Two Hydrated Ionic Cocrystals of Phenylphosphonate Exhibiting Second-Harmonic Generation and Proton Conductivity

Abstract

Phosphates and phosphonates have attracted extensive research owing to their diverse functional capabilities. In this work, two new hydrated ionic cocrystals, [C(NH2)3]2(PhPO3)·2H2O (1) and (C5H7N2)2(PhPO3)·5H2O (2), were synthesised using a simple solution evaporation method. Single-crystal structural analyses revealed that both compounds crystallise in polar space groups and feature phenylphosphonate anions, guanidinium or 4-aminopyridinium cations, and lattice water molecules. Compounds 1 and 2 exhibit pronounced second-harmonic generation (SHG) responses of approximately 0.74 × KH2PO4 (KDP) and 2.0 × KDP, respectively, together with high laser damage thresholds of 1.44 GW cm−2 (1.82 × KDP) and 1.23 GW cm−2 (1.56 × KDP). The two compounds exhibit UV absorption edges at short wavelengths of 285 nm and 330 nm, respectively. Notably, substantial proton conductivities of 1.25 × 10−3 S cm−1 and 3.94 × 10−3 S cm−1 were measured for compounds 1 and 2, respectively. Overall, compound 2 outperforms compound 1 and many reported phosphonate-based materials in terms of SHG and proton conductivity. Theoretical calculations attribute these exceptional properties to the synergistic interaction between the phosphonate anion with a large hyperpolarizability, the push-pull π-conjugated cation, and the extended hydrogen-bond network. This study establishes a new structural paradigm for the design of advanced multifunctional materials.