Chiral photonics, utilizing chiroptical phenomena (differential interactions with left- and right-circularly polarized light), have broad applications in opto-spintronics, optical information processing, biosensing, and quantum computing. While chiral organic materials are common, their chiroptical activity is often limited to the near-UV region, and their charge-transfer capabilities are poor. Low-dimensional organic-inorganic hybrid perovskites (OIHPs) offer a new platform with superior chiroptical performance and spin-related optoelectronic properties such as strong spin-orbit coupling, large Rashba splitting, and long spin lifetimes. However, the chirality transfer mechanism from chiral organic cations to the achiral inorganic framework remains unclear. Four mechanisms have been proposed: (i) crystallization into a chiral crystal structure; (ii) chiral distortion on the inorganic semiconductor surface; (iii) chiral dislocations; and (iv) electronic interactions between chiral organic molecules and inorganic semiconductors. This study aims to elucidate the role of electronic interactions, focusing on the effects of the π-electrons in chiral organic spacer cations on the chiroptical activity of chiral 2D OIHPs by modulating the interaction using spatially confined growth within nanopores.

Previous research has shown that chiral OIHPs exhibit circular dichroism (CD) and circularly polarized photoluminescence (CPPL), indicating chiroptical activity. Studies have explored the use of these materials in various forms, including nanocrystals, co-gels, nanoplatelets, and thin films. While significant chiroptical performance has been observed, the underlying mechanisms remain debated. Some studies suggest a correlation between crystal structure and chiroptical activity, highlighting the importance of the spatial interactions between chiral organic molecules and the inorganic framework. However, the role of electronic interactions between these components has been less studied, although recent work suggests that the π-electron system of organic spacers can modify the electronic configuration of 2D OIHPs. This study builds upon previous work by exploring the use of nanoconfined growth to precisely control the electronic interaction and micro-strain, allowing for a detailed investigation into the origin of chiroptical activity.

The study utilized strain-engineered chiral 2D OIHPs grown within nanoporous anodized aluminum oxide (AAO) templates with varying pore sizes. Chiral 2D OIHPs (MBA₂PbI₄(₁₋ₓ)Br₄ₓ, x = 0.325) were grown on both planar glass substrates and AAO templates. The precursor solution concentration was carefully controlled to prevent overlayer formation on the AAO templates. The samples were characterized using circular dichroism (CD) spectroscopy to measure the differential absorption of left- and right-circularly polarized light. The asymmetry factor (g-factor) was calculated from the CD spectra and absorption spectra. X-ray diffraction (XRD) was used to analyze the crystal structure and micro-strain in the perovskite lattice using a modified Williamson-Hall method. The degree of micro-strain was calculated by comparing the XRD peak broadening in the nanoconfined samples to that of a strain-free single crystal (MBA₂PbI₄). Density functional theory (DFT) calculations were performed to investigate the structural properties and hydrogen-bonding interactions in the perovskite structure under different micro-strain conditions. Circularly polarized photoluminescence (CPPL) spectroscopy was used to measure the emission of circularly polarized light, and the asymmetry factor (gCPPL) was calculated. The effects of the incident angle of the excitation laser on CPPL were also studied to eliminate the Rashba effect. Deconvolution of CD spectra and analysis of excited-state splitting were performed to further analyze the electronic structure.

Nanoconfined growth significantly enhanced the chiroptical activity of chiral 2D OIHPs. A 5.12-fold improvement in the g-factor was observed in the 100 nm pore-sized AAO template compared to planar samples. Strikingly, sign conversion of the Cotton effect and spectral shape conversion from unisignate to bisignate were observed in the AAO-templated samples. XRD analysis showed only marginal peak shifts, indicating that the enhanced chiroptical activity is not solely due to changes in crystal structure. Micro-strain analysis revealed a zigzag relationship between micro-strain and pore size, attributed to the flexible nature of 2D OIHPs and the conformational changes in the organic spacers. DFT calculations demonstrated that the asymmetric hydrogen-bonding interaction between the chiral organic spacers and the inorganic framework is amplified by the micro-strain. The CPPL measurements showed a remarkable anisotropy factor (gCPPL) of 6.4 × 10⁻² at room temperature for the 100 nm pore-sized AAO template samples, significantly higher than previously reported values. The sign conversion observed in CPPL is consistent with the CD results, confirming that the chiroptical phenomena arise from the same electronic transitions. Analysis of the ratio of free exciton (FE) to self-trapped exciton (STE) emissions indicates that the micro-strain does not significantly distort the inorganic framework structure, supporting the conclusion that the enhanced chiroptical activity is primarily due to changes in hydrogen bonding rather than structural distortion.

The findings demonstrate that the nanoconfined growth significantly enhances the chiroptical activity of chiral 2D OIHPs primarily through the modulation of asymmetric hydrogen bonding between the chiral organic spacer cations and the inorganic framework. The observed sign conversion and spectral shape changes in CD and CPPL highlight the importance of electronic interactions in chirality transfer. The correlation between micro-strain, hydrogen-bonding asymmetry, and chiroptical activity underscores a novel pathway to enhance chiroptical properties. The high CPPL anisotropy factor observed at room temperature is particularly noteworthy and opens avenues for developing high-performance spin-polarization-based optoelectronic devices. This study challenges the prevailing view that crystal structure is the sole determinant of chirality transfer in these materials and suggests that electronic interactions and hydrogen bonding should also be carefully considered.

This study demonstrates control over chiroptical phenomena in chiral 2D OIHPs through strain engineering using nanoconfined growth. Modulation of the π-electron system in the chiral organic spacers via micro-strain enhances asymmetric hydrogen bonding, facilitating efficient chirality transfer to the inorganic framework. The significantly amplified chiroptical activity, including a record-high room-temperature CPPL anisotropy factor, underscores the importance of considering electronic interactions alongside crystal structure in designing chiral materials. Future research could explore other nanoconfinement methods and different chiral organic spacers to further optimize chiroptical properties and develop new applications in spintronics and polarization-based optoelectronics.

The study focused on a specific range of bromide compositions (x = 0.325 to x = 0.400) in MBA₂PbI₄(₁₋ₓ)Br₄ₓ. While the findings are consistent across this range, further investigation is needed to determine the generality of these effects across a broader range of compositions. The DFT calculations were performed on a simplified model system (MBA₂PbI₄), and the inclusion of bromide ions in the calculations might refine the results. The interpretation of the CPPL results relies on the assumption that the Rashba effect does not significantly contribute to the observed anisotropy, which needs to be further investigated.