Stability and reactivity of alkylidene dihydropyridines.

Alkylidene dihydropyridines (ADHPs) are electron-rich nucleophilic intermediates that can be readily prepared by dearomatization of 4-alkylpyridines using chloroformate reagents and mild base. Their stability and reactivity can be tuned with the chloroformate reagent used as evidenced by NMR chemical shifts and oxidation potentials. ADHPs prepared with ethyl, allyl and trichloroethyl chloroformate undergo decomposition under an oxygen atmosphere at different rates (ethyl > allyl > trichloroethyl), predominantly to the corresponding 4-acylpyridine.

Adiabat-to-Diabat Angle in Seam Space: Renner-Teller-Type and Pseudo-Jahn-Teller-Type Problems.

In this study, we examine the adiabat-to-diabat (ATD) angles for trajectories in 2-dimensional vibrational subspace of the seam space of two degenerate states. In circulating around the tangential touching degeneracy center, the ATD angle is changed by or 0, similar to the Renner-Teller problem and the pseudo-Jahn-Teller problem, respectively. These ATD angle profiles may be indistinguishable from those of circulating multiple conical intersections or a pseudo-Jahn-Teller center.

The Lack of Triplet Fusion for an Intramolecular Singlet Fission Chromophore: The Expected, the Unexpected, and a Reconciliation.

Singlet fission (SF) has the potential to play a key role in photovoltaics since it generates a larger number of longer-lived triplet excitons after photoabsorption. Intramolecular SF (iSF) is of special interest since it enables tuning of SF efficiency by adjusting interchromophore configuration through covalent interaction. However, as elaborated in the present work, iSF chromophores are doomed to dissatisfy one general thermodynamic criterion for all SF chromophores, intramolecular or not: () ≥ 2(), and therefore, the fusion of two triplet excitons to one triplet exciton is thermodynamically favorable.

The Unified Hamiltonian Formalism of Spin-Orbit Jahn-Teller and Pseudo-Jahn-Teller Problems in All Axial Symmetries.

Spatial degeneracy of electronic states closely connects spin-orbit coupling and vibronic coupling, which together determine properties of materials, especially heavy element compounds. Accurate description of those materials entails accurate mathematical formulas for spin-orbit vibronic Hamiltonians. For the first time ever, we in this work derive the Hamiltonian formalism to describe all spin-orbit Jahn-Teller and pseudo-Jahn-Teller vibronic problems in all axial symmetries.

BF-Catalyzed Intramolecular Fluorocarbamoylation of Alkynes via Halide Recycling.

A BF-catalyzed atom-economical fluorocarbamoylation reaction of alkyne-tethered carbamoyl fluorides is reported. The catalyst acts as both a fluoride source and Lewis acid activator, thereby enabling the formal insertion of alkynes into strong C-F bonds through a halide recycling mechanism. The developed method provides access to 3-(fluoromethylene) oxindoles and γ-lactams with excellent stereoselectivity, including fluorinated derivatives of known protein kinase inhibitors.

Exploring the Lewis Acidity and Reactivity of Neutral Pentacoordinate Dithienophospholes.

A series of luminescent, neutral pentacoordinate dithieno[3,2-b:2',3'-d]phosphole compounds was synthesized by [4+1] cycloaddition of o-quinones with the corresponding trivalent phospholes. The electronic and geometrical modification of the π-conjugated scaffold implemented here impacts the aggregation behavior of the species in solution. It proved successful in generating species with improved Lewis acidity of the phosphorus center that was then leveraged for small-molecule activation.

Triplet Separation after the Fastest Intramolecular Singlet Fission in the Smallest Chromophore.

Singlet fission is of key importance in harvesting solar energy in solar cells, as it generates a pair of triplet excitons on the incidence of a photon. This phenomenon is not yet widely employed in the organic photovoltaics industry mostly because of the rarity of singlet fission chromophores. Pyrazino[2,3-]quinoxaline-1,4,6,9-tetraoxide was recently designed as the smallest intramolecular singlet fission chromophore, and it undergoes the fastest singlet fission with a 16 fs time scale.

Design of the Smallest Intramolecular Singlet Fission Chromophore with the Fastest Singlet Fission.

We designed an intramolecular singlet fission (iSF) chromophore, pyrazino[2,3-]quinoxaline-1,4,6,9-tetraoxide. Appropriate substitutions into anthracene enhance the tetraradical character, so that the molecule accommodates a pair of triplet excitons in its lowest singlet excited state. Our simulation showed a 16 fs fast iSF of the design, which is a new record.

Unified one-electron Hamiltonian formalism of spin-orbit Jahn-Teller and pseudo-Jahn-Teller problems in tetrahedral and octahedral symmetries.

Heavy element compounds with high symmetries often feature both spin-orbit coupling and vibronic coupling. This is especially true for systems with tetrahedral and octahedral symmetries, whose electronic states may be threefold degenerate and experience complicated Jahn-Teller and pseudo-Jahn-Teller interactions. To accurately describe these interactions, high quality spin-orbit vibronic Hamiltonian operators are needed.

Design of singlet fission chromophores by the introduction of N-oxyl fragments.

Singlet fission is a highly desired process in organic photovoltaic devices. It splits one singlet exciton into two triplet excitons and enhances the power-conversion efficiency. However, the exploitation of this process in photovoltaic devices is plagued by the small number of singlet fission chromophores.

Unified one-electron Hamiltonian formalism of spin-orbit Jahn-Teller and pseudo-Jahn-Teller problems in axial symmetries.

Spin-orbit coupling and vibronic coupling are both closely related to orbital degeneracy of electronic states. Both types of coupling play significant roles in determining properties of heavy element compounds and shall be treated on the same footing. In this work, we derive a unified one-electron Hamiltonian formalism for spin-orbit and vibronic interactions for systems in all axial symmetries.

The synthesis, properties, and reactivity of Lewis acidic aminoboranes.

The evolution of frustrated Lewis pair chemistry has led to significant research into the development of new Lewis acidic boranes. Much of this has focused on modifying aryl substituents rather than introducing heteroatoms bound to boron. We recently reported that bis(pentafluorophenyl)phenothiazylborane (1) could be used as a Lewis acid catalyst for the heterolytic dehydrocoupling of stannanes.

Structure-reactivity studies on hypervalent square-pyramidal dithieno[3,2-b:2',3'-d]phospholes.

A series of neutral pentacoordinate dithieno[3,2-b:2',3'-d]phosphole compounds were synthesized by [4 + 1] cycloaddition with o-quinones. Counter to the expected trigonal bipyramidal geometry, the luminescent hypervalent dithienophospholes exhibit square pyramidal geometry with inherently Lewis acidic phosphorus center that is stabilized via supramolecular π-stacking interactions in the solid state and in solution. Due to their Lewis-acid character, the compounds react with nucleophiles, suggesting their potential as mediator in organic transformations.

Diboron- and Diaza-Doped Anthracenes and Phenanthrenes: Their Electronic Structures for Being Singlet Fission Chromophores.

We used quantum chemistry methods at the levels of mixed-reference spin-flip time-dependent density functional theory and multireference perturbation theory to study diboron- and diaza-doped anthracenes and phenanthrenes. This class of structures recently surged as potential singlet fission chromophores. We studied electronic structures of their excited states and clarified the reasons why they satisfy or fail to satisfy the energy criteria for singlet fission chromophores.

Bis(pentafluorophenyl)phenothiazylborane - an intramolecular frustrated Lewis pair catalyst for stannane dehydrocoupling.

We synthesized a novel Lewis acidic aminoborane containing a phenothiazyl substituent and demonstrated its potential to catalytically promote the dehydrocoupling of tin hydrides. The observed reactivity would imply a homolytic frustrated Lewis pair type mechanism, however computational analysis suggests a heterolytic mechanism for this reaction. This result represents one of the first frustrated Lewis pair systems to dehydrocouple stannanes in a heterolytic fashion.

Non-adiabatic molecular dynamics with ΔSCF excited states.

Accurate modelling of nonadiabatic transitions and electron-phonon interactions in extended systems is essential for understanding the charge and energy transfer in photovoltaic and photocatalytic materials. The extensive computational costs of the advanced excited state methods have stimulated the development of many approximations to study the nonadiabatic molecular dynamics (NA-MD) in solid-state and molecular materials. In this work, we present a novel ▵SCF-NA-MD methodology that aims to account for electron-hole interactions and electron-phonon back-reaction critical in modelling photoinduced nuclear dynamics.

Charge transfer dynamics at the boron subphthalocyanine chloride/C interface: non-adiabatic dynamics study with Libra-X.

We report a study on the non-adiabatic molecular dynamics (NA-MD) of the charge transfer (CT) process in the boron subphtalocyanine chloride (SubPc)/fullerene (C60) interface using our newly implemented Libra-X software package, which is based on an interface of the Libra NA-MD library and the GAMESS electronic structure software. In particular, we address the following aspects of the simulation protocol: (a) the choice of the potential used to treat interatomic interactions and its effect on the structures of the complex and CT rates; (b) the choice of the electronic structure methodology used; and (c) the choice of the trajectory surface hopping (TSH) methodology used. From our analysis of the electronic structure, we suggest that the distortion of the SubPc conical structure affects orbital localization and that the "breathing" motion of SubPc drives the CT process in SubPc/C60.

A ground state potential energy surface for HONO based on a neural network with exponential fitting functions.

The minimum energy structures, i.e., trans-HONO, cis-HONO, HNO, and OH + NO, as well as the corresponding transition states, i.

Scaling relationships for nonadiabatic energy relaxation times in warm dense matter: toward understanding the equation of state.

Understanding the dynamics of electron-ion energy transfer in warm dense (WD) matter is important to the measurement of equation of state (EOS) properties and for understanding the energy balance in dynamic simulations. In this work, we present a comprehensive investigation of nonadiabatic electron relaxation and thermal excitation dynamics in aluminum under high pressure and temperature. Using quantum-classical trajectory surface hopping approaches, we examine the role of nonadiabatic couplings and electronic decoherence in electron-nuclear energy transfer in WD aluminum.

Vibrational energies for HFCO using a neural network sum of exponentials potential energy surface.

A six-dimensional potential energy surface (PES) for formyl fluoride (HFCO) is fit in a sum-of-products form using neural network exponential fitting functions. The ab initio data upon which the fit is based were computed at the explicitly correlated coupled cluster with single, double, and perturbative triple excitations [CCSD(T)-F12]/cc-pVTZ-F12 level of theory. The PES fit is accurate (RMSE = 10 cm(-1)) up to 10 000 cm(-1) above the zero point energy and covers most of the experimentally measured IR data.

Ab initio potential energy and dipole moment surfaces for CS2: determination of molecular vibrational energies.

The ground state potential energy and dipole moment surfaces for CS2 have been determined at the CASPT2/C:cc-pVTZ,S:aug-cc-pV(T+d)Z level of theory. The potential energy surface has been fit to a sum-of-products form using the neural network method with exponential neurons. A generic interface between neural network potential energy surface fitting and the Heidelberg MCTDH software package is demonstrated.