Diiodine-Induced Dimensionality Evolution in Two Antimony(III) Halides for Optimal-Bandgap Photovoltaics

Developing lead-free organic-inorganic hybrid metal halides with strong light-harvesting capabilities and superior stability, while precisely tuning their crystalline phases and electronic structures, remains a key challenge in optoelectronics. Herein, we report a neutral iodine-induced structural transition from a 1D chain-like (C₆H₁₁NH₃)₂SbI₅ architecture to a 0D dimeric (C₆H₁₁NH₃)₃[Sb₂I₉]·I₂ supramolecular host-guest complex. This transformation is achieved via a controlled solution-cooling crystallization process, yielding high-quality bulk single crystals. Crystallographic analysis reveals that N-H···I hydrogen-bonding networks stabilize the organic cations, while halogen bonding interactions anchor the I₂ guests within the lattice cavities of the [Sb₂I₉]^3- dimeric host. Experimental characterizations, including XRD, TGA, and XPS, confirm the high phase purity and thermal stability of the (C₆H₁₁NH₃)₃[Sb₂I₉]·I₂ hybrid and determine its electronic band structure. To further elucidate the underlying mechanisms, theoretical calculations were performed, revealing that strong sp-orbital hybridization yields a high absorption coefficient. The associated dimensional transition narrows the direct optical bandgap to 1.46 eV, approaching the Shockley-Queisser limit and demonstrating strong potential for visible-light harvesting. This work elucidates the role of supramolecular host-guest interactions in modulating the lattice evolution of lead-free antimony-based materials, presenting halogen guest engineering as an effective approach for optoelectronic material design.