Mechanical Switching Processes

Positional changes of atoms in a molecule or supermolecule correspond on the molecular scale to mechanical processes at the macroscopic level. One may therefore imagine the engineering of molecular machines that would be thermally, photochem-ically or electrochemically activated [1.7,1.9,8.3,8.109,8.278]. Mechanical switching processes consist of the reversible conversion of a bistable (or multistable) entity between two (or more) structurally or conformationally different states. Hindered internal rotation, configurational changes (for instance, cis-trans isomerization in azobenzene derivatives), intercomponent reorientations in supramolecular species (see Section 4.5) embody mechanical aspects of molecular behaviour. 

Although they are random and not oriented, the motions of a substrate inside a cyclodextrin cavity (see Section 4.5) have molecular ball-bearing flavour. Correlat- 

Mechano-ionoswitching takes place, for instance, in polytopic ligands presenting ion binding induced allosteric changes [8.201, 8.276] or when an ion binds to a catenand to form a catenate [8.281, 8.282]. Modulation of the binding of a second substrate may be brought about. 

In general, optically, electrically or chemically triggered switches would seem to be preferable to mechanically activated ones, as are photo-, electro- and chemo devices with respect to mechano devices and electronic or photonic computing with respect to mechanical computing. The ultimate in (nano)mechanical manipulation of a molecular device is represented by the realization of a bistable switch based on the motion of a single atom by means of the scanning tunnelling microscope [8.295] (see also Section 9.9). 

A highly relevant question is whether the goal should be to operate at the level of the single component or not. In order to minimize the risk of device failure the incorporation of some redundancy or repair procedures appears unavoidable, as in biological processes, so that it will be necessary to develop suitable addressing and detection procedures, error-checking and self-healing mechanisms for multidevice arrays. 

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These observations must be taken into account when considering the mechanism of halogen addition They force the conclusion that a simple one step bond switching process of the following type cannot be correct A process of this type requires syn addi tion It IS not consistent with the anti addition that we actually see... 

The substrate carries information defined by its molecular recognition features and constitutes a signal that may be modulated by the switching process. The stimulus to which the switch responds and the substrate on which it operates may both be either of photonic, electronic, ionic, magnetic, thermal or mechanical nature. Thus, a whole set of switches based on (substrate-stimulus) pairs, is defined by the combination of the various types opto-photo, opto-electro, electro-photo, etc. 

It must be admitted that 0-0 and a-n switch processes are of limited stereochemical interest. Given the prototypes (92) or (97), the reactions must be syw-stereospecific any other stereochemical result would in fact be used as a criterion of a multi-step mechanism. This is also true of the mixed types, (101)-(104). But these reactions are exceedingly useful as models for orbital-based calculations or estimates of free energies of activation, with the use of extended HMO theory. The four- or six-center process (92)-(104) does not appear to be more complex than the Diels-Alder reaction, which has been investigated theoretically (Herndon and Hall, 1967). 

Polystyrene capacitors have exceptionally low tan S values (< 10 q, making them well suited for frequency-selective circuits in telecommunications equipment. Polymer capacitors are widely used for power-factor correction in fluorescent lighting units, and in start/run circuitry for medium-type electric motors used in washing machines, tumble-dryers and copying machines for example. They are also used in filter circuits to suppress radio frequencies transmitted along main leads. Such interference noise may originate from mechanical switches, furnace controllers and switch mode power supplies it not only spoils radio and television reception but can also cause serious faults in data-processing and computer equipment. 

Next we consider the case where a low overpotential exists at some point in the electrochemical switching process, as is necessarily the case in cyclic voltammetric or chronopotentiometric experiments. Now, four additional mechanisms become possible on the time scale of the electron/ion transfer process. This is so because the electron/ion transfer step, E, is no longer constrained to be the first. Consequently, either one or both of the chemical steps may precede the coupled electron/ion transfer. During the oxidation of R to Of, if solvation of Ra is more rapid than its reconfiguration, the two mechanisms, CEC and CC E may occur. If reconfiguration of R is more rapid than its solvation, two additional mechanisms, C EC and C CE, become possible. 

To answer all these questions about the distinctive features of dream consciousness, it was necessary to know not only that the brain was switched back on in sleep, but also that this switch process was effected by quite different mechanisms from those of waking. After all, we usually don t wake up during REM and we already knew, from Jouvet s early work, part of the reason why we don t move our muscles are actively inhibited. This guarantees the continuation of immobility and it helps us understand why we often awaken feeling paralysed in our dreams. 

In real situations (Sections 3.1 and 3.5) sequential one-electron transfers precede the formation of electron-rich or electron deficient multi-electron catalytic complexes. Thus, such systems may be considered as devices for switching processes from the multistep one-electron mechanism to the multi-electron mechanism. 

P.R. Ashton, R. Ballardini, V. Balzani, S.E. Boyd, A. Credi, M.T. Gandolfi, M. Gomez Lopez, S. Iqbal, D. Philp, J.A. Preece, L. Prodi, H.G. Ricketts, J.R Stoddart, M.S. Tolley, M. Venturi, A. J.R White, D.J. Williams, Simple Mechanical Molecular and Supramolec-ular Machines Photochemical and Electrochemical Control of Switching Processes , Chem. Eur. J., 3, 152 (1997)... 

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]. 

This model of the switching process was confirmed by Kamitsos et al. Using Raman spectroscopy, they showed that a fraction of 10-15 mol% neutral TCNQ exists in the high conducting state immediately after switching [7]. A field-induced redox reaction was postulated by Potember [8] to describe the phase transition described above. Many publications deal with the switching mechanism but the details are still not yet fully understood. 

The alternative, much easier, mechanism to cause the switching process was based on the use of an external sacrificial redox reactant that operates after the photoinduced deactivation of the Ax station, as illustrated in Fig. 15. Such a sacrificial mechanism is based on the following four operations [7f] ... 

The most common mechanism for the switch is the OTC review process. An advisory review panel makes a recommendation to the FDA. If the FDA agrees with the recommendation, it publishes a final OTC drug monograph about the switch. The switch process can also result in a product being switched from OTC status to prescription. In 1977, hexachlorophene was switched from OTC status to prescription status because of overdoses in infants in France. 

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