| Summary: | 碩士 === 國立臺灣大學 === 化學研究所 === 101 === Metal string complexes (MSCs), in which the metal centers are co-linear and stably coordinated by four oligopyridylamido ligands, have been demonstrated a unique category of conductive molecular wires. Bond orders which describe the strength of metal-metal interactions are qualitatively well correlated with the single-molecule conductance of the MSCs. Previous examples are limited to homometallic string complexes in which the metal centers are composed of the same element. Explored in this thesis work are the conductive properties of heterometallic string complexes (HMSCs). Specifically, the sequences of the metal cores are RhRhNi, NiRhNi, RhRhNiNiNi, and RhRhRhNiNi. Also examined are Ni3 and Ni5 MSCs. Methods of compound purification and crystallization are improved. The experimental protocols and data analysis are also modified. The conductance measurements are carried out through electrode-molecule-electrode junctions created by an STM (scanning tunneling microscope) tip which repeatedly impinges into and retracts from the substrate. In this study, the tip-substrate contact is ensured by an external device, while it was not monitored in our earlier work. The conductance histograms are prepared by pooling all acquired conductance traces while, in the past, only those with step-like features were selectively used. The results show that the conductance histograms are featureless and that it is very difficult to determine the conductance values for pentametallic string complexes, even for Ni5 MSC. X-ray crystallographic data reveal that solvent molecules, present in the unit cell by previous purification methods, are absent for the new samples. Accordingly, mixed solvents exhibiting a range of polarity are utilized to improve the solubility of MSCs. The conductance peaks are slightly more pronounced in a more polar environment. The conductance peaks of Ni3, Ni5, and NiRhNi are solvent-independent, while the peak positions of asymmetric RhRhNi, RhRhNiNiNi, and RhRhRhNiNi shift associated with the solvent utilized. According to published researches in the effect of environment on molecular junction, we proposed that solvent molecules will affect the head group and alter gold-sulfur contact.
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