Monomeric Porphyrins

Porphyrin is a useful scaffold for developing molecular recognition elements, since it shows highly sensitive UV-vis absorption and fluorescence emission. By combining porphyrin and boronic acid, one can construct supramolecular systems that exhibit unique guest-induced spectroscopic changes. 

Imada et al. synthesized a porphyrin derivative bearing four boronic acid moieties (12). It was confirmed that 12 forms a one-dimensionally stacked aggregate in water/DMSO mixture at pH 6.9. After adding saccharides to the solution, CD (circular dichroism) spectra were measured. In the presence of saccharides (except fructose), the solutions of 12 become CD-active 

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Interpreting these results on a detailed molecular basis is difficult because we have at present no direct structural data proving the nature of the split Co(IIl/lI) voltammetry (which seems critical to the electrocatalytic efficacy). Experiments on the dissolved monomeric porphyrin, in CH-C solvent, reveal a strong tendency for association, especially for the tetra(o-aminophenyl)porphyrin. From this observation, we have speculated (3) that the split Co(III/II) wave may represent reactivity of non-associated (dimer ) and associated forms of the cobalt tetra(o-aminophenyl)porphyrins, and that these states play different roles in the dioxygen reduction chemistry. That dimeric cobalt porphyrins in particular can yield more efficient four electron dioxygen reduction pathways is well known (24). Our results suggest that efforts to incorporate more structurally well defined dimeric porphyrins into polymer films may be a worthwhile line of future research. 

LDMS is particularly well suited for the analysis of porphyrins.35-39 The heme molecule—a 22 rc-electron conjugated protoporphyrin system (Figure 8.1)—is an efficient photo-absorber in the visible and near UV (with an absorption maximum—the Soret band—near 400nm). This feature, concurrently with its low ionization potential, warrants that direct LDMS will possess extremely low limits for heme detection. The uses of IR or UV LDMS for structural characterization of natural porphyrins and their metabolites, synthetic monomeric porphyrins (e.g., used in photodynamic therapy), porphyrin polymers, and multimeric arrays, have been well documented.41148 In addition fast atom bombardment MS has been used to characterize purified hemozoin, isolated from the spleens and livers of Plasmodium yoelii infected mice.49... 

A similar catalytic activity with a monomeric porphyrin of iridium has been observed when adsorbed on a graphite electrode.381-383 It is believed that the active catalyst on the surface is a dimeric species formed by electrochemical oxidation at the beginning of the cathodic scan, since cofacial bisporphyrins of iridium are known to be efficient electrocatalysts for the tetraelectronic reduction of 02. In addition, some polymeric porphyrin coatings on electrode surfaces have been also reported to be active electroactive catalysts for H20 production, especially with adequately thick films or with a polypyrrole matrix.384-387... 

An elegant synthesis of a monomeric porphyrin intermediate bearing one pyridyl and three glycosyl units, precursor of cationic dimers, has also been described (Scheme 18).153 Thus pyrrole, p-formylphenyl 2,3,4,6-tetra-<9-acetyl-/i-D-... 

Fig. 7. NMR of the hematoporphyrin model (A), its acetate (B), and the purified but methylated diporphyrin 7 (C). The two a H can be seen clearly at 6.1 ppm (adjacent to MeO) and at 7.6 ppm (adjacent to the ring-connecting ester). The a-H next to the methoxy group has a chemical shift very similar to that of the hydroxy porphyrin as verified by monomeric porphyrins.

Some of the key properties of a photosensitizer are high efficiency of singlet-oxygen generation, strong absorption in the red and particularly near-IR (660-800 nm), preferential affinity for tumor rather than healthy tissue, and rapid clearance from the body. Porphyrin and phthalocyanine dyes fit these criteria best, and much work has been done on these dye types [76], Photofrin, a hemato-porphyrin derivative, was the first photosensitizer to be approved for clinical use. It is a complex mixture of monomeric porphyrins (protoporphyrin, hematopor-phyrin (84), and hydroxyethylvinyldeuteroporphyrin) and oligomers of these porphyrins. 

The aforementioned representative examples of chirogenic supramolecular systems based on monomeric porphyrinoids clearly demonstrate their vital importance and wide applicability in various fields, while many aspects of the operating mechanisms and especially chiroptical properties have yet to be comprehensively investigated and well understood. However, in many cases just a monomeric porphyrin cannot function properly without the synergetic assistance of another porphyrin unit (or several porphyrins) that give rise to chirogenic supramolecular assemblies built upon the dimeric/oligomeric porphyrinoids, which will be discussed in subsequent sections. 

Chirogenic supramolecular systems based on dimeric/oligomeric porphyrinoids can also be classified according to their complexity and structural features. First, the assemblies consisting of two or more monomeric porphyrins fixed in an asymmetrical fashion by chiral bridges via supramolecular binding will be examined. 

Liu prepared a sandwich type coordination compound (103) from porphyrin and phthalocyanine with the assistance of a microwave. The resulted compounds showed good solubility in conventional organic solvents. The photoelectric conversion properties have been tested with a Gratzel type cell. The results revealed that the sandwich type compound showed better photo-electric conversion efficiency than the corresponding monomeric porphyrin or phthalocyanine precursors. The short-circuit photocurrent of the solar cell with this sandwich type compound as sensitizer, was, as high as 691.31 A cm-2, which was much better, than those of porphyrin or phthalocyanine monomers [100]. 

Fig. 7 Molecular structures of monomeric porphyrins used as semiconductor of OFETs...

Table 4 Performance of some monomeric porphyrin-based -type OFET devices (mEn means m x 10 )... 

In analogy with the results found for many Co and Fe monomeric porphyrins, RRDE experiments in deaerated solutions containing peroxide showed that Co2FTF 4—2, 1N-FI, in addition to analogue species with shorter and longer struts, were not active for peroxide reduction nor for peroxide decomposition. 

The meso-[tetrakis(benzo-15-crown-5)] porphyrin (73) and its metal (Mg(II), Ni(II), Cu(II), Zn(II), and Mn(III)) derivatives also dimerize in the presence of K" ", NH4+, and Ba +. It has been established that the photophysical and redox properties of these dimers are different from those of the monomeric porphyrin units <85IC3253>. 

These formation and dissociation equilibria can be used to manipulate the multi-porphyrin assemblies, as was demonstrated in the following experiments. A mixture of coordination polymer 52 and dimer 53 was dissociated by the addition of MeOH followed by simple evaporation. The GPC elution curves before (Fig. 24a) and after (Fig. 24b) the evaporation were completely different. The new peaks (1, 2, and 3) in Fig. 24b were separated and identified as oligomers terminated by monomeric porphyrins 54 (n = 0, 1, 2, and 3). This result demonstrates the possibility of controlling the length of the porphyrin array and terminating it with other imidazolyl zinc porphyrins... 

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