Efficient synthesis of light-harvesting arrays composed of eight porphyrins and one phthalocyanine

Effective light-harvesting arrays require multiple photoactive:energy donors that funnel energy to an energy acceptor. Porphyrins and phthalocyanines are attractive components for light-harvesting arrays due to their strong absorption in the blue and red regions, respectively, and because energy transfer can occur from porphyrin to phthalocyanine regardless of their respective metalation states. Star-shaped light-harvesting arrays comprised of eight peripheral porphyrins and one core phthalocyanine have been prepared by a streamlined synthesis involving minimal reliance on protecting groups, a high degree of convergence, and facile chromatographic purification. The synthesis involves three distinct stages of complementary chemistries (porphyrin formation, Pd-mediated porphyrin dimer formation, phthalocyanine formation). Statistical reaction of p-iodobenzaldehyde, a phthalonitrile-linked benzaldehyde, and 5-mesityldipyrromethane afforded the desired trans-iodo/phthalonitrile-substituted porphyrin, which underwent Pd-mediated coupling with a monoethynyl porphyrin to give the porphyrin dimer bearing a phthalonitrile unit. Reaction of the dimer in 1-pentanol in the presence of MgCl2 and DBU for 48 h at 145 degrees C afforded the all-magnesium (porphyrin)(8)-phthalocyanine nonamer (MgP)(8)MgPc in 5.0% yield. The same reaction with lithium pentoxide in 1-pentanol for 2 h at 145 OC gave the all-free base nonamer (H2P)(8)H(2)Pe in 34% yield. The all-zinc nonamer (ZnP)(8)ZnPc was prepared by addition of zinc acetate at the end of the reaction. Similar treatment of a monomeric porphyrin-phthalonitrile afforded the pentameric (ZnP)(4)ZnPc in 58% yield. The (MgP)(8)MgPe was also obtained by magnesium insertion of(H2P)(8)H2Pc; The three nonamers were readily purified and are soluble in solvents such as toluene, THF, and CH2Cl2. Each nonamer absorbs strongly across the solar spectrum and exhibits efficient energy transfer from the porphyrins to the phthalocyanine.