Photoinduced electron transfer supramolecular assemblies

Albeit the substantial progress in the bioelectrochemical activation of enzymes, one could identify two important future challenges in the field (i) The active relay units wiring the redox centers of the enzymes with the electrodes could be generated by photoinduced electron transfer. This could pave the way to the photochemical wiring of enzymes and to the development of photobiofuel cells, (ii) DNA scaffolds provide unique templates for the ordered self-assembly of molecular or biomolecular units through dictated hybridization. The ordering of relay units and enzymes, or of relay units photosystems, on DNA templates associated with electrodes may yield attractive new supramolecular nanostructures for bioelectronics and optobioelec tronic s. 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

A kinetic scheme can be written, Scheme 1, in which the rates of back electron transfer from the contact, solvent-separated, and free ions must each be individually considered. Clearly, the greater the tendency of a given solvent to separate the ions, the less significant will be the electrostatic interaction between them, and the greater tendency to suppress back electron transfer for a period sufficiently long to allow radical ion chemistry to ensue. A major concern of this article will be in defining the efficacy of supramolecular assemblies which create inhomogeneous arrays to controll the secondary chemistry of radical ions formed via photoinduced electron transfer. 

Figure 33. Supramolecular system that mimics the function played by a macroscopic extension [140]. The two pseudorotaxane-type connections between the three molecular components can be controlled independently by acid/base and red/ox stimulation, respectively. In the fully assembled [3]pseudorotaxane, a photoinduced electron transfer process occurs from the excited state of the [RLiibpy) moiety of 35 to the bipyridinium unit of 36H +, which, in turn, is plugged into crown ether 2.

Recently, we have shown that the supramolecular approach (a non-covalent two-fold extra-ligation in conjunction with Zn-porphyrin chemical dimers and trimers) can be successfully employed to create a variety of multiporphyrin structures capable of the fast effective intra-complex energy migration and photoinduced electron transfer [5]. Following this self-assembly principle we have succeeded to anchor in pyridyl substituted porphyrin molecules on CdSe/ZnS QD surfaces. In this case, CdSe/ZnS QDs show luminescence quenching induced by dynamic and/or static interactions between QD and porphyrins depending critically on sample stability, temperature and solvent polarity [6]. The general focus of this contribution is devoted to demonstrate the... 

Similar to calix[n]arenes, porphyrins have been known to be one of the support pillars of supramolecular chemistry attributing suitable photoactive and electroactive properties to the molecular structures designed around them for building artificial molecular devices. Thus, various metallated and free base porphyrin-calixarene assemblies could afford attractive scaffolds for application in the areas of multipoint molecular recognition, receptors, host-guest chemistry, catalysis and photoinduced electron transfers. 

D Souza, F., G.R. Deviprasad, M.E. Zandler, M.E. El-Khouly, M. Fujitsuka, and O. Ito (2002). Electronic interactions and photoinduced electron transfer in covalently linked porphyrin-C6o(pyridine) diads and supramolecular triads formed by self-assembling the diads and zinc porphyrin. J. Phys. Chem. B 106(19), 4952-4962. 

Gyclodextrin cavities form the early models of host molecules involved in supramolecular assemblies. There are many other molecules known as cryptands which can be designed to offer a cavity of fairly precise dimensions to accommodate various ions or metal complexes. It may be possible to locate not just one, but two, guest molecules inside a cryptand cavity, and this may lead to new electron transfer reactions in restricted environments another step towards synthetic photoinduced biochemical reactions. 

Ward, M. D. Photoinduced electron and energy transfer in noncovalently bonded supramolecular assemblies ,... 

Election transfer remains one of the most important processes explored when using interfacial supramolecular assemblies and given the emerging area of molecular electronics, this trend is set to continue. Therefore, Chapter 2 outlines the fundamental theoretical principles behind the electiochemically and photochemi-cally induced processes that are important for interfacial supramolecular assemblies. In that chapter, homogeneous and heterogeneous electron transfer, photoinduced proton transfer and photoisomerizations are considered. 

Electron, energy and proton transfer or molecular rearrangements are the most important events that occur in interfacial supramolecular assemblies. In this chapter, the general theories of electron transfer, both within ISAs and across the film/electrode interface, are described. Moreover, photoinduced electron, energy and proton transfer processes are discussed. As this book focuses on supramolecular species, the treatment is restricted to intramolecular or interfacial processes without the requirement for prior diffusion of reactants. 

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

Hunter C. A. and Hyde R. K. Photoinduced Energy and Electron Transfer in Supramolecular Porphyrin Assemblies. Angew. Chem., Int. Ed. Engl. 35 (1996) pp. 1936-1939. 

IV. FUNCTIONAL ASPECTS OF SUPRAMOLECULAR ASSEMBLIES A. Photoinduced Electron and Energy Transfer... 

Irradiation of 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine or its copper derivatives in degassed ethanol containing triethanolamine causes its reduction by a mechanism which involves a higher excited state of the substrate,and a plot of the log of the rate constants for the photoinduced intramolecular electron transfer in a supramolecular assembly of Zn and Au porphyrins bridged by diphenylphenanthroline has been found to decrease linearly with diminishing energy difference between the HOMO and LUMO of the participating orbitals. ... 

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