Engineering Multiple Hydrogen‐Bond Sites With D‐π‐A Motifs for Strong Optical Nonlinearity and Giant Birefringence in Organic‐Inorganic Halides

ABSTRACT Organic‐inorganic metal halides (OIMHs) have emerged as promising nonlinear optical (NLO) materials, but simultaneously achieving large birefringence (Δ n ) and strong second‐harmonic generation (SHG) remains challenging owing to the propensity of π‐conjugated organic motifs to favor centrosymmetric packing. Herein, we develop a molecular design strategy by constructing a donor‐π‐acceptor (D‐π‐A) long‐chain cation, [DA7ClQ] 2+ , featuring multiple hydrogen‐bonding sites. Hybridization with inorganic [Sb 2 Cl 9 ] 3− anions yields the non‐centrosymmetric crystal (DA7ClQ)SbCl 5 , which exhibits a record birefringence of 0.46 at 550 nm among OIMHs‐based NLO crystals, coupled with a robust SHG response of 0.4 × KTP at 1550 nm and exceptional thermal and environmental stability. Theoretical calculations reveal that the D‐π‐A motif possesses superior first‐order hyperpolarizability (12.1 × 10 −29 esu) and polarizability anisotropy (739 a.u.), while a robust hydrogen‐bond network locks the acentric alignment. This work establishes that engineering extended π‐conjugated systems with augmented asymmetry and precise orientational control provides a versatile pathway toward advanced hybrid NLO materials.