Chiral-Unit-Driven Short-Wave Ultraviolet Nonlinear Optical Acetates with Balanced Overall Optical Properties

A persistent challenge in developing novel short-wave ultraviolet (UV) nonlinear optical (NLO) crystals is how to strike a balance between large second-harmonic generation (SHG) coefficients, moderate birefringence, and a wide band gap. Combining chiral non-π-conjugated 2-methylpiperazine and π-conjugated acetate groups affords two polar acetates, R-/S-(C5H14N2)(CH3COO)2. They exhibit balanced optical properties of large SHG responses of 1.5/1.4 × KH2PO4 (KDP), moderate birefringence of 0.0871/0.0838@546 nm, and a short UV cutoff edge below 200 nm, which make them promising short-wave UV NLO crystals. Compared with the previously reported non-π-conjugated R-/S-(C5H14N2)​(HPO4)·H2O, these acetates show a 3.0/2.3 times stronger SHG response and 7.9/7.0 times higher birefringence. Hirshfeld surface analysis reveals strong hydrogen bonding interactions between cations and anions, which play important roles in directing the corresponding homochiral structures. Theoretical calculations and structural analyses elucidate the structure–property relationship. Combining chiral groups with NLO-active oxyanions is expected to efficiently develop more short-wave UV NLO crystals.