Cluster photochemical properties

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. 

Of the large number of /u.3-alkylidyne clusters known, only the title complexes have had their photochemical properties reported (194). The structure of CH3CCo3(CO)9 (163) consists of a triangle of metal-metal... 

Nanometer sized semiconductor dusters, expected to have properties different from those of molecular and bulk semiconductors of the same composition, represent a new class of materials. Interest in their preparation and potential applications as photocatalysts and device components used in quantum electronics and nonlinear optics is growing rapidly. Isolated, so-called nanophase semiconductors can be considered as zero and one dimensional quantum dots and quantum wires. Their electronic, optical, and photochemical properties change with cluster size. Wider electronic band gaps and new absorption maxima in the electronic spectra have been observed as the size of these materials decreases and have been interpreted as quantum size effects. For example, as the dimensions of the semiconductor particle are reduced, a shift to higher energy in the absorption spectrum relative to that of the bulk is generally observed. 

In a series of papers, Ford and co-workers described the photophysical and photochemical properties of these luminescent clusters in detail [46-58]. In addition to complexes 4a and 4b, time-resolved emission spectra of the tetranu-clear copper(I) iodide clusters [Qi4l4(L)4] with a series of substituted pyridines [L = 4-tert-butylpyridine (4c), 4-benzylpyridine (4d), pyridine-dj (4e), 4-phe-nylpyridine (4f), 3-chloropyridine (4g), piperidine (4h), P"Bu3 (4i)] have also been studied [49]. The photophysical data are summarized in Table 1. In general, in toluene solution at 294 K, the complexes revealed a low-energy emission at 678-698 nm and a weaker, higher energy emission at 473-537 nm. The emission spectrum of 4a in toluene at 294 K is shown in Fig. 2... 

The information available is discussed in light of the effects of excitation energy and the environment on the photofragmentation process of several transition metal cluster complexes. The photochemical information provides a data base directly relevant to electronic structure theories currently used to understand and predict properties of transition metal complexes (1,18,19). 

The step-growth polymerization strategy used to incorporate metal-metal bonded units into polymers can also be used to incorporate metal clusters into polymers. One of only a few examples of this type of reactivity is shown by equation 11.35 It is noteworthy that metal clusters also undergo photochemical reactions.36,37 These reactions should also occur when the clusters are incorporated into polymer backbones. If polymers containing metal clusters can be shown to have unusual properties or applications (photochemical or otherwise), then the synthesis of these polymers will likely burgeon in coming years. 

Photochemical behaviour of coordination compounds described in previous chapters results mainly from electronic interactions between the central metal atom or ion and ligands in the hrst coordination sphere. An increased size of molecular systems to clusters and nanosized crystals expands the possibility of photoinduced electron transfer between the discrete electronic states to excitation within bands. Furthermore, interactions of nanoparticles with molecules yield unique materials, combining structural versatility of molecular species with collective properties of solids. 

Ford, P. C. Vogler, A. Photochemical and photophysical properties of tetranuclear and hexa-nuclear clusters of metals with d10 and s2 electronic configurations. Acc. Chem. Res. 1993,26,220-226. 

Of special significance are the catalytic properties of small metal clusters. At their surface such clusters have a large number of atoms with a low coordination number to which substrates bind. Catalytic reactions are being studied in hydrogenation, hydrosilylation, hydration, and the Heck reaction. Metal clusters are also of importance with regard to redox and electron transfer processes such as the photochemical decomposition of water (fuel cells) and photocatalytic hydrogenation. 

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