Photophysical properties of aqueous solutions of a styryl dye in the presence of cucurbit[<Emphasis Type="Italic">n</Emphasis>]uril (<Emphasis Type="Italic">n</Emphasis> = 5, 6, 8)

Photophysical properties of aqueous solutions of the styryl dye 4-(E)-2-(3,4-dimethoxyphenyl)-1-ethylpyridinium] perchlorate (1) in the presence of cucurbitn]urils (CBn]; n = 5, 6, 8) have been studied by fluorescent spectroscopy methods. The fluorescence intensity of a 10^–6 mol L^–1 solution of 1 increases by a factor of 12.6 upon the formation of 1 : 1 inclusion complexes with CB6] or 1.3 in complexes with CB8]. Upon the formation of inclusion complexes, the average lifetime of the electronically excited state of 1 increases to about 1 ns for both CB6] and CB8]. On the basis of fluorescence anisotropy measurements, the rotational relaxation times were estimated to be 408, 314, and 183 ps for the complexes with CB6], CB8], and for unbound 1, respectively. Using the fluorescence titration method developed for the case of poorly soluble cavitands, the binding constant of 1 with CB6] was determined to be 1.1 × 10⁵ L mol^–1. The addition of CB5] does not lead to changes in the photophysical properties of a solution of 1, indicating the absence of complexes between CB5] and 1. It has been found on the basis of the experimental data that the fluorescence rate constant of 1 decreases about twice in the complex with CB8], but doubles in the complex with CB6].