Cucurbituril Complexes of Viologens Bound to TiO₂ Films

Methylviologen (1,1'-dimethyl-4,4'-bipyridinium dichloride, MV(2+), 1) and a newly synthesized viologen derivative (1-methyl-1'-p-tolyl-4,4'-bipyridinium dichloride, MTV(2+), 2) were encapsulated in a macrocyclic host, cucur[7]bituril, CB[7]. The complexes MV(2+)@CB[7] and MTV(2+)@CB[7] were physisorbed on the surface of TiO(2) nanoparticles films. Viologens 1 or 2, which do not have anchoring group substituents, did not bind to the films in the absence of CB[7]. The complexation into CB[7] was monitored by (1)H NMR spectra in D(2)O solutions, which showed an upfield shift of the viologen protons upon encapsulation. TiO(2) films functionalized with the complexes were studied by FT-IR-ATR and UV-vis absorption. The electrochemical and spectroelectrochemical properties of MV(2+)@CB[7] and MTV(2+)@CB[7] were studied in solution and in electrochromic windows, where the complexes were bound to TiO(2) films cast on FTO. The windows prepared from MV(2+)@CB[7]/TiO(2)/FTO and MTV(2+)@CB[7]/TiO(2)/FTO electrodes showed reversible, sharp (colorless to purple), and fast color switching upon application of -0.8 V. Electrochromic behavior was not observed in control windows prepared in the absence of CB[7].