Donor-acceptor complexes photoinduced electron transfer

The ability to switch a molecular unit on and off is a key component of an efficient molecular device, since it allows modulation of the physical response of such a device by external physical or chemical triggers. A molecular device, based on a trinuclear metal complex, shown in Figure 59, functions as an electroswitchable-photoinduced-electron-transfer (ESPET) device.616 Electrochemical switching of the redox state of a spacer intervening between a donor-acceptor pair can dictate the type of the observable charge separation and the lifetime of the resulting ion pair.616... 

Photoinduced intramolecular electron transfer in the donor-acceptor complex 87 (R = H) generates transient charge-separated open-shell species with the remarkably long lifetime of about 75 ps [89]. Dyads that contain Tt-extended tetrathiafulvalene units also form stable cationic species upon oxidation [90]. The dumbbell shaped triad 91 [91-93] (Scheme 4.13) was obtained by carrying out the reaction with the in situ generated bis-diene at room temperature, in the dark and in o-dichlorobenzene as a solvent in 50% yield. The product is thermally unstable and easily undergoes a retro-Diels-Alder reaction [91]. 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

In chemical terms the photoinduced electron transfer results in transfer of an electron across the photosynthetic membrane in a complex sequence that involves several donor-acceptor molecules. Finally, a quinone acceptor is reduced to a semiquinone and subsequently to a hydroquinone. This process is accompanied by the uptake of two protons from the cytoplasma. The hydroquinone then migrates to a cytochrome be complex, a proton pump, where the hydroquinone is reoxidized and a proton gradient is established via transmembrane proton translocation. Finally, an ATP synthase utilizes the proton gradient to generate chemical energy. Due to the function of tetrapyrrole-based pigments as electron donors and quinones as electron acceptors, most biomimetic systems utilize some... 

For example, in 1963 the photochemistry of magnesium phthalocyanine with coordinated uranium cations was studied in pyridine and ethanol and indicated the occurrence of PET to the uranium complex . A rapid photoinduced electron transfer (2-20 ps) followed by an ultrafast charge recombination was shown for various zinc and magnesium porphyrins linked to a platinum terpyridine acetylide complex . The results indicated the electronic interactions between the porphyrin subunit and the platinum complex, and underscored the potential of the linking para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance. 

Photoinduced Electron Transfer. Monolayer organizates are particularly suited for the investigation of photoinduced electron transfer, since the molecules are fixed and the distance between the planes at which the donor and the acceptor molecules, respectively, are located can be well defined. Therefore, complex monolayers have been arranged in order to study the distance dependence of electron transfer in these systems (2, 20). This strategy has also been used to elucidate the relative contributions of electron injection and energy transfer mechanisms in the spectral sensitization of silver bromide (21). 

From the point of view of light stability and range of absorptivity, inorganic redox systems might be more interesting. Photoinduced electron transfer in an aqueous solution of tris-(2, 2 -bipyridine) ruthenium (II) has been found to decompose water in to a mixture of H2 and 02. The Complex can serve both as an electron donor and electron acceptor in the excited state. The efficiency is low because of barrier to electron transfer. SVhen spread as a monolayer on glass slides after attaching to a surfactant... 

We can now consider specific examples of micellar influence on the course of photoinduced electron transfer [25, 26]. The simplest photoinduced electron transfer involves photoionization, i.e., the transfer of an electron from the excited state of a solute to the solvent. The next level of complexity involves electron transfer from an excited donor to an acceptor or from an excited acceptor to a donor. Such reactions can occur either within a photoinert micelle, in a functionalized assembly, or through multicomponent arrays. 

The fundamental theories behind electron transfer were discussed above in Section 2.1. Indeed, some of the most important empirical proofs for these theories have originated from photoinduced electron transfer in supramolecular donor-acceptor complexes. The difference between thermally and photochemi-cally induced electron transfer lies in both the orbitals participating in the reaction and in the additional thermodynamic driving force provided by the excited state. It is therefore important to consider the redox properties of excited-state species. 

Aromatic substrates are by far the most commonly used substrates in the rapidly expanding area of photoinduced electron transfer [1,2]. This is obviously due to the favourable location of the frontier molecular orbitals in such compounds. The same factor facilitates the formation of electron transfer donor-acceptor (EDA) complexes both in the ground state (these possibly are intermediates in some thermal reactions, e.g. selected electrophilic substitutions), and in the excited state (exciplexes). 

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. 

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