Toward the Strongest Nonlinear Optical Response and Largest Birefringence in Solvent‐Free Organic–Inorganic Metal Halides by Hydrogen Bond Engineering

Abstract Organic–inorganic metal halides (OIMHs) have emerged as a pivotal structural framework for discovering novel nonlinear optical (NLO) crystals, but the dominant role of inorganic components in NLO responses has become a major bottleneck to performance improvement. In this study, a protonation‐induced hydrogen‐bond engineering strategy is proposed to introduce robust hydrogen‐bonding sites into the organic group of 4‐N,N‐dimethylamino‐4′‐N'‐methyl‐stilbazolium (DAMS + ) with strong NLO effects. This approach promotes tight bonding and dense stacking between DAMS + and the inorganic chain [Cd 2 Cl 6 ] n 2n− , leading to the synthesis and growth of a new OIMHs‐type NLO crystal [DAMS + ‐Protonation]Cd 2 Cl 6 ([DASH]Cd 2 Cl 6 ). Enhanced hydrogen bonding improves crystallinity and prevents solvent infiltration, ensuring high stability below its melting point (232 °C). Additionally, the protonation process induces a twist in the dimethylamino group of DAMS⁺, which weakens the π‐conjugation effect. This alteration effectively broadens the optical bandgap and shifts the UV cutoff edge to 380 nm. Consequently, [DASH]Cd 2 Cl 6 exhibits the strongest second‐harmonic generation (SHG) response (19 × KH₂PO₄ @1064 nm) and the highest birefringence (0.36@550 nm) among reported solvent‐free OIMHs‐type NLO crystals. This study demonstrates the feasibility of tuning NLO properties through the organic component of OIMHs, offering new insights into the structural regulation and performance enhancement of OIMHs‐type NLO crystals.