New Fluorescent Probes, Their Spectroscopic Properties, and an Iterative Analysis of Their Complexation with Cyclodextrins

Spectroscopic properties of a series of substituted 7-phenylamino-3-(2-pyridyl)coumarins have been characterized, and their ability to form host-guest inclusion complexes with cyclodextrins has been evaluated by the determination of the corresponding host-guest association constants. It has been found that the pyridylcoumarins form 1:1 host-guest complexes with sulfobutylated β-cyclodextrin (Captisol). Their host-guest association constants vary in the range 17-122 dm³ mol⁻¹, depending on the type of substituent. The association constant decreases with an increase of the electron-withdrawing character of the substituents, which suggests that the pyridylcoumarins interact with positively charged sites within the cyclodextrin cavity. Moreover, the problem associated with the Benesi-Hildebrand method, commonly used for determination of the host-guest association constants, has been clearly demonstrated and the use of an alternative data workup method, based on consecutive iterations methodology, is presented and explained in detail to enable the application of this methodology also to other experimental data sets, not necessarily related to the Benesi-Hildebrand equation.