New strategy unleashes double perovskites’ hidden chirality

Chiral optical materials have garnered considerable interest across various disciplines due to their versatile applications in remote sensing, three-dimensional displays, information communication, and optical information storage. In response to the growing demand for stable and environmentally friendly materials, there is a keen anticipation surrounding two-dimensional, chiral, lead-free halide double perovskites, which are poised to exhibit rich chiroptical and optoelectronic properties.

Despite their promising potential, chiral lead-free double perovskites are notably scarce. The primary obstacle lies in the limited variety of organic cation A present in the interlayer of double perovskites A4BIBIIIX8 (where A is an organic cation, BI and BIII are metallic cations, and X is a halogen). The selective incorporation of chiral cations A into double perovskites faces constraints imposed by the width of the organic interlayer.

In a recent study published in Chem, a research team led by Prof. Luo Junhua from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences introduced an innovative strategy called achiral-chiral cation intercalation. This approach aims to systematically design a series of new enantiomeric lead-free halide double perovskites endowed with asymmetric and chiral bifunctional features.

By employing the achiral-chiral cation intercalation strategy, the researchers achieved charge conservation and a balanced overall steric hindrance. This resulted in a transformation from the arrangement of the original intercalated cations A to a diverse arrangement represented as A+A¢, where A’ denotes abundant achiral cations.

The team successfully synthesized six novel enantiomeric lead-free halide double perovskites, namely (R/S-PPA)2(BA)2AgBiBr8, (R/S-PPA)2(BrPA)2AgBiBr8, and (R/S-PPA)2(Br-EA)2AgBiBr8, thus validating the feasibility of their innovative synthesis strategy.

Detailed analysis using single crystal X-ray diffraction revealed an alternating arrangement of achiral and chiral cations connected through various non-covalent intermolecular interactions such as CH···π, π···π, and CH···Br. These interactions facilitated the stable and harmonious coexistence of chiral organic cations and achiral organic cations in chiral halide double perovskites.

Further investigations indicated that chiral compounds tend to exhibit a more distorted structure due to their inherent asymmetric features. Larger structural distortions typically lead to lower crystal symmetries, triggering asymmetry breaking and creating a platform for the generation of circular dichroism (CD) and second harmonic generation (SHG) signals.

Taking the compounds (R/S-PPA)2(Br-EA)2AgBiBr8 as examples, the researchers observed a robust nonlinear optical response, surpassing that of state-of-the-art KH2PO4 nonlinear crystals. Additionally, these compounds exhibited prominent circular dichroism signals in the visible region.

This study introduces a novel approach to explore the realm of chiral lead-free halide double perovskites, opening avenues for further advancements in the field.

Source: Chinese Academy of Sciences

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