Formation kinetics and electrochemical behavior of an azobenzene self-assembled monolayer on a gold electrode

The formation kinetics of the self-assembled monolayer (SAM) of an azobenzenealkanethiol (HS-CH₂CH₂NHC(O)-(|)-N=N-(|) denoted as AzoC2SH) on gold surface was studied using electrochemical techniques, while the variation of the voltammetric behavior of AzoC2SH monolayers prepared with different assembly time was investigated. A two-step adsorption kinetics has been confirmed for the self-assembly of AzoC2SH molecules on gold surface: the fast adsorption process and the following long-term reorganization. The equilibrium constant (K_eq) for the adsorption and the interaction factor between adsorbate-adsorbate molecules (S) were evaluated based on the Frumkin isotherm and determined to be (3.17-0.13)xl06 M^-1 and (-0.34±0.04), respectively. We have also determined the adsorption rate constant (k_ad) of (1.90±0.4)×10³ M^-1 s^-1 on the basis of the Langmuir model. The value of k_ad is significantly larger than that of underivatized alkanethiols or ferrocenylalkanethiols, partially due to the strong attractive interaction between AzoC2SH molecules in the monolayer, which is evidenced by the negative interaction factor. Moreover, the electrochemical behavior shows a unique change when we prepare differently packed AzoC2SH monolayers by controlling the self-assembling time. The apparent electron transfer rate constant (k_app) was found to show a remarkable decrease with the surface concentration of the adsorbate molecules in the monolayer increasing. Taking into account the molecular structural change associated with the reduction/oxidation of an azobenzene group, the decrease of k_app can be understood by considering that there is an increase of the structural inhibition effect when the packing density of the AzoC2SH monolayer on gold surface increases.