Controlling the Charge State of a Single Redox Molecular Switch

Scanning tunneling microscopy and dynamic force microscopy in the noncontact mode are used in combination to investigate the reversible switching between two stable states of a copper complex adsorbed on a NaCl bilayer grown on Cu(111). The molecular conformation in these two states is deduced from scanning tunneling microscopy imaging, while their charge is characterized by the direct measurement of the tip-molecule electrostatic force. These measurements demonstrate that the molecular bistability is achieved through a charge-induced rearrangement of the coordination sphere of the metal complex, qualifying this system as a new electromechanical single-molecular switch.

Figure 1

Bis-dibenzoylmethanato-copper ${\mathrm{C}}_{30}{\mathrm{H}}_{22}{\mathrm{O}}_4\mathrm{Cu}$ (a) square-planar complex $[\mathrm{Cu}(\mathrm{II})(dbm{)}_2{]}^0$ and (b) tetrahedral complex $[\mathrm{Cu}(\mathrm{I})(dbm{)}_2{]}^{-1}$. STM images of $\mathrm{Cu}(dbm{)}_2$ on a bilayer of NaCl on Cu(111). (c) Tunneling current $It=1\mathrm{pA}$; bias voltage $Vt=1\mathrm{V}$. (d) $It=0.7\mathrm{pA}$; $Vt=1.2\mathrm{V}$.

Figure 2

(a)–(e) Sequence of STM images showing the reversible switching of two molecules. The position of the tip above the molecule during the application of the bias voltage is marked by a black circle. (f) 3D representation of image (b). Image size: $6.9\mathrm{nm}\times 5.5\mathrm{nm}$. $It=0.4\mathrm{pA}$; $Vt=100\mathrm{mV}$.

Figure 3

$I(V)$ and $dI/dV$ spectroscopy curves obtained on the SP (black lines) and the Td (red line) species. The numbers that appear on the $I\mathrm{\text{−}}V$ curves refer to the numbering of the STM images shown below the graph (size: $3.5\mathrm{nm}\times 3.5\mathrm{nm}$). They are located at the corresponding imaging bias voltage. 1, 2, and 6 (3, 4, and 5) correspond to the SP (Td) species.

Figure 4

STM images before (a) and after (b) the switching of a molecule. (c) The difference between (b) and (a).

Figure 5

(a) Frequency shift $\Delta f$ as a function of $V$ above NaCl, a SP, and a Td molecule. $\Delta z=100\mathrm{pm}$. Imaging conditions: Oscillation amplitude $A=240\mathrm{pm}$, $It=0.7\mathrm{pA}$, and $Vt=400\mathrm{mV}$. Each measurement is fitted with a parabola (red lines). (b) $V^{*}(\Delta z)$ above NaCl, the SP, and the Td molecule.

Figure 6

(a) Frequency shift at $V=V_{\mathrm{NaCl}}^{*}=-0.4\mathrm{V}$ and (b) forces above NaCl, a SP, and a Td molecule as a function of $\Delta z$.