Dual functional fluorescent sensor for selectively detecting acetone and Fe³⁺ based on {Cu₂N₄} substructure bridged Cu(I) coordination polymer

Abstract

A novel red emitting coordination polymer (CP), [Cu(tpp)·H₂O]_2n (1) [Htpp = 1-(4-tetrazol-5′′-yl)benzyl-3-(pyraziny-l)pyrazole], has been successfully constructed using a nitrogen heterocyclic ring ligand and characterized by single crystal X-ray diffraction, infrared (IR) studies, elemental analysis and thermogravimetric analysis (TGA). 1 is a rare 3D supramolecular assembly based on a 1D polymeric chain substructure of {Cu₂N₄} bridged by the Htpp ligand, which is propagated to form an extended 3D structure with the help of π−π stacking and C–H⋯π hydrogen-bonded packing. Based on the phosphorescence of Cu(I)-CP, 1 is chosen as a probe for sensing different small molecules and metal ions. The luminescence properties of 1 well dispersed in different solvents have been investigated systematically, which demonstrates distinct solvent-dependent luminescence spectra with emission intensities that are significantly quenched by acetone (detection limit: 0.0842 vol%). Besides that, 1 with open Lewis basic nitrogen atoms on the chain surface was exploited for the binding and specific sensing of metal ions via Lewis base interactions, and shows high selectivity and sensitivity (K_sv = 4.6 × 10⁴ L mol⁻¹) for Fe³⁺ ions with luminescence quenching. Solid-state 1 displays excellent red lighting properties, which can be comparable with rare earth compounds. Subsequently, 1 blended poly(methyl methacrylate) PMMA film demonstrates comparable intensity with the solid state at concentrations of 1.5%, accompanied by an improvement of the luminescence lifetimes.

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Introduction

Acetone is recognized as a highly volatile organic solvent, and humans can readily absorb acetone in many ways, such as ingestion, inhalation, and dermal exposure. The absorbed acetone distributing throughout the body, particularly in organs with high water content, would cause metabolism disorder and introduce serious damage to the human body. Considering the wide range of harm of acetone, it is crucial to design new sensors to detect it. Since the first example of an acetone responsive coordination polymer (CP) Eu(BTC)(H₂O)·1.5H₂O has been reported,¹ a handful of rare earth CPs has been constructed for sensing of acetone.^2–4 Nevertheless, considering the cost of luminescent probes, rare earth CP probes with high cost are not extensively employed in practical applications. It is urgent to develop low-cost CPs to replace them, and phosphorescence based Cu(I) CPs can be as outstanding candidates.

On the other hand, as an essential biological metal ion, iron(III) plays a critical role in a variety of fundamental processes in living biological systems.^5,6 However, iron(III) is harmful and even fatal when in excess or deficiency, underlining the need for accurate detection of iron ions.⁷ Until now, various analytical and spectroscopic methods have been employed to detect small molecules and metal ions,^8,9 but high cost, low sensitivity, intricate operation, and high levels of interference by other analytes associated with these methods have restricted their widespread use.¹⁰ Recently, fluorescence method based detection of small molecules or metal ions by using luminescent polymers has attracted much attention because of the selectivity, sensitivity, short response time, and convenient visual detection.^11,12

From the simple and economic perspective, of importance is the development of integrating dual-functional fluorescent sensor into one single system, not only for sensing small molecules but also for detecting metal ions. Particular attention has been paid to the photochemical and photophysical properties of copper(I) coordination polymers in light of the d¹⁰ electronic configuration which diversifies their luminescent behavior.^13,14 In our continuing research on luminescent coordination polymers, we focused our study on the development of Cu(I)-based dual-functional fluorescent sensor. In general, an effective strategy to synthesize Cu(I)-based sensors is to use π-conjugated organic molecules with Lewis basic sites, in which the π-conjugated organic molecules provide luminescence and the Lewis basic sites provide the binding site. Large conjugate nitrogen heterocyclic ring ligands are the first choice. Compared with rigid ligands, flexible ligands have various coordination modes and their final structures are various.^15,16 So flexible ligands have received considerable attention for the construction of Cu(I)-CPs, due to the assembled products possess several unique advantages.^17,18

In the construction of coordination polymers, transition metal copper is often used as metallic synthon due to its high affinity to N donor.¹⁹ In order to prepare Cu(I)-CPs, we choose nitrogen heterocyclic ring 1-(4-tetrazol-5′′-yl)benzyl-3-(pyraziny-l)pyrazole (Htpp) as ligand (Fig. 1), a novel 3D supramolecule [Cu(tpp)·H₂O]_2n (1) has been obtained successfully. Due to the ligand chosen with large π-conjugated systems and multifunctional coordination sites, they can participate in π⋯π stacking and hydrogen bonding interactions. 1 displays a 1D structure, which contains six-membered {Cu₂N₄} ring and further achieves a stable 3D supramolecular architecture by π⋯π stacking and C–H⋯π. Amazingly, 1 can serve as a dual functional fluorescent sensor through quenching the luminescence, not only selectively detecting acetone, but also possessing relatively high sensitivity for Fe³⁺. 1 was developed as highly selective and sensitive fluorescence probe targeting acetone, which could be proved by the fact that 1 demonstrates distinct solvent-dependent luminescent spectra and the detection limit is 0.0842 vol%. The further luminescent explorations revealed that 1, with open Lewis basic nitrogen atoms on the chain surface, belongs to the luminescent probe for Fe³⁺ possessing relatively high selectivity, and high sensitivity (K_sv = 4.6 × 10⁴ L mol⁻¹) among 10 kinds of metal ions. Up to now, a number of CPs have been successfully applied for the sensing of acetone and Fe³⁺ ions, which are summarized in Tables S1 and S2, ESI.† From the comparison with the reported literatures, most sensors are lanthanide coordination polymers,^20–22 and a small amount of IIB²³ and main group coordination polymers.²⁴ However, IB group metal Cu(I)-coordination polymer is very rare. In addition, 1 can be a good candidate for the red luminescent materials, due to it shows bright red luminescence in the solid state at 298 and 77 K. Then, 1 demonstrates the comparable intensity with solid state at low concentrations only of 1.5% (τ = 23.41 μs) in poly(methyl methacrylate) (PMMA). Development of easy-to-prepare hybrid materials with bright red luminescence in daily applications is meaningful.^25,26

Fig. 1 Ligand 1-(4-tetrazol-5′′-yl)benzyl-3-(pyraziny-l)pyrazole (Htpp).

Experimental section

Materials and methods

All reagents were analytical grade (99.7%) from commercial sources and were used directly without any further purification. Ligand 1-(4-tetrazol-5′′-yl)benzyl-3-(pyraziny-l)pyrazole (Htpp) and metal salt CuI were purchased from Ji Nan Henghua Sci. & Tec. Co. Ltd. (Shandong, China). Solvents for photophysical studies were dried and freshly distilled under dry nitrogen gas before using. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 240c elemental analyzer. IR spectra were recorded by Nicolet Impact 410 FTIR spectrometer (range in 4000–400 cm⁻¹). The solid-state and solution photoluminescence analyses were carried out on an Edinburgh FLS920 fluorescence spectrometer in the range of 200–800 nm. An Edinburgh Xe900 400 W Xenon arc lamp was used as exciting light source. The visible detector as well as the lifetime setup was red-sensitive photomultiplier (type r928). Low temperature analyses were carried out at 77 K with an Oxford Optistat DN™ cryostat (with liquid nitrogen filling). Lifetime studies were performed using photon-counting system with a microsecond pulse lamp as the excitation source. Data were analyzed through the nonlinear least squares procedure in combination with an iterative convolution method. The emission decays were analyzed by the sum of exponential functions. The decay curve is well fitted into a double exponential function: I = I₀ + A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂), where I and I₀ are the luminescent intensities at time t = t and t = 0, respectively, whereas τ₁ and τ₂ are defined as the luminescent lifetimes. The average lifetime was calculated according to the following equation:

Synthesis of [Cu(tpp)·H₂O]_2n (1)

A mixture of CuI (0.010 g, 0.05 mmol) and Htpp (0.030 g, 0.10 mmol) with a mole ratio of 1 : 2 was dissolved in 8 mL CH₃OH. When the pH value of the mixture was adjusted to ca. 8.0 with NH₃·H₂O, the solution was transferred into a Teflon-lined stainless steel vessel (20 mL) under autogenous pressure and heated at 120 °C for 72 h. The mixture was washed with distilled water and red pillared-shaped crystals were filtered off and dried at room temperature (yield ca. 74%, based on CuI). Elemental analysis (%): calc. for C₃₀H₂₆Cu₂N₁₆O₂ (M_r: 769.75): C, 46.81; N, 29.12; H, 3.40%. Found: C, 46.32; N, 29.05; H, 3.36%. IR (cm⁻¹): 3437 (br, s), 3065 (w), 2934 (w), 1623 (m), 1536 (s), 1438 (s), 1405 (vs), 1241 (vs), 1153 (s), 1055 (s), 848 (m), 793 (s), 739 (s), 531 (w), 433 (w).

The solvent sensing experiment

The solvent sensing experiment was performed as follows: finely ground samples of 1 (5 mg) were immersed in different organic solvents (5 mL), treated by ultrasonication for 1 hour, and then aged to form stable emulsions before their fluorescence was measured.

The metal ion sensing experiment

The samples 1 contain H₂O solvent molecules. In order to investigate the metal ion sensing properties, 1 was activated by heating at 105 °C under vacuum one day for full activation, producing 1a. 1a powders (50 mg) were immersed in an aqueous solution (8 mL) of MCl_x (M = K⁺, Na⁺, Mg²⁺, Ba²⁺, Ag⁺, Zn²⁺, Cd²⁺, Mn²⁺, Pb²⁺, Fe³⁺) for 48 h.

Synthesis of 1-blended PMMA polymer films

The PMMA polymer was blended with the 1 in the proportions 0.3, 0.6, 0.9, 1.2, 1.5, and 1.8% (w/w). The PMMA powder was dissolved in 6 mL N,N′-dimethylformamide (DMF), followed by addition of the required amount of 1 in DMF solution, and the resulting mixture was heated at 50 °C for 60 min. The polymer film was obtained after evaporation of excess solvent at 60 °C.

X-ray crystal structure determination

The X-ray diffraction data taken for coordination polymer 1 is collected on a Rigaku R-AXIS RAPID IP diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structure of 1 is solved by direct methods and refined on F² by the full-matrix least squares using the SHELXTL-97 crystallographic software.^27,28 Anisotropic thermal parameters are refined to all of the non-hydrogen atoms. The hydrogen atoms are held in calculated positions on carbon atoms and nitrogen atoms and that are directly included in the molecular formula on water molecules. The CCDC 1504779 contains the crystallographic data 1 of this paper. Crystal structure data and details of the data collection and the structure refinement are listed as Table 1. Selected bond lengths and bond angles for 1 are summarized in Table S3, ESI.†

Table 1 Crystal data and structure refinement parameters of coordination polymer 1

a R1 = ∑||Fo| − |Fc||/∑|Fo|.b wR2 = [∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]]^1/2.
Identification code	1
Empirical formula	C30H26Cu2N16O2
Formula mass	769.75
Crystal system	Monoclinic
Space group	P21/c
a (Å)	19.0235(14)
b (Å)	18.0362(13)
c (Å)	9.6480(7)
α (°)	90.00
β (°)	104.398(2)
γ (°)	90.00
V (Å^3)	13 206.4(4)
Z	4
Dc/(g cm^−3)	1.595
μ (Mo Kα)/mm^−1	1.385
F(000)	1568
Crystal size	0.26 × 0.22 × 0.22 mm
θ range (°)	1.11–25.00
Limiting indices	−22 ≤ h ≤ 22
	−20 ≤ k ≤ 21
	−11 ≤ l ≤ 7
Data/restraints/parameters	5645/0/451
GOF on F^2	1.090
Final R indices [I > 2σ(I)]
R1^a	0.0510
wR2^b	0.1422
R indices (all data)
R1	0.0816
wR2	0.1692
CCDC	1504779

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Results and discussion

Crystal structure of [Cu(tpp)·H₂O]_2n (1)

Single-crystal X-ray diffraction analysis reveals coordination polymer 1 is a 3D supramolecular assembly based on a 1D polymeric chain substructure of {Cu₂N₄} bridged by Htpp ligand, and its asymmetric unit contains two Cu(I) ions and two deprotonated tpp⁻ ligand. As shown in Fig. S1, ESI,† two crystallographically independent Cu(I) cations are bridged by a pair of symmetry-related tetrazole groups in syn–syn bridging mode to form a six-membered {Cu₂N₄} ring with the Cu⋯Cu separation is 3.555 Å. The central metal Cu(I) ions are four coordinated by four N atoms from two tpp⁻ ligands. Using Addison's model,^29,30 the coordination geometry around the copper atom in Cu1 (τ₄ = 0.904) and Cu2 (τ₄ = 0.900) can be better described as two tetrahedrons. In particular, the Cu(I) cation coordinates with nitrogen atoms from ligand tpp⁻ to form a five membered ring, which further extends system conjugation. Due to there exists –CH₂–, Htpp ligand is serious twisted and the dihedral angles are 83.75° and 80.87°, respectively, which is close to vertical. The uncoordinated nitrogen atoms of pyrazine ring prevent the linkage between the neighbouring chains into a higher dimensional structure. Luckily, intermolecular π⋯π stacking and hydrogen bond C–H⋯π make the 1D chains into a 3D supramolecular framework. It is clear from the geometrical feature that the 2D layer is directed by π⋯π (3.577 Å) stacking interaction (Fig. 2a and c), and the framework is further enforced by C–H⋯π hydrogen-bond packing (C20–H20A–Cg1 = 2.657 Å) among the organic building blocks (Fig. 2b and d). The collaborative π⋯π stacking and hydrogen bonding C–H⋯π have featured the 3D supermolecular architecture as the rare example of potentially robust SMOFs (Fig. 2e).

Fig. 2 (a) The π⋯π stacking interaction in 1; (b) the C–H⋯π interaction in 1; (c) the π⋯π stacking interaction to form 2D layer of 1; (d) the C–H⋯π interaction to form 2D layer of 1. (e) A 3D supramolecular structure of 1 constructed by π⋯π and C–H⋯π interactions.

PXRD and thermal analysis

In order to check the phase purity of 1 the X-ray powder diffraction (PXRD) pattern was checked at room temperature. The simulated and experimental PXRD patterns of 1 are in good agreement with each other (Fig. S3, ESI†), indicating the phase purity of the products. The differences in intensity may be due to the preferred orientation of the powder samples.

To estimate the stability of the coordination polymer 1, thermogravimetric analyses (TGA) in purified air were carried out and the TGA curves are shown in Fig. S4, ESI.† Host framework of 1 could keep until 316.4 °C, and the loss 18.74% occurring at the first step is attributed to remove benzene ring of Htpp ligand (calculated 18.96%). The second weight-loss of 56.18% from 340.2 to 688.6 °C, which corresponds to the decomposition of tpp⁻ ligand completely (calculated 56.59%). It's demonstrated when the ligand coordinated to metal, it will make the framework more stable. The remaining weight of 20.08%, which is in good agreement with the calculated value (19.96% for 1), indicating that the final product is CuO.

Luminescence property

Coordination polymers constructed from d¹⁰ metal ions and conjugated organic linkers are promising candidates for potential photoactive materials.^31–33 The luminescence properties of 1 in different solvent emulsions were discussed. The solvents used were dichloromethane (CH₂Cl₂), acetonitrile (CH₃CN), methanol (CH₃OH), ethanol (CH₃CH₂OH), n-hexane, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), dimethyl sulphoxide (DMSO) and acetone. The most interesting feature is that the photoluminescent spectra are largely dependent on the solvent molecules, particularly in the case of acetone, which exhibit the most significant quenching effects (Fig. 3). Such solvent-dependent luminescence properties are of interest for the sensing of acetone solvent molecules. The efficient quenching of acetone in this system can be ascribed to the physical interaction of the solute and the solvent, which induces the electron transfer from the excited 1 to electron-deficient acetone.³⁴ These encouraging results reveal that 1 could be promising luminescent probes for detecting small molecules of acetone.

Fig. 3 (a) The emission spectra of 1 in different organic solvents; (b) comparison of the luminescence intensity of 1 in different organic solvents; inset: the emission spectra of 1 in acetone.

Due to the miscibility between acetone and water, it is particularly important to detection acetone in aqueous solutions. In the compared reports, most of them are sensing acetone in organic solvent (such as DMF, CH₃CN, alcohol and so on).^2,35,36 The unusual selectivity of coordination polymer 1 to acetone prompts us to study it on the detection limit of acetone. The solid sample 1 was immersed in different concentrations of acetone for 24 h, and the luminescence spectra were recorded. As demonstrated in Fig. 4a, the emission intensity of 1 suspension increased in accordance with the increase of acetone amounts from 0% to 3 vol%. When the acetone solvent was gradually and increasingly added to the 1 standard emulsions, the fluorescence intensities of the standard emulsions gradually decreased with the addition of the acetone solvent. The fluorescence decrease was nearly proportional to the acetone concentration and the system ultimately reached the equilibrium state. In other words, the fluorescence response of 1 towards acetone is linear when measured in the range 0–3 vol% of acetone (inset of Fig. 4b). To calculate the standard deviation and detection limit of this detection method, 1 with fine particles was made into a suspension. Then, acetone was added into the suspension and the fluorescent intensities were recorded. Standard deviation (σ) was calculated from five blank tests of 1 suspension and the detection limit was calculated via the formula: 3σ/m (m: the slope of the linear region).³⁷ The standard deviation of this detection method and acetone detection limit are calculated to be 0.44 and 0.0842 vol%, respectively (Table S4, ESI†), which is comparable with the occupational exposure limit of acetone (750 ppm or 0.075 vol%) that is stipulated by the American Conference of Governmental Industrial Hygienists (ACGIH).³⁸ In addition, the detection limit of our sensor is lower than those in previously reported (0.1–0.5 vol%) (Table S1, ESI†). The low detection concentration reveals that 1 is a potential sensor for the sensitive and selective detection of acetone.

Fig. 4 (a) Fluorescence intensity of 1 at ca. 600 nm (λ_ex = 360 nm); (b) comparison of the luminescence intensity of 1 in different concentrations of acetone in aqueous solution; inset: linear relationship of 1 quenched by acetone.

The existence of open Lewis basic nitrogen atoms active sites in the framework makes the structure of 1 more interesting, which means 1 is a promising candidate for sensing and detecting metal ions. In this regard, 1a (active 1) was simply immersed in an aqueous solution of 0.001 mol L⁻¹ MCl_x (M = K⁺, Na⁺, Mg²⁺, Ba²⁺, Ag⁺, Zn²⁺, Cd²⁺, Mn²⁺, Pd²⁺, Fe³⁺ respectively) for 48 h for luminescence studies. The photoluminescence properties are recorded. The corresponding luminescence curve shows the emission peaks at ca. 595 nm were monitored under the perturbation of various cations, as shown in Fig. 5a and S5, ESI.† Interestingly, Fe³⁺ exhibits a drastic quenching effect on the luminescence of 1a, while other metal ions have no significant effect on the emission. The high selectivity for Fe³⁺ is probably attributed to several integrated factors, such as the suitable coordination geometry conformation of the nitrogen heterocyclic ring, the suitable open window size of the structure, the soft Lewis acid and the partially filled d-orbital. The differential effects of other metal ions on the luminescence intensity of 1a may be attributed to their electronic nature and charge transfer. For example, Na⁺, K⁺, Mg²⁺, Ba²⁺ having the closed-shell electron configuration, display essentially no quenching effects.^39,40 And Ag⁺, Zn²⁺, Cd²⁺ possess the saturated state (d¹⁰) could not accept additional electrons from other atoms, which have a negligible effect on the luminescence intensity. The interaction between the Fe³⁺ and 1a may induce charge transfer through the partially filled d-orbital based on ligand field transitions, which displays quenching effects. Meanwhile, 1a with open Lewis basic nitrogen atoms on the chain surface was exploited for the binding and specific sensing of Lewis acid (Fe³⁺ ion) via Lewis base interactions, and thus, shows high selectivity and sensitivity for Fe³⁺ ions. The different effects on the emission between Fe³⁺ and other cations are clearly observed, indicative of the fact that the title compound can be considered as a promising luminescent probe for Fe³⁺ ions.

Fig. 5 (a) Results of the competition experiments between Fe³⁺ and selected metal ions. (b) The fluorescence emission spectra of 1 in addition of Fe³⁺ salt aqueous solutions.

Furthermore, to study the influence of mixed cations on the emission of 1a, 1 mL of Fe³⁺ (1 × 10⁻³ M) and 1 mL of other cations (1 × 10⁻³ M) were slowly dropped into a 6 mL suspension of 1a in aqueous solution, respectively. The quenching effect of Fe³⁺ on 1a is not influenced by the introduced metal ions (Fig. 5a). The results indicated that the coexisting metal ions produced negligible interferences in comparison with Fe³⁺. The high selectivity of 1 to Fe³⁺ is ascribed to the fact that Fe³⁺ possesses a much higher binding ability to the N atoms than other metal ions.

In addition, to determine effect of the counter anions on Fe³⁺ recognition based on 1a, a range of aqueous solutions of Fe³⁺ salts containing the anions NO₃⁻, Cl⁻ and SO₄²⁻ were analyzed. Fig. 5b shows that the luminescence intensity of 1a decreased upon addition of Fe³⁺ salt solutions containing different counter anions, which demonstrates that the nature of the anion had a negligible effect on the luminescence intensity of 1a.

Moreover, to explore the detection limit of 1a as Fe³⁺ probes, Fe³⁺ with different concentrations in aqueous solution were dropped into suspension of 1a to form a series of suspensions of Fe³⁺–1a in aqueous solution in the range of approximately 10⁻⁶ to 10⁻² mol L⁻¹. The luminescence intensity of 1 gradually decreases with increasing concentration of Fe³⁺ (Fig. 6 and S6†). The fluorescence quenching efficiency can be quantitatively explained using the Stern–Volmer (SV) equation:^41,42 (I₀/I) = 1 + K_sv[Q], in which K_sv is the quenching constant (M⁻¹), [Q] is the molar concentration of the analyte (mM), I₀ and I are the fluorescence intensities before and after the addition of the analyte, respectively. As shown in inset Fig. 6, the luminescent intensity vs. [Fe³⁺] plots can be curve−fitted into (I₀/I) − 1 = 45 982.71[Fe³⁺], and the value of K_sv are 4.6 × 10⁴ L mol⁻¹ for Fe³⁺, thus demonstrating its obvious quenching effect on the luminescent intensity of 1. The detection limit of the sensor is in the 10⁻⁵ M level. Compared with other reported sensors (K_sv = 3.83 × 10⁴ to 3.54 × 10³), the K_sv (4.6 × 10⁴ L mol⁻¹) of our sensor is larger. Meanwhile, further experiments showed that the quenching effect of Fe³⁺ on our probe is not influenced by the introduced metal ions and counter anions (Table S2, ESI†).

Fig. 6 The relative intensities of 1 at 597 nm dispersed in different concentrations aqueous solutions of Fe³⁺. Inset: Stern–Volmer plot of compound 1 quenched by Fe³⁺ aqueous solution.

We speculate that it may be due to the open Lewis basic nitrogen atoms of CP 1a on the chain surface was exploited for the binding and specific sensing of Fe³⁺ ion via Lewis base interactions. Further analysis was studied to confirm the sensing mechanism. The association constant and stoichiometry for the formation of complex was calculated using the following Benesi–Hildebrand (B–H) eqn (2):^43–45

here F₀ is the fluorescent intensity of 1 in the absence of Fe³⁺, F is the fluorescent intensity recorded in the presence of added Fe³⁺, F_max is the fluorescent intensity in presence of added [Fe³⁺]_max, K is the association constant, and n is the binding stoichiometry ratio between 1 and Fe³⁺. The association constant (K) could be determined from the slope of the straightline of the plot of 1/(F − F₀) against 1/[M]ⁿ. According to the linear Benesi–Hildebrand expression, the measured fluorescence [1/(F − F₀)] at 597 nm showed a linear relationship with a change of 1/[Fe³⁺] (R = 0.9894), indicating the 1 : 1 stoichiometry of 1–Fe³⁺ complex (Fig. 7).

Fig. 7 (a) The fluorescence emission spectra of 1 in concentrations of Fe³⁺; (b) Benesi–Hildebrand plot of 1 assuming 1 : 1 stoichiometry between 1 and Fe³⁺ in water. The association constant of 1–Fe³⁺ is 7.08 × 10⁴ M⁻¹.

The solid-state emission spectra of 1 in this work were measured at room temperature and 77 K, results of which are given in Fig. 8a. When excited at 360 nm, the strong luminescent emissions of 1 are centered at 607 and 610 nm displaying strong red light, Commission Internacionale d’Eclairage (CIE) coordinates codes are (0.65, 0.35) and (0.64, 0.36) for 1 (Fig. 8a), respectively, which should be due to the metal-to-ligand charge transfer (MLCT). The temperature effects on the light emissions were investigated for 1. When the temperature decreases to 77 K, the intensity of the emission band is sharp increase twice for 1 at 298 K. That may be due to the fact that reducing temperature impedes the intramolecular rotations and fortify the restriction of the intramolecular rotation (RIR) effect,⁴⁶ thus it activates the radiative transitions and boosts the emission intensity. Besides that, 1 exhibits intense red light in the solid state, which shows better fluorescent performances in materials applications. We use 1 featuring a larger conjugate nitrogen heterocyclic ring to blend with PMMA. With increased of the content of 1, the intensity and lifetimes of the 1–PMMA films increase moderately. It can be attributed to coordination polymer 1 can disperse uniformly in the PMMA matrix and the PMMA effectively sensitizes the luminescence of 1. When the content of 1 reaches at 1.5%, the intensity of 1–PMMA is comparable with 1 in the solid state (Fig. 8b). Hence, taking the transparency and economics of 1–PMMA into account, the proper blending concentration is 1.5%. Additionally, the lifetimes of 1–PMMA films increase with the enhancement of the content of 1, τ reaches 23.41 μs at 1.5%, which is twice than that in the solid state (τ = 12.58 μs). The emission is boosted as a result of the organic polymer (PMMA) enhancing the light absorption cross section so that more energy can be transferred to central Cu(I) ions based on the higher excitation and emission intensities. The hybrid materials can serve as ideal candidates in the pursuit of application in farm plastic-film and organic luminescent glass.^47,48

Fig. 8 (a) Emission spectra of 1 and Htpp in the solid state at 298 K and 77 K; inset: the corresponding color coordinate diagram of emission for 1 and Htpp (blue symbol for 1 at 298 K, red symbol for 1 at 77 K and black symbol for Htpp at 298 K); (b) the emission spectra of PMMA polymer blended with 1 in 0.3–1.8% at 298 K.

Conclusions

In summary, a new red emission Cu(I)-CP as luminescent probe with dual-functional sensitivity to detect acetone and iron(III) ions has been yielded successfully by introducing the large nitrogen heterocyclic ring ligand. Structural analysis shows that the combination of π⋯π stacking and C–H⋯π interactions endow 1 with a 3D supramolecular array. More importantly, luminescence studies revealed that 1 has high selectivity for acetone and Fe³⁺ ions through luminescence quenching, and thus, should be an excellent candidate for probing these pollutant or excessive substance. In addition, 1 displays the strong red emission in the solid state. Hence, 1 was blended with PMMA film and the results illustrate that the comparable intensity with solid and enhanced the luminescent lifetime can be realized at a relative lower concentrate.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant 21371040 and 21571042), the National Key Basic Research Program of China (973 Program, No. 2013CB632900).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1504779. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra23694d

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