Porphyrins and phthalocyanines

MgTPP has also been examined as a substrate for constructing porphyrin sponges, i.e., lattice clathrates that can reversibly absorb and release guest molecules. Such guests as methyl benzoate, propanol and (R)-phenethylamine) have been structurally authenticated other examples are known.  

The solid state structure of the centrosymmetric dilithium tetraphenylporphyrin bis(diethylethe-rate) differs from the salt-like compounds, in that the [Li(Et20)] moiety is coordinated to both faces of the porphyrin in a square pyramidal fashion (Li-N = 2.23-2.32 A). A related motif is found in the case of sodium octaethylporphyrinate X-ray crystallography reveals two Na(THF)2 moieties symmetrically bound to all four nitrogen atoms, one on each face of the porphyrin ring (Na—N (ay) = 2.48 A). The structure of the potassium derivative K2(py)4(OEP) is similar (K—N (av) = 2.84A).  

Crystallization of [Li(TPP) ] from dichloromethane and diethyl ether yields purple crystals the solid state structure indicates that the lithium atom is bound in the plane of the porphyrin. The porphyrin macrocycle is slightly ruffled, with opposite pyrrolic carbons up to 0.3 A above or below the mean porphyrin plane.  

Molecules which have been rendered amphiphilic by one of the techniques discussed above are deposited in the Y mode so that the planes of the molecules are nearly vertical with respect to the film plane. 

In some cases it is possible to use materials which have fourfold symmetry. It is not entirely clear how such materials can be deposited by the LB technique. 

The ring structure has long hydrocarbon chains attached at the corners so that they stand up on one side. These chains provide the hydrophobic component and the polarisable ring structure provides the hydrophilic moiety. 

All these methods have been used successfully and examples will now be discussed. Films in which amphiphilic porphyrins alternate with some other molecule are discussed in the next chapter. 

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LB Films of Porphyrins and Phthalocyanines. The porphyrin is one of the most important among biomolecules. The most stable synthetic porphyrin is 5,10,15,20-tetraphenylporphyrin (TPP). Many porphyrin and phthalocyanine (PC) derivatives form good LB films. Both these molecules are important for appHcations such as hole-burning that may allow information storage using multiple frequency devices. In 1937 multilayers were built from chlorophyll (35). 

Porphyrin systems therefore obey Hiickel s rule in having An + 2 n = A) TT-electrons in a planar, cyclic, conjugated array. Both major tautomeric forms have delocalization pathways with opposite N-Hs (trails tautomers), as shown in 71a 71b. It is already known (76AHCS1) that tautomers with inner hydrogens adjacent (cis tautomers) are much less stable, playing an important role only in the mechanism of proton transfer in porphyrins and phthalocyanines. 

Macrocyclic effect and specific character of complex formation with rigid macrocyclic ligands such as porphyrins and phthalocyanins 97MI8. 

Samsonova and Nikiforov, 1984), and porphyrin and phthalocyanine metal complexes (Becker et al., 1985a, 1986b Becker and Grossmann, 1990) were tested. That a series of relatively simple anions such as the oxalate monoanion, tetraphenyl bor-anate (Ph4B ), bromide, chloride, and even tetrafluoroborate can act as donors is, at least for the last mentioned anion, surprising, but Becker et al. (1985 b) were able to trap aryl radicals and in some cases also donor radicals (Cl, COO ) by spin trapping with nitrosodurene and phenyl-tert-butylnitrone. The photochemical effect is postulated to be due to ion pairs ArNJ X-. 

Two classes of stiff polyazamacrocycles with intra-annular basic functions should not be forgotten porphyrins and phthalocyanines. In these molecules, the perimeter is fixed. The pXa value may be influenced only by substituents but not by variation of the ring size of the cavity. In this context we will therefore not discuss porphyrins and phthalocyanines. 

Vasudevan P, Santosh, Mann N, Tyagi S. 1990. Transition metal complexes of porphyrins and phthalocyanines as electiocatalysts for dioxygen reduction. Transition Metal Chemistry, 15, 81-90. 

The development of such a reaction proceeding under mild conditions is a technological challenge constituting one of the key points for the finalizing of efficient and low cost fuel cells. The catalytic properties of macrocyclic complexes like porphyrins and phthalocyanines for the reduction of molecular oxygen have been well known for four decades350,351 and numerous papers are devoted to this area. Here only some relevant and recent work in this field is described. 

For systemic administration, the photosensitizer usually has to be delivered into the bloodstream by intravenous injection. Since the photosensitizer is a solid, this means that a solution or a stable suspension has to be provided. Metal complexes of the basic porphyrin and phthalocyanine nuclei are insoluble in water, so that some effort has to be made to render the system water soluble, or at least amphiphilic, by placing various substituents (e.g., S03H, C02H, OH, NR3+, polyether, aminoacid, sugar) on the periphery of the molecule. The aromatic character of the ligand offers a suitable opportunity for such substitutions to be made. Examples will appear frequently in the following sections. 

Bocian, Lindsey and co-workers studied sandwich complex nanocapacitors comprised of porphyrin and phthalocyanine ligands separated by lanthanide metals [133]. A triple-decker sandwich of phthalocyanine-Eu-phthalocyanine-Eu-porphy-rin, with two phenylethynyl linker wires from the porphyrin, potentially has up to nine accessible oxidation states (—4 to +4). SAMs of monomers, dimers, trimers, and oligomers of this sandwich, anchored at one or both ends by thioacetyl groups, gave charge densities up to 10 10 mol cm-2, electron-transfer rates up to 105 electrons s-1, and charge-dissipation half-lives in the 10-50 s range. 

Triplet energy transfer measurements from porphyrin and phthalocyanine sensitizers give the triplet energies of six (Z)-A4,A4-diethyl-2-(alkyl, aryl)-A1-(3-phenyl-77/-pyrazolo[5,l-r-][l,2,4]triazol-7-ylidene)benzene-l,4-diamine azomethine dyes 154 with adsorption maxima in ethanol at 546-633 nm to lie in the range of 115-88 kJmoF1 <2003PPS563> (Figure 24). 

Caughey, W. S., Raymond, L. D., Horiuchi, M., and Caughey, B. (1998). Inhibition of protease-resistant prion protein formation by porphyrins and phthalocyanines. Proc. Natl. Acad. Sci. USA 95, 12117-12122. 

B. D. Berezin, Coordination Compounds of Porphyrins and Phthalocyanines, John Wiley Sons, New York (1981). 

Porphyrazines (153) are porphyrins with the four =CH— inter-ring bridges replaced by =N—, thus forming a halfway house between porphyrins and phthalocyanines (next section). The octaethylporphyrazine complexes of iron(II) and of iron(III) have been reviewed. ... 

The authors are most grateful to Dr. J. G. Jones for his valuable assistance with the preparation of the sections on porphyrin and phthalocyanine complexes. 

The dithionite reduction of the micelle encapsulated aqua (hydroxo) ferric hemes at pH 10 (in inert atmosphere) gives an iron (II) porphyrin complex whose optical spectrum [21] shows two well-defined visible bands at 524 and 567 nm and a Soret band split into four bands (Fig. 10). Such spectral features are typical of four-coordinate iron (II) porphyrins. The magnetic moment (p = 3.8 + 0.2 Pb) of this sample in the micellar solution is also typical of intermediate spin iron(II) system and is similar to that reported for four-coordinate S = 1 iron(II) porphyrins and phthalocyanine [54-56]. The large orbital-contribution (ps.o. = 2.83 p for S = 1) observed in this iron(II) porphyrin... 

Bonnett R (1995) Photosensitizers of the porphyrin and phthalocyanine series for photodynamic therapy. Chem Soc Rev 24 19... 

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