Electron rich supramolecular polymers as fluorescent sensors for nitroaromatics

Abstract

Three π-electron rich fluorescent supramolecular polymers (1–3) have been synthesized incorporating 2-methyl-3-butyn-2-ol groups in reasonable yield by employing Sonagashira coupling. They were characterized by multinuclear NMR (¹H, ¹³C), ESI-MS and single crystal X-ray diffraction analyses [1 = 1-(2-methyl-3-butyn-2-ol)pyrene; 2 = 9,10-bis(2-methyl-3-butyn-2-ol)anthracene; 3 = 1,3,6,8-tetrakis(2-methyl-3-butyn-2-ol)pyrene]. Single crystal structures of 1–3 indicated that the incorporation of hydroxy (–OH) groups on the peripheral of the fluorophores helps them to self-associate into an infinite supramolecular polymeric network via intermolecular hydrogen bonding interactions between the adjacent discrete fluorophore units. All these compounds showed fluorescence characteristics in chloroform solution due to the extended π-conjugation and were used as selective fluorescent sensors for the detection of electron deficient nitroaromatics. The changes in photophysical properties of fluorophores (1–3) upon complex formation with electron deficient nitroaromatic explosives were studied in chloroform solution by using fluorescence spectroscopy. All these fluorophores showed the largest quenching response with moderate selectivity for nitroaromatics over various other electron deficient/rich aromatic compounds tested (Chart 1). Analysis of the fluorescence titration profile of 9,10-bis(2-methyl-3-butyn-2-ol)anthracene fluorophore (2) with 1,3,5-trinitrotoluene/2,4-dinitrotoluene provided evidence that this particular fluorophore detects nitroaromatics in the nanomolar range [2.0 ppb for TNT, 13.7 ppb for DNT]. Moreover, sharp visual color change was observed upon mixing nitroaromatic (DNT) with fluorophores (1–3) both in solution as well as in solid phase. Furthermore, the vapor-phase sensing study of thin film of fluorophores (1–3) showed efficient quenching responses for DNT and this sensing process is reproducible. Selective fluorescence quenching response including a sharp visual color change for nitroaromatics make these tested fluorophores (1–3) as potential sensors for nitroaromatic compounds with a detection limit of ppb level.