Conformational Restriction Unlocks Record Birefringence in Sulfonate Ultraviolet NLO Crystals

ABSTRACT

Optimizing second–harmonic generation (SHG), birefringence, and bandgap simultaneously within a single material remains a formidable challenge due to their intrinsic mutual trade‐offs. Herein, we propose a conformational restriction strategy that replaces a flexible monocyclic cation with a rigid polycyclic analogue, which affords two organic ionic sulfonate crystals, [C ₅ H ₆ NO][SO ₃ (C ₇ H ₅ O ₃ )]·2H ₂ O ( HPSA ) and [C ₁₂ H ₉ N ₂ ][SO ₃ (C ₇ H ₅ O ₃ )] ( OPSA ). HPSA adopts a highly ordered centrosymmetric (CS) crystal structure, Pbca , stabilized by strong and directional hydrogen bonds. Whereas OPSA crystallizes in a noncentrosymmetric (NCS) space group P 2 ₁ arising from steric hindrance and weak, asymmetric supramolecular interactions, thereby endowing OPSA with a stable SHG response (1.1 × KDP), with no observable intensity degradation upon heating up to 533 K. Both compounds possess wide bandgaps and pronounced optical anisotropy, notably OPSA with record‐high birefringence ( Δ n = 0.93@546 nm) among all known NLO crystals containing π‐conjugated groups with bandgap exceeding 3.1 eV. Moreover, OPSA achieves single‐phase white light with an ultrabroad dual‐band spectrum spanning 400–800 nm, enabling a prototype white‐light‐emitting diode with a color rendering index of 87, CIE chromaticity coordinates of (0.31, 0.32), and correlated color temperature of 7185 K. This work expands the structural diversity of sulfonate‐based materials and establishes a rational design strategy for multifunctional optical crystals.