Host-Guest Inclusion Complexation of α-Cyclodextrin and Triiodide Examined Using UV-Vis Spectrophotometry.

The historically relevant host-guest complexation of α-cyclodextrin (α-CD) and triiodide (I3(-)) in aqueous solution was examined using a systematic UV-vis spectrophotometric approach. This particular system is experimentally challenging because of the coupled equilibria, namely, I2 + I(-) ⇌ I3(-) and α-CD + I3(-) ⇌ α-CD·I3(-). We therefore developed a unique experimental approach that allowed us to determine the concentration of all iodine species. This enabled us to unequivocally demonstrate that the large increase in the UV absorbance with added α-cyclodextrin is due to an increase in the overall triiodide concentration as α-CD essentially converts iodine to triiodide according to the coupled equilibria. Herein we report (a) the complexation stoichiometry is 1:1 (i.e., the host-guest complex is α-CD·I3(-)), (b) the binding constant is KH-G = (1.35 ± 0.05) × 10(5) M(-1) at room temperature, and (c) the binding constant is temperature dependent with ΔH = -31.0 ± 0.9 kJ/mol.