Porphyrin photochemistry electron transfer systems

In this section, some basic theoretical approaches to understanding electron transfer will be reviewed. Although more complex and more quantitatively accurate theories exist (see Volume I of this handbook), a discussion of these is beyond the scope of this review. The simple theories discussed below are intended only to provide a useful framework for thinking about the photophysics and photochemistry of the porphyrin based donor-acceptor systems reviewed in this chapter. 

The majority of the research on the photochemistry of porphyrins linked to other moieties has been in the area of photoinduced electron transfer, and the systems studied are all in some sense mimics of the photosynthetic process described above. The simplest way to prepare a system in which porphyrin excited states can act as electron donors or acceptors is to mix a porphyrin with an electron acceptor or donor in a suitable solvent. Experiments of this type have been done for years, and a good deal about porphyrin photophysics and photochemistry has been learned from them. Although these systems are easy to construct, they have serious problems for the study of photoinduced electron transfer. In solution, donor-acceptor separation and relative orientation cannot be controlled. As indicated above, electron transfer is a sensitive function of these variables. In addition, because electron transfer requires electronic orbital overlap, the donor and acceptor must collide in order for transfer to occur. As this happens via diffusion, electron transfer rates and yields are often affected or controlled by diffusion. As mentioned above, porphyrin excited singlet states typically have lifetimes of a few nanoseconds. Therefore, efficient photoinduced electron transfer must occur on a time scale shorter that this. This is difficult or impossible to achieve via diffusion. Thus, photoinduced electron transfer between freely diffusing partners is confined mainly to electron transfer from excited triplet states, which have the required long lifetimes (on the micro to the millisecond time scale). 

A special situation is encountered with metal-porphyrin or metal-phthalo-cyanine molecules that can be either deposited by sublimation under UHV conditions or in solution environments. For these macrocyclic compounds, free-base species exist, i.e., the metal centers are not required per se as a construction unit. The building of supramolecular structures that incorporate porphyrin subunits is of great interest to many research groups. The rich photochemistry and redox properties (e.g., photoinduced electron transfer, luminescence, and light harvesting) of porphyrins have driven this interest. Porphyrins or phtalocyanines have a rich coordination chemistry that allows the inclusion of many different metal centers at their macrocycle. They serve in many respects as a model system since this constitutes a low-coordination complex. Recent STM studies report on the organization of metal-coordinated or free-base porphyrins as well as phthalocyanines on... 

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