Supramolecular Complexation of Azobenzene Dyes by Cucurbit[7]uril

This report describes cucurbit[7]uril (CB7) complexationof azobenzenedyes that have a 4-(N,N '-dimethylamino)or 4-amino substituent. Absorption and NMR data show that CB7 encapsulatesthe protonated form of the azobenzene and that the complexed dye existsas its azonium tautomer with a trans azo conformationand substantial quinoid resonance character. Because CB7 complexationstabilizes the dye conjugate acid, there is an upward shift in itspK (a), and in one specific case, the pK (a) of the protonated azobenzene is increasedfrom 3.09 to 4.47. Molecular modeling indicates that the CB7/azobenzenecomplex is stabilized by three major noncovalent factors: (i) ion-dipoleinteractions between the partially cationic 4-(N,N '-dimethylamino) or 4-amino group on the encapsulatedprotonated azobenzene and the electronegative carbonyl oxygens onCB7, (ii) inclusion of the upper aryl ring of the azobenzene withinthe hydrophobic CB7 cavity, and (iii) a hydrogen bond between theproton on the azo nitrogen and CB7 carbonyls. CB7 complexation enhancesazobenzene stability and increases azobenzene hydrophilicity; thus,it is a promising way to improve azobenzene performance as a pigmentor prodrug. In addition, the striking yellow/pink color change thataccompanies CB7 complexation can be exploited to create azobenzenedye displacement assays with naked eye detection.