Photoinduced Electron Transfer and Hole Migration in Nanosized Helical Aromatic Oligoamide Foldamers

A series of photoactive triads have been synthesized and investigated in order to elucidate photoinduced electron transfer and hole migration mechanism across nanosized, rigid helical foldamers. The triads are comprised of a central helical oligoamide foldamer bridge with 9, 14, 18, 19, or 34 8-amino-2-quinolinecarboxylic acid repeat units, and of two chromophores, an N -terminal oligo(para-phenylenevinylene) electron donor and a C-terminal perylene bisimide electron acceptor. Time-resolved fluorescence and transient absorption spectroscopic studies showed that, following photo-excitation of the electron acceptor, fast electron transfer occurs initially from the oligoquinoline bridge to the acceptor chromophore on the picosecond time scale. The oligo(para-phenylenevinylene) electron donor is oxidized after a time delay during which the hole migrates across the foldamer from the acceptor to the donor. The charge separated state that is finally generated was found to be remarkably long-lived (>80 mu s). While the initial charge injection rate is largely invariant for all foldamer lengths (ca. 60 ps), the subsequent hole transfer to the donor varies from 1 X 10(9) s(-1) for the longest sequence to 17 X 10(9) s(-1) for the shortest. In all cases, charge transfer is very fast considering the foldamer length. Detailed analysis of the process in different media and at varying temperatures is consistent with a hopping mechanism of hole transport through the foldamer with individual hops occurring On the subpicosecond time scale (k(ET) = 2.5 X 10(12) s(-1) in CH2Cl2). This work demonstrates :the possibility of fast long-range hole transfer over 300 angstrom. (through bonds) across a synthetic modular bridge, an achievement that had been, previously observed principally with DNA structures.