Electrochemical and exchange studies of self-assembled monolayers of biphenyl based thiols on gold

Capacitance and charge transfer rates of self-assembled monolayers of ω-(4′-methyl-biphenyl-4-yl)-alkanethiols (CH3–C6H4–C6H4–(CH2)n–SH, n=0–6) adsorbed on polycrystalline gold were investigated. For n=1–6 the reciprocal capacity exhibits a linear dependence on the length of the alkane spacer. Its change of 0.054±0.0036 cm2 μF−1 per CH2 group and the calculated dielectric constant of ε=2.5 agrees well with the values of n-alkane thiols. ε of the aromatic tail group is around 5 which is substantially larger than the bulk value of biphenyl. Measurements of the charge transfer rate yield an exponential dependence on the number of methylene units but reveal significant differences from alkanethiols. Transfer rates are more than a factor of 100 larger than those for alkane thiols of comparable length and the tunneling constant of 1.48±0.15 Å−1 is higher compared to the value of about 1 Å−1 for pure alkane thiols. In contrast to the film structure which alternates with the number of CH2 units, no odd–even-effect is observed either for the capacity or for the charge transfer rate. However, the structural differences are reflected in the exchange kinetics. Exposure of the biphenyl monolayers to a hexadecane thiol solution reveals a significant dependence of the exchange rate on n. SAMs with n=even are displaced more easily compared to n=odd with differences being largest for shorter spacer chains.