Research ArticleAPPLIED SCIENCES AND ENGINEERING

Powering rotary molecular motors with low-intensity near-infrared light

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Science Advances  28 Oct 2020:
Vol. 6, no. 44, eabb6165
DOI: 10.1126/sciadv.abb6165
  • Fig. 1 Concept and design of NIR light–sensitized molecular motor 1.

    (A) Concept, design, and potential energy diagram of NIR light–driven artificial rotary molecular motor. Two photons of NIR light excite a covalently bound sensitizer that transfers the energy to the motor core to initiate rotation. This process can be studied by ultrafast transient absorption (TA) and time-resolved photoluminescence (PL) spectroscopy. The formation of 1s and 1m following photoexcitation is expected to pass through two separate CIs as has been shown for a closely related motor (31). (B) Structure of stable and metastable isomers of motor 1 and their interconversion by light and heat leading to unidirectional rotation. (C) Absorption spectra (1.1 × 10−5 M, CHCl3, 20°C) of 2s (blue), AF-343 (red), 1s (green), and the mixture of 1s and 1m at the photostationary state (PSS) reached after irradiating 1s with a 455-nm LED for 5 min (magenta). The dashed curve shows a normalized PL spectrum of AF-343 using an excitation wavelength of 780 nm. arb.u., arbitrary units.

  • Fig. 2 Comparison of absorption spectra before and after NIR light irradiation.

    (A) Difference absorption spectra (ΔOD) of 1 (green), 2 (blue), and AF-343 (red) after irradiation at 800 nm for 30 min and keeping the sample in the dark for 10 s. The inset shows the dependence of ΔOD of 1 around 510 nm on the irradiation intensity. The black line shows the fit to a power law function, y = axn with n = 2.0 ± 0.5. (B) Time evolution of difference absorption spectra of a sample of 1 after irradiation at 800 nm for 30 min and keeping it in the dark between 10 s and 160 min. The magenta arrows depict the thermal recovery of 1s via THI from 1m. The inset shows the decrease of ΔOD around 510 nm over time together with an exponential fit (black line) with a lifetime, τ, of 43 ± 3 min. In both (A) and (B), the irradiation intensity was 0.3 W/cm2. ΔOD data for the insets were averaged in the 500- to 520-nm spectral window; the error bars refer to the SD. The molar concentration of all compounds was set as ~1.7 × 10−5 M with CHCl3 as the solvent.

  • Fig. 3 Excited-state quenching of the sensitizer domain in 1s.

    TA traces starting from 1s (green), 2s (blue), and AF-343 (red) at 620-nm probe wavelength under two-photon excitation at 800 nm. All transients are solvent-corrected (for details, see the Supplementary Materials). The excitation intensity of all compounds was set as 9 W/cm2. The gray curves represent fits to multiexponential functions, convoluted with the apparatus function (for the fitting parameters, see table S3). The black arrow indicates excited-state depletion upon attaching the sensitizer to the motor. The inset depicts the ΔOD dependence of AF-343 on the excitation intensity at a delay of 10 ps. The black line shows the fit to a power law function, y = axn with n = 1.9 ± 0.1. The molar concentration of all compounds was set to be similar to ~6 × 10−4 M with CHCl3 as the solvent.

  • Fig. 4 Ultrafast dynamics of the motor domain in 1s after RET, leading to formation of 1m.

    TA traces starting from 1s (green) and 2s (blue) at 510-nm probe wavelength under two-photon excitation at 800 nm (9 W/cm2 excitation intensity). All transients are solvent-corrected (for details, see the Supplementary Materials). The magenta arrow indicates the formation of the photochemically generated metastable isomer 1m. The inset shows the TA transient of 1 at early time with the contribution of AF-343 subtracted. The black curve is the fit to a function y=1etτ with τ = 0.9 ps. The gray curve represents the fit to biexponential function (for the fitting parameters, see table S5). The molar concentration of all compounds was set to be similar to ~6 × 10−4 M with CHCl3 as the solvent.

Supplementary Materials

  • Supplementary Materials

    Powering rotary molecular motors with low-intensity near-infrared light

    Lukas Pfeifer, Nong V. Hoang, Maximilian Scherübl, Maxim S. Pshenichnikov, Ben L. Feringa

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    • Sections 1 to 10
    • Figs. S1 to S30
    • Tables S1 to S6
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