Synthesis and properties of star-shaped multiporphyrin-phthalocyanine light-harvesting arrays

Light-harvesting arrays containing four porphyrins covalently linked to a p hthalocyanine in a starshaped architecture have been synthesized. Cyclotetr amerization of an ethyne-linked porphyrin-phthalonitrile in l-pentanol in t he 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 rea ction using Zn(OAc)(2). 2H(2)O afforded the all-zinc porphyrin-phthalocyani ne pentad in 15% yield. Arrays with different metals (free base, Mg, Zn) in the porphyrin and phthalocyanine macrocycles have been prepared by selecti ve demetalation and metalation steps. This approach provides rapid and conv ergent access to multiporphyrin-phthalocyanine arrays in diverse metalation states. The arrays are reasonably soluble in organic solvents such as tolu ene, THF, and CH2Cl2. The arrays exhibit strong absorption in the blue and red regions. Time-resolved and static optical measurements indicate that in tramolecular singlet-excited-state energy transfer from the porphyrin to th e phthalocyanine moiety is extremely rapid (picoseconds) and efficient. Gro und-state electronic communication among the porphyrins is indicated by rap id hole/electron hopping among the metalloporphyrins in the arrays as detec ted by EPR measurements on the singly oxidized pentads. These physical meas urements indicate that the porphyrin-phthalocyanine pentads possess favorab le characteristics for light harvesting and other photonics applications.