Chiral manganese halide isomers: decoding spatial stacking effect on second-Harmonic generation circular dichroism

Abstract

Chiral hybrid metal halides (CHMHs) represent promising candidates for chiral optoelectronics and nonlinear optics (NLO). However, the effect of spatial stacking on second-harmonic generation circular dichroism (SHG-CD) in CHMHs has not been well understood. Herein, we constructed two pairs of chiral manganese(II) halide isomers (R)-α-Mn, (S)-α-Mn, (R)-β-Mn and (S)-β-Mn, which crystallize in chiral space groups C2221 and P21, respectively. They exhibit near-unity photoluminescence quantum yields and efficient circularly polarized luminescence with asymmetry factors (glum) of ~1.0×10-3. Additionally, these isomers show significant NLO responses with SHG intensity of 2.03 and 1.30 times that of KH2PO4 and polarization ratios of up to 0.87 and 0.57 for (R)-α-Mn and (R)-β-Mn, respectively. More importantly, (R)-α-Mn demonstrates an intense SHG-CD response with a SHG-CD factor (gSHG-CD) value of –0.56, about 1.86 times larger than that of (R)-β-Mn (–0.30). Compared to (R)- and (S)-β-Mn, the stacking mode of (R)- and (S)-α-Mn generates a more dense asymmetric hydrogen-bonding network, which greatly distorts [MnBr4]2- tetrahedra and enhances the dipole moment, thereby significantly improving the SHG-CD value. This work elucidates the pivotal role of spatial stacking in chiral NLO materials.