The enhancement of fluorescence quantum yields of anilino naphthalene sulfonic acids by inclusion of various cyclodextrins and cucurbit[7]uril

The association constants (K) for the inclusion complexation of four kinds of cyclodextrins (CDs (β- And γ-), 2,6-di-O-methylated β- CD, and 2,3,6-tri-O-methylated β- CD) and cucurbit[7]uril (CB[7]) with 1,8- And 2,6- Anilinonaphthalene sulfonic acids (ANSs) were determined from fluorescence spectra enhanced by inclusion. Various CDs and CB[7] form stable 1:1 inclusion complexes with 1,8- And 2,6-ANSs: K = 80-11700 M^-1for 2,6-ANS and 50-195 M^-1for 1,8-ANS. The high stability of the inclusion complexes of 2,6-ANS with CB[7] and 2,6-di-O-methylated b-CD is shown. Further, we determined the fluorescence quantum yields (φvalues) for the inclusion complexes of ANSs by using a fluorescence spectrophotometer equipped with a half-moon unit. Theφvalues of 1,8- And 2,6-ANSs were largely enhanced by the inclusion of methylated β-CDs and did not correlate with the degree of stability (K) of the inclusion complexes. We characterized the structures of the inclusion complexes by 2D ROESY-NMR measurements. In addition, the microenvironmental polarity inside the hydrophobic CD and CB[7] cavities was evaluated using the fluorescence probe 2,6-ANS. Based on the emission mechanism and the aspect of inclusion in a hydrophobic cavity, we have suggested that the microenvironmental polarity and viscosity for the excited state of ANS plays an important role for the φ values of inclusion complexes.