Command monolayer

In the example shown in Fig. 4.39, a photoisomerizable command mono-layer was immobilized on a sohd surface and hquid crystalline layers were then deposited on the monolayer. Photoisomerization of the command monolayer can then change the orientation of the thick hquid crystalhne layer. Molecular... 

Scheme 10 Reversible photoswitchable activation/deactivation of the electrical contact between cytochrome c and the electrode and the secondary activation/deactivation of the COx-biocata-lyzed reduction of oxygen using a thiolated nitrospiropyran and thiolated pyridine mixed monolayer as a command interface.

A further approach to controlling electrical communication between redox proteins and their electrode support through a photo-command interface includes photo stimulated electrostatic control over the electrical contact between the redox enzyme and the electrode in the presence of a diffusional electron mediator (Scheme 12).[58] A mixed monolayer, consisting of the photoisomerizable thiolated nitrospiropyran units 30 and the semi-synthetic FAD cofactor 25, was assembled on an Au electrode. Apo-glucose oxidase was reconstituted onto the surface FAD sites to yield an aligned enzyme-layered electrode. The surface-reconstituted enzyme (2 x 10-12 mole cm-2) by itself lacked electrical communication with the electrode. In the presence of the positively charged, protonated diffusional electron mediator l-[l-(dimethylamino)ethyl]ferrocene 29, however, the bioelectrocatalytic functions of the enzyme-layered electrode could be activated and controlled by the photoisomerizable component co-immobilized in the monolayer assembly (Figure 12). In the... 

This section will demonstrate the first sergeants and soldiers-type helix command surface experiment, in which thermo-driven chiroptical transfer and amplification in optically inactive polysilane film from grafted (or spin-coated) optically active helical polysilane onto quartz substrate [92]. Although helix and optical activity amplification phenomena based on the sergeants and soldiers principle was mainly investigated in polymer stereochemistry, the orientation and physical properties of a thick layer deposited onto a solid surface and controlled by a monolayer command film based on command surface principles was established in photochemical material and surface science [93,94]. Both sergeants and soldiers and command surface experiments appear to have been developed independently. 

K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki, K. Aoki, Reversible Change in Alignment Mode of Nematic Liquid Crystals Regulated Photochemically by Command Surfaces Modified with Azobenzene Monolayer , Langmuir, 4, 1214 (1988)... 

Command surfaces based on isomerizable monolayers have also been used to detect various signals (temperature change [204], pH change [205]) by the variation of redox cofactor regeneration rates (therefore by the control of enzymatic activity). Thus, they represent examples of biocatalytic switches. All the systems described above represent examples of bioelectronic devices that can be used for the trans-... 

Fe(CN)6] , electrochemically contacted at a photoisomerizable command interface (lla/llb). Figure 7.15 shows the impedance features (as Nyquist plots) of the nitrospiropyran (11a) and protonated nitromerocyanine (lib) electrodes in the presence of [Fe(CN)6] as a redox probe. The impedance spectra show a larger resistance to interfacial electron transfer when the monolayer is in the neutral dinitrospiropyran state (Ret = 60 kll) than when it is in the positively charged protonated merocyanine state (Ret = 48 kQ) (Figure 7.15, curves b and a). The heterogeneous rate constants for electron transfer between the electrode and the redox probe were calculated to be 0.82 X 10" and 1.1 x 10" cm s" for the 11a and 1 lb-monolayer modified Au-electrodes, respectively. 

Doron, A., Katz, E., Portnoy, M., and Willner, I. An electroactive photoisomcnzable monolayer-electrode A command surface for the amperometric transduction of recorded optical signals. Angew. Chem. Int. Ed. Engl., 1996, 35,1535-1537. 

Ichimura, K., Suzuki, Y., Seki, T., Hosoki, A., and Aoki, K. Reversible change in alignment mode of nematic liquid crystals regulated photochemically by command surfaces modified with an azobenzene monolayer. Langmuir, 4, 1422 (1988). 

Ubukata, T., Seki, T, and Ichimura, K. Modeling the interface region of command surface. Parr 1. Structural evaluations of azobenzene/liquid crystal hybrid Langmuir monolayers. /. 

Keywords Molecular machines, Molecular switches, Molecular memory, Molecular computing, Supramolecular chemistry, Molecular optoelectronics, Command surfaces, Monolayers, Nanotechnology... 

Another approach to the organization of integrated optoelectronic switches is schematically detailed in Fig. 23, and involves the organization of a photoisomerizable command interface on the solid support [86]. The command surface controls the interfacial electron transfer to a solution-state redox species. In one photoisomeric state, electron transfer to a redox probe solubilized in the electrolyte solution is prohibited (e.g. by repulsive interactions), whereas in the complementary state of the monolayer the interfacial electron transfer is allowed (e.g. because of associative interactions). Various interactions, such as electrostatic interactions, host-guest or donor-acceptor interactions, contribute to the selective contacting of the redox probe to one state of the photoisomerizable monolayer. 

Thus, the photoisomerization of the monolayer between the 46a-state and the protonated nitromerocyanine 46b-state provides a means to control the electrical features of the electrode surface, thereby regulating electron transfer at the electrode interface. The 46a-monolayer results in a neutral electrode surface while the 46b-monolayer gives a positively charged surface, causing the formation of an electrical double-layer at the electrode interface. Photoisomerization of the command interface resulting from the different electrochemical kinetics of the soluble redox probe can also be probed by Faradic impedance spectroscopy [90]. A small electron transfer resistance is found for the system when there is an attractive interaction between the charged redox probe and the command interface. Much larger electron transfer resistances are found upon photoisomerization to the state when repulsive interactions exist. 

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]. 

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