Molecular electronics, rotaxanes

This remarkable molecular memory was preceded by two other related examples of addressable molecular electronics from the Stoddart and Heath groups in collaboration with Hewlett-Packard scientists. First in 1999 a [2] rotaxane based system was configured into molecular AND and OR logic gates,63 then in 2000 a [2] catenane based molecular switch was reported.64 The molecular basis for the operation of this switch is the same as in 11.67, namely reorientation in response to reduction of... 

Rotaxanes were proposed as challenging materials in areas covering molecular electronics like for example switches [66], molecular wires [14], and finally, logic gates [14], These molecules are already known for their binary reversible states arising from shuttling of the macrocycle along the thread. 

Interlocked structures such as catenanes, rotaxanes and knots have intrigued synthetic chemists not only because of their beauty but also because of their potential applications in materials for molecular electronics. A number of excellent reviews on catenanes and rotaxanes are available [12, 13]. The simplest catenane,... 

Before going on to discuss molecular electronic machines, it will be useful to describe their structural foundation at a molecular level, namely those based on interlocked molecules. Interlocked molecules can take on a variety of forms, the most common being catenanes, rotaxanes, knots [16], and carceplexes [17]. For the purpose of this review, only catenanes, rotaxanes and their geometrically related complexes - pseudorotaxanes [18] - will be discussed. When conferred with the ability to undergo some mechanical motion as a result of an applied stimulus - be it chemical, electrochemical, or photochemical - these interlocked molecular and interpenetrated supramo-lecular systems often take on the characteristics of molecular machines [19]. 

Stoddart, Heath, and coworkers developed a 160-kilobit molecular electronic memory circuit fabricated at a density of 10 bits cm (pitch 33 mn memory cell size 0.0011 m ) using a monolayer of bistable [2]rotaxane (Figure 77) with both hydrophobic and hydrophilic regions as the data storage elements in 2007. This memory circuit has achieved the dimensions of a dynamic random access memory circuit projected to be available by 2020. This proves that it is feasible and promising to use molecules as nanoscale components to create miniaturized electric circuits and develop molecular computing. 

FIGURE 4 The hysteretic response (top) and switch cycling (bottom) of a molecular-electronic switch consisting of a bistable [2]rotaxane sandwiched between a polysilicon bottom electrode and a Ti/Al top electrode. The device dimensions are approximately 50 X 50 nm. Control data from the dumbbell component (the [2]rotaxane without the TCP4+ ring) are included. 

Like rotaxanes, catenanes are mechanically interlocked molecules. However, instead of interlocking one ring shaped macrocycle and a dumbbell shape, catenanes consist of interlocked macrocycles. The number of macrocycles contained in a catenane is indicated by the numeral that precedes it. Catenanes have bistable and multistable forms and a switchable, bistable [2]catenane is commonly exploited in nanotechnology and molecular electronics because its behavior can be controlled by electrochemical processes [89]. Collier et al. was the first to demonstrate the electroactivity of interlocked catenanes [90]. The authors affixed phospholipid counterions to a monolayer of [2]catenanes and then sandwiched this system between two electrodes. This work resulted in a molecular switching device that opened at a positive potential of 2 V and closed at a negative potential of 2 V. 

It should also be recalled that a full electrochemical, as well as spectroscopic and photophysical, characterization of complex systems such as rotaxanes and catenanes requires the comparison with the behavior of the separated molecular components (ring and thread for rotaxanes and constituting rings in the case of catenanes), or suitable model compounds. As it will appear clearly from the examples reported in the following, this comparison is of fundamental importance to evidence how and to which extent the molecular and supramolecular architecture influences the electronic properties of the component units. An appropriate experimental and theoretical approach comprises the use of several techniques that, as far as electrochemistry is concerned, include cyclic voltammetry, steady-state voltammetry, chronoampero-metry, coulometry, impedance spectroscopy, and spectra- and photoelectrochemistry. 

Structurally related to these species are the triply branched compound 56+ and its rotaxanes 66+, 76+, and 86+ (Fig. 13.6)9, in which one, two, or three acceptor units are encircled by the electron donor macrocyclic compound 2. Although these rotaxanes cannot behave as degenerate molecular shuttles because of their branched topology, they are nevertheless interesting from the electrochemical viewpoint. 

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. 

The first example of electrochemically driven molecular shuttles is rotaxane 284+ (Fig. 13.25) constituted by the electron-deficient cyclophane 124+ and a dumbbellshaped component containing two different electron donors, namely, a benzidine and a biphenol moieties, that represent two possible stations for the cyclophane.10 Because benzidine is a better recognition site for 124+ than biphenol, the prevalent isomer is that having the former unit inside the cyclophane. The rotaxane... 

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. 

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