A highly sensitive benzimidazole-based chemosensor for the colorimetric detection of Fe(II) and Fe(III) and the fluorometric detection of Zn(II) in aqueous media

Abstract

A new dual chemosensor 1 for the colorimetric detection of Fe^2+/3+ and the fluorometric detection of Zn²⁺ has been developed and characterized. The sensor 1 has proven to be highly selective to Fe^2+/3+ with a color change from colorless to dark green in a near-perfect aqueous solution. 1 had low detection limits (1.21 μM for Fe³⁺ and 1.18 μM for Fe²⁺), which are lower than the World Health Organization guideline (5.36 μM) for drinking water. Moreover, 1 showed a selective detecting ability for Zn²⁺ by ‘OFF–ON’ fluorescent response in aqueous solution. The detection limit (1.05 μM) of 1 for Zn²⁺ was much lower than World Health Organization guideline (76 μM) for drinking water. Interestingly, it could be recycled simply through treatment with an appropriate reagent such as EDTA (ethylenediaminetetraacetic acid). The sensing mechanism of 1 for Zn²⁺ was explained by the theoretical calculations. In particular, the sensor 1 could be used to quantify iron and zinc in water samples.

---

1. Introduction

The development of colorimetric and fluorescent sensors for biologically active metal ions has attracted a great deal of attention due to their potential applications in chemistry, life science, medicine and biotechnology.^1–6 In addition, colorimetric and fluorescent methods in conjunction with sensors are attractive approaches for the measurement of metal ions because of their visual simplicity, instant response, as well as on site and real time monitoring.^2–4

Iron is one of the most essential metal ions and plays a pivotal role for many living organisms by participating in a wide range of metabolic processes such as cellular metabolism and enzyme catalysis.^7–9 Especially, iron acts as an oxygen carrier in hemoglobin and is involved in several electron transfer reactions.¹⁰ Fe³⁺ deficiency is the most common cause of anemia in the world.¹¹ Based on the various functions of iron, detecting traces of iron in medicinal and environmental chemistry and in biology is an important topic.^12,13

In human body zinc is the second most abundant element after iron.^14–16 Zn²⁺ plays pivotal roles in numerous biological processes including brain activity, gene transcription, and immune function, etc.^17–20 However, excessive Zn²⁺ ion would cause imbalance in cellular processes, resulting in neurodegenerative diseases such as Menkes and Wilson diseases, Alzheimer's disease, Parkinson's disease, prostate cancer and diabetes.^21–24 Therefore, several studies on improving the sensitivity and selectivity of fluorescent probes for Zn²⁺ have been reported in the literatures.^25–31 In particular, the development of chemosensors that can discriminate Zn²⁺ from Cd²⁺ is still a huge challenge as they are in the same group of the periodic table and have similar properties.^32–34 Thus, low cost and easily prepared Zn²⁺ fluorescence chemosensors are needed for convenience.^35–42

Benzimidazole derivatives provide a good possibility to chelation with transition metal ions, which displays excellent fluorogenic properties.^43–45 Also, 4-diethylaminosalicylaldehyde moiety is a well-known fluorophore and many chemosensors with this moiety are water-soluble.^46,47 Therefore, we designed and synthesized a new chemosensor 1 based on the benzimidazole and 4-diethylaminosalicylaldehyde moieties, and tested its sensing ability to various metal ions.

Herein, we report a dual chemosensor 1 based on benzimidazole and 4-diethylaminosalicylaldehyde, which can be used for chromogenic and fluorescent recognition. Receptor 1 could detect Fe²⁺ and Fe³⁺ by color change from colorless to dark green in a near-perfect aqueous environment, and showed a fluorescent response toward Zn²⁺ in aqueous media. In order to understand their sensing mechanisms, various analytical studies such as Job plot, UV-vis titration, ESI-mass spectrometry analysis and ¹H NMR titration were carried out.

2. Results and discussion

2.1. Synthesis

A new chemosensor 1 was obtained by the condensation reaction of 2-(aminomethyl)benzimidazole dihydrochloride with 4-diethylaminosalicylaldehyde in methanol at room temperature (Scheme 1), and characterized by ¹H NMR, ¹³C NMR, ESI-mass spectrometry analysis and elemental analysis.

Scheme 1 The synthetic procedure for receptor 1.

2.2. Colorimetric sensing of 1 for iron

The colorimetric selective sensing abilities of receptor 1 with various metal ions in bis-tris buffer solution (10 mM, pH 7.0) were monitored by UV-vis absorption spectrometry (Fig. 1a). Only the addition of iron(II) and iron(III) ions induced distinct spectral changes while other metal ions such as Al³⁺, Ga³⁺, In³⁺, Zn²⁺, Cd²⁺, Cu²⁺, Mg²⁺, Cr³⁺, Hg²⁺, Co²⁺, Ni²⁺, Na⁺, K⁺, Ca²⁺, Mn²⁺ and Pb²⁺ did induced little or small spectral changes. Consistent with the change of the UV-vis spectra, the solution colors of 1 in the presence of the iron(II) and iron(III) ions changed immediately from colorless to dark green (Fig. 1b). This result indicated that receptor 1 can serve as a potential candidate of a “naked-eye” chemosensor for iron(II) and iron(III) in aqueous solution. These peaks (445 nm) with molar extinction coefficients in the thousands, 2.6 × 10³ M⁻¹ cm⁻¹ (ε_{445 nm}) for Fe²⁺ and 3.0 × 10³ M⁻¹ cm⁻¹ (ε_{445 nm}) for Fe³⁺, are too large to be Fe based d–d transition. Therefore, the peaks might be attributed to a metal-to-ligand charge-transfer (MLCT),⁴⁸ which is responsible for the dark green color of the solution.

Fig. 1 (a) Absorption spectral changes of receptor 1 (20 μM) in the presence of 1.2 equiv. of various metal ions in bis-tris buffer solution (10 mM, pH 7.0). (b) Photographs of receptor 1 (20 μM) in the presence of 1.2 equiv. of various metal ions in bis-tris buffer solution (10 mM, pH 7.0).

UV-vis spectral variation of receptor 1 was monitored during titration with different concentrations of Fe³⁺ (Fig. 2). Upon the addition of Fe³⁺ to the solution of 1, the absorbance at 380 nm decreased significantly, while the one at 445 nm increased up to 1.0 equiv. of Fe³⁺, which induced a color change from colorless to dark green. An isosbestic point was observed at 409 nm, indicating the formation of the only one product.

Fig. 2 UV-vis spectra of receptor 1 (20 μM) upon the addition of different concentrations of Fe³⁺ in bis-tris buffer solution (10 mM, pH 7.0). Inset: absorption at 445 nm versus the number of equiv. of Fe³⁺ added.

To examine the ratio of the complexation of 1 with Fe³⁺, the Job plot⁴⁹ was established from UV-vis spectra of 1 and Fe³⁺. As shown in Fig. S1,† the result indicated a 2 : 1 binding interaction between the receptor 1 and Fe³⁺, which was further confirmed by ESI-mass spectrometry analysis. The positive ion mass spectrum of 1 upon the addition of 1 equiv. of Fe³⁺ indicated the formation of the 2·1 + Fe³⁺–2H⁺ complex [m/z: 698.30; calcd, 698.28] (Fig. 3). Based on the UV-vis titration, Job plot and ESI-mass analysis, we propose the sensing mechanism of Fe³⁺ by 1, as shown in Scheme 2.

Fig. 3 Positive-ion electrospray ionization mass spectrum of 1 (0.1 mM) upon addition of Fe³⁺ (0.1 mM).

Scheme 2 The proposed structures for Fe²⁺–2·1, Fe³⁺–2·1, and Zn²⁺–2·1 complexes.

On the basis of the UV-vis titration of 1 with Fe³⁺, the association constant (K) of 1 with Fe³⁺ was found to be 6.0 × 10⁹ M⁻¹ by using Li's equation (Fig. S2†).⁵⁰ The detection limit⁵¹ (DL) of receptor 1 was calculated to be 1.18 μM for the Fe³⁺ (DL = 3σ/k, k = slope, σ is standard deviation) (Fig. S3†). DL of 1 for Fe³⁺ is lower than that (5.36 μM) recommended by WHO in drinking water.

To examine the interference of other metal ions on Fe³⁺–2·1 complexation, the competition experiments were performed in the presence of Fe³⁺ mixed with other metal ions such as Al³⁺, Ga³⁺, In³⁺, Zn²⁺, Cd²⁺, Cu²⁺, Mg²⁺, Cr³⁺, Hg²⁺, Co²⁺, Ni²⁺, Na²⁺, K⁺, Ca²⁺, Mn²⁺, and Pb²⁺ (Fig. 4). Upon the addition of 1.0 equiv. of Fe³⁺ ion in the presence of the same concentration of other metal ions, the coexistent metal ions did not interfere with naked-eye detection of Fe³⁺ by the receptor 1 in aqueous media, while Cu²⁺ inhibited slightly.

Fig. 4 Competitive selectivity of 1 (20 μM) towards Fe³⁺ (1.2 equiv.) in the presence of other metal ions (1.2 equiv.) in bis-tris buffer solution (10 mM, pH 7.0). (b) Colorimetric competitive photographs of 1 (20 μM) in the presence of Fe³⁺ (1.2 equiv.) and other metal ions (1.2 equiv.) in bis-tris buffer solution (10 mM, pH 7.0).

The reversibility of the chemosensor is one of the essential aspects for sensor applications. Therefore, we performed the reversibility of receptor 1 by the alternate addition of Fe³⁺ and a metal chelator DFO (deferoxamine) (Fig. S4†). The Fe³⁺–2·1 complex was reversible with DFO for several cycles.

The effect of pH on the absorption response of receptor 1 to Fe³⁺ ions was investigated in the pH range of 2 to 12 (Fig. 5). The color of the 1–Fe³⁺ complex was intense and stable between pH 6 and pH 11.

Fig. 5 Absorbance (445 nm) of Fe³⁺–2·1 complex at different pH values (2–12) in bis-tris buffer solution (10 mM, pH 7.0).

In order to examine the applicability of the chemosensor 1 in environmental samples, we constructed a calibration curve for the determination of Fe³⁺ (Fig. S5†), which exhibited a good linear relationship between the UV-vis intensity of 1 and Fe³⁺ concentration (0–5.0 μM) with a correlation coefficient of R² = 0.9934 (n = 3). Then, the chemosensor was applied for the determination of iron in water samples which were prepared by adding various metal ions known as being in industrial processes into deionized water. The results were summarized in Table 1, which exhibited a satisfactory recovery and R.S.D. values for the water samples.

Table 1 Determination of Fe(III) in water samples

Sample	Fe(III) added (μmol L^−1)	Fe(III) found (μmol L^−1)	Recovery (%)	R.S.D. (n = 3) (%)
a Synthesized by deionized water, 6.00 μmol L^−1 Zn(II), 10 μmol L^−1 Cd(II), Pb(II), Na(I), K(I), Ca(II), Mg(II). Conditions: [1] = 10 μmol L^−1 in bis-tris buffer solution (10 mM, pH 7.0).
Water sample^a	4.00	3.66	91.5	1.6

---

Next, UV-vis titrations of 1 with Fe²⁺ were performed to understand the binding property of 1 with Fe²⁺ (Fig. S6†). The results were nearly identical to those obtained in the sensing test of 1 with Fe³⁺. Based on these results and our previous study, we propose that as soon as Fe²⁺ reacts with 1, Fe²⁺ of the 2·1–Fe²⁺ complex might be oxidized to Fe³⁺. To prove for the oxidation of Fe²⁺ to Fe³⁺, we carried out ESI-mass spectrometry analysis (Fig. S7†). The positive-ion mass spectrum of 1 upon the addition of 1 equiv. of Fe²⁺ showed similar ESI-mass patterns as shown in the Fe³⁺–2·1 complex.

Stoichiometry of the 1 with Fe²⁺ ion was determined by Job plot which revealed a 2 : 1 ratio as shown in the Fe³⁺–2·1 complex (Fig. S8†). Based on the UV-vis titration, Job plot and ESI-mass analysis, we also propose the sensing mechanism of Fe²⁺ by 1, as shown in Scheme 2. Based on UV-vis titration, the association constant (K) of 1 with Fe²⁺ ion was calculated as 3.0 × 10⁹ using Li's equation⁵⁰ (Fig. S9†), which is nearly identical to that of the 1–Fe³⁺ complex. The detection limit⁵¹ of receptor 1 as a colorimetric sensor for the analysis of Fe²⁺ ions was found to be 1.21 μM (Fig. S10†), which is also nearly close to that of the Fe³⁺–2·1 complex.

On the other hand, we carried out the FT-IR measurements for 1, Fe²⁺–2·1, Fe³⁺–2·1 and Zn²⁺–2·1 species (Fig. S11†), in order to further examine a possibility for the oxidation of the phenolic ring of sensor 1 in the presence of Fe³⁺ and O₂. Fe²⁺–2·1 complex showed the band at 1065 cm⁻¹ associated with the counter anion ClO₄⁻. Fe³⁺–2·1 and Zn²⁺–2·1 complexes displayed the bands at 1280 cm⁻¹ and 1294 cm⁻¹ related to the counter anion NO₃⁻. There was no significant band for the oxidized form quinone of the phenolic ring.

2.3. Fluorometric sensing of 1 for zinc

We have also examined the selectivity of 1 toward a variety of metal ions through fluorescence emission in DMF : bis-tris buffer solution (v/v, 1 : 1, 10 mM, pH 7.0) at room temperature. As shown in Fig. 6a, the receptor 1 alone has a very weak fluorescence emission with an excitation of 365 nm. When 1.1 equiv. of various metal ions such as Al³⁺, Ga³⁺, In³⁺, Zn²⁺, Cd²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mg²⁺, Cr³⁺, Hg²⁺, Co²⁺, Ni²⁺, Na⁺, K⁺, Ca²⁺, Mn²⁺ and Pb²⁺ was added to the receptor 1, it was found that the solution of 1 exhibited no or a small significant increase of the fluorescence. By contrast, the addition of Zn²⁺ resulted immediately in a drastic enhancement of the emission intensity at 425 nm (11 folds, Fig. 6b). These results indicate that receptor 1 could be used as a fluorescence chemosensor for Zn²⁺. Importantly, 1 can clearly distinguish Zn²⁺ from Cd²⁺, while the discrimination of Zn²⁺ from Cd²⁺ is well known to be a major problem.^52–54 The selective fluorescence enhancement by Zn²⁺ might be due to the effective coordination of Zn²⁺ with 1 over other metal ions, which causes the zinc complex to be a more rigid structure than 1 itself (this refers to chelation-enhanced fluorescence (CHEF) effect).^55,56 In addition, receptor 1 is poorly fluorescent in part due to isomerization of the C=N double bond in the excited state and in part due to excited-state intramolecular proton transfer (ESIPT) involving the phenolic protons.⁵⁷ Upon stable chelation with zinc ion, the C=N isomerization and ESIPT might be inhibited, leading to the fluorescence enhancement.

Fig. 6 (a) Fluorescence spectral changes of receptor 1 (20 μM) in the presence of different metal ions (1.1 equiv.) such as Al³⁺, Ga³⁺, In³⁺, Cd²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mg²⁺, Cr³⁺, Hg²⁺, Co²⁺, Ni²⁺, Na⁺, K⁺, Ca²⁺, Mn²⁺ and Pb²⁺ with an excitation of 365 nm in DMF : bis-tris buffer solution (v/v, 1 : 1, 10 mM, pH 7.0). (b) Bar graph shows the relative emission intensity (at 425 nm) of 1 upon treatment with various metal ions.

To further investigate the chemosensing properties of 1 toward Zn²⁺, fluorescence titration of the receptor 1 with Zn²⁺ ion was performed (Fig. 7). Upon the addition of Zn²⁺ to 1, fluorescence intensity increased gradually and was saturated at 1.1 equiv. of Zn²⁺. The photophysical properties of 1 were also examined using UV-vis spectrometry (Fig. S12†). Upon addition of Zn²⁺ ions to a solution of 1, the absorption band at 362 nm decreased and a new absorbance intensity at 370 nm increased with an isosbestic point at 406 nm, which indicates that only one product was generated from the interaction of 1 with Zn²⁺.

Fig. 7 Fluorescence spectral change of receptor 1 (20 μM) upon gradual addition of Zn²⁺ in DMF : bis-tris buffer solution (v/v, 1 : 1, 10 mM, pH 7.0). Inset: fluorescence at 425 nm versus the number of equiv. of Zn²⁺ added.

The binding mode between 1 and Zn²⁺ was determined by using Job plot analysis.⁴⁹ As shown in Fig. S13,† the Job plot exhibited a 2 : 1 complexation stoichiometry for 1 with Zn²⁺. The formation of Zn²⁺–2·1 complex was further confirmed by ESI-mass spectrometry analysis (Fig. S14†). The m/z peak at 707.30 appeared, which corresponds to the molecular ion peak of the 2·1 + Zn²⁺–H⁺ complex [calcd, 707.28]. From the results of fluorescence titration, the association constant of the Zn²⁺–2·1 complex was determined as 4.0 × 10⁹ M⁻¹ on the basis of Li's equation⁵⁰ (Fig. S15†). This value is within the range of those (1.0–1.0 × 10¹²) reported for Zn²⁺-chemosensors.⁵⁸ For practical application, the detection limit⁵¹ was also an important parameter. Thus, we calculated the detection limit of receptor 1 for the analysis of Zn²⁺ (DL = 1.05 μM) (Fig. S16†), which is far lower than the WHO guideline (76 μM) for Zn²⁺ in drinking water.⁵⁹

To check the practical applicability of 1 as a selective fluorescence sensor for Zn²⁺, the competition experiments were conducted in the presence of Zn²⁺ mixed with other relevant metal ions, such as Al³⁺, Ga³⁺, In³⁺, Cd²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mg²⁺, Cr³⁺, Hg²⁺, Ag⁺, Co²⁺, Ni²⁺, Na⁺, K⁺, Ca²⁺, Mn²⁺ and Pb²⁺. When 1 was treated with 1.1 equiv. of Zn²⁺ in the presence of the same concentration of other metal ions (Fig. 8), the coexistent metal ions had a small and negligible effect on the emission intensity of the Zn²⁺–2·1 complexation, except for Cu²⁺, Fe²⁺ and Fe³⁺. They inhibited about 34, 61 and 43% of the emission intensity, respectively, but are still discernible. In particular, it is worth noting that cadmium ion hardly inhibited the fluorescence intensity of the Zn²⁺–2·1 complex. These results indicate that 1 could be a good Zn²⁺ sensor which could distinguish Zn²⁺ from Cd²⁺ commonly having similar properties.

Fig. 8 (a) Competitive selectivity of 1 (20 μM) towards Zn²⁺ (1.1 equiv.) in the presence of other metal ions (1.1 equiv.) in DMF : bis-tris buffer solution (v/v, 1 : 1, 10 mM, pH 7.0).

We investigated the effect of pH on the fluorescence response of receptor 1 to the Zn²⁺ ion in a series of buffers with pH values ranging from 2 to 12 (Fig. S17†). An intense and stable fluorescence of Zn²⁺–2·1 found in the pH range of 7.0–11.0 warrants its application under physiological conditions, without any change in detection results.

In order to examine the practical applicability of 1 for environmental samples, we constructed a calibration curve for the determination of Zn²⁺ (Fig. S18†). The curve showed a good linear relationship between the fluorescence intensity of 1 and Zn²⁺ concentration (0–9.0 μM) with a correlation coefficient of R² = 0.9975 (n = 3). Then, we prepared water samples by adding various metal ions known as being involved in industrial processes into deionized water. The chemosensor was applied for the determination of zinc in the water samples. As shown in Table 2, a satisfactory recovery and R.S.D. values of the water samples were obtained.

Table 2 Determination of Zn(II) in water samples

Sample	Zn(II) added (μmol L^−1)	Zn(II) found (μmol L^−1)	Recovery (%)	R.S.D. (n = 3) (%)
a Synthesized by deionized water, 10 μmol L^−1 Cd(II), Pb(II), Na(I), K(I), Ca(II), Mg(II). Conditions: [1] = 10 μmol L^−1 in DMF : bis-tris buffer solution (v/v, 1 : 1, 10 mM, pH 7.0).
Water sample^a	10.00	10.40	104	1.2

---

To examine the reversibility of sensor 1 toward Zn²⁺ in DMF : bis-tris buffer solution (v/v, 1 : 1, 10 mM, pH 7.0), ethylenediaminetetraacetic acid (EDTA, 1.1 equiv.) was added to the complexed solution of receptor 1 and Zn²⁺. As shown in Fig. 9, the intensity at 425 nm disappeared immediately. Upon addition of Zn²⁺ again, the intensity was recovered. The intensity changes were almost reversible even after several cycles with the sequentially alternative addition of Zn²⁺ and EDTA. These results indicate that receptor 1 could be recyclable simply through treatment with a proper reagent such as EDTA.

Fig. 9 (a) Fluorescence spectral changes of receptor 1 (20 μM) after the sequential addition of Zn²⁺ and EDTA in DMF : bis-tris buffer solution (v/v, 1 : 1, 10 mM, pH 7.0). (b) Reversible changes in fluorescence intensity (at 425 nm) of 1 (20 μM) after the sequential addition of Zn²⁺ and EDTA.

The interaction between receptor 1 and Zn²⁺ was further studied through ¹H NMR titration (Fig. 10). Upon addition of 0.5 equiv. of Zn²⁺, the H₁₃ of the phenol group and the H₁ of the benzimidazole moiety disappeared. H₇ of C=N moiety at 8.55 ppm and H₆ of methylene moiety at 4.99 ppm showed downfield shifts by 0.04 ppm and 0.22 ppm, respectively. These results suggest that the imine and the amine N atoms and the oxygen atom of the phenyl group might coordinate to the Zn²⁺. There was no shift in the position of proton signals on further addition of Zn²⁺ (>0.5 equiv.). Based on the Job plot, ESI-mass spectrometry analysis, and ¹H NMR titration, we propose the structure of a 2 : 1 complex of 1 and Zn²⁺, as shown in Scheme 2.

Fig. 10 ¹H NMR titration of receptor 1 with Zn²⁺.

To get understanding on the electronic structures of 1 and Zn²⁺–2·1 complex, we optimized energy-minimized structures of chemosensor 1 and Zn²⁺–2·1 complex at DFT/B3LYP/6-31G** level (Fig. S19†). The bond lengths and dihedral angles were compared between 1 and Zn²⁺–2·1 complex. The energy-minimized structure of 1 indicated the hydrogen bond in N1–H7 (Fig. S19a†). In case of Zn²⁺–2·1 complex, Zn²⁺ was coordinated to N1, O6, N9, N1′, O6′, and N9′, which indicated a hexa-coordinated zinc(II) complex (Fig. S19b†). To investigate the sensing mechanism of 1 with Zn²⁺, the frontier molecular orbitals of 1 and Zn²⁺–2·1 complex were compared in Fig. S20.† In case of 1, the energy gap between HOMO and LUMO was assigned to π → π* transition of 4-diethylaminosalicylaldehyde moiety. For Zn²⁺–2·1 complex, the energy gap between HOMO and LUMO was also assigned to π → π* transition of 4-diethylaminosalicylaldehyde moiety. There were no obvious changes of HOMO–LUMO transitions between 1 and Zn²⁺–2·1 complex. These results suggested that the sensing mechanism of 1 toward Zn²⁺ was originated by the rigid structure of Zn²⁺–2·1 complex, which might inhibit the non-radiative process such as C=N isomerization. Thus, the CHEF effect and the inhibition of C=N isomerization play important roles in the fluorescence enhancement.⁶⁰

3. Conclusion

We have developed a new chemosensor 1 based on the Schiff base with the benzimidazole and 4-diethylaminosalicylaldehyde moieties. The receptor 1 can be used as a multifunctional chemosensor for the highly selective detection of Fe²⁺ and Fe³⁺ by colorimetry and Zn²⁺ by fluorimetry in a 2 : 1 stoichiometric manner in aqueous media. 1 could be successfully applied to real samples for the quantification and qualification of Fe³⁺ and Zn²⁺ with the satisfactory recovery and R.S.D. values, respectively. The detection limits of 1 for Fe^2+/3+ and Zn²⁺ were far below the guidelines of the WHO. The addition of EDTA to the Zn²⁺–2·1 complex regenerated the free 1, indicating that the receptor 1 could be recyclable simply through treatment with a proper reagent such as EDTA. The sensing mechanism of 1 for Zn²⁺ was explained by the theoretical calculations. Consequently, the receptor 1 showed the possibility to be used as a dual-mode sensor in aqueous environment.

4. Experimental section

4.1. General information

All solvents and reagents (analytical grade and spectroscopic grade) were obtained from Sigma-Aldrich and used as received. Both ¹H NMR and ¹³C NMR were recorded on a Varian 400 MHz and 100 MHz spectrometer, respectively. The chemical shifts (δ) were recorded in ppm. Absorption spectra were recorded at room temperature using a Perkin Elmer model Lambda 2S UV/Vis spectrometer. Electrospray ionization mass spectra (ESI-mass) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQTM Advantage MAX quadrupole ion trap instrument. Fluorescence measurements were performed on a Perkin Elmer model LS45 fluorescence spectrometer. Elemental analysis for carbon, nitrogen, and hydrogen was carried out by using a Flash EA 1112 elemental analyzer (thermo) in Organic Chemistry Research Center of Sogang University, Korea.

4.2. Synthesis of 1

2-(Aminomethyl)benzimidazole dihydrochloride (0.22 g, 1 mmol) was dissolved in 15 mL of methanol and neutralized by sodium hydroxide (0.04 g, 1 mmol). Then, 4-diethylamino salicylaldehyde (0.20 g, 1 mmol) was added into the solution. The reaction mixture was stirred for 1 h at room temperature. The solvent was removed under reduced pressure to obtain dark brown oil, which was purified by silica gel column chromatography (10 : 1 v/v CH₂Cl₂ : CH₃OH). Yield: 0.19 g (60.0%). ¹H NMR (DMSO-d₆, 400 MHz): δ 13.21 (s, 1H), 12.42 (s, 1H), 8.46 (s, 1H), 7.51 (d, J = 32 Hz, 2H), 7.19 (d, J = 8 Hz, 1H), 7.16 (d, J = 8 Hz, 2H), 6.24 (d, J = 8 Hz, 1H), 6.00 (d, J = 2.4 Hz, 1H), 4.87 (s, 2H), 3.35 (m, 4H), 1.09 (t, J = 8 Hz, 4H). ¹³C NMR (DMF-d₇, 100 MHz): δ 189.28, 164.56, 161.78, 160.66, 149.19, 132.77, 130.13, 105.76, 101.75, 100.45, 95.87, 94.08, 57.49, 52.42, 43.15, 42.22, 41.88 ppm. ESI-mass: m/z calcd for C₁₉H₂₂N₄O + H⁺ ([M + H⁺]), 323.19; found, 323.20. Anal. calc. for C₁₉H₂₂N₄O: C 70.78; H, 6.88; N, 17.38; found: C, 70.82; H, 6.96; N, 17.58.

4.3. Chromogenic iron sensing

4.3.1. UV-vis titration measurements. For Fe²⁺, the receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL) and 20 μL of the receptor 1 (3 mM) was diluted to 2.98 mL of bis-tris buffer solution (10 mM, pH 7.0) to make the final concentration of 20 μM. Fe(ClO₄)₂ (10.4 mg, 0.04 mmol) was dissolved in bis-tris buffer solution (2 mL). 0–3.6 μL of the Fe²⁺ solution (20 mM) was transferred to the receptor solution (20 μM, 3 mL) prepared above. After mixing them for a few seconds, UV-vis spectra were taken at room temperature.

For Fe³⁺, the receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL) and 20 μL of the receptor 1 (3 mM) was diluted to 2.98 mL of bis-tris buffer solution (10 mM, pH 7.0) to make the final concentration of 20 μM. Fe(NO₃)₃ (16.5 mg, 0.04 mmol) was dissolved in bis-tris buffer solution (2 mL). 0–3.6 μL of the Fe²⁺ solution (20 mM) was transferred to the receptor solution (20 μM, 3 mL) prepared above. After mixing them for a few seconds, UV-vis spectra were taken at room temperature.

4.3.2. Job plot measurements. For Fe²⁺, the receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL) and Fe(ClO₄)₂ (10.4 mg, 0.04 mmol) was dissolved in bis-tris buffer solution (10 mM, pH 7.0, 2 mL), respectively. 700 μL of the receptor 1 solution was diluted to 29.3 mL of bis-tris buffer solution (10 mM, pH 7.0) to make the concentration of 70 μM. 105 μL of Fe(ClO₄)₂ solution was diluted to 29.09 mL of bis-tris buffer solution (10 mM, pH 7.0). 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL of the receptor 1 solution were taken and transferred to vials. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the Fe²⁺ solution were added to each receptor solution separately. Each vial had a total volume of 3 mL. After shaking the vials for a few seconds, UV-vis spectra were taken at room temperature.

For Fe³⁺, the receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL) and Fe(NO₃)₃ (16.5 mg, 0.04 mmol) was dissolved in bis-tris buffer solution (10 mM, pH 7.0, 2 mL), respectively. 700 μL of the receptor 1 solution was diluted to 29.3 mL of bis-tris buffer solution (10 mM, pH 7.0) to make the concentration of 70 μM. 105 μL of Fe(NO₃)₃ solution was diluted to 29.09 mL of bis-tris buffer solution (10 mM, pH 7.0). 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL of the receptor 1 solution were taken and transferred to vials. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the Fe³⁺ solution were added to each receptor solution separately. Each vial had a total volume of 3 mL. After shaking the vials for a few seconds, UV-vis spectra were taken at room temperature.

4.3.3. Competitive test toward iron. For Fe²⁺, the receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL). MNO₃ (M = Na, K, 0.04 mmol) or M(NO₃)₂ (M = Zn, Cd, Cu, Mg, Hg, Co, Ni, Ca, Mn, Pb, 0.04 mmol) or M(NO₃)₃ (M = Al, Ga, In, Fe, Cr, 0.04 mmol) or M(ClO₄)₂ (M = Fe, 0.04 mmol) was separately dissolved in bis-tris buffer solution (10 mM, pH 7.0, 2 mL). 3.6 μL of each metal solution (20 mM) was diluted to 2.973 mL of bis-tris buffer solution (10 mM, pH 7.0), separately. 3.6 μL of the Fe²⁺ solution (20 mM) was taken and added to the solutions prepared above. Then, 20 μL (3 mM) of the receptor 1 was taken and added to the mixed solutions. Each vial had a total volume of 3 mL. After shaking the vials for a few seconds, UV-vis spectra were taken at room temperature.

For Fe³⁺, the receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL). MNO₃ (M = Na, K, 0.04 mmol) or M(NO₃)₂ (M = Zn, Cd, Cu, Mg, Hg, Co, Ni, Ca, Mn, Pb, 0.04 mmol) or M(NO₃)₃ (M = Al, Ga, In, Fe, Cr, 0.04 mmol) or M(ClO₄)₂ (M = Fe, 0.04 mmol) was separately dissolved in bis-tris buffer solution (10 mM, pH 7.0, 2 mL). 3.6 μL of each metal solution (20 mM) was diluted to 2.973 mL of bis-tris buffer solution (10 mM, pH 7.0), separately. 3.6 μL of the Fe³⁺ solution (20 mM) was taken and added to the solutions prepared above. Then, 20 μL (3 mM) of the receptor 1 was taken and added to the mixed solutions. Each vial had a total volume of 3 mL. After shaking the vials for a few seconds, UV-vis spectra were taken at room temperature.

4.3.4. pH effect toward iron. For Fe²⁺, a series of buffers with pH values ranging from 2 to 12 was prepared by mixing sodium hydroxide solution and hydrochloric acid in bis-tris buffer. After the solution with a desired pH was achieved, receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL), and then 20 μL of the receptor 1 (3 mM) was diluted with 2.98 mL buffers to make the final concentration of 20 μM. Fe(ClO₄)₂ (10.4 mg, 0.04 mmol) was dissolved in bis-tris buffer (10 mM, pH 7.0, 2 mL). 3.6 μL of the Fe²⁺ solution (20 mM) was transferred to each receptor solution (20 μM) prepared above. After mixing them for a few seconds, UV-vis spectra were obtained at room temperature.

For Fe³⁺, a series of buffers with pH values ranging from 2 to 12 was prepared by mixing sodium hydroxide solution and hydrochloric acid in bis-tris buffer. After the solution with a desired pH was achieved, receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL), and then 20 μL of the receptor 1 (3 mM) was diluted with 2.98 mL buffers to make the final concentration of 20 μM. Fe(NO₃)₃ (16.5 mg, 0.04 mmol) was dissolved in bis-tris buffer (10 mM, pH 7.0, 2 mL). 3.6 μL of the Fe³⁺ solution (20 mM) was transferred to each receptor solution (20 μM) prepared above. After mixing them for a few seconds, UV-vis spectra were obtained at room temperature.

4.4. Fluorogenic zinc sensing

4.4.1. Fluorescence titration measurements. The receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL) and 20 μL of the receptor 1 (3 mM) was diluted to 2.98 mL of DMF : bis-tris buffer solution (1 : 1, v/v, 10 mM, pH 7.0) to make the concentration of 20 μM. Zn(NO₃)₂·6H₂O (11.9 mg, 0.04 mmol) was also dissolved in DMF (2 mL) and 0–3.3 μL of the Zn²⁺ solution (20 mM) was transferred to the solution of receptor 1 (20 μM, 3 mL) prepared above. After mixing them for a few seconds, fluorescence spectra were taken at room temperature.

4.4.2. UV-vis titration measurements. The receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL) and 30 μL of the receptor 1 (3 mM) was diluted to 2.97 mL of DMF : bis-tris buffer solution (1 : 1, v/v, 10 mM, pH 7.0) to make the final concentration of 30 μM. Zn(NO₃)₂·6H₂O (11.9 mg, 0.04 mmol) was dissolved in DMF (2 mL). 0–4.5 μL of the Zn²⁺ solution (20 mM) was transferred to the solution the receptor 1 (30 μM, 3 mL) prepared above. After mixing them for a few seconds, UV-vis spectra were taken at room temperature.

4.4.3. Job plot measurements. The receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL) and Zn(NO₃)₂·6H₂O (11.9 mg, 0.04 mmol) was dissolved in DMF (2 mL), respectively. 600 μL of the receptor 1 solution was diluted to 29.4 mL of DMF : bis-tris buffer solution (1 : 1, v/v, 10 mM, pH 7.0) to make the concentration of 60 μM. 90 μL of Zn(NO₃)₂·6H₂O solution was diluted to 29.01 mL of bis-tris buffer solution (10 mM, pH 7.0). 2.7, 2.4, 2.1, 1.8, 1.5, 1.2, 0.9, 0.6 and 0.3 mL of the receptor 1 solution were taken and transferred to vials. 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4 and 2.7 mL of the Zn²⁺ solution were added to each receptor solution separately. Each vial had a total volume of 3 mL. After shaking the vials for a few seconds, UV-vis spectra were taken at room temperature.

4.4.4. Competitive test toward zinc. The receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL). MNO₃ (M = Na, K, 0.04 mmol) or M(NO₃)₂ (M = Zn, Cd, Cu, Mg, Hg, Co, Ni, Ca, Mn, Pb, 0.04 mmol) or M(NO₃)₃ (M = Al, Ga, In, Fe, Cr, 0.04 mmol) or M(ClO₄)₂ (M = Fe, 0.04 mmol) was separately dissolved in bis-tris buffer solution (10 mM, pH 7.0, 2 mL). 3.3 μL of each metal solution (20 mM) was diluted to 2.973 mL of DMF : bis-tris buffer solution (1 : 1, v/v, 10 mM, pH 7.0), separately. 3.3 μL of the Zn²⁺ solution (20 mM) was taken and added to the solutions prepared above. Then, 20 μL of the receptor 1 (3 mM) was taken and added to the mixed solutions. Each vial had a total volume of 3 mL. After shaking the vials for a few seconds, fluorescence spectra were taken at room temperature.

4.4.5. pH effect toward zinc. A series of buffers with pH values ranging from 2 to 12 was prepared by mixing sodium hydroxide solution and hydrochloric acid in bis-tris buffer. After the solution with a desired pH was achieved, receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL), and then 20 μL of the receptor 1 (3 mM) was diluted with 2.98 mL buffers to make the final concentration of 20 μM. Zn(NO₃)₂·6H₂O (11.9 mg, 0.04 mmol) was dissolved in bis-tris buffer (10 mM, pH 7.0, 2 mL). 3.3 μL of the Zn²⁺ solution (20 mM) was transferred to each receptor solution (20 μM) prepared above. After mixing them for a few seconds, fluorescence spectra were obtained at room temperature.

4.4.6. Reversible test of 1 toward zinc by using EDTA. The receptor 1 (1.9 mg, 0.006 mmol) was dissolved in DMF (2 mL) and 20 μL of the 1 (3 mM) was diluted to 2.98 mL of DMF : bis-tris buffer (1 : 1, v/v, 10 mM, pH 7.0) to make the concentration of 20 μM. Zn(NO₃)₂·6H₂O (11.9 mg, 0.04 mmol) was also dissolved in DMF (2 mL) and 3.3 μL of the Zn²⁺ solution (20 mM) was transferred to the solution of 1 (20 μM, 3 mL) prepared above. After mixing them for a few seconds, fluorescence spectrum was taken at room temperature. Ethylenediaminetetraacetic acid disodium salt dehydrate (EDTA, 0.04 mmol) was dissolved in buffer solution (2 mL) and 3.3 μL of the EDTA solution (20 mM) was added to the solution of Zn²⁺–2·1 complex (20 μM) prepared above. After mixing it for few seconds, fluorescence spectrum was taken. For the reversibility study, another 3.3 μL of the Zn²⁺ ion solution (20 mM) was added to the above solution. After mixing it for a few seconds, fluorescence spectrum was taken. The same experimental procedure was repeated several times at room temperature.

4.4.7. Theoretical calculation methods of 1 toward Zn²⁺. All theoretical calculations were performed by DFT method with the hybrid exchange-correlation functional B3LYP^61,62 applying the 6-31G**^63,64 basis set without any symmetry restrictions. The energy-minimized structure of 1 was obtained in various geometric forms. On the basis of the optimized ground (S₀) structure of 1, the optimized structures of Zn³⁺–2·1 complex were also obtained. In vibrational frequency calculations, there was no imaginary frequency for the optimized geometries of 1 and Zn²⁺–2·1 complex, suggesting that these geometries represented local minima. For all calculations, the solvent effect of water was considered by using the Cossi and Barone's CPCM (conductor-like polarizable continuum model).^65,66 The GaussSum 2.1 was used to calculate the contribution of molecular orbitals in electronic transitions.⁶⁷ All the calculations were performed with Gaussian 03 suite.⁶⁸

Acknowledgements

Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2014R1A2A1A11051794 and NRF-2015R1A2A2A09001301) are gratefully acknowledged. We thank Nano-Inorganic Laboratory, Department of Nano & Bio Chemistry, Kookmin University to access the Gaussian 03 program packages.

References

1. R. P. Haugland, The Molecular Probes Handbook: A Guide to Fluorescent Probes and Labeling Technologies, Invitrogen, Carlsbad, CA, 10th edn, 2005.
2. K. Kaur, R. Saini, A. Kumar, V. Luxami, N. Kaur, P. Singh and S. Kumar, Coord. Chem. Rev., 2012, 256, 1992–2028.
3. N. Kaur and S. Kumar, Tetrahedron, 2011, 67, 9233–9264.
4. B. N. Giepmans, S. R. Adams, M. H. Ellisman and R. Y. Tsien, Science, 2006, 312, 217–224.
5. S. Aoki, D. Kagata, M. Shiro, K. Takeda and E. Kimura, J. Am. Chem. Soc., 2004, 126, 13377–13390.
6. K. K. Upadhyay and A. Kumar, Org. Biomol. Chem., 2010, 8, 4892–4897.
7. Z. Xu, S. J. Han, C. Lee, J. Yoon and D. R. Spring, Chem. Commun., 2010, 46, 1679–1681.
8. K. Komatsu, Y. Urano, H. Kojima and T. Nagano, J. Am. Chem. Soc., 2007, 129, 13447–13454.
9. S. Yoon, E. W. Miller, Q. He, P. H. Do and C. J. Chang, Angew. Chem., 2007, 119, 6778–6781.
10. B. He, A. B. Pun, D. Zherebetskyy, Y. Liu, F. Liu, L. M. Klivansky, A. M. McGough, B. A. Zhang, K. Lo, T. P. Russell, L. Wang and Y. Liu, J. Am. Chem. Soc., 2014, 136, 15093–15101.
11. P. Aisen, M. Wessling-Resnick and E. A. Leibold, Curr. Opin. Chem. Biol., 1999, 3, 200–206.
12. S. DuBois and D. J. Kearney, Am. J. Gastroenterol., 2005, 100, 453–459.
13. Y. S. Kim, G. J. Park, J. J. Lee, S. Y. Lee, S. Y. Lee and C. Kim, RSC Adv., 2015, 5, 11229–11239.
14. G. R. You, G. J. Park, S. A. Lee, K. Y. Ryu and C. Kim, Sens. Actuators, B, 2015, 215, 188–195.
15. J. M. Berg and Y. Shi, Science, 1996, 271, 1081–1085.
16. G. J. Park, M. M. Lee, G. R. You, Y. W. Choi and C. Kim, Tetrahedron Lett., 2014, 55, 2517–2522.
17. S. C. Burdette, G. K. Burdette, B. Springler, R. Y. Tsien and S. J. Lippard, J. Am. Chem. Soc., 2001, 123, 7831–7841.
18. E. L. Que, D. W. Domaille and C. J. Chang, Chem. Rev., 2008, 108, 1517–1549.
19. C. J. Frederickson and A. I. Bush, Biometals, 2001, 14, 353–366.
20. G. Zhang, H. Li, S. Bi, L. Song, Y. Lu, L. Zhang, J. Yu and L. Wang, Analyst, 2013, 138, 6163–6170.
21. A. Voegelin, S. Pfister, A. C. Scheinost, M. A. Marcus and R. Kretzschmar, Environ. Sci. Technol., 2005, 39, 6616–6623.
22. S. Kury, B. Dreno, S. Bezieau, S. Giraudet, M. Kharfi, R. Kamoun and J.-P. Moisan, Nat. Genet., 2002, 31, 239–240.
23. M. P. Cuajungco and G. J. Lees, Neurobiol. Dis., 1997, 4, 137–169.
24. A. I. Bush, W. H. Pettingell, M. D. Paradis and R. E. Tanzi, J. Biol. Chem., 1994, 269, 12152–12158.
25. Y. Mei, C. J. Frederickson, L. J. Giblin, J. H. Weiss, Y. Medvedeva and P. A. Bentley, Chem. Commun., 2011, 47, 7107–7109.
26. Z. C. Xu, K. H. Baek, H. N. Kim, J. N. Cui, X. H. Qian, D. R. Spring, I. Shin and J. Yoon, J. Am. Chem. Soc., 2010, 132, 601–610.
27. K. Li, X. Y. Wang and A. J. Tong, Anal. Chim. Acta, 2013, 776, 69–73.
28. Y. S. Cho, K. M. Kim, D. Lee, W. J. Kim and K. H. Ahn, Chem.–Asian J., 2013, 8, 755–759.
29. L. Xue, G. Li, C. Yu and H. Jiang, Chem.–Eur. J., 2012, 18, 1050–1054.
30. Y. Zhou, Z. X. Li, S. Q. Zang, Y. Y. Zhu, H. Y. Zhang, H. W. Hou and T. C. W. Mak, Org. Lett., 2012, 14, 1214–1217.
31. D. L. Lu, L. G. Yang, Z. D. Tian, L. Z. Wang and J. L. Zhang, RSC Adv., 2012, 2, 2783–2789.
32. Z. Li, M. Yu, L. Zhang, M. Yu, J. Liu, L. Wei and H. Zhang, Chem. Commun., 2010, 46, 7169–7171.
33. Y. J. Jang, Y. H. Yeon, H. Y. Yang, J. Y. Noh, I. H. Hwang and C. Kim, Inorg. Chem. Commun., 2013, 33, 48–51.
34. Y. K. Jang, U. C. Nam, H. L. Kwon, I. H. Hwang and C. Kim, Dyes Pigm., 2013, 99, 6–13.
35. K. B. Kim, G. J. Park, H. Kim, E. J. Song, J. M. Bae and C. Kim, Inorg. Chem. Commun., 2014, 46, 237–240.
36. N. Singh, N. Kaur, C. N. Choitir and J. F. Callan, Tetrahedron Lett., 2009, 50, 4201–4204.
37. S. Sen, S. Sarkar, B. Chattopadhyay, A. Moirangthaem, A. Basu, K. Dhara and P. Chattopadhyay, Analyst, 2012, 137, 3335–3342.
38. D. Y. Lee, N. Singh and D. O. Jang, Tetrahedron Lett., 2011, 52, 3886–3890.
39. J. J. Lee, Y. W. Choi, G. R. You, S. Y. Lee and C. Kim, Dalton Trans., 2015, 44, 13305–13314.
40. D. H. Kim, Y. S. Im, H. Kim and C. Kim, Inorg. Chem. Commun., 2014, 45, 15–19.
41. U. A. Fegade, S. K. Sahoo, A. Singh, N. Singh, S. N. Attarde and A. S. Kuwar, Anal. Chim. Acta, 2015, 872, 63–69.
42. E. J. Song, J. Kang, G. R. You, G. J. Park, Y. Kim, S. J. Kim, C. Kim and R. G. Harrison, Dalton Trans., 2013, 42, 15514–15520.
43. K. Skonieczny, A. I. Ciuciu, E. M. Nichols, V. Hugues, M. B. Desce, L. Flamigni and D. T. Gryko, J. Mater. Chem., 2012, 22, 20649–20664.
44. M. Wang, J. G. Wang, W. J. Xue and A. X. Wu, Dyes Pigm., 2013, 97, 475–480.
45. L. J. Tang, M. J. Cai, Z. L. Huang, K. L. Zhong, S. H. Hou, Y. J. Bian and R. J. Nandhakumar, Sens. Actuators, B, 2013, 185, 188–194.
46. Y. J. Na, Y. W. Choi, J. Y. Yun, K.-M. Park, P.-S. Chang and C. Kim, Spectrochim. Acta, Part A, 2015, 136, 1649–1657.
47. I. H. Hwang, Y. W. Choi, K. B. Kim, G. J. Park, J. J. Lee, L. Nguyen, I. Noh and C. Kim, New J. Chem., 2016, 40, 171–178.
48. S. Cui, S. Pu and Y. Dai, Anal. Methods, 2015, 7, 3593–3599.
49. P. Job, Ann. Chim., 1928, 9, 113–203.
50. G. Grynkiewcz, M. Poenie and R. Y. Tsein, J. Biol. Chem., 1985, 260, 3440–3445.
51. C. R. Lohani, J. M. Kim, S. Y. Chung, J. Yoon and K. H. Lee, Analyst, 2010, 135, 2079–2084.
52. Z. Shi, Q. Han, L. Yang, H. Yang, X. Tang, W. Dou, Z. Li, Y. Zhang, Y. Shao, L. Guan and W. Liu, Chem.–Eur. J., 2015, 21, 290–297.
53. Y. Mikata, K. Kawata, S. Takeuchi, K. Nakanishi, H. Konno, S. Itami, K. Yasuda, S. Tamotsu and S. C. Burdette, Dalton Trans., 2014, 43, 10751–10759.
54. P. S. Hariharan and S. P. Anthony, Anal. Chim. Acta, 2014, 848, 74–79.
55. V. Bhalla, R. Kumar and M. Kumar, Dalton Trans., 2013, 42, 975–980.
56. N. C. Lim, J. V. Schuster, M. C. Porto, M. A. Tanudra, L. Yao, H. C. Freake and C. Brückner, Inorg. Chem., 2005, 44, 2018–2030.
57. L. Wang, W. Qin, X. Tang, W. Dou and W. Liu, J. Phys. Chem. A, 2011, 115, 1609–1616.
58. J. H. Kim, I. H. Hwang, S. P. Jang, J. Kang, S. Kim, I. Noh, Y. Kim, C. Kim and R. G. Harrison, Dalton Trans., 2013, 42, 5500–5507.
59. Y. P. Kumar, P. King and V. S. K. R. Prasad, Chem.–Eng. J., 2006, 124, 63–70.
60. Y. W. Choi, G. R. You, J. J. Lee and C. Kim, Inorg. Chem. Commun., 2016, 63, 35–38.
61. A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
62. C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37, 785–789.
63. P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 28, 213–222.
64. M. M. Francl, W. J. Petro, W. J. Hehre, J. S. Binkley, M. S. Gordon, D. F. DeFrees and J. A. Pople, J. Chem. Phys., 1982, 77, 3654–3665.
65. V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102, 1995–2001.
66. M. Cossi and V. Barone, J. Chem. Phys., 2001, 115, 4708–4717.
67. N. M. O'Boyle, A. L. Tenderholt and K. M. Langner, J. Comput. Chem., 2008, 29, 839–845.
68. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery Jr, T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian 03, Revision D.01, Gaussian, Inc., Wallingford CT, 2004.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10086d

---