Molecular actuator

An important issue associated with molecular machines is the detection of actuations on the nanoscale level. When a chemical stimulus induces movement in a machine, several spectroscopic techniques, such as nuclear magnetic resonance (NMR) spectroscopy, UV-Vis spectroscopy, emission spectroscopy and X-ray photoelectron spectroscopy (XPS) can be used to detect their outputs. More intri-guingly, electrochemical and photochemical inputs often provide [6, 8g] a two-fold advantage by inducing the mechanical movements and detecting them. Additionally, the dual actions of the these two types of stimuli can be exploited when the time-scale of the molecular actuations, which ranges from picoseconds to seconds, falls within the detection time-scale of the apparatus. 

An electrochemical stimulus provides a two-fold advantage since it can stimulate molecular actuations through clean redox reactions and also detects simultaneously the resulting molecular movements by sensing the changes in the electromechanical properties of a bistable molecule. CV is often employed to both write and read the electromechanical movements in the redox-active bistable... 

Conformational changes induced by the complexation of alkali cations have been analyzed experimentally and theoretically in oligothiophenes 89 and 90 specifically tailored to serve as model compounds for molecular actuation [197]. 

Jousselme, B., P. Blanchard, E. Levillain, J. Delaunay, M. AUain, R Richomme, D. Rondeau, N. Gallego-Planas, and J. Roncali. 2003. Crown-annelated ohgothiophenes as model compounds for molecular actuation. / Am Chem Soc 125 1363—1370. 

The described theoretical model involves a basic molecular actuator constituted by an ideal and lineal polymeric chain. We can imagine this basic chain coimected to a metallic electrode and immersed in an electrolyte (Figure 16.5). The strong intramolecular interactions produce a coil-compact structure of the chain. During oxidation, consecutive electrons are extracted from the chain, positive charges are... 

Both the theoretical model and this basic molecular actuator include electric pulses, ions and water interchanges between the polymer and the solution, chemical reactions, stimulation of the conformational movements along polymeric chains, and changes in the inter- and intramolecular interactions. Those processes occurring in soft and wet materials mimic, at the molecular level, the consecutive events involved in the actuation of a natural anisotropic muscle. 

Figure 2.2 A simplified scheme of events that define and influence a bio-responsive hydrogel system. An input of a stimulus (step 1) leads to the binding of the receptor to the ligand within the gel network resulting in molecular biorecognition events (step 2). Molecular actuation is stimulated in step 3 to trigger further micro and macroscopic events, i.e. swelling. Molecular actuation results in a response (step 4) that can be mechanical, chemical, optical or electronic.

A quantitative measure of a molecular actuator is the magnitude of geometrical change as function of the amount and sign of the charge transferred to the molecule, q, in terms of the number of electrons added (X , reduction) or removed (X +, oxidation), and the following formula can be used to characterize them ... 

Fig. 45 Proposed components of molecular actuators in which formation of n dimers induces a change in conformation...

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