Ultrafast aggregate-to-aggregate energy transfer within self-assembled light-harvesting columns of zinc phthalocyanine tetrakis(perylenediimide)

Light harvesting in photosynthetic antenna proteins involves a series of highly efficient ultrafast energy transfers between spectroscopically different populations of chlorophylls. Several strategies have recently been employed to mimic this natural energy transfer process, including polymers, dendrimers, and oligomeric porphyrin arrays linked by covalent bonds or by self-assembly. In all of these systems, excitation energy transfer occurs from one molecule to another, while very few of them involve energy transfer from one very strongly interacting chromophore aggregate to another such aggregate. Here we report the synthesis and characterization of a covalent zinc phthalocyanine-2,3,9,10,16,17,23,24-octacarboxytetraimide in which all four imide nitrogen atoms are substituted with N-octyl-N′-(4-aminophenyl)-1,7(3′,5′di-tert-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (ZnPcIm₄-PDI₄). The individual molecules self-assemble into stacked heptamers in solution as evidenced by small-angle X-ray scattering and form long fibrous structures in the solid as evidenced by TEM. The ZnPcIm₄ and PDI molecules both stack in register with the same components in an adjacent covalent building block. Ultrafast energy transfer occurs with τ = 1.3 ps from the aggregated peripheral PDI chromophores to the core ZnPcIm₄ chromophore aggregate. Exciton hopping between the ZnPcIm₄ chromophores occurs with τ = 160 fs.