Photoisomerizable materials

The immobilization of a photoisomerizable material that can be switched by light between redox-active and redox-inactive or conductive and insulating states offers an encouraging route toward integrated molecular memory devices. Figure 7.2 shows a photoisomer state A in which the molecular unit is redox-inactive and no electronic signal is transduced. Photoisomerization of the chemical component to state B generates a redox-active assembly, and the electron transfer between the electrode and the chemical modifier yields an amperometric (electrochemical) indicator of the state of the system. 

In another example, a mixed monolayer composed of a photoisomerizable component and an electrochemical catalyst was applied to switch the electrocatalytic properties of a modified electrode between ON - and OFF -states. A gold electrode surface functionalized with a spiropyran-monolayer and pyrroloquinoline quinone (PQQ) moieties incorporated into the mono-layer was applied to control the electrocatalytic oxidation of NADH by light [92]. The positively charged merocyanine-state interface resulted in the repulsion of Ca2+ cations (promoters for the NADH oxidation by the PQQ), thus resulting in the inhibition of the electrocatalytic process. In the nitrospiropyran-state the monolayer does not prevent association of the PQQ-catalyst and Ca2+-promoter, so provides efficient electrocatalytic oxidation of NADH. Similar results have been achieved by a combination of the photo- and thermal effects resulting in the isomerization of the spiropyran-monolayer with the incorporated PQQ-catalyst [93], Other photoisomerizable materials such as an azobenzenealkanethiol derivative mixed with a ferrocene-redox component have also been used to control the electrocatalyzed electron transfer process between a command interface and a dissolved redox probe [94]. 

LIGHT-SWITCHABLE MODIFIED ELECTRODES BASED ON PHOTOISOMERIZABLE MATERIALS... 

Photoresponsive molecules are important for the fabrication and photomodulation of photoresponsive CLC materials, no matter if they are employed as chiral mesogens, achiral nematic LC hosts, or chiral/achiral guest molecules. The most important property of these molecules in photoresponsive CLC materials is the changes in molecular shape (geometry/conformation) as a result of the light-induced isomerization, which constitutes the basis for photomning of the properties of CLCs [18], The first example of using photoresponsive molecules to modulate the properties of CLCs was reported by Sackmann in 1971 [19], where azobenzene was employed as the dopant molecule. To date, various kinds of photoisomerizable molecules have been developed for this purpose (Fig. 5.3). 

To effect the conformational changes illustrated above, one must introduce a stimulus such as au acid, a base, a metal ion, or a ligand into a system. The accumulation of materials on multiple switching cycles makes such systems of Utde practical value. Therefore, the use of a noninvasive stimulus such as light or heat is highly desirable. Azobenzene, a very conunon photoisomerizable molecule, is often used as a switch to photomodulate molecular properties. For instance, Hecht and coworkers incorporated an azobenzene moiety into the center of an mPE oligomer that was... 

Ueda M., Kim H.-B., Ikeda T., Ichimura K. Photoisomerizability of an azobenzene covalently attached to siUca-gel matrix. J. Non-Cryst. Solids 1993 163 125-132 Ueda M., Kim H.-B., Ichimura K. Photochemical and thermal isomerization of azobenzene derivatives in sol-gel bulk materials. Chem. Mater. 1994 6 1771-1775 Uchida Y., Baba H., Nakamura Y., Matsui K. Spectroscopic characterization of sol-gel silica derived from 3-aminopropyl-triethoxysilane. Trans. Mater. Res. Soc. Jpn. 2002 27 771-774 Wei Y., JinD., DingT. Optical rotatory silica materials prepared via sol-gel processes. J. Phys. Chem. B 1997 101 3318-3323... 

The introduction of photoisomerizable groups, such as azobenzene derivatives, in dendrimers enables the controlled induction of a structural change, especially when those units are attached to the core or implemented in the branches. If attached at the periphery, these photoisomerizable groups, can be used to close the surface of a dendrimer by means of a photoinduced increase of steric hindrance at the periphery. This type of dendrimers can be used as carriers of small moleeules, while a eontrolled release of those guest molecules is possible using light, whieh indueed the isomerization from cis to tram. In addition, azobenzene-fiinetionalized materials are widely used in the field of datastorage. 

---