Chiral 2D Cu(I) Halide Frameworks

Hybrid multifunctional materials have great potential in a wide variety of applications due to their flexible combination of organic and inorganic components. Introducing chiral organic modules into the metal halide frameworks can effectively generate multifunctional materials, achieving new functionalities with noncentrosymmetric structures. Here, by incorporating (R)- or (S)-piperidine-3-carboxylic acid (R/S-PCA) as the templating cation, we report the synthesis and characterization of three pairs of new 2D chiral hybrid Cu(I) halides, namely, (R/S-PCA)CuBr2, (R/S-PCA)CuBr2·0.5H2O, and (R/S-PCA)CuI2. These chiral Cu(I) halides crystallize in the noncentrosymmetric space group C2 and belong to a new structural type similar to layered silicates. The optical absorption edges of these chiral materials can be tuned by changing the halide or upon the absorption of water and range from 2.70 to 3.66 eV. A dynamic conversion between (R/S-PCA)CuBr2 and (R/S-PCA)CuBr2·0.5H2O occurs through exposure to moisture or vacuum drying along with changes in the reversible bandgap and photoluminescence. Chiroptical properties such as circular dichroism, circular polarized light emission, and second harmonic generation are investigated. Density functional theory calculations (DFT) show the indirect and direct bandgap natures of these Cu(I) halides and reveal the mechanism for the broadband self-trapped exciton emission at the excited state. The fascinating structural type, chiroptical properties, and reversible hydrochromic behavior of these Cu(I)-based halides make them viable candidates for next-generation multifunctional optoelectronic materials.