Inverted Conformation Stability of a Motor Molecule on a Metal Surface

Abstract

Molecular motors have been intensely studied in solution, but less commonly on solid surfaces that offer fixed points of reference for their motion and allow high-resolution single-molecule imaging by scanning probe microscopy. Surface adsorption of molecules can also alter the potential energy surface and consequently preferred intramolecular conformations, but it is unknown how this affects motor molecules. Here, we show how the different conformations of motor molecules are modified by surface adsorption using a combination of scanning tunneling microscopy and density functional theory. These results demonstrate how the contact of a motor molecule with a solid can affect the energetics of the molecular conformations.

Figure 1

(a) Chemical structure of the MM1. (b) STM image (8 pA, 400 mV) of motor molecules on a terrace of the Cu(111) surface. The bright lobes are individual MM1, while the dark spots are CO molecules. (c) Zoom-in STM image (10 pA, 200 mV) showing the three molecular conformations (named A, B, and C with their relative abundance indicated). Deposition was done at 5 K sample temperature.

Figure 2

Comparison of the calculated structures on the Cu(111) surface (at the left: two side views and one top view) with simulated and experimental STM images [all 1.8 nm × 1.8 nm in size with 9 pA/200 mV (A), 10 pA/100 mV (B) and 60 pA/−50 mV (C)] for all three molecular motor conformations (A, B, and C) as indicated. All structures and images show the (S) enantiomer (see Figure 3 below). The simulated STM images were artificially blurred (see Figure S1).

Figure 3

(a,b) Chemical structures of the two enantiomers (R) and (S) of MM1. (c–j) STM images [all 2.26 nm × 2.26 nm in size with 11 pA/300 mV (c,d), 10 pA/150 mV (e), 10 pA/100 mV (f) and 60 pA/−50 mV (g−j)] individual molecules in conformation A (c,d), B (e,f) and C (g,h) on the Cu(111) surface, which are assigned to the (R) (upper row) and (S) enantiomer (lower row) from their characteristic appearances. (i–j) Same STM images as in (g,h), but with a multicolor contrast to enhance the visibility of the chiral appearance with respect to the approximate symmetry axis of the molecular appearance (indicated by a dashed line).

Figure 4

(a) Relative abundance of the three conformations (A, B, and C) on the surface for different sample temperatures during molecular deposition, as determined from the analysis of many STM images. (b) STM image (30 pA, −50 mV) of different individual molecules and small molecular assemblies of the motor molecules on Cu(111). Molecules were deposited onto the sample at a maximum temperature of 273 K. (c) Calculated total energies (see also Figure S2) of the three conformations on the Cu(111) surface, plotted as the energy difference ΔE with respect to conformation C (ΔE > 0 indicates a less preferred configuration).

Figure 5

(a) Full 360° rotary cycle of the molecular motor. (b) Scheme of the molecular adsorption on the surface from the gas phase and the origin of the on-surface conformations A, B, and C (the important methyl group, see main text, is indicated by circles). Note that while all structures in (a,b) show the same enantiomer (S) for the sake of clarity, the same mechanisms are valid for the other enantiomer (R).