Host-guest interaction of 1,4-dihydroxy-9,10-anthraquinone (quinizarin) with cyclodextrins

The interaction of 1,4-dihydroxy-9,10-anthraquinone, (quinizarin; QZ), with alpha-, beta- and gamma-cyclodextrin (CD) hosts, has been investigated using absorption, and steady-state and time-resolved fluorescence measurements, in order to understand the effects of cavity size of CDs on the binding of QZ molecule and the changes in the photophysical properties of QZ in the microenvironment of the hosts. The results demonstrate that QZ forms inclusion complexes with all the CDs. The low binding constants as well as the thermodynamic parameters indicate that the mode of interaction between QZ and CDs is mainly hydrophobic in nature. The relative magnitudes of the binding constants and the differential enhancements in the fluorescence intensity of QZ upon complexation with the CDs can be explained by considering the relative dimensions of the host cavity and the guest molecule, as well as the orientation of the guest molecule inside the CD cavity. It is proposed that the unsubstituted benzene ring of QZ is encapsulated within alpha- and beta-CD cavities whereas the dihydroxy-substituted aromatic ring is encapsulated within the gamma-CD cavity. This is further supported by the complexation studies of the QZ.CD systems with Al(III) ions. It is observed that the complexation of QZ with the metal ion is enhanced in the QZ.alpha-CD and QZ.beta-CD systems whereas it is significantly reduced in the QZ.gamma-CD system, due to shielding of the chelating groups of the dye inside the CD cavity in the latter case.