Electrochromic properties of alkylviologen-cyclodextrin systems

Abstract

Electrochromic properties of methyl, ethyl, n-hexyl and n-heptyl viologens, and α-, β-, γ-cyclodextrin systems were studied using cyclic voltammetry and absorption spectroscopy. Colourations of red-violet (λ _max = 530 or 560 nm), blue (λ _max = 600nm) and light purple (λ _max = 560 and 600 nm) could be obtained by changing the length of the alkyl chain of viologens or the radius of cyclodextrin. The intermolecular interaction between viologen radicals is weakened when the radicals form inclusion complexes so the blue colour inherent in the monomer of viologen radical appears. The red-violet colour is ascribed to the dimer of the reduced viologen. With the addition of γ-cyclodextrin, viologens were found to be included as dimer. The light purple colour is ascribed to the mixed colour of the monomer and the dimer. Furthermore, since the ratio of monomer and dimer depends upon the concentration of cyclodextrin, any colour between the red-violet of the dimer and the blue of the monomer can be obtained by adjusting the concentration of cyclodextrin. The redox reaction of an inclusion complex was found to depend upon the length of the alkyl viologen and the size of the cavity in cyclodextrin.