Inelastic electron tunneling spectroscopy of difurylethene-based photochromic single-molecule junctions

Abstract

Diarylethene-derived molecules alter their electronic structure upon transformation between the open and closed forms of the diarylethene core, when exposed to ultraviolet (UV) or visible light. This transformation results in a significant variation of electrical conductance and vibrational properties of corresponding molecular junctions. We report here a combined experimental and theoretical analysis of charge transport through diarylethene-derived single-molecule devices, which are created using the mechanically controlled break-junction technique. Inelastic electron tunneling (IET) spectroscopy measurements performed at 4.2 K are compared with first-principles calculations in the two distinct forms of diarylethenes connected to gold electrodes. The combined approach clearly demonstrates that the IET spectra of single-molecule junctions show specific vibrational features that can be used to identify different isomeric molecular states by transport experiments.

Keywords: inelastic electron tunneling spectroscopy; molecular junction; photochromic; single molecule.

Figure 1

(a) Structures of the open and closed forms of difurylethene-thio-methyl (C5F-ThM) molecules, which switch states under the illumination with UV and visible light. Shown at the bottom is a scanning electron microscopy image of a MCBJ device. (b) Variation of UV–visible absorption spectra under UV light irradiation (wavelength of 313 nm with an intensity of ca. 0.15 mW/cm⁻²) as a function of illumination time. The black curve corresponds to the open form, the red one to the closed form. (c) Conductance traces recorded while breaking C5F-ThM molecular junctions, containing the open form (black) and the closed form (red). Curves are shifted along the displacement axis for clarity. (d) Conductance histograms constructed from around 1000 curves of breaking and forming processes. The lowest conductances for the open and closed forms are (1.4 ± 1.0) × 10⁻⁷ G₀ and (8.3 ± 4.5) × 10⁻⁷ G₀, respectively. Histogram curves of both forms (black and red lines) are displaced along the counts axis for better visibility. The inset shows the corresponding conductance histograms after subtracting the exponential backgrounds indicated by green dashed lines in the main panel.

Figure 2

(a) The geometric structure used in the DFT calculations is displayed for the open and closed forms of Au-C5F-ThM-Au junctions. (b) The computed transmission curves for the open (black) and the closed form (red). (c) The wavefunction of the dominant transmission eigenchannel for electrons coming in from the left side for the open (left) and the closed form (right). Red and turquois indicate different signs of the wavefunction.

Figure 3

Experimentally measured IET spectra (gray) are displayed for (a) the open and (b) the closed form. For each form twelve curves were averaged (solid black line) and the average was symmetrized (pink-dotted line), using the function y = [f(x) − f(−x)]/2. (c) Averaged and symmetrized IET spectra for the open (black) and the closed (red) form, displayed only in the positive bias range. The IET spectrum of the closed form is magnified by a factor of 2. (d) Theoretically computed IET spectra for the open (upper panel, black) and closed (lower panel, red) form. Each mode, indicated with Roman numerals, is listed in Table 1.