Photochemistry clusters

Vergeer FW, Kleverlaan CJ, Matousek P et al (2005) Redox control of light induced charge separation in a transition metal cluster photochemistry of a methyl viologen-substituted [Os3(CO)io(alpha-diimine)] cluster. Inorg Chem 44 1319-1331... 

Leadbeater NE (1995) The generation and reactivity of versatile ruthenium carbonyl organometallic intermediates by cluster photochemistry. J Chem Soc Dalton Trans 2923-2934... 

Roterya, V., Profant, V., Farnik, M., Slavicek, R, 8c Buck, U. (2007). Experimental and theoretical study of the pyrrole cluster photochemistry Closing the ndissociation pathway by com-plexation. Journal of Chemical Physics, 127(6), 064307. doi 10.1063/1.2754687. 

Low-temperature, photoaggregation techniques employing ultraviolet-visible absorption spectroscopy have also been used to evaluate extinction coefficients relative to silver atoms for diatomic and triatomic silver in Ar and Kr matrices at 10-12 K 149). Such data are of fundamental importance in quantitative studies of the chemistry and photochemistry of metal-atom clusters and in the analysis of metal-atom recombination-kinetics. In essence, simple, mass-balance considerations in a photoaggregation experiment lead to the following expression, which relates the decrease in an atomic absorption to increases in diatomic and triatomic absorptions in terms of the appropriate extinction coefficients. 

The photochemistry of Ru3(CO)y2 has been investigated in our laboratory (3-5) and others (6-11) and has been shown to involve both photofragmentation of the cluster (Equations 1 and 2) and photolabi-lization of carbonyls to give substituted trinuclear clusters Ru3(C0)] ] L (Equation 3). 

It has been our goal for some time to run photochemical energy storage reactions without relay molecules or separate catalysts. We have concentrated on the photochemistry of polynuclear metal complexes in homogeneous solutions, because we believe it should be possible to facilitate multielectron transfer processes at the available coordination sites of such cluster species. 

One problem we have had to overcome in developing metal-cluster oxidation-reduction photochemistry is the tendency of excited clusters to dissociate into radical fragments (for... 

Explain the photochemistry of organometallic compounds from the point of view of photodissociation of simple organometallics, metal-to-metal bond cleavage and the photosubstitution of cluster species. 

Ozin GA, Flugues F, Mattar SM, McIntosh DF (1983) Low nuclearity silver clusters in faujasite-type zeolites optical spectroscopy, photochemistry and relationship to the photodimerization of alkanes. J Phys Chem 87 3445-3450... 

DR. ANTHONY POE (University of Toronto) As Dr. Geoffroy mentioned, there is a fair bit of work being done on dinuclear metal-metal bonded carbonyls but rather less on metal clusters [Geoffroy, G. L. Wrighton, M. S., "Organometallic Photochemistry," Academic Press New York, 1979]. We have been interested for some time in the thermal fragmentation of metal clusters, and have recently looked at some photochemical reactions as well. I would like to present some results here today which are very preliminary. 

The last example of a sequential approach is from Sanov (excerpt 130). A series of increasingly complex experiments is proposed to study the photochemistry of 02, and OCS . Sanov begins with the easier diatomic anions (02 and which will serve as prototypes for subsequent experiments. Next, he will study a larger, polyatomic anion (OCS ) and its cluster ions, 0CS (H20)]j. In the future, he will study even larger dimers and trimers (OCS)n (n > 2) and their hydrated counterparts. 

Photochemistry can be used to demonstrate solvent effects in supercritical fluids. The analysis revealed trimodal fluorescence lifetime distributions near the critical temperature, which can be explained by the presence of solvent-solute and solute-solute clustering. This local aggregation causes an increase in nonradiative relaxations and, therefore, a decrease in the observed fluorescence lifetimes. Concentration and density gradients are responsible for these three unique lifetimes (trimodal) in the supercritical fluid, as contrasted with the single lifetime observed in a typical organic solvent. The... 

The above processes describe the possible unimolecular reactions occurring after photoexcitation of aldehydes. For assessing the effect of solvation on the photochemistry, the following model systems have been used. Pinonic acid has been studied with one and five water molecules, and a pentanal cluster using five identical pentanal molecules has been built. 

Finally, we would like to discuss the effect of molecular aggregates on the photochemistry of pentanal. As discussed already earlier, a cluster of five pentanal... 

Dividing the total reaction channels into monomeric and cross-molecular reactions, it is found that about 73% of the reaction channels involve cross-molecular reactions, compared to only 27% of monomeric reactions. The conclusion that follows from these numbers is that the molecular aggregate has a large effect on the photochemistry of pentanal and that it yields a much larger number of different products. It is predicted that longer simulation timescales or larger clusters will even more increase the cross-molecular reactions. Experimental evidences provided in the same Ref. [32] support most of the predicted reaction channels. 

D. Shemesh, S. L. Blair, S. A. Nizkorodov, R. B. Gerber. Photochemistry of aldehyde clusters cross-molecular versus unimolecular reaction dynamics, Phys. Chem. Chem. Phys., 16 23861-23868 (2014). 

STM experiments are in general agreement with the picture above. STM directly observes the two different molecular precursor states at low Ts and observes their dissociation by thermal annealing, photochemistry and viatunneling electrons [153,326]. The molecularly adsorbed states and dissociated show a strong Ts dependent clustering at low Ts, and this is evidence for a very mobile physisorbed precursor since the molecularly chemisorbed states are not significantly mobile at these Ts [326,327]. 

As might be expected this cluster shows interesting photochemical properties that seem to arise from partial photodissociation of the unique Cu(III) ion followed by a two-electron transfer from reducing substrates. Work in progress is aimed at understanding the interesting photochemistry of this cluster. 

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