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Synthesis and Properties of Star-Shaped Multiporphyrin−Phthalocyanine Light-Harvesting Arrays

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journal contribution
posted on 20.11.1999, 00:00 by Junzhong Li, James R. Diers, Jyoti Seth, Sung Ik Yang, David F. Bocian, Dewey Holten, Jonathan S. Lindsey
Light-harvesting arrays containing four porphyrins covalently linked to a phthalocyanine in a star-shaped architecture have been synthesized. Cyclotetramerization of an ethyne-linked porphyrin−phthalonitrile in 1-pentanol in the presence of MgCl2 and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) afforded the all-magnesium porphyrin−phthalocyanine pentad in 45% yield. Similar reaction using Zn(OAc)2·2H2O afforded the all-zinc porphyrin−phthalocyanine pentad in 15% yield. Arrays with different metals (free base, Mg, Zn) in the porphyrin and phthalocyanine macrocycles have been prepared by selective demetalation and metalation steps. This approach provides rapid and convergent access to multiporphyrin−phthalocyanine arrays in diverse metalation states. The arrays are reasonably soluble in organic solvents such as toluene, THF, and CH2Cl2. The arrays exhibit strong absorption in the blue and red regions. Time-resolved and static optical measurements indicate that intramolecular singlet-excited-state energy transfer from the porphyrin to the phthalocyanine moiety is extremely rapid (picoseconds) and efficient. Ground-state electronic communication among the porphyrins is indicated by rapid hole/electron hopping among the metalloporphyrins in the arrays as detected by EPR measurements on the singly oxidized pentads. These physical measurements indicate that the porphyrin−phthalocyanine pentads possess favorable characteristics for light harvesting and other photonics applications.