Chiral Molecules Induce Enantiomorphic Lattice Helicity in Chiral 0D Tin Bromide Crystals

Chiral (R-MBA)₂SnBr₆ and (S-MBA)₂SnBr₆ metal halides crystallize in enantiomorphic helical space groups P6₁ and P6₅, respectively. The helical orientation of chiral molecules in the lattice arises due to strong N─H···Br and C─H···π interactions and is visualized via sixfold symmetry. The crystals show mirror-image CD spectra (g ≈ 3.5 × 10^{− 2}) and circularly polarized SHG with a dissymmetry factor (gCP–SHG) up to 0.44.

Abstract

Chiral organic–inorganic hybrid metal halides have emerged as a promising class of materials for spin-controlled optical and optoelectronic effects and related applications. Chiral hybrid metal halides generally crystallize in non-helical space groups. Herein, we report the discovery of zero-dimensional (0D) chiral (R/S-MBA)₂SnBr₆ (MBA: methylbenzylammonium cation) single crystals with enantiomorphic lattice helicity. The S-enantiomer of the chiral molecule induces right-handed helicity with the P6₁ space group (right-handed, P-helix), while the R-enantiomer induces right-handed helicity with the P6₅ space group (left-handed, M-helix). The chiral molecules induce the helical twist of inorganic units in the lattice through N─H···Br and C─H···π interactions. Density functional theory (DFT) calculations indicate that the strong electronic coupling between chiral molecules and SnBr₆ ²⁻ subunits is responsible for the generation of chirality. The chiral crystals exhibit circular dicroism (CD) spectra with a high dissymmetry factor (g _CD) of 3.5 × 10^{− 2} and no Cotton effect, maintaining the same CD sign throughout the spectrum. In addition, they exhibit broadband second harmonic generation (SHG) over a broad excitation range, with a g _CP-SHG up to 0.44. Furthermore, we find that the alloying of Sn with Pb leads to a change in dimensionality from 0D to non-helical 1D structures. These crystals with helical lattices and interesting CD responses are expected to open new avenues for spin-controlled applications.

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