Photophysical properties of cavity-controlled host–guest assembly of cucurbit[8]uril and cucurbit[10]uril

This study investigates cavity-size-dependent host–guest interactions between cucurbit[n]urils (Q[8] and Q[10]) and a cationic guest molecule G1. Using 1H NMR, X-ray diffraction, UV–vis, and fluorescence spectroscopy, we demonstrated that both hosts form 1:2 complexes with G1 but exhibit distinct behaviours. The smaller cavity of Q[8] confines two G1 molecules in an antiparallel, head-to-tail arrangement, enhancing fluorescence quantum yield via charge–transfer interactions and restricted molecular motion. In contrast, the larger cavity of Q[10] fully encapsulates G1, favouring π−π stacking that resulting in red-shift fluorescence emission. These results highlight how cavity dimensions dictate supramolecular recognition and photophysical responses and advances understanding of Q[n]-based host–guest systems.