Metal oxides reactions yielding electronically excited

The second photochemical reaction which was studied was the reaction of CotCO NO with Lewis base ligands L (J 6 ). The observed solution phase photochemical reaction is carbonyl photosubstitution. This result initially did not appear to be related to the proposed excited state bending. Further reflection led to the idea that the bent molecule in the excited state is formally a 16 electron coordinatively unsaturated species which could readily undergo Lewis base ligand association. Thus, an associative mechanism would support the hypothesis. Detailed mechanistic studies were carried out. The quantum yield of the reaction is dependent on both the concentration of L and the type of L which was used, supporting an associative mechanism. Quantitative studies showed that plots of 1/ vs. 1/[L] Were linear supporting the mechanism where associative attack of L is followed by loss of either L or CO to produce the product. These studies support the hypothesis that the MNO bending causes a formal increase in the metal oxidation state. 

Primary photoreactions leading to net oxidation or reduction reactions of coordination compounds are well known and are often the result of decay paths accessible only from CT states. A number of coordination compounds yield photoelectron production in solution, the Ru(2,2 -bipyridine)3+ ion has been shown to be an electron donor from its electronically excited state, and photoreduction of several metal complexes has been studied in detail. Discussion of these three areas should reveal most of the important principles associated with photoredox and CT state chemistry. 

The partitioning of the POM excited state between productive processes (photoredox reactions involving substrate) and nonproductive processes (radiationless decay including internal conversion, bimolecular quenching, or emission) depends on the structure and elemental composition of the POM, and to a lesser extent other parameters. The quantum yields for the photoredox processes of POMs in solution can be quite high (well over 50%). The presence of -electron containing metal centers lowers the quantum yields for substrate oxidation by quenching the excited states. 

The original diode device has even been successfully applied to monitor chemicurrents under the steady-state catalytic reaction of CO oxidation [28-34]. The initially reported electron yields of three electrons for the production of four CO2 molecules [29,30] were unphysically high. In the meantime, yields of the order 10 to 10 [33,34] were determined, which values fit into the general picture. Since supported catalysts often consist of metal nanoparticles on an oxide support, it is argued that these chemicurrents may be used to monitor the electronic excitations associated with "real" catalysts. 

In summary, the triplet (do po) excited states of the d -d metal dimers [Ir(p-pz)(C0D)]2 and Pt2(pop)4 " undergo a variety of photochemical reactions. Electron transfer to one-electron quenchers such as pyridinium cations or halocarbons readily occurs with acceptors that have reduction potentials as negative as -2.0 V. With the latter reagents, net two-electron, photoinduced electron transfer yields d -d oxidative addition products. Additionally, the triplet (da pa) excited state of Pt2(pop)4 apparently is able to react by extracting a hydrogen atom from a C-H bond of an organic substrate. 

In an aqueous medium, luminol reacts with a potent oxidizing agent in the presence of a catalyst (generally a metal, a metal-containing compound or an enzyme) in alkaline solution to yield 3-aminophthalate in an excited electronic state, which returns to ground state with the production of light. Metal ions such as Co(ii), Cu(n) and Fe(ii) or enzymes like HRP and microperoxidase catalyse the luminol reaction efficiently (Figure 27.3). The CL reaction can be summarized as follows ... 

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