Ligand-Directed Hydrogen Bond Symmetry Regulates Ultraviolet Absorption in Organic–Inorganic Metal Halides

We report a series of Sb-based hybrid metal halides templated by isomeric bipyridinium ligands (3,3′-bipyridine, 2,3′-bipyridine, 2,2′-bipyridine, 2,4′-bipyridine, and 4,4′-bipyridine), exhibiting a systematic color evolution from white to deep yellow. Structural and electronic analyses reveal that the gradual band gap reduction (from ∼3.36 to ∼2.70 eV) originates from enhanced electron localization at C and N sites, driven by hydrogen-bonding (N–H···Cl and C–H···Cl) environmental changes around the organic cations. This ligand–isomer-directed “hydrogen bond engineering” regulates the electronic structure and light absorption ability of hybrid metal halides without altering the metal center. Leveraging this tunability, we fabricate solution-processed coatings exhibiting >70% visible-light transmittance with strong ultraviolet (UV) absorption (cutoff ∼400 nm). This work establishes bipyridinium isomerism as a novel strategy for band gap control in organic–inorganic metal halide (OIMH) optical absorption crystal materials and demonstrates their promise as high-clarity UV shielding layers.