Pyrrolidin-1-ium-2-carboxylate·4-hydroxybenzoic acid co-crystal: a novel phase-matchable acentric organic material for efficient second-harmonic generation

Acentric organic crystals that combine strong second-harmonic generation (SHG) with phase-matchability are of particular interest for advanced nonlinear-optical (NLO) devices. We report the growth of a 1 : 1 co-crystal of pyrrolidin-1-ium-2-carboxylate and 4-hydroxybenzoic acid (P2C4HBA), a novel organic material in which protonated pyrrolidinium (NH⁺) and deprotonated carboxylate (COO⁻) groups coexist with neutral carboxylic acid (–COOH) units in a non-centrosymmetric space group P2₁2₁2₁. The co-crystals are optically transparent (∼87% transmittance across 300–1100 nm) with a bandgap of 4.18 eV, as determined from Tauc's plot. Kurtz–Perry powder measurements show that P2C4HBA exhibits an SHG intensity (I_SHG) 3.74 × KDP and 2.28 × urea; the signal saturates at ∼135 μm particle size, demonstrating effective phase-matchability. The co-crystal also exhibits a low dielectric constant (ε_r = 2.97 at 1 kHz, 313 K), a high laser-damage threshold (5.29 GW cm⁻² at 1064 nm), and thermal stability up to 182 °C. In addition, quantum chemical calculations were performed on isolated P2C4HBA molecules to evaluate their electronic structure and NLO properties. Hirshfeld surface analysis was employed to quantitatively and visually interpret the intermolecular interactions governing the crystal packing. The synergistic combination of competitive, phase-matchable SHG response, wide optical transparency, dielectric robustness, and high thermal and laser resilience positions P2C4HBA as a promising candidate for next-generation NLO and optoelectronic applications, including terahertz generation, frequency conversion, and electro-optic modulation.