Dumbbell component

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... 

Rotaxanes Containing Identical Recognition Sites for the Ring in their Dumbbell Component... 

If during the template-directed synthesis of a rotaxane, the location of two identical recognition sites ( stations ) within its dumbbell component can be arranged (Fig. 13.3a and b), a degenerate, coconformational equilibrium state is obtained in which the macrocyclic component spontaneously shuttles back and forth between... 

Dumbbell 34+ exhibits two bielectronic and reversible processes that can be attributed to the simultaneous first and second reduction of the two bipyridinium units contained in its axle-like section. The bielectronic nature of the processes indicates, as expected, that the bipyridinium units are equivalent and behave independently. Also, model rotaxane 44+ shows two bielectronic and reversible processes that are straightforwardly assigned to the bipyridinium units contained in its dumbbell component they are, however, shifted to more negative potentials compared to dumbbell 34+. These shifts can be attributed to the CT interactions with the electron donor ring that make the electron acceptor bipyridinium units more difficult to reduce, whereas the bielectronic nature of the processes indicates the such units are noninteracting and equivalent—both of them are surrounded by a ring—in full agreement with the supramolecular structure of 44+. 

When a rotaxane contains two different recognition sites in its dumbbell component, it can exist as two different equilibrating coconformations, the populations of which reflect their relative free energies as determined primarily by the strengths of the two different sets of noncovalent bonding interactions. In the schematic representation shown in Fig. 13.8, it has been assumed that the molecular ring... 

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. 

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... 

In dumbbell-shaped component 326+, all the redox processes of the incorporated units are present at almost the same potentials as in the separated units (Fig. 13.30) this finding shows that there are no substantial intercomponent electronic interactions. On going from the dumbbell component to rotaxane 316+, some processes are affected while others are not (Fig. 13.30). 

In one such TTF/DNP-based bistable [2]rotaxane 54+ (Fig. 8.6a), a long, rigid p-ter-phenyl spacer was employed between the TTF and DNP recognition sites and two tetraarylmethane stoppers were employed [22a] on both ends of the dumbbell component for the solution-phase switching studies. UV-Vis spectroscopy (Fig. 8.6b), which reveals the precise location of the CBPQT4+ ring on the dumbbell... 

A TTF/DNP-based [2]rotaxane [15e] 94+ was functionalized (Fig. 8.10a) with a disulfide-based anchoring group at the DNP end of the dumbbell component in order to allow its self-assembly on to gold surfaces. The CV of the SAM/Au was recorded at a scan rate of 300 mV s 1 at 288 K. The first CV cycle (Fig. 8.10b, green trace) displays a higher positive potential (+490 mV as opposed to +290 mV for the dumbbell) for the first one-electron oxidation of the TTF unit... 

The simplest rotaxane morphology, as defined in the Introduction and represented in Figure 2.1, is noted as [2]-rotaxane to express that it is made up with two components a ring and a dumbbell.13 More generally, an [n]-rotaxane contains n — 1 rings threaded onto a dumbbell component. Figure 2.4 shows several relatively simple rotaxane morphologies that have been realized. 

Rotaxanes 145 <1993CC1269>, 146 <1993CC1274>, and 147 <1996JA4931> incorporating Jt-electron-deficient bipyridinium-based dumbbell components and one or more 7t-electron-rich hydroquinone-based (and/or dioxy-naphthalene-based <1995CC747>) macrocyclic polyether counterparts have been assembled and their spectroscopic and electrochemical properties investigated in connection with the potential fabrication of chemically, photochemi-cally, and electrochemically active molecular devices <1996JA4931>. 

It is possible to synthesize [2]rotaxanes containing two identical recognition sites within their dumbbell component. The result is a degenerate equilibrium state in which the macrocyclic component can shuttle back and forth along the linear portion of the dumbbell. Such a system constitutes a molecular shuttle. Two examples [lla,b, 39] of [2]rotaxanes that behave as degenerate molecular shuttles are shown in Figure 12. 

In the clipping method the macrocycle is constructed around the linear or dumbbell component. Obviously the precursor to the macrocycle must be bound to the linear component prior to the cyclization step. This may be achieved either covalently (schemes 5 and 6) or non-covalently (scheme 7). 

In the second case, non-covalent interactions are used to create a complex between the precursor to the macrocycle and the dumbbell component (scheme 7). Therefore the latter templates the macrocyclization reaction which may occur either intramo-lecularly, if the precursor bears two complementary ftinctions, or intermolecularly if a third molecular fragment (i.e. Y-Y) is involved. 

Slippage experiments involved extremely hard reaction conditions the macrocycle was melted at 350°C in the presence of the dumbbell component during very short periods of time (10 min). A [2]-rotaxane was obtained in 3% yield by this method and was shown to be stable at room temperature in dmf solution [106]. 

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