Research ArticlePHYSICS

Electric field–catalyzed single-molecule Diels-Alder reaction dynamics

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Science Advances  20 Jan 2021:
Vol. 7, no. 4, eabf0689
DOI: 10.1126/sciadv.abf0689
  • Fig. 1 Characterization of single-molecule devices.

    (A) Schematic diagram of the single-molecule electrical monitoring platform. (B) Demonstration of a single-molecule connection. During the interaction between single-molecule maleimide and fluorescein-substituted furan, 5000 photos taken by a superresolution fluorescence microscopy with 50-ms exposure are reconstructed to obtain a single-molecule resolution photograph and an enlarged image without the background. (C) Synchronous monitoring of current and fluorescent signals during the 5-s reaction. 10−5 M fluorescein-substituted furan in an ethanol/toluene (v/v: 2/3) solution was added to the reaction cell at 298 K and 300 mV. a.u., arbitrary units.

  • Fig. 2 Effect of an electric field on the Diels-Alder reaction.

    (A) Bias voltage–dependent experiments under 100 to 600 mV at 393 K. The right side is the zoom-in picture of the concerted reaction process. The left side is the statistical histogram of the highest conductivity state [zwitterionic intermediate (ZI)] under 300 to 600 mV. (B) Statistical histograms and reaction mechanism under 100 mV and the corresponding attribution of the six conductance states obtained from Gaussian fittings of I-t measurements. (C) Transmission spectra of six species, where the dominated transmission orbitals (p-HOMOs) are displayed. (D) Quantitative analysis of the EEF effect, showing that the calculated Gibbs free energies of each species (left) and transition states (right) change with the given electric field strength. This indicates that the strong EEF can overcome the huge disadvantage of the stepwise mechanism. (E) Gibbs free energies for the concerted and stepwise pathways at −2.57 V/nm.

  • Fig. 3 Effects of temperature and polarity on the Diels-Alder reaction.

    (A to E) Typical I-t curves and corresponding enlarged images and statistical histograms at 193 K (A), 293 K (B), 343 K (C), 393 K (D), and 443 K (E). (F to K) I-t curves and corresponding enlarged images and statistical histograms measured in the solutions with 0% (F), 20% (G), 40% (H), 60% (I), 80% (J), and 100% (K) toluene in dimethyl sulfoxide (DMSO) at 298 K and 300 mV.

  • Fig. 4 Thermodynamic and kinetic properties of the single-molecule Diels-Alder reaction.

    (A) Time sequence of the transitions among different species during the Diels-Alder reaction. (B) Plots of time intervals of each conductance state and the lifetimes of each state obtained from single-exponential fittings. (C) Plots of the thermodynamic parameters (lnK versus 1000/T) deduced from temperature-dependent measurements. ΔH of the two concerted pathways was derived from Van’t Hoff equation. (D) Plots of kinetic parameters (lnk versus 1000/T). The activation energies of the forward and reverse reactions of the two concerted pathways were obtained by Arrhenius equation. (E) Dependence of the reaction rate (lnk) of the concerted pathway on the gradient ratio of toluene at 298, 318, 338, and 358 K. (F) Dependence of the product yield (PS/CT) of the concerted pathway on the gradient ratio of toluene at 298, 318, 338, and 358 K. (G) The approximately linear relationship of the equilibrium of ZI to other species (lnK) with the applied bias voltage and the exponential correlation between its lifetime and bias voltage at 393 K.

Supplementary Materials

  • Supplementary Materials

    Electric field–catalyzed single-molecule Diels-Alder reaction dynamics

    Chen Yang, Zitong Liu, Yanwei Li, Shuyao Zhou, Chenxi Lu, Yilin Guo, Melissa Ramirez, Qingzhu Zhang, Yu Li, Zhirong Liu, K. N. Houk, Deqing Zhang*, Xuefeng Guo

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