Drum-like Metallacages with Size-Dependent Fluorescence: Exploring the Photophysics of Tetraphenylethylene under Locked Conformations

Materials with aggregation-induced emission (AIE) properties are of growing interest due to their widespread applications. AIEgens, such as tetraphenylethylene units, display varying emission behaviors during their conformational changes. However, the structure-property relationships of the intermediate conformations have rarely been explored. Herein, we show that the conformational restriction on TPE units can affect the structural relaxation in the excited state and the resulting photophysical behaviors. Specifically, three metallacages of different sizes were prepared via the coordination-driven self-assembly of a TPE-based tetrapyridyl donor with length-increasing Pt(II) acceptors. While the metallacages share similar scaffolds, they exhibit a trend of red-shifted fluorescence and attenuated quantum yield with the increase of their sizes. Furthermore, spectroscopic and computational studies together with a control experiment were conducted, revealing that the degree of cage tension imposed on the excited-state conformational relaxation of TPE moieties resulted in their distinct photophysical properties. The precise control of conformation holds promise as a strategy for understanding the AIE mechanism as well as optimizing the photophysical behaviors of materials on the platform of supramolecular coordination complexes.