Photochemistry of porphyrins

II. BASIC PHOTOCHEMISTRY OF PORPHYRINS A. General Concepts and Theoretical Background... 

Porphyrin derivatives have been extensively tested as photosensitizers for the PDT of cancer for two sets of reasons. First, their strong absorption of light in the phototherapeutic window and efficient photoinduced reactions with molecular oxygen offer a photochemical tool to induce localized cytotoxicity in targeted tissues. Second, porphyrin derivatives have an intrinsic affinity for tumors (4-6). Whereas the spectroscopy and photochemistry of porphyrin derivatives are very well understood, the same is not (yet) true for the mechanisms that contribute to their preferential localization and accumulation in tumors. This latter subject is outside the scope of this work, and it will only be briefly mentioned in the context of in vivo studies with porphyrin derivatives. 

The rates of radiationless transitions between electronic states of porphyrins and their derivatives play a dominant role in their photochemistry because they are the major decay channels of the electronically excited states. Radiative channels, such as fluorescence, rarely exceed 10% of the overall decay rate constant at room temperature. The lifetimes of the lowest electronic states of free-base porph3nins and closed-shell metalloporphyrins vary by more than 10 orders of magnitude with the nature of the substituents. The understanding of such variations is essential to design and control the photochemistry of porphyrins and justifies an incursion on the fundamentals of radiationless transitions. 

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). 

For understanding the photochemistry of porphyrin compounds, characteristics of absorption spectrum, fluorescence quantum yield, lifetime of singlet excited state, phosphorescence quantum yield, and excited triplet state lifetime are most crucial. These data are actually helpful for a more sophisticated design of a photo-responsive molecular system. In this section, comprehensive data on excited states of porphyrins, mainly on tetraphenylporphyrin (TPP), octaethylporphyrin (OEP), and fiieir metal complexes, are presented. Some empirical rules for estimating those data will also be described. 

III. MOLECULAR PHOTOCHEMISTRY OF PORPHYRINS A. Excited Energy Transfer [281-284]... 

---