From the Bulk to the Nanoscale: Strong Evolution in the Structure and Chiroptical Properties of Chiral Halide Double Perovskites

Abstract In the quest for lead‐free chiral metal‐halide materials, 2D double perovskites appear as very promising candidates especially for spintronic applications. However, compared to highly crystalline Pb ²⁺ compounds, the more challenging synthetic procedure for compounds based on Ag ⁺ and Bi ³⁺ /Sb ³⁺ so far limited the number of these chiral materials to very few reported examples, as highlighted by the absence of crystal structure based on the extensively used ( S/R )‐1‐phenylethylammonium ( S/R ‐MBA) cations in chiral metal halides. This study presents the successful preparation and characterization of a complete series of 2D bromide‐based and iodide‐based double perovskites based on the cations ( S/R )‐1‐(4‐bromophenyl)ethylammonium ( S/R ‐4BrMBA), which allows to describe their structures, thin film processing, and resulting chiroptical properties. The strong modulation of the structure and circular dichroism (CD) properties at the nanoscale is unprecedented, since the thin films of ( S / R ‐4BrMBA) ₄ AgBiBr ₈ strongly evolve from a single‐phase compound with small intrinsic CD _iso to a polymorphic material showing a strong increase in the CD signal due to macroscopic effects.