The Light-Induced Transformation

The molecular event that triggers the visual process is the light-induced transformation of the 11-c form of retinal in rhodopsin to the aJi-trans form. 

The formation and the dissociation of the supramolecular donor-acceptor complex between the xanthene dye and the bipyridinium unit can therefore be triggered by the light-induced transformation of the latter component to photoisomers exhibiting high or low affinities for the electron donor, respectively. 

Many previous studies have shotm that the light-induced transformation of certain pollutants Is greatly accelerated In natural waters compared to distilled water. Oxidations via photoproduced H2O2 may account for the results of some of these Investigations. 

Light-induced transformations over fluorinated titania (TiOi/F) cannot be initiated either by =Ti— 0 (OHads), due to the lack of =Ti—OH groups, or by SET from a surface complexed substrate, due to the fluoride competition. In addition to these major effects, the adsorption of molecular oxygen can be affected also and the surface charge is dramatically decreased. The last effect may be important particularly for charged substrates and intermediates and for the possibility of interfacial electron transfer. 

Photoswitchable enzymes could have an important role in controlling biochemical transformations in bioreactors. Various biotechnological processes generate an inhibitor, or alter the environmental conditions (pH, for example) of the reaction medium. Photochemical activation of enzymes that adjust environmental conditions or deplete the inhibitor to a low concentration may maintain the bioreactor at optimal performance. More specifically, integration of the photoswitchable biocataly-tic matrix with a sensory electrode might yield a feedback mechanism in which the sensor element triggers the light-induced activation/deactivation of the photosensitive biocatalyst. 

Diarylethenes are promising synthetic photoswitchable molecules because of their outstanding fatigue-resistance in reversible light-induced transformation between two isomers with different absorption spectra.21,22 The tt-conjugation extends over the entire molecular in the closed form, whereas it is restricted to each half of the molecule in the open form. As a consequence, the closed form is expected to exhibit better electrical conductance than the open form. The two forms are referred to as the on and off states of the switch, respectively. The UV-vis spectra of this molecular switch in toluene show that the wavelengths that can be used for on to off switching are 420 < A< 650 nm, and 300 < A< 350 nm for the reverse operation. 

Figure 10 shows the photo-induced transformation of radical species in irradiated PMMA [43], The photobleaching with visible light at 77 K causes the liberation of an electron from the anion radical and the splitting of a methyl radical from the ester side group ... 

It was only after Woodward and Hoffmann in 1965 had predicted a conrotatory mode for the thermal cyclopropyl-allyl anion transformation that a new interest developed in this reaction. By means of the iso-7c-electronic aziridine 310 Huisgen and coworkers succeeded in demonstrating that the thermal recation gave a conrotatory formation of azomethine ylid (311) and that the light-induced reaction resulted in a disrotation to give 312. 

The light-induced control of the electroactivity of charged redox-active substrates allows the functionalized electrode to be used to control electrochemical transformations. A system composed of two oppositely charged diffusional redox probes—positively charged 2,5-bis[[2-(dimethylbutylammo-nio)ethyl]amino]-l,4-benzoquinone (14) and negatively charged pyrrolo-quinoline quinone (PQQ) (15)—was used to demonstrate this behavior... 

Azobenzene derivatives constitute a family of dye molecules well known for their photochromic properties, which are due to the reversible cis-trans photoisomerization. " Figure 16.11 shows a photoisomerization process of azobenzene. Azobenzene derivatives have two geometric isomers the trans and the cis forms. The isomerization reaction is a light- or heat-induced inter-conversion of the two isomers. Because the trans form is generally more stable, the thermal isomerization is generally in the direction of from cis to trans. Light induces transformations in both directions. 

For insulating polymers, a local activation can be achieved by appropriate laser light. Cleavage of chemical bonds is followed by local electronic conductivity and the possibility of metal ion discharge. Another type of activation is the light-induced polyimide transformation from imide to amide producing free electrons [6.132]. 

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