Ultraviolet Photochemistry and ex Situ Ozonolysis of Alkanethiol Self-Assembled Monolayers on Gold

Citations Over TimeTop 13% of 1999 papers

Abstract

The ultraviolet (UV)-photoinduced chemistry at low irradiances in O2 and the O3 thermal chemistry of alkanethiolate SAMs have been studied. At the low irradiances typical of low-pressure Hg lamps, UV photolytic generation of products soluble in polar solvents from hexadecanethiol (HDT) SAMs requires both O2 and irradiation below 200 nm. Significantly, exposure of HDT SAMs to ex situ generated O3 in the dark produced the same results as UV irradiation of HDT SAMs in the presence of O2. Both of these facts suggest a leading role for photogenerated O3 in the UV photochemistry of alkanethiol SAMs. Further, O3 was found to be at least an order of magnitude more effective than other active oxygen-containing species at generating labile products under the conditions employed. The products of reaction from either O3 exposure or UV irradiation in O2 appear to be oxidized sulfur headgroups, which are easily removed by subsequent rinsing in a polar solvent. The heterogeneous reaction of O3 with alkanethiols is remarkably efficient with one oxidation event occurring for every 3300 O3 molecules transiting the flow cell. Finally oxidation at the S atom dominates reactions in the interior of the film under the conditions used. Taken together, these results are convincing evidence that UV photooxidation of HDT SAMs at low irradiances proceeds principally by generation of O3 followed by ozonolysis of the sulfur headgroup to produce solvent-labile species which can be subsequently removed.

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