Chiral Two-Dimensional Cu–Pb Bromides: Circularly Polarized Luminescence and Pressure-Enhanced Optical Properties

The search for high-performance double perovskite-related materials remains constrained by the limited synthetic accessibility of bimetallic halides compared to their conventional halide double perovskite counterparts, leaving substantial unexplored territory in this domain. A promising structural modification strategy involves the incorporation of chiral organic moieties into the metal halide frameworks, enabling precise engineering of noncentrosymmetric structures toward targeted functional properties. Here, we report a pair of chiral two-dimensional (2D) Cu(I)-Pb bimetallic bromides (R/S-PCA)₄Cu₂PbBr₈·H₂O (R/S-CuPbBr, R/S-PCA = R/S-3-piperidinecarboxylic acid) and investigate their behavior under external stimuli including pressure and temperature. The R/S-CuPbBr compounds crystallize in a noncentrosymmetric monoclinic C2 space group, consisting of inorganic bimetal [Cu₂PbBr₈] layers and organic layers formed via hydrogen bonding interactions. For comparison, another pair of 2D Pb-based bromides (R/S-PCA)₃Pb₂Br₇·H₂O (R/S-PbBr) was synthesized, crystallizing in the noncentrosymmetric orthorhombic P2₁2₁2₁. These materials exhibit broadband yellow emission and circularly polarized luminescence emission at room temperature. The g_lum values of R/S-CuPbBr and R/S-PbBr are 8.63 × 10^-3 and -7.99 × 10^-3, 4.33 × 10^-3 and -3.52 × 10^-3, respectively. Density functional theory (DFT) calculations reveal R/S-CuPbBr and R/S-PbBr are indirect and direct bandgap semiconductors, respectively. More importantly, R-CuPbBr exhibits dramatic enhancements in optical properties under high pressure, with an 8-fold increase in photoluminescence and 44-fold boost in second-harmonic generation at elevated pressure.