Excimer Emission in Inclusion Complexes of Dibenzofuran and 5-Dibenzosuberenone with α- and β-Cyclodextrins

The supramolecular self-assemblies of dibenzofuran (DBF) and 5-dibenzosuberenone (DBS) with alpha-cyclodextrin (alpha-CD) and beta-cyclodextrin beta-CD) were investigated by absorption, steady-state, and time-resolved fluorescence, Fourier transformation infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), proton nuclear magnetic resonance ((HNMR)-H-1), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and molecular modeling methods. In all the solvents, the absorption and emission maxima of DBF is red shifted from that of DBS. DBF and DBS show single exponential in water and biexponential in CD solution. In both CD solutions, the change in the emission intensity indicates 1:1 inclusion complex is formed. At higher beta-CD concentrations, excimer emission (2:2 stoichiometry, guest(2):beta-CD2) is detected. Binding constants were determined from the analysis of F-o/F versus [CD] plots, resulting in a stronger association for beta-CD. In the absorption and fluorescence spectra, the different trend displayed in alpha-CD seems to be related to a shallow inclusion of both drugs, stabilized by hydrogen bonding between the oxygen atom of the guests and the hydroxy groups of CDs. Self-assembled nanosphere and nanosheet morphology of the inclusion complexes were observed by TEM. The possible modes of theoretical calculations suggest the complex formation is favorable to an enthalpy driven process.