A novel 2-(2′-aminophenyl)benzothiazole derivative displays ESIPT and permits selective detection of Zn²⁺ ions: experimental and theoretical studies

Abstract

Synthesis of a novel 2-(2′-aminophenyl)benzothiazole based probe (1) and demonstration of excited state intramolecular proton transfer (ESIPT) with a large Stokes shift (∼246 nm) are presented. Selective interactions of probe 1 with DMSO and Zn²⁺ result in the inhibition of ESIPT operating in 1. Inhibition of ESIPT by only Zn²⁺ and not by other common metal ions permits the selective detection of Zn²⁺ at concentrations as low as 4.5 × 10⁻⁹ M in aqueous medium.

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Introduction

Over the past few decades, excited state intramolecular proton transfer (ESIPT) has been exploited for the development of molecular logic gates,¹ bioimaging,² ultraviolet stabilizers,³ light-emitting materials⁴ and molecular probes.^1,4 Generally, ESIPT occurs in molecules containing intramolecular hydrogen bonds through tautomerization during excitation. The excited state is typically characterized by very short lifetimes and low fluorescence quantum yields. However, the large Stokes shift associated with the ESIPT process enables fluorescence detection at longer wavelengths without self-absorption and interference from auto-fluorescence.⁵

Molecules containing 2-(2′-hydroxyphenyl)benzothiazole (HBT), 2-(2′-hydroxyphenyl)benzoxazole (HBO), 2-(2′-hydroxyphenyl)benzimidazole or 2-(2′-aminophenyl)benzimidazole (APBI) moiety display ESIPT process, and they have been applied to elucidate different analytes.^6,7 Recently, the ESIPT process has been studied in a number of interesting Schiff bases that interact with different analytes.^8–10 To our knowledge, 2-(2′-aminophenyl)benzothiazole (APBT) derivatives, which display both ESIPT and –CH=N– isomerization processes simultaneously, have not been explored for the detection and estimation of trace amounts of analytes.

We report, herein, a new ESIPT based 2-(2′-aminophenyl)benzothiazole (APBT) probe 1 – (E)-N-(2-benzo[d]thiazol-2-yl)phenyl)-2-((2-hydroxybenzilidene)amino)acetamide – with intramolecular hydrogen bond and isomerizable imine moiety. The synthesis of 1 was readily achieved in four steps as shown in Scheme 1. The structure of 1 was determined using ¹H, ¹³C-NMR, HMBC and ESI-MS data (Fig. S1–S4†). Incorporation of a phenol-Schiff base with the APBT through an amide linkage conferred N–N–N–O combination of coordinating atoms for chelation with specific metal ions. As with the APBI probes,¹¹ the intramolecularly hydrogen bonded form of 1 is likely to be the stable form in the ground state, and the tautomer form could become dominant in the excited-state (Scheme 1). Interaction of 1 with metal ions can be expected to inhibit ESIPT and –CH=N– isomerization processes in 1 and, thereby, permit the detection of specific metal ions.

Scheme 1 Synthesis of probe 1.

Results and discussion

We first examined the effect of different solvents on the absorption and fluorescence profiles of 1 to assess the ESIPT phenomenon. The absorption spectrum of 1 in all the solvents chosen for the study was characterized by a broad band in the range from 275 to 375 nm (Fig. 1), except in methanol in which an additional absorption band at ∼400 nm was observed (inset to Fig. 1). The band at ∼400 nm is presumably due to deprotonation of amide hydrogen.^7c Another interesting feature of 1 was that the absorption maximum was only slightly sensitive to solvent polarity. These results indicate that the normal form of 1 is the major species at the ground state and that probe 1 is almost insensitive to the surrounding medium.

Fig. 1 The absorption (a) and fluorescence (b) spectra of 1 (100 μM) in different solvents.

When excited at the respective absorption maximum of 1 in different solvents, the fluorescence spectra showed comparable emission patterns (Fig. 1b) with a prominent emission band at ∼550, displaying a large Stokes shift (∼240 nm). The huge Stokes shift can only be attributed to the ESIPT process at the excited state and tautomerization to the other form of 1. Strikingly, the fluorescence spectrum of 1 in dimethyl sulfoxide displayed a strong emission at ∼451 nm with a Stokes shift of only 139 nm, presumably due to disruption of intramolecular hydrogen bond needed for ESIPT process.

To validate the solvent dependent variation in ESIPT process, ¹H NMR spectra of 1 in CDCl₃, DMSO-d₆ and CD₃CN were examined (Fig. 2). Interestingly, in DMSO-d₆, the resonance positions of H₁ and H₉ are shifted downfield (to 8.78 and 12.71 ppm, respectively), while those of H₂ and H₈ are shifted upfield (to 8.57 and 11.93 ppm, respectively) compared to their resonance positions in CD₃CN and CDCl₃. These results are consistent with the observed absorption and fluorescence emission profiles (Fig. 1), and revealed the existence of the intramolecular hydrogen bond and the influence of DMSO on ESIPT process in 1.

Fig. 2 ¹H NMR spectrum of 1 in (a) CDCl₃, (b) DMSO-d₆ and (c) CD₃CN-d₃.

Interruption of ESIPT process in 1 by DMSO prompted us to explore common di- and trivalent metal ions for their ability to disrupt the intramolecular hydrogen bond in 1 and inhibit the ESIPT process. Consequently, the absorbance and fluorescence profiles of 1 (10 μM) in aqueous medium (H₂O–CH₃CN, 3 : 7, v/v) in the absence and presence of different metal ions (10 equiv.) such as Na⁺, K⁺, Zn²⁺, Co²⁺, Ca²⁺, Ni²⁺, Ba²⁺, Mg²⁺, Pb²⁺, Cu²⁺, Mn²⁺, Hg²⁺, Cd²⁺, Fe²⁺, Fe³⁺, Cr³⁺ and Al³⁺ were analysed. To our delight, only Zn²⁺ induced a significant change in the absorption and fluorescence profiles of 1, while no significant changes were observed upon addition of other metal ions (Fig. 3). Upon addition of Zn²⁺ ions, the absorption value at ∼320 nm decreased with the appearance of a new band at ∼355 nm (Fig. 3a), presumably due to deprotonation of the amide moiety in 1. An analogous addition of Zn²⁺ ions produced a strong emission band at ∼459 nm in the fluorescence spectrum, corresponding to the normal form of probe 1 without ESIPT and –CH=N– isomerization processes (Fig. 3b). Since the probe 1 shows weak emission at ∼556 nm associated with ESIPT and –CH=N– isomerization process, the observed Zn²⁺ induced emission at ∼459 nm could only be ascribed to the formation of 1-Zn²⁺ complex and absence of ESIPT and –CH=N– isomerization processes in the complex.

Fig. 3 Change in the absorption (a) and fluorescence (b) spectra of 1 (10 μM) upon addition of 10 equiv. of various metal ions in H₂O : CH₃CN (3 : 7, v/v) medium.

To support the experimental results, time dependent density functional theory (TDDFT) CAM-B3LYP/6-31++G and CAM-B3LYP/6-31G** basis sets¹² were used for calculating electronic excitation energy, absorption wavelength and oscillator strength of 1 in different solvents. The calculated absorption maxima for 1 in different solvents¹³ (ESI, Table S1†) are in agreement with the experimentally observed values for 1 (Fig. 1a), and confirm the normal form of 1 as the predominant species in the ground state (Scheme 1).¹¹

To establish the involvement of –NH, –CH=N– and –OH groups in ESIPT and –CH=N– isomerization processes, FT-IR and ¹H-NMR studies were carried out. In the FT-IR spectrum of 1, the stretching vibrations of the –OH and –NH groups occur at 3203.50 and 3180.43 cm⁻¹ (Fig. S5†), and they disappear upon complexation with Zn²⁺ ions. In addition, the vibrational stretching band of C=O observed at 1689.23 cm⁻¹ and that of C=N observed at 1632.06 cm⁻¹ are shifted to 1644.67 and 1592.09 cm⁻¹, respectively. These observations are convincing proof to indicate the coordination of amide and imine N-atoms. Further, the 1-Zn²⁺ complex formation was evidenced from ¹H-NMR spectra of 1 and 1-Zn²⁺ complex in CDCl₃ (Fig. S6†). While the amide (H₈) and hydroxyl (H₉) proton resonances of 1 disappeared completely upon addition of 1 equiv. of Zn²⁺, the imine (H₁) proton resonance at 8.54 ppm was shifted upfield significantly (Δδ = 0.74 ppm). In addition, the resonance positions of methylene (H₁₀) and aromatic (H₂, H₃, H₆ and H₇) protons showed distinct differences (Fig. S7†). These observations clearly show the involvement of –CH=N– (H₁), –OH (H₉) and –NH (H₈) groups, as well as the N-atom of benzothiazole ring, in the formation of 1-Zn²⁺ complex.

To gain more insight about the inhibition of ESIPT process in 1-Zn²⁺ complex, we used the B3LYP/6-31++G and B3LYP/6-31G** basis sets¹⁴ embedded in Gaussian 09 to calculate the electronic structures of 1 and 1-Zn²⁺ complex. Fig. 4 shows the optimized ground state geometries of probe 1 and its 1 : 1 complex with Zn²⁺. The LUMO of 1 shows large orbital interactions between the amide and benzothiazole groups in APBT moiety. The electron density localized at the amide group is lesser than that localized at the benzothiazole group (Fig. S7†). This difference in electron density indicates that the acidity of the amide N-atom and the basicity of N-atom of benzothiazole are increased upon excitation, which could facilitate proton transfer in the excited state. This proposition is consistent with the experimental finding that 1 displays only ESIPT emission at ∼550 nm.

Fig. 4 Optimized geometries of 1 (front view) and 1–Zn²⁺ complex as predicted by quantum-chemical calculations.

The optimized structure of 1-Zn²⁺ complex shows the participation of four donor atoms, viz., –CH=N– (H₁), –OH (H₉) –NH– (H₈) and N-atom of benzothiazole as shown in Fig. 4. The electron density localized at the N-atom of benzothiazole group in the LUMO becomes weaker in the LUMO+1 of 1-Zn²⁺ complex (Fig. S7†). As the N-atom of benzothiazole becomes less basic, the ESIPT process is inhibited in 1-Zn²⁺ complex. Such transitions of the HOMO/LUMO orbitals in Zn²⁺ complexes have been reported.^9h Consequently, 1-Zn²⁺ complex shows transitions from HOMO to LUMO+1 and HOMO−1 to LUMO, contributing mainly to the absorption bands at ∼321 and ∼350 nm, respectively (ESI, Table S2†).

The observed selective interaction of 1 with Zn²⁺ assumes importance as zinc is the second most abundant transition metal ion present in human body (2–3 g) and plays critical roles in gene expression, regulation of metalloenzymes, apoptosis, DNA binding proteins, neurotransmission and pathological diseases.¹⁵ Zinc(II) is also a harmful environmental pollutant.¹⁶ Therefore, selective sensing of Zn²⁺ and non-interference of other common metal ions were assessed (Fig. S8†).

Spectrophotometric and fluorometric titrations of 1 with serial concentrations of Zn²⁺ (0–10 equiv.) were carried out in aqueous medium (H₂O–CH₃CN, 3 : 7, v/v). As shown in Fig. 5a, the absorption value at ∼320 nm gradually decreased with a concomitant increase in the absorption at ∼355 nm (absorption of deprotonated form of 1). Under similar experimental conditions, the fluorescence emission intensity at 459 nm (Fig. 5b) showed a linear change in the concentration range from 0–20 μM (R² = 0.9973, Fig. S9†). The limit of detection was found to be 4.5 × 10⁻⁹ M (detection limit, DL = 3σ/S, where σ is the standard deviation of the blank solution, and S is the slope of the calibration curve).¹⁷ The association constant calculated using Benesi–Hildebrand method was 2.4 × 10⁴ M⁻¹ (R² = 0.9978, Fig. S10†).¹⁸ The 1 : 1 stoichiometry of the 1-Zn²⁺ complex formed was confirmed using Job plot (Fig. S11†). Further, the ESI-MS spectrum of 1-Zn²⁺ complex (Fig. S12†) provided additional evidence for the 1 : 1 binding stoichiometry (m/z = 449.20).

Fig. 5 Changes in the absorption (a) and fluorescence spectra (b) of 1 (10 μM) upon serial addition of Zn²⁺ ions (0–10 equiv.) in H₂O : CH₃CN (3 : 7, v/v) medium.

The thermal stability of 1 and 1-Zn²⁺ complex was also evaluated by thermogravimetric analysis (TGA). The TGA curve of 1 exhibited good thermal stability displaying the onset of decomposition at 175.49 °C (corresponding to 0.89% weight loss), while decomposition of 1-Zn²⁺ complex was observed at 259.38 °C with a weight loss of 1.15% (Fig. S13†). The good thermal stability and minimal weight loss demonstrates the suitability of 1 for environmental applications.

Conclusions

Synthesis of a novel APBT based probe 1 with a tunable ESIPT process is reported. We have also provided experimental and theoretical evidence for the ESIPT process operating in 1. Selective binding of 1 with Zn²⁺ and formation of 1-Zn²⁺ complex have been demonstrated using fluorescence, UV-Vis, IR and NMR spectroscopic methods. Binding of Zn²⁺ with active coordination sites in 1 prevents –CH=N– isomerization and inhibits the ESIPT process, as evidenced by the spectroscopic studies and DFT/TDDFT calculations. The observed very low limit of detection of Zn²⁺ (4.5 × 10⁻⁹ M in aqueous medium) and good thermal stability of 1 would be useful for designing new APBT derivatives for tuneable fluorescent sensors.

Experimental

Material and methods

All the materials for synthesis were purchased from Sigma Aldrich, USA, and used as received. Column chromatography was performed on silica gel (200–400 mesh). ¹H and ¹³C NMR spectra were obtained using BRUKER 400 MHz NMR spectrometer. ESI-MS spectra were obtained on a Thermo Finnigan LCQ Advantage MAX 6000 ESI spectrometer. IR spectra (4000–400 cm⁻¹) were recorded using KBr pellets on a Perkin-Elmer Fourier transform infrared spectrophotometer. Analytical grade solvents and double distilled water were used in all experiments. The solutions of different metal ions (Na⁺, K⁺, Zn²⁺, Co²⁺, Ca²⁺, Ni²⁺, Ba²⁺, Mn²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Hg²⁺, Fe²⁺, Fe³⁺, Cr³⁺, and Al³⁺) were prepared using their chloride salts, and for Mg²⁺ solution, magnesium sulfate was used. Absorption spectra were recorded on a SPECORD 200 PLUS UV-Visible spectrophotometer. Fluorescence measurements were performed on a Cary Eclipse spectrofluorometer (excitation wavelength: 310 nm, excitation and emission slit width: 5 nm). All measurements were carried out at room temperature.

The geometries of 1 and 1-Zn²⁺ complex were optimized using DFT B3LYP/6-31++G basis set.¹⁴ The vibrational frequency analysis of the optimized geometry confirmed that the optimized geometry corresponded to the minima on the potential energy surface by exhibiting all real frequencies. Based on the optimized ground state geometries, the vertical excitation energies and associated oscillator strengths were obtained by performing TDDFT calculations using the long-range corrected Coulomb attenuated method (CAM-B3LYP functional). The CAM-B3LYP functional was used to get more accurate results for the excited state properties than the B3LYP functional as the compounds 1 and 1-Zn²⁺ complex show significant charge transfer characteristics.¹² The solvent effects on absorption properties was included using the polarizable continuum model (PCM).¹³ The TDDFT CAM-B3LYP/6-31++G and CAM-B3LYP/6-31G** basis sets were also used to predict the absorption properties in different solvents. All calculations were performed using Gaussian 09 package.

Synthesis of ESIPT probe 1

The probe 1 was synthesized in a four step procedure. To the DMF solution of N-(2-(benzothiazol-2-yl)phenyl)-2-bromoacetamide obtained from the reaction of 2-(2′-aminophenyl)benzothiazole with bromoacetyl bromide,¹⁹ sodium azide (0.43 g, 2.6 mmol) was added, and the mixture was stirred at room temperature. After completion of the reaction, the reaction mixture was extracted using DCM, concentrated and purified through column chromatography to obtain 2-azido-N-(2-(benzothiazol-2-yl)phenyl)acetamide (A). To the solution of A (0.63 g) in 10 mL THF : water (7 : 3), triphenylphosphine (0.73 g, 2.8 mmol) was added, and the mixture was refluxed at 80 °C for 24 h. After completion of the reaction, the reaction mixture was concentrated and subjected to silica gel column chromatography to obtain compound B as pale yellow solid. Salicylaldehyde (0.14 mL, 1.34 mmol) was added to a solution of B (0.32 g, 1.12 mmol) in ethanol at room temperature, with constant stirring. The reaction was monitored through TLC. After completion of the reaction, the pale green solid obtained was filtered, washed with ethanol and dried under vacuum to obtain 1 in 60% (0.26 g) yield. ¹H NMR (400 MHz, CDCl₃): δ (ppm) 12.70 (s, 1H), 12.31 (s, 1H), 8.75 (d, J = 8 Hz, 1H), 8.54 (s, 1H), 7.83 (d, J = 8 Hz, 1H), 7.51–7.46 (m, 2H), 7.40–7.30 (m, 3H), 7.22–7.16 (m, 2H), 6.99–6.93 (dd, J = 14, 8 Hz, 1H), 4.57 (s, 2H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 168.89, 168.26, 168.03, 160.92, 153.07, 137.01, 133.32, 133.21, 132.13, 131.70, 130.14, 126.32, 125.59, 123.95, 123.03, 121.75, 121.16, 120.35, 119.12, 118.79, 117.42, 64.47. IR (KBr, cm⁻¹): 3204, 3180, 3118, 3055, 2922, 2852, 1689, 1632, 1583, 1530, 1489, 1456, 1441, 1408, 1298, 1277, 1213, 974, 845, 754, 739, 729. Mass calculated for [C₂₂H₁₇N₃O₂S + H]⁺: 388.46; found: 388.33.

Acknowledgements

One of the authors, S. J., thanks the DST, New Delhi, India for the INSPIRE research fellowship. Financial support from CSIR 12^th plan project CSC0201 is acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09713h

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