Self-Assembly of Supramolecular Light-Harvesting Arrays from Covalent Multi-Chromophore Perylene-3,4:9,10-bis(dicarboximide) Building Blocks

We report on two multi-chromophore building blocks that self-assemble in solution and on surfaces into supramolecular light-harvesting arrays. Each building block is based on perylene-3,4:9,10-bis(dicarboximide) (PDI) chromophores. In one building block, N-phenyl PDI chromophores are attached at their para positions to both nitrogens and the 3 and 6 carbons of pyromellitimide to form a cross-shaped molecule (PI-PDI(4)). In the second building block, N-phenyl PDI chromophores are attached at their para positions to both nitrogens and the 1 and 7 carbons of a fifth PDI to produce a saddle-shaped molecule (PDI(5)). These molecules self-assemble into partially ordered dimeric structures (PI-PDI(4))(2) and (PDI(5))(2) in toluene and 2-methyltetrahydrofuran solutions with the PDI molecules approximately parallel to one another primarily due to pi-pi interactions between adjacent PDI chromophores. On hydrophobic surfaces, PDI(5) grows into rod-shaped nanostructures of average length 130 nm as revealed by atomic force microscopy. Photoexcitation of these supramolecular dimers in solution gives direct evidence of strong pi-pi interactions between the excited PDI chromophore and other PDI molecules nearby based on the observed formation of an excimer-like state in <130 fs with a lifetime of about 20 ns. Multiple photoexcitations of the supramolecular dimers lead to fast singlet-singlet annihilation of the excimer-like state, which occurs with exciton hopping times of about 5 ps, which are comparable to those observed in photosynthetic light-harvesting proteins from green plants.