Stabilization of the assemblies of short alkyl chain asymmetric viologens on alkanethiol-coated electrodes

Abstract

Self-assembled monolayers (SAMs) of n-alkanethiol (CH 3(CH 2) n SH, n=3–17)-coated electrodes have been used to stabilize the assemblies of N-ethyl- N′-tetradecylviologen ( 1) and N-ethyl- N′-dodecylviologen ( 2) on the electrode surface. The redox behavior of 1 and 2 on the bare Au electrode was unstable because of the weak adsorption of their assemblies on the Au electrode surface. On the other hand, both 1 and 2 are strongly confined and compact in the SAM domain of alkanethiol and showed a stable redox response. The compactness of the assemblies of 1 and 2 on alkanethiol-coated electrodes can be understood from its blocking effect on the redox reaction of [Ru(NH 3) 6] 2+/3+ and EQCM measurements. EQCM measurements demonstrated that the ingress of water molecules into the assembly of 1 and 2 was markedly suppressed at the assembly of 1 and 2 bound to alkanethiol-coated electrodes. Furthermore, the redox behavior of 1 and 2 on the bare and alkanethiols-coated electrodes was also studied in the presence of weakly hydrated anions of ClO 4 − and PF 6 −. For the ClO 4 − ion, an additional oxidation peak was observed for the first reduction of 1 and 2 along with a usual redox wave. While, in the presence of PF 6 − ions, the electrochemical behavior was largely dependent on the alkyl chain length of thiol. Two redox waves were observed on short chain thiol ( n=3)-coated electrode whereas a single redox wave was observed on the electrodes coated with alkanethiols of n=5, 7 and 9. An irreversible redox response was observed on the longer alkyl chain ( n>9) thiol-coated electrodes. The observed irreversible response has been attributed to the effective blocking of water molecules at the assembly of 1 and 2 on the long chain alkanethiol-coated electrodes in the presence of weakly hydrated PF 6 - ions. At these electrodes, 1 and 2 may form an insoluble salt with PF 6 − ion and are electroinactive. When the same electrodes were transferred to solutions of Cl − or ClO 4 − ions, the PF 6 − ions could be readily exchanged with Cl − or ClO 4 − ions and 1 and 2 became electroactive. Such a situation may not arise at the assembly of 1 and 2 bound to short chain thiol ( n<9) coated-electrodes because of the free flow of water molecules into the disordered monolayers of short chain thiols.