Thermodynamic Parameters for Formation of Through-Ring α-Cyclodextrin Complexes of Donor–Acceptor Linked Compounds: Comparison between D2O and H2O Solutions of Anthracene–Viologen Systems with Intramolecular Charge-Transfer Absorption Spectra

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Abstract
On the addition of α-cyclodextrin (CD), an intramolecular charge-transfer absorption band of an anthracene-viologen linked compound rapidly disappeared with elapsed time. The reason was ascribed to formation of a through-ring CD complex, which hinders direct interaction between the donor- and acceptor moieties. Thermodynamic parameters, including the energy of activation for complexation of α-CD, were evaluated by the use of the charge-transfer absorption band in D2O- and H2O solutions. Solvent deuterium isotope effects were observed with the free energy change on complexation, but not with the energy of activation for formation of the through-ring CD complex. An open, extended conformer of the linked compound was proposed as the model of the activated state for forming the through-ring CD complex.