Spacer group, electronic couplings

This relation was indeed verified experimentally in a series of organic molecules with naphthyl and biphenylyl groups linked by rigid spacers. Assuming that each rate constant is proportional to the square of an electronic coupling element gives ... 

The reduction potential of the anthraquinone moiety is more positive than that of any of the Co3+/= + couples in the sarcophaginates with apical aromatic substituents. Hence the opportunity now exists to place an electron donor (for example, anthracene in its excited state linked to the cage by a longer spacer group) and an electron acceptor (anthraquinone) at opposite ends of a cobalt(III) cage. After photolysis, charge separation could be achieved in a cascade, mediated by the encapsulated metal ion [137]. 

Comparison of compounds 1 + and 2 + shows that, despite the longer metal-metal distance, the forward electron transfer is faster across the phenylene spacer (k = 3.0 X 10 s ) than across the two methylene groups (k = 1.7 X 10 s ). This result can be related to the lower energy of the lowest unoccupied molecular orbital (LUMO) of the phenylene group, which facilitates electronic coupling. In the homogeneous family of compounds 2 +-4 +, the rate constant decreases exponentially with increasing metal-metal distance. 

A series of slipped cofacial zinc(II)porphyrin dimers formed through axial coordination of a wieso-methyl-imidazolyl substituent and possessing an appended ferrocenyl as electron donor groups were realized as photosynthetic models (Fig. 9b) [52]. Directly bond and methyl-bridged ferrocene derivatives show good electronic coupling whereas phenylene-ethylene and phenylene-ethynylene spacers decreased the electronic communication. Those electronic effects were reflected on the absorption and fluorescence properties, as well as on the porphyrin ring redox properties. 

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