New Research


“Open-Shell Character” Pioneers a Novel Class of Nonlinear Optical Materials

Prof. Masayoshi Namkano, Assoc. Prof. Yasutaka Kitagawa, Assist. Prof. Ryohei Kishi

Most of materials in chemistry are composed of molecules, which consist of plural atoms held together by “chemical bond”.  Namely, the first step to understand the structure, property, and reactivity of materials is to elucidate the nature of “chemical bond”.  The fundamental understanding of “chemical bond” can be done by quantum theory.  In our group, we introduce a chemical index, “open-shell character”, which is quantum chemically well-defined and indicates a bond weakness or the degree of electron correlation, in order to clarify the nature of chemical bonds in various molecular systems.  Furthermore, using quantum chemical calculations, we reveal the relationships of the open-shell character with the structure, reactivity, and property of molecules, and then design highly active functional materials from a unified viewpoint of “open-shell character”.  This approach will lead to a genuine breakthrough in materials science and engineering.  For instance, using the “open-shell character” concept, we perform theoretical and computational quantum chemical design for highly efficient nonlinear optical (NLO) molecular systems, which are expected to play a central and fundamental role in future photonic applications.  In particular, we first presented “a theory of novel NLO materials based on open-shell singlet molecular systems”, where we clarified the open-shell (diradical) character (y) dependences of the second hyperpolarizability g (the third-order NLO property at the molecular scale): open-shell singlet molecular systems with intermediate diradical character exhibit larger second hyperpolarizability than conventional closed-shell and pure diradical molecular systems.  On the basis of this principle, we also carried out design for real molecules in collaboration with experimentalists, and found that several polycyclic aromatic hydrocarbons including phenalenyl diradical systems exhibit significantly large two-photon absorption (TPA) cross sections (a typical third-order NLO property).  Recently, several experimental research groups synthesize various novel open-shell singlet molecules and verify our principle, which contributes to pioneering a new research field “Open-Shell Singlet NLO Materials”. 

              Please visit the following URL for details and other research activities in our group.
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