Zinc tetrapyrroles

A number of studies concerning zinc complexes of porphyrins or related ligands have also been reported 1193-1196 cadmium complexes of a number of porphyrins have also been investigated,1197-1199 a number of NMR studies of zinc porphyrin complexes have been made1200,1201 and a number of donor-acceptor complexes of zinc tetrapyrrole species with 1,3,5-trinitrobenzene have been described.1202 Recent developments in metallo-phthalocyamine chemistry, including aspects of zinc complexes, have been reviewed.9-15,12026,1468 11... 

The first investigation into the excited states of ZnPc based on first-principles methods is the TDDFT/SAOP study by Ricciardi et al [135], where the UV-vis and the vacuum ultraviolet region of the electronic spectrum of ZnPc are described in detail. Subsequently, Nguyen and Pachter, in the context of a TDDFT/B3LYP study of the electronic spectroscopy of the zinc tetrapyrrole series [140], ZnP, ZnPz, ZnTBP, and ZnPc, came to a somewhat different interpretation of the Uv-vis spectrum of this phthalocyanine. 

One must, however, take into account that zinc porphyrin complexes have a strong tendency to coordinate axially Lewis bases, thus sometime affording pentacoordinate geometries. Figure 51 shows the example of [Zn(TPP)(H20)], in which the axial coordination of the water molecule causes the zinc atom to move out of the tetrapyrrolic plane.93... 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

At the first stage of the research, several zinc(II) tetrapyrroles (porphyrin, phthalocyanine and naphthalocyanine derivatives) were investigated [27] in the systems containing sacrificed compounds. The results obtained contributed to the development of the theory of redox reactions and energy-transfer processes, they were, however, of little practical significance. 

The first class of tetrad molecules discussed in the previous section focused on electron transfer between quinone moieties, whereas the second featured electron transfer involving two tetrapyrrole moieties. Phenomena of both types are observed in the natural photosynthetic apparatus. Conceptually, both lines of research can be fused via construction and study of C-P-P-Q-Q pentad molecules. The first of these, 22, has recently been synthesized [66]. The carotenodiporphyrin portion of the molecule resembles that of tetrad 18, but the porphyrin bearing the carotenoid contains a zinc ion. The diquinone is related to that of 15, but it is joined to the porphyrin by a different linkage. 

Figure 5.34 Schematic X-ray structure of the bis-helical zinc octaethylformylbiliver-dinate. Dimerization is caused by the tendency of the zinc ion to arrange nitrogen ligands as a tetrahedral ligand field. This cannot be achieved here with a single tetrapyrrole ligand.

Reaction with f-butyl alkoxide in the presence of zinc (II), which acts as a template and can be readily removed with acid from the cyclized ligand system that is formed, finally results in the formation of bonellin dimethylester (85) from the linear tetrapyrrole (84). 

The application of X-ray absorption spectroscopy (XAS), X-ray absorption nearedge structure (XANES), and EXAFS to tetrapyrroles that are not iron porphyrins is as yet relatively limited. Studies include EXAFS of 3-propenamide-zinc(II)-mcso-tetraphenylporphyrin complexes [201], XANES of vanadyl-phthalocyanine and vanadyl-meso-tetraphenylporphyrin [202], and XANES of iron(III)-phthalocyanine and its CO adduct [203]. 

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