Lewis acid-base interactions enhance explosives sensing in silacycle polymers

Area of Science:

• Materials Science
• Polymer Chemistry
• Analytical Chemistry

Background:

• High sensitivity of silole- and silafluorene-containing polymers for explosive detection is observed.
• Existing explanations focus on hydrophobic interactions and fluorescence quenching via delocalization.
• The role of silicon's Lewis acidity in these polymer sensors is underexplored.

Purpose of the Study:

• To investigate the contribution of silicon's Lewis acidity to the sensing mechanism of conjugated poly(silafluorene-vinylene)s for explosives.
• To correlate Lewis acidity with analyte binding and sensor performance.
• To elucidate the interaction between explosive nitro groups and the polymer's silacycle structure.

Main Methods:

• Synthesis and characterization of a model trimer fragment of poly(silafluorene-vinylene).
• Nuclear Magnetic Resonance ((29)Si NMR) spectroscopy to determine chemical shifts and Lewis acidity.
• UV-Vis absorption spectroscopy to monitor changes upon analyte interaction.
• Scatchard analysis to calculate association constants for analyte binding.
• Spin-lattice relaxation time (T(1)) measurements to confirm analyte-polymer interactions.

Main Results:

• Peripheral and central silicon atoms in the trimer exhibit distinct chemical shifts.
• Silicon resonances shift down-field in the presence of Lewis bases, correlating with their basicity.
• An association constant (K(a)) of 0.12 M(-1) was determined for acetonitrile binding to peripheral silicon.
• Spin-lattice relaxation times of acetonitrile protons significantly changed upon addition of the silafluorene trimer.
• UV-Vis absorption spectra showed significant changes, supporting analyte-polymer interactions.

Conclusions:

• The Lewis acid character of silicon in strained silacycles is a critical factor in the high sensitivity of these polymer sensors for explosives.
• Nitro groups of high explosives can act as Lewis bases, donating to silacycles.
• This donor-acceptor interaction likely facilitates analyte orientation and efficient electron transfer quenching, enhancing detection sensitivity.