Efficient blue-emitting silafluorene–fluorene-conjugated copolymers: selective turn-off/turn-on detection of explosives

Abstract

The synthesis and spectroscopic characterization of a series of new blue-emitting silafluorene–fluorene copolymers is described. The polymers are synthesized using kinetically controlled hydrosilylation copolymerization of 1,1-dihydridosilafluorene with a series of 9-substituted 2,7-diethynylfluorenes. The polymers contain a trans-only framework with molecular weights in the range of 13 000–20 000, as determined by gel permeation chromatography (GPC) using polystyrene standards, and by ¹H NMR spectroscopy using dimethylphenylsilane as an end-capping marker group. The three stereoregular polymers synthesized include a 9,9-dihydridofluorene (PSF1), a 9,9-dimethyl-9H-fluorene (PSF2), and a 9,9′-spirobifluorene (PSF3) comonomer with the frameworks. These fluorenyl units are conjugated through the silicon center of the silafluorene moiety by bridging vinylene groups. Quantum yields of fluorescence range from 20 to 100% with PSF3 having the highest quantum efficiency. Polymers PSF1-3 emit in the blue region of the spectrum (∼475 nm), showing good color purity with little change in luminescence properties between the solution and solid-state phases. The polymers were tested for explosives detection properties by a fluorescence-quenching mechanism. Targeted explosives include laboratory prepared TNT, DNT, picric acid, RDX, HMX, PETN, TNG, and Tetryl, as well as production line PETN and C-4. All three polymers exhibit detection of explosive particulates with limits as low as 1 pg cm⁻² for Tetryl. Polymer PSF1 simultaneously acts as a selective fluorescence “turn-on” sensor for nitrate ester explosives when irradiated with UV light. In the presence of nitrate ester-based explosives such as PETN, PSF1 initially exhibits fluorescence quenching, but continued exposure to UV-light (302 nm), promotes a photochemical reaction forming a luminescent green fluorenone copolymer. This is the first example of a single material acting as both a turn-off and turn-on selective fluorescent sensor for an explosive material.