Rotaxane-based molecular shuttle

Umehara T, Kawai H, Fujiwara K, Suzuki T (2008) Entropy- and hydrolytic-driven positional switching of macrocycle between imine- and hydrogen-bonding stations in rotaxane-based molecular shuttles. J Am Chem Soc 130 13981-13988... 

Upon snpramolecnlar complexation, neutral hydrophobic orgauic compounds and hydrophilic metal cations usually show entirely different, often opposite, solvent polarity dependencies. Snch contrasting behavior is very generally observed when one compares the solvophobically versus electrostatically driven complexation for detailed discussion, see, for example, the study by Mizutani et al This difference in solvation property is exploited as a conventional tool for creating a vast variety of supramolecular architectures and devices, including rotaxane-based molecular shuttles and other so-called molecular machines. " ... 

Several examples that may be considered mechanical molecular devices have been reported including die rotaxane based molecular shuttle [16], the chir-optical molecular switch [17], light-activated bianthrone molecules [18], mole-eular brakes [19], molecular ratchets [20], molecular propellers [21], and eonformationally invertable columnar mesophases [22]. 

Altieri A, Aucagne V, Carrillo R, Clarkson GJ, D Souza DM, Dunnett JA, Leigh DA, Mullen KM (2011) Sulfur-containing amide-based [2]rotaxanes and molecular shuttles. Chem Sci 2 1922-1928... 

Numerous interlocked assemblies were designed in connection with attempts to gain control of rotational and translational motions at the molecular level, a stepping stone toward possible outstanding development. [ -As-re ards light-operated cases, a representative shuttling scheme for a rotaxane-based molecular abacus is depicted in Fig. As active components. 

The earliest light-driven CD-based molecular shuttle consisted of an azobenzene moiety in the axis and a-CD (Fig. 4). NMR spectroscopy supported the formation of a rotaxane complex where a-CD stays at the central trans-azobenzene site. Trans-cis conversion with 360 nm light caused a shift of a-CD to the methylene spacer, due to the exclusion of the cts-azobenzene moiety from the CD cavity. Back photoreaction and repositioning of the macrocycle on the trarzs-azobenzene unit was obtained with irradiation at 430 nm. 

Another recent example using PET reaction-based molecular shuttle has also been designed with a CBPQT-based rotaxane, with the thread composed of a porphyrin (Por) as the photoactive unit, Ceo as the electron acceptor, tetrathiafiilvalene (TTF) and dioxynaphthalene as the two tt-electron-donating stations, and substituted 2,6-diisopropylbenzene as the stopper (Figure 36). ° In... 

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. 

Figure 11.38 A [2] rotaxane-based, pH- and redox-switchable molecular shuttle.

Fig. 9 (a) Molecular shuttles based on reversible Diels-Alder reaction (al-a6) and sulfur-containing molecular shuttles (a7-al2) (b) methoxy-cycloheptatriene- and methoxy-acridane-based rotaxanes (c) imine-based rotaxanes (see text for details)... 

Control of Motional Features of Molecular Shuttles based on Reversible Covalent Changes. Rotaxanes... 

Rotaxanes-Threading Molecular Rings Rotaxanes are obtained by threading linear polymers through molecular rings such as cyclodextrins, crown ethers and cyclophanes. Molecular shuttles based on the rotaxane structure have been proposed. 

Photoisomerizations can often occur by several different mechanisms. Systems that isomerize via a controlled mechanism are potential candidates for molecular machines [184]. Energy in the form of light is absorbed and converted to controlled mechanical force on the molecular scale. Examples of a mono-directional rotor [185, 186], a switchable rotor [187], and a molecular shuttle [188] have been demonstrated. These systems are light-controlled, but there are also examples of systems which control molecular motion based on electro- and/or chemical modulation, such as the threading/imthreading of (pseudo)rotaxanes [189-196]. 

Murakami, H, A Kawabuchi, K Kotoo, M Kunitake and N Nakashima (1997). A light-driven molecular shuttle based on a rotaxane. Journal of American Chemical Society, 119(32), 7605-7606. 

Fig. 3 A chemically controllable molecular shuttle the macrocyelic ring can be switched between the two stations of the dumbbell-shaped component by base/acid inputs. Additionally, in the deprotonated rotaxane, the ring can be displaced from the bipyridinium station through reduction of such unit. (View this art in color at www.dekker.com.)...

This opposite solvent polarity dependence is generally observed for solvophobically versus electrostatically driven complexation processes see, for instance, the recent study by Mizutani et al. This general phenomena is used to create a wide variety of supramolecular devices, such as molecular shuttles based on rotaxanes as well as other so-called. molecular machines. ... 

Figure 3.39 A molecular shuttle, based around a rotaxane with two potential binding sites. A change in affinities leads the macrocycle to be shifted to the alternate site.

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