Cyclodextrin Complexation of the Tetracyanoquinodimethane Anion Radical: The Chemical Consequences of Cavity size and Alkyl Derivatization

Optical spectroscopy shows that in aqueous solution the tetracyanoquinodimethane anion radical (TCNQ^-) exists in equilibrium with its π-π dimer (Boyd and Phillips). A study of the effect of various cyclodextrins on this equilibrium shows strikingly different results depending on cavity size and on alkyl derivatization. The equilibrium is unaffected by the presence of α-cyclodextrin. In contrast, γ-cyclodextrin complexes and stabilizes the π-π dimer. EPR and optical spectroscopy show that the anion radical is included and stabilized by heptakis(2,6-di-o-methyl)β-cyclodextrin and a 2-hydroxypropyl derivative of β-cyclodextrin. Of great interest is the observation that the complex formed by TCNQ^- and β-cyclodextrin rapidly reacts to form a diamagnetic product. Aqueous solutions of this diamagnetic product give evidence of appreciable concentrations of TCNQ^- when heated or when low dielectric solvents like methanol are added. NMR indicates that the diamagnetic product is a dianion formed by the bonding of two β-cyclodextrin complexed anion radicals. σ-Bonded dimers of TCNQ^- have been previously observed only in the solid phase (Vu Dong et al., Harms et al.). The tetrafiuorotetracyanoquinodimethane anion radical exhibits the same general cyclodextrin chemistry as TCNQ^-.