The role of halogens in structural diversity and chirality enhancement of 1D chiral hybrid metal halides

Abstract

The impact of halogens on chiral crystal structures and properties has been a significant focus. In this study, we utilize protonated R/S-tetrahydro-2H-pyran-3-ammonium (R/S-3ATHP) as an organic cation to synthesize four pairs of one-dimensional (1D) helical chiral hybrid metal halides. The replacement of the halogen anions (X = Cl⁻, Br⁻, and I⁻) and variation of the cation/anion ratio induce changes in the 1D chain packing mode and helicity of the inorganic components in the crystals. Notably, the Br-based chiral hybrid metal halides exhibit two structures, analogous to the Cl (zigzag, z-) and I (linear, l-) counterparts. Structural variations resulting from halogen adjustments further modulate the chiroptical activity, semiconducting, and photoluminescence properties of the crystals. Particularly noteworthy is the helicity of the [PbX₆] octahedral-based chains that is significantly influenced by the asymmetric hydrogen bonding interactions which are determined by the halogens and the number of organic cations. The regular change observed in g_CD values: z-(R/S-3ATHP)₃PbBr₅ > z-(R/S-3ATHP)₃PbCl₅ > l-(R/S-3ATHP)PbI₃ > l-(R/S-3ATHP)PbBr₃, is correlated with helicity of the helical 1D chains in the crystals, indicating that more helicity corresponds to stronger chiroptical activity. This work establishes a model system for comprehending the interplay between halogens and structural diversity as well as helicity and chiroptical activity, contributing to a deeper understanding of these relationships in related fields.