Porphyrin photochemistry photochemical reactions

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

Several investigations of photochemical reactions of high-spin Fe porphyrins have been reported, and the subject has been reviewed see Photochemistry of Transition Metal Complexes). When bound to anions such as CH or OH, Fe porphyrins are photochemically reduced to Fe , with production of a radical (Ch or In the case of a cofacial Fe -Zrf diporphyrin, however, the photochemical reaction produced PFe JCl... 

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

The photochemistry of true organomagnesium compounds remains almost completely unexplored. A literature search in preparation of this work found only a few scattered examples of photochemical studies, mostly in relation to Grignard reactions and 1,3-diketonate chelates Similar to the situation with organozinc compounds magnesium tetrapyrrole chelates, i.e. magnesium porphyrins 1, 5,10,15,20-tetraazaporphyrins (por-phyrazines) 2 and phthalocyanines 3 have found more interest. This is primarily related... 

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. 

In the study of protein electron transfer (ET), radiolytic and photochemical techniques have indeed proven highly complementary. Between them, these techniques provide a range of reaction types and reaction free energies [cf. Zn porphyrin triplets (F° - 0.8 V) versus Fe porphyrins ( ° - 0 V)], Of particular interest in the current study is the different dynamic range(s) of the techniques. Photochemistry is subject to a natural time window set by the excited state lifetime only reactions faster than the excited state decay can be observed. Conversely, the bimolecular nature of radiolysis sets an upper hmit on the observed rates that is often determined by the rate of electron capture. 

Using this strategy, four gold(III) porphyrins have been assembled into a single cluster [17], This tetrameric array shows interesting photochemical behavior and appears to be an improved model for photosynthetic reaction center complexes since it combines the features of exciton annihilation and light-induced electron transfer. Below, we contrast the photochemistry of the bis- and tetrakis- complexes. 

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