Single and Double Threading of Congo Red into γ-Cyclodextrin. Solution Structures and Thermodynamic Parameters of 1:1 and 2:2 Adducts, as Obtained from NMR Spectroscopy and Microcalorimetry

Detailed NMR studies of aqueous solutions (pH 7) of γ-cyclodextrin (γCD) and the azo dye Congo Red (CR) show distinct, concentration-independent 1H NMR signals for different species. A very stable 1:1 pseudorotaxane (K 11=38,000±1100 M-1) is formed. In addition, a second complex corresponding to a 2:2 adduct (K 22=13±3 M -1) is produced by dimerisation of the 1:1 species. The structure of the 1:1 pseudorotaxane involves fast motion of the γCD ring along the CR backbone, leaving the outer naphthalene rings free. This entity undergoes structural reorganisation and dimerises to form the 2:2 adducts. Variable-temperature spectra did not lead to coalescence and allowed for the calculation of K 11 and K 22 at each temperature and also of the corresponding thermodynamic parameters. Therefore, formation of the 1:1 complex is favourable (ΔG=-26.1±0.1 kJ/mol) and exothermic (ΔH=-21.7±1.0 kJ/mol), whereas formation of the 2:2 entity is also favourable (ΔG=-6.36±0.58 kJ/mol) but endothermic (ΔH=+43.3±8.7 kJ/mol). The...