Electrostatic interactions between viologens and a sulfated β-cyclodextrin; formation of insoluble aggregates with benzyl viologens

Abstract

Viologens are important compounds and are used in several applications. The interactions between viologens and a negatively charged cyclodextrin, sulfated β-cyclodextrin (sβ-CD), were studied using electrochemistry, 1H NMR and UV–Vis spectroscopy. Weak electrostatic interactions were found between the dicationic viologens (V2+) and the anionic cyclodextrin. The diffusion coefficient of ethyl viologen (EV2+) was reduced from 5.33 × 10−6 to 1.98 × 10−6 cm2 s−1 with an excess of sβ-CD. More significant electrostatic interactions were found between benzyl viologen (BV2+) and sβ-CD. The H atoms in the benzyl substituent, which appear as a singlet in the NMR experiment, were split into a multiplet in the presence of the sβ-CD, while the chemical shift of the H in the β position to the quaternary nitrogen, was shifted by 0.17 ppm in the presence of sβ-CD. Greater electrostatic interactions were evident between the benzyl radical cation (BV·+) and sβ-CD. Using cyclic voltammetry, reduction of the radical cation to the neutral benzyl viologen (BV0) was shifted by 230 mV to a lower potential, indicating that the reduction of the radical cation becomes considerably more difficult in the presence of sβ-CD. This was attributed to the formation of an insoluble BV·+:sβ-CD aggregate, which was also evident in rotating disc voltammetry, where the typical diffusion-limited currents were not observed and in spectroelectrochemistry experiments, where the deposition of the aggregate at the ITO electrode increased the absorbance of the radical species.