posted on 2016-09-21, 00:00authored byXuesong Li, Nagula Markandeya, Gediminas Jonusauskas, Nathan
D. McClenaghan, Victor Maurizot, Sergey A. Denisov, Ivan Huc
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 bis-imide
electron acceptor. Time-resolved fluorescence and transient absorption
spectroscopic studies showed that, following photoexcitation 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 μs). 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 × 109 s–1 for the longest sequence to 17 × 109 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 helix, with individual hops
occurring on the subpicosecond time scale (kET = 2.5 × 1012 s–1 in CH2Cl2). This work demonstrates the possibility of
fast long-range hole transfer over 300 Å (through bonds) across
a synthetic modular bridge, an achievement that had been previously
observed principally with DNA structures.