Structures and energetics of protonated bipyridine-cucurbituril complex isomers in the gas phase

We investigated the gas-phase isomerism and stability of host-guest complexes formed between cucurbiturils (CB[6] and CB[7]) and three n,n'-bipyridine regioisomers (n = 2, 3, and 4), focusing on how molecular geometry and charge distribution influence complex formation. Ion mobility spectrometry-mass spectrometry and collision-induced dissociation experiments, supported by density functional theory (DFT) calculations, reveal distinct inclusion and exclusion complex isomers. Singly-protonated bipyridines tend to form exclusion complexes with CB[6], while doubly-protonated forms enable stable inclusion through enhanced charge-portal interactions. CB[7], with its larger and more flexible cavity, consistently supports inclusion for all bipyridine isomers, regardless of charge state. These findings emphasize the importance of charge localization, host flexibility, and phase-specific effects in supramolecular assembly, which may further offer valuable insights for designing bipyridine- or bipyridinium-based materials.