Use of alkanethiol-coated electrodes to study the importance of water content oil the electrochemical behavior of N-ethyl-N '-octadecyl viologen on agold electrode surface

The electrochemical behavior of an N-ethyl-N'-octadecyl viologen (1) assemb ly on Au and bound to alkanethiol-coated electrodes was studied. The wet an d dry self-assembled monolayers (SAMs) of 1 on Au electrodes showed a singl e redox wave for the first reduction in the presence of KCl, NaNO3, Na2SO4, and NaClO4. On the, other hand,in the presence of NH4PF6, the wet SAM of 1 showed sharp shoulder-reduction peaks at -0.50 and -0.57 V and two oxidati on peaks at -0.50 and -0.42 V, and the dry SAM of 1 showed only a single re dox wave. On the basis of in situ Fourier transform infrared reflection spe ctroscopy studies, we assigned the sharp reduction peak of -0.50 V to the r eduction of strongly interacted dications of I surrounded by water molecule s and the more negative potential reduction peak (-0.57 V) to the reduction of the dications of 1 ion-paired with PF6- ions. Among the two oxidation p eaks, the oxidation peak of -0.50 V was assigned to the usual oxidation of the radical cation whereas the oxidation peak of -0.42 V was assigned to th e oxidation of the radical cation dimer. Surprisingly, the assembly of 1 sh owed an irreversible response on long chain n-alkanethiols [CH3(CH2)(n)SH, n = 11, 13, 15, and 17]-coated Au electrodes in the presence of 0.1 M NH4PF 6. Meanwhile, transfer of the same electrode to other supporting electrolyt es (typically, KCl, Na(2)SQ(4), and NaClO4) gave a clear redox wave. The ob served irreversible response of 1 on long chain alkanethiol-coated Au elect rodes is explained by the blocking effect of water molecules by the assembl y of 1 on alkanethiol-coated Au electrodes in addition to the presence of v ery weakly hydrated PF6- ions. It is proposed that because of the absence o f water molecules in the assembly of 1 on long-chain alkanethiol-coated Au electrodes. the dications of 1 form an "insoluble salt" with PF6- ions at t he electrode surface. Whereas the same electrode was transferred to the aqu eous solution of hydrophilic anions such as Cl- and SO42-, the dications of 1 were well-solvated due to the ingress of water molecules along with the hydrophilic anions of Cl- or SO42- into the assembly of 1 on alkanethiol-co ated electrodes, and thus, it showed a clear redox response. On the other h and, the assembly of 1 on short-chain n-alkanethiol (n < 9)-coated electrod es showed a well-resolved redox wave in the presence of 0.1 M NH4PF6. The i ngress/egress of water and anions into/from the assembly of 1 on Au and alk anethiol-coated Au electrodes during the redox reaction were studied by ele ctrochemical quartz crystal microbalance. It is found that the ingress of w ater molecules into the assembly of 1 on alkanethiol-coated Au electrodes w as less than that of 1 on Au electrodes. The ingress of water molecules als o strongly depends on the identity of the anion that is present within the monolayer.