Majority Rule of Enantiomeric Excess in Chiral Halide Perovskites for Continuous Tuning of Chiroptical Properties

Abstract Chiral halide perovskites have emerged as a promising platform for spin-optoelectronic applications, yet the role of chiral precursor composition in governing their chiroptical properties remains poorly understood. Here, we investigate how enantiomeric excess (ee) controls the chiroptical response of the prototypical two-dimensional (2D) chiral halide perovskite (R/S-MBA)2PbI4 (MBA = methylbenzylammonium) across a compositional series spanning ee = −100% to +100%, corresponding to R-MBA mole fractions ranging from 0% to 100%. By combining experimental spectroscopy with first-principles simulations, an approach we demonstrate to be transferable to tin-based analogues as well, we establish a quantitative structure-chiroptical property relationship across the full ee range. Circular dichroism (CD) spectra and absorption dissymmetry factors (gabs) exhibit a progressive, nonlinear evolution with increasing ee, consistent with cooperative majority-rule behavior governing chirality transfer from the organic cation to the inorganic framework. This effect is most pronounced in the 400–450 nm spectral region, where the CD signal tracks enantiomeric composition with high fidelity. Ab initio molecular dynamics coupled with time-dependent density functional theory (TD-DFT) quantitatively reproduces both the experimental CD spectra and gabs values across the entire compositional series, revealing that the macroscopic chiroptical response originates from the collective arrangement of chiral ligands, acting as a continuous dial for simultaneously modulating optical activity. This positions ee as a general, accessible design parameter for chiroptical engineering across the family of halide perovskites.