Theoretical Study on Open-Shell Nonlinear Optical Systems
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Theoretical Study on Open-Shell Nonlinear Optical Systems Masayoshi Nakano,1 Ryohei Kishi,1 Nozomi Nakagawa,1 Tomoshige Nitta,1 Takashi Kubo,2 Kazuhiro Nakasuji,2 Kenji Kamada,3 Koji Ohta,3 and Benoît Champagne,4 Edith Botek,4 Satoru Yamada,2 and Kizashi Yamaguchi2 1
Division of Chemical Engineering, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan 2 Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan 3 Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan 4 Laboratoire de Chimie Théorique Appliquée Facultés Universitaires Notre-Dame de la Paix, rue de Bruxelles, 61, 5000 Namur, Belgium ABSTRACT The static second hyperpolarizabilities (γ) of open-shell organic nonlinear optical (NLO) systems composed of singlet diradical molecules are investigated using ab initio molecular orbital (MO) and density functional theory (DFT) methods. It is found that neutral singlet diradical systems with intermediate diradical characters tend to enhance γ as compared to those with small and large diradical characters. This suggests that the diradical character is a novel control parameter of γ for singlet diradical systems. INTRODUCTION So far, the investigation of nonlinear optical (NLO) properties has been mostly limited to the closed-shell systems though pioneering studies have been carried out to enhance the fieldinduced charge fluctuations in charged radical states [1-4] in view of designing multi-functional materials, for example, combining magnetic and optical properties [5-7]. On the basis of the perturbational analysis of virtual excitation processes of γ, the charged radical systems having symmetric resonance structure with invertible polarization (SRIP) [8-10] are expected to have unique negative second hyperpolarizability (γ), which is the microscopic origin of the third-order NLO property, in the off-resonant region and to exhibit large electron-correlation dependences [1,11]. Some crystals composed of these charged radical systems are predicted to present unique third-order NLO properties (negative γ) as well as high electrical conductivity [3,9]. Further, the open-shell molecules and their clusters involving nitronyl nitroxide radicals, which are important in realizing organic ferromagnetic interaction, also belong to the SRIP systems and are predicted to give negative off-resonant γ values. The characteristic of open-shell NLO systems has also been determined from the viewpoint of the chemical bonding nature [12-14]. In previous studies [12,14], we have predicted a remarkable variation in γ of H2 model with increasing the bond distance and have suggested a significant enhancement of γ in the intermediate correlation (intermediate bond breaking) regime. This feature is understood by the fact that the intermediate bonding electrons are sensitive to the applied field, leading to large fluctuation of el
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