Electrochemical stability of self-assembled monolayers of biphenyl based thiols studied by cyclic voltammetry and second harmonic generation

The reductive desorption of self-assembled monolayers (SAMs) of ω-(4′-methyl-biphenyl-4-yl)-alkanethiols (CH₃-C₆H₄-C₆H₄-(CH₂)_n-SH, BPn) on Au(1 1 1) on mica was studied in 0.5 M KOH solution as a function of the length of the aliphatic spacer chain (n = 1-6 and 12) and for two different preparations temperatures (295 K and 343 K). Second harmonic generation (SHG) was applied in situ parallel to cyclic voltammetry (CV). Odd-even differences in the structure of the BPn monolayers are clearly reflected in the electrochemical stability, as well as by the charge and shape of the desorption peak. For n = 1-5 a single desorption peak is detected whereas multiple peaks occur for BP6 similar to hexadecane thiol which was also studied for comparison. An increased preparation temperature affects the shape and width of the desorption peak but not the position. BP1 exhibits a temperature dependence different from the other homologues. The relationship between coverage monitored by SHG and desorption charge determined from the CVs is found to be linear and surprisingly independent from the details of the SAMs. The combined SHG and CV experiments suggest that capacitive and faradaic current are always closely coupled even for BP6 and hexadecane thiol which exhibit multiple desorption peaks.