Exponential Distance Dependence of Photoinitiated Stepwise Electron Transfer in Donor–Bridge–Acceptor Molecules: Implications for Wirelike Behavior

Donor–bridge–acceptor (D–B–A) systems in which a 3,5-dimethyl-4-(9-anthracenyl)­julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5-bis­(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN_<i>n</i>) bridges (<i>n</i> = 1–3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ^+•-An^–•, and An^–• acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ^+•-An^–•–FN_<i>n</i>–NI → DMJ^+•-An–FN_<i>n</i>^–•–NI → DMJ^+•-An–FN_<i>n</i>–NI^–•. The charge-shift reaction from An^–• to NI^–• exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 Å^–1, which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN_<i>n</i>^–• radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D^+•–B^–•–A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D–B–A molecule

Conformationally Gated Charge Transfer in DNA Three-Way Junctions

Molecular structures that direct charge transport in two or three dimensions possess some of the essential functionality of electrical switches and gates. We use theory, modeling, and simulation to explore the conformational dynamics of DNA three-way junctions (TWJs) that may control the flow of charge through these structures. Molecular dynamics simulations and quantum calculations indicate that DNA TWJs undergo dynamic interconversion among “well stacked” conformations on the time scale of nanoseconds, a feature that makes the junctions very different from linear DNA duplexes. The studies further indicate that this conformational gating would control charge flow through these TWJs, distinguishing them from conventional (larger size scale) gated devices. Simulations also find that structures with polyethylene glycol linking groups (“extenders”) lock conformations that favor CT for 25 ns or more. The simulations explain the kinetics observed experimentally in TWJs and rationalize their transport properties compared with double-stranded DNA

Charge Transport across DNA-Based Three-Way Junctions

DNA-based molecular electronics will require charges to be transported from one site within a 2D or 3D architecture to another. While this has been shown previously in linear, π-stacked DNA sequences, the dynamics and efficiency of charge transport across DNA three-way junction (3WJ) have yet to be determined. Here, we present an investigation of hole transport and trapping across a DNA-based three-way junction systems by a combination of femtosecond transient absorption spectroscopy and molecular dynamics simulations. Hole transport across the junction is proposed to be gated by conformational fluctuations in the ground state which bring the transiently populated hole carrier nucleobases into better aligned geometries on the nanosecond time scale, thus modulating the π–π electronic coupling along the base pair sequence