Bistable catenanes

More interesting observations were made on incorporation of the bistable catenane 134+ (Fig. 17.12) into a solid-state device.115 A monolayer of the catenane was transferred onto a photolithographically patterned polycrystalline silicon electrode. The patterning was such that the Langmuir film was deposited along several parallel lines of poly-Si on the electrode. A second set of orthogonally oriented wires was then deposited on top of the first set such that a crossbar architecture is obtained. This second set of electrodes consisted of a 5 nm thick layer of Ti, followed... 

Mechanically interlocked molecules, such as bistable catenanes [13] and [2]rotax-anes [14], constitute some of the most appropriate candidates to serve as nanoscale switches and machines in the rapidly developing fields of nanoelectronics [15] and nanoelectromechanical systems (NEMS) [16]. The advantages of using mechanically interlocked molecules in the fields of molecular electronics and... 

Fig. 22 Design of a bistable catenane that performs as a molecular rotary motor controlled by two pairs of independent stimuli (Si and S2)- The working scheme is based on the potential energy changes expected for the chemical reactions and co-conformational rearrangements brought about by stimulation with independent inputs. A, B recognition sites A site A is switched on, K hindering group, X blocking group...

Conductivity differences were seen in a bistable [3]catenane closed-loop molecule, with a naphthalene group as one station, and tetrathiafulvalene as the second station, decorated by a tetracationic catenane salt that could travel on the catenane, like a train between two stations, making a nano-switch [141]. 

The earliest examples of mechanically bonded molecular switches were two-station rotaxanes and catenanes (a station is a recognition site for the macrocyclic component) that could be switched by these common stimuli. Our group reported a bistable rotaxane in 1994 that could be switched chemically and electrochemi-cally, utilizing a n-n donor/acceptor recognition motif (see Scheme 2 and explanation in corresponding caption) [107]. Many of the recognition motifs in MIMs that... 

Derivative 178 containing a dibenzo-34-crown-10 ring interlocked with macrocycles incorporating two 4,4 -dipyr-idyl moieties tethered by different aryl spacers acts as bistable [2]catenane <20060L2119>. Variable-temperature (VT) NMR studies were used to determine the activation energy required for the conformational interconversions and to demonstrate that, by appropriate incorporation of bulky groups on one or both of the aryl linkers, it was possible to block one or both of the two circumrotation pathways. 

Stoddart and co-workers have developed molecular switch tunnel junctions [172] based on a [2]rotaxane, sandwiched between silicon and metallic electrodes. The rotaxane bears a cyclophane that shuttles along the molecular string toward the electrode and back again driven by an electrochemical translation. They used electrochemical measurements at various temperatures [173] to quantify the switching process of molecules not only in solution, but also in self-assembled monolayers and in a polymer electrolyte gel. Independent of the environment (solution, self-assembled monolayer or solid-state polymer gel), but also of the molecular structure - rotaxane or catenane - a single and generic switching mechanism is observed for all bistable molecules [173]. 

FIGURE 332 A catenane molecule developed by Professor Stoddart and colleagues to function as a controllable (bistable) nanoswitcb (see text). [Reprinted witb permission from Accounts of Chemical Research 1 (2001), copyright (2001) American Cbemical Society.]... 

Photo-driven molecular devices (light-powered bistable rotaxanes and catenanes) 07CSR77. 

In Milestone Six, a change in electrical conductivity, presumably tied to a molecular translation, was reported. A cyclic bistable [2]calcnane, with two different electron donor moieties (the closed "track") and a cyclic acceptor traveling on the catenane (the "train" on the closed track) (Fig. 8) was measured as a LB monolayer between polysilicon and Ti electrodes. The two donor moieties are naphthalene, a weak donor, and tetrathiafulvalene, a much stronger donor (in the sense that it is more easily... 

The results of these condensed-phase switching experiments provide us with the impetus to incorporate the bistable [2]catenane 4" into solid-state electronic devices. In collaboration with members of the Heath group in the California NanoSystems Institute (CNSI) here at UCLA, the [2]catenane 4 4DMPA was introducedfirst of all into two-terminal devices to form molecular switch tunnel junctions (MSTJs) in which each MSTJ functions as an electronically reconfig-... 

Fig. 1.6 Proposed electromechanical mechanism for the operation of the device based on the bistable [2]catenane 4". ...

Another similar bistable side-chain poly[2]catenane 64 was rejxjrted by Stoddart and coworkers (Scheme 17.19), using a method similar to that employed when preparing 59 [122]. The [2]catenane monomer 60 bearing an alkyne group was first prepared, as described above. As shown in Scheme 17.19, monomer 60 was bistable and composed of two switchable isomers a ground-state co-conformation (GSCC) and the metastable-state co-conformation (MSCC), which corresponded to the locations of the tetrathiafulvalene (TTF) and dioxynaphthalene (DNP) units of... 

Scheme 17.19 Synthesis of bistable side-chain poly[2]catenane 64.

A significant number and variety of molecular machines have been created from the oxidation-driven movement of a TTF" " station out of the tetracationic CBPQT " " host, as discussed above in conjunction with Figure 10. A key example is the bistable [2]catenane Cat-2 + (Figure 19). The CV published in the original paper has a very similar form as the pseudorotaxane in Figure 10, indicating the movement of the TTF" " monocation out of the center of the... 

Stoddart et al. also incorporated a bistable [2]catenane (Figure 39) into macromolecules to form a side-chain poly[2]catenane (Figure 82). This poly[2]catenane could also behave as a molecular switch that can be addressed in an on or off state electrochemically by the virtue of the redox properties of the TTF moiety. Furthermore, the switching properties remain even in spherical aggregates. This stndy provides a good mechanically interlocked switchable polymeric scaffold for the construction of solid-state molecular electronic devices. 

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