Molecular electrochemically controlled

In a film, the cooperative effort of the different molecular motors, between consecutive cross-linked points, promotes film swelling and shrinking during oxidation or reduction, respectively, producing a macroscopic change in volume (Fig. 18). In order to translate these electrochemically controlled molecular movements into macroscopic and controlled movements able to produce mechanical work, our laboratory designed, constructed, and in 1992 patented bilayer and multilayer103-114 polymeric... 

After this first report, a remarkable number of electrochemically controllable molecular shuttles have been designed, constructed, and studied. Rotaxane 294+ (Fig. 13.26), for instance, incorporates the electron-deficient cyclophane 124+ and a dumbbell containing two kinds of electron-rich units, namely, one 2,6-dioxyanthra-cene and two 1,4-dioxybenzene moieties.34 In solution, the rotaxane is present as the isomer with the 2,6-dioxyanthracene unit inside the cyclophane, owing to the fact that this unit is a better station in comparison to the 1,4-dioxybenzene recognition sites. 

Figure 14.6 Schematic representation of electrochemically controlled molecular motion.

Fig. 15. A chemically and electrochemically controllable molecular shuttle 104+. When the dumbbell is in its unperturbed state, the cyclophane exists in an 84 16 ratio (CD3CN, -44 °C) encircling preferentially the comparatively more 7t-electron-rich benzidine site. The cyclophane can be enticed to translate to the biphenol site exclusively either chemically -through protonation of the benzidine nitrogen atoms - or electrochemically - through oxidation of the benzidine unit to its radical cation. Both of these processes are completely reversible, through addition of base, or electrochemical reduction, respectively...

Fig. 16. A photo- and electrochemically controllable molecular shuttle. The unperturbed rotaxane 116+ exists preferentially in the translational isomer in which the BPP34C10 crown ether resides around the bipyridinium unit, a Photochemical excitation of the Ru(bipy)3 unit results in PET to the bipyridinium site, and consequent translation of the crown ether to the 3,3dimethylbipyridinium unit, which is a less efficient recognition site for the cyclophane CBPQT4+ than a bipyridinium system. This process occurs only in the presence of a sacrificial reductant which reduces the Ru(III) center back to its Ru(II) state in order to prevent charge recombination, b Conversely, upon electrochemical reduction of the bipyridinium unit, the crown ether takes up residency around the 3,3 -dimethylbipyridi-nium site. This process is reversed through electrochemical oxidation of the bipyridinium radical cation back to the dication...

Korybut-Daszkiewicz and coworkers developed a new electrochemically controlled molecular shuttle (Figure 37) based on a [2]catenane comprised of DB24C8... 

He J, Fu Q, Lindsay S, Ciszek JW, Tour JM (2006) Electrochemical origin of voltage-controlled molecular conductance switching. J Am Chem Soc 128 14828-14835... 

As discussed in Section 13.2.2, when a rotaxane contains two different recognition sites in its dumbbell component, it can behave as a controllable molecular shuttle, and, if appropriately designed by incorporating suitable redox units, it can perform its machine-like operation by exploiting electrochemical energy inputs. Of course, in such cases, electrons/holes, besides supplying the energy needed to make the machine work, can also be useful to read the state of the systems by means of the various electrochemical techniques. 

As discussed in Section 13.2.4, when one of the two rings of a catenane carries two different recognition sites, the dynamic processes of one ring with respect to the other can be controlled. In particular, if redox units are incorporated into the catenane structure, there is the possibility of controlling these processes upon electrochemical stimulation. Catenanes that exhibit such a behavior can be seen as electrochemically driven molecular rotors. An example is offered by catenane 384+ (Fig. 13.33a), which incorporates macrocycle 2 and a tetracationic cyclophane comprising one bipyridi-nium and one trans-l,2-bis(4-pyridinium)ethylene unit.19,40... 

Another synthetic strategy is based on self-assembly driven by molecular recognition between complementary 7t-donors and 7T-acceptors. Examples include the synthesis of catenanes and rotaxanes that can act as controllable molecular shuttles (6,236). The 7t-donors in the shuttles are located in the dumb-bell shaped component of the rotaxane and the 7T-acceptors in the macrocydic component, or vice versa. The shuttles may be switched by chemical, electrochemical, or photochemical means. 

The electrochemical behavior of 14+ is particularly clean and interesting, since only the 4- and the 5-coordinate geometries can be obtained on translating the metal-complexed ring from the phen site to the terpy site)841 The electrochemically induced molecular motions (square scheme1851), similar to those represented in Figure 10 but now involving stopped compounds, can be monitored by cyclic voltammetry (CV) and controlled potential electrolysis experiments)851... 

So far, several example of the chemically and electrochemically controlled switching of bistable linear molecular machines have been presented. The final section of this chapter will be dedicated to illustrating how such molecular switches and motors, when designed ingenuously, can also be powered by nature s most abundant and powerful energy source - light. 

P.R. Ashton, R. Ballardini, V. Balzani, S.E. Boyd, A. Credi, M.T. Gandolfi, M. Gomez Lopez, S. Iqbal, D. Philp, J.A. Preece, L. Prodi, H.G. Ricketts, J.R Stoddart, M.S. Tolley, M. Venturi, A. J.R White, D.J. Williams, Simple Mechanical Molecular and Supramolec-ular Machines Photochemical and Electrochemical Control of Switching Processes , Chem. Eur. J., 3, 152 (1997)... 

Electrochemically controlled SAMs of the alkanethiol class characterized to high voltammetric resolution and to molecular and sub-molecular structural in situ STM resolution have been reviewed recently [60, 151]. We note here first some issues of importance to functionalized alkanethiols as linker molecules for gentle immobilization of fully functional redox metalloprotein monolayers on singlecrystal Au(lll) electrode surfaces. We discuss next specifically the functionalized alkanethiols cysteine (Cys) and homocysteine (Hey). These two molecules represent a core protein building block and a core metabolite, respectively. The former has been used to display unique sub-molecular in situ STM resolution [152]. The latter shows a unique dual surface dynamics pattern that could be followed both by single-molecule in situ STM and by high-resolution capacitive voltammetry. 

Zhang, J.D., WeHnder, A.C., Chi, Q., and Ulstrup, J. (2011) Electrochemically controlled self-assembled monolayers characterized with molecular and sub-molecular resolution. Physical Chemistry Chemical Physics, 13, 5525-5545. 

Figure 7-25. Measurements of the single molecule conductance of 6V6 molecular wires performed under electrochemical control in 0.1 M phosphate buffer solution. The electrode potential dependence of the conductance at constant tip-sample bias (Ut = 0.2 V) of a single 6V6 molecule is shown. " ...

Figure 8-7 shows a schematic view of a redox molecule enclosed in a nano- or molecular scale gap between two electrochemically controlled metallic electrodes. 

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