Solvo-thermal synthesis of an ultra-stable 3D luminescent cadmium (II) hybrid and its application as a recyclable luminescent sensor for Fe (III)

2.1 Materials and physical measurements

All chemicals and solvents were of A.R. grade(≥99%) and used without further purification. The IR spectrum was recorded on a Shimazu IR435 spectrometer using the KBr disk technique (400–4000 cm^–1). Photoluminescent measurement of 1 and the cation L1 in the solid state were conducted on a HITACHI F-4600 spectrophotometer and the data was collected at room temperature. Powder X-ray diffraction (PXRD) data were measured on a Philips X’ Pert Pro MPD X-ray diffractometer (Mo-Kα= 0.71073) with an X’ Celerator detector.

2.2 Synthesis of compounds 1

1, 3, 5-tris(imidazol-1-ylmethyl)-2, 4, 6-trimethylbenzene was synthesized according to the literature [19]. L1·Br ₃ was synthesized according to the literatures [20, 21].

Synthetic procedure and single crystal growth for 1: Acetonitrile and water solution of cation L1 Br₃ (0.0076 g, 0.01 mmol) was added to a stirring colorless solution of CdI₂ (0.0036 g, 0.01 mmol) dissolved in 1.5 mL acetonitrile solution with excess of KI. The resulting mixture was stirred for 5 min and filtered. Then the solution was slowly evaporated in a vial at room temperature. Before all solvent was evaporated enough ideal crystals formed in the bottom of the vial were harvested and regarded as the main product. With this method transparent well-formed single crystals of dimensions 0.22 mm×0.08 mm×0.07 mm suitable for X-ray analysis were obtained within a week in about 30% yield. The product was not soluble in common solvents such as acetonitrile, methanol, ethanol, chloroform, methylene chloride, acetone, tetrahydrofuran, water, and the characteristic photograph of the granular crystals is shown in Fig. 1. IR(KBr): 3416.16(m), 3109.35(m), 2974.46(m), 1617.83(m), 1566.02(s), 1400.03(m), 1328.64(m), 1129.64(s), 807.81(m), 763.12(m), 719.01(w), 617.30(s), 500.21(w) cm^–1.

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Fig.1

Photograph of {[CdI₄]₃·(L1)₂} (1) and {(L1) [HgI₄]·I} (2).

The crystallographic data for compound 1 was collected on the Bruckner SMART CCD diffractometer with graphite-monochromatic Mo-Kα radiation (λ= 0.71073 Å) at 293K. The structure was solved by direct methods and expanded using Fourier techniques. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by using geometrical constraints. The structure was refined with full-matrix least-squares techniques on F² using the SHELXTL-97 program package. The information regarding data collection and structure refinement is summarized in Table 1. Selected bond lengths and bond angles are listed in Table 2. For compound 1 the as-synthesized PXRD patterns closely match the simulated patterns generated from the results of the single crystal diffraction data, indicative of pure products which were shown in Fig. 2.

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Fig.2

Power X-ray Diffraction (PXRD) of complex 1.

Powdered sample (20 mg) of 1 was dispersed in an aqueous solution (3 mL) of M(NO₃)_c (M^c+ = K⁺, Cu²⁺, Ba²⁺, Al³⁺, Co²⁺, Cd²⁺, Ni²⁺, Fe²⁺, Cr³⁺, Zn²⁺ and Fe³⁺ (0.01 mol/L) at room temperature. Luminescence data of the solution were collected after ultrasonication for 30 min [22, 23].

2.5 Recyclable and selective luminescence experiments

The recyclability and selectivity of 1 for sensing Fe³⁺ was also investigated. 3 mL Fe³⁺ (0.005M) was added into the suspension of corresponding ions and samples in a selective experiment. Luminescence data of the solution were collected after ultrasonication for 30 min. After the first quenching experiment, the powder of 1 was centrifuged and washed several times with deionized water. The recovered solid was collected and then used in the successive quenching experiments.

3.1 Description of crystal structure

3.1.1 Structure of compound {₃·(L1)₂} (1)

Compound 1 crystallizes in a monoclinic system with P2₁/c space group. Its smallest repeating unit consists of two parts: organic cation L1 and anion [CdI₄]^2–. In (CdI₄)^2– anion, the central Cd atom coordinates with four I atoms to form an irregular tetrahedral structure (Fig. 3(a)). The key Cd-I bond length is between 2.6977(16)–2.8261(12). I-Cd-I key bond angle is in a range from 103.44(6)–118.17(4)°. The distance of benzene rings is 5.103 Å and 5.126 Å (Fig. 3(b)). The three imidazolium rings are not arranged symmetrically, the angles of them are 37.93, 41.73 and 79.65°, and the distances of the 2-C atoms of the imidazolium are 4.553, 4.601 and 4.697 Å, respectively. the 2-C distance in the other cation are 4.488, 4.566, and 4.693 Å (Fig. 3(c)). As shown in Fig. 3(d), there is hydrogen bonding between I atom and imidazole ring (C - H⋯I = 2.777–3.474 Å). Through hydrogen bonds, van der Waals force, and electrostatic interactions compound 1 form a three-dimensional (3D) supramolecular structure.

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Fig.3

(a) Smallest repeating unit of 1. The H atoms were omitted for clarity; (b) and (c) The unit diagram of 1; (d) 2D stacking diagram of compound 1 and hydrogen bonds (red dash lines) between adjacent cations and anions in 1.

A compound most similar to 1 is {(L1) [HgI₄]·I} (2), which crystallizes in a monoclinic system with P21/c space group. Its smallest repeating unit consists of the organic cation L1, anion [HgI₄]^2– and a free I^–. Hg²⁺ also exhibits four coordination mode with I^–, forming a tetrahedral structure (Fig. 4(a)). The distance of the benzene rings is 5.0821 Å. Figure 4(b) shows the weak hydrogen bonding among the cationic L1 and anion [HgI₄]^2– [24]. Comparison of some important conformational parameters of L1, 1 and 2 are listed in Table 3. The vertical height of the cages L1 in 2 is smaller than that in 1, and the cyclophane cage become flat.

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Fig.4

(a) Structure unit diagram of 2. The H atoms were omitted for clarity. (b) The hydrogen bonding in compound 2.

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Table 3

Comparison of some important conformational parameters of L1, 1 and 2

Compounds	The vertical height of the cages (Å)	Dihedral angles between imidazole rings (°)	Distance between the 2-C (Å)	Refs
L1·Br3	5.14	31.6,76.0,44.4	4.548,4.716,4.613	this article
{(L1)2 [CdI4]3}	5.126, 5.103	37.93,79.65,41.73	4.553,4.697,4.601	this article
{(L1) [HgI4]·I}	5.0821	37.35, 60.04,82.60	4.561,4.744,4.660	46

Our previous studies showed that the cationic conformation plays an important role in the anion assembly and final architectures [25–27]. Tris(imidazolium)cyclophane cages possessing strong rigidity and insufficient inner space which is expected to depress its template effect to induce or include an anion guest. The assembly product with electron-poor metallic halides exhibit 0D mononuclear or polynuclear structure.

3.3 Infrared spectroscopy (IR)

The infrared absorption of main functional groups in 1 is shown in Fig. 5. The peak at 3109.35cm^–1 is the C-H stretching vibration of the imidazole ring. The multiple peaks between 1600-1400 cm^–1 are the respiratory vibration peaks of the benzene ring molecular skeleton. The stretching vibration peak of –CH₂ is around 2000 cm^–1. The C-N stretching vibration peak and the in-plane bending vibration peak of the imidazole ring are around 1150 cm^–1 and 1129.64 cm^–1, respectively.

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Fig.5

Infrared spectroscopy (IR) of complex 1.

1 Exhibited high stability under different chemical conditions. As can be seen from the infrared spectra, compound 1 did not change before and after exposing to various solvents (Fig. 6), pH (Fig. 7) and sensing (Fig. 8) conditions by centrifuging the suspension and washing several times with water and measured IR after 24 hours.

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Fig.6

IR spectra for 1 after treatment with different solvent environments at room temperature for 24 hours.

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Fig.7

IR spectra of 1 after exposure to different PH from 2 to 13 at room temperature for 24 hours.

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Fig.8

IR diagram of compound 1 before and after detecting metal ions.

Metal complexes are promising luminescent materials. Until now luminescent properties of tris(imidazolium)cyclophane L1 and its supramolecular compounds in solid state have not yet been reported. Herein, the luminescent properties and the CIE diagram of free cations L1 and compound 1 were studied (Figs. 9, 10). The CIE (Commission International edel’Eclairage1931) diagram used to show that the fluorescence color of the free cation L1 is deep blue (0.1782, 0.2207) and the fluorescence color of 1 is light blue (0.2325, 0.2413). The UV-Vis spectra of compounds 1 is shown in Fig. 11. At ambient temperature, the emission spectrum of L1 shows a peak at 454.8 nm upon excitation at 409 nm. The maximum emission peak of compound 1 is located at 438.4 nm upon excitation at 370 nm with a blue shift of 16.4 nm in contrast to cation L1; The luminescence of L1 may be due to the charge transfer π*-π transition. The resemblance between the emission spectrum of compound 1 and that of free cation L1 reveals that the luminescence of compound 1 is L1-based emission [28–30]. This fluorescence emission phenomenon also indicates that the conformations of the cationic cage have certain changes, which leads to the change of the energy level difference between the ground state and the excited state.

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Fig.9

The fluorescence spectra of L1 in solid state and CIE chromaticity diagram of L1.

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Fig.10

The fluorescence spectra of 1 in solid state (Ex = 409 nm) and CIE chromaticity diagram of 1.

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Fig.11

The solid UV spectra of compounds 1.

Detection of Fe³⁺ in aqueous media is of great importance as iron is one of the most important elements in environmental and biological systems and plays a central role in the biosphere. In addition, Fe³⁺ plays essential roles in oxygen uptake, oxygen metabolism, and electron transfer. Thus, it is very important to detect Fe³⁺ rapidly and sensitively in environmental protection and food safety [31–35].

To explore the potential applications in fluorescent aspects, compound 1 was selected for probing all kinds of metal ions, such as K⁺, Cu²⁺, Ba²⁺, Al³⁺, Co²⁺, Cd²⁺, Fe²⁺, Ni²⁺, Cr³⁺, Zn²⁺ and Fe³⁺. The luminescence intensity of 1 was quenched sharply in Fe³⁺. The reasons may be related to the stronger static electric interaction between 1 and Fe³⁺. Ion recognition maps are shown in Fig. 12.

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Fig.12

Photoluminescence intensity 1 treated with 0.01 mol/L different metal ions and the fluorescence spectra of detecting metal ions excited at 270 nm.

As shown in Fig. 13, the results reveal that different metal ions display markedly different effects on the luminescence of 1. The luminescence intensities of the samples in Fe³⁺ solution drastically decrease while in other ions have little change, which indicated that 1 could be used as a fluorescence sensor for Fe³⁺ ions with high sensitivity and selectivity. Moreover, compounds 1 is easily recovered by centrifuging the suspension after detecting Fe³⁺ ions and washing several times with water (Fig. 14). The sample can be regenerated and reused for three cycles, and the quenching efficiencies of the three cycles maintained high values which are comparable to that of other similar materials [31]. All results reveal that 1 could be employed as a fluorescence sensor for detecting Fe³⁺ ions with high sensitivity and recyclability.

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Fig.13

Fluorescence spectrum of 1 (10 mg) in the presence of various metal ions (0.01 M) and Fe³⁺ (0.005 M) (λ= 270 nm).

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Fig.14

Cycle diagram of compound 1.