Wires, molecular photoinduced electron transfer

Ito O, Yamanaka K (2009) Roles of molecular wires between fullerenes and electron donors in photoinduced electron transfer. Bull Qiem Soc Jpn 82 316—332... 

Oike T, Kurata T, Takimiya K et al (2005) Polyether-bridged sexithiophene as a complexa-tion-gated molecular wire for intramolecular photoinduced electron transfer. J Am Chem Soc 127 15372-15373... 

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. 

Mateusz Wielopolski, Carmen Atienza, Timothy Clark, Dirk M. Guldi, Nazario Martin. p-Phenyleneethynylene Molecular Wires Influence of Structure on Photoinduced Electron-Transfer Properties. Chem. Eur. J. 2008, 14, 6379. 

A doubly metallated 15 base-pair double helix containing ruthenium and rhodium at each end of the strands [106] showed the efficiency of DNA for coupling electron donors and acceptors over a very long range, greater than 40 A. The DNA double helix was found to behave like a piece of molecular wire with fast electron-transfer rates (>1010 s l) for the photoinduced electron transfer between the metallointercalators [107-109] and semiempirical Hartree-Fock calculations of HAB for DNA mediated electron transfer [110] were described. 

Fig. 25 The series of dyads, 29(n), possessing the oligo-p-phenylenevinylene bridge that were used to investigate the switchover from superexchange characteristics to molecular wire behaviour in the photoinduced electron transfer reaction, from the locally excited state of tetracene (TET) donor to the pyromellitimide (PI) acceptor.148 Also, shown are a schematic of the photoinduced charge separation rate versus, donor-acceptor distance (lower left-hand side) and the LUMO energies of TET and the various bridges (lower right-hand side).

Electron injection from MLCT-excited Ru-polypyridine complexes are used to investigate electron transfer along DNA strands, that is to decide whether DNA can behave as a molecular wire [358-360]. In these studies, derivatives of [Ru(phen)2(dppz)] + act as excited-state electron donors and [Rh (phi)2(bpy)] + as a ground-state electron acceptor. Both complexes are anchored at different DNA sites and the rate of Ru —> Rh photoinduced electron transfer is measured. In another study [361], a [Ru (bpy)2(im)(NH2-)] + unit attached to a terminal ribose of a DNA duplex acted as an excited-state oxidant toward a [Ru (NH3)4(py)(NH2-)] " unit attached at the other end. 

The 7i-stacked bases of ds DNA might be expected to provide a better medium for bridge-mediated electron transfer than the sigma bonds of proteins or hydrocarbons. It has in fact been proposed by Turro and Barton [18d] that ultrafast photoinduced electron transfer processes involving intercalated donors and acceptors can occur with little or no distance dependence. According to this paradigm, duplex DNA can function as a molecular wire or r-way . 

As in the past two years, fullerenes continue to be a source of considerable photochemical interest and photoinduced electron transfer is a central theme of numerous studies. The field of molecular-scale electronic devices continues to promote interest in the photophysical properties of novel molecular architectures and such aspects of photoactive rotaxanes and catenates have been reviewed (Benniston and Chambron et ai), while Harriman and Ziessel have outlined the design principles associated with the construction of photo-activated molecular wires. 

The construction of intramolecular molecular system whose photo active molecule linked with conducting molecular wire is an important subject in realization of. molecular electronic or photonic devices. For such objectives, systematization of donor-photosensitizer-acceptor triad molecules into large molecular systems is one of the feasible approaches because the exquisite incorporation of the photosensitizer and a suitable electron donor and/or acceptor into a conducting polymeric chain is useful for various molecular systems based on the photoinduced electron transfer. With this in mind, we synthesized symmetrical donor-acceptor-donor triad molecules which can be polymerized by the normal electrochemical oxidation. By the polymerization, one-dimensional donor-acceptor polymers with porphyrin moieties separated by ordered oligothienyl molecular wire which is considered as a proto-type molecular device was obtained. 

T. Oike, T. Kurata, K. Takimiya, T. Otsubo, Y. Aso, H. Zhang, Y. Araki and O. Ito, Polyether-bridged sexithio-phene as a complexation-gated molecular wire for intramolecular photoinduced electron transfer, J. Am. Chem. Soc., 127, 15372-15373 (2005). 

Specifically, energy- and charge-transfer properties of several different molecular-wire systems have been studied within the framework of photoinduced charge separation and solar-energy conversion. Up front, the conductance behavior of wire-like molecules was of particular interest. Such features have been carefully examined in view of possible applications in the fields of molecular electronics and/or photovoltaic devices. Among the tested systems, 7t-conjugation played a crucial role. 

It should be recalled, however, that even an interaction of a few cm (which cannot be noticed in spectroscopic experiments) may be sufficient to cause intercomponent energy transfer or electron transfer processes. As already mentioned, the nature and length of the bridging ligand can contribute strongly to the rate of the photoinduced processes. Many compounds have been labeled wire molecules, but in most cases the wire-type behavior could not be observed. However, one should first define what is a molecular wire and what are the expectations for such a system. 

The simplest component of an electrical circuit is a wire, and the design of molecular wires has received a great deal of attention. In a broad sense, this term can be used to designate any molecular" structme able to mediate the transfer of electrons between appropriate electron donor and acceptor sites (electrodes, photoactive, and redox-active molecular components). In practice, different "conduction mechanisms may apply, depending on the molecular structure of the wire and on the type of experimental setup used (see below). Molecular wires were studied in a variety of experimental conditions, depending on the nature of the donor and acceptor terminals the wire is connected to. and on the method used to detect the electron flow. Available methods can be broadly divided into the following (Fig. 1) photoinduced eiectron transfer in donor-wire-acceptor systems (dyads), fast electrochemistry of adsorbed wire-electroactive group assemblies, and conductance [/(V)] measurements on metal-wire-metal junctions. 

MOLECULAR WIRES FOR PHOTOINDUCED ENERGY AND ELECTRON TRANSFER... 

Figure 1. Schematic representation of a molecular-level wire (a) and examples of photoinduced energy (b) and electron (c) transfer processes.

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