Manipulating Chirality of 2D Lead Perovskites via Achiral Isomer Cations

Hybrid organic-inorganic perovskites have emerged as an important family of materials for optoelectronics with broader applications in solar cells, photodetectors, light-emitting diodes (LEDs), and lasers. As ionic semiconductors, the flexible crystal structures and tunable compositions make it possible to rationally design hybrid perovskites with desired properties. Chiral perovskites demonstrate such possibility and the breakthrough with respect to the development of a new class of chiral semiconductors, which opens up new applications of hybrid perovskites to chiroptoelectronics, ferroelectrics, and spintronics. One type of chiral perovskites is based on two-dimensional (2D) hybrid organic-inorganic perovskites by incorporating chiral organic ligands between achiral inorganic single layers composed of corner-sharing metal-halide octahedra. As a new class of chiral semiconductors, the chirality of hybrid 2D perovskites is attributed to the symmetry-breaking in the inorganic framework induced by the enantiopure chiral organic cations via asymmetric hydrogen bonding interactions, which transfers the structural chirality across the organic-inorganic interface. In this work, we introduced achiral alkyl isomer cations, n-butylammonium (n-BA⁺) and iso-butylammonium (iso-BA⁺) into the organic layer with chiral organic cations (R/S-MBA⁺). Single phase 2D (R/S-MBA_xnBA_1-x)₂PbI₄ and (R/S-MBA_yisoBA_1-y)₂PbI₄were obtained with x = 0.5 and y = 0.7. The CD signals became strong, and the polarity was flipped by the incorporation of nBA in (R/S-MBA_0.5nBA_0.5)₂PbI₄. We performed density functional theory (DFT) calculations to understand the interaction between organic and inorganic layers and its impact on electronic band structure and band gap. We performed temperature dependent powder XRD and pair distribution function (PDF) measurements using NSLS-II synchrotron at the Brookhaven National Laboratory to investigate structural variations with temperature to help understanding the observation of the unique behavior exhibited in the temperature dependent circular polarized photoluminescent (CPPL) measurements. We also conducted conventional and circular polarized transient absorption spectroscopy (TAS) to understand the photophysics embedded in these new chiral perovskites. Unlike (R/S-MBA)₂PbI₄that exhibit red-shifted and broaden CPPL spectra with the increasing of temperature, (R/S-MBA_0.5nBA_0.5)₂PbI₄ and (R/S-MBA_0.7isoBA_0.3)₂PbI₄ show non-shifted CPPL peak and conserved peak width from 10 K -300 K. DFT calculations show stronger hydrogen bonding and more tilted octahedron due to the asymmetric cations on the opposite sides of the inorganic framework. While many phenomena are unknown and needed further investigations, this work provides a new way to manipulate chirality of 2D perovskites, which could lead to broader applications.