| Abstract
| - The excited-state behavior of synthetic DNA dumbbells possessing stilbenedicarboxamide (Sa) linkers separatedby short A-tracts or alternating A−T base-pair sequences has been investigated by means of fluorescenceand transient absorption spectroscopy. Electronic excitation of the Sa chromophores results in conversion ofa locally excited state to a charge-separated state in which one Sa is reduced and the other is oxidized. Thissymmetry-breaking process occurs exclusively via a multistep mechanismhole injection followed by holetransport and hole trappingeven 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 ofa single A−T base pair results in a pronounced decrease in both the rate constant and efficiency of chargeseparation. Hole trapping by Sa is highly reversible, resulting in rapid charge recombination that occurs viathe reverse of the charge separation process: hole detrapping, hole transport, and charge return to regeneratethe locally excited Sa singlet state. These results differ in several significant respects from those previouslyreported for guanine or stilbenediether as hole traps. Neither charge separation nor charge recombinationoccur via a single-step superexchange mechanism, and hole trapping is slower and detrapping faster when Saserves as the electron donor. Both the occurrence of symmetry breaking and reversible hole trapping by ashallow trap in a DNA-based system are without precedent.
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