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

Organic-inorganic metal halides (OIMHs) have emerged as promising nonlinear optical (NLO) materials, but simultaneously achieving large birefringence (Δn) 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 [Sb2Cl9]3- anions yields the non-centrosymmetric crystal (DA7ClQ)SbCl5, 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.