Study on Electronic Properties of N-Heterocyclic Carbene Metal Complexes at the Single Molecule Level

概要

This thesis describes the electronic properties of N-heterocyclic carbene (NHC) metal complexes, consisting of two chapters.

The first chapter is devoted to summarizing the background of molecular electronics and NHC, and describing the research objectives. Molecular electronics is a research field in which single molecules or small groups of molecules are designed to perform electronic functions.1 Initiated by Aviram’s proposal of molecular rectifier,2 a device that allows the current to flow in one direction only, single molecular electronic systems with various functions such as switching, transistor, and wire have been rapidly developed.3 Metal-containing single molecule devices are particularly attractive because of multiple redox states and spin states of metals. Several ligands have been reported to be used for metal-containing single molecules, e.g. bis-(terpyridine), porphyrin, and phenanthroline. The ligands mentioned above are nitrogen-based multidentate ligands where there is no empty site on the metal, so they are less reactive and have less structural variation. The weakly binding of nitrogen-based ligands to metals makes it difficult to form stable metal complexes only in monodentate coordination. To design stable new metal complexes with monodentate ligands, it is considered to select a structural design that can bind firmly to the metal and still have a vacant site in the metal.

N-heterocyclic carbenes (NHCs) have been recognized as privileged ligands that can bind firmly to metals due to their strong sigma donor properties. NHC metal complexes are commonly used for organometallic catalysts and some organic light-emitting diodes.4-5 Several applications of NHC metal complexes in the field of nanoscience, such as nanoparticles, self-assembled monolayers, and single molecule electronics, have been reported.6 Based on the NHC's strong binding properties to metals and its stability in monodentate fashion, the author designed a new metal-containing single molecule using NHC ligands. It is expected that the NHC single molecule could form a stable molecular junction, and the metal on the NHC could tune the molecular orbital in the molecule. This metal-containing single molecule design was accomplished by synthesizing a series of NHC metal complexes with two anchoring groups in the NHC core and metal in the center, and constructing metal-molecule-metal complex junctions, and measuring the conductance of single molecules (Figure 1). With the anchoring group attached to the para position of aryl, it is expected that the current passes through the backbone of the NHC molecule. The metal in the NHC can adjust the molecular orbitals of the junction, hence tune the conductance.

The second chapter is devoted to the synthesis and conductance properties of single-molecule NHC metal complexes. The author synthesized two NHC copper complexes with different anchoring groups, namely alkyne and methyl sulfide, to compare the junction stability during measurement. The author also synthesized three different NHC complexes with different metals (Cu, Ag, and Au) to elucidate the metal effect on the single molecule conductance. Lastly, the author synthesized three NHC copper complexes, NHC copper chloride with one and two ethynyl anchors, and NHC copper carbazolide complexes to investigate the junction conformation. The single-molecule conductance properties of these series of NHC metal complexes were measured by using scanning tunneling microscopy–break junction (STM-BJ) methods.

The stability of metal-molecule-metal junctions was investigated by comparing NHC copper chloride complexes with methyl-sulfide and alkyne as anchoring groups (Figure 2a). NHC copper chloride with alkyne anchoring group gave a higher conductance value around 5 × 10–5 G0 than the corresponding methyl sulfide complex did (around 1.5 × 10–5 G0, Figure 2b). Meanwhile, the 2D histograms showed longer plateau for methyl sulfide anchor than alkyne anchor (Figure 2 c, d). The longer plateau suggests a stronger binding for methyl sulfide anchor to the gold electrode and a higher stability of the single molecule junction.

For the single molecule junctions of NHC metal complexes, there are two possible type of conformation as shown in Figure 3a. To distinguish them, measurement of two NHC complexes with one or two ethynyl anchors was conducted. The results show the similar conductance with the value of 5.25 × 10–5 G0 and 5.00 × 10–5 G0 for one and two ethynyl anchors, respectively. These results suggest that the molecular junction is formed in type A conformation, wherein the metal on the NHC core binds to the electrode.

To make type B junction conformation, it is important to prevent the contact between the gold electrode and the metal atom on the NHCs. Therefore, the synthesis of NHC-metal complex is carried out by replacing chloride atom with bulky carbazole. From the 1D histogram in Figure 5, the conductance of the NHC copper carbazole is different from that of the NHC copper chloride (6 × 10–6 G0 vs 1.5 × 10–5 G0), implying the formation of type B junction. These results indicated that by limiting the NHC metal center's contact to the electrodes, the junction conformation can be controlled. The conductance value of the conformation B is smaller than the conformation A, possibly due to the torsion angle between the imidazole ring and the aryl rings in the NHC.

The author also investigated the metal effect on the single-molecule conductance value of NHC complexes using methyl sulfide anchoring group. The NHC metal chloride complexes exhibited different conductance, with a value of 3.5 × 10–5 G0 for Au, 1.9 × 10–5 G0 for Cu, and 1.4 × 10–5 G0 for Ag (Au > Cu > Ag, Figure 5). Theoretical calculation suggested that the conductance value is also affected by the binding site of the metals, and the comparison with experimental results implied the binding geometries are different for metals (Table 1). Interestingly, 2D histograms (Figure 5 b–d) indicate that the plateau length also depends on the metals with the order Au > Ag > Cu. These results imply that the binding strength to the electrode depends on the metals and their binding geometries.