Porphyrin Supramolecules for Artificial Photosynthesis and Molecular Photonic/Electronic Materials

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Abstract
Porphyrin supramolecules were constructed by ligand coordination to central metal ions, hydrogen bonding, or chelate complexation to external metal ions. meso-N-Methyl-2-imidazolyl-substituted porphyrin afforded a special pair model and a light-harvesting antenna mimic by complementary coordination of imidazolyl to Zn and Mg, respectively. Characteristic splits of the Soret band in absorption spectra and large upfield shifts of imidazolyl and pyrrolic β protons in the 1H NMR spectra suggested the close proximity of two porphyrin π–π planes. Such upfield shifts in the 1H NMR spectra were also observed for coordination organization of 2,5-dihydroxyphenyl-substituted porphyrinatoMg. As an example of organization by hydrogen bonding, the supramolecular assembly between imidazolyl substituents afforded stacks of porphyrins with antenna function in solution and gave rise to the formation of stable antenna liposome without any lipid components such as lecitin. Mono- and bis(8-hydroxy-5-quinolyl) substituted porphyrins were coordinated with Ga(III) to afford tris(oxinato) chelate and poly(oxinato) chelate, respectively. The fluorescence intensity of each Ga complex was increased significantly compared with that of each starting monomer. At the same time, efficient excitation energy transfer among three porphyrins has been observed to realize the light-harvesting function. Novel bipyridylene-bridged bisporphyrin was synthesized by a nickel(0)-mediated homocoupling reaction of bromopyridyl-porphyrinatoZn. Spatial geometries of two porphyrins were regulated by reversible complexation of the bipyridyl part with PdCl2. Bis(imidazolylporphyrinatoZn), which was directly linked at meso-position, afforded a giant linear multi-porphyrin array by successive links of complementary coordination of imidazolyl to zinc having molecular weight of 105 at a distribution maximum corresponding to 80 bisporphyrin units and at the molecular length of 110 nm. On the other hand, the polyporphyrin can be dissociated to bisporphyrin unit and scrambled with monomeric imidazolylporphyrinatoZn in the presence of MeOH or pyridine to produce oligomers terminated by the monomeric porphyrin. This reorganization process could apply to the systems generating efficient photocurrents and large third-order optical nonlinearity.

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