Static Secondary Ion Mass Spectrometry Studies of Self-Assembled Monolayers:  Influence of Adsorbate Chain Length and Terminal Functional Group on Rates of Photooxidation of Alkanethiols on Gold

The rates of photooxidation of self-assembled monolayers (SAMs) of alkanethiols containing methyl, hydroxyl, and carboxylic acid terminal groups have been studied using static secondary ion mass spectrometry (SIMS). SIMS has revealed a wealth of structure-specific peaks that enable the progress of photochemical reactions within the SAM to be monitored accurately and effectively. The addition of eight methylene groups to the alkyl chain leads to a reduction in the rate of reaction by a factor of approximately 4. The nature of the adsorbate terminal group exerts a profound influence over the reactivity of the SAM, with rate constants for the photooxidation reaction being in the ratio 4:2:1 for CH 3:OH:COOH, for both long and short chain alkanethiols. The reduction in the rate constant observed for SAMs with polar terminal groups is attributed to the existence of strong hydrogen bonding interactions that stabilize the SAM, either by increasing adsorbate order or by reducing the rate of diffusion of active oxygen species to the S−Au interface. Tail group interactions may be more generally important in determining the structures and reactivities of SAMs.