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Reverse intersystem crossing (RISC), the uphill spin-flip process from a triplet to a singlet excited state, plays a key role in a wide range of photochemical applications. Understanding and predicting the kinetics of such processes in vastly different molecular structures would facilitate the rational material design. Here, we demonstrate a theoretical expression that successfully reproduces experimental RISC rate constants ranging over five orders of magnitude in twenty different molecules. We show that the spin flip occurs across the singlet–triplet crossing seam involving a higher-lying triplet excited state where the semi-classical Marcus parabola is no longer valid. The present model explains the counterintuitive substitution effects of bromine on the RISC rate constants of previously unknown molecules, providing a predictive tool for material design. Understanding and predicting the kinetics of reverse intersystem crossing (RISC) facilitates the design of materials. Here, the authors demonstrate a theoretical expression that reproduces experimental RISC rate constants ranging over five orders of magnitude in selected molecules.

Hund’s multiplicity rule states that a higher spin state has a lower energy for a given electronic configuration 1 . Rephrasing this rule for molecular excited states predicts a positive energy gap between spin-singlet and spin-triplet excited states, as has been consistent with numerous experimental observations over almost a century. Here we report a fluorescent molecule that disobeys Hund’s rule and has a negative singlet–triplet energy gap of −11 ± 2 meV. The energy inversion of the singlet and triplet excited states results in delayed fluorescence with short time constants of 0.2 μs, which anomalously decrease with decreasing temperature owing to the emissive singlet character of the lowest-energy excited state. Organic light-emitting diodes (OLEDs) using this molecule exhibited a fast transient electroluminescence decay with a peak external quantum efficiency of 17%, demonstrating its potential implications for optoelectronic devices, including displays, lighting and lasers. A fluorescent molecule is described that does not follow Hund’s rule and instead shows singlet and triplet excited states with inverted energy levels, leading to high-efficiency OLEDs with potential implications for optoelectronic devices.

Azobenzene has been widely explored as a photoresponsive element in materials science. Although some studies have investigated the force-induced isomerization of azobenzene, the effect of force on the rupture of azobenzene has not been explored. Here we show that the light-induced structural change of azobenzene can also alter its rupture forces, making it an ideal light-responsive mechanophore. Using single-molecule force spectroscopy and ultrasonication, we found that cis and trans para -azobenzene isomers possess contrasting mechanical properties. Dynamic force spectroscopy experiments and quantum-chemical calculations in which azobenzene regioisomers were pulled from different directions revealed that the distinct rupture forces of the two isomers are due to the pulling direction rather than the energetic difference between the two isomers. These mechanical features of azobenzene can be used to rationally control the macroscopic fracture behaviours of polymer networks by photoillumination. The use of light-induced conformational changes to alter the mechanical response of mechanophores provides an attractive way to engineer polymer networks of light-regulatable mechanical properties. Light-induced azobenzene cis / trans isomerization has been extensively investigated, but the mechanical strength of its cis / trans structure is not well understood. Now it has been shown that cis azobenzene is mechanically less stable than the trans isomer due to its regiochemical structure, as revealed by single-molecule force spectroscopy.

Ab initio kinetic studies are important to understand and design novel chemical reactions. While the Artificial Force Induced Reaction (AFIR) method provides a convenient and efficient framework for kinetic studies, accurate explorations of reaction path networks incur high computational costs. In this article, we are investigating the applicability of Neural Network Potentials (NNP) to accelerate such studies. For this purpose, we are reporting a novel theoretical study of ethylene hydrogenation with a transition metal complex inspired by Wilkinson’s catalyst, using the AFIR method. The resulting reaction path network was analyzed by the Generative Topographic Mapping method. The network’s geometries were then used to train a state-of-the-art NNP model, to replace expensive ab initio calculations with fast NNP predictions during the search. This procedure was applied to run the first NNP-powered reaction path network exploration using the AFIR method. We discovered that such explorations are particularly challenging for general purpose NNP models, and we identified the underlying limitations. In addition, we are proposing to overcome these challenges by complementing NNP models with fast semiempirical predictions. The proposed solution offers a generally applicable framework, laying the foundations to further accelerate ab initio kinetic studies with Machine Learning Force Fields, and ultimately explore larger systems that are currently inaccessible.

Selecting diverse molecules from unexplored areas of chemical space is one of the most important tasks for discovering novel molecules and reactions. This paper proposes a new approach for selecting a subset of diverse molecules from a given molecular list by using two existing techniques studied in machine learning and mathematical optimization: graph neural networks (GNNs) for learning vector representation of molecules and a diverse-selection framework called submodular function maximization. Our method, called SubMo-GNN, first trains a GNN with property prediction tasks, and then the trained GNN transforms molecular graphs into molecular vectors, which capture both properties and structures of molecules. Finally, to obtain a subset of diverse molecules, we define a submodular function, which quantifies the diversity of molecular vectors, and find a subset of molecular vectors with a large submodular function value. This can be done efficiently by using the greedy algorithm, and the diversity of selected molecules measured by the submodular function value is mathematically guaranteed to be at least 63% of that of an optimal selection. We also introduce a new evaluation criterion to measure the diversity of selected molecules based on molecular properties. Computational experiments confirm that our SubMo-GNN successfully selects diverse molecules from the QM9 dataset regarding the property-based criterion, while performing comparably to existing methods regarding standard structure-based criteria. We also demonstrate that SubMo-GNN with a GNN trained on the QM9 dataset can select diverse molecules even from other MoleculeNet datasets whose domains are different from the QM9 dataset. The proposed method enables researchers to obtain diverse sets of molecules for discovering new molecules and novel chemical reactions, and the proposed diversity criterion is useful for discussing the diversity of molecular libraries from a new property-based perspective.

1,2-Bis(diphenylphosphino)ethane (DPPE) and its synthetic analogues are important structural motifs in organic synthesis, particularly as diphosphine ligands with a C 2 -alkyl-linker chain. Since DPPE is known to bind to many metal centers in a bidentate fashion to stabilize the corresponding metal complex via the chelation effect originating from its entropic advantage over monodentate ligands, it is often used in transition-metal-catalyzed transformations. Symmetric DPPE derivatives (Ar 1 2 P−CH 2 −CH 2 −PAr 1 2 ) are well-known and readily prepared, but electronically and sterically unsymmetric DPPE (Ar 1 2 P−CH 2 −CH 2 −PAr 2 2 ; Ar 1 ≠Ar 2 ) ligands have been less explored, mostly due to the difficulties associated with their preparation. Here we report a synthetic method for both symmetric and unsymmetric DPPEs via radical difunctionalization of ethylene, a fundamental C 2 unit, with two phosphine-centered radicals, which is guided by the computational analysis with the artificial force induced reaction (AFIR) method, a quantum chemical calculation-based automated reaction path search tool. The obtained unsymmetric DPPE ligands can coordinate to several transition-metal salts to form the corresponding complexes, one of which exhibits distinctly different characteristics than the corresponding symmetric DPPE–metal complex. DPPEs are fundamental bidentate ligands with a C2-alkyl-linker chain for many transition-metal-catalyzed reactions. Here, authors utilize the AFIR method to develop a practical synthetic method for both symmetric and unsymmetric DPPEs with ethylene.

Photoinduced concerted multiple-bond rotation has been proposed in some biological systems. However, the observation of such phenomena in synthetic systems, in other words, the synthesis of molecules that undergo photoinduced multiple-bond rotation upon photoirradiation, has been a challenge in the photochemistry field. Here we describe a chalcogen-substituted benzamide system that exhibits photoinduced dual bond rotation in heteroatom-containing bonds. Introduction of the chalcogen substituent into a sterically hindered benzamide system provides sufficient kinetic stability and photosensitivity to enable the photoinduced concerted rotation. The presence of two different substituents on the phenyl ring in the thioamide derivative enables the generation of a pair of enantiomers and E / Z isomers. Using these four stereoisomers as indicators of which bonds are rotated, we monitor the photoinduced C–N/C–C concerted bond rotation in the thioamide derivative depending on external stimuli such as temperature and photoirradiation. Theoretical calculations provide insight on the mechanism of this selective photoinduced C–N/C–C concerted rotation. Although photoinduced concerted multiple-bond-rotation processes are known in photoactive biological systems, the synthesis of compounds exhibiting similar behaviour has proven challenging. Now a thioamide-based system featuring chalcogen substituents has been shown to exhibit photoinduced C–N/C–C rotation; the rotation mode can be switched depending on external stimuli such as temperature and light irradiation.

The development of sustainable and efficient routes to β‐ and γ‐amino acids is urgent due to their pivotal importance in pharmaceuticals and materials science. This study presents a novel approach utilizing the CO2 radical anion as a key intermediate for the synthesis of β‐ and γ‐amino acids from N‐Ac‐enamides and N‐Ac‐allylamines, respectively. Both electrochemical and photochemical techniques were employed to generate the CO2 radical anion under mild and environmentally friendly conditions. The feasibility of the synthesis of β‐amino acids was confirmed through an automated reaction path search using the AFIR method. This work highlights the potential of CO2 as a versatile building block in organic synthesis and provides a sustainable alternative to conventional methods for producing β‐ and γ‐amino acids. The development of sustainable methods for synthesizing β‐ and γ‐amino acids is crucial due to their significance in pharmaceuticals and materials science. This study introduces a novel strategy employing the CO2 radical anion as a key intermediate to access these amino acids from N‐Ac‐enamides and N‐Ac‐allylamines. Both electrochemical and photochemical approaches were used to generate CO2•− under mild, environmentally friendly conditions. The feasibility of β‐amino acid formation was supported by automated reaction path searches using the artificial force induced reaction method. This work demonstrates the potential of CO2 as a versatile building block and offers a sustainable alternative to conventional methods.

An orbital energy-based reaction analysis theory is presented as an extension of the orbital-based conceptual density functional theory. In the orbital energy-based theory, the orbitals contributing to reactions are interpreted to be valence orbitals giving the largest orbital energy variation from reactants to products. Reactions are taken to be electron transfer-driven when they provide small variations for the gaps between the contributing occupied and unoccupied orbital energies on the intrinsic reaction coordinates in the initial processes. The orbital energy-based theory is then applied to the calculations of several S N2 reactions. Using a reaction path search method, the Cl− + CH3I → ClCH3 + I− reaction, for which another reaction path called “roundabout path” is proposed, is found to have a precursor process similar to the roundabout path just before this SN2 reaction process. The orbital energy-based theory indicates that this precursor process is obviously driven by structural change, while the successor SN2 reaction proceeds through electron transfer between the contributing orbitals. Comparing the calculated results of the SN2 reactions in gas phase and in aqueous solution shows that the contributing orbitals significantly depend on solvent effects and these orbitals can be correctly determined by this theory.

Quantum chemical calculations are mainly regarded as a method for mechanistic studies in organic chemistry, whereas their use for the simulation of unknown reactions could greatly assist in reaction development. Here we report a strategy for developing multicomponent reactions on the basis of the results of computational reaction simulations. In silico screening of multicomponent reactions with difluorocarbene using the artificial force induced reaction method suggested that cycloadditions between an azomethine ylide and a variety of coupling partners would proceed to generate the corresponding α,α-difluorinated N -heterocyclic compounds. The predicted reaction was successfully realized experimentally, leading to a multicomponent N -difluoroalkylative dearomatization of pyridines involving a pyridinium ylide-mediated 1,3-dipolar cycloaddition with a diverse range of electrophiles such as aldehydes, ketones, imines, alkenes and alkynes. Moreover, the performance of the cycloaddition could be explained by comparing the energy barrier of the desired pathway with that of the competitive undesired pathway, which was also identified by the artificial force induced reaction search.