Cucurbituril Complexes of Viologens Bound to TiO2 Films

Methylviologen (1,1'-dimethyl-4,4'-bipyridinium dichloride, MV2+, 1) and a newly synthesized viologen derivative (1-methyl-1'-p-tolyl-4,4'-bipyridinium dichloride, MTV2+, 2) were encapsulated in a macrocyclic host, cucur[7]bituril, CB[7]. The complexes MV2+@)CB[7] and MTV2+@CB[7] were physisorbed on the surface of TiO2 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 H-1 NMR spectra in D2O solutions, which showed an upfield shift of the viologen protons upon encapsulation. TiO2 films functionalized with the complexes were studied by FT-IR-ATR and UV-vis absorption. The electrochemical and spectroelectrochemical properties of MV2+@CB[7] and MTV2+@CB[7] were studied in solution and in electrochromic windows, where the complexes were bound to TiO2 films cast on FTO. The windows prepared front MV2+@CB[7]/TiO2/FTO and MTV2+@CB[7]/TiO2/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].