Fluorescent aggregates of AIEE active triphenylene derivatives for the sensitive detection of picric acid

Abstract

Unprecedented fluorescent aggregates of triphenylene derivatives 3 and 5 having aggregation induced emission enhancement (AIEE) characteristics have been developed which selectively detect picric acid in mixed aqueous media.

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Introduction

Triphenylene based materials are important due to their unique optoelectronic properties and their capability to undergo self-assembly in the bulk as well as in solution phase.¹ Self-assembly of triphenylene derivatives in solution phase is an important fabrication tool for building complex functional architectures.² However, in solution these triphenylene derivatives form non-fluorescent H-aggregates due to efficient intermolecular π–π stacking.³ On the contrary, for the fabrication of optical and electronic devices such as organic light emitting diodes (OLEDs), organic light-emitting field-effect transistors (OLEFETs) and organic fluorescent sensors, luminescent π-conjugated organic molecules are required.⁴ Though there are several reports regarding the development of fluorescent aggregates of triphenylene derivatives in organic media⁵ there is still no report regarding the development of fluorescent aggregates of triphenylene derivatives in aqueous media. Thus, preparation of triphenylene derivatives which undergo self-assembly to form luminescent supramolecular aggregates is still a challenge.

Our research work involves development of new fluorescent assemblies in mixed aqueous media and their utilization for detection of various analytes such as metal ions, anions, amino acids and nitroaromatic explosives.⁶ In continuation of this work, we were then interested in the development of fluorescent supramolecular assemblies based on triphenylene derivatives in aqueous media and for this purpose, we designed and synthesized triphenylene derivative 3 having cyano groups at the periphery. We envisioned that molecules of derivative 3 will be packed in slipped arrangement due to twist enforced by bulky cyano groups at the periphery.⁷ The packing of molecules in slipped fashion will prevent the π–π intermolecular stacking, thus, making the molecule more emissive in the aggregated state. We further envisioned that presence of phenyl groups with rotatable C–C/C–N single bonds will impart aggregation induced emission enhancement (AIEE) characteristics to the derivative 3 and will make the molecule emissive in the aggregated state.⁸ To our pleasure, derivatives 3 exhibits AIEE characteristics and formed fluorescent aggregates in mixed aqueous media. To the best of our knowledge, this is the first report where triphenylene derivative undergoes aggregation to generate fluorescent supramolecular aggregates in mixed aqueous media. To study the influence of electronic nature of substituents on aggregation behaviour of molecule, we have also synthesized derivative 5 having electron rich amino groups at the periphery. Interestingly, derivative 5 exhibited marginally higher emission enhancement (1.6 folds) in aqueous media in comparision to that of derivative 3 (1.5 folds) and formed fluorescent aggregates in aqueous media. Additionally, fluorescent aggregates of derivatives 3 and 5 have been used for simple, fast, sensitive and selective detection of picric acid (PA). Picric acid is a strong irritant and allergen and is very harmful to human beings.⁹ Unfortunately, the widespread military and non-military use of PA has made it a significant environmental pollutant.¹⁰ The concern over the adverse effects of PA on environment and health provides the sufficient impetus to develop cost efficient, selective, portable, fast and sensitive methods for trace detection of PA in aqueous media. Recently, several fluorescent chemosensors have been reported for the sensitive detection of picric acid¹¹ however application of triphenyelene based derivatives for detection of explosives is still very less explored. Triphenylene derivatives are electron rich, however, formation of non-fluorescent H-aggregates in aqueous media impairs the utility of triphenylene derivatives as chemosensors for nitroaromatics in aqueous media. Interestingly, fluorescent aggregates of derivatives 3 and 5 can detect the PA in nanomolar range in mixed aqueous media and the solution coated paper strips of derivative 5 could detect picric acid in the range of femtogram level. To the best of our knowledge, this is the first report where fluorescent aggregates of triphenylene derivatives have been used for the detection of picric acid in aqueous media.

Results and discussion

Suzuki–Miyaura coupling of boronic ester 2¹² with 2,3,6,7,10,11-hexabromotriphenylene 1 furnished triphenylene derivative 3 in 50% yield (Scheme 1). The structure of derivative 3 was confirmed from its spectroscopic and analytical data. The ¹H NMR spectrum of derivative 3 showed two doublets (12H each) at 7.40 and 7.64 ppm and a singlet (6H) at 8.68 ppm corresponding to aromatic protons (Fig. S18 ESI†). The MALDI-TOF mass spectrum of derivative 3 showed a peak at m/z 834.163 (M⁺) (Fig. S20, ESI†). These spectroscopic data corroborates the structure 3 for this compound.

Scheme 1 Synthesis of triphenylene derivative 3.

Derivative 3 shows an absorption band at 306 nm in pure THF (Fig. S1, ESI†). Upon increasing water fraction from 0 to 70%, a red shift of 12 nm is observed in the absorption spectrum along with appearance of levelling of tails in the visible region of the spectrum. The fluorescence spectrum of derivative 3 exhibits an emission band at 393 nm (φ = 0.29) (Fig. 1A). Upon increasing the water fraction from 0 to 50%, 1.5 folds enhancement (φ = 0.44) in the emission signal was observed. Further increase in the water fraction from 60 to 90% resulted in red shifting with broadening of emission band along with the decrease in emission intensity (Fig. S2, ESI†).¹³ The observed behaviour is due to the presence of electron deficient cyano groups at the periphery of triphenylene which make it donor–acceptor system and such type of donor–acceptor systems tend to adopt twisted conformation in more polar solvents and exhibit twisted intramolecular charge transfer (TICT) in excited state which is susceptible to various non-radiative quenching processes.¹⁴ Scanning electron microscope (SEM) image of derivative 3 in H₂O : THF (1 : 1) mixture showed flake like morphology of aggregates (Fig. 1B).

Fig. 1 (A) Fluorescence emission spectrum of derivative 3 (10 μM) in different fraction of water in THF; λ_ex = 306 nm. (B) SEM image of derivative 3 in H₂O : THF (1 : 1) mixture.

On the basis of absorption and fluorescence studies, we believe that molecules of derivative 3 are packed in slipped face to face fashion in aggregated state¹⁵ and formation of non-fluorescent H-aggregates is restricted. The dynamic light scattering (DLS) studies of derivative 3 in 30% and 50% H₂O : THF solvent mixture showed aggregates of average size 350 nm and 580 nm, respectively (Fig. S3, ESI†). Thus, the enhancement of the emission signal is also related to the aggregation driven growth of aggregates of derivative 3.¹⁶ We also carried out the fluorescence studies of derivative 3 in different fractions of triethylene glycol (TEG) in THF. Upon addition of 90% volume fraction of TEG to the THF solution of derivative 3, fluorescence enhancement was observed (Fig. 2A). Such an increase in fluorescence intensity suggests restriction of intramolecular restriction (RIR) of phenyl rings with respect to rigid triphenylene core upon increasing the viscosity of the medium.¹⁷ Further, we also carried out time resolved fluorescence studies of derivatives 3 in THF and in aggregated state. The life time of derivative 3 was found to be higher in its aggregated state in comparison to its solution in THF (Fig. 2B and Table 1 in ESI†). This increase in lifetime of derivative 3 suggests the formation of ordered aggregates.¹⁸

Fig. 2 (A) Fluorescence emission spectrum of derivative 3 (10 μM) upon increasing the TEG fraction in THF. (B) Time resolved fluorescence spectrum of derivative 3 (10 μM) in pure THF and H₂O : THF (1 : 1) mixture; (λ_ex = 377 nm).

Further to evaluate the electronic nature of the substituents on the AIEE behaviour of triphenylene derivative, we synthesized symmetrically substituted triphenylene derivative 5 having electron rich amino groups at the periphery by the reported method (Fig. 3A).¹⁹ The fluorescence spectrum of derivative 5 showed emission at 428 nm in pure THF (φ = 0.34)²⁰ when excited at 322 nm (Fig. 3B). Upon addition of water fractions up to 70%, fluorescence enhancement (φ = 0.54) was observed along with the bathochromic shift of 30 nm. Further, addition of more than 70% water fraction to THF solution of derivative 5 resulted in decrease in fluorescence emission intensity. This phenomenon is often observed in derivatives with AIEE properties as after the aggregation only the molecules on the surface of aggregate emit light and contribute to the fluorescent intensity upon excitation and this leads to a decrease in emission intensity. The SEM image of derivative 5 in H₂O : THF (7 : 3) mixture shows the formation of irregular shaped aggregates (Fig. 3C). DLS studies of derivative 3 in 30%, 50% and 70% H₂O : THF solvent mixture showed aggregates of average size 210 nm, 615 nm and 1281 nm, respectively. (Fig. S4A–C, ESI†) and linear relationship between emission enhancement and size of aggregates of derivative 5 was observed (Fig. S4D, ESI†). Further, the lifetime of derivative 5 in aggregated state was found to be higher than that in the molecular state which suggests the formation of ordered aggregates (Fig. 3D).

Fig. 3 (A) Structure of derivative 5 (B) fluorescence emission spectra of derivative 5 (10 μM) in different fractions of H₂O in THF; λ_ex = 322 nm. (c) Scanning electron microscopic (SEM) image of derivative 5 in H₂O : THF (7 : 3) mixture. (D) Time resolved fluorescence spectrum of derivative 5 (10 μM) in THF and H₂O : THF (7 : 3) mixture (λ_ex = 377 nm).

We also carried out the concentration dependent ¹H NMR studies of derivatives 3 and 5. In case of derivative 3, broadening of aromatic signals was observed upon increasing the concentration (Fig. S5, ESI†). However, upfield shifting in the signals corresponding to the aromatic protons was observed in case of derivative 5 which suggests the intermolecular π–π stacking between molecules of derivative 5²¹ (Fig. S6, ESI†). On the basis of absorption, fluorescence, DLS, time resolved and NMR studies we believe that molecules of derivatives 3 and 5 are packed in slipped fashion and restriction of intramolecular rotation along with aggregation driven growth are the main reasons for the observed emission enhancement in these derivatives.

In comparison, derivative 5 having electron rich substituents, exhibits higher emission enhancement due to AIEE phenomena that too in presence of higher volume fraction of water (70%) in comparison to that in case of derivative 3 (H₂O : THF, 1 : 1). Formation of TICT state, as in case of donor–acceptor system 3 seems to be the reason behind this observation. Thus, we believe that the presence of electron rich substituents should be beneficial for the development of triphenylene based AIEE active materials.

Further, the high emission intensity of aggregates of triphenylene derivatives 3 and 5 prompted us to evaluate their application as potential chemosensors for nitroaromatic explosives. Thus, keeping this in mind, we studied the recognition behaviour of these AIEE active triphenylene derivatives in H₂O : THF mixture toward various nitroaromatic compounds (NACs) such as picric acid (PA), 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), 1,4-dinitrobenzene (DNB), nitrobenzene (NB), 4-nitrotoluene (4-NT), 1,4-benzoquinone (BQ), nitromethane (NM) and 2,3-dimethyl-2,3-dinitrobutane (DMNB) by fluorescence spectroscopy. Among the various nitroaromatic compounds tested, the addition of 20 equiv. of picric acid to the solution of derivative 3 in H₂O : THF (1 : 1) mixture results in 98% quenching of fluorescence emission (Fig. 4A). The quenching of fluorescence of derivative 3 was studied by Stern–Volmer plot (Fig. S7, ESI†). The Stern–Volmer plot was found to be linear at lower concentration of picric acid (upto 5 equiv. Inset Fig. S7, ESI†) with Stern–Volmer constant of 1.11 × 10⁵ M⁻¹ but at higher concentration it bends upward due to the superamplified quenching effect.²² The detection limit of derivative 3 was found to be 40 nM (Fig. S8, ESI†) which is better than other chemosensors reported in literature (Table 3, ESI†). To get insight into mechanism of fluorescence quenching of derivative 3 with picric acid, we have carried out the fluorescence lifetime studies of derivative 3 at different concentrations of picric acid. The fluorescence lifetime of aggregates of derivative 3 was found to be invariant (Fig. S9, ESI†) at different concentration of picric acid which suggests the static mechanism for the fluorescence quenching. Despite the presence of electron withdrawing cyano groups, the aggregates of derivative 3 exhibit sensitive response towards picric acid. We believe that flake like morphology of aggregates of derivative 3 provide the large number of channels for the migration of excitons which results in higher sensitivity towards picric acid. Thus, aggregates of derivative 3 displays morphology assisted sensing of picric acid.²³

Fig. 4 (A) Fluorescence spectra of derivative 3 (10 μM) showing the variation of fluorescence intensity upon the addition of picric acid from 0–20 equiv. in H₂O : THF (1 : 1) mixture (λ_ex = 306 nm). (B) Fluorescence spectra of derivative 5 (10 μM) showing the variation of fluorescence intensity upon the addition of picric acid from 0–26 equiv. in H₂O : THF (7 : 3) mixture (λ_ex = 322 nm).

Under similar set of conditions as used for aggregates derivative 3, we also carried out the fluorescence studies of derivative 5 with picric acid and quenching of emission was observed upon the addition of 26 equiv. of picric acid (Fig. 4B). The detection limit of aggregates of derivative 5 was found to be in the range of 35 nM (Fig. S10, ESI†). The Stern–Volmer constant was found to be 1.95 × 10⁵ M⁻¹. To the best of our knowledge, the Stern–Volmer constant shown by aggregates of derivative 5 is better/comparable than other chemosensors reported in literature (Table 4, ESI†). Picric acid is a strong acid²⁴ and readily undergoes dissociation to give free protons which results in the protonation of basic NH₂ groups of aggregates of derivative 5. We believe that electrostatic interactions between aggregates of protonated form of derivative 5 with picrate anions resulted in fluorescence quenching as shown in Scheme 2.²⁵

Scheme 2 Mechanism of quenching of fluorescence of derivative 5 with picric acid.

To get insight into the quenching mechanism, we carried out ¹H NMR studies of derivative 5 in the presence of picric acid in DMSO-d₆ (Fig. S11, ESI†). In the presence of picric acid, signal corresponding to NH₂ protons disappeared completely and downfield shift is observed in the signals corresponding to aromatic protons (Table 2, ESI†). Similar downfield shift in the signals was observed in the ¹H NMR studies of derivative 5 in DMSO-d₆ in the presence of trifluoroacetic acid (TFA). These studies validate the proposed mechanism involving the electrostatic interaction between the protonated form of derivative 5 and picrate ions. Under similar set of conditions, we also studied the recognition behaviour of aggregates of derivative 3 and 5 in H₂O : THF mixture toward other nitroaromatic compounds (NACS) such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), 1,4-dinitrobenzene (DNB), nitrobenzene (NB), 4-nitrotoluene (4-NT), 1,4-benzoquinone (BQ), nitromethane (NM) and 2,3-dimethyl-2,3-dinitrobutane (DMNB). The quenching was also observed with other NACs, however much higher equivalents of nitroderivatives were used. The relative percentage quenching with various nitroaromatics is summarized in bar diagram by taking 20/26 equiv. of picric acid for derivative 3 and 5, respectively (Fig. S12, ESI†). From bar diagram, it is clear that aggregates of derivative 3 and 5 showed sensitive response towards picric acid. High sensitivity of aggregates of derivatives 3 and 5 towards picric acid may be attributed to the electron deficient nature and high polarizability of picric acid.²⁶ Moreover, the large spectral overlap of emission spectra of aggregates of derivative 3 with absorption spectrum of picric acid is observed which further supports the energy transfer as the probable mechanism of quenching (Fig. S13, ESI†). We believe that quenching mechanism in case of picric acid is long range energy transfer process²⁷ whereas in case of other nitroaromatics, the main quenching mechanism is short range charge transfer process. This explains the higher sensitivity of aggregates of derivative 3 towards picric acid in comparison to other nitroaromatics. Further, electrostatic interactions in case of derivative 5 account for its high selectivity towards picric acid. To confirm the selectivity of derivative 3 and 5 towards picric acid we have also carried out the quenching experiment in the presence of other nitroaromatic compounds such as 4-NT, TNT, DNT, DNB, NB, DMNB, BQ and NM. For this 20/26 equiv. of each NAC is added to the solution of derivative 3/5 respectively followed by the addition of same equivalents of PA. It has been observed that initial addition of various NACs results in the negligible quenching of fluorescence whereas it showed drastic quenching upon the addition of picric acid (see Fig. S14–S15; ESI†). Thus, derivatives 3 and 5 showed sensitive response towards picric acid even in the presence of other NACs. Further, we have also checked the fluorescence response of derivative 3 and 5 with various metal ions (Cu²⁺, Fe³⁺, Zn²⁺, Hg²⁺, Ni²⁺ as their perchlorate/chloride salts) and anions (F⁻, CN⁻, OAc⁻, Cl⁻, Br⁻, I⁻, EDTA⁻⁴, HSO₄⁻, NO₃⁻, H₂PO₄⁻, ClO₄⁻) in H₂O : THF (1 : 1 and 7 : 3) mixture respectively however insignificant quenching was observed with these metal ions and anions (Fig. S16 and S17 in ESI†)

During the packing of explosive materials, contamination of the clothes, body and surrounding materials may occur²⁸ thus, to detect the contamination; we have also prepared the test strips by dip coating the solution of aggregates of derivatives 5 in H₂O : THF (7 : 3) which can detect the trace amounts of picric acid in solid state. For the detection of trace amounts of picric acid, we prepared the aqueous solution of picric acid of varying concentration. The 10 μl of these solutions was placed over the solution coated strips of aggregates of derivative 5 covering an area of about 1 cm⁻². The minimum amount of picric acid that can be detected by naked eye was found to be 10⁻¹¹ M which corresponds to 22.9 × 10⁻¹⁵ g cm⁻² of picric acid (Fig. 5). These results show the practical applicability of derivatives 5 for the fast, reliable and visual on site trace detection of picric acid.

Fig. 5 Photographs of solution coated paper strips of derivative 5 showing fluorescence quenching (under UV light of 365 nm); (i) blank paper strip (ii) a paper strip with a drop of water; paper strips upon the addition of 10 μl solution of PA of different concentrations (iii) 10⁻³ M (iv) 10⁻⁵ M (v) 10⁻⁷ M (vi) 10⁻⁹ M (vii) 10⁻¹¹ M.

Conclusion

In conclusion, we designed and synthesized triphenylene derivatives 3 and 5 which form fluorescent aggregates in aqueous media due to the synergetic effect of slipped packing of molecules, intramolecular restriction of rotation and aggregation driven growth of the aggregates. Additionally, aggregates of these discotic molecules act as potent chemosensor for picric acid and can detect the trace amount of picric acid explosive in the nanomolar range in aqueous medium. Further, the solution coated test strips of derivative 5 can detect upto 22.9 × 10⁻¹⁵ g cm⁻² of picric acid which provides a cheap, portable and easily accessible method of onsite trace detection of picric acid.

Experimental section

(a) General experimental information and instrumentation

All reagents were purchased from Aldrich and were used without further purification. THF (AR grade) was used to perform analytical studies. Silica gel 60 (60–120 mesh) was used for column chromatography. All the UV-Vis spectra were recorded on SHIMADZU UV-2450 spectrophotometer with a quartz cuvette (path length, 1 cm). The cell holder was thermostatted at 25 °C. All the fluorescence spectra were recorded on SHIMADZU RF 5301 PC spectrofluorometer. ¹H and ¹³C NMR spectra were recorded on JEOL-FT NMR-AL 300 MHz and BRUKER-AVANCE-II FT-NMR-AL 500 MHz spectrophotometer using CDCl₃ as solvent and TMS as internal standards. Data are reported as follows: chemical shifts in ppm (δ), multiplicity (s = singlet, d = doublet, br = broad singlet, m = multiplet), coupling constants (Hz), integration, and interpretation. Elemental analysis (C, H, N) was performed on a Flash EA 1112 CHNS–O analyzer (Thermo Electron Corp.). Scanning electron microscope (SEM) images were obtained with a field-emission scanning electron microscope (FESEM) JEOL JSM-6610LV. Time resolved fluorescence studies were performed on Horriba Time Resolved Fluorescence Spectrometer instrument using time correlated single photon counting (TPSPC) technique. The dynamic light scattering (DLS) data was recorded using Zetasizer Nano, ZS Malvern Instrument Ltd., U.K.

(b) Calculation of Stern–Volmer constants

The sensitivity of derivatives 3 and 5 toward the nitroaromatic explosives was estimated from their Stern–Volmer constants, K_sv as determined from equation

I_o/I = 1 + K_sv [Q]

where I_o and I are the fluorescence intensities in the absence and presence of nitroaromatics respectively and Stern–Volmer plots were plotted as a function of nitroaromatic concentration [Q]. The Stern–Volmer constants, K_sv can be calculated from the slope of Stern–Volmer plots.

(c) Preparation of paper strips

Paper strips for solid state detection of picric acid were prepared by coating the solution of derivatives 5 on Whatman filter paper™ followed by the removal of solvent under vacuum. The TLC strips were then cut into 1 cm × 1 cm dimensions, which were used as test strips for explosive sensing.

(d) Synthesis of derivative 3

To the mixture of suspension of 4-cyanophenylboronic acid 2 (350 mg, 2.38 mmol), bis(triphenylphosphine)dichloro-palladium(II) (63 mg, 0.090 mmol) in 1,4-dioxane (12 ml) was added a suspension of 2,3,6,7,10,11-hexabromotriphenylene 1 (253 mg, 0.36 mmol) in an aqueous solution (2 ml) of potassium carbonate (600 mg, 4.32 mmol). The mixture was refluxed overnight under N₂ and allowed to cool to room temperature. The mixture was extracted with CHCl₃. The organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The crude product was then purified by column chromatography (95 : 5; Hex : CHCl₃) to give the product 3 in 50% yield. ¹H NMR (300 MHz, CDCl₃, δ ppm): 7.40 (d, J = 8.1 Hz, 12H, ArH); 7.64 (d, J = 8.4 Hz, 12H, ArH); 8.68 (s, 6H, ArH) ¹³C NMR (75.45 MHz, CD₂Cl₄): 111.27, 118.93, 126.00, 129.64, 130.83, 132.40, 132.58, 138.79, 145.08. MS (MALDI-TOF) m/z 834.1631 (M)⁺. Elemental analysis: calculated for: C₆₀H₃₀N₆: C, 86.31; H, 3.62; N, 10.07 found: C, 86.30; H, 3.61; N, 10.09.

Acknowledgements

V.B. is thankful to DST, New Delhi (ref. no. SR/S1/OC- 69/2012), CSIR, New Delhi [ref. no. 02(0083)/12/EMR-II] and UGC, New Delhi [ref. no. 40-75/2011 (SR)] for financial support. We are also thankful to UGC, New Delhi for providing infrastructural facilities under the UPE scheme. HA is thankful to CSIR for senior research fellowship (SRF).

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Footnote

† Electronic supplementary information (ESI) available: The contents of the SI section include ¹H, ¹³C, mass spectra, fluorescence lifetime, DLS studies etc. of compounds 3 and 5. See DOI: 10.1039/c5ra04337a

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