Controlled oligomeric guest stacking by cucurbiturils in water
Résumé
Previously, we have reported a guest molecule containing a viologen (V), a phenylene (P) and an imidazole (I) fragment (VPI) forming a host:guest 2:2 complex with cucurbit[8]uril (CB[8]) and an unprecedented 2:3 complex with cucurbit[10]uril (CB[10]). To better address the structural features required to form these complexes, two VPI analogues were designed and synthesized, the first with a tolyl (T) group grafted on the V part (T-VPI), and the second with a naphthalene (N) fused on the imidazole (I) part (VPI-N). While VPI-N afforded a discrete well-defined 2:2 complex with CB[8] and a 2:3 complex with CB[10], T-VPI organized also as a 2:2 complex with CB[8] but no well-defined complex was obtained with CB[10]. These complexes were studied by NMR notably DOSY, and allowed to estimate binding constants for 2:2 complex formation with CB[8] pointing to more stable 2:2 complexes with more hydrophobic guests. UV-vis and fluorescence spectroscopies confirmed complex formation, suggesting host-stabilized charge-transfer interactions. Therefore, the simple addition of CB[8] or CB[10] enabled to control the level of guest stacking (dimer or trimer) using relevant pairs of synthetic hosts through spontaneous, host:guest quaternary or quinary self-assembly.