Porphyrins analogs

MRI contrast agents. Therefore the possibility of combined diagnostic imaging and photodynamic therapy arises (324). Lanthanide texaphy-rin complexes are more kinetically stable than the porphyrin analogs. 

Phtalocyanine dyads (28a,b) and sub-phtalocyanine dyads (30a-c) (Fig. 14) are porphyrin analogs with unique electrochemical properties. Comparison of the redox potentials of (28a) and (28b) with those of model compounds (25) and (29a) and (29b) (see Table 15) indicate that the dyads retain the electronic properties of the individual units. However, all the phtalocyanine-centered processes in both dyads are positively shifted with respect to those of the model phtalocya-nines, (29a,b). Additionally, the observed C6o-centered reduction potentials of both dyads are shifted to more negative values... 

Porphyrin analogs, homologs, and porphyrinoids - The class of tetrapyrrole chromophores can be enlargened by alteration of the numbers of carbon atoms linking the pyrrole rings [101]. A beautiful porphyrin isomer, porphycene, has been prepared by Vogel and coworkers [102], This porphyrin isomer, H2(Pyc),... 

Variations on the basic structural theme of 1 have led to a plethora of unusual macrocyclic systems, collectively known as porphyrin analogs or porphyrinoids. These molecules, often nontrivial to synthesize, exhibit remarkable physical and chemical properties and their chemistry has been extensively reviewed [2-18], With the impressive range of structural modifications introduced so far, the term porphyrinoid has ultimately expanded to encompass a wide range of often exotic macrocycles, some of which contain no pyrrole rings at all, or have a structural outline barely resembling that of porphyrin. Some of the generic modification types are shown in Fig. 1. Combination of these design concepts provides a virtually inexhaustible source of structural diversity. 

A great number of porphyrin analogs possess circular conjugation pathways, and often exhibit aromaticity comparable with that of the parent system 1. Moreover, the JT-electron delocalization of many porphyrin-like molecules is strongly affected by structural detail, redox chemistry, and prototropic tautomerism. The aim of the present review is to provide a description of porphyrinoid aromaticity and its connection with tautomeric equilibria. Our main focus will be on the physical manifestations of aromaticity, with a special emphasis on NMR spectroscopy. The reactivity of porphyrin analogs, including their coordination chemistry, will be discussed only to the extent it has a bearing on their aromaticity. 

Some representative examples of fullerene-porphyrin dyads are shown in Scheme 9. In other examples, porphyrin analogs such as phthalocyanines and subphthalocyanines have been used for the construction of efficient dyads. Again, the most straightforward approach for their synthesis involved 1,3-dipolar cycloaddition of the appropriate azomethine ylides to C60 [203-205]. Also, with the aid of the Bingel reaction, other phthalocyanine-fullerene systems have been prepared [206,207] with the most prominent example being the one that contains a flexible linker possessing an azacrown subunit [208]. The novelty of this dyad can be found in the nature of the linker that could, in principle, induce conformational changes in the multicomponent system when certain ions (e.g., alkaline ions) are present. As a direct consequence this would potentially allow an external control over the electronic interactions between the phthalocyanine and fullerene units. 

Axial coordination of metallo-phthalocyanines has been relatively little studied compared with that of the porphyrin analogs. Nevertheless, this approach has been employed to construct various phthalocyanine-based hetero-arrays as described below. 

Di- and trisulfide linked oligopyrrolic macrocycles 805, 807, and 809 are obtained when appropriate 2,5-free pyrrolic precursors 804, 806, and 808 are reacted with disulfur dichloride (Equations 198-200). The isolated yields were 21%, 34% (R = Me), and 27%, respectively <2005CC2122>. These systems represent the first examples of what might be a general new class of porphyrin analogs. 

Because of the potential relationship to the naturally occurring porphyrins and porphyrin-analog macrocycles, the tetraaza macrocycles have been the focus of much attention. 

Since the initial disclosure of the basic corrin structure, there has been a considerable body of effort devoted toward the synthesis of macrocycles related to this chromophore that may be considered as being intermediates between porphyrin and corrin. These macrocycles, namely the dehydrocorrins (e.g., tetradehydrocor-rin 2.3) and the corroles (e.g., 2.4), represent interesting classes of contracted porphyrins that warrant specific mention here. The interest in these molecules derives in part from the fact that they could represent milestones along the biosynthetic pathway leading to vitamin B12. They are, however, also of interest from a non-biological perspective. Simply stated, this is because corrole-type macrocycles possess unique electronic and chemical characteristics, the study of which can help one to understand better the chemistry of all porphyrin analogs. 

The UV-vis absorption spectra of homoporphyrins 4.16a and 4.16b resemble those of the parent porphyrin analogs. Both the endo and exo isomers exhibit Soret-like absorbance bands in the 450 nm region (s = 87 000 M cm ), and two Q-type transitions at 584 and 688 nm (e = 6350 and 17 300 M cm , respectively). A single crystal X-ray structure of the endo-epimer 4.16a confirmed the sp -hybridiza-tion at the inserted meso-caxhon (Figure 4.1.1). It also revealed a highly distorted... 

The photophysical properties of a Ceo-linked phytochlorin (31) [344, 373], a porphyrin analog, are quite different from those of conventional Cgo-linked porphyrins. The phytochlorin-Cfio exciplex is formed in both toluene and benzonitrile via either the singlet excited states of the phytochlorin or the Ceo- The exciplex relaxes directly to the ground state in toluene, whereas it undergoes a conversion to the charge-separated state, followed by the decay to the ground state in benzonitrile. A similar proposal for the exciplex formation has been forwarded by the report on a free-base porphyrin Ceo dyad [374, 375]. 

Synthesis, structural features, and acid-base interactions in non-central symmetric thiadiazole and triazole porphyrin analogs 04MI24. 

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