Mechanisms of Energy and Electron Transfer

The photoinduced transfer of energy (excitation) or electrons is of fundamental importance in molecular devices. Excitation transfer requires some kind of electronic interaction between donor and acceptor. It occurs via one of two mechanisms  

The electron exchange (Dexter) mechanism which involves the overlap of wavefunctions of the donor and acceptor groups. This is a short-range excitation transfer mechanism that operates by exchange of electrons. 

The coulombic (Forster) mechanism, a dipole-dipole mechanism that can operate over distances as long as 10 nm. 

In addition photoexcitation can also result in the transfer of an excited state electron to a distant acceptor group resulting in charge separation. This process can be understood within the framework of Marcus theory and subsequent more sophisticated theoretical treatments. - The rate of electron transfer (kei) drops with distance according to an attenuation factor exp(-y8 Tab) where r B 

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The amazing structure with precise molecular dignment and reaction mechanism of energy and electron transfer in natural photosynthesis undoubtedly affords one of the most promising examples for photochemically active supramo-lecular systems. In the following section, recent approaches involving molecular systems linked by covalent bonds and chromophoric assemblies will be surveyed. 

Sensitization Via Electron Transfer. Pappas and Jilek analyzed the energetics of energy and electron transfer sensitization.(5j ) Their evidence indicated that inmost cases, sensitization could be explained by an electron transfer mechanism (see Figure 5). Sensitization is believed to involve the transfer of an electron from an excited photosensitizer (S ) to an onium molecule, which may involve the formation of an excited state complex. Product analysis and irreversible polarography previously determined from electrochemical studies, indicate that the reduced iodonium salt undergoes homolytic bond cleavage to form a phenyl radical and iodobenzene. ( ) The reduced sulfonium salt produces phenyl sulfide and a phenyl radical.(10)... 

Electronic properties of chlorophylls and related systems are of fundamental interest to understand the molecular mechanisms of energy and charge transfer in complex antenna and photosynthetic reaction centers [104]. Several studies were dedicated to investigate the electronic absorption spectra of photosynthetic chromophores (see Konig and Neugebauer [105] for a recent review). Most of the available experimental information on the electronic properties of chlorophylls is determined in solution. This feature fostered theoretical studies on the electronic properties of photosynthetic chromophores in solution, [106-114] or in interaction with hydrogen bonding species, [113, 115-122] or with the protein environment [123, 124]. 

Pulsed radiolyses with electrons of pyrene doped polyethylene samples have been examined over a wide temperature range. ° Time-dependent changes in concentration of pyrenyl excited states and ions were used to study mechanisms of energy- and charge-transfer within the polymer matrix. Fast exciton migration is followed by molecular ion rearrangement and recombination to form pyrene excited states (step 28 in Scheme 4). ... 

Under optimal conditions this layer can be transferred to a solid substrate (glass or metal) and several monomolecular layers can be deposited in this way. These L-B films represent highly organized molecular assemblies on a macroscopic scale, since not only the distance between neighbouring molecules but also the relative orientations of their chromophores can be determined. The distance dependence of photoinduced energy and electron transfers have been investigated in L-B films. The R6 dependence of the Forster dipole-dipole mechanism has been confirmed, but it must be realized that some questions remain concerning the possible role of defects in the film structures. 

Figure 2.14 Schematic representations of the mechanisms of photoinduced (a) electron transfer, (b) Dexter (electron-exchange) energy transfer, and (c) Fdrster (dipole-dipole) energy transfer mechanism processes in the supramolecular dyad A-L-B spheres represent electrons, while curved arrows indicate the directions of transfer...

Much more complex functions can be achieved by assembling molecules into supramolecular systems. Upon excitation with chemical species, electrons, and photons, suitably designed supramolecular systems can indeed perform a variety of useful functions related to energy- and electron-transfer processes and to mechanical movements. 

D-A molecules relate to (i) the mechanism of the radiative and nonradiative charge recombination CT So, (ii) the electronic structure and conformation of the molecules in the fluorescent CT state and (iii) the role of the charge-transfer triplet states ( CT) in the intramolecular energy and electron transfer. 

The tris(diimine)Ru(II) complexes are potential mediators in the conversion of solar energy to chemical energy via electron-transfer mechanisms (see 13.4). Their spectra display intense visible-range absorption bands and the relative long lifetimes of the lowest-energy ES, MLCT in character, allow efficient bimolecular energy and electron-transfer processes in solution. The photolability of certain bis(diimine)Ru(II) complexes is of synthetic utility ... 

It should also be noted that it is difficult to demonstrate conclusively an electron transfer quenching mechanism. In several cases, both energy and electron transfer are allowed, and both processes may lead to the same final products. A classical example is the quenching of the ( CT)Ru-(dipy)a excited state by Fe ions (Figure 4). (The validity of the orbital (29) and spin (30) labels of the excited states of metal chelates of... 

The electronic effects in energy and electron transfer reactions, including excited state systems, have been discussed in a review by Endicott. The trends observed in the rate constants for the quenching of the doublet E) excited state of [Cr(bpy)3] by a series of organochromium complexes, [Cr(H20)5R], indicate an outer-sphere electron transfer mechanism. The different reactivity patterns found for the oxidations of [(H20)Co([14]aneN4)R] complexes by [Ru(Bpy)3] and [ Cr(bpy)3] point to electron and energy transfer mechanisms, respectively. The reductive quenching of [ Cr(bpy)3] by Fe produces [Cr(bpy)3], which also quenches the excited state in the absence of added... 

Tryptophan (Trp), tyrosine (Tyr), cystine (Cys), and phenylalanine (Phe) moieties play a determinant role regarding UV light-induced chemical alterations in many proteins. After the absorption of light by these moieties, in most cases mainly by Trp and Tyr, they undergo photoionization and participate in energy-and electron-transfer processes. This not only holds for structural proteins such as keratin and fibroin [11], but also for enzymes in aqueous media such as lysozyme, trypsin, papain, ribonuclease A, and insulin [7]. The photoionization of Trp and/or Tyr residues is the major initial photochemical event, which results in inactivation in the case of enzymes. A typical mechanism pertaining to Trp residues (see Scheme 8.3) commences with the absorption of a photon and the subsequent release of an electron. In aqueous media, the latter is rapidly solvated. By the release of a proton, the tryptophan cation radical Trp is converted to the tryptophan radical Trp. ... 

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