Electron Transport Behavior Analysis of Bis(terpyridine) Metal Complex Wires on Electrodes

Abstract

Molecular wires with rapid and smooth charge transport ability are desired to achieve high performance and low energy consumption of molecular electronic devices. Various kinds of molecular wires with superior electron transport abilities have been reported. They exhibit the tiny dependencies of electron transfer rate constants on the wire lengths, in other words the small attenuation factors, β. We have reported that bis(terpyridine)metal complex [M(tpy)2] (M=Fe, Co, tpy=2,2’:6’,2’’-terpyridine) oligomer wires show the small β values because M[(tpy)2] unit works as an electron hopping site and promotes intrawire electron transport.  The quantitative evaluation of charge transport is important to develop superior electron transport system. In this research, we prepared two types of gold electrode-[Fe(tpy)2]-ferrocene molecular arrays as the simplest electron transport model of [M(tpy)2] wire system. The prepared arrays were characterized using cyclic voltammetry and scanning tunneling microscope. The electron transfer between the gold electrode and the terminal ferrocene moiety via [Fe(tpy)2] was evaluated by the potential step chronoamperometry (PSCA) and electrochemical impedance spectroscopy (EIS). The analysis of Tafel plots derived from PSCA based on Marcus theory revealed the greater electron transfer rate constant and smaller reorganization energy than a ferrocene-terminated alkanethiol self-assembled monolayer system. The parameters extracted by the fitting of the observed impedance spectra using an electric circuit model suggested the additional factor for the rapid electron transport of [Fe(tpy)2] system. On the other hand, in the present electronics field, semiconductors are used as major materials for electronic devices. Therefore, the investigation of the electron transport behaviors of Fe(tpy)2 wires on a semiconductor electrode will contribute to develop molecular devices based on the present electronics. Ferrocene- and triarylamine-terminated [Fe(tpy)2] wires were prepared on hydrogen-terminated silicon electrodes using the stepwise coordination method. Their electron transport behaviors between the terminal redox species and silicon electrodes via [Fe(tpy2)] wires were analyzed based on the cyclic voltammograms recorded at various scan rates. The ferrocene-terminated wire gave the redox wave derived from Fc+/Fc with largely-separated oxidation and reduction peaks indicating slow electron transfer while the triarylamine-terminated wire exhibited the reversible redox couple of triarylamine cation radical/triarylamine derived from the rapid electron transport. In addition, the separation of oxidation and reduction peak potentials of Fc+/Fc redox wave showed the linear dependency on the natural logarithm of scan rates at the high scan rate region whereas the that of triarylamine redox couple did not. These different electron transport phenomena were discussed based on the band structure of silicon electrode and the energy levels of wire components. At the redox potential of ferrocene moiety, the band structure of silicon is largely bended, and the electron injection is prevented. In the case of triarylamine moiety, the band deformation is insignificant. In addition, the applied potential is close to the HOMO level of Fe(tpy)2, and might allow the smooth electron injection from the wire to the electrode.