Unlocking Strong Second-Harmonic Generation in Deep-UV-Transparent Polar Organic Sulfonates through Connectivity Regulation

The development of high-performance ultraviolet nonlinear optical (UV NLO) crystals requires both highly efficient NLO-functional primitives and their optimal alignment within non-centrosymmetric structures—a dual challenge difficult to address. In this study, we present a connectivity-regulation approach for construction of polar organic salts, which has been experimentally verified by systematically varying either the counter-cations or, more efficiently, the anionic alkyl tails of aliphatic sulfonates. Compositional evolution drives change in alignment of the sulfonate anions from antiparallel in the parent centrosymmetric compound Li[SO₃(CH₂)₂X](H₂O) (X = Cl and Br) to staggered antiparallel in the polar analogues Na[SO₃(CH₂)₂X](H₂O) and then to parallel in Li[SO₃(CH₂)₂OH], affording a connectivity-dependent enhancement in linear and nonlinear optical properties. Li[SO₃(CH₂)₂OH] simultaneously exhibits an ultrawide bandgap (> 6.53 eV) and the largest second-harmonic generation among deep-UV-transparent sulfonates (3.0 × KH₂PO₄ @ 1064 nm), with sufficient birefringence to enable phase-matched fourth-harmonic generation at 266 nm from Nd:YAG lasers. Theoretical calculations and crystal structure analyses suggest that the parallel alignment of the [SO₃(CH₂)₂OH]⁻ anions, facilitated through hydrogen-bonding interactions and ionic bonding, is responsible for the strong optical performance. This study highlights that structural connectivity change can profoundly influence key NLO properties, initiating a new avenue for development of high-performance UV NLO organic salts.