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.