(C4H8N5)(PhPHO2)·H2O: a polar ionic cocrystal with strong second-harmonic generation and high birefringence

Abstract

A polar ionic cocrystal, 2,4-diamino-6-methyl-1,3,5-triazinium phenylphosphinate monohydrate (AMTPP), (C₄H₈N₅)(PhPHO₂)·H₂O, was successfully synthesized. The material exhibits exceptional nonlinear optical (NLO) properties, including a second-harmonic generation (SHG) response approximately 2.0 times that of KH₂PO₄ (KDP), a high laser-induced damage threshold of 1.27 GW cm⁻² (1.61 × KDP), a wide band gap of 4.3 eV, and a high birefringence of 0.219 at 1064 nm, enabling phase matching. Structural analyses and theoretical calculations indicate that the intense SHG primarily originates from the (PhPHO₂)⁻ anions, which possess significant hyperpolarizability, while the high birefringence mainly stems from the planar aromatic cations and their π–π interactions. This study demonstrates that phosphinate salts are a promising class of materials for use in ultraviolet nonlinear optics.