Chiral-Unit-Oriented Design of Non-π-Conjugated Noncentrosymmetric Phosphates with Short Absorption Edges

Deep ultraviolet (UV) nonlinear optical (NLO) crystals are important optoelectronic materials in high-tech applications. How to effectively design a noncentrosymmetric (NCS) structure, which is the rigorous prerequisite for second-order NLO crystals, remains a great challenge. Introducing chiral methylpiperazine into phosphates achieved two polar phosphates, R- and S-(C₅H₁₄N₂)(HPO₄)·H₂O. The chiral units not only serve as structure-directing agents for generating NCS structures but also do not have a negative impact on UV absorption, making the phosphates maintain short absorption edges below 200 nm. They also exhibit moderate NLO activities of ∼0.5/0.6 KH₂PO₄ (KDP). Hirshfeld-surface analysis reveals that hydrogen-bonding along with Coulomb interactions rivet the crystallization of the units in the polar space group. Dipole moment calculations confirm the direction of macroscopic polarization. The first-principles calculations indicate that the optical properties mainly originate from the synergistic interaction of inorganic and organic units. This study will provide useful insights into the targeted design of NCS optical materials.