Organic nanofibrils based on linear carbazole trimer for explosive sensingw

Detection of trace explosives is of great concern for homeland security, battlefield protection, and industrial and environmental safety control. Fluorescence quenching based sensing has proven to be one of the most promising approaches for trace explosives detection, for which various conjugated polymers, molecular imprinted polymers, dye doped silica and metal–organic frameworks have been fabricated into films and fluorescence quenching upon exposure to the vapor of nitroaromatic explosives (e.g., TNT, DNT) has been explored. However, the quenching efficiency of these materials is often limited by short exciton diffusion due to poor molecular organization and/or weak intermolecular electronic interactions as usually observed for polymer based materials. Recently, our group has reported an alternative approach to increase fluorescence sensing efficiency by fabricating rigid aromatic molecules into nanofibrils. These novel one-dimensional nanostructures possess long range exciton diffusion due to the extended intermolecular p p electronic interaction. Upon deposition onto a substrate, the nanofibrils form a nanoporous film in a variable range of porosity through entangled piling of the individual fibrils. The large surface area to volume ratio and porosity thus formed, in combination with the amplified fluorescence quenching relied on the enlarged exciton diffusion, usually enable expedient, effective vapor detection of nitroaromatic explosives. Particularly, for the building-block molecules containing carbazole as the conjugation unit, the nanofibrils fabricated demonstrate not only high sensitivity in vapor sensing, but also strong selectivity towards nitrobased explosives against other common chemical liquids and solids. However, the macrocyclic conjugation structure of these molecules imposes much difficulty to synthesis because it demands special catalysts for making the precursors. To make the nanofibrils more practical in vapor sensing, it is imperative to find alternative building-block molecules that are easier to synthesize but still maintain the chemical properties and features suited for fabrication into nanofibrils and fluorescence sensing of nitrobased explosives. Here, we report an easy synthesis of a 2,7-linked carbazole trimer (Scheme 1) and fabrication into nanofibril structures. The nanofibril film thus fabricated demonstrated efficient, fast fluorescent sensing not only for nitroaromatic explosives (e.g., TNT, DNT), but also for nitroaliphatic explosives (e.g., nitromethane), which remain difficult to detect due to their high volatility and low chemical binding to many sensory materials. The carbazole trimer was synthesized from commercial available 2,7-dibromo-9-octyl-9H-carbazole and 9H-carbazole2-boronic acid pinacol ester in one step by Suzuki-coupling reaction (Scheme 1). The 2,7-substituted carbazoles, compared to 3,6-substituted ones, are ideal building blocks for rigid, rodlike molecules with a longer conjugation distance. Introducing one octyl substituent gives the trimer necessary solubility for solution processing while maintaining minimal sterical hindrance to facilitate the intermolecular arrangement and one-dimensional self-assembly into nanofibrils. Indeed, welldefined nanofibrils could feasibly be fabricated from this molecule through a bisolvent phase transfer process (see ESIw). In comparison, when substituting all the three carbazoles with alkyl chains, the self-assembly through the same process gave only ill-defined big chunks. Fig. 1(a) shows a SEM image of the nanofibrils fabricated from the carbazole trimer deposited on a silicon substrate. The nanofibrils feature a high aspect ratio, with diameter of only ca. 30 nm and length of several microns. The deposited nanofibrils formed a nanoporous film consisting of entangled fibril networks. The nanofibrils exhibit strong blue fluorescence upon excitation under UV light as shown by fluorescence microscopy imaging (Fig. 1(b)). The maximum wavelength of the fluorescence was measured at 438 nm, 44 nm longer than that measured for the molecules dissolved in solution (Fig. S1, ESIw), which is characteristic of strong intermolecular electronic interaction typically observed in molecular aggregates. Such strong intermolecular interaction is also indicated by the bathochromic shifted absorption peak Scheme 1 Synthesis of 2,7-linked carbazole trimer.