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

Abstract

Abstract 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[SO 3 (CH 2 ) 2 X](H 2 O) (X = Cl and Br) to staggered antiparallel in the polar analogues Na[SO 3 (CH 2 ) 2 X](H 2 O) and then to parallel in Li[SO 3 (CH 2 ) 2 OH], affording a connectivity‐dependent enhancement in linear and nonlinear optical properties. Li[SO 3 (CH 2 ) 2 OH] simultaneously exhibits an ultrawide bandgap (> 6.53 eV) and the largest second‐harmonic generation among deep‐UV‐transparent sulfonates (3.0 × KH 2 PO 4 @ 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 [SO 3 (CH 2 ) 2 OH] − 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.