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.