Phase-Transition-Enabled Switching of Chiroptical Properties in 2D Hybrid Perovskites

Chiral materials typically possess chiroptical properties that are fixed upon fabrication or synthesis. However, the ability to reversibly modulate these properties on demand could unlock new opportunities in sensing, photonics, and information storage. In this work, we exploit solid-solid phase transitions in chiral two-dimensional hybrid perovskites (2DHPs) to achieve dynamically switchable chiroptical properties within a single material system. By incorporating the chiral additive S/R-[1,1'-binaphthalene]-2,2'-diyl bis(trifluoromethansulfonate) (S/R-BTS) into n-pentylammonium lead iodide (PA₂PbI₄) and n-hexylammonium lead iodide (HA₂PbI₄), we induce a large chiroptical response in otherwise achiral 2DHPs, as revealed by circular dichroism (CD) spectroscopy. Importantly, the inclusion of S/R-BTS preserves the characteristic solid-solid phase transitions of the parent 2DHPs. These phase transitions drive pronounced shifts in the CD spectra, including a region in the CD spectrum that reversibly inverts across the transition. Together, these results establish control of 2DHP phase behavior as a powerful strategy for creating dynamically tunable chiral materials.