Thermodynamics of the inclusion of naphthoxazole derivatives in cucurbit[7]uril

Abstract

The inclusion of two naphthoxazole derivatives in cucurbit[7]uril has been studied employing steady-state fluorescent methods. These guest present an unusual behavior of the emission, showing a decrease of the fluorescence band corresponding to the locally excited state, concomitant with the on set of a new red-shifted fluorescence band, that increases when the cucurbit[7]uril concentration increases. This result was interpreted in terms of anintramolecular charge-transfer complex formed during the lifetime of the excited dye, after the inclusion complex has been produced. The equilibrium constants for the inclusion complex were determined from the fluorescence increase at different temperatures. Thermodynamic parameters obtained from these experiments indicate that inclusion of 2-methylnaphtho[1,2-d]oxazole is determined by enthalpic factors whereas classical entropic driving forces are dominant in the inclusion process of 2-phenylnaphtho[2,1-d]oxazole.