Octa-Substituted Porphyrin

Ultimately, all porphyrin syntheses start from pyrroles (which themselves need to be made), so that a special section with this title might appear pointless. However, this section really deals with those porphyrins where the number of chemical manipulations that the pyrrole has to go through prior to porphyrin formation, is minimal the porphyrin is synthesised directly from the final pyrrole. This means that such synthetic routes are usually confined to those porphyrins with a symmetrical arrangement of peripheral substituents, e.g., 5,10,15,20-meso-tetrakis(aryl)porph)rrin (e.g. TPP) and 2,3,7,8,12,13,17,18-octa-substituted porphyrins (e.g. OEP). 

Phthalocyanines and metallophthalocyanines have been intensively studied as discotic liquid crystals,but porphyrins have received less attention. The first examples of porphyrin discotic phases were based on naturally occurring uroporphyrins but showed a very narrow discotic LC range. However, octa—substituted porphyrins can show discotic meso-phases over a broad temperature range, the LC phase being stabilised by metal insertion into the porphyrin, e.g. zinc. These materials are organic semiconductors whose LC properties allow them to form highly ordered thin films in the solid state. Emission, excitation and absorption spectra in... 

The orientation of n-nonoxyethyl octa-substituted liquid-crystalline material was explored by X-ray diffraction. The molecules were found to structurally order in columns such that the macrocyclic planes tilt at an angle of 46° to the columns (Figure 2) The intricacy of the structure was further elaborated for the n-decoxyl derivative. The interfacial porphyrin-porphyrin distance was found to be 3.98 A, and the intercolumn. separation was 23.59 A (Figure 3). This structure has been suggested as a one-dimensional molecular wire motif for electronic conduction. ... 

Figure 3. Schematic representation of the coiumnar arrangement of discotic zinc rr-decyl ether octa-substituted porphyrin stacks. Reprinted with permission from Liu, C, Y. Pan, H. L Tang, H. Fox, M. A. Bard, A. J. /. Phys. Chem. 1995, 99, 7632 1995 American Chemical Society,...

Template condensation of l,2-dicyano-l,2-his(methylthio)ethylene in the presence of magnesium propylate results in the magnesium complex of the octa-substituted tetraaza porphyrin [Mg(L580)] (Eq. 2.223) [414]. 

Khene S, Cammidge AN, Cook MJ, Nyokong T (2007) Electrochemical and photophysical characterization of non-peripherally-octa alkyl substituted dichlorotin(IV) phthalocyanine and tetrabenzotriazaporphyrin compounds. J Porphyrins Phthalocyanines 11(10) 761-770... 

Matemadombo F, Maree MD, Ozoemena K I, Westbroek P, Nyokong T (2005) Synthesis, electrochemical and spectroelectrochemical studies of octa phenyl thio-substituted phthalocyanines. J Porphyrins Phthalocyanines 9(7) 484-490... 

Eventual irreversible oxidation of both the picket-fence and the capped porphyrins occurs because steric encumbrance is present only on one side of the molecule. To avoid this, octa-orf/io-substituted TPP complexes have been prepared. By choosing the correct steric bulk for the ortho substituents a protected pocket may be formed on both sides of the porphyrin ring. The pockets could still be penetrated by axial bases and diatomic molecules but would form a barrier to the close approach of two metal centres, thus stabilizing the oxygenated porphyrin. Amundsen and Vaska have prepared the hemes Fe (P) (where (P ) is the dianion of tetrakis[2,4,6-tris(methoxy)phenyl]porphyrin or tetrakis[2,4,6-tris(ethoxy)phenyl]porphyrin) by condensation of the appropriate trisub-stituted benzaldehyde with pyrrole Balch has shown by H-NMR that although the... 

The iron porphyrin models discussed above all share a nearly planar porphine core, similar to the hone in Mb, whereas the heme in Cyt c is distorted from the planar structure by the covalent bonds and hydrogen bonds with surrounding protein residues. To address the effect of environmentally induced heme distortion on the mechanism of VER, rather than including the surrounding protein residues explicitly (too expensive for any reasonable high level of theory), we replaced the heme s central iron atom by nickel. Due to the smaller size of nickel relative to iron, the porphine structure is distorted in the Ni-substituted Ni-hane in a manner similar to the heme distortion observed in cytochrome c. Another nickel porphyrin model, nickel octa-ethylporphyrin (NiOEP), was also optimized to a structure with similar nonplanar porphine core geometry [90]. 

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