Excited State Atom Transfer

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

While electron-transfer processes are common in inorganic photochemistry, excited-state atom transfer is limited to a small class of inorganic complexes. For U022 , the diradical excited state ( U-OO is active in alcohol oxidation (2). The primary photoprocess is hydrogen atom abstraction by the oxygen-centered radical. Photoaddition to a metal center via atom transfer has been observed for binuclear metal complexes such as Re2(CO)io (3-5). The primary photoprocess is metal-metal bond homolysis. The photogenerated metal radical undergoes thermal atom-abstraction reactions. Until recently, atom transfer to a metal-localized excited state had not been observed. 

An alternative pathway to outer-sphere electron transfer, which yields similar photoredox products with alkyl halides, is excited-state atom transfer (Figure 3b). Data obtained for Pt2(P20sH2)4 " indicate that alkyl... 

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

As in isolated phenol and in phenol-ammonia/water clusters, the OH bond is broken homolytically in 7HQ-A3, resulting in the transfer of a hydrogen atom rather than proton transfer. As found for phenol-A /W and naphthol-A /W clusters, ammonia is a better hydrogen acceptor than water. Excited-state hydrogen transfer processes are thus strongly favoured in an ammonia environment. 

Figure 10-16. Reactant (Watson-Crick) and product GC structures of the excited state proton transfer reaction. The atom numbering scheme is illustrated for the initial Watson-Crick configuration...

Encapsulation of a Ru atom into a caged [109, 110] or hemicaged [299] tris-bipyridine ligand extends the MLCT excited state lifetime and improves photostability relative to [Ru(bpy)3] ", while retaining the fast (diffusion controlled) bimolecular excited state electron transfer reactivity. In contrast, the [Ru(bpy)3] + in the core of a dendrimer [248] has about the same inherent lifetime as the free complex but the rate of electron transfer quenching rapidly decreases with increasing the number and size of dendrimer branches. 

As an alternative to y-ray emission, an excited nuclear state can also lose its energy by internal conversion electron emission. In this parallel process, the energy of the excited state is transferred directly to an electron of the atomic shell, without previous y-ray emission. The energy of internal conversion electrons depends on the nuclear transition energy and also on the ionization energy of the K, L, M,... electrons. 

Transition Energies [29] Corresponding to Excited States of Some Atoms and Ions, as Calculated within HE Theory and Using LSD and MLSDSIC Functionals for Excited State Electron Transfer from s Orbital to d Orbital... 

Wright, J.C., 1976, Upconversion and Excited State Energy Transfer in Rare-Earth Doped Materials, in Fong, F.K., ed.. Radiationless Processes in Molecules and Crystals, Vol. 15 (Springer-Verlag, Heidelberg, Germany). Wybourne, B.G., 1970, Symmetry Principles and Atomic Spectroscopy (Wiley, New York). Yamada, N., S. Shionoya, and T. Kashida, 1972, J. Phys. Soc. Japan 32, 1577. 

In each case the mechanism likely involves chlorine atom abstraction by the excited state of the It2 complex, although an excited state electron transfer (SrnI) pathway cannot be discounted. 

Bifunctional azaaromatic compounds in which ESIPT is not possible due to tmfavorable arrangement of pyrrole NH and pyridine-type nitrogen atoms may stiU undergo self-assisted" intermolecular excited-state proton transfer if this process is thermodynamically allowed [37]. Examples of known doubly hydrogen-... 

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