Insertion of individual dendrimer molecules into self-assembled monolayerson gold: A mechanistic study

The insertion process of individual dendrimer sulfide 1 molecules into self -assembled monolayers (SAMs) of 11-mercapto-1-undecanol has been investigat ed by atomic force microscopy (AFM), wettability, and electrochemical measu rements. Immersion of 11-mercapto-1-undecanol SAMs in solutions of increasi ng concentrations of 1 leads to an increase in the number of dendrimer mole cules inserted into the thiol layer. Similarly, longer contact times of the original SAM with the solvent (dichloromethane) also lead to a higher numb er of inserted dendrimers. However, 11-mercapto-1-undecanol monolayers prep ared by immersion into 1 mM solutions of the thiol for 2, 5, 10, 30, 60, 12 0, and 180 min, followed by a 60 min exposure to a solution of dendrimer 1 did not show significant differences in the number of inserted dendrimers. This indicates that the initial layer quality is not a determining factor f or the insertion process. A mechanism consisting of rapid dissociation of s urface thiols, possibly as disulfides, followed by slow dendrimer adsorptio n, is proposed. The rate-determining step of the process is the insertion o f the individual dendrimer molecules. For the dissociation step, first-orde r kinetics for the fraction of covered substrate versus time of exposure to solvent was found, with a rate constant, k, of (8.0 +/- 1.5) x 10(-4) min( -1).