Open-shell Electronic States Research Articles

Thienothiophene-centered ladder-type π-systems that feature distinct quinoidal π-extension

Quinoidal π-conjugated structures, a kind of fundamental subunits for organic π-systems, may produce some intriguing optical, electronic and magnetic properties of polycyclic hydrocarbons (PHs). Herein, we report two thienothiophene-centered ladder-type polycyclic molecules (1 and 2), which possess one quinoidal thienothiophene moiety and two para-quinodimethane (p-QDM) subunits, respectively. As theoretically and experimentally studied, while 1 is a fully closed-shell molecule, 2 owns an open-shell structure along with partial contribution of tetraradical state that is induced by the resonance of p-QDM. Moreover, although 2 has a larger π-conjugated skeleton and open-shell electronic state, it exhibits larger bandgap and blue-shifted absorption. On the other hand, the reversible oxidation activity of 1 enables the preparation of its dication 12+, and the studies on its single-crystal and aromatic structures demonstrate that its two positive charges are delocalized onto the oxygen atoms, thus achieving fully π-extended structure and near-infrared absorption. This study not only gains insight into quinoidal π-subunits, but also provides an important basis for the development of antiaromatic and open-shell π-electron materials.

Bis(9-Boraphenanthrene) and Its Stable Biradical.

Selective and site-specific boron-doping of polycyclic aromatic hydrocarbon frameworks often give rise to redox and/or photophysical properties that are not easily accessible with the analogous all-carbon systems. Herein, we report ligand-mediated control of boraphenanthrene closed- and open-shell electronic states, which has led to the first structurally characterized examples of neutral bis(9-boraphenanthrene) (2-3) and its corresponding biradical (4). Notably, compounds 2 and 3 show intramolecular charge transfer absorption from the 9-boraphenanthrene units to p-quinodimethane, exhibiting dual (red-shifted) emission in solution due to excited state conjugation enhancement (ESCE). Moreover, while boron-centered monoradicals are ubiquitous, biradical 4 represents a rare type of open-shell singlet compound with 95% biradical character, among the highest of any reported boron-based polycyclic species with two radical sites.

Algebraic Diagrammatic Construction Theory for Simulating Charged Excited States and Photoelectron Spectra.

Charged excitations are electronic transitions that involve a change in the total charge of a molecule or material. Understanding the properties and reactivity of charged species requires insights from theoretical calculations that can accurately describe orbital relaxation and electron correlation effects in open-shell electronic states. In this Review, we describe the current state of algebraic diagrammatic construction (ADC) theory for simulating charged excitations and its recent developments. We start with a short overview of ADC formalism for the one-particle Green's function, including its single- and multireference formulations and extension to periodic systems. Next, we focus on the capabilities of ADC methods and discuss recent findings about their accuracy for calculating a wide range of excited-state properties. We conclude our Review by outlining possible directions for future developments of this theoretical approach.

大気安定な発光性ポリクロロトリアリールメチルラジカルの開発と機能創出

Stable radicals possess open-shell electronic states and exhibit vivid physical properties such as electrical conductivity and magnetism attributed to unpaired electrons. Their luminescence properties, however, have yet to be well examined, mainly due to the rarity of emissive radicals and chemical reactivity in their excited states. I have developed highly-photostable luminescent organic radicals and luminescent open-shell metal complexes with luminescent radicals as ligands, and investigated photofunctions based on their doublet or multiplet states. The studies suggest that the radicals can demonstrate photofunctions unique to their open-shell electronic states, such as magnetic-field-controlled luminescence, which are unexpected or impossible to achieve for conventional closed-shell molecules. Such functions will expand the scope of the radicals in molecular photonics.

Monometallic Endohedral Azafullerene.

Azafullerenes derived from nitrogen substitution of carbon cage atoms render direct modifications of the cage skeleton, electronic, and physicochemical properties of fullerene. Gas-phase ionized monometallic endohedral azafullerene (MEAF) [La@C81N]+ formed via fragmentation of a La@C82 monoadduct was detected in 1999, but the pristine MEAF has never been synthesized. Here, we report the synthesis, isolation, and characterization of the first pristine MEAF La@C81N, tackling the two-decade challenge. Single-crystal X-ray diffraction study reveals that La@C81N has an 82-atom cage with a pseudo C3v(8) symmetry. According to DFT computations, the nitrogen substitution site within the C82 cage is proposed to locate at a hexagon/hexagon/pentagon junction far away from the encapsulated La atom. La@C81N exists in stable monomer form with a closed-shell electronic state, which is drastically different from the open-shell electronic state of the original La@C82. Our breakthrough in synthesizing a new type of azafullerene offers a new insight into the skeletal modification of fullerenes.

Photostable Luminescent Triarylmethyl Radicals and Their Metal Complexes: Photofunctions Unique to Open-shell Electronic States

Stable organic radicals with unpaired electrons exhibit vivid physical characteristics such as magnetic and electron-conducting properties, while being assumed as non-emissive chemical species. This highlight review introduces the luminescence properties of radicals and radical-ligated metal complexes as emerging functions of open-shell molecular systems. Their unique photofunctions will expand the utility of radicals in molecular photonics.

Scaling reducibility of metal oxides

The reducibility of bulk metal oxides in which the cation is in its highest oxidation state (MgO, Sc2O3, Y2O3, TiO2, m-ZrO2, m-HfO2, CeO2, V2O5, Nb2O5, Ta2O5, WO3, CrO3, Al2O3, β-Ga2O3, SiO2, SnO2 and ZnO) has been studied by standard periodic density functional theory. We have defined and calculated descriptors able to describe and quantify semi-quantitatively the extent of reduction: electronic band gap, oxygen vacancy formation energy and electronic localization. We find that there is no single criterion for characterizing the reducibility. We discuss the advantages and limitations of each method, and we apply them to classify the materials with the PBE+U and B3LYP functionals. Typical irreducible oxides such as MgO show a large band gap, high oxygen vacancy formation energy and electronic localization of the reduction electrons forming and F-center, with a diamagnetic singlet electronic state. Reducible oxides such as TiO2 present small band gaps, small oxygen vacancy formation energy and electron localization of the reduction electrons in the cations, decreasing their oxidation state and presenting open-shell electronic states. Intermediate or ambivalent behavior is found for ZrO2, HfO2, β-Ga2O3, ZnO and SnO2.

What Factors Control the Reactivity of Cobalt–Imido Complexes in C–H Bond Activation via Hydrogen Abstraction?

Metal–imido complexes are critical intermediates in transition metal-catalyzed C–H amination reactions. Discerning the factors that control their reactivity, however, remains largely open for exploration, particularly for the territory of cobalt–imido’s. Herein we describe a systematic computational exploration of this new frontier via the C–H activation mechanisms of typical well-defined cobalt–imido complexes, whose formal oxidation states cover an extremely wide range from Co(II) to Co(V). Hydrogen atom abstraction (HAA) is found to be the rate-limiting step in all these systems, with the open-shell electronic states of radical character consistently bearing kinetic advantage over the closed-shell ones. Surprisingly, there is no correlation found between the cobalt oxidation state and the HAA reactivity. To render a more accessible HAA channel, the dichotomous EER/anti-EER electron-shift scenarios for the open-shell electronic structure are dependent on the cobalt oxidation states [Co(III), different f...

A reduced form of the spin–spin centrifugal distortion Hamiltonian for orthorhombic asymmetric top molecules

The spin–spin centrifugal distortion Hamiltonian for an asymmetric top molecule in a given vibrational level of an open-shell electronic state with S ⩾ 1 may contain more parameters than can be determined from the observed energy levels. This paper describes the reduction of the Hamiltonian by means of a unitary transformation to a form suitable for fitting to observed energies. It is established that there are six determinable spin–spin centrifugal distortion parameters for a molecule of orthorhombic symmetry; the corresponding matrix elements are derived for both Hund’s case (a) and case (b) coupling schemes.

Iterative CI general singles and doubles (ICIGSD) method for calculating the exact wave functions of the ground and excited states of molecules

The iterative configuration-interaction general singles and doubles (ICIGSD) method was applied to various closed- and open-shell electronic states of molecules within finite basis sets and was shown to give the exact results that are identical to the full CI ones. The structure of the ICIGSD is unique among the ICI formalisms, that is, the singularity problem intrinsic to atomic and molecular Hamiltonians can be avoided. The convergence of the ICIGSD method was fairly good regardless of the characters of the electronic states and the qualities of the basis sets; only several iterations were enough for obtaining microhartree accuracy. These favorable properties are attributed to the unique GSD structure. The present method was shown to be applicable to various spin states and to quasidegenerate states appearing in bond dissociation process. We have also applied the ICIGSD-CI method to calculate the excited states simultaneously. We have confirmed that the ICIGSD-CI method is accurate for calculating the excited states the symmetries of which are not only similar to but also different from that of the ground state.

Nonlinear optical properties of open-shell polychlorotriphenylmethyl radicals

The second-order nonlinear optical (SONLO) response of a series of the highly chemically and thermally stable polychlorotriphenylmethyl monoradicals 1–8 has been studied by Hyper-Rayleigh Scattering. These radicals exhibit relatively high nonlinear optical (NLO) responses with β values ranging from 118×10−30 to 755×10−30 esu, which may result from an enhancement of the SONLO activities according to their open-shell electronic state. Hyperpolarizability data for radicals 1–8 have also been correlated with their redox potentials, correlation that has turned out to be linearly proportional either with the oxidation or the reduction potentials. These data have been justified in terms of molecular orbital energies. Finally we have successfully explored the capacity of radicals 1–8 to participate in a molecular switching array involving two main steps, an acid/base reaction and a redox process. The presence of open-shell character for this family of compounds makes them attractive candidates to design new multifunctional materials that would simultaneously possess magnetism and NLO.

Crystal Structures of Tetrathiafulvalene Multiannulated Macrocycles in Open-Shell Electronic State

Crystal structures of dimethylthio-tetrathiafulvalene (DMT-TTF) multiannulated macrocycles, bis(DMT-TTF) and tris(DMT-TTF), were examined. The intramolecular dimerization was a common structural feature for these multi-TTF macrocycles in open-shell electronic state.

Charge Transfer Mechanism and Electronic States of Acceptor-Type Graphite Intercalation Compounds

A new model for the charge transfer mechanism and electronic states of the acceptor-type graphite intercalation compounds is proposed. The essential points of this model are the following two. One is the transformation of the molecular intercalate into another molecular form with the open shell electronic state. The other is the existence of the Fermi surface originated in the intercalate due to the incomplete charge transfer to its singly occupied molecular orbital.

Tuning of Intramolecular π-π Overlap Mode of Tetrathiafulvalene Bisannulated Macrocycles in the Open-Shell Electronic State

Abstract Charge-transfer complexes of bis(methylthio)tetrathiafulvalene (DMT-TTF) bisannulated macrocycles with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) were prepared. The crystal structural analysis revealed an intramolecular dimer structure with the π-π overlap of DMT-TTF units. The magnitude of intramolecular π-π overlap depends on the ring size of macrocycles.

Large Second-Order Optical Nonlinearities in Open-Shell Chromophores. Planar Metal Complexes and Organic Radical Ion Aggregates

Attempts to discover molecular chromophores with large hyperpolarizabilities have largely focused on closed-shell organic species. This contribution explores the use of sum over states (SOS) perturbation theory and the computationally efficient intermediate neglect of differential overlap (INDO) model Hamiltonian, combined with single configuration interaction (SCI) techniques, to calculate frequency-dependent quadratic molecular hyperpolarizabilities of both coordination complexes and organic radical ion chromophore aggregates having open-shell doublet ground state electronic structures. The correct evaluation of the lowest-lying excited states has been tested by comparison with experimental optical spectra. In the cases considered here, the INDO/SCI-SOS method provides both a good description of linear optical transitions and an adequate prediction of second-order nonlinearity. It is seen that the larger second-order nonlinearities of systems having open-shell electronic states, as compared to those having analogous closed-shell structures, are a consequence of accessible lower-lying charge-transfer excited electronic states. The INDO/SCI-SOS model is attractive for designing new, highly efficient open-shell second-order nonlinear optical chromophores.

A reduced form of the spin-rotation Hamiltonian for asymmetric-top molecules, with applications to HO 2 and NH 2

The spin-rotational Hamiltonian for an asymmetric-top molecule in a given vibrational level of an open-shell electronic state may contain more parameters than can be determined from the observed energy levels. This paper describes the reduction of the Hamiltonian by means of a unitary transformation to a form suitable for fitting to observed energies. It is shown that, for molecules of lower than orthorhombic symmetry, there are fewer determinable quadratic spin-rotation parameters than have been used previously. For example, for a molecule belonging to the group C 8, there are four, not five, determinable spin-rotation constants, ϵ αβ . Similar indeterminacies exist among the quartic terms of the spin-rotation Hamiltonian. The case of a molecule of orthorhombic symmetry, for which there are six determinable quartic parameters, is considered in detail. The results are applied to the experimental data available on the spin-rotation splittings of the HO 2 and NH 2 radicals in their ground vibrational and electronic states.

Generalization of Dewar's half-electron method for calculating energies of open-shell electronic states

Dewar's “half-electron” model for calculating electronic energies of certain open-shell doublet and triplet states is extended so as to be applicable to the lowest-energy open-shell state of any given symmetry and multiplicity.