From Octahedral to Pseudo-Seven-Coordinate: Halogen Bonding Reshapes Pb ²⁺ Geometry and Exciton Dynamics

To address the limited understanding of how the Pb²⁺ coordination geometry governs photoluminescence (PL) in low-dimensional lead halides, we employ halogen bonding (XB) as a strategic tool to systematically modify one-dimensional (1D) hybrid perovskite structures. By varying the halide composition and incorporating XB donors, single-crystal diffraction and density functional theory (DFT) calculations uncover a fundamental restructuring of the coordination environment. The iodoplumbate-based system without XB forms a conventional octahedral geometry, whereas its chloroplumbate analogue adopts an asymmetric "5 + 1" arrangement. Introducing XB dramatically reshapes both, driving them toward a previously unreported "5 + 2" pseudo-seven-coordinate structure, where an added Pb···X-C interaction imposes significant lattice distortion. Low-temperature (77 K) PL resolves distinct emission bands near 500 and 600 nm, assigned to free exciton (FE) and self-trapped exciton (STE) emission, respectively. Crucially, XB-modified iodoplumbate exhibits a pronounced enhancement of the STE emission alongside a subtle suppression of the FE band, whereas the chloride analogue lacks this prominent long-wavelength feature. This contrast underscores the role of the specific "5 + 2" coordination, which fosters carrier localization and stabilizes STE formation. This work demonstrates that precise coordination control via XB is a powerful method for enhancing structural complexity and deliberately tuning optoelectronic performance in low-dimensional perovskite materials.