Fluorescent sensors for molecules. Guest-responsive monomer and excimer fluorescence of 6A,6B-; 6A,6C-; 6A,6D-; and 6A,6E-bis(2-naphthylsulfonyl)-gamma-cyclodextrins

Flexible hosts, 6A,6B-; 6A,6C-; 6A,6D-; and 6A,6E-bis(2-naphthylsulfonyl)-gamma-cyclodextrins (gamma-1, gamma-2, gamma-3, and gamma-4, respectively) were used as fluorescent sensors with which a variety of organic compounds were detected by naphthalene excimer and monomer emissions. In a 10 vol% ethylene glycol aqueous solution, gamma-1 exhibits almost pure monomer fluorescence while gamma-2, gamma-3, and gamma-4 exhibit both monomer and excimer emissions. The intensities of the emissions changed upon addition of guest species, particularly in the case of gamma-2 and gamma-3, and the guest-induced intensity variations were used as sensitivity factors of the sensors. When (-)-borneol (5), cyclohexanol (6), cyclododecanol (7), and 1-adamantanecarboxylic acid (8) were added to each host solution, gamma-2, gamma-3, and gamma-4 increased the excimer emission intensity but decreased the monomer one, the absolute intensity variations being 6 < 5 approximate to 8 < 7. When geraniol (9), nerol (10), and (-)-menthol (11) were added, the hosts decreased intensities in both monomer and excimer emissions for 9 and 10 while their emission variations for 11 were similar to those of 5. For steroids such as cholic acid (12), deoxycholic acid (13), chenodeoxycholic acid (14), and ursodeoxycholic acid (15), gamma-4 showed depression in the excimer emission and enhancement in the monomer one while gamma-2 and gamma-3 showed complicated features in which the excimer emission was enhanced with the order of 15 < 14 < 13 approximate to 12 but the monomer one was depressed or enhanced depending on the hosts. All these data demonstrate that the hosts can be used as sensors for molecular recognition.