Photochemically triggered physical

Photochemically Triggered Physical Amplification of Photoresponsiveness in Molecular Aggregate Systems... 

Figure 1. Examples of photochemically triggered physical amplification.

S. Tazuke, S. Kurihara, H. Yamaguchi, and T. Ikeda, Photochemically triggered physical amplification of photoresponsiveness, 1. Phys. Chem. 91, 249-251 (1987). 

Fig. 1. Examples of photochemically triggered physical amplification. a) Spherical micelle, read-out by change in surface tension, b) Plate-like micelle, read-out by light scattering, c) Vesicle, read-out by circular dichroism. d) Liquid crystals, read-out by polarized light.

The two most important methods of both probing and stimulating supramolecular devices are photochemical and electrochemical techniques. The most prevalent events to occur in such devices are electron, energy and proton transfer, as well as molecular rearrangement. Any of these events can provide the basis for forms of transducable output on which molecular electronic devices may be based. Therefore, the theories most commonly applied to such electrochemically and photochemically triggered events are outlined in this chapter. In addition, an overview of the mechanisms by which such events occur is provided, identifying the molecular or physical parameters required to make such events feasible in a supramolecular structure. 

The active compound within the bacillary layer is retinal. To simplify the photo-physics within the rods and cones hugely, absorption of a photon initiates a series of conformational changes that lead ultimately to photo-isomerization of retinal from the 11-cis isomer to the 11-trans isomer see Figure 9.20. The uncoiling of the molecule following photo-excitation triggers a neural impulse, which is detected and deconvoluted by the brain. The photochemical reaction is breakage and, after rotation, re-formation of the C=C bond. 

Work by Warburg and Bodenstein (1912-1925) clarified earlier confusions between photon absorption and observed chemical change. Molecules which absorb photons become physically excited , and this must be distinguished from becoming chemically active. Excited molecules may lose their energy in nonchemical ways, or alternatively may trigger off thermal reactions of large chemical yield. The socalled law , therefore, rarely holds in its strict sense, but rather provides essential information about the primary photochemical act. 

Photochemical reactions proceed in the gas phase, in solution, and even in the solid state. Photochemical reactions in the solid state are especially important when considering the applications of photofunctional molecules in materials, where photophysical processes and photochemical reactions are used to trigger some change in a given physical property of a system, such as its color, solubility, or electroconductivity. 

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