Zinc porphyrin complexes self-assembly

Hunter (60) reported a self-assembled open polymer formed by a zinc porphyrin bearing one para-aniline substituent at the meso position. The ortho- and mela-analogs discussed above form closed dimers, but the geometry of the para-derivative precludes this, and polymerization is the only alternative (76, Fig. 31). Although the dilution experiments could be fitted to a non-cooperative polymerization model with a pairwise association constant (K = 190 M 1) practically identical to that found for simple aniline-zinc porphyrin complexes (K = 130 M 1), broadening of the 4H NMR spectrum at high concentrations is characteristic of oligomerization. 

A Thermodynamic Model Self-Assembly of Zinc Porphyrin Complexes... 

Self-assembled monolayers of an Os3(CO)s CN(CH2)3Si(OEt)3)(ft3-ri rf if-C6o) cluster on indium-tin oxide or gold surfaces have been shown to be electrochemically stable, being reducible to tetra-anionic species in their cyclic voltammograms. Further derivatization through tethering a zinc porphyrin complex to the cluster generated a high-performance photovoltaic cell with potential applications in artificial photosynthesis. ... 

The aniline-zinc porphyrin interaction has also been exploited to form dimers. Hunter (60) reported the dimerization of porphyrins functionalized at one meso position with ortho or meta aniline groups (47, 48, Fig. 15). Both compounds showed concentration-dependent H NMR spectra with large upfield shifts for the aniline protons. The dimerization constants are 160 and 1080 M-1 respectively for 47 and 48, and these values are an order of magnitude higher than the association constants of simple reference complexes (K — 10 and 130 M 1 respectively), which is indicative of cooperative self-assembly. The complexa-tion-induced changes in chemical shift were used to obtain three-dimensional structures of the dimers. 

Crossley (89) described the self-assembly of a spherical cage-like structure made up of two zinc porphyrin dimers bound to a tetramine ligand. The 2 1 complex is stable at 10-6m concentrations, but addition of excess of ligand causes dissociation of the capsule with formation of a 1 1 complex. 

Zinc(II) porphyrins form stable complexes with nitrogen donors. This complementary binding motif has been used in a number of variations for the self-assembly combinatorial construction of phosphine and phosphite ligand libraries. 

Figure 5.56 Schematic illustrating the self-assembled multilayers of sodium 3-mercaptoethane-sulfonate (layer 1 ), hexacationic homooxacalix[3]arene-fullerene (2 1) complex (layer 2 ), with either the anionic porphyrin polymer ( 3 ) or its zinc porphyrin analogue ( 4 ) forming the third layer. Reprinted with permission from A. Ikeda, T. Hatano, S. Shinkai, T. Akiyama and S. Yamada, /. Am. Chem. Soc., 123,4855 (2001). Copyright (2001) American Chemical Society...

A different way to introduce thiophene rings into a porphyrin is to attach them to the porphyrin core giving 162 and its zinc complex 162-Zn <2005CC1974>. Self-assembling at the solid-liquid interface was studied and some calculations performed. An interesting approach toward the chiral porphorynoids 163 and 164 was described <2001TL1%9>. 

Finally, a DEER study on models for molecular wires made from butadiyne-linked zinc porphyrin oligomers, end-labeled with nitroxide radicals, was performed by Lovett, Anderson, and coworkers [107]. Unlike in [104—106], one can control the conformations of these metalloporphyrin-based strucmres by self-assembly with multidentate amine ligands, which bend the rigid oligomeric structure. The experimentally found end-to-end distances in these complexes match the predictions from molecular dynamics calculations. This study thus presents a proof-of-principle that DEER spectroscopy is also well suited for understanding more complex supramolecular stmctures. 

The cage structure 6 is composed of two face-to-face porphyrins linked at both sides via six hydrogen bonds to two triaminopyridine entities. The distance between the cofacial porphyrins is estimated to be 10 A. The initial nieso 5,15-diuracil-substituted porphyrin has two rotameric forms, a syn and an anti, due to the relative orientations of the two uracil groups with respect to the porphyrin plane. In the self-assembling process, the syn rotamer yielded the major component, the cage structure, whereas the anti conformer yielded a zig-zag strand structure. As for the properties expected by such systems, metalation of one of the porphyrins allows for the creation of a self-assembled donor-acceptor system while complexation of the porphyrins with zinc(Il) allows for the cage cavity to be used as a... 

As we have just seen, electrostatic interactions can reinforce the formation of axial base complexes in metalloporphyrins, but other thermodynamically favored processes, such as chelation around a bidentate base like pyrazine, can be used. This approach has led Kuroda to the preparation of self-assembled dendrimer-type structures 86-89 represented in Figure 13.50 ". In these structures, the zinc porphyrin dimer formed by multihydrogen bonding of tetracar-boxylate derivatives of TPP is considered as a chelate. 

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