Proceedings Article | 18 November 2019
Proc. SPIE. 11194, Plasmonics IV
KEYWORDS: Plasmons, Molecules, Nitrogen, Laser scattering, Raman spectroscopy, Laser damage threshold, Molecular spectroscopy, Molecular lasers, Temperature metrology, Oxidation
A gap mode plasmon induced photocatalytic oxidation of p-alkyl thiophenol (p-AlTP), encompassing methyl, iso-propyl and tert-butyl groups, to p-mercaptobenzoic acid (p-MBA), for which excitation 532 nm laser (<1 μW/μm2) impinged on a sample of silver (Ag) nanoparticles/p-AlTP/Ag films/BK-7 prism. Apparently, the oxidation of p-AlTP yielded p-MBA without leaving any signs of the intermediated species like p-mercaptobenzyl-aldehyde or -alcohol (p-MBAl) in the air. In contrast to p-AlTP, o-methyl TP and m-methyl TP were not oxidized, indicating preferential reactivity of the para position in TP molecules. Nonetheless, the site selectivity is not always valid in this type of photocatalytic reactions, as o-, m-, and p-MBAl molecules were oxidized to corresponding o-, m- and p-MBA. Similarly, all of o-, m-, and p-MBA molecules were decarboxylated to TP in the air. With respect to the reaction mechanism, we confirmed that the oxidation of p-MeTP is not induced by thermal heating of the samples up to 373 K. Subtle temperature increase (<10 K) during the gap mode-induced oxidation was also corroborated by the observed Stokes and anti-Stokes scattering intensity of p-MeTP. Oxygen molecules accelerated the oxidation of p-MeTP at room temperature, whereas nitrogen atmosphere generated an intermediate species attributable to p-mercaptobenzyl radical.
