title: Reversible bridge-mediated excited-state symmetry breaking in stilbene-linked DNA dumbbells.
abstract: The excited-state behavior of synthetic DNA dumbbells possessing stilbenedicarboxamide (Sa) linkers separated by short A-tracts or alternating A-T base-pair sequences has been investigated by means of fluorescence and transient absorption spectroscopy. Electronic excitation of the Sa chromophores results in conversion of a locally excited state to a charge-separated state in which one Sa is reduced and the other is oxidized. This symmetry-breaking process occurs exclusively via a multistep mechanism-hole injection followed by hole transport and hole trapping-even at short distances. Rate constants for charge separation are strongly distance-dependent at short distances but become less so at longer distances. Disruption of the A-tract by inversion of a single A-T base pair results in a pronounced decrease in both the rate constant and efficiency of charge separation. Hole trapping by Sa is highly reversible, resulting in rapid charge recombination that occurs via the reverse of the charge separation process: hole detrapping, hole transport, and charge return to regenerate the locally excited Sa singlet state. These results differ in several significant respects from those previously reported for guanine or stilbenediether as hole traps. Neither charge separation nor charge recombination occur via a single-step superexchange mechanism, and hole trapping is slower and detrapping faster when Sa serves as the electron donor. Both the occurrence of symmetry breaking and reversible hole trapping by a shallow trap in a DNA-based system are without precedent.
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA. fdl@northwestern.edu
authors.forename: Frederick D
authors.initials: FD
authors.lastname: Lewis
authors.forename: Pierre
authors.initials: P
authors.lastname: Daublain
authors.forename: Ligang
authors.initials: L
authors.lastname: Zhang
authors.forename: Boiko
authors.initials: B
authors.lastname: Cohen
authors.forename: Josh
authors.initials: J
authors.lastname: Vura-Weis
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski
authors.forename: Vladimir
authors.initials: V
authors.lastname: Shafirovich
authors.forename: Qiang
authors.initials: Q
authors.lastname: Wang
authors.forename: Milen
authors.initials: M
authors.lastname: Raytchev
authors.forename: Torsten
authors.initials: T
authors.lastname: Fiebig
mesh_terms: D004247:DNA / Q000737:chemistry*; D007700:Kinetics; D009690:Nucleic Acid Conformation; D013267:Stilbenes / Q000737:chemistry*
chemical_list: D013267:Stilbenes; D004247:DNA