Shuttling chemically controlled

Figure 6.7 A chemically controllable molecular shuttle. The macrocydic ring can be switched between the two stations of the dumbbell-shaped component by acid-base inputs.

No example has so far been reported of a shuttling process controlled by electron transfer chemical reactions. There are, however, very interesting examples of shuttling processes controlled by acid/base reactions. One case is that of the previously discussed compound 13 " (see Figure 14), in which the shuttling of the macrocycle component can be controlled not only electrochemically, but also by protonation/ deprotonation of the benzidine unit [43]. 

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.)...

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

The last example is a molecular shuttle with two different states between which one can switch electrochemically and chemically [25]. The more electron-rich part of the axle is the bisaniline moiety and, consequently, the electron-poor wheel tends to bind more strongly to this side (Scheme 6.5.9, center). On oxidation and/ or protonation charge repulsion moves the wheel to the bisphenol part of the axle. Thus, the switching between the two states can be controlled by two different... 

Fig. 11. (A) Design of the Langmuir trough used in the current studies. The trough is shuttled under computer control for IR illumination of the background surface and monolayer covered surface. (B) Schematic of the IRRAS instrument displaying the sections of the accessory that rotate in concert under computer control to provide the desired angle of incidence. Taken from Ref. [81] with permission from American Chemical Society.

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]. 

Fig. 15. A chemically and electrochemically controllable molecular shuttle 104+. When the dumbbell is in its unperturbed state, the cyclophane exists in an 84 16 ratio (CD3CN, -44 °C) encircling preferentially the comparatively more 7t-electron-rich benzidine site. The cyclophane can be enticed to translate to the biphenol site exclusively either chemically -through protonation of the benzidine nitrogen atoms - or electrochemically - through oxidation of the benzidine unit to its radical cation. Both of these processes are completely reversible, through addition of base, or electrochemical reduction, respectively...

Fig. 6 A molecular shuttle controllable either chemically or electrochemically- resting state, t e w ee is ocate on e...

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