Role of viologen substituents and host size in the gas-phase fragmentation of cucurbituril–viologen host–guest complexes

Viologens are widely used in the development and fabrication of electrochromic devices. One strategy to enhance their stability in solution upon electrochemical reduction is to incorporate them within the cavity of an appropriate host molecule. In solution, the formation of host-viologen complexes is primarily driven by hydrophobic interactions and the displacement of water molecules from the host cavity. Studying these complexes in the gas phase provides insights into the intrinsic factors determining their stability towards fragmentation in the absence of solvent molecules. In this work, we used collision induced dissociation (CID) to assess the relative stability of host-guest complexes formed between cucurbiturils (CBs) and viologen dications. Methyl-, heptyl-, and benzyl-disubstituted viologens were selected as guest species to investigate how alkyl substituents of viologen guests influence the stability of host-guest complexes. Because these complexes are stabilized by ion-dipole interactions, the release of the doubly charged guest is not a preferred fragmentation pathway. Instead, fragmentation occurs via either the release of a charge-reduced guest or the loss of neutral substituents. Substituent loss is primarily observed for viologen guests with bulky substituents that extend beyond the host cavity. Meanwhile, the release of a charge reduced guest is a competing pathway for complexes with benzyl or methyl viologen and for complexes in which the guest is not fully encapsulated within the host. We also examined the effect of host size on the strength of host-guest interactions. Our results indicate that the number of effective ion-dipole interactions between the host and the guest is a key factor determining the stability of the complex. Specifically, CB(7) forms the most stable complexes, while complexes with CB(6) exhibit lower stability because the smaller CB(6) host cannot fully accommodate the guest. Meanwhile, the effectiveness of ion-dipole interactions is diminished for CB(8) due to its large cavity size. Overall, this study provides valuable insights into the intrinsic factors, such as host size and guest substitution, that determine the stability of cucurbituril-viologen host-guest complexes towards gas phase fragmentation.