Rhodamine appended hexaphenylbenzene derivative: through bond energy transfer for sensing of picric acid

Abstract

A rhodamine appended hexaphenylbenzene based derivative 5 has been designed and synthesized, which displays through bond energy transfer in the presence of picric acid in MeOH and serves as “turn on” sensor for picric acid. Furthermore, derivative 5 exhibits an intense colorimetric response towards picric acid and can selectively detect picric acid with a detection limit of 104.9 pg cm⁻².

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Introduction

Picric acid (PA) (2,4,6-trinitrophenol) is considered as an environmental contaminant and is toxic to living organisms. It can cause skin/eye irritation and can affect the organs involved in the respiratory system.^1,2 PA is used in dye manufacturing, pharmaceuticals, and chemical laboratories; moreover, it is used as a sensitizer in photographic emulsions and as a component in matchsticks.^3,4 Furthermore, PA is well known for its explosive characteristics as it is widely used in rocket fuel and fireworks, and just a booster is required to create the explosion. Furthermore, it is highly soluble in water and can contaminate soil and groundwater when exposed. Unfortunately, the degradation of PA is difficult in the biosystem due to its electron-deficient character.⁵ Therefore, the development of cost-effective, selective, sensitive, fast and portable methods for the detection of PA is a priority for a healthy and green environment.^6–9 Fluorescence signalling is one of the first choices because of its sensitivity and selectivity, and a variety of fluorescent sensors have been reported, most of which display a ‘turn off’ response towards PA.^10–15 However, chemosensors showing ‘turn on’ responses are advantageous because their sensing mechanism leads to a significant increase in emission intensity and the changes are clearly visible to the naked eye. Very recently, we¹⁶ and others^17–20 reported the development of new probes for the ‘turn on’ detection of picric acid. However, the reported chemosensors suffer from some limitations such as very small Stokes shift, weak intensity of the final emission signal, emission changes centred around the same wavelength, low selectivity towards PA, use of organic solvents for sensing process and irreversibility of the probe. For real life applications in chemistry, biology, medicine and material science, sensors with large Stokes shift and emission shifts are required. However, the development of organic dyes with these desirable photophysical properties is not easy. Recently, fluorescence resonance energy transfer (FRET)^21–23 and through bond energy transfer (TBET)^24–26 based probes have been developed with more than one fluorophore, linked through a linker with an energy donor–acceptor combination wherein energy from one fluorophore, called the donor, is transferred to another fluorophore, called the acceptor, without emission of a photon. However, FRET systems require spectral overlap between the donor emission and the acceptor absorption, whereas TBET based probes are not limited by the constraint of spectral overlap between the donor emission and the acceptor absorption. In addition, high energy transfer efficiencies, fast energy transfer rates and large pseudo-Stokes' shifts enable applications of TBET based systems as optical materials,^27,28 photosynthetic models,²⁹ chemosensors,^30,31 and in biotechnology.^32,33 TBET systems have a conjugated spacer between the donor and the acceptor moiety, and these systems absorb at a wavelength characteristic of the donor and then emit via the acceptor. Recently, TBET systems have been developed for use in biotechnology and fluorescent sensing of metals.^24–26 Keeping in view the several advantages of TBET based probes, we were interested in a fluorescent probe that involves the phenomenon of TBET for the detection of PA. Thus, we have designed and synthesized a hexaphenylbenzene (HPB) based derivative 5, having a rhodamine moiety connected to it through a phenyl spacer. We have chosen HPB moiety as a TBET donor because it emits at 430 nm, and a rhodamine moiety as acceptor because it absorbs at 554 nm, and thus there is a minimum spectral overlap between HPB and rhodamine (Fig. S1 in ESI†), a condition suitable for TBET to take place. Furthermore, HPB has a rotor rotating around its own axis, which could impede the electron conjugation between donor and acceptor moieties, thus fulfilling the requirements for TBET to occur. The rhodamine moiety was selected as TBET acceptor due to its target-triggered “turn on” response, which could be easily distinguished even by the naked eye. We envisioned that electron deficient PA, being a strong acid,³⁴ could trigger the ring opening of the spirolactam ring of the rhodamine moiety, which will make TBET operative. As expected, derivative 5 displays through bond energy transfer in the presence of PA in MeOH and serves as a “turn on” sensor for PA. Furthermore, derivative 5 exhibits an intense colorimetric response towards PA and can selectively detect PA with a detection limit of 104.9 pg cm⁻². To the best of our knowledge, this is the first report where a TBET based “turn on” probe has been developed for the naked eye detection of picric acid. This probe has several advantages; first, in the absence of PA, derivative 5 is non emissive, while in the presence of PA, an emission signal appears at 578 nm with a large Stokes shift of 264 nm. Second, the color change from colorless to pink can be distinguished easily by the naked eye. Third, the probe exhibits high selectivity towards PA as compared to other nitroaromatics. Most importantly, derivative 5 serves to be a better probe for the detection of PA in comparison to the previously reported chemosensors for PA.³⁵

Results and discussion

Suzuki–Miyaura cross coupling of compound 1³⁶ with 4-(3,3,4,4-tetramethylborolan-1-yl)benzenamine 2³⁷ catalyzed by Pd(II), furnished compound 3 in 55% yield (Scheme 1). The structure of derivative 3 was confirmed from its spectroscopic and analytical data (pS16 to S18 in ESI†). The ¹H NMR spectrum of compound 3 showed one broad signal at 3.65 ppm corresponding to –NH₂ protons, one multiplet at 6.8–6.85 ppm, two doublets at 7.05 and 6.65 ppm and one broad signal at 7.25 ppm corresponding to aromatic protons. The mass spectrum showed a parent ion peak at 626.26 (M + 1⁺) corresponding to compound 3. These spectroscopic data corroborate the structure 3 for this compound.

Scheme 1 Synthesis of derivative 5; ^a(i) THF, K₂CO₃ (aq.), PdCl₂(PPh₃)₂; (ii) THF, Et₃N.

Furthermore, the reaction of compound 3 with 2-(2,7-bis(diethylamino)-9,10-dihydroanthracen-10-yl)benzoylchloride 4³⁸ in dry THF furnished compound 5 in 56% yield. The ¹H NMR spectrum of compound 5 showed one quartet at 3.33 ppm corresponding to –NCH₂CH₃ protons, one triplet at 1.14 ppm corresponding to –NCH₂CH₃ protons, eight doublets at 7.99, 7.19, 7.13, 7.00, 6.62, 6.29, 6.28 and 6.25 ppm, and two multiplets at 7.47–7.49 and 6.78–6.83 ppm corresponding to aromatic protons. The mass spectra showed a parent ion peak at 1050.48 (M + 1⁺) corresponding to compound 5. These spectroscopic data corroborate the structure 5 for this compound (pS19 to S21 in ESI†).

The recognition behaviour of compound 5 toward different nitroaromatics, such as picric acid (PA), 4-nitrophenol (NP), 2,4-dinitrophenol, 4-nitrotoluene (NT), 2,4-dinitrotoluene (DNT), 1,4-benzoquinone (BQ), nitromethane (NM), 1,4-dinitrobenzene (DNB), benzoic acid (BA) and 4-nitrobenzoic acid (NBA) was investigated by UV-vis and fluorescence spectroscopy studies. The absorption spectrum of compound 5 in MeOH exhibits two absorption bands at 274 and 314 nm (Fig. S2 in ESI†). The absence of any absorption band in the 400–600 nm region and the appearance of a colorless solution indicates the spirolactam conformation of rhodamine in the compound. Upon the gradual addition of PA (0–30 equiv.) to the solution of receptor 5, two new bands appear at 348 nm and 554 nm. The absorption band at 348 nm is due to picric acid, whereas the absorption band at 554 nm is due to the ring-open form of rhodamine (Fig. 1A). These spectral changes are accompanied by a gradual change of color from colorless to pink, thus allowing the calorimetric detection of PA by the naked eye (inset, Fig. 1A). The formation of a new band at 554 nm is attributed to the interaction of PA with the receptor 5, thus leading to the conversion of the rhodamine initial spirolactam form to its ring opened amide form, which facilitates the protonation. No such observation was found in the presence of other nitroaromatic derivatives (Fig. S3 in ESI†).

Fig. 1 (A) UV-visible spectra of derivative 5 (5 μM) in methanol with the addition of 30 equiv. of PA. Inset showing change in colour of derivative 5 (i) before and (ii) after the addition of PA; (B) fluorescence emission spectra of derivative 5 (5 μM) in methanol with the addition of 30 equiv. of PA; λ_ex = 314 nm. Inset showing normalized fluorescence intensity (i) before and (ii) after the addition of PA.

The fluorescence spectrum of derivative 5 in MeOH does not exhibit any emission band when excited at 314 nm (Fig. S4 in ESI†). This may be due to the photoinduced electron transfer (PET)^39–41 from the nitrogen atom of the spirolactam ring to the photoexcited hexaphenylbenzene unit. In the absence of PA, the acceptor moiety (rhodamine moiety) remains in the closed ring, non-fluorescent spirolactam form and no energy transfer is observed from donor to acceptor. Upon the addition of incremental amounts of picric acid (30 equiv.) to the solution of derivative 5 in methanol, an emission band appears at 578 nm (ϕ = 0.82)⁴² when excited at 314 nm (Fig. 1B). We believe that in the presence of picric acid, proton transfer takes place from picric acid to the nitrogen atom of the spirolactam ring, which results in opening of the spirolactam ring of the rhodamine group to the amide form, thus indicating the TBET process in derivative 5, i.e., via the conjugated linker from donor to acceptor as shown in Scheme 2. The characteristic emission of the hexaphenylbenzene moiety at ∼430 nm was not observed, suggesting nearly 100% energy-transfer efficiency.⁴³ These spectral changes are accompanied by the color change of the solution from colourless to orange with the addition of PA (365 nm UV light, inset Fig. 1B). Furthermore, it is very important for practical purposes that the fluorescence intensity of the acceptor in the covalently linked donor–acceptor system with a conjugated spacer should be greater than that of the acceptor without the donor when it is excited at the donor absorption wavelength. The fluorescence enhancement factor for compound 5 is 147-fold compared to the rhodamine hydrazide when excited at 314 nm (Fig. S5 in ESI†). This result corroborates the idea of through bond energy transfer. Furthermore, we also carried out life-time fluorescence studies of derivative 5 at corresponding emission maxima, which shows a shorter decay time (0.822 ns); however, in the presence of PA (30 equiv.), a long lived component (0.943 ns) is observed (Fig. S6 and pS6 in ESI†). This result suggests the formation of a new fluorescent species due to the interaction between derivative 5 and PA. We have also tested the response of derivative 5 to other analogous acids such as trifluoroacetic acid (TFA) (Fig. S7 in ESI†). We observed that upon the addition of 120 equivalents of TFA (10⁻² M) to the solution of derivative 5, a band appeared at 578 nm, indicating the ring opening of the rhodamine moiety, but response with TFA was minimal compared to that of PA. We believe that in comparison to electron deficient and acidic PA, TFA has only acidic character, hence, less sensitive response is observed.

Scheme 2 Ring opening of derivative 5 in the presence of PA.

To test if the protonation of the rhodamine group of derivative 5 in the presence of PA could be reversed, we also carried out a reversibility experiment. The addition of tetrabutylammonium hydroxide (10⁻¹ M) (TBAOH) to the solution of 5-PA (6 equiv. in methanol) resulted in the quenching of the respective fluorescence intensity (Fig. 2). The quenching of fluorescence is due to the strong affinity of hydroxide ions for the H⁺ ions, which is responsible for the deprotonation of the species 5′ complex, i.e., H⁺ ions are not available for binding with the nitrogen atom of the spirolactam ring. The reversibility and reusability of derivative 5 has been demonstrated by carrying out the titration of derivative 5 with PA, followed by the addition of OH⁻ ions in alternate cycles (Fig. S8 in ESI†). The repeated “On/Off” fluorescence behavior suggests that derivative 5 is a reusable sensor for PA when the addition of PA is followed by the addition of OH⁻ ions.

Fig. 2 Fluorescence emission spectra of derivative 5-PA (5 μM) in pure methanol with the addition of 6 equiv. of TBAOH (10⁻¹ M) ions; λ_ex = 314 nm.

Under the same set of conditions as those used for compound 5, we recorded the fluorescence of an equimolar solution of derivative 3 (donor) and rhodamine hydrazide (acceptor) in methanol. The fluorescence spectrum exhibits the individual emission band at 430 nm, corresponding only to derivative 3 (Fig. S9 in ESI†). No fluorescence corresponding to the rhodamine group was observed at λ_ex = 314 nm, which shows that there is no intermolecular resonance energy transfer between the hexaphenylbenzene (donor) and the rhodamine (acceptor) (Fig. S9 in ESI†). Thus, the advantage of the TBET system for energy transfer is obvious.

To get an insight into the interactions between derivative 5 and PA, we carried out ¹H NMR studies of derivative 5 in CDCl₃ in the presence of PA. The ¹H NMR titration of derivative 5 in the presence of PA shows downfield shift of 0.25 and 0.14 ppm for protons corresponding to –NCH₂CH₃ and –NCH₂CH₃ respectively, and an average downfield shift of 0.25 ppm corresponding to aromatic protons, along with the appearance of a new broad signal at 3.02 ppm due to the –NH– group, which indicates that the spirocyclic ring opening mechanism prevails in the system with the addition of picric acid (Fig. S10 in ESI†). Under the same set of conditions as was used for the detection of PA, we also carried out fluorescence studies of derivative 5 in the presence of 4-nitrophenol (NP), 2,4-dinitrophenol (DNP), 2-nitrotoluene (NT), 2,4-dinitrotoluene (DNT), 1,4-benzoquinone (BQ), nitromethane (NM), 1,4-dinitrobenzene (DNB), benzoic acid (BA) and 4-nitrobenzoic acid (NBA), but no new band corresponding to the ring-open form of the rhodamine moiety was observed (Fig. S11 in ESI†). Thus, derivative 5 is selective towards picric acid amongst the other nitroderivatives, organic acids and phenol derivatives tested. Competitive experiments further demonstrated that the probe can also detect PA in the presence of various potentially interfering analytes (Fig. S12 in ESI†). Furthermore, the detection limit of derivative 5 for PA was found to be 35 nM, which is sufficiently low for the detection of the submillimolar concentration of PA found in many chemical systems (Fig. S13 in ESI†). We also tested the fluorescence response of 5 to other metal ions such as Fe³⁺, Fe²⁺, Pb²⁺, Cd²⁺, Cu²⁺, Zn²⁺, Ni²⁺, Ag⁺, Co²⁺, Mg²⁺, Li⁺, Na⁺ and K⁺ in MeOH; however, no significant variation in the fluorescence spectra of 5 was observed with any other metal ion (Fig. S14a and b in ESI†), except Hg²⁺ ions, which also induce similar fluorescence emission (Fig. S15 in ESI†). However, the addition of tetrabutylammonium hydroxide to the solution of 5-Hg²⁺ (12 equiv. in methanol) could not restore the original fluorescence spectra of the molecule (Fig. S16 in ESI†). Thus, this study can be utilized to differentiate the sensory response of derivative 5 towards PA from that of Hg²⁺ ions. We also carried out pH studies of derivative 5 at different pH values and it was found that the emission spectra of derivative 5 in MeOH is independent of pH in the range of 5 to 12 (Fig. S17 in ESI†).

To study the role of spacer in the energy transfer process, we synthesized the model compound 6²⁶ in which the rhodamine moiety is directly linked to the HPB core without any spacer. The fluorescence spectrum of compound 6 in MeOH is non-emissive and the addition of incremental amounts of picric acid (30 equiv.) to the solution of derivative 6 in MeOH does not lead to any change in the emission spectrum, suggesting that no energy transfer takes place from the donor HPB unit to the acceptor rhodamine moiety (Fig. S18 in ESI†). This result shows the importance of the presence of a spacer between HPB and the rhodamine moiety.

The trace detection of PA is a major concern in the field of analytical and forensic sciences because PA can contaminate the human body, clothing and other materials in the surroundings during the manufacture of rocket fuel and fireworks. Thus, we prepared TLC strips by dip-coating a solution of derivative 5 onto TLC strips, followed by drying them in a vacuum to check for residual contamination in contact mode. We prepared several samples of solution-coated TLC strips and studied the response of their fluorescence towards picric acid in contact mode and in solution phase. PA crystals were placed over coated TLC strips for 15 s, and upon illumination with a UV lamp, reddish pink spots were observed in the contact area [Fig. 3(i)]. We also checked the effect of various concentrations of PA on the fluorescent TLC strips by applying small spots of different concentrations of analyte to the TLC strips [Fig. 3(ii)]. The minimum amount of PA detectable by the naked eye was in the picogram range.

Fig. 3 Images of derivative 5 coated TLC strips under different experimental conditions (i) (a) PA crystals on the top of derivative 5 coated TLC strips (b) removal of PA crystal after 15 s (ii) application of small spots of different concentrations of PA [(a) 1.049 mg, (b) 0.1049 mg, (c) 1.049 μg, (d) 10.49 ng, and (e) 104.9 pg] on the test strips of 5. All the images were taken under 365 nm UV illumination.

Conclusion

We have designed and synthesized TBET based derivative 5, which exhibits high selectivity, reusability, fast response time and efficient energy transfer in the presence of PA. In addition, the solution coated TLC strips of derivative 5 can serve as a simple, fast, and portable approach for the detection of PA in picogram range.

General experimental methods

All the fluorescence spectra were recorded on a SHIMADZU 5301 PC spectrofluorometer. UV spectra were recorded on a Shimadzu UV-2450 PC spectrophotometer with a quartz cuvette (path length: 1 cm). The cell holder was thermostated at 25 °C. Elemental analysis was performed using a Flash EA 1112 CHNS/O analyzer from Thermo Electron Corporation. ¹H and ¹³C NMR spectra were recorded on a JEOL-FT NMR-AL 300 MHz spectrophotometer and a Bruker (Avance II) FT-NMR 500 MHz spectrophotometer using CDCl₃ as a solvent and tetramethylsilane (Si(CH₃)₄) for internal standards. Data are reported as follows: chemical shifts in parts per million (δ), multiplicity (s = singlet, br = broad signal, d = doublet, m = multiplet), coupling constants (Hz), integration and interpretation. All the spectrophotometric titration curves were fitted with SPECFIT 32 software. All the spectral characterizations were carried out in HPLC-grade solvents at 20 °C within a 10 mm quartz cell.

Procedure for analyte sensing

UV-vis and fluorescence titrations were performed on 5 μM solutions of ligand (3 μl of THF are used to dissolve) in MeOH with different metal ions such as Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, Co²⁺, Pb²⁺, Zn²⁺, Ni²⁺, Cd²⁺, Ag⁺, Ba²⁺, Mg²⁺, K⁺, Na⁺ and Li⁺, as their perchlorate salts. In titration experiments, each time a 3 ml solution of 5 was filled in a quartz cuvette (path length: 1 cm) and spectra were recorded.

Fluorescence quantum yield

9,10-Diphenylanthracene (ϕ_f = 0.95) in cyclohexane has been used as a standard in the measurement of fluorescence quantum yield by using eqn (1), in which ϕ_s is the radiative quantum yield of the sample; ϕ_fr is the radiative quantum yield of the reference; A_s and A_r are the absorbance of the sample and the reference, respectively; D_s and D_r are the areas of emission for the sample and reference; L_s and L_r are the lengths of the absorption cells of sample and reference, respectively; and N_s and N_r are the refractive indices of the sample and reference solutions (pure solvents were assumed), respectively.

Experimental details of finding the detection limit

To determine the detection limit, a fluorescence titration of compound 5 with picric acid was carried out by adding aliquots of a picric acid solution of micromolar concentration and plotting the fluorescence intensity as a function of picric acid. From this graph, the concentration at which there was a sharp change in the fluorescence intensity multiplied with the concentration of receptor gave the detection limit.

Preparation of TLC strips

TLC strips (5 × 2 cm²) were prepared by coating with derivative 5 (5 × 10⁻⁶ M), followed by the removal of solvent under vacuum at room temperature. The ensemble coated filter papers were then cut into 8 pieces (1 × 1 cm²) to obtain the test strips and were used for the detection of explosives.

Experimental details

Synthesis. Compounds 1,³⁶ 2³⁷ and 6²⁶ were synthesized according to literature procedures.

Synthesis of compound 3. To a solution of 1 (0.5 g, 0.82 mmol) and 2 (0.21 g, 0.98 mmol) in THF, K₂CO₃ (0.90 g, 6.52 mmol), distilled H₂O (3 ml) and [Pd(Cl)₂(PPh₃)₂] (0.34 g, 0.49 mmol) were added under argon and the reaction mixture was refluxed overnight. THF was then removed under vacuum and the residue thus obtained was treated with water, extracted with dichloromethane and dried over anhydrous Na₂SO₄. The organic layer was evaporated and the compound was purified by column chromatography using ethyl acetate as an eluent to obtain compound 3, which was further recrystallized from methanol to provide 0.30 g of yellow solid (yield 59%): mp = 250 °C; ¹H NMR (500 MHz, CDCl₃): δ 3.65 (br, 2H, NH₂), δ 6.65 (d, 2H, J = 5 Hz, ArH), 6.81–6.85 (m, 27H, ArH), 7.05 (d, 2H, J = 5 Hz), 7.25 (br, 2H, ArH);¹³C NMR (125 MHz, CDCl₃): 115.3, 124.3, 125.1, 126.5, 126.6, 127.5, 131.4, 131.7, 140.3, 140.7; MALDI-MS: 626.26 (M + 1⁺); anal. calcd for C₄₈H₃₅N: C, 92.12; H, 5.64; N, 2.24. Found: C, 92.10; H, 5.62; N, 2.23%.

Synthesis of compound 5. The acid chloride 4 (0.04 g, 0.08 mmol) was dissolved in dry THF (10 ml). To the abovementioned solution, the solution of compound 3 (0.05 g, 0.08 mmol) in dry THF and triethylamine was added. The reaction mixture was stirred overnight at room temperature. The mixture thus obtained was treated with water, extracted with dichloromethane and dried over anhydrous Na₂SO₄. The organic layer was evaporated under reduced pressure and the crude product was purified by column chromatography (EtOAc–hexane, 2 : 8) and recrystallised from methanol to obtain 0.045 g of white solid 5 (yield 56.25%): mp > 260 °C; ¹H NMR (500 MHz, CDCl₃): δ 1.14 (t, 12H, J = 5.0 Hz, NCH₂CH₃), 3.33 (q, 8H, J = 10 Hz, NCH₂CH₃), 6.25 (d, 2H, J = 5 Hz, ArH), 6.28 (d, 1H, J = 5.0 Hz, ArH), 6.29 (d, 1H, J = 5 Hz, ArH), 6.62 (d, 2H, J = 10.0 Hz, ArH), 6.78–6.83 (m, 29H, ArH), 7.00 (d, 2H, J = 10 Hz, ArH), 7.13 (d, 1H, J = 5 Hz, ArH), 7.19 (d, 1H, J = 10.0 Hz, ArH), 7.47–7.49 (m, 2H, ArH), 7.99 (d, 2H, J = 5 Hz, ArH); ¹³C NMR (125 MHz, CDCl₃): 12.6, 44.3, 97.8, 106.3, 108.1, 124.9, 125.2, 126.6, 126.6, 126.7, 126.9, 131.4, 140.3, 140.5, 140.6, 148.7, 153.0; MALDI-MS: 1050.48 (M + 1⁺); anal. calcd for C₇₆H₆₃N₃O₂: C, 86.91; H, 6.05; N, 4.00; O, 3.05. Found: C, 86.88; H, 6.03; N, 3.09; O, 3.03%.

Acknowledgements

M. K. is thankful to DST (ref. no. SR/S1/OC-69/2012) and V. B. is thankful to CSIR (ref. no. 02(0083)/12/EMR-II) for financial support. We are thankful to UGC (New Delhi, India) for the “University with Potential for Excellence” (UPE) project.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00436e

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