Ultraviolet photochemistry and ex situ ozonolysis of alkanethiol self-assembled monolayers on gold

The ultraviolet (UV)-photoinduced chemistry at law irradiances in O-2 and the O-3 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 O-2 and irradiation below 200 nm. Significantly, exposure of HDT SAMs to ex situ generated O-3 in the dark produced the same results as UV irradiation of HDT SAMs in the presence of Oz. Both of these facts suggest a leading role for photogenerated O-3 in the UV photochemistry of alkanethiol SAMs. Further, O-3 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 O-3 exposure or UV irradiation in Oz appear to be oxidized sulfur headgroups, which are easily removed by subsequent rinsing in a polar solvent. The heterogeneous reaction of O-3 with alkanethiols is remarkably efficient with one oxidation event occurring for every 3300 O-3 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 O-3 followed by ozonolysis of the sulfur headgroup to produce solvent-labile species which can be subsequently removed.