Synthesis and fluorescent molecular sensing at exciplex emission of pyrene- and cyanobenzene-modified γ-cyclodextrins

Decrease of exciplex emission of gamma -2 with increase of ursodeoxycholic acid concentration (1: 0, 2: 1.0 x 10(-5), 3: 2.0 x 10(-5), 4: 4.0 x 10(-5), 5: 6.0 x 10(-5), 6: 8.0 x 10(-5), 7: 1.0 x 10(4) M). Flexible regioselectively hetero-substituted hosts, 6(A)-O-4-pyrenylbutanoyl-6(X)-O-p-cyanobenzoyl-modified gamma -cyclodextrins (X = B or H, C or G, D or F, and E for gamma -1, gamma -2, gamma -3, and gamma -4, respectively) have been synthesized in order to investigate their fluorescence sensing properties for application to organic compounds such as bile acids and cyclic alcohols. The hosts, gamma -1, gamma -2 and gamma -3, exhibit both monomer and exciplex fluorescence, whereas gamma -4 exhibits only monomer emission. In guest-induced emission of gamma -1, gamma -2 and gamma -3, exciplex emission is suppressed while monomer emission is increased. However, gamma -4 exhibits only a negligible change in monomer fluorescence in the presence of guests. The extent of exciplex fluorescence variation of gamma -1, gamma -2 and gamma -3 with guests is recognized as the manifestation of the sensing ability of the hosts. A sensing parameter (DeltaI(ex)/I-ex(0)) was used to describe the sensing ability of three hosts. Host gamma -analogs, gamma -1, gamma -2 and gamma -3, are able to detect ursodeoxycholic acid, deoxycholic acid, chenodeoxycholic acid, and (-)-borneol with high sensitivity by exciplex emission. The sequence of the binding ability of these hosts is gamma -2 > gamma -1 > gamma -3. The behaviors of the appended moieties of these hosts during the formation of host-guest complexes were studied using induced circular dichroism (ICD) and fluorescence spectra. The host gamma -analogs gamma -1-gamma -3 exhibit different ICD patterns to gamma -4 before and after addition of ursodeoxycholic acid. The guest-induced variations of ICD and fluorescence spectra changes suggest that the pyrene and cyanobenzene moieties move, altering the spatial relationship between them.