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

Self-assembled monolayers (SAMs) of n-alkanethiol (CH3(CH2)(n)SH, n = 3-17) -coated electrodes have been used to stabilize the assemblies of N-ethyl-N' -tetradecylviologen (I) and N-ethyl-N'-dodecylviologen (2) on the electrode surface. The odor behavior of 1 and 2 on the bare Au electrode was unstabl e because of the weak adsorption of their assemblies on the Au electrode su rface. 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 comp actness of the assemblies of I and 2 on alkanethiol-coated electrodes can b e understood from its blocking effect on the redox reaction of [Ru(NH3)(6)] (2+/3+) and EQCM measurements. EQCM measurements demonstrated that the ingr ess 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. Further more, the redox behavior of 1 and 2 on the bare and alkanethiols-coated ele ctrodes was also studied in the presence of weakly hydrated anions of ClO4- and PF6-. For the ClO4- ion, an additional oxidation peak was observed for the first reduction of 1 and 2 along with a usual redox wave. While, in th e presence of PF6- 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 obser ved on the electrodes coated with alkanethiols of n = 5, 7 and 9. An irreve rsible redox response was observed on the longer alkyl chain (n > 9) thiol- coated electrodes. The observed irreversible response has been attributed t o the effective blocking of water molecules at the assembly of 1 and 2 on t he long chain alkanethiol-coated electrodes in the presence of weakly hydra ted PF6- ions. At these electrodes, 1 and 2 may form an insoluble salt with PF6- ion and are electroinactive. When the same electrodes were transferre d to solutions of Cl- or ClO4- ions, the PF6- ions could be readily exchang ed with Cl- or ClO4- ions and 1 and 2 became electroactive. Such a situatio n may not arise at the assembly of 1 and 2 bound to short chain thiol (n < 9) coated-electrodes because of the free how of water molecules into the di sordered monolayers of short chain thiols. (C) 1999 Elsevier Science Ltd. A ll rights reserved.