Highly selective and sensitive colorimetric chemosensors for Hg²⁺ based on novel diaminomaleonitrile derivatives

Abstract

Five novel soluble chemosensors in EtOH/H₂O based on Schiff-base diaminomaleonitrile derivatives which were modified by different aromatic functional moieties have been synthesized. These chemosensors were fully characterized by ¹H NMR, ¹³C NMR spectroscopy, mass spectrometry, and single crystal X-ray diffraction. The recognition abilities of these chemosensors with a range of metal ions at different pH values were evaluated and their photophysical properties have been systematically investigated. DFT theoretical calculations were employed to understand the behaviors of the chemosensors toward Hg²⁺. The sensing mechanism was derived through experimental and theoretical calculations. All the results consistently indicated that these diaminomaleonitrile derivatives were ideal chemosensors for the detection of Hg²⁺ in aqueous solutions.

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Introduction

In the past decades, many metals and their alloys have been widely used in industrial, military, civil and other fields. However, some heavy metal ions and transition metal ions are harmful to the environment. In particular, the Hg²⁺ ion is regarded as one of the most toxic metal ions. Accumulation of Hg²⁺ over time in the bodies of humans and animals can lead to serious debilitating illnesses and central nervous system damage.^1,2 Therefore, the development of increasingly selective and sensitive methods for the detection of Hg²⁺ is currently receiving considerable attention.^3–7 Current detection methods for the Hg²⁺ ion use atomic absorption spectrometry and inductively coupled plasma-mass spectrometry and share some drawbacks of needing expensive instruments and time-consuming procedures.⁸ Colorimetric chemosensors for the Hg²⁺ ion have attracted considerable attention due to their simple naked eye detection, inexpensive instrumentation, and potential environmental applications.^9–15 Consequently, a broad range of colorimetric chemosensors for the Hg²⁺ ion have been reported.^9–15 However, it is highly challenging to synthesize new reversible colorimetric chemosensors for the Hg²⁺ ion with ideal water solubility, excellent selectivity and outstanding sensitivity.¹⁶

In recent years, many researchers reported diaminomaleonitrile (DMN) derivatives^17–19 as fluorescent probes for Hg²⁺ ion detection in ethanol–water solution with their high detection abilities. Our synthetic strategy was to create an extensive donor–acceptor system by incorporating electron donor (aromatic) and a strong electron acceptor (diaminomaleonitrile) within the same framework. The extended π-conjugation enhanced the intramolecular charge transfer (ICT), which is expected to be highly sensitive towards external perturbations such as metal ion proximity resulting into optical and spectral changes.^20–22 The diaminomaleonitrile was chosen not only for its ability to act as diamine and form simple Schiff-base ligands but also for the fact that the electron-withdrawing CN groups, which clearly affect the coordinating capacity of DMN itself.²³ In addition, an imine group, which was easily dissolved in aqueous solution was brought in to improve the solubility of the chemosensors and enlarge their extent of applications, concurrently modulate the coordinating properties of molecules cooperatively with DMN.²⁴ Inspired by this idea, we have synthesized a series of chemosensors by coupling diaminomaleonitrile with different functionalized aromatic aldehydes (Scheme 1). These organic ligands displayed dramatic color change with Hg²⁺ in aqueous solutions at room temperature immediately.

Scheme 1 Syntheses of ligands A1–A5.

Experimental

Materials and instruments

1-Pyrenecarboxaldehyde was purchased from TCI (Shanghai) Development Company. 4-Fluorobenzaldehyde, 4-tert-butylbenzaldehyde, 2,3,4-trimethoxybenzaldehyde and diaminomaleonitrile were purchased from J&K Chemial. EtOH was spectrometric grade. Metal salts LiNO₃, NaNO₃, KNO₃, Mg(NO₃)₂, Ca(NO₃)₂, Ba(NO₃)₂, Cu(NO₃)₂, Zn(NO₃)₂, Co(NO₃)₂, Ni(NO₃)₂, Cd(NO₃)₂, AgNO₃, Hg(NO₃)₂, Pd(NO₃)₂, Fe(NO₃)₃·9H₂O, Cr(NO₃)₃ were purchased from Aldrich.

All of the chemicals were commercially available, and used without further purification. Elemental analyses were performed with an EA1110 CHNS-0 CE elemental analyzer. Absorbance spectra were collected by a Perkin Elmer Lambda 25 UV-vis spectrophotometer using quartz cells of 1.0 cm path length. ¹H, ¹⁹F and ¹³C NMR experiments were carried out on a MERCURY plus 400 spectrometer operating at resonance frequencies of 100.63 MHz. Electrospray ionization mass spectra (ESI-MS) were recorded on a Finnigan LCQ mass spectrometer using dichloromethane/methanol as mobile phase. All electron DFT calculations were performed using a Dmol3 package in Gaussian 09W.

Synthesis

Preparation of N-(4-fluorobenzyl)-2-aminomalonitrile (A1). To a solution of 4-fluorobenzaldehyde (0.57 g, 4.6 mmol) in ethanol (20 mL) was added diaminomaleonitrile (0.50 g, 4.6 mmol) dissolved. The mixture was heated under reflux for 8 h. The yellow precipitate obtained was filtered off, washed with ethanol 3 times and water 3 times, and dried overnight with oven. Yield = 0.94 g, 87%. ¹H NMR (400 MHz, DMSO) δ 8.30–7.82 (m, 5H), 7.40–7.21 (m, 2H). ¹⁹F NMR (376 MHz, DMSO) δ −108.25 (s). ¹³C NMR (101 MHz, DMSO) δ 165.77, 163.28, 154.22, 131.81, 127.46, 116.06, 114.85, 114.21, 102.92, 97.22, 54.95. EI-MS: m/z calcd for 214 (M + 1)⁺, found 214. Element analysis (%): anal. calc. for C₁₁H₇FN₄: C, 61.68; H, 3.29; F, 8.87; N, 26.16. Found: C, 61.59; H, 3.51; F, 8.76; N, 26.14.

Preparation of N-(pentafluorobenzyl)-2-aminomalonitrile (A2). To a solution of pentafluorobenzaldehyde (0.90 g, 4.6 mmol) in ethanol (20 mL) was added diaminomaleonitrile (0.50 g, 4.6 mmol) dissolved. The mixture was heated under reflux for 8 h. The yellow precipitate obtained was filtered off, washed with ethanol 3 times and water 3 times, and dried overnight with oven. Yield = 1.07 g, 76%. ¹H NMR (400 MHz, DMSO) δ 8.50 (s, 1H), 8.12 (d, J = 37.4 Hz, 2H). ¹⁹F NMR (376 MHz, DMSO) δ −141.50 (s), −142.96 (s), −151.47 (s), −162.63 (s), −162.78 (s). EI-MS: m/z calcd for 286 (M + 1)⁺, found 286. Element analysis (%): anal. calc. for C₁₁H₃F₅N₄: C, 46.17; H, 1.06; F, 33.20; N, 19.58. Found: C, 46.09; H, 1.37; F, 33.02; N, 19.52.

Preparation of N-(4-tert-butylbenzyl)-2-aminomalonitrile (A3). To a solution of 4-tert-butylbenzaldehyde (1.08 g, 4.6 mmol) in ethanol (20 mL) was added diaminomaleonitrile (0.50 g, 4.6 mmol) dissolved. The mixture was heated under reflux for 10 h. The yellow precipitate obtained was filtered off, washed with ethanol 3 times and water 3 times, and dried overnight with oven. Yield = 0.83 g, 87%. ¹H NMR (400 MHz, CDCl₃) δ 8.40 (s, 1H), 7.75 (d, J = 7.6 Hz, 2H), 7.48 (d, J = 7.7 Hz, 2H), 5.25 (s, 2H), 1.34 (s, 9H). EI-MS: m/z calcd for 252.10 (M + 1)⁺, found 252.95. Element analysis (%): anal. calc. for C₁₅H₁₆N₄: C, 71.40; H, 6.39; N, 22.21. Found: C, 71.48; H, 6.45; N, 22.17.

Preparation of N-(2,3,4-trimethoxybenzyl)-2-aminomalonitrile(A4). To a solution of 2,3,4-trimethoxybenzaldehyde (0.91 g, 4.6 mmol) in ethanol (20 mL) was added diaminomaleonitrile (0.50 g, 4.6 mmol) dissolved. The mixture was heated under reflux for 14 h. The yellow precipitate obtained was filtered off, washed with ethanol 3 times and water 3 times, and dried overnight with oven. Yield = 1.03 g, 73%. ¹H NMR (400 MHz, CDCl₃) δ 8.70 (s, 1H), 7.72 (d, J = 8.3 Hz, 1H), 7.26 (s, 1H), 6.72 (d, J = 9.3 Hz, 1H), 5.11 (s, 2H), 4.01–3.83 (m, 9H). ¹³C NMR (101 MHz, DMSO) δ 157.19, 154.34, 150.81, 141.91, 126.24, 123.39, 121.63, 114.88, 114.18, 108.81, 103.81, 62.19, 61.19, 56.89. EI-MS: m/z calcd for 286.1 (M + 1)⁺, found 286.9. Element analysis (%): anal. calc. for C₁₄H₁₄N₄O₃: C, 58.73; H, 4.93; N, 19.57; O, 16.77. Found: C, 58.60; H, 5.09; N, 19.51; O, 16.80.

Preparation of N-(pyrenyl)-2-aminomalonitrile (A5). To a solution of 1-pyrenecarboxaldehyde (1.06 g, 4.6 mmol) in ethanol (20 mL) was added diaminomaleonitrile (0.50 g, 4.6 mmol) dissolved. The mixture was heated under reflux for 10 h. The yellow precipitate obtained was filtered off, washed with ethanol 3 times and water 3 times, and dried overnight with oven. Yield = 0.94 g, 63.5%. ¹H NMR (400 MHz, CDCl₃) δ 9.39–9.22 (s, 1H), 9.01–8.92 (s, 1H), 8.80–8.66 (s, 1H), 8.45–7.94 (m, 9H). ¹³C NMR (101 MHz, DMSO) δ 152.52, 133.44, 131.30, 130.46, 130.24, 129.57, 127.91, 127.72, 127.25, 127.06, 126.88, 126.66, 125.47, 124.38, 124.10, 122.07, 115.08, 114.30, 104.28, 97.22, 54.95, 49.08. EI-MS: m/z calcd for 321 (M + 1)⁺, found 321. Element analysis (%): anal. calc. for C₂₁H₁₂N₄: C, 78.73; H, 3.78; N, 17.49. Found: C, 78.69; H, 3.86; N, 17.45.

Preparation of [Hg(A4)₂] (B4). A mixture of Hg(NO₃)₂, (0.08 mmol, 25.96 mg), A4 (0.04 mmol, 11.48 mg), H₂O (0.2 mL), DMF (0.2 mL) and EtOH (2 mL) in a capped vial was heated at 80 °C for one day. Yellow block like crystals of B4 suitable for single-crystal X-ray diffraction were collected, washed with ether and dried in air. Yield = 12.61 mg, 82%. Element analysis (%): anal. calc. for C₂₈H₂₄N₈O₆Hg: C, 43.69; H, 3.12; N, 14.56. Found: C, 43.71; H, 3.10; N, 14.57.

Results and discussions

Colorimetric study

Colorimetric sensing by naked eye is the simplest way to observe the selectivity of chemosensor to various metal cations. As shown in Fig. 1, under daylight lamp irradiation, upon the addition of 1.0 equiv. of K⁺, Ca²⁺, Ba²⁺, Co²⁺, Cd²⁺, Mg²⁺, Ag⁺, Na⁺, Fe³⁺, Hg²⁺, Cu²⁺, Ni²⁺, Al³⁺, Zn²⁺, Cr³⁺ and Pd²⁺ metal ions, chemosensors A1–A5 displayed a dramatic color change with Hg²⁺ in EtOH/H₂O at room temperature immediately, whereas no significant changes were found with other tested metal cations. The selective color change can be used for the “naked eye” detection of Hg²⁺ in aqueous solutions.²⁵

Fig. 1 Solutions of five chemosensors upon addition of different metal ions in EtOH/H₂O (v/v = 4 : 1). The color changed of (a), (b), (c), (d), (e) is the diagram of A1, A2, A3, A4 and A5, respectively. [A1–A5] = 40 μM, [Mⁿ⁺] = 200 μM.

UV-vis spectrophotometric estimation of Hg²⁺ binding with A1–A5

The UV-vis spectra of the chemosensors A1–A5 in EtOH/H₂O (v/v = 4 : 1, 1.0 × 10⁻⁵ M) solutions were characterized by two bands. The strong absorption peaks centered (max) at 362, 364, 365, 377 and 421 nm, respectively. The band centered at about 250 to 300 nm can be tentatively assigned to a charge-transfer band from donor to acceptor, corresponding to the π–π* absorbance of diaminomaleonitrile.^26,27 For the chemosensors based on ICT, electron-donating functional groups can greatly enhance the electron-donating ability of electron donors and result in a red-shift in the absorption spectra. As shown in Fig. 2, due to the conjugation pyrene ring, A5 has the largest red-shift amongst the five chemosensors. This phenomenon can be ascribed to the varied electron-donating ability of the donators. The trimethoxybenzyl group also has an effect on D–π–A conjugate system causing about a decade nanometers red-shifted in A1–A3. Fluorobenzyl group and pentafluorobenzyl group are electron-withdrawing groups which make the π–π* absorbance of A1 and A2 blue-shifted to shorter wavelength.

Fig. 2 Changes in absorption spectra of A1–A5 by alternating irradiation with UV-vis in EtOH/H₂O (v/v = 4 : 1, 1.0 × 10⁻⁵ M) at room temperature.

The ability of chemosensor A4 to form complex with metal ions was also studied using UV-vis spectroscopy (Fig. 3). The addition of Hg²⁺ to A4 caused a 28 nm red-shifted (from 377 to 405 nm), which changed the color from grey to yellow (Fig. 1). This phenomenon is similar to the addition of Hg²⁺ to chemosensors A1, A2, A3 and A5, respectively (Fig. S16–S19, in the ESI†). Hg²⁺ ion binding with chemosensors A1–A5 caused identical red-shifts and color changes. Hg²⁺ ion can induce deprotonation of active NH groups, especially those conjugated to aromatic or carbonyl groups. Such deprotonation process caused by complexation can be used for metal ions recognition and sensing.²⁸ For the chemosensors based on ICT, the deprotonation of those coordinated atoms in electron-donating groups can greatly enhance their electron-donating ability and result in a red-shifted in absorption spectra. These results indicate the occurrence of coordination between the five chemosensors and Hg²⁺.

Fig. 3 Absorption changes of A4 in EtOH/H₂O (v/v = 4 : 1, 1.0 × 10⁻⁵ M) upon addition of 1 equiv. of different nitrate salts (1.0 × 10⁻⁵ M).

To study the influence of other metal ions (Ag⁺, Al³⁺, Ba²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, K⁺, Mg²⁺, Na⁺, Ni²⁺, Pb²⁺ and Zn²⁺) on Hg²⁺ the binding of with A1–A5, competitive absorption spectroscopic were experiments performed with Hg²⁺ (1.0 × 10⁻⁵ M) in the presence of other metal ions (5.0 × 10⁻⁵ M). Take A4 for example (Fig. 4), bars represent for the absorbance at the biggest intensity band of chemosensers. Green bars represent the addition of 2 equivalent of metal ions (Ag⁺, Al³⁺, Ba²⁺, Ca²⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, K⁺, Mg²⁺, Na⁺, Ni²⁺, Pb²⁺, Zn²⁺ and blank, respectively) to the chemosensor solution. Red bars represent the subsequent addition of 1 equivalent of Hg²⁺ to the solution in the presence of competitive cations. Absorption changes caused by the mixture of Hg²⁺ with other metal ions were similar to that of Hg²⁺ alone. These facts confirmed that the selectivity of A1–A5 for Hg²⁺ ion detection is higher than other metal ions.

Fig. 4 Absorbance of A4–Hg²⁺ at the new band upon addition of various cations. The low bars represent A4 (1.0 × 10⁻⁵ M) with cations (5.0 × 10⁻⁵ M) without Hg²⁺; the high bars represent A4 (1.0 × 10⁻⁵ M) with cations (5.0 × 10⁻⁵ M) upon the subsequent addition of Hg²⁺ (1.0 × 10⁻⁵ M) in EtOH/H₂O(v/v = 4 : 1).

Detection limits and association constants

In order to prove the selectivity of A1–A5 towards Hg²⁺, the detection limits²⁹ were calculated as shown in Table 1. Assuming a 2 : 1 molar ratio forms the ligand/Hg²⁺ complex, the association constant (K) was calculated on the basis of the titration curves of A1–A5 with Hg²⁺. To further confirm the superior selectivities of A1–A5 towards Hg²⁺, the pH effects of their responses are evaluated as explained below.

Table 1 Calculated stability constant and detection limit data for A1–A5

	K	Detection limit
A1	6.88 × 10^2	2.40 × 10^−6
A2	4.89 × 10^2	3.13 × 10^−6
A3	3.11 × 10^3	2.74 × 10^−6
A4	3.62 × 10^3	1.93 × 10^−6
A5	2.12 × 10^3	3.13 × 10^−6

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Stoichiometries and affinity constants of chemosensors with Hg²⁺

To explore the quantitative interrelation and the interaction mode of chemosensors with Hg²⁺ ion, the coordination reaction of A4 with Hg²⁺ ion was monitored by UV-vis spectroscopic titration (Fig. 5). After the addition of Hg²⁺ ion (0.5 equimolar per drop), the peak of A4 at 279 nm showed significant red-shifted (about 9 nm) to the peak of B4 at 288 nm. The peak at 377 nm was found to become weaker and red-shifted with increasing amounts of Hg²⁺ ion. A new band centered at 382 nm started to develop with a distinct isosbestic point at 405 nm, indicating strong interactions between A4 and the Hg²⁺. No difference was observed in the absorption spectra once the 2 : 1 molar ratio of A4/Hg²⁺ was reached, which clearly indicated the formation of [Hg(A4)₂]_n.

Fig. 5 UV-vis titration curves of A4 in EtOH/H₂O in the presence of different amounts of Hg²⁺.

In order to further ensure the binding sites of A1–A5, take A4 for example, the stoichiometries of A4 + Hg²⁺ were calculated through Job's plots as shown in Fig. 6 the stoichiometries of A4 + Hg²⁺ was established by Job's plots between the mole fraction (X_M) and maximum absorption intensities at 430 nm, respectively. Upon the addition of 0–30 μM of Hg²⁺ (with an equal span of 3 μM), the absorption maxima of A4 were quenched rapidly up to 10 μM, afterwards they were found to be restored again. Therefore, the Job's plots intensities displayed a absorption maxima at a molar fraction of ca. 0.66 (A4 + Hg²⁺), as shown in Fig. 6, representing their 2 : 1 stoichiometric complexation.³⁰

Fig. 6 Absorbance spectral changes of stoichiometry calculations based on absorbance intensities at 430 nm; X_M = [Hg²⁺]/([Hg²⁺] + [A4]); where X_M = mole fraction, [Hg²⁺] and [A4] are concentrations of Hg²⁺ and A4; A4 + Hg²⁺ = 2 : 1 stoichiometry (ca. 0.66).

Benesi–Hildebrand (B–H) method is a widely used approach in physical chemistry for the determination of equilibrium constant K and stoichiometry of non-bonding interactions. This approach could generate stoichiometric conclusions for bonding interaction, such as charge-transfer complexes and host–guest molecular complexation.

nL + M → ML_n

According to Benesi–Hildebrand equation.

Taking logarithm on both side of equation

here in: A₁ is the absorbance of Hg²⁺ ion solution balanced. A₀ is the absorbance of Hg²⁺ ion absent solution. [Hg²⁺] is the concentration of Hg²⁺ ion, K is the stability constant, and n is the stoichiometry of ligand and metal ion.

Based on the 2 : 1 stoichiometry and the Job's plot, the stability constant of the complex between A1 and Hg²⁺ was estimated to be 6.88 × 10² by plotting lg[(A_l − A₀)/(A − A₀)] against lg{1/[Hg²⁺]}, all the experiment-based stability constants are calculated listed in Table 1. From Table 1, A3 has the biggest stability constant of the five chemosensors, meaning A3 has the greatest potential application as Hg²⁺ chemosensor. Due to its wide absorbance peak and poor red-shift ability, A5 does not perform as well as the other four ligands (Fig. 7).

Fig. 7 Benesi–Hildebrand plot of the Hg²⁺–A4 complexes in EtOH/H₂O solution.

pH effects

To demonstrate the Hg²⁺-induced deprotonation of the imine group in A1–A5, pH titration experiments were carried out. First, the influence of pH on chemosensor A4 was studied using UV-vis spectroscopy. Over a pH range of 5–10, the visible absorption band centered at 405 nm stays unchanged. A decrease in pH from 5 to 1 engendered a shift in the maximum absorption wavelength, which was due to protonation of the imine group.¹⁶ The effect of pH on Hg²⁺ binding to A4 was further studied by monitoring A4–Hg²⁺ complex at a wavelength of 405 nm (Fig. 8). The absorbance suddenly increased at pH 5.0 and reached a maximum over a pH range of 5.0–10.0, indicating that the formation of A4–Hg²⁺ complex is a deprotonation process.³¹ When the pH value exceeded 10, the absorbance gradually decreased during to the dissociation of A4–Hg²⁺ complex, which resulted in lower absorbance at 405 nm. The absorbance was almost negligible at pH values less than 3 clearly that evidently the A4–Hg²⁺ complex do not exist over this pH range.³² Because the five chemosensors were modified by different electron functional aromatic moieties, their abilities of protonation and deprotonation were different. Therefore, the five chemosensors' detection ranges of pH values were different.

Fig. 8 The stable range of pH values on A4, chemosensor A4 (1.0 × 10⁻⁵ M) was added in 1.0 mL EtOH/H₂O solution (v/v = 4 : 1, 20 mM buffer). The buffers were: pH 1.0–2.0, HCl; pH 2.5–4.0, KHP/HCl; pH 4.5–6.0, KHP/NaOH; pH 6.5–10.0 Hepes/NaOH, pH 11.0–12.0 NaOH.

Single crystal diffraction study

Single crystals of A4 were obtained by diffusion of EtOH to dichloromethane solution of A4. Single-crystal X-ray diffraction study reveals that A4 crystallized in the monoclinic crystal system with P2₁/c space group. The crystallographic details are provided in Table 2. A large planar can be seen between benzene and aminomalonitrile, which are quite conjugated as shown in Fig. 9a. The C–C and C–N bonds distances are in the range of 1.370(20)–1.458(19) and 1.285(18)–1.392(18) Å, respectively. There exists weak non-classical C–H⋯O intramolecular hydrogen bonds involving methoxy groups with methenyl group (C⋯O = 2.804–3.464 Å) as well as N–H⋯N intramolecular interaction between the amino and imino group (N⋯N = 2.791 Å), which play a vital role in the consolidation of the A4 molecule. Via N–H⋯N intermolecular interactions between the amino and neighboring cyano groups (N⋯N = 3.020–3.088 Å, N–H⋯N = 137–169°) as well as the nonclassical C–H⋯O hydrogen bonds interaction between benzene rings and neighboring methoxy groups (C⋯O = 3.393 Å, C–H⋯O = 151°), the A4 molecules are interlinked into a two-dimensional supramolecular network (Fig. 9c).

Table 2 Crystal data and structure refinement for A4 and B4

Identification code	A4	B4
a R1 = ∑||Fo| − |Fc||/∑|Fo|.b wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Empirical formula	C14H14N4O3	C28H24HgN8O6
Mr/g mol^−1	286.29	769.14
T/K	110(2)	153(2)
Crystal system	Monoclinic	Monoclinic
Space group	P21/c	C2/c
a/Å	14.022(4)	17.206(6)
b/Å	7.417(2)	13.808(3)
c/Å	13.758(4)	13.936(4)
α/°	90.00	90.00
β/°	90.41(3)	122.01(3)
γ/°	90.00	90.00
V/Å^−3	1430.77(7)	2807.35(14)
Z	4	4
ρc/g cm^−3	1.329	1.820
μ/mm^−1	0.097	5.541
Reflections collected	5133	16 304
Unique reflections	2810	2757
Rint	0.021	0.038
R1^a (I > 2σ(I))	0.036	0.018
wR2^b (all data)	0.099	0.042

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Fig. 9 (a) Molecular structure of A4. (b) Coordination structure of complex B4. H atoms omitted for clarity. (c) Perspective views of hydrogen-bonding interactions in A4.

It also reveals that complex B4 crystallized in the monoclinic crystal system with C2/c space group. The asymmetric unit contains one half of the formula unit and consists of one crystallographically unique Hg atom and one A4 (Fig. 9b). Hg atom lies in a special position (SOF = 0.5), being four-coordinated by two amino and two imino nitrogen atoms from two different A4 (Hg–N = 2.034(2)–2.647(2) Å). The two A4 molecules have a dihedral angle ca. 19.69° (based on the benzene rings), being located at the same side of Hg atom. There exists weak non-classical C–H⋯O intramolecular hydrogen bonds between methoxy groups (C⋯O = 2.826–3.168 Å), which play a significant role in the construction of the mononuclear Hg unit. Via C–H⋯N intermolecular interactions between the methoxy groups and neighboring cyano groups (C⋯N = 3.106–3.457 Å, N–H⋯N = 102–149°), the B4 molecules are interlinked into a two-dimensional supramolecular network (Fig. 10).

Fig. 10 2D supramolecular structure of B4 mediated by hydrogen bonds.

Theoretical calculation study

In order to further understand the behavior of A4 with the Hg²⁺, DFT calculations have been carried out. As Hg²⁺ is known for its four-coordination complex, the input structures were designed in the same way to have bidentate coordination with DMN. The structural optimization was done using the Gaussian 09W package adopting the B3LYP method with SDD as the basic set. TD-DFT calculations investigate the change of the absorption spectra upon addition of Hg²⁺. Electron density of both HOMO and LUMO of A4 and B4 lies on the electron-withdrawing part. From the energy level diagram (Fig. 12), it can be seen that addition of the metal ions leads to the stabilization of the HOMO and LUMO of these chemosenors. Both HOMO and LUMO of the B4 and A4 were shown in Fig. 11, which are mainly dominated by molecular orbitals from A4. The contribution from Hg²⁺ ion to all these orbitals is significant. The electron density of the HOMO is localized on the whole conjugate part of the molecular, whereas the LUMO distribute on the diaminomaleonitrile and the imine moiety. For complex B4, its HOMO is more strongly localized than that in A4, and the localization on the cyano group of the diaminomaleonitrile from Hg atom is disappeared. The LUMO of complex B4 is more located in the benzene ring and its localization on the cyano group of diaminomaleonitrile from Hg atom is also disappeared. Therefore, it can be concluded that the electronic π–π* transition in these complexes are localized on the 2-substituted diaminomaleonitrile ligands.

Fig. 11 HOMO and LUMO orbitals of A4 (a) and B4 (b) calculated on the DFT level.

The calculated HOMO–LUMO energy gaps (E_g) are summarized in Fig. 12. From the energy level diagram, complex B4 has a lower LUMO energy level of −4.59 eV, while ligand A4 has that of −2.50 eV. So it can be concluded that the coordination of Hg stabilized the LUMO of the complex. The HOMO energy levels of A4 and B4 are calculated to be −6.01 and −5.33 eV, respectively, showing that the HOMO energy level of A4 is a bit higher than that of B4. The calculated HOMO–LUMO energy gaps (E_g) obtained from DFT calculations were 3.51 and 0.74 eV for A4 and B4, respectively. The complex B4 has much smaller energy gap and longer wavelength of emission than that of A4. These data are consistent with the optical energy gaps as well as the band gap trend is in good agreement with the optical data.

Fig. 12 Energy diagrams of HOMO and LUMO orbitals of A4 and B4 complex calculated on the DFT level.

Conclusion

In summary, five diaminomaleonitrile-based chemosensors were designed for naked-eye detection of Hg²⁺, which were investigated with much higher selectivity over other competitive metal ions. They displayed strong colorimetric responses with slight red-shift absorptions that were useful for easy detection of Hg²⁺ by naked eyes visualization. Adequate characterization and studies were carried out to confirm the chemosensors A1–A5 and their sensing behaviors toward Hg²⁺ ion. The binding mode study showed that the diaminomaleonitrile chemosensor formed a 2 : 1 complex with Hg²⁺ where the imine group played a critical role in the selective binding of Hg²⁺ ion. Their recognition behavior can function well over a wide range of pH value, making them suitable for detection of Hg²⁺ ion in industrial and environmental fields.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21172047 and 21372051), the Excellent Young Teacher Development Project of Universities in Guangdong Province (261532106) and the Guangzhou Science and Technology Program (201510010156).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 994864 and 994865. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra21474b

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