Influence of Adsorbate Ordering on Rates of UV Photooxidation of Self-Assembled Monolayers

The photooxidation of a range of methyl terminated straight chain alkylthiol (CH 3 (CH 2 ) n - 1 SH) self-assembled monolayers (SAMs) on Au has been investigated using X-ray photoelectron spectroscopy (XPS). The photooxidation process has been found to involve the oxidation of the thiolate group to a sulfonate by an active O 2 species (possibly singlet oxygen). The rate of photooxidation was found to vary strongly with alkyl chain length and has been correlated with the amount of disorder within the monolayers. Short-chain SAMs ( n ≤ 8) have the highest disorder and oxidize the fastest, with the rate of oxidation increasing sharply with decreasing n, while close packed long-chain SAMs ( n ≥ 12) oxidize much more slowly, with the rate of oxidation decreasing slowly with increasing n .