Photoinduced Charge Separation in Linear Arrays

When electron transfer processes are targeted, one of two different options can be chosen. The first deals with photoinduced charge separation. This requires the connection of the photoactive unit, the porphyrin or muitiporphyrin architecture, to a nonporphyrinic eiectron donor and/or acceptor. The second option concerns electron conduction in muitiporphyrin arrays. The donor and acceptor in this case can be eiectrode materiais to which the array is connected on both sides. 

For discrete, well characterized species, Ceo derivatives are almost exclusively utilized as electron acceptors due to their small reorganization energies (0.48 and A simple comparison between porphyrin- 

Fluorescence quenching studies carried out on the first series of porphyrin dimer 53a, trimer 53b, and tetramer 53c show that fluorescence of the porphyrin arrays is quenched approximately 100 times compared to arrays not bearing fullerenes. Due to the properties of the arrays, namely strong excitonic coupling and red shifted, split Soret bands, no conclusions are drawn about the mechanism (ET or eT) of the quenching. 

Further studies on a series of conformationaUy isomeric fused dimers, 54a, and 54b, show that in this case, the quenching is due to photoinduced electron transfer from the zinc porphyrins to the Ceo acceptors, generating a lifetime of 

630 ps for the CS state of 54a. Up to 15 possible redox processes, including zinc porphyrin oxidations, and reductions of both the zinc porphyrins and the C6os, are possible in the fused conformers 54a-b. Differences in the redox potentials are observed, depending on the side of the porphyrins with which the fullerenes interact. Interestingly, excitation of the Ceo acceptor, by irradiation at 330 nm, induces ET from the carbon sphere to the porphyrins, without further charge separation. A typical porphyrin-based fluorescence is observed in this case. 

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