Manipulating Chiroptical Activities in 0D Chiral Hybrid Manganese Bromides by Solvent Molecular Engineering

Abstract

0D hybrid metal halides (0D HMHs) with fully isolated inorganic units provide an ideal platform for studying the correlations between chiroptical activities and crystal structures at atomic levels. Here, through the incorporation of different solvent molecules, a series of 0D chiral manganese bromides (RR/SS-C₂₀H₂₈N₂)₃MnBr₈·2X (X = C₂H₅OH, CH₃OH, or H₂O) are synthesized to elucidate their chiroptical properties. They show negligible circular dichroism signals of Mn absorptions due to C_2v-symmetric [MnBr₄]^2- tetrahedra. However, they display distinct circularly polarized luminescence (CPL) signals with continuously increased luminescence asymmetry factors (g_lum) from 10^-4 (X = C₂H₅OH) to 10^-3 (X = H₂O). The increased g_lum value is structurally revealed to originate from the enhancement of [MnBr₄]^2- tetrahedral bond-angle distortions, due to the presence of different solvent molecules. Furthermore, (RR/SS-C₂₀H₂₈N₂)MnBr₄·H₂O enantiomers with larger bond-angle distortions of [MnBr₄]^2- tetrahedra are synthesized based on hydrobromic acid-induced structural transformation of (RR/SS-C₂₀H₂₈N₂)₃MnBr₈·2H₂O enantiomers. Therefore, such (RR/SS-C₂₀H₂₈N₂)MnBr₄·H₂O enantiomers exhibit enhanced CPL signals with |g_lum| up to 1.23 × 10^-2. This work provides unique insight into enhancing chiroptical activities in 0D HMH systems.

Keywords: 0D hybrid metal halide; circularly polarized luminescence; magnetic dipole transition moment; solvent molecules.