Subject tetrapyrrole

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. 

The subject of this Section is photochemical processes in which polynuclear tetrapyrrole complexes with metal-metal or metal-bridge-metal bonds are formed from or decomposed to mononuclear tetrapyrrole ligand containing... 

The present Volume 84 of Structure and Bonding is entitled "Metal Complexes with Tetrapyrrole Ligands III" and completes a series of three volumes dedicated to this general topic which started with Volume 64 and continued with Volume 74. The first volume contained topics such as stereochemistry of metal lotetrapyrroles, infrared and Raman spectra, biomimetic porphyrins, or metal loporphyrins with metal-carbon single bonds and metal-metal bonds. In the second volume, subjects like extended X-ray absorption fine structure or metal tetrapyrroles with special electrical and optical properties were covered. 

Two bacteriochlorophyll monomers, and Bg, are located next to the primary donor D, but they are buried deeper in the membrane. Their positions are fixed by helices B, C, D, and de of the L- and M-subunits, respectively. How these so-called accessory bacteriochlorphylls are involved in the ET has been the subject of a long debate (Holzapfel et al., 1990 Kirmaier and Holten, 1991). Some evidence for their function as true electron carriers has been provided by subpicosecond absorption spectroscopy (Arlt et al., 1993 Zinth et al., 1996). The Bg molecule facilitates the triplet energy transfer between D and the carotenoid (Frank and Violette, 1989). B and Bg follow the local Cj symmetry. Their tetrapyrrole rings are superimposed by a rotation (M on L) of -175.8 (Deisenhofer and Michel, 1989a,b) which is not as perfect as for the D /Dgpair. As in the case of Dg,the phytyl side chain of Bg interacts with the M-subunit... 

Porphyrinuria, characterised by the excretion of uro- and other porphyrins, which impart a red or brown colour to the urine. The condition appears to be due to a failure of the mechanism for transforming cyclic tetrapyrrols into bile pigments. The accumulation of porphyrins in the blood renders the subject photosensitive to strong light. 

---