Marián Palcut^1,* and Ladislav Rábara^2,3,4

¹ Institute of Materials Science, Faculty of Materials Scienceand and Technology, Slovak University of Technology, SK-917 24 Trnava, Slovakia
² International Laser Centre, Ilkovi~ova 3, SK-841 04 Bratislava, Slovakia
³ Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, Comenius University, Mlynská dolina, SK-842 15 Bratislava, Slovakia
⁴ present address: Department of Chemistry, Faculty of Natural Sciences, University of Ss. Cyril and Methodius, Nám. J. Herdu 2, SK-917 01 Trnava, Slovakia
* Corresponding author: E-mail: marian.palcut@gmail.com

Abstract
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.

Kewords: β-cyclodextrin, coumarin 6, fluorescent probe, NMR titration, inclusion complex