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The focus of the second year of the grant has been the electrochemical study of cucurbituryl complexes of viologens bound to titanium dioxide nanoparticle films for electrochromic materials applications.  In electrochromic molecules, reversible redox states are associated with color changes. This property is useful to develop a variety of devices.  Viologen (1,1'-dimethyl-4,4'-bipyridinium dichloride) and its derivatives are excellent electrochromic materials because possess fast switching times, and the reduced state is long-lived, stable and intensely colored.  Methyl viologen (MV2+) is colorless, and a one-electron reduction forms a deeply blue radical anion (MV+•). The bleached and colored state can be reversibly switched upon application of a potential (-0.8 V).  Viologens bound to TiO₂ nanoparticle films through anchoring group substituents, for instance phosphonic acid groups, and cast onto transparent conductive electrodes, are key components of devices such as electrochromic windows.  Some of the challenges associated with this design include the need for synthetic modifications of the viologen framework to introduce the anchor group. Furthermore, dimerization of MV^+•, aggregation of MV (the product of double reduction), desorption, and other side reactions can make the switching process not completely reversible or can lead to degradation.

In this work, we encapsulated unsubstituted MV²⁺ into a macrocyclic host, specifically cucur[7]bituril (CB[7]), and anchored the MV²⁺@host onto the surface of the semiconductor.  In all cases the counterion is chloride ion. There are very few examples of dyes or redox active molecules incapsulated in molecular containers and bound to the surface of semiconductors through the host.  This strategy is a new method to develop dye/semiconductor materials, which have found applications in a variety of field, ranging from sensors to dye sensitized solar cells.

Cucurbiturils, macrocycles consisting of repeating glycoluril units, typically 5 to 8, encapsulate positively charged molecules, including methylviologens, through charge-dipole interactions as well as hydrogen-bonding interactions.  In the second year of the grant  we have studied the commercially available methyl viologen and in addition synthesized a variety of viologen derivatives, with our without anchoring groups (COOH, phosphonate) for further comparisons. Finally we have fabricated and studied electrochromic windows. In summary, the inclusion of viologen derivatives in cucurbituril[7], their binding to metal oxide surfaces and the testing of electrochromic windows from them, one of the original goals of the proposal, have been successfully  developed.

The graduate student who has carried out the project Marina Freitag (nee’ Kaiser) has  spent  2 months (june-july 2009) in the group of Prof. Carlo Bignozzi at the University of Ferrara to investigate the application of the viologens@CB[7] complexes for the development of photovoltaics and for their electrochemical study utilizing cobalt complexes as the redox mediator in the electrolyte.  The student has learned a variety of electrochemical and spectroscopic techniques. The key achievement of year 2 of the grant are summarized below:

(a) Study of viologens@CB[7] in solution and bound to TiO₂ Cucurbituril CB[7] forms a 1:1 host-guest complex with viologens in deionized water (the concentration of the solutions is typically0.05 mM). The complex formation is monitored mainly  by 1H NMR, as well as IR and UV-VIS.  We found that CB[7] can is bound or physisorbed to TiO₂ films and that viologens@CB[7] complexes also bind to TiO2 films. The binding was followed by FT-IR-ATR.  This is important because unsubstituted viologens do not bind to TiO2.

(b) Development of novel viologen derivatives.
We have used the commercially available methyl viologen  (MV²⁺) as the reference compound, but we have developed a variety of viologen derivatives characterized by the presence of COOH and phosphonate anchoring groups (for a comparison) as well as a  newly synthesized viologen derivative 1-methyl-1'-p-tolyl-4,4'- bipyridinium dichloride, MTV2+, which is fluorescent (PL lambda max = 480 nm). In all cases the counterion is Cl-. Viologen derivative MTV²⁺ and CB [7] were dissolved in water and the formation of the complexes in water (MTV²⁺@CB) was monitored by ¹H NMR.  The complex binds to TiO2 films, and it shows reversible redox behavior on the surface.  In summary, the newly developed MTV2+ exhibited improved properties compared to the MV2+. We found that the derivative with COOH anchoring group had limited stability, possibly because of decarboxylation processes during the redox process.  This reinforces the idea that the proposed strategy (encapsulate an unsubstituted viologen in a host and bind the host) is valid.

(c) Study of electrochemical properties of viologens@CB[7] and viologens@CB[7]/TiO2/FTO by cyclic voltammetry.

The studies were done 1) in solutions (no TiO2 present),2) on TiO2/ITO electrodes and 3) in functioning electrochromic cells. (1) In Solution. All viologens both free and as CB[7] complexes (MV2+@CB[7]).exhibited the characteristic two reversible one-electron reductions. The complex, however exhibited  significant shifts to more negative potentials. The electrolyte was 0.05 M LiClO₄ in g-butyrolactone (2) Bound to TiO2/ITO.  The viologen@CB[7] complex-modified TiO₂ electrodes were measured versus Ag/AgCl (1.0 mol dm^-3 KCl) in electrolyte solution (0.05 mM LiClO₄  in anhydrous  g-butyrolactone). Reversible behavior and switching between the colorless-colored state  was observed for  MV2+ and MTV2+ while the carboxylate derivatives were irreversible  (3)  Functionin Electrochromic windows were prepared . For the measurement of absorption spectra  a  potential of -0.8 V was applied in order to reduce the species to the colored state and at +0.1 V in order to oxidize the species back to the bleached state.  Ferrocene was used as the redox shuttle either in solution or anchored to the counter electrode as Fc-COOH. This is also a novel development. The coloration was always reversible during switching between bleached and colored state.

(d) The work started at the University of Ferrara involved:  (a) Development of New Cathode for Solar Cells Applications. replacement of Pt-based cathodes would reduce the cost and improve the durability of DSSCs. The MV²⁺@CB[7] bound  to the counter electrode showed to be  a promising replacement. (b) study of improved redox shuttles in the electrochromic windows. In particular it was found that cobalt complexes of bpy were excellent redox carriers.
In summary: a new anchoring method of redox groups to semiconductor was developed and windows prepared from MTV2+@CB[7]/TiO2 exhibited excellent electrochromic properties.