a new generation of spintronic devices and have been used in constructing prototypical field effect transistors and nonlinear optical devices. Open-shell systems, however, are also reactive, thus requiring design strategies to "protect" their radical character from the environment, especially when they are embedded in solid-state devices. Here, we report the persistence on a metallic surface of the diradical character of a diindeno[b,i]anthracene (DIAn) core protected by bulky end-groups. Our scanning tunneling spectroscopy measurements on single-molecules detected singlet-triplet excitations that were absent for DIAn species packed in assembled structures. Density functional theory simulations unravel that the molecular geometry on the metal substrate can crucially modify the value of the singlet-triplet gap via the delocalization of the radical sites. The persistence of the diradical character over metallic substrates is a promising finding for integrating radical-based materials into functional devices.