# Chiral manganese halide isomers: decoding the spatial stacking effect on second-harmonic generation circular dichroism

## TL;DR

This paper explores how the spatial stacking of chiral manganese halide isomers affects their nonlinear optical properties, particularly second-harmonic generation circular dichroism.

## Contribution

The study reveals how spatial stacking influences SHG-CD in chiral manganese halides, offering new insights into designing chiral optoelectronic materials.

## Key findings

- The (R)-α-Mn isomer shows a significantly higher SHG-CD factor compared to (R)-β-Mn.
- A denser asymmetric hydrogen-bonding network in (R)-α-Mn enhances dipole moments and SHG-CD response.
- The SHG intensities of the isomers are higher than KH2PO4, indicating strong nonlinear optical responses.

## 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 an asymmetry factor (glum) of ∼1.0 × 10−3. Additionally, these isomers show significant NLO responses with SHG intensities 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.

The denser asymmetric hydrogen-bonding network of chiral (R)-α-Mn isomer greatly distorts [MnBr4]2− tetrahedra, resulting in a superior SHG-CD response compared to that of (R)-β-Mn isomer.