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 t he O-3 thermal chemistry of alkanethiolate SAMs have been studied. At the l ow irradiances typical of low pressure Hg lamps, UV photolytic generation o f products soluble in polar solvents from hexadecanethiol (HDT) SAMs requir es both O-2 and irradiation below 200 nm. Significantly, exposure of HDT SA Ms to ex situ generated O-3 in the dark produced the same results as UV irr adiation of HDT SAMs in the presence of Oz. Both of these facts suggest a l eading 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 effe ctive than other active oxygen-containing species at generating labile prod ucts 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 het erogeneous reaction of O-3 with alkanethiols is remarkably efficient with o ne oxidation event occurring for every 3300 O-3 molecules transiting the fl ow cell. Finally oxidation at the S atom dominates reactions in the interio r of the film under the conditions used. Taken together, these results are convincing evidence that UV photooxidation of HDT SAMs at low irradiances p roceeds principally by generation of O-3 followed by ozonolysis of the sulf ur headgroup to produce solvent-labile species which can be subsequently re moved.