Strong Optical Nonlinearity in Polar Hypophosphites via Chiral Cation Assembly

Simultaneously optimizing second-harmonic generation (SHG) response, optical bandgap, and birefringence is essential to meet application requirements and advance the development of nonlinear optical (NLO) materials, yet it remains a significant challenge, particularly for hypophosphites. We report herein the first polar semiorganic hypophosphites (L-C6H10N3O2)(H2PO2) (L-HPO) and (D-C6H10N3O2)(H2PO2) (D-HPO) by a chiral cation assembly strategy. The two compounds are isostructural and possess three-dimensional framework structures composed of [H2PO2] anions and chiral histidinium [L/D-C6H10N3O2] cations. Both L-HPO and D-HPO simultaneously exhibit multiple key NLO parameters for applications in the ultraviolet region: short cutoff edges [233 nm (L-HPO) and 234 nm (D-HPO)], strong phase-matchable SHG responses [2.1 × KH2PO4 (KDP) (L-HPO) and 2.0 × KDP (D-HPO), the highest values among solar-blind UV hypophosphites] at 1064 nm, and sufficient birefringence [calcd: 0.077 (L-HPO) and 0.082 (D-HPO) at 546 nm]. Structural analyses and first-principles calculations suggest that the chiral [L/D-C6H10N3O2] cations and [H2PO2] anions, connected by salt-bridge hydrogen-bonding interactions, are responsible for the strong optical performance. This study suggests that the introduction of chiral organic cations not only provides a facile route to stable polar structures of hypophosphites but also affords a paradigm for the development of efficient NLO materials.