A pH‐ and Metal‐Actuated Molecular Shuttle in Water

Abstract

The structure of the Viologen-Phenylene-Imidazole (VPI) guest, previously shown to be bound by cucurbit[7]uril (CB[7]) with binding modes depending on pH and silver ions, has been extended by adding hydrophobic groups on the two extremities of VPI before investigations of CB[7] binding by NMR, ITC, X-ray diffraction, UV-vis and fluorescence spectroscopies. With an imidazole station extended by a naphthalene group (VPI-N), binding modes of CB[7] are similar to those previously observed. However, with the viologen extended by a tolyl group (T-VPI), CB[7] preferentially sits on station T, shuttling between the T and P stations at acid pH or after Ag + addition. The CB[7] • T-VPI complex thus behaves as a metal-actuated thermodynamic stop-andgo molecular shuttle featured by fast and autonomous ring translocation between two stations and a continuum for fractional station occupancy solely and easily controlled by Ag + concentration.

Controlled oligomeric guest stacking by cucurbiturils in water

“…The synthesis of VPI-N and T-VPI was described previously. 46 Briefly, a tolyl group was considered to be added because this group is well-recognized by CB[ n ]s when directly grafted on viologens and expected to make VPI fluorescent, by analogy with the emissive ditolyl-viologen. 47,48…”

“…In parallel, the benzimidazole unit was replaced with a naphthimidazole ( VPI-N ) group to investigate the impact of hydrophobic extension of station I on complex formation. During initial work with CB[7], 46 several results were obtained suggesting possible VPI-N dimerization (Fig. 2a).…”

“…We assigned this to the de-oligomerization of VPI-N that tends to form dimers in water as discussed above. 46…”

The Diversity of Cucurbituril Molecular Switches and Shuttles

“…Moreover, a second ring translocation was observed when large excesses of CB[7] were added onto solutions of CB[7] ⋅ VPI‐H + and CB[7] ⋅ VPI‐Ag + since a second CB[7] could be stabilized on the benzimidazole station, therefore inciting the first CB[7] to sit back on the viologen station. Changing the benzimidazole station to the more hydrophobic naphthimidazole group in VPI−N afforded similar results, except that stoichiometric amounts of hosts enabled ring translocation for the CB[7] 2 ⋅ VPI−N‐H + and CB[7] 2 ⋅ VPI−N‐Ag + complexes [76] . On the other hand, increasing hydrophobic contribution by grafting a tolyl group on the other end of VPI (T‐VPI) resulted in marked changes in CB[7] binding behavior (Figure 31).…”

“…However, the imidazole station can again interact with H + or Ag + ions, increasing the CB[7] binding affinity on the phenylene station, not sufficiently to make the ring quantitatively switching, but enough for the ring to compete for tolyl and phenylene accommodation. Further addition of CB[7] permitted to occupy both tolyl and phenylene stations, thereby stopping the autonomous shuttling process [76] . To summarize, metal‐coordination as a stimulus allowed for ring translocation or autonomous ring shuttling but the systems remain at thermodynamic equilibrium.…”