Second-order optical nonlinearity exists in a noncentrosymmetric crystal. We investigated a new molecular concept for noncentrosymmetric crystal structure. The Λ-shaped molecules have been found to easily form noncentrosymmetric crystals by stacking along one direction. The configuration of Λ-shaped molecules is close to an optimum condition for crystal phase-matched nonlinearity. We demonstrate that methanediamine derivatives forming Λ-shaped conformation are promising as efficient χ(2) materials.

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