Several polymers have been used to detect nitroaromatic explosives by a variety of transduction schemes. Detection relies on both electronic and structural interactions between the sensing material and the analyte. Quenching of luminescent polymers by electron deficient nitroaromatic explosives, such as trinitrotoluene, may be monitored to detect explosives. Resistive sensing using carbon black particles that have been coated with different organic polymers and deposited across metallic leads can also be used to detect nitroaromatic vapors in an electronic nose approach. Frequency changes in surface acoustic wave devices may be monitored to detect nitroaromatics after their adsorption into polymer coatings. Luminescent polymetalloles have recently been investigated for sensing explosives in aqueous-based solutions and for improved visual detection of trace particulates on surfaces.

http://nathan.instras.com/ResearchProposalDB/doc-151.pdf

Polymer sensors for nitroaromatic explosives detection

The synthesis, spectroscopic characterization, and fluorescence quenching efficiency of polymers and copolymers containing tetraphenylsilole or tetraphenylgermole with Si-Si, Ge-Ge, and Si-Ge backbones are reported. Poly(tetraphenyl)germole, 2, was synthesized from the reduction of dichloro(tetraphenyl)germole with 2 equivs of Li. Silole-germole alternating copolymer 3 was synthesized by coupling dilithium salts of tetraphenylsilole dianion with dichloro(tetraphenyl)germole. Other tetraphenylmetallole-silane copolymers, 4-12, were synthesized through the Wurtz-type coupling of the dilithium salts of the tetraphenylmetallole dianion and corresponding dichloro(dialkyl)silanes. The molecular weights (M(w)) of these metallole-silane copolymers are in the range of 4000 approximately 6000. Detection of nitroaromatic molecules, such as nitrobenzene (NB), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and picric acid (PA), has been explored. A linear Stern-Volmer relationship was observed for the first three analytes, but not for picric acid. Fluorescence spectra of polymetalloles or metallole-silane copolymers obtained in either toluene solutions or thin polymer films displayed no shift in the maximum of the emission wavelength. This suggests that the polymetalloles or metallole-silanes exhibit neither pi-stacking of polymer chains nor excimer formation. Fluorescence lifetimes of polymetalloles and metallole-silanes were measured both in the presence and absence of TNT, and tau(o)/tau is invariant. This requires that photoluminescence quenching occurs by a static mechanism.

Detection of nitroaromatic explosives based on photoluminescent polymers containing metalloles.

Synthesis, properties, and application of new σ- and π-electron systems consisting of the silole rings are described. A series of 2,5- and 1,1-difunctionalized siloles have been prepared based on the intramolecular reductive cyclization of diethynylsilanes. Starting from these functionalized siloles, oligo(2,5-silole)s and oligo(1,1-silole)s have been synthesized as model compounds for poly(2,5-silole)s and poly(1,1-silole)s, respectively, which are still veiled target molecules in this field. Some silole-containing π-conjugated cooligomers and copolymers with thiophene, pyrrole, and acetylene π-electron systems have also been prepared. They all have unique photophysical properties such as long-wavelength absorption in the UV/VIS absorption spectra. Some silole-based π-conjugated compounds have also been found to work as new useful materials for organic electroluminescent devices.

Silole-containing σ- and π-conjugated compounds

Nitroaromatic explosives can be detected in a simple and rapid method by the quenching of the photoluminescence of fluorescent polysiloles. Quenching is achieved by electron transfer from the conduction band of the polysiloles to electron-poor molecules such as picric acid, nitrobenzene, 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT). Dilute polymer solutions can also be employed as a forensic spray-on reagent to visualize TNT or picric acid residues under a UV lamp (see picture of the print of a nitrile-gloved hand that had been in contact with TNT).

Detection of TNT and Picric Acid on Surfaces and in Seawater by Using Photoluminescent Polysiloles

A series of 2,5-diaryl-3,4-diphenylsiloles, with various mono-substituted phenyl groups, extended pi-conjugated groups, and heteroaryl groups as aryl groups at the 2,5-positions, has been prepared by a one-pot synthesis from bis(phenylethynyl)silanes based on the intramolecular reductive cyclization followed by the palladium-catalyzed cross-coupling with aryl halides. Crystal structures and chemical reactivities toward the alkaline desilylation reactions have been studied on the 2,5-bis(p-mono-substituted phenyl)silole derivatives to elucidate the effects of the p-substituents. The UV-visible absorption and fluorescence spectra, and cyclic voltammetry of the 2,5-diarylsiloles have been systematically evaluated. Their photophysical properties as well as their electronic structures significantly depend on the nature of the 2,5-aryl groups.

Toward New Materials for Organic Electroluminescent Devices: Synthesis, Structures, and Properties of a Series of 2, 5-Diaryl-3,4-diphenylsiloles

Dilithium derivative of tetraphenylsilole : an eta 1-eta 5 dilithium structure

Synthesis, solid-state structure, and reduction of 1,1-dichloro-2,3,4,5-tetramethylsilole

The previously unknown 1,1-dichloro-2,3,4,5-tetramethyl-1-silacylopenta-2,4-diene ( 3 ) was prepared in a three-step synthesis starting from zirconocene dichloride, 2-butyne, and n-butyllithium via bis(cyclopentadienyl)-2,3,4,5-tetramethyl-1-zirconacyclopenta-2,4-diene ( 1 ) and 1,4-diiodo-1,2,3,4-tetramethylbuta-1,3-diene ( 2 ). 3 was characterized by NMR spectroscopy and single-crystal X-ray diffraction. Reaction of the new silole 3 with 4 equivalents of lithium in THF gives the corresponding dianion 4 . 1 H-, 13 C-, and 29 Si-NMR chemical shifts of 4 compared with 3 indicate some aromatic character for the ionic species. Trapping of 4 with Me 3 SiCl gives 1,1-bis(trimethylsilyl)-2,3,4,5-tetramethysilole ( 5 ) in quantitative yield.

Uwe Bankwitz

Papers

Dilithium derivative of tetraphenylsilole : an eta 1-eta 5 dilithium structure

Synthesis, solid-state structure, and reduction of 1,1-dichloro-2,3,4,5-tetramethylsilole

Synthesis, Solid-State Structure, and Reduction of 1,1-Dichloro-2,3,4, 5-tetramethylsilole.