Molecular shuttling

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

Leigh DA, Perez EM (2006) Dynamic Chirality Molecular Shuttles and Motors. 265 185-208... 

XH NMR was as fast as 300000 times a second. It is interesting if the shuttling speed can be controlled by light or an electrochemical method [96]. Bissell et al. [97] obtained the molecular shuttle shown in Fig. 29, in which benzidine and bisphenol units act as the stations. At 229 K the tetracation bead was found to stay on the benzidine side at a probability of 86%, but when the compound was treated with an acid or oxidized electrochemically it turned out that the bead can move to the bisphenol side at a higher probability. 

Supermolecular interlocked macromolecules have been paid much attention as candidates of smart materials. Polyrotaxane (PRX) is a typical example. PEG/ cyclodextrin (CD)-based polyrotaxane was firstly reported by Harada and coworkers by attachment of stoppers to pseudopolyrotaxane (pPRX) consisting of a PEG and CDs [263]. Subsequently, many CD-based PRXs have been designed and prepared as smart materials such as biomaterials, light-harvesting antennae, insulating polymers, stimuli-responsive molecular shuttles etc. [264—268]. 

Aucagne V, Bema J, Crowley JD, Goldup SM, Hanni KD, Leigh DA, Lusby PJ, Ronaldson VE, Slawin AMZ, Viterisi A, Walker DB (2007) Catalytic active-metal template synthesis of [2]rotaxanes, [3]rotaxanes, and molecular shuttles, and some observations on the mechanism of the Cu(I)-catalyzed azide-alkyne 1, 3-cycloaddition. J Am Chem Soc 129 11950-11963... 

The possibility of fine-tuning electron transfer processes through molecular shuttling was finally shown by introducing ferrocene electron donors on the macrocycle (Scheme 9.13).56... 

Interestingly, the dumbbell component of a molecular shuttle exerts on the ring motion the same type of directional restriction as imposed by the protein track for linear biomolecular motors (an actin filament for myosin and a microtubule for kinesin and dynein).4 It should also be noted that interlocked molecular architectures are largely present in natural systems—for instance, DNA catenanes and rotaxanes... 

Figure 13.3 (a) Operation of a two-station rotaxane as a degenerate molecular shuttle,... 

Figure 13.4 Structure formulas of (a) the two-station rotaxane 14+ that behaves as a degenerate molecular shuttle and (b) its molecular components 2 and 34 + and model rotaxane 44 +.

An example of rotaxane that behaves as degenerate molecular shuttle is represented... 

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. 

Figure 13.8 Schematic operation of a two-station rotaxane as a controllable molecular shuttle, and idealized representation of the potential energy of the system as a function of the position of the ring relative to the axle upon switching off and on station A. The number of dots in each position reflects the relative population of the corresponding coconformation in a statistically significant ensemble. Structures (a) and (c) correspond to equilibrium states, whereas (b) and (d) are metastable states. An alternative approach would be to modify station through an external stimulus in order to make it a stronger recognition site compared to station A.

Figure 13.9 Structure formula of rotaxane 9H3+ and representation of its operation as a pH controllable molecular shuttle.

When one of the two rings of a catenane carries two different recognition sites, then the opportunity exists to control the dynamic processes (Fig. 13.21) in a manner reminiscent of the already discussed controllable molecular shuttles (Fig. 13.8). 

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. 

Rotaxane 316+ was specifically designed36 to achieve photoinduced ring shuttling in solution,37 but it also behaves as an electrochemically driven molecular shuttle. This compound has a modular structure its ring component is the electron donor macrocycle 2, whereas its dumbbell component is made of several covalently linked units. They are a Ru(II) polypyridine complex (P2+), ap-terpheny 1-type rigid spacer... 

More recently, the second-generation molecular shuttle 374+ (Fig. 13.32) was designed and constructed.38 The system is composed of two devices a bistable redox-driven molecular shuttle and a module for photoinduced charge separation. In the stable translational isomer, the electron-accepting cyclophane 124+, which is confined in the region of the dumbbell delimited by the two stoppers Tj and T2, encircles the better electron donor tetrathiafulvalene (TTF) station. 

A second molecular shuttle 10(4) +, has been synthesized in our group38 and is represented in Fig. 14.15. The mobile ring is the nonhindering 39-membered ring 9 containing one 8,8 -diphenyl-3,3 -biisoquinoline chelate, and its linear part contains, as in previous molecular shuttle 7(4)+, one bidentate 1,10-phenanthroline and one tridentate terpyridine units. Another difference is the phenylene group as a linker, which makes this thread more rigid. 

Figure 14.15 Second copper-based molecular shuttle 10(4) +, containing a sterically nonhindering macrocycle.

Naphthalimide can be reduced to the corresponding radical anion. Compared to naphthalimide, its radical anion exhibits high hydrogen binding affinity with hydrogen donors. By using these properties of naphthalimide, a molecular shuttle based on... 

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