A novel cyclometalated Ir(III) complex based luminescence intensity and lifetime sensor for Cu²⁺

Abstract

We report the synthesis and characterization of two luminescent cyclometalated Ir(III) complexes [Ir(dfppy)₂(bpy-DPA)]PF₆ (Ir-1) and [Ir(dfppy)₂(bpy-BiDPA)]PF₆ (Ir-2), where dfppy, bpy-DPA and bpy-BiDPA represent 2-(2,4-difluorophenyl)pyridine, N-([2,2′-bipyridine]-4-yl)-2-(bis(pyridin-2-ylmethyl)amino)acetamide and N,N′-([2,2′-bipyridine]-4,4′-diyl)bis(2-(bis(pyridin-2-ylmethyl)amino)acetamide), respectively. Their photophysical properties and sensing properties towards various metal ions were investigated at room temperature. The two complexes both possessed good photophysical properties, and Ir-2 had quite a high quantum yield (93.83%) and a long lifetime (101.17 μs), which is impressive among Ir(III) complexes. Ir-2 exhibited both high selectivity and sensitivity towards Cu²⁺ over other metal ions. The Job curve and mass spectra suggested the formation of a 1 : 2 bonding mode between Ir-2 and Cu²⁺. The photoluminescence (PL) intensity quenching curve indicated that Ir-2 could enable rapid and reversible detection of Cu²⁺ with a low detection limit of 0.85 ppb (13 nM). Furthermore, a linear relationship could be observed between the PL lifetime value of Ir-2 and the concentration of Cu²⁺ ions in the range of 0–8 μM. These results suggested that Ir-2 might have use as not only a promising photoluminescence intensity sensor, but also a promising lifetime sensor for Cu²⁺ in aqueous solution.

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Introduction

Since many metal ions play essential roles in industry, agriculture and human life activities, the design and synthesis of sensitive and selective chemosensors for the detection of particular metal ions have drawn more and more attention.^1–5 The copper ion (Cu²⁺) is one of the most abundant heavy metal elements in the human body, and it is an indispensable component for many biological processes. Cu²⁺ ion depletion or excess in a human body may cause some illnesses such as Menkes, Alzheimer’s, Wilson’s and prion diseases.⁶ Furthermore, a Cu²⁺ imbalance is also harmful to flora and fauna. For its extensive applications in agriculture, industry and electronics, Cu²⁺ is considered to be an essential environmental pollutant.⁷ As a consequence, the detection of Cu²⁺ is necessary and many efforts have been made towards Cu²⁺ detection. The use of luminescence methods instead of classical methods such as inductively coupled plasma mass spectroscopy, inductively coupled plasma-atomic emission spectrometry and atomic absorption spectrometry, may be an advisable choice^8–10 due to their simplicity, high sensitivity, and high speed.¹¹ Many luminescence Cu²⁺ sensors with high selectivity, high sensitivity, good water solubility, excellent photostability, good reversibility, pH stability and low biotoxicity have been designed and prepared. However, only a few sensors possess all these advantages,^12–16 and most are based on traditional organic luminophores, suffering from water insolubility, short lifetime, low stability and luminescence quenching by water.¹⁷

Luminescent cyclometalated Ir(III) complexes have proven to be promising optical materials, due to their good water solubility, high quantum yields, relatively long lifetimes, large Stokes shifts and low biotoxicity.^18–20 Ir(III) complexes have been frequently utilized as phosphorescent emitters with extensive applications in organic light emitting diodes (OLED).^21–23 In recent years, Ir(III) complexes have also found a use in the luminescence detection of many analytes such as pH, amino acids, DNA, inorganic ions and transition metal ions.^24–29 In this paper, we designed and prepared two Ir(III) complexes functionalized with ion recognition group DPA (bis(pyridin-2-ylmethyl)amine) (Scheme 1), in which an amide group was inserted as a linker between ligand 2,2′-bipyridine and DPA. Probes based on DPA are known to be turn-on sensors for Zn²⁺, but the luminescence of many Zn²⁺ sensors based on DPA can be quenched by Cu²⁺ after complexation with Zn²⁺ ions. These results might indicate that DPA is more suitable for the detection of Cu²⁺ than Zn²⁺.^30–32 The synthesized complex Ir-2 possesses much better photophysical properties and cation recognition properties than Ir-1, and Ir-2 has been shown to act as a promising luminescence intensity and lifetime sensor for the detection of Cu²⁺.

Scheme 1 Synthetic route to Ir(III) complexes.

Results and discussion

Photophysical properties

The absorption spectra of two complexes in aqueous solution (1 vol% of MeCN, 10 μM) are shown in Fig. 1. Intense bands are observed in the ultraviolet part of the spectra, between 244 nm and 306 nm, which are ascribed to spin-allowed ligand-centered (LC)¹ π–π* transitions of the C⁁N ligand of dfppy. Weak bands observed at 358–446 nm arise from spin allowed [dπ(Ir)–π*(ligand)] ¹MLCT transitions. Some extremely inconspicuous bands observed at lower energies (more than 446 nm) are assigned to spin forbidden ³MLCT transitions.

Fig. 1 The absorption and PL spectra of two complexes in aqueous solution (1 vol% of MeCN, 10 μM).

Photoluminescence (PL) emission spectra of Ir-1 and Ir-2 were recorded by excitation at their excited maxima 340 nm and 343 nm, respectively. As shown in Fig. 1, the emission maxima of the two complexes are at 530 nm and 505 nm in aqueous solution (1 vol% of MeCN, 10 μM), emitting strong green light. As is well known, the photoluminescence quantum yield (Φ) and lifetime (τ) are very important factors for luminescent dyes. Photophysical parameters were also measured and summarized in Table 1. The Φ and τ values of the bpy–BiDPA complex Ir-2 (93.83%, 101.17 μs) are much higher than that of the bpy–DPA complex Ir-1 (30.04%, 10.85 μs). These results may be due to the fact that the insertion of two functional groups can enhance the phosphorescence emission by increasing the probability of the triplet-ground-state transitions and reducing the non-radiative decay rates. Furthermore, the high quantum yield and strong emission of the aqueous solution of Ir-1 and Ir-2 indicate that the two complexes can efficiently inhibit luminescence quenching by water.

Table 1 Photophysical parameters for Ir(III) complexes

Complex	λabs^a (nm), (ε, 10^4 M^−1 cm^−1)	λem^a (nm)	Φ^a (%)	τ^a (μs)
a Aqueous solution (1 vol% of MeCN, 1.0 × 10^−5 M).
Ir-1	244 (4.52), 304 (1.91), 358 (0.57), 446 (0.14)	530	30.04	10.85
Ir-2	258 (9.99), 306 (3.63), 359 (1.20), 445 (0.18)	505	93.83	101.17

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Influence of Cu²⁺ on the PL intensities of Ir-1 and Ir-2

To evaluate the sensing properties of the novel complexes towards Cu²⁺ ions, luminescence titration experiments of the two complexes with Cu²⁺ in varying concentrations were performed using PL spectroscopy. Upon the addition of Cu²⁺ ions to the aqueous solutions of the complexes (10 μm), the PL emission intensities both decreased (Fig. 2). The PL intensity of Ir-1 dropped by 67.9% upon addition of 2.0 equiv. Cu²⁺, while the PL emission of Ir-2 was almost completely quenched. This high quenching phenomenon might result from photoinduced electron transfer (PET) or energy transfer.^33,34 Furthermore, a linear relationship was observed between the ln(I₀/I) value of Ir-2 and the concentration of Cu²⁺ ions in the range of 0–10 μM. The limit of detection (LOD) value of Ir-2 based on 3S/K was calculated to be 13 nM (0.85 ppb), where S is the standard deviation of five replicate measurements of the blank sample, and K is the slope in the plot of the intensity versus the sample concentration.³⁵ This LOD value is much lower than most Cu²⁺ sensors in many former reports. The PL quenching curves of Ir-1 also showed a similar linear relationship in a narrow concentration range (0–3 μM) of Cu²⁺ ions (Fig. 2), but the sensitivity of Ir-1 was much lower than that of Ir-2. The relative detection limit value of Ir-1 was 48 nM. Therefore, Ir-2 will be the main focus of the sections below.

Fig. 2 (a) The PL emission spectra of Ir-1 (10 μM) in aqueous solution with variable additions of Cu²⁺ (λ_ex = 340 nm). (b) The plot of ln(I₀/I) vs. the concentration of Cu²⁺ (Ir-1), where I₀ and I are the PL intensities at the maximum emission wavelength in the absence and presence of Cu²⁺; inset, the linear relationship of ln(I₀/I) vs. the concentration of Cu²⁺ in the range of 0–3 μM. (c) The PL emission spectra of Ir-2 (10 μM) in aqueous solution with variable addition of Cu²⁺ (λ_ex = 343 nm). (d) The plot of ln(I₀/I) vs. the concentration of Cu²⁺ (Ir-2); inset, the linear relationship of ln(I₀/I) vs. the concentration of Cu²⁺ in the range of 0–10 μM.

To verify selective luminescence quenching by Cu²⁺, the PL emission spectra of Ir-2 in the presence of individual different metal ions such as Ag⁺, Al³⁺, Fe³⁺, Sn⁴⁺, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Cr³⁺, Ca²⁺, K⁺, Mg²⁺, Na⁺, and Hg²⁺ (all the metal salts used were chlorates except for silver nitrate and mercuric nitrate) were recorded in pH 7.0 aqueous buffer solutions (10 μM, 25 mM PIPES containing 1 vol% of MeCN) (Fig. 3a). As shown in Fig. 3a, only the addition of Cu²⁺ could lead to a nearly complete luminescence quenching, and the PL intensity of Ir-2 showed little response to the addition of other metal ions. The obvious luminescence quenching by Cu²⁺ might be attributed to the paramagnetic nature of Cu²⁺.^36,37 Owing to the unfilled d orbital of Cu²⁺, the transitions of the product after coordination with Cu²⁺ would be mainly forbidden d–d transitions with the lowest energy, giving no luminescence. Furthermore, red shifts were observed after the addition of Zn²⁺ and Cd²⁺, and this phenomenon may result from coordination behavior between DPA groups and the two metal ions, just like the Zn²⁺ probes based on DPA mentioned above. To further investigate the specificity of the probe molecules for the detection of Cu²⁺, competition experiments in the presence of other metal ions were performed (Fig. 3b). Ir-2 showed a good selectivity towards Cu²⁺ even in the presence of other metal ions. These observations indicate that Cu²⁺ is the only metal ion that affects the luminescence of this Ir(III) complex based chemosensor, and that Ir-2 serves as a potential luminescence material for the selective detection of Cu²⁺ in aqueous solution.

Fig. 3 (a) PL emission spectra of Ir-2 (10 μM) in aqueous solution upon the addition of 4 equiv. of metal ions such as Ag⁺, Al³⁺, Fe³⁺, Sn⁴⁺, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Cr³⁺, Ca²⁺, K⁺, Mg²⁺, Na⁺, and Hg²⁺. (b) Column diagrams of the PL intensity of Ir-2 + Mⁿ⁺ (metal ions) at 505 nm. Black bars represent the addition of various metal ions (40 μM) to the blank solution and red bars represent the subsequent addition of 2 equiv. Cu²⁺ to the above solutions (λ_ex = 343 nm).

Effect of pH value

With the purpose of finding a suitable pH span in which Ir-2 can selectively detect Cu²⁺ efficiently, the pH effect on the PL response of Ir-2 in the presence and absence of Cu²⁺ was investigated in a PIPES aqueous buffer solution (25 mM). As seen in Fig. 4, the PL intensities of Ir-2 in the presence and absence of Cu²⁺ showed no obvious change over a pH range of 4 to 10. This result indicates the pH independence of sensors based on Ir-2, which is applicable to biological systems (pH 7.4).

Fig. 4 The luminescence intensity of Ir-2 (10 μM) in the presence and absence of Cu²⁺ (20 μM) as a function of pH in a PIPES (25 mM) aqueous buffer solution (λ_ex = 343 nm).

Mechanism

To determine the interaction between Ir-2 and Cu²⁺, Ir-2 was dissolved in MeCN-d₃ solution and mixed with 4.0 equiv. of CuCl₂. The resulting mixture and a MeCN-d₃ solution with the same concentration of free Ir-2 were characterized by ¹H NMR (Fig. S1†) and ESI-HRMS (Fig. S2†). Although from Fig. S1,† it can be seen that the addition of Cu²⁺ led to a suppression of the ¹H NMR features of Ir-2, ESI-HRMS analysis provided clear evidence to support a reaction between Ir-2 and Cu²⁺ (Fig. S2†). After being mixed with CuCl₂, the original peak at m/z 1237.3601 assigned to [M − PF₆]⁺ of Ir-2 disappeared and a new signal was observed at m/z 1433.1354, which could be assigned to [M − PF₆ + 2Cu + 2Cl − 2H]⁺ (calculated, 1433.1375). This suggests a 1 : 2 stoichiometry between Ir-2 and Cu²⁺ ions in accordance with a Job’s plot and a Benesi–Hildebrand linear analysis plot (Fig. S3†), and the relative association constant calculated from the emission titration data was 1.68 × 10¹¹ M⁻² according to the Benesi–Hildebrand equation.³⁸ To verify the reversibility of the interaction between Ir-2 and Cu²⁺, 4.0 equiv. EDTA–Na₂ was added to the mixture of Ir-2 (10 μM) and Cu²⁺ (20 μM). As expected, the PL intensity at 505 nm was almost completely recovered after the introduction of EDTA–Na₂ (to 97.7% of the original, Fig. 5), indicating the chemical reversibility of the Cu²⁺ detection. Many reports show the use of Cu²⁺ sensors containing mono-functional groups for the indirect detection of S²⁻.^39–42 Hence, a reversible experiment using Na₂S was also performed (Fig. 5). To our surprise, the addition of excess Na₂S only led to a slight PL intensity recovery (less than 11%), unlike the previous reports. We imagine that this phenomenon is probably due to the stronger copper-binding ability of Ir-2 containing two DPA groups than that of the sensors containing one DPA group, which may minimize interference of other analytes.

Fig. 5 (a) The PL emission spectra of Ir-2 (10 μM; blank; black line), Ir-2 (10 μM) + Cu²⁺ (20 μM; Cu²⁺; red line), Ir-2 (10 μM) + Cu²⁺ (20 μM) + EDTA (40 μM; Cu²⁺, EDTA; blue line) and Ir-2 (10 μM) + Cu²⁺ (20 μM) + S²⁻ (40 μM; Cu²⁺, S²⁻; cyan line) in aqueous solution (1 vol% of MeCN), λ_ex = 343 nm. (b) Fluorescence photos of the aqueous solutions irradiated with an ultraviolet light at 365 nm.

Phosphorescence lifetime based Cu²⁺ sensing

To explore the effect of Cu²⁺ on the PL lifetime, lifetime titration of Ir-2 was also performed on a time-resolved fluorescence spectrofluorometer. The lifetime decays of Ir-2 were found to be double-exponential, and this result might be attributable to two MLCT (dπ(Ir) → π*(bpy-BiDPA) and dπ(Ir) → π*(dfppy)) excited states.^43,44 The lifetime decays of Ir-2 with varying concentration of Cu²⁺ ions are shown in Fig. 6a. With the gradual addition of Cu²⁺, the lifetime decays of Ir-2 changed significantly. The average phosphorescence lifetime of the 505 nm decay trace was calculated using equation τ_avg = ∑α_iτ_i²/α_iτ_i (i = 1–2).⁴⁵ The relevant curve of τ_avg vs. the concentration of Cu²⁺ is shown in Fig. 6b, and a linear relationship was obtained between the PL lifetime value of Ir-2 and the concentration of Cu²⁺ ions in the range of 0–8 μM. These results indicate that Ir-2 could also be used as a phosphorescence lifetime sensor for Cu²⁺, and the combination of the PL intensity and lifetime methods would make the detection of Cu²⁺ more reliable.

Fig. 6 (a) Phosphorescence lifetime titration of Ir-2 (10 μM, aqueous solution containing 1 vol% MeCN) with the addition of CuCl₂ (λ_ex = 343 nm, λ_em = 505 nm). An arrow indicates the change in the decay trace with increasing CuCl₂ concentration. (b) The plot of τ₅₀₅ (the average PL lifetime at 505 nm) vs. the concentration of Cu²⁺; inset, the linear relationship of τ₅₀₅ vs. the concentration of Cu²⁺ in the range of 0–8 μM.

Experimental

Instrumentation

¹H-NMR and ¹³C NMR spectra were measured on a BRUKER AMX 300 or 500 MHz instrument with tetramethylsilane as the internal standard. All experiments used CDCl₃, DMSO-d₆ and MeCN-d₃ from Sigma-Aldrich referenced to solvent resonances at 7.26, 2.49 and 1.94 ppm, respectively. Molecular masses were determined by electrospray ionization-mass spectrometry (ESI-MS) using a FINNIGAN LCQ instrument or an Agilent 1260-6224 liquid chromatography-high-resolution mass spectrometer. Absorption spectra of the target compounds were measured by a SHIMADZU UV-2450 spectrophotometer. Photoluminescence (PL) emission spectra were performed on a HORIBA FLUOROMAX-4 spectrophotometer. The measurements of lifetime (τ) and the solution PL quantum yield (Φ) were conducted on a Horiba Jobin Yvon Inc. Fluorolog 3-TSCPC.

Materials

Acetonitrile (MeCN), diisopropylethylamine (DIPEA) and triethylamine (TEA) were of analytical grade, dried by calcium hydride and distilled before use, and other reagents were used as purchased without further purification. All manipulations involving air-sensitive reagents were performed under a dry nitrogen atmosphere. 2-(2,4-Difluorophenyl)pyridine (dfppy), N,N-bis(2-pyridylmethyl)amine (DPA), and [Ir(dfppy)₂Cl]₂, were prepared by our group according to literature procedures.^46–48

Synthesis of [2,2′-bipyridine]-4-amine (1)

[2,2′-Bipyridine]-4-diamine was synthesized according to reported methods.^49,50 Under a nitrogen atmosphere, 4-nitro-2,2′-bipyridine N-oxide (1.09 g, 5.0 mmol) and 10% Pd–C (200 mg) were diffused in methanol (20 mL). After cooling down to 0 °C, sodium borohydride (0.95 g, 25.0 mmol) was added in small portions to the above mixture and maintained with stirring. The mixture was stirred at 0 °C for 6 h. After filtration, methanol was removed under reduced pressure. Water was added to the residue, and the resulting solution was extracted with diethyl ether (100 mL × 3). The combined organic phase was dried over anhydrous Na₂SO₄ and concentrated to give a white solid (86%). ¹H NMR (300 MHz, DMSO) δ 8.61 (d, J = 3.9 Hz, 1H), 8.30 (d, J = 7.9 Hz, 1H), 8.09 (d, J = 5.5 Hz, 1H), 7.86 (td, J = 7.8, 1.8 Hz, 1H), 7.62 (d, J = 2.2 Hz, 1H), 7.48–7.26 (m, 1H), 6.51 (dd, J = 5.5, 2.3 Hz, 1H), 6.13 (s, 2H). ESI⁺-MS: calculated for C₁₀H₁₀N₃ [M + H]⁺ 172.1, found 172.1.

Synthesis of [2,2′-bipyridine]-4,4′-diamine (2)

[2,2′-Bipyridine]-4,4′-diamine was synthesized according to the reported literature.⁵¹ A mixture of 4,4′-dinitro-[2,2′-bipyridine]-1,1′-dioxide (2.0 g, 7.0 mmol) and 10% Pd/C (1.0 g) in 150 mL ethanol was heated to reflux under nitrogen. Then 8 mL of hydrazine hydrate in 20 mL ethanol was dropped into the above solution. The resulting solution was refluxed for 12 h. After hot filtration and concentration, the residue was purified by recrystallization from ethanol and water to give a faintly yellow solid (78%). ¹H NMR (300 MHz, DMSO) δ 8.00 (d, J = 5.5 Hz, 1H), 7.51 (d, J = 2.1 Hz, 1H), 6.42 (dd, J = 5.5, 2.3 Hz, 1H), 5.99 (s, 2H). ESI⁺-MS: calculated for C₁₀H₁₁N₄ [M + H]⁺ 187.1, found 187.1.

Synthesis of N-([2,2′-bipyridine]-4-yl)-2-chloroacetamide (L1)

[2,2′-Bipyridine]-4-amine (0.34 g, 2 mmol) was placed in a round bottomed flask under argon. MeCN (dry, 30 mL) and TEA (1.01 g, 1.39 mL, 10 mmol) were added and the suspension was stirred for 30 min. The mixture was cooled to 0 °C before chloroacetyl chloride (1.13 g, 0.80 mL, 10 mmol) in MeCN (5 mL) was added dropwise. The mixture was left stirring overnight at room temperature. After concentration, 5% NaHCO₃ aqueous solution (100 mL) was added to the residue, and stirred for 0.5 h. The resulting solution was filtrated, and washed with water and diethyl ether to yield a brown solid (60%). ¹H NMR (300 MHz, DMSO) δ 13.57 (s, 1H), 8.83 (d, J = 4.3 Hz, 1H), 8.38–8.08 (m, 3H), 7.71–7.64 (m, 1H), 7.57 (s, 1H), 6.89 (d, J = 6.9 Hz, 1H), 4.27 (s, 2H). ESI⁻-MS: calculated for C₁₂H₉ClN₃O [M − H]⁻ 246.1, found 246.1.

Synthesis of N,N′-([2,2′-bipyridine]-4,4′-diyl)bis(2-chloroacetamide) (L2)

L2 was prepared similarly to L1 using [2,2′-bipyridine]-4,4′-diamine (0.37 g, 2.0 mmol), triethylamine (2.02 g, 2.78 mL, 20.0 mmol) and chloroacetyl chloride (2.26 g, 1.60 mL, 20.0 mmol). It was isolated as a brown solid (55%). ¹H NMR (300 MHz, DMSO) δ 11.33 (s, 2H), 8.74–8.63 (m, 4H), 7.87 (d, J = 5.6 Hz, 2H), 4.46 (s, 4H). ESI⁻-MS: calculated for C₁₄H₁₁Cl₂N₄O₂ [M − H]⁻ 337.0, found 337.0.

Synthesis of [Ir(dfppy)₂(bpy-NHCO-Cl)]PF₆ (3)

[Ir(dfppy)₂Cl]₂ (0.22 g, 0.18 mmol) and L1 (0.11 g, 0.45 mmol) were dissolved in a mixture of CH₂Cl₂ (15 mL) and EtOH (15 mL) and then refluxed under N₂ for 12 h. This was then cooled down to room temperature, excess potassium hexafluorophosphate (KPF₆) was added and the resulting solution was stirred for another 1 h. After filtration and concentration, the crude product was purified by column chromatography (silica gel; eluent: CH₂Cl₂/CH₃OH) to give complex 3 as a powder in 70% yield. ¹H NMR (300 MHz, DMSO) δ 11.38 (s, 1H), 9.00 (s, 1H), 8.61 (d, J = 8.2 Hz, 1H), 8.34 (d, J = 8.1 Hz, 3H), 8.08 (t, J = 7.8 Hz, 2H), 7.96 (s, 1H), 7.87–7.70 (m, 5H), 7.35–7.17 (m, 3H), 7.09–6.93 (m, 2H), 4.46 (s, 2H). ESI⁺-MS: calculated for C₃₄H₂₂ClF₄IrN₅O [M − PF₆]⁺ 820.1, found 820.1.

Synthesis of [Ir(dfppy)₂(bpy-Bi(NHCO-Cl))]PF₆ (4)

This compound was prepared according to the procedure for the synthesis of 3, with a yield of 61%. ¹H NMR (300 MHz, DMSO) δ 11.46 (s, 2H), 8.93 (s, 2H), 8.33 (d, J = 7.9 Hz, 2H), 8.07 (s, 2H), 7.96–7.67 (m, 8H), 7.29 (s, 2H), 6.96 (d, J = 9.8 Hz, 2H), 4.45 (s, 4H). ESI⁺-MS: calculated for C₃₆H₂₄Cl₂F₄IrN₆O₂ [M − PF₆]⁺ 911.1, found 911.1.

Synthesis of [Ir(dfppy)₂(bpy-DPA)]PF₆ (Ir-1)

[Ir(dfppy)₂(bpy-NHCO-Cl)]PF₆ (0.19 g, 0.20 mmol), DPA (0.08 g, 0.40 mmol), DIPEA (0.06 g, 0.48 mol), and KI (0.08 g, 0.48 mol) were dissolved in MeCN and then refluxed under N₂ for 12 h. After concentrating, water (50 mL) was added to the residue. The resulting solution was extracted with CH₂Cl₂ (40 mL × 3). Each organic phase was combined, and then the mixture was dried by Na₂SO₄, and purified by column chromatography (silica gel; eluent: CH₂Cl₂/CH₃OH), giving [Ir(ppy)₂(bpy-DPA)]PF₆ as a powder in 70% yield. ¹H NMR (300 MHz, DMSO) δ 11.49 (s, 1H), 9.06 (s, 1H), 8.59 (s, 3H), 8.29 (s, 3H), 8.05 (s, 3H), 7.93 (s, 1H), 7.84–7.66 (m, 8H), 7.45 (d, J = 7.7 Hz, 2H), 7.26 (s, 4H), 6.96 (s, 2H), 3.96 (s, 4H), 3.61 (s, 2H). ¹³C NMR (75 MHz, DMSO) δ 170.36, 160.23, 157.58, 155.98, 154.35, 154.00, 149.18, 148.99, 147.25, 138.81, 138.07, 136.01, 127.45, 123.27, 122.39, 122.13, 121.40, 116.18, 112.33, 97.54, 57.70, 56.80. ESI⁺-HRMS: calculated for C₄₆H₃₄F₄IrN₈O [M − PF₆]⁺ 983.2421, found 983.2406. Anal. calcd for C₄₆H₃₄F₁₀IrN₈OP: C, 48.98; H, 3.04; N, 9.93; found: C, 49.05; H, 3.11; N, 9.86.

Synthesis of [Ir(dfppy)₂(bpy-BiDPA)]PF₆ (Ir-2)

This compound was prepared according to the procedure for the synthesis of Ir-1, with a yield of 59%. ¹H NMR (300 MHz, DMSO) δ 11.60 (s, 2H), 8.95 (s, 2H), 8.60 (s, 4H), 8.29 (s, 2H), 8.04 (s, 2H), 7.86–7.65 (m, 12H), 7.44 (d, J = 7.5 Hz, 6H), 7.26 (s, 8H), 6.95 (s, 2H), 3.97 (s, 8H), 3.63 (s, 4H). ¹³C NMR (75 MHz, DMSO) δ 170.67, 162.30, 156.42, 154.64, 153.11, 149.23, 147.71, 147.81, 147.60, 147.33, 147.19, 137.90, 135.50, 135.26, 126.06, 123.10, 122.33, 122.17, 121.92, 121.75, 121.14, 120.73, 120.36, 116.08, 112.31, 112.09, 111.81, 97.38, 57.94, 56.91. ESI⁺-HRMS: calculated for C₆₀H₄₈F₄IrN₁₂O₂ [M − PF₆]⁺ 1237.3589, found 1237.3601. Anal. calcd for C₆₀H₄₈F₁₀IrN₁₂O₂P: C, 52.13; H, 3.50; N, 12.16; found: C, 52.23; H, 3.35; N, 12.11.

Conclusions

In summary, two novel luminescent Ir(III) complexes Ir-1 and Ir-2 were designed and prepared for Cu²⁺ detection in this paper. Only Ir-2 with two functional groups (DPA) showed good Cu²⁺ detection properties with high sensitivity, high selectivity, good reversibility and broad-range pH stability (from pH 4 to 10) even in the presence of other metal ions. Furthermore, Ir-2 exhibited not only a PL intensity response but also a PL lifetime response to Cu²⁺ ions in aqueous solution. We consider that Ir-2 may be used as a promising reversible intensity and lifetime sensor for Cu²⁺ in aqueous solution. Furthermore, it can also act as a green-emitting dye in other areas due to its excellent photophysical properties.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (KYLX15_0125) and National Major Scientific Instruments and Equipment Development Projects (2014YQ060773).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27189d

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