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, (C12H8N2)Pb(H2PO3)2 (PNPP) and (C12H8N2)PbCl2 (PNPC), we reveal that the side-occupying H2PO3 - 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 Δn = 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.