Tethered Redox Center

One approach for achieving multiphoton and multielectron reactions is to synthesize cluster complexes where the metal centers are in close conununication. One of the simplest bridging species that can be used to link two Ru(bpy)2 -type centers in close proximity to achieve this goal is a cyanide ion. The ruthenium(II) bimetallics NC-Ru(bpy)2-CN-Ru(bpy)2-CN , NC-Ru(bpy)2-Ru(phen)2-CN, and the trimetallic NC-Ru(bpy)2 N-Ru(bpy)2-NC-Ru(bpy)2-CN have been synthesized, and each shows a shift of the emission band to low energy as compared to the monomeric complex Ru(bpy)2(CN)2. This lowest d-n triplet excited 

In addition to being bound to T1O2 by carboxylate groups, this triruthenium(II) complex can be bound by peripheral sulfonate groups. These complexes absorb over a very broad range in the visible region of the spectrum, and monochromatic incident photon-to-current conversion efficiencies of over 80% have been observed 

Fi g u re 5.16. Photophysics of the cyanide-bridged triruthemum(II) complex adsoibed onto Ti02. 

Cyanide bridges can also be used to bridge Ru(bpy)2 groups to other ruthenium centers. Examples of such multimetallic compounds are the triruthenium complexes X(NH3)4Ru-NC-Ru(bpy)2 N-Ru(NH3)4Y (X = NH3, py Y=NH3, py n = 4-6), which have a central Ru(II) bpy complex, and outer ammine or pyridine complexes that have Ru2(II), Ru(II, III), or Ru2(III) metal centers. These complexes are nonemissive with the Ru(bpy)2(CN)2 chromophore being completely quenched by the Ru(NH3)r and Ru(NH3)4py moieties. This quenching is a result of radiationless deactivation of the excited state via low-lying remote d-n or intervalence states. For the semioxidized complex an additional possibility is that electron transfer steps convert the intermediate states to the end-to-end intervalence state. 

Bridging groups other than cyanide can also be used to link Ru(bpy)2 centers to other metal complexes. A series of bridging groups of differing chain length and conjugation have been used to synthesize a variety of new polymetallic complexes 

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Mao F, Mano N, Heller A. 2003. Long tethers binding redox centers to polymer backbones enhance electron transport in enzyme wiring hydrogels. J Am Chem Soc 125 4951 -4957. 

Willner et al. [52] have created some elegant interfacial supramolecular assemblies to address this issue by removing the non-covalently bound flavin adenine dinucleotide (FAD) redox center from glucose oxidase and immobilizing the enzyme on a tether consisting of cystamine chemisorbed on a gold surface, a pyrroloquinoline quinone (PQQ) link and FAD. The mediator potential and electron transfer distances of this assembly were carefully chosen so that transfer of electrons from the FAD to the PQQ and to the electrode is very fast. A maximum rate of 900 150 s-1 for the enzymatic reaction within this monolayer assembly was obtained, which is indistinguishable from the value of about 1000 s-1 obtained for the enzyme in solution. While monolayers can offer molecular-level control of the interfacial structure, the... 

Figure 39. Electrical communication between an enzyme redox center and a photoexcited species attaining light-induced biocatalyzed transformations (A) direct electrical wiring of the protein by its chemical modification with tethered electron-relay units (B) electrical communication by the immobilization of the protein into a redox-functionalized polymer matrix.

Eor heterogeneous electron transfer, the use of ordered organic monolayers (self-assembled monolayers or SAMs) at electrode surfaces as blocking films either with electroactive species in the electrolyte [26] or with electroactive groups tethered at the opposite end of the blocking molecule from the covalent attachment end [27] has provided a method to study the effect of electrode-redox center distance and the effect of the electrode potential on the electron-transfer rate. 

There are numerous reports of the existence of electrocatalysis via an attached redox center on a SAM (see Sect. 4.3), but few reports in which the rates of electron transfer between the electrode and the attached redox molecule and between the attached redox molecule and the solution redox molecule are measured. It would be interesting to study the electron-transfer kinetics in SAMs with multiple redox molecules linked along a single tether (such as porphyrins [157] or metal-terpyridine complexes) [122]. From such a system, one could derive the rate of electron transfer between two redox molecules connected by a molecular bridge and check the considerable data available on intramolecular transfer obtained by other methods [254]. For a variety of applications, measurements of the rates of electron transfer between an electrode and metal nanoparticles tethered to the SAM are also of interest [255]. 

Newkome et al. have synthesized several dendrimers which contain a core of four diaminoanthraquinone molecules tethered together. Dendritic branches grow from these redox centers, and various isomers of the anthraquinones (AQs) have been examined. There were differences noted in the electrochemical behavior for the various isomers of diaminoanthraquinone, and, as might be expected, the electrochemical response of the redox centers became more irreversible as the dendrimer became larger [56]. 

In configuration A, ET from the enzyme redox center to the electrode is made possible by addition of a diffusional electron mediator that rapidly shuttles the electron (s) to and from the electrode. In configuration B, redox labels are covalently tethered to the enzyme surface and relay the electrons to and from the electrode. 

To exploit the 2 term in Equation 9.13, Mao et al. introduced an osmium redox polymer in 2003 in which the redox center was a pendant on the polymer backbone at the end of a 13-atom molecular tether [59]. The increased range of motion for the redox site led to an increase in D pp (see panel G of Table 9.1). A version with a higher redox potential, more appropriate to mediate a cathode enzyme, was also produced [74]. 

Figure 17.6 Redox hydrogel approach to immobilizing multiple layers of a redox enzyme on an electrode, (a) Structure of the polymer, (b) Voltammograms for electrocatalytic O2 reduction by a carbon fiber electrode modified with laccase in the redox hydrogel shown in (a) (long tether) or a version with no spacer atoms in the tether between the backbone and the Os center (short tether). Reprinted with permission fi om Soukharev et al., 2004. Copyright (2004) American Chemical Society.

A further approach to electrically wire redox enzymes by means of supramolecular structures that include CNTs as conductive elements involved the wrapping of CNTs with water-soluble polymers, for example, polyethylene imine or polyacrylic acid.54 The polymer coating enhanced the solubility of the CNTs in aqueous media, and facilitated the covalent linkage of the enzymes to the functionalized CNTs (Fig. 12.9c). The polyethylene imine-coated CNTs were covalently modified with electroactive ferrocene units, and the enzyme glucose oxidase (GOx) was covalently linked to the polymer coating. The ferrocene relay units were electrically contacted with the electrode by means of the CNTs, and the oxidized relay mediated the electron transfer from the enzyme-active center to the electrode, a process that activated the bioelectrocatalytic functions of GOx. Similar results were observed upon tethering the ferrocene units to polyacrylic acid-coated CNTs, and the covalent attachment of GOx to the modifying polymer. 

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