Investigation of the Interfacial Electron Transfer Kinetics in Ferrocene-Terminated Oligophenyleneimine Self-Assembled Monolayers

In this article, the synthesis, characterization, and cyclic voltammetry (CV) measurements are reported for ferrocene-terminated oligophenyleneimine (OPI_Fc) and ferrocene-terminated conjugation-broken oligophenyleneimine (CB-OPI_Fc) self-assembled monolayers (SAMs) in two different electrolytes, namely, 1-ethyl-3-methylimidazolium-bis (trifluoromethyl-sulfonyl) imide (EMITFSI) ionic liquid and tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile (0.1 M solution). The SAMs were synthesized on Au surfaces by the sequential imine condensation reactions. CV was used to investigate the kinetics of electron transfer (ET) to the ferrocene, and it was observed that the standard ET rate constant (k0) is a strong function of the electrolyte nature as well as the chemical composition of the SAM. Interestingly, when 0.1 M Bu4NPF6 in acetonitrile was used as the electrolyte, all of the SAMs exhibited quite similar k0 values. However, in the case of the ionic liquid, we found that k0 dramatically varies for each SAM and trends as OPI 6_Fc > CB3-OPI 6_Fc > CB5-OPI 6_Fc > CB3,5-OPI 6_Fc. We also examined the temperature dependence of ET kinetics for OPI 2_Fc, OPI 4_Fc, OPI 8_Fc, CB3-OPI 6_Fc, CB5-OPI 6_Fc, and CB3,5-OPI 6_Fc SAMs in EMITFSI ionic liquid. It was found that the activation energies of the ET in these SAMs are very similar (∼0.2 eV). Moreover, it was observed that ln k0 varies linearly with the molecular length for three SAMs, OPI 2_Fc, OPI 4_Fc, and OPI 8_Fc. These findings suggest that the ET to the ferrocene in OPI_Fc and CB-OPI _Fc SAMs takes place via a direct tunneling mechanism.