http://nathan.instras.com/documentDB/paper-12.pdf

Applications of functionalized transition metal complexes in photonic and optoelectronic devices

The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.

Chemical approaches to artificial photosynthesis. 2.

Electron transfer from electron-donor to electron-acceptor molecules via a molecular ‘bridge’ is a feature of many biological andchemical systems. The electronic structure of the bridge component in donor–bridge–acceptor (DBA) systems is known to play a critical role in determining the ease of electron transfer1,2. In most DBA systems, the rate at which electron transfer occurs scales exponentially with the donor–acceptor distance — effectively the length of the bridge molecule. But theory predicts that regimes exist wherein the distance dependence may be very weak, the bridge molecules essentially acting as incoherent molecular wires3,4,5,6. Here we show how these regimes can be accessed by molecular design. We have synthesized a series of structurally well-defined DBA molecules that incorporate tetracene as the donor and pyromellitimide as the acceptor, linked by p -phenylenevinylene oligomers of various lengths. Photoinduced electron transfer in this series exhibits very weak distance dependence for donor–acceptor separations as large as 40 A, with rate constants of the order of 1011 s−1. These findings demonstrate the importance of energy matching between the donor and bridge components for achieving molecular-wire behaviour.

Molecular-wire behaviour in p-phenylenevinylene oligomers

Subnanosecond Relaxation Dynamics in TiO2 Colloidal Sols (Particle Sizes Rp = 1.0-13.4 nm). Relevance to Heterogeneous Photocatalysis

We have studied the current vs voltage curves (I–V characteristics) of a mesoscopic device consisting of two electrodes and a molecular wire. The wire Hamiltonian includes both electronic tunneling and Coulomb repulsion within a Hubbard model that is treated at the Hartree–Fock level. The inclusion of electron repulsion is an extension of our previous work that only considered the case of noninteracting electrons. We have found several important features in the calculated characteristics of the wire. These include (1) a staircaselike structure that strongly resembles that associated with Coulomb blockade in heterostructures and quantum dots, but that in the case of the wire is associated with the discrete nature of the molecular resonances; (2) regions of negative differential resistance associated with increased localization of the molecular resonances. Our theoretical model includes a consistent treatment of the conduction in the linear and nonlinear regimes which remains valid even when the device is o...

Current‐voltage characteristics of molecular wires: Eigenvalue staircase, Coulomb blockade, and rectification

This book presents proceedings in which the important achievements and latest advancements of photochemical energy conversion are summarized. Various aspects of photochemical solar energy, including the basic principles of energy and electron transfer, charge separation, energy conversion in homogeneous and heterogeneous systems, photocatalysis, photoelectrochemisty, photochemistry in organized assemblies, biomimetic photochemistry, photosynthesis, and new strategies to the production of fuels and useful chemical through environmentally clean renewable sources are examined.

D. Meisel

Papers

Photochemical energy conversion