Bacteriochlorin properties

The intense near infrared absorption exhibited by bacteriochlo-rophyll a is a most interesting property for a sensitizer, but bac-teriochlorophyll a is known to be extremely sensitive to oxidation. For many years, the lability of the bacteriochlorins available directed the research efforts on new photosensitizers... 

Chlorins, Bacteriochlorins and Isobacteriochlorins. 2.1 General properties. 2.2 Spectroscopic properties. 2.3 Syntheses... 

There has been substantial recent interest in the structures of reduced porphyrin derivatives. Reduced derivatives mean the reduction of the porphyrin system by the addition of one or more moles of hydrogen. This chemical change can have important effects on physical properties. However, in this review, the effect of such modified macrocycles on the coordination parameters of the metalloderivatives is the major question. Characterized species include three levels of reduction, di-, tetra- and hexahydroporphyrins. Examples of the dihydroporphyrins are chlorins (G) and porphodimethenes (H). Tet-rahydroporph)nins include bacteriochlorins (I) and isobacteriochlorins (J). Characterized hexahydroporphyrins are the pyrrocorphins (K) and hexahydroporphyrins (L). 

Most of the naturally occurring bacteriochlorins (e.g., 132) have absorptions between 760 and 780 nm and are extremely sensitive to oxidation, resulting in a rapid transformation into the chlorin state 133 which generally has an absorption maximum at or below 660 nm (Scheme 37). Furthermore, if a laser is used to excite the bacteriochlorin in vivo, oxidation may result in the formation of a new chromophore absorbing outside the laser window, reducing its photodynamic efficacy. Due to the desirable photophysical properties of bacteriochlorins, there has been increasing interest in the synthesis of stable bacteriochlorins from bacteriochlorophyll a or other similar tetrapyrrolic systems. 

In bacteriochlorin dimers 1 and 2, a through space overlap of the unreduced pyrroles LUMOs explains the enhanced mixing of monomer-like Qx transitions. This overlap is in addition to r-7r interactions through the phenylene bridge that are also observed for 3 and 4. This is important regarding the use of porphyrin chromophores to model the properties of natural systems, because it implies that BC dimers in the photosynthetic apparatus are able to perform excitation energy transfer via the LUMO. This is not possible in model porphyrin dimers. 

Recent reconstitution experiments open the possibility of selectively introducing other metallo bacteriochlorins (or chlorins) at the acceptor BChl or BPheo sites. Zinc substitutions are obvious choices since the optical and coordination properties of Zn complexes resemble those of Mg derivatives but the former are easier to reduce (and harder to oxidize) than the latter. Perhaps a less obvious candidate for metal replacement is nickel which offers several advantages Ni complexes are also easier to reduce and harder to oxidize than Mg compounds but, in addition, redox potentials of Ni porphyrins and chlorins are close to those of the metal-free derivatives the optical properties of Ni chlorins are similar to those of Mg ones they possess shortlived excited states as a result of which they have proved to be valuable for resonance Raman studies. A more practical advantage of Ni complexes is their enhanced stability towards demetallation, compared to Mg or Zn complexes, a property of merit if the chromophores are to be used in reconstitution reactions. (Ni protoporphyrins IX have been successfully exchanged for the histidine-bound hemes in hemoglobin.) ... 

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