Turning a Fluorescence Quencher into an Aggregation-Induced Emission Promoter: The Role of Iodide Ions

Abstract Aggregation-induced emission (AIE)-generating molecules (AIE luminogens) have created an enormous amount of interest in various fields, such as photo- and electroluminescent device fabrication, bio- and chemosensing, imaging studies, and therapeutics. Strategic molecular design plays a critical role in regulating the self-assembly and luminescent properties of the aggregated molecular system. Iodide, owing to its inherent heavy-atom effect, which facilitates intersystem crossing and emission quenching, has never been a part of the molecular design for AIE luminogens. In the present study, the iodide ion is utilized as an AIE promoter to significantly enhance the fluorescence of polycyclic aromatic hydrocarbons (quantum yield (QY) enhancement >40%) by coordinating to an acyl hydrazone linkage of a molecular framework in tetrahydrofuran (THF). Spectroscopic and photophysical analyses revealed that iodide interacts noncovalently with the acyl hydrazone linkage in the framework, leading to the suppression of E–Z isomerism and subsequently acting as a noncovalent spacer that reorganizes the molecular packing into larger and more ordered aggregates, resulting in significant aggregation-induced enhanced emission. The findings indicate for the first time that the iodide ion can act similarly to an AIE luminogen through coordination to an acyl hydrazone molecular linkage in a suitable dielectric medium.