Experimental investigations of photoinduced charge transport in synthetic DNA
capped hairpins possessing electron acceptor and donor stilbene chromophores at
either end have established the mechanism, dynamics, and efficiency of charge
transport in DNA. The mechanism for charge transport in repeating A-T base pairs
(A-tracts) was found to change from single-step superexchange at short distances
to multistep incoherent hole hopping at longer distances. The rate constants for
base-to-base hole hopping in longer A- and G-tract sequences are 1.2
× 109 s–1 and 4.3 × 109 s–1,
respectively, considerably slower than the rate constants associated with
molecular wires. Even slower rate constants are observed for alternating or
random base sequences such as those encountered in natural DNA. The efficiency
of charge separation in capped hairpins with A-tract sequences is also low as a
consequence of the competition of hole hopping with charge recombination.
Significantly higher efficiencies for charge separation are possible using
diblock purine base sequences consisting of two or three adenines followed by a
larger number of guanines. The short A-block serves as a molecular rectifier,
slowing down charge recombination. More efficient charge separation can also be
achieved using non-natural bases or by using the triplet acceptor anthraquinone
for hole injection.
Charge-Separation and Charge-Recombination Rate Constants in a Donor–Acceptor Buckybowl-Based Supramolecular Complex: Multistate and Solvent Effects
