Porphyrin self-assembled systems

A four-component self-assembling system was described by Kuroda (46). Two rhodium porphyrins are coordinated by the terminal pyridine groups of an extended ligand constructed from a tartrate derivative. 

The energetics of peptide-porphyrin interactions and peptide ligand-metal binding have also been observed in another self-assembly system constructed by Huffman et al. (125). Using monomeric helices binding to iron(III) coproporphyrin I, a fourfold symmetric tetracarboxylate porphyrin, these authors demonstrate a correlation between the hydropho-bicity of the peptide and the affinity for heme as well as the reduction potential of the encapsulated ferric ion, as shown in Fig. 12. These data clearly demonstrate that heme macrocycle-peptide hydrophobic interactions are important for both the stability of ferric heme proteins and the resultant electrochemistry. 

After examining the aggregates consisting of achiral/racemic porphyrins, it is reasonable to discuss the self-assembled systems based on chiral por-phyrinoid. 

While supramolecular chemistry of porphyrins has been extensively studied over the last few decades, related study of phthalocyanines is still in its infancy. Despite the great potential of phthalocyanine-based supramolecular arrays in various applications as mentioned earlier, only a handful systems have been reported so far. Self-assembled systems held by hydrogen bonding, donor-acceptor interactions, and host-guest interactions are still very rare. There is certainly much room for further investigation in the chemistry of this important class of compounds, particularly on their structural and functional aspects. We hope this article can provide the grounding for further studies. 

The covalently linked or self-assembled systems constructed from free-base porphyrins or metalloposphyrins and various quenchers and related studies of excited-state energy-transfer and electron-transfer processes in such systems are a widely studied topic of contemporary supramolecular chemistry. There are two major reasons for the exceptional interest in the topic a challenge to synthesize artificial systems for harvesting solar energy of... 

In the case of self-assembling systems, the porphyrin parts and the recognition site of the system may be designed separately making consideration of assembly-component variation possible. Some typical assembly modes which have appeared in artificial poiphyrin assemblies are shown in Figure 17. 

Figure 21. The quaternary self-assembling system composed of two kinds of porphyrins and the tartrate linker.

A significant research effort has been devoted to the design and application of various supramolecular self-assembled systems in photoelectrochemical solar cells. Fullerenes, fullerene derivatives, and carbon nanotubes are typically used as electron acceptor components of such systems. Porphyrins, phthalocyanines, ruthenium complexes, conjugated oligomers, and polymers are applied as electron donor counterparts. 

Figure 20 Redox gradient in the hybrid self-assembled system composed of fullerene derivative, positively charged porphyrin, negatively charged porphyrin and ferrocene with positively charged pyridinium anchor. (Reproduced from Ref. 102. Royal Society of Chemistry, 2004.)...

By peripheral functionalization, we refer to those porphyrins whose structures have been modified at the pyrrole )8-position or at the phenyl rings in tetraphenylporphyrins, with the aim of incorporating them into self-assembled systems. The examples presented are not meant to be an exhanstive list of peripherally angmented porphyrins... 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

Shinkai (65) exploited the boronic acid-diol motif mentioned above to self-assemble a polymer based on pyridine-magnesium porphyrin interactions. In this case, the evidence for polymerization comes from lightscattering experiments in dilute solution which yielded an average molecular weight of 109 g mol-1 for this system. 

Most helicates have linear axes, though a few helicates with circular axes are known - indeed the chiral (D4) molecular squares formed from Zn2+ and 2,5 -bis(2,2 -bipyridin- 6 -yl)pyrazine, 22, may be regarded as circular helicates (450). The formation of circular or linear forms seems to depend on balances between kinetic and thermodynamic control iron(II)-poly-2,2/-diimine systems with their substitutionally-inert metal centers provide useful systems for disentangling thermodynamic and kinetic contributions. The mechanism of formation of circular helicates of this type is believed to entail a kinetically favored triple helicate intermediate (484). Self-assembly of chiral-twisted iron(III)-porphyrin dimers into extended polynuclear species takes place through the intermediacy... 

---