Host-guest Interactions of Coumarin 6 with β-cyclodextrin in Polar Solvents

The complexation of coumarin 6 (3-(benzothiazol-2-yl)-7-diethylaminobenzopyran-2(2H)-one) in β-cyclodextrin media was studied by the fluorescence emission spectroscopy and NMR titrations in polar solvents. The excitation wavelength used to monitor the fluorescence was 445 nm. The emission spectra were recorded at 450–650 nm in water. The additions of β-cyclodextrin resulted in a substantial enhancement of the fluorescence emission of coumarin 6, especially at concentrations of 1 × 10–4 to 1 × 10–3 mol dm–3. The shift of the fluorescence maximum towards higher frequencies was also observed. The formation of the inclusion complex of coumarin 6 with β-cyclodextrin is proposed. Furthermore, the effect of the hydrophobic cavity on spectral properties of the guest molecule is discussed. The inclusion phenomenon was further studied by liquid state NMR in dimethylsulfoxide. The measured 1H NMR shifts showed that coumarin molecule provided a significant electron shielding of protons located inside the β-cyclodextrin cavity. The geometry of the host-guest complex is discussed and the possible location of the guest molecule inside the cavity is suggested.