Lewis acid–base interactions enhance explosives sensing in silacycle polymers

Abstract

The high sensitivity of silole-and silafluorenecontaining polymers for detecting organic nitro, nitrate, and nitramine explosives cannot be solely attributed to favorable analyte-polymer hydrophobic interactions and amplified fluorescence quenching due to delocalization along the polymer chain. The Lewis acidity of silicon in conjugated poly(silafluorene-vinylene)s is shown to be important. This was established by examining the 29 Si NMR chemical shifts (Δ) for the model trimer fragment of the polymer CH 3 -silafluorene-(trans-C 2 H 2 )-silafluorene-(trans-C 2 H 2 )-silafluorene-CH 3 . The peripheral and central silicon resonances are up-field from a TMS reference at −9.50 and −18.9 ppm, respectively. Both resonances shift down-field in the presence of donor analytes and the observed shifts (0 to 1 ppm) correlate with the basicity of a variety of added Lewis bases, including TNT. The most basic analyte studied was acetonitrile and an association constant (K a ) of 0.12 M −1 was calculated its binding to the peripheral silicon centers using the Scatchard method. Spin-lattice relaxation times (T 1 ) of 5.86(3) and 4.83(4)s were measured for the methyl protons of acetonitrile in benzene-d 6 at 20°C in the absence and presence of the silafluorene trimer, respectively. The significant change in T 1 values further supports a binding event between acetonitrile and the silafluorene trimer. These studies as well as significant changes and shifts observed in the characteristic UV-Vis absorption of the silafluorene group support an important role for the Lewis acid character of Si in polymer sensors that incorporate strained silacycles. The nitro groups of high explosives may act as weak Lewisbase donors to silacycles. This provides a donor-acceptor interaction that may be crucial for orienting the explosive analyte in the polymer film to provide an efficient pathway for inner-sphere electron transfer during the electrontransfer quenching process.

On the flocculation and re-dissolution of trivalent lanthanide metal ions by sodium dodecyl sulfate in aqueous solutions

“…The experimental complex Gd 3+ concentration data can be fitted by using the Lorentz equation from Origin 8.0 software (Fig. 2) [37]. The data show a maximum for a SDS molar fraction (0.75 ± 0.01) which is in complete agreement with the formation of a complex with a predominant stoichiometry 3:1 (DS À :Gd 3+ ).…”

Applications of nuclear shielding

“…Both resonances shift down-field in the presence of donor analytes and the observed shifts (0-1 ppm) correlate with the basicity of a variety of added Lewis bases, including TNT. 268 The quantitative structureproperties relationship (QSPR) was applied to modeling and predicting the 29 Si-NMR chemical shifts of a series of silicate species (on the Q2 sites). The descriptors that were selected by stepwise multiple linear regression technique were square of alpha polarisability, Moran autocorrelation-lag3/ unweighted by Sanderson electronegativities, square of asphericity and topological path/walk 2-Randic shape index.…”

Preparation of triazatriphenylene cations, promising chemosensors for nitro compounds

“…According to literature data, the efficiency of this electronic interaction can be increased by the introduction of Lewis acid fragments, e.g., metals or azinium cations into the poly(hetero)aromatic sensor molecule [3]. The structure of the polynitro compounds include Lewis base fragments (the oxygen atoms of the nitro groups), and hence an interaction with cations and other Lewis acids accelerates the "donor-acceptor" electronic communication, thus enhancing the photoluminescence quenching [4].…”

“…Contemporary synthetic methods frequently use hard to obtain alkenes or arylacetylenes in transition metal-catalyzed cycloaddition reactions with aromatic substrates [7,8]. On the other hand, a method of synthesizing aliphatic triphenylene precursors by cyclotrimerization of cyclohexanone [9] is widely known, and we have previously reported the preparation of 2-R-5,6,7,8,9,10,11,12-octahydro[1, 2,4]triazino[1,6-f]phenanthridin-13-ium-4-olates by reaction of 3-R-1,2,4-triazin-5(4H)-ones with cyclohexanone under acid catalysis conditions [10]. The 1,2,4-triazines 1a,b prepared in this way are of interest as triazatriphenylene precursors.…”

“…Fluorescence titration of compound 2a,b solutions with dichloromethane solutions of 2,4-dinitrotoluene and 2,4,6-trinitrophenol (picric acid) showed strong fluorescence quenching with values of the Stern-Volmer quenching constant of K sv ~10 3 -10 4 M -1 [4]. Cocrystallization of the triazatriphenylene 2a with 2,4,6-trinitrophenol in chloroform solution led to formation of the picrate 3a, the elemental analysis of which indicated a 1:1 stoichiometry.…”