Spectroscopic studies on exchange properties in through-ring cyclodextrin complexes of carbazole-viologen linked compounds: Effects of spacer chain length

Analysis of ¹H NMR spectra (400 MHz) revealed a novel mode of interaction between cyclodextrin (CD) and carbazole-viologen linked compounds (CACnV), where the spacer chain was consisted of n methylene units (n = 4, 6, 8, 10, and 12). In the case of α-CD, the complexed species lived long enough to afford distinct proton signals, when the spacer chain was relatively long (n ≥ 8). As to CAC12V, the equilibrium constant for the 1:1 complex was 4.9 × 10⁴ M^-1 at 30°C and coalescence temperatures for the proton signals exceeded 100°C. Clear NOEs were observed to prove strong interaction between the protons in the CD cavity and the spacer methylene groups of CAC12V. The spacer was concluded to be encased in the cavity of α-CD. In the case of β-CD, essentially the same "through-ring CD complex" was formed. The line shape analysis indicated that the free energies of activation at 70°C for complexation and decomplexation were 11.6 and 17.2 kcal/mol, respectively. Activation parameters for the α-CD complexes were evaluated by the rate of disappearance of intramolecular charge-transfer absorption (420 nm) on the addition of α-CD. The free energy of activation for decomplexation was found to exceed 22 kcal/mol in the α-CD complexes for CACnV (n = 8, 10, and 12). The viologen moiety of CACnV was concluded to be the site of entrance for forming "through-ring CD complex", and the large activation energies were ascribed to dehydration of viologen units to go through the CD cavity.