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Photoluminescent metalloles for chemical sensing of nitroaromatic explosives and chromium(VI)

  • Author(s): Toal, Sarah Josepha;
  • et al.

Photoluminescent metallacyclopentadienes, or metalloles, are shown to be highly sensitive and selective sensors for oxidant molecules. The method of detection is through electron-transfer quenching of the luminescence of the metallole by the electron-deficient analytes. The capability of metalloles to act as redox sensors results from their low energy LUMO and-small HOMO-LUMO band gap energy. An improved catalytic dehydrogenative coupling synthesis of the explosives sensing polysilole and polygermole is detailed. This new method requires less hazardous reagents and offers significantly improved product yields over traditional Wurtz coupling syntheses. Addition of a hydrogen-accepting alkene co-reagent further improves polymer yield and molecular weight. Dehydrogenative coupling proceeds through either a catalytically homogeneous or heterogeneous mechanism, depending on the catalyst used. Homogeneous catalysts, RhCl(PPh3) 3 and Pd(PPh3)4, produce a significant amount (40%) of the silole dimer, thus limiting chain length and overall yield. The heterogeneous catalyst, H2PtCl6, produces higher molecular weight polymer product. Reaction mechanisms are proposed to account for this difference. Polymetalloles and novel copolymers of diethynylbenzene and dihydrometalloles are investigated as detectors of solid-state nitroaromatic explosives, namely trinitrotoluene, dinitrotoluene, and picric acid. Thin films of the polymers are highly photoluminescent and show visual luminescence quenching in the presence of the explosives. Thin films are prepared via spray coating an organic solution of the polymers and the presence of explosives is confirmed by visual inspection of the film when illuminated with near UV radiation. Detection limits as low as 5 ng are possible. Aqueous TNT detection is possible at 20 parts per billion (ppb) using colloidal polysilole nanoparticles. Fluorescence quenching efficiencies improve by 400% for the nanoparticles relative to the dissolved polymer. Nanoparticles, formed by the precipitation of an organic solution of the polymer with water, are approximately 80 nm in diameter, and have fluorescence lifetimes near 3.8 ns. Functionalization of metalloles can produce redox sensors specific to other analytes. Thus, catalytic hydrosilation of allylamine by methylhydrosilole yields a chemoselective chromate sensor. A colloid of siloleamine nanoparticles is able to detect 100 ppb CrO42-. The sensor is selective to chromate and relatively insensitive to other oxoanion interferents commonly found in water supplies

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