Photoinduced electron transfer donor-acceptor compounds

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). 

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

Photoinduced electron-transfer reaction of aromatic compounds with amines is one of the most fundamental reactions in the electron-donor-acceptor systems, which was recently reviewed by Lewis [35], Because of the low oxidation potentials of the amines, the photoinduced one-electron transfer from the amines to the excited singlet states of aromatic hydrocarbons ( Aril ) readily occurs to give the radical cations of amines and the radical anions of aromatic compounds even in the less polar solvents. 

Further work used a similar system to inhibit the formation of a second ion pair completely, using the electric field of an initial ion pair. In compound 14, Zn3PN and 9-(N-pyrrolidinyl)perylene-3,4-dicarboximide (pyr-PMI) are the electron donors, while NI and PI are once again electron acceptors.11701 Photoinduced electron transfer from Zn3PN to PI with 416 nm laser pulses occurs with t = 27 ps however, if a 645 nm laser pulse is used to excite pyr-PMI first, this event is completely inhibited. 

Ogawa and coworkers have examined peptide y -strand mimics of the general type 24 (R = CH(CH3)2, n = 1-3), consisting of a ruthenium(II) polypyridyl electron donor tethered to a cobalt(III) pentammine electron acceptor by an polyvaline peptide chain [107]. A related parallel ) -sheet mimic has also been studied [108]. These compounds adopt the conformational properties found within the individual strands of a y -pleated sheet in both aqueous and methanol solutions. Emission lifetime measurements and HPLC product analyses suggest that the binuclear donor-acceptor compounds undergo photoinduced electron transfer. The values of et decrease with increasing donor acceptor distance according to y = 1.1 A, which is observed for electron transfers both in water at 298 K and in ethanol-... 

Fig. 16.8 Charge recombination lifetimes in the compounds shown in the inset in dioxane solvent. (J. M. Warman, M. P. de Haas, J. W. Verhoeven, and M. N. Paddon-Row, Adv. Chem. Phys. 106, Electron transfer—from isolated molecules to bio-molecules, Part I, edited by J. JortnerandM. Bixon (Wiley, New York, 1999). The technique used is time-resolved microwave conductivity (TRMC), in which the change in dielectric response of a solution is monitored following photoinduced electron transfer—a charge separation process that changes the solute molecular dipole. The lifetimes shown as a function of bridge length (number of a-bonds separating the donor and acceptor sites in the compounds shown in the inset) are for the back electron transfer (charge recombination) process.

Kosower and co-workers have found the photoinduced, barrierless charge separation processes of substituted polyaromatics to be controlled by solvent relaxation behavior over a large temperature range in alcohol solvents. Heitele and Michel-Beyerle reported on the complex solvent- and temperature-dependent electron transfer fluorescence quenching in some covalently linked aromatic donor-acceptor compounds in viscous solvents. These authors have attempted a critical comparison between current theoretical models and their experimental results, and the limitations of current theoretical models are discussed. 

Syntheses of oligoporphyrins have targeted the construction of model systems to miinic the assembly of chlorophylls in photosynthesis of green plants and bacteria. In earlier times, a chlorophyll dimer was considered the key unit of antenna systems which gather sunlight for production of carbohydrate. Many model systems that elucidate the mechanism of photoinduced electron-transfer reaction of photosynthesis have been reported and well documented in many reviews. Syntheses of covalently linked electron donors and acceptors have been extended to model compounds appended to successive electron acceptors such as quinone derivatives. 

Excitation of either porphyrin subunit in this bisporphyrin results in electron transfer from zinc to gold porphyrins to form the respective %-radical ions. For the zinc porphyrin, both singlet and triplet excited states function as electron donors whereas for the gold porphyrin intersystemcrossing to the triplet manifold is quantitative and the triplet acts as an electron acceptor. Consequently, virtually every photon absorbed by the bisporphyrin results in charge separation and the compound provides a unique opportunity to compare singlet and triplet state photoinduced electron transfer processes. 

For this discussion, our goal is (1) to describe the properties of typical organometallic compounds relevant to both EDA complex formation and electron transfer, (2) to discuss the electronic and structural charges attendant upon complex formation, and (3) to consider the consequences of the photoinduced electron transfer from the donor to the acceptor by irradiation of the CT band of the EDA complex according to Scheme II. Sensitized organometallic electron-transfer [25] and EDA complexation among inorganic coordination complexes [26] have been previously reviewed and will not be discussed. +... 

The HOMO —> LUMO electronic transition observed in the electronic spectra corresponds to a photoinduced electron transfer from the donor naphthalene moiety to the acceptor TCNQ ring. Since donor and acceptor moieties linked by a Ti-system can provide a strong hyperpolarizability in such molecules, these compounds could be used, in principle, to devise nonlinear optical materials. 

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