Metal-organic dyads electron transfer

Photoinduced Electron Transfer in Metal-Organic Dyads Kirk S. Schanze and Keith A. Walters... 

Photoinduced Electron Transfer in Metal-Organic Dyads... 

In most of the metal-organic dyads described in this review the metal center has a d6 electronic configuration. Further, the lowest excited state typically has a metal-to-ligand charge transfer (MLCT) configuration arising from promotion of a metal centered -electron into a ligand based -tt level, e.g.,... 

Figure 1 Photoinduced electron transfer schemes for type 1 and type 2 metal-organic dyads. Key L is a diimine ligand such as 2,2 -bipyridine M is a transition metal dn indicates the electron count in the valence shell d-orbitals of M A is an organic electron acceptor D is an organic electron donor FET is forward ET BET is back ET.

VI. PHOTOINDUCED INTRAMOLECULAR ELECTRON TRANSFER IN METAL-ORGANIC DYADS ... 

We wish to acknowledge the National Science Foundation for supporting our work on electron transfer in metal-organic dyads (Grant No. CHE-9401620). 

Schanze KS, Walters KA (2000) Photoinduced electron transfer in metal-organic dyads. In Ramamurthy V, Schanze KS (Eds) Organic and Inorganic Photochemistry. Molecular and Supramolecular Photochemistry Series, Vol 2, Chap 3. Marcel Dekker, New York, p 75... 

Schanze, K. S. Walters, K. A. Photoinduced electron transfer in metal-organic dyads. In Molecular and Supramolecular Photochemistry, Ramamurthy, V. Schanze, K. S., Eds. Marcel Dekker New York, 1999, Vol. 8, Chapter 3, pp 75-126, and references therein. 

The MLCT basis for the reactive excited state leads to interesting consequences with respect to the orbitals involved in photoinduced forward and back ET in metal complex dyads. In order to categorize this difference we define two categories of dyad systems Type 1 dyads contain an electron acceptor covalently attached to the d6 metal chromophore and type 2 dyads contain an electron donor covalently attached to the d6 metal chromophore (see Fig. 1). In the type 1 dyads, photoinduced forward ET involves transfer of an electron from a tt orbital localized on the acceptor ligand, L, to a ir orbital on the organic electron acceptor, A. Back ET involves transfer of an electron from a it orbital of the organic electron acceptor, A, to the -shell of the transition metal center. By contrast, in the type 2 dyads photoinduced forward ET involves transfer of an electron from a tt orbital on the organic donor, D, into the hole in the d-shell of the... 

Related molecular dyads have been constructed in which a metal complex, often ruthenium(II) tris(2,2 -bipyridine) or similar, functions as chromophore and an appended organic moiety acts as redox partner. Other systems " have been built from two separate metal complexes. Each of these systems shows selective intramolecular electron transfer under illumination. Rates of charge separation and recombination have been measured in each case and, on the basis of transient spectroscopic studies, the reaction mechanism has been elucidated. The results are of extreme importance for furthering our understanding of electron-transfer reactions and for developing effective molecular-scale electronic devices. The field is open and still highly active. 

Hybrid systems have been constructed in which a metal complex is covalently linked to an organic species so as to produce a donor-acceptor dyad, with either subunit functioning as the chromophore. Thus, ruthenium(II) tris(2,2 -bipyridyl) complexes have been synthesized bearing appended anthraquinone or tyrosine functions. Both systems enter into intramolecular electron-transfer reactions. With an appended anthraquinone moiety, direct electron transfer occurs from the triplet excited state of the metal complex to the quinoid acceptor. This is not the case with tyrosine, which is an electron donor, but the metal complex can be photooxidized by illumination in the presence of an added acceptor. The bound tyrosine residue reduces the resultant ruthenium(III) tris(2,2 -bipyridyl) complex... 

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]. 

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