title: Crossover from superexchange to hopping as the mechanism for photoinduced charge transfer in DNA hairpin conjugates.
abstract: The mechanism and dynamics of photoinduced charge separation and charge recombination have been investigated in synthetic DNA hairpins possessing donor and acceptor stilbenes separated by one to seven A:T base pairs. The application of femtosecond broadband pump-probe spectroscopy, nanosecond transient absorption spectroscopy, and picosecond fluorescence decay measurements permits detailed analysis of the formation and decay of the stilbene acceptor singlet state and of the charge-separated intermediates. When the donor and acceptor are separated by a single A:T base pair, charge separation occurs via a single-step superexchange mechanism. However, when the donor and acceptor are separated by two or more A:T base pairs, charge separation occurs via a multistep process consisting of hole injection, hole transport, and hole trapping. In such cases, hole arrival at the electron donor is slower than hole injection into the bridging A-tract. Rate constants for charge separation (hole arrival) and charge recombination are dependent upon the donor-acceptor distance; however, the rate constant for hole injection is independent of the donor-acceptor distance. The observation of crossover from a superexchange to a hopping mechanism provides a "missing link" in the analysis of DNA electron transfer and requires reevaluation of the existing literature for photoinduced electron transfer in DNA.
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60201, USA. lewis@chem.northwestern.edu
authors.forename: Frederick D
authors.initials: FD
authors.lastname: Lewis
authors.forename: Huihe
authors.initials: H
authors.lastname: Zhu
authors.forename: Pierre
authors.initials: P
authors.lastname: Daublain
authors.forename: Torsten
authors.initials: T
authors.lastname: Fiebig
authors.forename: Milen
authors.initials: M
authors.lastname: Raytchev
authors.forename: Qiang
authors.initials: Q
authors.lastname: Wang
authors.forename: Vladimir
authors.initials: V
authors.lastname: Shafirovich
mesh_terms: D000225:Adenine / Q000737:chemistry; D004247:DNA / Q000737:chemistry*; D009690:Nucleic Acid Conformation; D010777:Photochemistry; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet; D013936:Thymidine / Q000737:chemistry
chemical_list: D004247:DNA; D000225:Adenine; D013936:Thymidine