Ferrocene photoisomerization

B. Redox, Optical, and Photoisomerization Properties of Azo-Bridged Ferrocene Oligomers... 

The study of the direct irradiation and triplet sensitized isomerization of styryl-ferrocene, reaction (46), is noteworthy.134) For either direct or benzophenone sensitized photoisomerization the photostationary state is exclusively the trans isomer. 

Figure 18.5 Control of mass transfer through an azobenzene-modified film, (a) Structure and photoisomerization of azobenzene (b) structures of ferrocene derivatives used to monitor anodic current at the film-electrode interface and (c) current as a function of light exposure for FDM and FDMDG redox probes.57 (Reprinted with permission from N. Liu et al., Nano Lett. 2004, 4, 551-554. Copyright 2004 American Chemical Society.)...

It has been recently reported that the E-form of bis(ferrocenylethynyl)-ethane, 97a, can undergo photoisomerization to the Z-form, 97b, by excitation of a charge transfer band with visible light [82]. This structural change affords a decrease in the through bond mixed-valence interaction between two ferrocenes. 

Photoswitchable electrical communication between enzymes and electrodes has also been achieved by the application of photoisomerizable electron-transfer mediators [195, 199]. DilTusional electron mediators (viologen or ferrocene derivatives) were functionalized with photoisomerizable spiropyran/merocyanine units. These mediators can be reversibly photoisomerized from the spiropyran state to the merocyanine state (360 < A < 380 nm) and back (A > 475 nm). An enzyme multilayer array composed of glutathione reductase or glucose oxidase was electrically contacted only when the photoactive group linked to the redox relay (viologen or ferrocene derivative, respectively) was in the spiropyran state. 

The photoisomerizable enzyme monolayer electrode also revealed photoswitchable bioelectrocatalytic activity (Figure 7.10). In the presence of ferrocene carboxylic acid (5) as a diffusional electron transfer mediator, the nitrospiropyran-tethered GOx (4a) revealed a high bioelectrocatalytic activity, reflected by a high electrocatalytic anodic current. The protonated nitromerocyanine-GOx (4b) exhibited a two-fold lower activity, as reflected by the decreased bioelectrocatalytic current. By the reversible photoisomerization of the enzyme electrode between the 4a- and 4b-states, the current responses are cycled between high and low values (Figure 7.10, inset). 

FiCr. 7.38 (A) The assembly and photnisomerization of a switchabie rotaxane on aAu-eleomode. (B) The chronoamperometric response of the monolayer in (a) the trans-estate, and (b) the ds-state. (C) The electron transfer rate constant between the ele[Pg.259]

The bioelectrocatalyzed oxidation of glucose in this system originates from the primary oxidation of the ferrocene carboxylic acid, (21), to the respective ferrocenylium cation that mediates the oxidation of the enzyme s redox center and its activation towards the oxidation of glucose. Photoisomerization of the enzyme monolayer to the MRH-GO state switched-off the bioelectrocatalytic functions of the protein monolayer, and only the electrical response of the diffusional electron mediator was observed, Fig. 3-31, curves (b) and (d). By the cyclic photoisomerization of the enzyme-monolayer electrode between the SP-GOx and MRlT-GOx states, the reversible photoswitching of the enzyme activity between ON and OFF states was demonstrated, Fig. 3-31 (inset). 

Quenching measurements with ferrocene or azulene have confirmed the triplet route as the major pathway in tram - cis photoisomerization of several nitrostilbenes [118, 188, 192]. As a typical example, , c and 4>c, of 4-nitrostilbene in benzene are shown as a function of the ferrocene concentration (Figure 7). A related case is NOz-StN where the triplet pathway also operates [199]. 

Photostationary trans/cis ratios in the presence and absence of ferrocene or azulene either under direct or sensitized excitation conditions give different results. The simplest explanation is that 4 is quenched in the former and 3t in the latter cases. Further evidence for a mere singlet mechanism is the linearity of the Stern-Volmer plot (l/0, c vs. [Q]). This is not in agreement with a quenching of both 4 and 3t in the direct trans - cis photoisomerization. 

The CyD resides preferentially on the trans-azobenzene component and photoisomerization to the cis-azobenzene state causes translocation of the Fc-y -CyD to the alkyl chain component of the assembly. This light-driven translocation is reversible and proceeds by isomerization of azobenzene between the trans and cis states. The chronoamperometric response of the redox-active ferrocene group associated with a CyD unit reflects a change of position on the molecular array. 

A photoswitchable bioelectrocatalytic device based on a similar azo-SAM with a PAA-g-CD coating was designed, able to catalyze the oxidation of glucose by glucose oxidase upon inclusion of ferrocene-methanol (Fc), as electron mediator, into the available free CD units of the PAA-g-CD film. Photoreversible activation and deactivation of the enzyme could be obtained by UV/Vis light irradiation. The immobilization and release of the redox polymer was driven by the trans-cis photoisomerization of the azobenzene units in the SAM. ... 

Fig. 31a). The native FAD cofactor was extracted from GOx and the semisynthetic FAD cofactor was reconstituted into the apo-GOx (apo-GOx) (Fig. 31b). This reconstituted enzyme includes a photoisomerizable unit directly attached to the redox center of the enzyme, and hence, the enzyme is predisposed for optimized photoswitchable bioelectrocatalytic properties. The photoisomerizable enzyme was assembled on an Au-electrode as described in Fig. 31(c). The bioelectrocatalytic oxidation of glucose was stimulated in the presence of ferrocene carboxylic acid as a diffusional electron-transfer mediator. The (28a)-state of the reconstituted GOx was inactive for the bioelectrocatalytic transformation, whereas photoisomerization of the enzyme to the (28b)-state activated the system (Fig. 32). By the cyclic photoisomerization of the enzyme mono-layer between (28a) and (28b) states, the bioelectrocatalyzed oxidation of glucose was cycled between the off and on states, respectively (Fig. 32, inset). It was also found that the direction of the photo-bioelectrocatalytic switch of the (28a/28b)-FAD-reconstituted GOx is controlled by the electrical properties of the diffusional electron-transfer mediator [385]. With ferrocene dicarboxylic acid as a diffusional electron-transfer mediator, the enzyme in the (28a)-state was found to correspond to the switched off biocatalyst, while the (28b)-state exhibits switched on behavior. In the presence of the protonated 1-[1-(dimethylamino)ethyl]ferrocene, the direction of the photobioelectrocatalytic switch is reversed. This control of the photoswitch direction of the photoisomerizable GOx was attributed to electrostatic interactions between the diffusional electron-transfer mediator and the photoisomerizable unit linked to the FAD. The (28b)-state attracts the oxidized negatively charged... 

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