Structural Tuning via Chain Length for Emergent Second Harmonic Generation in Hybrid Metal Halides

Ten new hybrid halides were synthesized using three pyridine-based organic cations with progressively increasing side chain lengths, including 2-(methylamino)pyridine (MAP), 2-[(methylamino)methyl]pyridine (MAMP), and 2-(2-methylaminoethyl)pyridine (MAEP). Systematic variation of the chain length, halide (Cl, Br, I), and metal center (Sb, Bi) enables controlled tuning of crystal symmetry, intermolecular interactions, local distortions, and optical behavior. The medium-length, doubly protonated MAMP cation guides orientational distortions along one-dimensional inorganic chains, stabilizing six noncentrosymmetric P2₁2₁2₁ phases exhibiting strong second-harmonic generation (up to ∼1.2× KDP). Halide substitution effectively narrows the band gap and tailors absorption and emission behavior, while all compounds display ns²-type broadband luminescence at 80 K. Thermal analysis confirms high stability (280-330 °C). Notably, (MAP)₂SbBr₅ exhibits a distinct melting event near 153 °C with an average entropy change of 108.12 J·mol⁻¹·K⁻¹. Density functional theory shows inorganic-dominated valence-band maximum (VBM) and organic-dominated conduction-band minimum (CBM) contributions, demonstrating strong coupling between the structure and electronic properties. This work establishes a structure-guided pathway for rationally designing multifunctional hybrid halides.