Star-shaped triazatruxene derivatives for rapid fluorescence fiber-optic detection of nitroaromatic explosive vapors

Abstract

Bright blue light-emitting molecules based on triazatruxene have been prepared and used for nitroaromatic explosive vapor detection. The fiber-optic probe using a star-shaped carbazole-functionalized triazatruxene derivative (TATCz3) as the sensing material exhibits promising potential as an extremely fast-response and highly sensitive sensor for detection of TNT and DNT vapors.

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Rapid detection of nitroaromatic explosive compounds, such as trinitrotoluene (TNT) and dinitrotoluene (DNT), has attracted much attention in recent years, because it is critical for homeland security, forensic analysis, and land mine detection.^1–4 Various spectroscopic instruments have been used for explosives detection, such as ion mobility spectrometry, gas chromatography coupled with mass spectrometry, and surface-enhanced Raman spectroscopy.^5–7 These instruments are usually bulky, expensive, complicated, and unsuitable for on-field testing. However, the detection of trace amounts of TNT and other nitroaromatic explosives remains a significant challenge due to the low volatility in solid state.⁸ Recently, fluorescence quenching based on photo-induced electron-transfer (PET) from excited fluorophores (electron-donor) to nitroaromatics (electron-acceptor) has been intensively investigated as one of the most sensitive and convenient methods for explosives detection.^1,9–14 Among various fluorescent small molecules and conjugated polymers, planar π-conjugated systems, such as pyrene, are believed to be highly beneficial for the binding to nitroaromatics through π–π interactions, which will facilitate the PET process.^4,15,16 Indeed, many pyrene and pyrene-based molecules and polymers have shown good selectivity and sensitivity for the detection of TNT.^4,15–20

Similar to pyrene, triazatruxene (TAT) is also a planar extended π-system comprising three carbazole units that share an aromatic ring.^21–23 In addition, its derivatives present good electron-donating capabilities, and have been widely studied as semiconducting liquid crystals and hole transporting materials.^22,24–28 Therefore, TAT derivatives could be promising sensor materials to detect nitroaromatics with high fluorescence quenching sensitivity. However, there is only one report of insoluble and unprocessable conjugated microporous polymer powders based on TAT for DNT vapor detection, which is unsuitable for thin film fabrication and the further incorporation into fluorescent sensing devices.²⁹

In the present study, we report the fluorescent film detection of TNT and DNT vapors with three TAT-based small molecules (TATC6, TATCz3 and TATF3 in Fig. 1) for the first time. Compared with the parent TAT, two star-shaped trifunctional TAT derivatives, with more extended π-conjugated chromophores, higher photoluminescence efficiencies and better film-forming abilities, have the potential to be used as fluorescent film sensor materials. The explosive vapors sensing properties are examined with their thin films coated on glass substrates. Fiber-optic sensors are suitable for portable, stealth, real-time remote monitoring, which is often important for explosives detection.^30,31 Thus, we further fabricated fiber-optic sensor with TATCz3 for explosive vapors detection.

Fig. 1 Chemical structures of triazatruxene derivatives.

The photophysical properties of TATs were characterized by absorption and photoluminescence (PL) spectroscopy in THF and in thin films (Fig. S1 and Table S1 in ESI†). Compared to the absorption maximum of TATC6 (318 nm), both TATCz3 and TATF3 show almost the same red-shifted absorption peak at ∼356 nm in THF due to the increase in conjugation length associated with fluorene and carbazole groups. Similarly, the emission peaks are red-shifted from 393 nm for TATC6 to 428 and 434 nm for TATCz3 and TATF3, respectively. Especially, while the emission of TATC6 is rather weak with a low fluorescence quantum yield (Φ_PL) of 7.0%, bright blue emission are observed for TATCz3 (Φ_PL = 71.4%) and TATF3 (Φ_PL = 49.8%) with much higher fluorescence quantum yields, which demonstrates the effect of the more extended conjugation lengths. It is worth noting that transparent smooth thin films of TATCz3 and TATF3 could be obtained by spin-coating from their THF solutions, while the homogeneous neat film of TATC6 is unavailable due to its poor film-forming ability. Thus, the solid-state photophysical properties of TATC6 were evaluated as its doped film in polystyrene. For all TATs, their thin film absorption and emission spectra show very small red shifts at their maxima relative to solutions, suggesting that intermolecular π–π stacking interactions between fluorophores are weak in these films.

To demonstrate the ability of TATs for nitroaromatics detection, fluorescence quenching titrations were performed by adding aliquots of various analytes to their THF solutions at first. As shown in Fig. 2a, clear fluorescent quenching can be observed upon the addition of TNT, DNT, 4-nitrotoluene (pNT), and nitrobenzene (NB). Electron-deficient benzophenone (BP) and duroquinone (DQ) also quenched the fluorescence of TATs modestly, but their quenching effects were lower than TNT and DNT. The other interferents such as toluene (Tol), ethanol (EtOH), and water have almost no effect on the emission of TATs. Especially, the parent TATC6 exhibits the highest fluorescence quenching responses to the nitroaromatic compounds over other analytes, indicating its good selectivity toward nitroaromatics. The quenching efficiencies of TATCz3 are slightly higher than those of TATF3. Anyway, in all cases, TNT is the best quencher. Linear Stern–Volmer plots were obtained from the fluorescence quenching titration profiles^32,34 and the calculated Stern–Volmer binding constants (K_SV) toward TNT were 815 ± 9 M⁻¹ for TATC6, 289 ± 5 M⁻¹ for TATCz3, 245 ± 5 M⁻¹ for TATF3 (Table S2 in ESI†), which are comparable with pentiptycene-derived conjugated polymers.¹⁰ Based on the PET process, the energy gaps between the lowest unoccupied molecular orbitals (LUMOs) of TATs and nitroaromatics are the key driving forces for electron-transfer.^8,10,33 The LUMO energy levels derived from the electrochemical and optical data (Fig. S5 and Table S1 in ESI†) follow the same order as their quenching constants: TATC6> TATCZ3> TATF3. The LUMO energy level of TATC6 (−1.73 eV) is about 0.3 eV higher than those of TATCz3 (−1.99 eV) and TATF3 (−2.03 eV), which results in its highest fluorescence quenching response in THF. In order to further understand the nature of fluorescent quenching of TATs with nitroaromatics, theoretical studies based on density functional theory calculations at the B3LYP/6-31G(d) level were performed. Both highest occupied molecular orbitals (HOMOs) and LUMOs of TATCZ3 and TATF3 spread to the surrounding carbazole and fluorene segments, indicating the extended delocalized π-conjugation (Fig. S6 in ESI†). Again, similar LUMO energy levels trend was found as the experimental results and all their LUMOs are much higher than those of nitroaromatics, supporting the PET process.

Fig. 2 (a) Quenching efficiency of various TAT derivatives for different analytes (0.6 mM). (b) Fluorescence quenching behaviours of TATC6 with gradual addition of TNT in THF. The inset shows the Stern–Volmer plot of TATC6 with different concentrations of TNT.

The sensing performances of thin films of these TATs doped in polystyrene on flat glass substrate were tested with incubation of DNT and TNT vapors. The thickness of these thin films was almost the same (∼90 nm). As shown in Fig. 3, all TATs exhibit obvious fluorescent quenching. In particular, upon exposure to DNT vapor, TATCz3 shows a fastest quenching of its blue emission by 48% in 60 s and nearly 89% in 600 s. The quenching response of TNT (21% quenching in 60 s and 50% quenching in 600 s) are not as high as DNT, perhaps due to its lower vapor concentration (ca. 5 ppb) relative to DNT (ca. 100 ppb). For TATC6 and TATF3, only 27% and 42% quenching were obtained upon 600 s exposure to TNT vapor. The quenching efficiencies of the three TATs are in the following order: TATCz3 > TATF3 > TATC6, which is different from their quenching behaviours in THF, perhaps due to higher exciton diffusion lengths for the star-shaped extended π-conjugation in TATCz3 and TATF3, which is especially important in film fluorescent sensors.^34–36 For comparison, we prepared pyrene-doped film at the same condition. In both DNT and TNT vapors, the fluorescence quenching of TATs are much better than pyrene, demonstrating the effect of the strong electron-donating property of the planar triazatruxene unit, which facilitates their interaction with nitroaromatics.

Fig. 3 Time-dependent fluorescence quenching of various TAT derivatives and pyrene doped polystyrene films on exposure to saturated (a) DNT and (b) TNT vapors.

Quenching experiments were further carried out for TATCz3 and TATF3 neat films (∼90 nm thickness) because of their good film-forming property. Much faster fluorescent responses were observed than their doped films (Fig. 4a). Notably, upon exposure to TNT vapor, the quenching of TATCz3 neat film reached 50% in just 10 s and 73% in 60 s, which is comparable to the best results for conjugated polymers.¹ For the more volatile DNT, TATCz3 neat film exhibited a more rapid response. Its emission can be quenched by 79% in 10 s and virtually disappeared completely after just 60 s. In comparison with TATCz3, TATF3 neat film showed lower sensitivities toward both TNT and DNT with 59% and 77% quenching in 60 s, respectively. Considering its good sensitivity and fast response, high fluorescence quantum yield and proper film-forming property, TATCz3 seems to be the most suitable material for explosive sensing.

Fig. 4 (a) Time-dependent fluorescence quenching of TATCz3 and TATF3 neat films on exposure to saturated DNT and TNT vapors. (b) A schematic drawing of an experimental setup for fiber-optic sensing of explosives vapors. Inset: beam paths of excitation and emission. (c) Time-dependent fluorescence quenching of TATCz3 film coated on fiber-optic tips upon exposure to saturated DNT and TNT vapors. Inset: fiber-optic tip images before (left) and after (right) exposure to DNT vapor. (d) Ten continuous cycles of quenching-recovery test of TATCz3 film coated on the fiber-optic tip. The quenching was performed by exposing the film to the saturated vapor of DNT for 15 s. After each cycle of quenching, the fluorescence of the film was recovered by air blowing for 1 min.

Finally, fiber-optic probes were fabricated by dip-coating the TATCz3 solutions in THF on clean optical fiber tips (Fig. 4b). The fiber-optic detection provides even faster quenching responses than the films coated on flat glass substrate. The fluorescence of TATCz3 is completely quenched (99% fluorescence quenching) in DNT vapor for just about 15 s. Upon exposure to TNT vapor, the fluorescence intensity also dropped by 66% in 20 s and 89% in 1 min (Fig. 4c). This good performance is among the best fluorescent film sensors for TNT vapor detection.^1,4,12,37,38TATCz3 also shows good selectivity to nitroaromatics compared to the interferential vapors, such as BP, DQ, toluene, ethanol, and water (Fig. S9 in ESI†). Furthermore, the fiber-optic detection showed highly reversible fluorescent response. The quenched fluorescence after exposure to DNT vapor can be recovered by simply blowing with an air blower for 1 min, and can be used for DNT vapor sensing again. These on-off cycle could be repeated for at least ten times (Fig. 4d), indicating high stability and reversibility of the fiber-optic detection system with TATCz3.

Conclusions

In conclusion, bright blue light-emitting molecules based on triazatruxene have been prepared and used for nitroaromatic explosive vapors detection for the first time. Among them, a star-shaped carbazole-functionalized triazatruxene derivative (TATCz3) exhibits extended π-conjugation, high photoluminescence efficiency and good film-forming property. The fiber-optic probe using TATCz3 as the sensing material exhibits promising potential as an extremely fast-response and highly sensitive sensor for detection of TNT and DNT vapors.

Acknowledgements

This paper was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12010200), the 973 Project (No. 2015CB655001), the Science Fund for Creative Research Groups (No. 20921061), and the National Natural Science Foundation of China (No. 91333205, 51173179, 21204080, 21074130, 21574131, and 21322403).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, synthetic route, absorption and fluorescence spectra, cyclic voltammograms, theoretical calculations. See DOI: 10.1039/c6ra04553g

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