Metal-Mediated Photochemistry

SCHEME 30.12 Photothermal cycUzation of vanadium (V) metaUoenediyne in the solid state. 

SCHEME 30.14 Reduction of the porphyrin backbone during photolysis of tetraphenyl octakis(j4ienylethynyl) porphyrin. 

SCHEME 30.15 Formation of picenoporphyrins from dialkynylporphyrinic enediynes. 

SCHEME 30.16 Suggested reaction energy profiles for ground- and excited-state Bergman cyclization of porphyrin-based enediynes. Reprinted with permission from Nath et al. [29a]. American Chemical Society. 

SCHEME 30.17 Inhibition of photo-Bergman cyclization upon Ru complexation. 

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The photochemistry of the BC has been largely expanded by metal-mediated strategies aiming to achieve control over enediyne reactivity. The effects of direct incorporation of metal centers in metalloenediynes, porphyrin-based enediynes, as well as inhibitory complexation of simple acyclic and cyclic enediynes are discussed in the following. 

The theoretical description of photochemistry is historically based on the diabatic representation, where the diabatic models have been given the generic label desorption induced by electronic transitions (DIET) [91]. Such theories were originally developed by Menzel, Gomer and Redhead (MGR) [92,93] for repulsive excited states and later generalized to attractive excited states by Antoniewicz [94]. There are many mechanisms by which photons can induce photochemistry/desorption direct optical excitation of the adsorbate, direct optical excitation of the metal-adsorbate complex (i.e., via a charge-transfer band) or indirectly via substrate mediated excitation (e-h pairs). The differences in these mechanisms lie principally in how localized the relevant electron and hole created by the light are on the adsorbate. 

We focus initially on the photochemical behaviour of complexes of Fe(III) with simple carboxylic acids and give particular attention to oxalic acid. This compound is prevalent in atmospheric aerosols [28], provides a simple example of environmentally important light-mediated ligand-to-metal charge transfer (LMCT) processes which result in ligand decarboxylation [27] and is used to initiate the degradation of contaminants both in the absence and presence of added hydrogen peroxide (via the so-called modified photo-Fenton process [29,30]). In addition, the photochemistry of Fe(III)-oxalate complexes has been studied in detail, as it is the basis of... 

Topics which have formed the subjects of reviews this year include photosubstitution reactions of transition-metal complexes, redox photochemistry of mononuclear and polynuclear" complexes in solution, excited-state electron transfer processes, transition-metal complexes as mediators in photochemical and chemiluminescence reactions, lanthanide ion luminescence in coordination chemistry, inorganic photosensitive materials," and photocatalytic systems using light-sensitive co-ordination compounds. Reviews have also appeared on the photoreduction of water.Finally, various aspects of inorganic photochemistry have been reviewed in a single issue of the Journal of Chemical Education. 

In summary, although the perturbation to the photochemistry induced by adsorbate-substrate interactions for insulators is not as large as in the case of metals, the perturbation can be considerable. New photochemical channels are opened, different branching ratios between established channels occur and energy partitioning between translation, internal modes and substrate is altered. New multilayer and substrate mediated effects become apparent and surface enhancements are observed. 

VVTith the exception of molecules in the gas phase at low pressures, the properties of any species are mediated to some degree by the environment. In particular, the photophysical processes within a transition metal ion complex are often markedly dependent on the surroundings in which the complex is embedded. Interest in transition metal ion photophysics has been high not only for the intrinsic importance e.g., phosphors and lasers) but also because photochemistry and photophysics are intimately interrelated. It is this connection between photochemistry and photophysics that will be emphasized in this discussion. 

There Is now considerable evidence that a variety of photo-chemlcally Induced redox processes occur In natural water systems. At the present time however, little Is known about how these processes Influence redox equilibria of minor elements such as copper or other transition metals In the water column. A variety of elements exist In the upper water column In valence states which are unstable with respect to the O2/H2O couple. However, since little kinetic data Is available for probable photoredox processes Involving these elements, It Is Impossible to assess the Importance of photochemistry In relation to biologically mediated processes. 

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