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Over the last decades, scientists have designed molecules and materials with unique nonlinear optical (NLO) properties targeting the realization of all-optical computing and signal processing. For many reasons (large response, short response time) organic compounds are of considerable interest. However, suitable materials for practical applications are still missing because the third-order NLO responses and the related molecular second hyperpolarizability (γ) are still too small. Increasing the conjugation length was a first strategy to optimize the third-order nonlinear optical responses while in subsequent approaches appropriate substituents with specific donor and acceptor strengths were selected. More recent strategies consist of varying the shape and dimensionality of the π-electron chromophore and of charging it, for instance by chemical doping. Although these systems are closed-shell molecules, recent studies have demonstrated that open-shell species show potential as materials with high NLO efficiency and in particular, singlet diradical systems with intermediate diradical character (y). For instance, ab initio molecular orbital and density functional theory calculations performed on the p-quinodimethane model showed that γ attains a maximum in the intermediate diradical character region and subsequently that i) π-conjugated diradical systems involving imidazole rings and ii) compounds of the family of thermally stable diphenalenyl diradical systems synthesized display significant enhancements of γ with respect to their analogous closed-shell systems. Then, the dependence of γ value on the spin states makes these compounds attractive for designing multifunctional materials. In this presentation, theoretical aspects on the evaluation and interpretation of the second hyperpolarizabilities of diradical species are discussed.

We study the (hyper)polarizabilities of a singlet diradical molecule (2-borapropane-1,3-diyl) which contains a boron atom. For the purpose to control the (hyper)polarizabilities by tuning the diradical character of the boron compound, we employ water solvent as an electron donor to the boron atom. A series of quantum chemical calculations reveal that the electron donation from the lone pair of electrons in a water molecule to the vacant p orbital of boron atom gives rise to the increase of the diradical character, leading to the large variation of the polarizability of the molecule. We also investigate the solvation effects of the bulk water by utilizing the polarizable continuum model (PCM). It is shown that the PCM environment significantly increases the degree of enhancement of the second hyperpolarizability in the intermediate diradical regime of the compound. These results suggest that a singlet diradical boron compound immersed in an electron donor solvent can exhibit efficient third-order nonlinear optical properties, which can be tuned by designing the solute-solvent interactions. We examine the reliability of the PCM results by performing the additional QM/MM simulations. It is found that the QM/MM solvation effects slightly decrease the polarizability in contrast to the PCM calculation, which implies the possibility that PCM tends to overestimate the polarizability due to the lack of the ability to represent the solvation structures.

The third-order nonlinear optical (NLO) properties of open-shell singlet molecular systems are investigated using model and real molecular systems by the ab initio molecular orbital (MO) and density functional theory (DFT) approaches. The diradical character dependence of the second hyperpolarizability (γ) - the microscopic origin of the third-order NLO properties - is elucidated as well as its spin state dependence: γ values for the singlet systems with intermediate diradical character are remarkably enhanced as compared to those of closed-shell and pure diradical systems, and the γ values for intermediate diradical systems are significantly reduced by changing from singlet to triplet state. Such features are exemplified by two-site diradical model systems, diphenalenyl diradicaloids, and nanographenes. The γ values of multi-radical systems including one-dimensional diphenalenyl diradical clusters are also examined, while the intermolecular interaction effects and the size dependence of γ per unit are discussed toward realizing a new class of open-shell NLO materials.

We have studied the properties of singlet open-shell molecules that are designed, synthesized, and isolated by us. The molecules contain p-quinodimethane or cumulene structures in the middle of their skeletons, and also equip spindelocalizing moieties, phenalenyl, in the both ends for thermodynamic stabilization. Crystal structures and electronic spectra revealed that these conjugated Kekule molecules actually had open-shell character despite closed-shell chemical structures can be drawn for the compounds.

The electronic structure of bisanthene has been considered as a bottom-up approach to provide experimental evidence for the edge-state in graphene ribbons. After several steps of chemical reactions and following recrystallization, bisanthene and its tert-butyl group substituted derivative were obtained as crystals. Physical measurements demonstrated no appreciable open-shell electronic structure, while chemical reactivity toward oxygen suggested the possibility of the presence of the spin-polarized electronic state at meso-positions.

Singlet open-shell character of polyperiacenes has been elucidated by theoretical analysis based on the Clar's aromatic sextet valence bond model and quantum chemical calculations. Anti-ferromagnetic ground state of large polyperiacenes was relevant to the sufficient stabilization energy derived from aromatic sextet formation to overcome the energetic penalty associated with π-bond cleavages.

One- and two-photon absorption cross sections of symmetric open-shell singlet molecular systems are investigated using a valence configuration interaction scheme. It is found for one-photon absorption (OPA) that the peak intensity decreases and the peak position moves to the low energy region as increasing the diradical character of the system. In contrast, a significant enhancement of two-photon absorption (TPA) peak is predicted for open-shell singlet systems with intermediate diradical character as compared to the closed-shell and pure diradical systems. It is revealed that the largest TPA peak intensities occur for open-shell singlet diradicals having a ferromagnetically-coupled ground state and strongly depends on the ratio between damping factors of the excited states.

Spin polarization and third-order nonlinear optical properties, second hyperpoalrizabilities (γ), of open-shell singlet graphene nanoflakes (GNFs) with hexagonal, rhombic and bow-tie shapes are investigated using a hybrid density functional theory method. The spatial spin polarization exhibits a strong structural dependence, i.e., edge shape effect, and the diradical character is also found to depend on the structure of zigzag-edged nanoflakes. These structural dependences of diradical characters of graphene nanoflakes are expected to provide a novel control scheme of the γ values.

The static second hyperpolarizabilities (γ) of 1,3-dipole molecules in singlet states are investigated using various ab initio molecular orbital (MO) electron correlation methods and density functional theory (DFT) methods with extended basis sets. Substantial dependences of γ as a function of the method of calculation are observed. At the UCCSD(T) level of theory, we observe the enhancement of γ in the intermediate diradical character regions as compared to those of small and pure diradical character regions.

Parameter adjustment of neural electrode for a target brain region is essential to obtain good signal‐to‐noise ratio recording of neuronal activities in the brain. However these parameters have been empirically decided so far. In this study, we develop a simulation system, which is composed of three‐dimensional models of electrode and neural tissue of the hippocampal CA3 region. We estimate the model parameters and evaluate the simulation results based on the fluctuation analysis of extracellular neuronal potentials recorded experimentally. This system would be useful for the design and test of the neural electrode.

A new concept on magnetic field of plasma production and confinement has been proposed to enhance efficiency of an electron cyclotron resonance (ECR) plasma for broad and dense ion beam source under the low pressure. The magnetic field configuration is constructed by a pair of comb‐shaped magnet which has opposite polarity each other, and which cylindrically surrounds the plasma chamber. This magnetic configuration suppresses the loss due to ExB drift, and then plasma confinement is enhanced. The resonance zones of the fundamental and the second harmonics for 2.45GHz microwaves detach from the wall of the chamber. The connection length of the magnetic field lines through the resonance zone is elongated, and the confinement is better than that of the simple multipole magnetic field. The 2.45 GHz microwaves are fed from the side wall by the rod antenna. The electron density attained to about four times cutoff density for the 2.45GHz microwave at the low Ar pressure below 0.08Pa and also the low microwave power below 300W. We compare profiles of the electron density and temperature in the comb‐shaped magnetic field configuration with those in the simple multipole magnetic field.