The inclusion complexes between [Zn(dmit)2]2− anion and cyclodextrins: studied by induced circular dichroism spectra and density functional theory calculations

Abstract

A series of inclusion complexes between cyclodextrins (α-, β-, HP-β-, and γ-cyclodextrin, HP-β-cyclodextrin = 2-hydroxypropyl-β-cyclodextrin) and [Zn(dmit)2]2− anion (dmit = 1,3-dithiole-2-thione-4,5-dithiolate) were investigated by electronic spectra, induced circular dichroism (ICD) spectra, 1H-NMR spectra, and quantum chemistry calculations. The stoichiometry of inclusion complexes [Zn(dmit)2]2−@cyclodextrins in solution was determined by the method of continuous variation. The experimental data showed that inclusion complex [Zn(dmit)2]2−@β-cyclodextrin adopted 1:2 and 1:1 guest-to-host inclusion ratios in solution, and the inclusion constants (Kinclu) were computed to be 5.85 × 107 M−2 and 1.63 × 104 M−1, respectively. In the case of [Zn(dmit)2]2−@γ-cyclodextrin, the continuous variation method showed the inclusion complex adopted 1:1 guest-to-host ratio and Kinclu was determined to be 3.27 × 104 M−1. Moreover, inclusion complex [Zn(dmit)2]2−@HP-β-cyclodextrin showed different inclusion modes in solution as well. Time-dependent density functional theory (TD-DFT) was used to assist assignment of the ICD signals in inclusion complexes [Zn(dmit)2]2−@cyclodextrins. The p → π* transition of [Zn(dmit)2]2− chromophore, which occurred in the cavity of cyclodextrin, was calculated to be perpendicular to C=C double bonds and thus parallel to the symmetric axis of the host. When subject to the well-known Harata and Kodaka’s rule, the TD-DFT calculations coincided very well with the spectroscopic data in elucidation of the p → π* transition relevant ICD signals of [Zn(dmit)2]2−@cyclodextrins.