Electron transfer metal-polymer complex

The degree of ionicity in the bond between a metal atom and a polymer, or molecule, is related to the ionization potential and electron affinities of the substituents. The metals we have studied are of interest as electron injecting contacts in electronic devices. These metals must have a low ionization potential (or work function), of the same order as the electron affinity of the polymer, in order for the charge transfer process to occur. If the ionization potential of the metal is lower than the polymery-electron affinity, spontaneous charge transfer occurs which is the signature of an ionic bond. Thus, the character of the charge distribution in the metal-polymer complexes we are studying is related to the situation in the electronic device. 

Similarly, this amphiphilic polymer micelle was also used to dismpt the complex between cytochrome c (Cc) and cytochrome c peroxidase (CcP Sandanaraj, Bayraktar et al. 2007). In this case, we found that the polymer modulates the redox properties of the protein upon binding. The polymer binding exposes the heme cofactor of the protein, which is buried in the protein and alters the coordination environment of the metal. The exposure of heme was confirmed by UV-vis, CD spectroscopy, fluorescence spectroscopy, and electrochemical kinetic smdies. The rate constant of electron transfer (fc°) increased by 3 orders of magnimde for the protein-polymer complex compared to protein alone. To establish that the polymer micelle is capable of disrupting the Cc-CcP complex, the polymer micelle was added to the preformed Cc-CcP complex. The observed for this complex was the same as that of the Cc-polymer complex, which confirms that the polymer micelle is indeed capable of disrupting the Cc-CcP complex. 

These discussions will embrace homogeneous solutions of polymer-metal complexes. Of course one of the important advantages offered by the use of a polymer ligand, especially a crosslinked polymer ligand, in catalysis is the insolubilization of the attached complexes the insolubility of the polymer catalyst makes it very easy to separate from the other components of the reaction mixture. Several polymer-metal complexes have been used for this purpose, although such applications are not covered in this article. The aim here is (1) to characterize polymer-metal complexes and their behavior in such simple but important elementary reactions as complex formation, ligand substitution, and electron transfer, and (2) to describe their catalytic activity. 

A Co(IH) complex is inert in ligand-substitution reactions, and its uniform structure is thus maintained even in an aqueous solution. The reaction mechanism of a Co(III) complex in solution is well known, so that a pendant-type polymer-Co(IU) complex, e.g. 17,19, is one of the most suitable compounds for a quantitative study of the effects of a polymer ligand on the reactivity of a metal complex. The reactivities of the polymer-Co(III) complexes are discussed here kinetically and compared with those of the monomeric Co(III) complexes in the following reactions electron-transfer reactions between the polymer complexes and Fe(II) [Eqs. (5) and (6)], and the ligand-substitution reaction of the polymer-Co(III) complex with hydroxy ions or water [Eqs. (7) and (8)J. One of the electron-transfer reactions proceeds via... 

It is thus expected that the conformation of the polymer-ligand chain will influence the reactivity of a metal complex. The influence of the conformational change of a poly(N-vinyl-2-methylimidazole)(PVMI 3) ligand has been studied in the electron-transfer reaction of a Co(III) complex in aqueous-alcoholic solvents87. ... 

Control of the electron-transfer step was also attempted by combining two metal species on a polymer ligand167. We prepared polymer-metal complexes involving both the Cu(II) and Mn(III) ions. The oxidative polymerization of XOH catalyzed by the PVP-Cu, Mn mixed complex or the diethylaminomethylated poly(styrene)(PDA)-Cu Mn mixed complex proceeded 10 times faster than the polymerization catalyzed by either PVP- or PDA-metal complex. The maxima of the activity observed at [Cu]/[Mn] = 1 and [polymer]/[Cu,Mn] moderately small where Cu and Mn ions were crowded within the contracted polymer chain. Cooperative interaction between Cu and Mn was inferred. The rate constant of the electron-transfer step (ke in Scheme 14) for Cu(II) -> Cu(I) was much larger than that for Mn(III) -> Mn(II). The rate constants of the reoxidation step (k0) were polymer-Mn ex polymer-Cu.Mn > polymer-Cu, so the rapid redox reaction... 

The theory of electron transfer in chemical and biological systems has been discussed by Marcus and many other workers 74 84). Recently, Larson 8l) has discussed the theory of electron transfer in protein and polymer-metal complex structures on the basis of a model first proposed by Marcus. In biological systems, electrons are mediated between redox centers over large distances (1.5 to 3.0 nm). Under non-adiabatic conditions, as the two energy surfaces have little interaction (Fig. 5), the electron transfer reaction does not occur. If there is weak interaction between the two surfaces, a, and a2, the system tends to split into two continuous energy surfaces, A3 and A2, with a small gap A which corresponds to the electronic coupling matrix element. Under such conditions, electron transfer from reductant to oxidant may occur, with the probability (x) given by Eq. (10),... 

In this chapter, we presented three different systems of molecular assemblies using molecular wires. The first involved the fabrication of the molecular wire system with metal complex oligomer or polymer wires composed of bis(terpyridine)metal complexes using the bottom-up method. This system showed characteristic electron transfer distinct from conventional redox polymers. The second involved the fabrication of a photoelectric conversion system using ITO electrodes modified with porphyrin-terminated bis(terpyr-idine)metal complex wires by the stepwise coordination method, which demonstrated that the electronic nature of the molecular wire is critical to the photoelectron transfer from the porphyrin to ITO. This system proposed a new, facile fabrication method of molecular assemblies effective for photoelectron transfer. The third involved the fabrication of a bioconjugated photonic system composed of molecular wires and photosystem I. The feasibility of the biophotosensor and the biophotoelectrode has been demonstrated. This system proposed that the bioconjugation and the surface bottom-up fabrication of molecular wires are useful approaches in the development of biomo-lecular devices. These three systems of molecular assemblies will provide unprecedented functional molecular devices with desired structures and electron transfer control. 

Figure 5.61 Schematic representation of a [Ru(bpy)3]2+/a-ZrP viologen structure on silica, plus the sequence of fast (1,2) and slow (3) electron transfer steps that follow photoexcitation of the photoactive ruthenium-containing polymer MDESA, p-methoxyaniline diethylsulfonate. Reprinted from Coord. Chem. Rev., 185-186, D. M. Kaschak, S. A. Johnson, C. C. Waraksa, J. Pogue and T. E. Mallouk, Artificial photosynthesis in lamellar assemblies of metal poly(pyridyl) complexes and metalloporphyrins, 403-416, Copyright (1999), with permission from Elsevier Science...

The visible absorption and luminescence properties of the [Ru(bpy)3]2+-like centre in the N-ethylated polymer closely resemble those of [Ru(bpy)3]2+ and depend only slightly on the loading of the metal complex centre on the polymer (Ennis et al, 1986). The electron transfer reactions of a 1 11 copolymer have been studied in most detail. 

Examples of the molecular catalysts for C02, H+, and 02 reductions were reviewed recently and a role for molecular aggregates composed of a simple metal complex and a functional polymer was emphasized. When using molecular aggregates as a catalyst, efficient catalysis by the complex via a multi-electron transfer reduction often takes place [108,109]. 

The reactions leading to the formation of these polymers—except polyphenylene—have one feature in common, although they otherwise differ greatly in mechanism the crucial step in the reaction sequence is a one-electron transfer from the monomer to a transition metal ion serving as an electron acceptor. In addition to being an electron acceptor the transition metal ion is probably also involved in the coupling reaction by complexation of radical-like intermediates produced. 

Double helical DNA is a water-soluble polymer that contains an electronically well-coupled stack of aromatic heterocyclic base pairs. This review describes efforts in our laboratory to characterize electron-transfer reactions between transition metal complexes bound by intercalation within the 7r-stack of DNA. Much information is available concerning the structure, synthesis, and methods of characterization of this polymer. Also, research in our laboratories has been directed toward describing the photophysical and photochemical properties of metal complexes bound to DNA. Using these metal complexes to probe the DNA 7r-way, we are now in a position to ask Is DNA a molecular wire ... 

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