Cucurbit[7]uril host-vologen guest complexes: electrochromic and photochemical properties

In this thesis, we demonstrated that a molecular host, cucurbit[7]uril, provides an alternative method of adsorbing molecules on semiconductors and shields the guest from the hetereogenous interface. These novel hybrid systems exhibited photophysical and electrochemical properties that differ from the properties of layers obtained by directly attaching the chromophore to the semiconductor through binding groups.This thesis describes the host-guest chemistry between cucurbit[7]uril (CB[7]) and various series of viologen guests. Methylviologen (1,1'-dimethyl-4,4'-bipyridinium dichloride, MV2+), 1-methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+), and 1,1'-dip-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+) were encapsulated in the macrocyclic host cucurbit[7]uril, CB[7]. The complexes MV2+@CB[7] and MTV2+@CB[7] were physisorbed to the surface of TiO2 nanoparticle films. The complexation into CB[7] was monitored by 1H NMR. 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 (ECDs), where the complexes were bound to TiO2 films cast on FTO. The ECDs prepared from MV2+@CB[7]/TiO2/FTO and MTV2+@CB[7]/TiO2/FTO electrodes showed reversible, sharp and fast color switching upon application of -0.8 V. Viologen derivative DTV2+ exhibited enhanced fluorescence upon encapsulation. Aqueous solutions of DTV2+ were weakly fluorescent (Φ = 0.02, τ < 20 ps), whereas the emission of the DTV2+@2CB[7] complex was enhanced by one order of magnitude (Φ = 0.29, τ = 0.7 ns) and was blue-shifted by 35 nm. DTV2+ in polymethylmethacrylate (PMMA) matrix was fluorescent with a spectrum similar to that observed for the complex in solution. DFT and CIS calculations suggested that the increased planarity of the aromatic rings and a quinonoid structure of the S1 state, induced by encapsulation in the host, can explain the observed emission enhancement. The absorption and emission spectra of DTV2+@2CB[7] in water exhibited a large Stokes shift (ΔSt ~ 10,000 cm-1) and no fine structure. 1H NMR and UV-Vis titration indicated that the DTV2+@2CB[7] complex is formed in aqueous solutions with a complexation constant of K1W = 1.2×104 M-1, K2W = 1.0×104 M-1 in water, and K1NaCl = 1.1×104 M-1, K2NaCl = 0.8×104 M-1in 0.05 M NaCl aqueous solution.Ph. D.Includes bibliographical referencesIncludes vitaby Marina Freita