Energy transfer supramolecular assembly

Other supramolecular structures such as dendrimers have also been synthesized with zinc-containing porphyrins. Sixteen free base and sixteen zinc porphyrin units were added at the fifth generation of dendritic poly(L-lysine) and intramolecular fluorescence energy transfer was observed.823 Assembly of supramolecular arrays in the solid state has been achieved with the incorporation of an amide group for hydrogen bonding. Zinc meso-tetra(4-amidophenyl)porphyrin... 

Fluorescence lifetime measurements on the aggregate have shown that the rate constant of the intermolecular energy transfer from the zinc porphyrin unit to the free-base porphyrin unit has been evaluated to be 3.0 x 109 s-1. This value is reasonable from a model in which dendritic donor 6b and acceptor 5a contact each other directly at their exterior surfaces (Scheme 2). Therefore, electrostatic assembly of positively and negatively charged dendrimers provides a promising supramolecular approach to construct photofunctional materials with nanometric precision. 

The discussion which follows is divided into two main sections. The first termed antenna sensitizers presents studies of polynuclear compounds with a surface bound unit that can accept energy from covalently linked chromophoric groups and inject electrons into the semiconductor from its excited state. The second describes supramolecular assemblies designed to promote intramolecular and interfacial electron transfer upon light absorption. 

The stronger excitonic interaction in EB assemblies than that of ACR or AMAC is apparently due to a greater hydrophobic surface area of the former, as estimated from computer modeling studies (MacSpartan). Such increased hydrophobic surface is not expected from their structures (three six-membered ring systems) it also results in an enhanced entropic contribution to the binding energy when the probe is transferred from the aqueous phase to the interior of BAZrP, where there is little or no water. Therefore, the formation of these supramolecular assemblies may indeed involve a large entropic component, but this needs to be demonstrated experimentally. 

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

Electrochemical and photochemical processes are the most convenient inputs and outputs for interfacial supramolecular assemblies in terms of flexibility, speed and ease of detection. This chapter provides the theoretical background for understanding electrochemical and optically driven processes, both within supramolecular assemblies and at the ISA interface. The most important theories of electron and energy transfer, including the Marcus, Forster and Dexter models, are described. Moreover, the distance dependence of electron and energy transfer are considered and proton transfer, as well as photoisomerization, are discussed. 

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 nature of the platform also impacts on the electrochemical and photophysical properties of the interfacial supramolecular assembly (ISA). For example, the density of states within gold, platinum and carbon electrodes are different, so causing subtle changes in the rate of electron transfer across the electrode/ISA interface. In addition, in terms of the photophysical properties, the nature of the platform can radically change the excited-state properties of a molecule upon adsorption. For example, if a adsorbate is located close to (<10 nm) a metal surface and is then pumped into an electronically excited state, efficient energy or electron transfer is expected which will lead to quenching of the excited state. This process can dramatically increase the photostability of compounds that would ordinarily photodecompose in solution. 

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]. 

In this report, we will describe some of our studies aimed at (i) obtaining new inorganic photosensitizers by second-sphere modification of known ones, and (ii) assembling photosensitizer units with other molecular components in discrete, covalently bound supramolecular structures. Studies of type (i), besides their intrinsic interest, have some relevance to the problem of how the properties of a photosensitizer are modified by inclusion in a supramolecular structure. Systems of type (ii) would be useful to study the basic processes of intramolecular electron and energy transfer involved in the performance of molecular photochemical devices. 

Finally, it should be noted that singlet-state lifetimes in [Re(L)(CO)3(bpy)], are long enough to allow for ultrafast electron or energy transfer in supramolecular assemblies, at surfaces or molecule/nanoparticle interfaces, see Sect. 7.3. Indeed, a hot electron injection has been seen with Ti02 nanoparticles [42] or in Re-labeled redox proteins [43],... 

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