Dual tuning of second-harmonic generation and Rashba-Dresselhaus spin-splitting in chiral hybrid perovskites via halogen-site engineering

Chiral organic–inorganic hybrid metal halides are emerging as promising candidates for nonlinear optical (NLO) and spintronic applications due to their intrinsic non-centrosymmetry and structural tunability. Here, we investigate how bromine substitution at the ortho, meta, or para positions of chiral organic cations tunes the structural topology, electronic structure, second-harmonic generation (SHG) response, and Rashba-Dresselhaus spin-splitting behavior in lead iodide perovskites. Using experimentally determined single-crystal structures as input, our first-principles calculations show that, within this small series, higher calculated SHG coefficients coincide with stronger Rashba-Dresselhaus splitting. Both quantities increase together with the Electronic Chirality Measure (ECM), consistently with a tunable chirality transfer linked to the degree of chirality of the organic cation. Among the series, the meta-substituted compound (S)-m-BrMBA2PbI4 [m-BrMBA = 1-(3-bromophenyl)-ethylamine] exhibits the strongest calculated SHG response, about 4.65 times that of benchmark KH2PO4 (KDP) and the largest Rashba-Dresselhaus parameter (α = 1.193 eV·Å), dependent on the local symmetry breaking induced by bromine site engineering. Moreover, the calculated spin polarizations of the left- and right-handed enantiomers display mirror-symmetric patterns in momentum space, and their detailed topology also depends on the bromine position. We find that ECM varies systematically across the series and provides a useful molecular descriptor of chirality transfer within this dataset. Together, these results suggest that halogen-site engineering can effectively tune the structural asymmetry, calculated SHG, and SOC-driven band splitting in chiral hybrid perovskites.