Electrochemistry and STM Observation of a Copper Complex-Based Molecular Rotor on Gold

Abstract

Molecular rotors demonstrate that switching and mechanical work can be achieved at the nanoscale. The development of a system whereby the switching behavior is controllable at room temperature is one of the ultimate goals in this research field. Further research in this area could lead to the development of methods of detecting or visualizing the motion of individual molecules, which would greatly aid the functionality of future single molecular machines and memory devices. Our group has recently developed a pyridylpyrimidine-ligated copper(I) complex system that acts as a molecular switch through the redox-driven rotation of a coordinated pyrimidine ring (Figure 1).1 Asymmetric substitution at the 4-position of the pyrimidine ring leads to the two switching isomers, in which different degrees of steric congestion result in distinct oxidation potentials. The rotation of this system can be driven by temperature and light, and furthermore, the motion can be detected as various kinds of outputs; for example, changes of rest potential, magnetic and optical properties. Note that it is the correlation between redox potential and structure that enables these functionalities within a simple and small molecular design. In this research, a new molecular switch, a copper complex (1•PF6 ), was synthesized. Cyclic voltammograms of 1•PF6 in solution taken at room temperature had two redox couples. Based on the electrochemistry of Cu complexes, these signals were assigned to the redox reactions of the o- and i-isomer from the negative to the positive potential. This complex was immobilized on the Au surface by immersing an Au-coated mica substrate in a CH2Cl2 solution of 1•PF6 ; the modified substrate is designated as SAM-1. Cyclic voltammetry of SAM-1 at room temperature shows two reversible copper(II/I) redox waves at potentials similar to those of 1•PF6 in solution. From the deconvolution of peak areas, the ratio of the two isomers functionalized on the Au surface was calculated. Different scan rates measurements of oxidation waves suggested that the ratio of the isomers is constant and does not deviate from the equilibrium molar ratio (i : o = 8 : 2). We next carried out scanning tunneling microscopy to observe the isomerization behavior of individual molecules. A mixed self-assembled monolayer (SAM) of 1•PF6 and 1-hexanethiol (mSAM-1) was used for STM observation at room temperature. The brightness of spots are measured as the apparent height. It was found that apparent heights of the complex molecules varied with time as they switched between the two states. Aggregated apparent height data show two Gaussian components. Least squares fitting of the two components indicates that 1 has two conformations which have an area ratio of 8 : 2. This ratio seems consistent with the Cu(I) equilibrium ratio of the i- and o-isomers (i : o = 8 : 2), which suggests a successful observation of the switching behavior of individual molecules. Reference M. Nishikawa, S. Kume, H. Nishihara, Phys. Chem. Chem. Phys. 2013, 15, 10549-10565. Figure 1