Toward Understanding the Composition–Structure Relationship of Hybrid Organic Lead Iodide Compounds: Impact from Secondary Structures of Organic Cations

In studies of low-dimensional hybrid organic lead halide compounds (HOLHCs), understanding the composition–structure relationship is important for controlling their optical, optoelectronic, spintronic, and ferroelectric properties. Prior knowledge usually considers the primary structure of organic cations to predict the dimensionality of the HOLHCs. However, with the existence of noncovalent interactions (especially hydrogen bonding), the structure-directing organic cations may form secondary structures, which change their shape and steric hindrance when the cations are packed inside the crystal structure. To the best of our knowledge, the role of secondary structures of organic cations has not been systematically investigated yet. Herein, we report a systematic investigation of the influence from the secondary structure of ammonium ions induced by hydrogen bonding. We use a series of alkoxy-ammoniums as the model system to investigate how the NH···O hydrogen bonding induces the folding of the organic cations into ring structures. The folding increases the steric hindrance around the NH3+ end and thus reduces the dimensionality of the hybrid organic lead iodide compounds. By changing the linker length between alkoxy and ammonium groups, we have determined that the seven-member ring forms the strongest intramolecular hydrogen bonding. More intriguingly, the folded secondary structures become chiral, which provides a new approach for creating symmetry-breaking chiral materials.