Host Properties of Cucurbit[7]uril:  Fluorescence Enhancement of Anilinonaphthalene Sulfonates

• Published in: The journal of physical chemistry. B 2003-10, Vol.107 (39), p.10741-10746
• Main Authors: Wagner, Brian D, Stojanovic, Natasa, Day, Anthony I, Blanch, Rodney J
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp034891j

This work describes the fluorescence enhancement of the probes 2,6- and 1,8-ANS via complexation with the macrocyclic host cucurbit[7]uril (Q7). The association of these two guests with the Q7 host has been studied using fluorescence, 1H NMR spectroscopy, and molecular modeling. In the case of 2,6-ANS, 1:1 inclusion complexes are formed via inclusion of the phenyl moiety into the Q7 cavity (as confirmed by NMR), with a large fluorescence enhancement of a factor of 25 ± 3 and an association constant of 600 ± 150 M-1. These values are significantly larger than those reported in the literature for 2,6-ANS inclusion into cucurbit[6]uril (Q6); for example, the association constant is larger by over an order of magnitude, indicating the superior host abilities of Q7 as compared to its smaller homologue. These results are significant, as they provide the first direct comparison of the host abilities of Q6 and Q7. In the case of 1,8-ANS, very large fluorescence enhancement was also observed upon addition of Q7. The enhancement as a function of Q7 concentration indicated the formation of a 2:1 host:guest complex. However, host−guest inclusion was not observed via NMR. Thus, a 2:1 complex where 1,8-ANS is sandwiched between the outer surface of two Q7 molecules is proposed. Such complexation is supported by semiempirical PM3 calculations, and the resulting minimized structures are reminiscent of the structure of the solid exclusion compound of 1,8-ANS and Q6 previously reported in the literature.