Odd–even effects in the cyclic voltammetry of self-assembled monolayers of biphenyl based thiols

Monolayers of ω-(4′-methyl-biphenyl-4-yl)-alkanethiols (CH3C6H4C6H4(CH2)mSH, m=1–6, BPm) adsorbed on polycrystalline gold were investigated by cyclic voltammetry. In sharp contrast to alkane thiols, the BPm monolayers show an electrochemical stability which alternates with the length of the alkane spacer. For m=even, reductive desorption takes place at potentials 83 mV more positive compared with m=odd. This odd–even effect is detected only for native layers and is lost after the first voltammetric cycle due to incomplete readsorption. In addition to the desorption potential the charge associated with the desorption also exhibits an odd–even behaviour. Beyond odd–even variations of intermolecular interactions and the thiol coverage which affect capacitive and Faradaic contributions to the desorption peak, the influence of an m-dependent charge distribution at the sulphur ∣ gold interface is discussed with respect to its influence on the Faradaic component and the assumption of a one electron transfer process in the reductive desorption of thiols