Dimensional Engineering in Hybrid Transition Metal Halide Materials through the Introduction of Chiral Organic Cations

Hybrid transition metal halides are an emerging class of materials whose structural diversity and compositional tunability offer a wide landscape for the discovery of new optoelectronic functions. Here we report the synthesis and structural evolution of crystalline hybrid Cu(I) and Ag(I) iodides, templated by chiral R/S-β-methylphenethylammonium (R/S-β-MPEA) and its achiral analog phenethylammonium (PEA). Substitution of PEA with β-MPEA introduces a pronounced structural transformation, reducing the dimensionality of the inorganic lattice from extended one-dimensional metal-iodide chains to isolated M₂I₆ dimers. Crystallographic analysis reveals that the dimensional reduction is driven by steric constraints imposed by the methyl group of β-MPEA, which favors the formation of discrete inorganic motifs. Circular dichroism spectroscopy reveals the emergence of chiroptical activity in the inorganic framework, evidence for effective chirality transfer from the organic cations to the metal-iodide dimers. Finally, metal alloying in (β-MPEA)₄Cu_2–xAg_xI₆ is shown to tune both bandgaps and chiroptical responses. Together, these results, supported by insights from crystallography, highlight a synthetic design strategy for accessing low-dimensional, chiral hybrid materials with tunable optoelectronic properties.