Large Exchange-Driven Intrinsic Circular Dichroism of a Chiral 2D Hybrid Perovskite

In two-dimensional (2D) chiral metal-halide perovskites (MHPs), chiral organic spacers endow structural and optical chirality to the metal-halide sublattice, enabling exquisite control of light, charge, and electron spin. The chiroptical properties of 2D-MHPs have been measured by transmissive circular dichroism (CD) spectroscopy, which necessitates thin-film samples, and the results depend on the film thickness and morphology. Here, by developing a reflection-based approach, we characterize the intrinsic, circular polarization-dependent complex refractive index (RI) for a prototypical 2D chiral lead-bromide perovskite. Remarkably, comparison with <i>ab initio</i> theory reveals the large CD arises from the inorganic sublattice rather than the chiral ligand and is an excitonic phenomenon driven by electron-hole exchange interactions, which breaks the degeneracy of transitions between Rashba-Dresselhaus-split bands. The obtained RIs between opposing circular polarizations suggest that previous CD data of spin-coated films largely underestimate the true optical chirality, which we attribute to their lower crystallinity in comparison to single crystals. Our study provides quantitative insights into the intrinsic optical properties of 2D chiral MHPs, paving the way for their prospective chiroptical and spintronic applications.