Giant Apparent Optical Circular Dichroism in Thin Films of Bismuth‐Based Hybrid Organic–Inorganic Metal Halide Semiconductor Through Preferred Orientation

Abstract Introducing chirality into organic/inorganic hybrid materials can impart chiroptical properties such as circular dichroism. The ability to tune chiroptical properties in self‐assembled materials can have important implications for spintronic and optoelectronic applications. Here, a chiral organic cation, ( R/S )‐4‐methoxy‐α‐methylbenzylammonium, is incorporated to synthesize the bismuth‐based hybrid organic–inorganic metal halide semiconductor, ( R/S ‐MeOMePMA)BiI 4 . Thin films of this Bi‐based compound demonstrate large chiroptical responses, with circular dichroism anisotropy (g CD ) values up to ≈0.1, close to the highest value observed in another chiral metal‐halide semiconductor, ( R ‐MBA 2 CuCl 4 ). Detailed investigation reveals that this large g CD in ( R/S ‐MeOMePMA)BiI 4 is caused by the apparent CD effect. Careful selection of deposition conditions and the concomitant thin‐film orientation enables the control of g CD , with maximum value observed when its thin film has a well‐crystallized preferred (001) orientation parallel to the substrate. The results support a growing body of evidence that low symmetry plays an important role in achieving unusually large g CD in these chiral metal–halide materials and provides design rules for achieving large chiroptical response via morphology control.