Lateral‐Site Unblocking Enables Bilateral π ‐Chromophore Organization for Giant Birefringence in One‐Dimensional Lead Hybrids

Birefringence in low‐dimensional organic–inorganic hybrids depends on both the intrinsic anisotropy of building units and their dense, coherent lattice organization. Herein, we identify lateral coordination‐site blocking as a structural bottleneck in one‐dimensional (1D) lead hybrids and demonstrate a lateral‐site unblocking strategy to overcome this restriction. Using a model pair, (C 12 H 8 N 2 )Pb(H 2 PO 3 ) 2 (PNPP) and (C 12 H 8 N 2 )PbCl 2 (PNPC), we reveal that the side‐occupying H 2 PO 3 − linkers enforce a sparse, single‐sided hanging mode of phenanthroline (phen) ligands. In contrast, the introduction of compact bridging Cl − ions preserves the 1D Pb‐based backbone while releasing the lateral coordination space, thereby enabling a bilateral, tightly interleaved organization of the phen π ‐chromophores. This structural switch induces a contraction of the interchain spacing and decreases the interchromophore separation from 6.79 Å in PNPP to 3.36 Å in PNPC, resulting in close π – π stacking in PNPC and enhanced spatial accumulation of the Pb‐centered and π ‐conjugated polarizability anisotropy tensors. Consequently, PNPC exhibits an exceptional birefringence of Δ n = 0.82 at 546 nm, establishing a record‐high value among all reported Pb‐based crystalline materials. This work highlights lateral‐site accessibility as a decisive structural parameter for regulating lattice‐scale polarization alignment, providing a design paradigm for next‐generation miniaturized visible‐to‐near‐infrared polarizers.