Multicolor Emissive Phosphorescent Iridium(III) Complexes Containing L-Alanine Ligands: Photophysical and Electrochemical Properties, DFT Calculations, and Selective Recognition of Cu(II) Ions

Abstract

Three novel Ir(III) complexes, (ppy)₂Ir(L-alanine) (Ir1) (ppy = 2-phenylpyridine), (F₄ppy)₂Ir(L-alanine) (Ir2) (F₄ppy = 2-(4-fluorophenyl)pyridine), and (F_2,4,5ppy)₂Ir(L-alanine) (Ir3) (F_2,4,5ppy = 2-(2,4,5-trifluorophenyl)pyridine), based on simple L-alanine as ancillary ligands were synthesized and investigated. Due to the introduction of fluorine substituents on the cyclometalated ligands, complexes Ir1–Ir3 exhibited yellow to sky-blue emissions (λ_em = 464–509 nm) in acetonitrile solution. The photoluminescence quantum yields (PLQYs) of Ir1–Ir3 ranged from 0.48–0.69, of which Ir3 with sky-blue luminescence had the highest PLQY of 0.69. The electrochemical study and density functional theory (DFT) calculations show that the highest occupied molecular orbital (HOMOs) energy of Ir1–Ir3 are stabilized by the introduction of fluorine substituents on the cyclometalated ligands, while L-alanine ancillary ligand has little contribution to HOMOs and lowest unoccupied molecular orbitals (LUMOs). Moreover, Ir1–Ir3 presented an excellent response to Cu²⁺ with a high selectivity, strong anti-interference ability, and short response time. Such a detection was based on significant phosphorescence quenching of their emissions, showing the potential application in chemosensors for Cu²⁺.

1. Introduction

Cu²⁺ participates in various biological processes in mammals, such as iron absorption, hematopoiesis, and metabolism [1,2]. Abnormal levels of Cu²⁺ can lead to acute hemolytic anemia and multiple neurodegenerative diseases [3,4]. Moreover, Cu²⁺ is also widely used in industry and agriculture, and the ecological environment could be damaged due to the emission of a large number of metal pollutants [5]. Thus, scientists are committed to developing accurate and dependable detective methods for Cu²⁺. Among various methods for the detection of Cu²⁺, fluorescence and phosphorescence detection methods based on changes in photophysical properties have received wide attention due to their high selectivity, rapid response, and simple operation [6,7,8]. Hence, the design and synthesis of novel luminescent materials to sense Cu²⁺ becomes a primary task.

Iridium complexes have gradually become the most promising phosphorescent materials due to their high PLQY (photoluminescence quantum yield) [9,10,11], easy modification of chemical structure [12,13], easy emission wavelength adjustment [14], and excellent chemical stability [15]. Thus, iridium complexes have been used as photosensitizers [16,17] and chemosensors for the detection of various analytes [18,19], showing excellent performance [20]. For example, Ir(III) complex ZIr2 based on di(2-picolyl)amine (DPA) as a Cu²⁺ receptor was designed and synthesized, which exhibited phosphorescence quenching for addition of Cu²⁺ with high selectivity and good reversibility [8]. In another study, Ir(III) complex Ir-L^14naph was designed with bidentate chelating pyrazolyl pyridine ligand as a copper-specific receptor, and the detection limit of Cu²⁺ was 20 nmol/L [21]. In these studies, the Cu²⁺ was recognized by combining with nitrogen heteroatoms in ancillary ligands of iridium complexes. However, the structure of ancillary ligands in the above complexes is complicated, which will increase the difficulty and cost of the synthetic process.

Amino acids, as cheap and easily available amphoteric substances containing oxygen and nitrogen atoms, can provide binding sites for sensing Cu²⁺. Hence, we selected the simple L-alanine as the ancillary ligand, which can simplify the synthesis method and reduce the material cost. Secondly, the introduction of C–F bonds into the cyclometalated ligands can reduce the rate of nonradiative process to enhance the PLQY and can increase the energy of the excited state to make the emission peaks shift to the blue region [22,23]. Herein, three novel Ir(III) complexes, (ppy)₂Ir(L-alanine) (Ir1) (ppy = 2-phenylpyridine), (F₄ppy)₂Ir(L-alanine) (Ir2) (F₄ppy = 2-(4-fluorophenyl)pyridine), and (F_2,4,5ppy)₂Ir(L-alanine) (Ir3) (F_2,4,5ppy = 2-(2,4,5-trifluorophenyl)pyridine), were synthesized (Figure 1b). Complexes Ir1–Ir3 presented yellow, green, and sky-blue emissions with high PLQYs up to 0.69. Furthermore, these complexes achieved as “turn-off” photoluminescence chemosensors of Cu²⁺ with high sensitivity.

2. Results and Discussion

2.1. Synthesis and Characterization

The cyclometalated ligands ppy and fluorine-substituted ligands F₄ppy and F_2,4,5ppy were prepared in high yields via the Suzuki coupling reaction. Complexes Ir1–Ir3 were obtained by the Ir(III) chloro-bridged dimers ([(ppy)₂Ir(μ-Cl)]₂, [(F₄ppy)₂Ir(μ-Cl)]₂, or [(F_2,4,5ppy)₂Ir(μ-Cl)]₂), L-alanine, and potassium tert-butoxide in DMF under nitrogen atmosphere with moderate yields. Complexes Ir1–Ir3 are air- and moisture-stable solids, and they are soluble in polar organic solvents including acetonitrile and dichloromethane. The structures of complexes Ir1–Ir3 were confirmed by ¹H, ¹³C, and ¹⁹F NMR spectroscopy (Figures S1–S8) and electrospray ionization mass spectroscopy (ESI-MS).

2.2. Photophysical Properties

The UV-vis absorption spectra and normalized emission spectra of the complexes Ir1–Ir3 in degassed acetonitrile (2 × 10⁻⁵ mol/L) are shown in Figure 2. The spectral data are summarized in

Table 1. Photophysical and electrochemical data for complexes Ir1–Ir3.

Complex	λabs (nm) a	λem (nm) b	PLQY	kr (105 s−1)	knr (105 s−1)	τ (μs)	EOX (V) c	EHOMO (eV) d	Ered (V) c	ELUMO (eV) d
Ir1	262, 404, 454	509	0.48	2.9	3.2	1.64	0.59	−5.19	−1.07	−3.70
Ir2	258, 380, 423	493	0.55	3.3	2.7	1.67	0.73	−5.33	−1.08	−3.52
Ir3	250, 311, 378	464, 490	0.69	4.7	2.1	1.46	0.91	−5.51	−1.07	−3.53

^{a, b} Measured in acetonitrile at room temperature with the concentration of phosphors at 2.0 × 10⁻⁵ mol/L. ^c Measured in degassed DCM: MeCN [1:1(v/v)] at a scan rate of 0.10 V·s⁻¹ versus Fc⁺/Fc using 0.10 mol/L [n-Bu₄N]PF₆ as a supporting electrolyte. ^d E_HOMO = −[E_ox − E_(Fc⁺_/Fc) + 4.8] eV; E_LUMO = −[E_red − E_(Fc⁺_/Fc) + 4.8] eV.

. Under 320 nm, the absorption bands of complexes Ir1–Ir3 with a high molar extinction coefficient in the level of 10⁴ L·mol⁻¹·cm⁻¹ can be attributed to the spin-allowed singlet ligand-centered (¹LC) electronic transitions. The weaker bands in the 320–550 nm ranges are designated for metal-to-ligand (MLCT) and ligand-to-ligand (LLCT) charge transfer [24]. In 320–550 nm, the lowest energy absorption maxima of Ir2 and Ir3 are blue shifted in comparison with that of complex Ir1. The results indicate that the introduction of fluorine substituents to the cyclometalated ligands can widen the energy gaps of the absorption bands in the lowest energy region.

The maximum emission peaks of complexes Ir1 and Ir2 are 509 nm and 493 nm, and that of Ir3 is 464 nm with the shoulder peak at 490 nm. The CIE color coordinates of Ir1 and Ir2 are (0.24, 0.67) and (0.14, 0.53) with yellow to green emission, those of Ir3 are (0.13, 0.24) with sky-blue emissions. Compared with the emission peak of Ir1, a hypsochromic shift of 16 nm in Ir2 and 45 nm in Ir3 was observed, which can be attributed to the introduction of fluorine substituents in cyclometalated ligands. The lifetimes (τ) of complexes Ir1–Ir3 measured at room temperature are 1.64, 1.67, and 1.46 μs, respectively, illustrating the phosphorescent character. The PLQYs of Ir1–Ir3 in acetonitrile solution were measured to be 0.48–0.69, of which Ir3 with F_2,4,5ppy ligands had the highest PLQY of 0.69. The radiative (k_r) and nonradiative decay rate constants (k_nr) can be calculated through the equations k_r = PLQY/τ and k_nr = (1 – PLQY)/τ (Table 1). With the increase in the number of fluorine substituents in complexes Ir2 and Ir3, the k_nr of Ir2 and Ir3 decreases and PLQYs of Ir2 and Ir3 increase. Therefore, the introduction of electron-withdrawing groups (-F) on the cyclometalated ligands can improve the energy of the MLCT state and reduce the rate of nonradiative process, which is beneficial to obtain iridium complexes with adjustable emission color and obtain phosphorescent materials with high PLQY.

2.3. Electrochemical Properties

The cyclic voltammograms of complexes Ir1–Ir3 are presented in Figure 3, and Table 1 contains the data gathered. As shown in Figure 3, complexes Ir1 and Ir2 both have a pair of reversible oxidation waves between 0.50 and 1.20 V (vs Ag⁺/Ag), which are attributed to the oxidation process of Ir³⁺/Ir⁴⁺ [25]. Complex Ir3 with F_2,4,5ppy as the cyclometalated ligands displays an irreversible oxidation wave, suggesting electrochemical stability decreases with the increasing number of fluorine substituents [26]. Ir3 > Ir2 > Ir1 is the trend of the oxidation potentials for complexes Ir1–Ir3, while the order of the highest occupied molecular orbital (HOMO) energy is Ir3 < Ir2 < Ir1. The findings suggest that the introduction of electron-withdrawing groups (-F) onto the cyclometalated ligands can increase its oxidation potential, so as to stabilize the HOMO energy. The complexes Ir1–Ir3 have similar reduction potentials, and the irreversible reduction peaks appear in −1.00~−1.10 V (vs Ag⁺/Ag). According to the DFT calculations, the lowest unoccupied molecular orbitals (LUMOs) of complexes Ir1–Ir3 are localized mainly on the pyridyl moieties of the cyclometalated ligands (

Table 2. The frontier orbital energy and electron density distribution for Ir1–Ir3.

Complex	Orbital	Energy (eV) (Calculated)	Eg (eV) (Calculated)	Composition %
				Ir	Cyclometalated ligands		Ancillary ligands
					phenyl group	pyridyl group
Ir1	HOMO	−5.22	3.65	52.28	35.88	6.24	2.60
	LUMO	−1.52		4.79	26.68	66.42	2.12
Ir2	HOMO	−5.37	3.80	50.50	36.23	7.57	5.71
	LUMO	−1.58		4.76	25.91	67.20	2.13
Ir3	HOMO	−5.64	3.96	50.82	38.30	4.62	6.26
	LUMO	−1.69		4.49	26.85	66.01	2.65
	LUMO+1	−1.68		5.05	27.71	65.26	1.99

), so the introduction of fluorine substituents has little influence on the reduction potentials and LUMO energy of the complexes.

2.4. Theoretical Calculation

DFT and TD-DFT methods were used to explore the lowest-energy electronic transition of the complexes Ir1–Ir3. The energy and surface distributions of HOMOs and LUMOs for complexes Ir1–Ir3 are presented in Figure 4 and Table 2, and Tables S4–S6 provide a summary of the calculated spin-allowed electronic transitions electron density distributions. The HOMOs of complexes Ir1–Ir3 are predominantly located on the phenyl moieties (35.88–38.30%) of cyclometalated ligands and d orbitals of the iridium atom (42.12–43.80%). The LUMOs are mostly distributed over the π* orbitals of the pyridyl moieties of the cyclometalated ligand (66.01–67.02%) and have a small distribution on the iridium atom (4.49–4.79%). Thus, the introduction of substituents onto the phenyl moieties of cyclometalated ligands significantly affects the HOMO energy of the iridium complexes. The HOMO energy of complexes Ir2 (−5.37 eV) and Ir3 (−5.64 eV) is lower than that of Ir1 (−5.22 eV), which can be attributed to the addition of fluorine substituents of iridium complexes. It can be proved that the introduction of fluorine substituents can stabilizes the HOMO effectively, while the LUMO is affected much less. Moreover, the ancillary ligand L-alanine has little contribution to HOMOs and LUMOs. These facts are consistent with the conclusions obtained in the electrochemical experiments. According to TD-DFT calculation, the low-energy absorption bands (320–550 nm) in the electron absorption spectra are mainly generated by HOMO → LUMO and HOMO → LUMO + 1 transitions, which are assigned to a mixture of MLCT and LLCT (Figure 4, Table 1, and Tables S1–S6).

2.5. Cation-Binding Properties

The photoluminescence response specificity of complexes Ir1–Ir3 (50 mmol/L) toward Cu²⁺ over other metal ions, including K⁺, Na⁺, Ag⁺, Ca²⁺, Cd²⁺, Co²⁺, Zn²⁺, Fe²⁺, Ni²⁺, Hg²⁺, Pb²⁺, Mg²⁺, and Cr³⁺, were systematically investigated using photoluminescence spectroscopy in acetonitrile solution. When the above metal ions (10.0 equiv.) were added to the solution of complexes Ir1–Ir3, the luminescence was quenched obviously only after the addition of Cu²⁺, whereas the addition of other cations caused tiny luminescence changes (Figures S7–S9). The spectral emission intensity of complexes Ir1–Ir3 were decreased by 94–96% by adding Cu²⁺. Moreover, the existence of Cu²⁺ ions were converted into a visual signal by Ir1–Ir3 that could be seen with the naked-eye under 365 UV light (Figure 5a–c).

The metal ions competitive experiments were carried out to study the anti-interference of complexes Ir1–Ir3 in sensing Cu²⁺, and the results are recorded in Figure 5d–f. For complexes Ir1–Ir3, the emission intensities were greatly quenched when 10.0 equiv. Cu²⁺ ions were added into the mixing solutions of Ir(III) complexes and coexisting metal ions, which are consistent with those obtained by adding Cu²⁺ ions alone to the solutions of iridium complexes. The results indicated that complexes Ir1–Ir3 can specifically discriminate Cu²⁺ from other metal ions and show an obvious “turn-off” response, indicating its good selectivity and anti-interference ability.

As shown in Figure S12, when Na₂EDTA was added to the mixture solution of Ir(III) complex and Cu²⁺ ions, the emission intensities of complexes Ir1–Ir3 were nearly entirely restored in two minutes. The results indicate that the free Ir(III) complexes can be regenerated by the addition of Na₂EDTA to the mixture of Ir(III) complexes and Cu²⁺ ions due to the high affinity between Na₂EDTA and the complexes, suggesting that the detection mechanism of Ir(III) complexes for Cu²⁺ may be binding-based mechanisms. Then, ¹H NMR spectrum of Ir2 with 1.0 equiv. Cu²⁺ in DMSO-d₆ was measured (Figure S13). It was found by analysis that the proton signal at 8.57 ppm corresponded to the H_b of pyridyl ring disappeared, and the other proton signals did not change significantly. We speculate that the detection mechanism of the complex may be due to the coordination between Cu²⁺ and the oxygen atom of L-alanine, which leads to the luminescence quenching of the Ir(III) complex by the paramagnetic effect from the spin-orbit coupling of the Cu²⁺. [27] The possible working mechanism of Ir(III) complex on Cu²⁺ detection is shown in Scheme S1.

In order to further evaluate the linear relationship between the photoluminescence response of complexes Ir1–Ir3 and the amount of Cu²⁺ added, emission titration experiments were investigated (Figure 6a–c). It was found that the photoluminescence intensity of complexes Ir1–Ir3 at their strongest emission peaks decreased along with the additional amount of Cu²⁺, and then gradually reached a plateau (Figure 6d–f). In a certain range, the luminescence intensity of complexes Ir1–Ir3 had a good linear relationship with copper ion concentration. According to the titration curves of complexes Ir1–Ir3 with Cu²⁺, the complexation constant (K) of Ir1–Ir3 with Cu²⁺ was determined by using the Benesi–Hildebrand equation [28], and the K value was 5.0 × 10⁴ L/mol, 4.9 × 10⁴ L/mol, and 2.9 × 10⁴ L/mol. According to the detection limit calculation formula, 3σ/k, where σ is the standard deviation of blank measurement, k is the slope of emission intensity of complex and Cu²⁺ titration curve, the detection limit of Ir1–Ir3 calculated is 1.9 × 10⁻⁶ mol/L, 1.1 × 10⁻⁶ mol/L, and 2.0 × 10⁻⁶ mol/L, which are lower than the maximum allowable copper content in drinking water 30 μmol/L set by the World Health Organization (WHO) [29].

2.6. Water Sample Analysis

In order to investigate the application ability of complexes Ir1–Ir3 towards Cu²⁺ in real samples, three water samples including tap, drinking, and lake water were analyzed by the spike-and-recovery method [30]. To remove large particles, all water samples were filtered through a 0.2 mm membrane. Cu²⁺ was added into the water sample to make the Cu²⁺ concentration 2.0 μmol/L, 4.0 μmol/L, and 6.0 μmol/L. The experimental results are shown in

Table 3. Application in real samples testing for Ir1.

Sample	[Cu2+] (μmol/L)	Found [Cu2+] (μmol/L)	Recovery (%)
Lake water	2.0	2.4	83.3
Lake water	4.0	3.3	121
Lake water	6.0	5.6	107
Tap water	2.0	2.4	83.3
Tap water	4.0	3.6	111
Tap water	6.0	5.5	109
Drinking water	2.0	2.3	87.0
Drinking water	4.0	3.5	114
Drinking water	6.0	5.7	105

,

Table 4. Application in real samples testing for Ir2.

Sample	[Cu2+] (μmol/L)	Found [Cu2+] (μmol/L)	Recovery (%)
Lake water	2.0	2.0	100
Lake water	4.0	3.9	102
Lake water	6.0	5.7	105
Tap water	2.0	2.0	100
Tap water	4.0	4.0	100
Tap water	6.0	5.5	109
Drinking water	2.0	1.8	111
Drinking water	4.0	4.4	90.9
Drinking water	6.0	6.1	98.3

and

Table 5. Application in real samples testing for Ir3.

Sample	[Cu2+] (μmol/L)	Found [Cu2+] (μmol/L)	Recovery (%)
Lake water	2.0	1.9	105
Lake water	4.0	4.0	100
Lake water	6.0	6.1	98.3
Tap water	2.0	2.1	95.2
Tap water	4.0	4.2	95.2
Tap water	6.0	5.9	101
Drinking water	2.0	2.1	95.2
Drinking water	4.0	4.1	97.5
Drinking water	6.0	6.1	98.3

. The recovery rates of Cu²⁺ for complexes Ir1–Ir3 in water samples are 83–121%, 90–111%, and 95–101%, respectively. These results indicate that complexes Ir2 and Ir3 containing fluorinated substituents show better recovery rates, and they have potential applications in monitoring the concentrations of Cu²⁺ in real samples.

3. Materials and Methods

3.1. Instruments

The chemicals used in this paper are all analytical grade reagents. ¹H and ¹³C NMR spectra were obtained by Bruker AM 400 MHz spectrometers using deuterated dimethyl sulfoxide (DMSO-d₆) and deuterated chloroform (CDCl₃) as recording solvents. ¹⁹F NMR spectra were obtained by Bruker AVANCE NEO 600 MHz using DMSO-d₆ as recording solvents. Mass spectra (MS) were obtained with ESI-MS (Agilent 6520Q-TOF LC/MS). Absorption spectra and photoluminescence spectra were measured by a UV-2700 spectrophotometer and Hitachi F-2700 spectrophotometer, respectively. The decay lifetimes of the complexes in deoxygenated acetonitrile solution were determined by an FLS920P fluorescence spectrometer. Cyclic voltammetry was performed on a CHI 760E electrochemical workstation with platinum thread; AgNO₃ (0.010 mol/L in CH₃CN)-Ag and a polished Pt plate were used as the counter electrode, reference electrode, and working electrode, respectively. Both density functional theory (DFT) and time-dependent DFT (TD-DFT) were carried out using the Gaussian 09 software package [31,32]. The PLQYs of the complex were calculated according to Equation (1) [33].

$${\Phi }_{\mathrm{unk}}={\Phi }_{\mathrm{std}}(\frac{I_{\mathrm{unk}}}{I_{\mathrm{std}}}\left)\right(\frac{A_{\mathrm{std}}}{A_{\mathrm{unk}}}\left)\right(\frac{{\eta }_{\mathrm{unk}}}{{\eta }_{\mathrm{std}}}{)}^2$$

(1)

where Φ_unk, I_unk, and A_unk represent the luminescent quantum yield, integrated emission intensities, and absorbance under excitation wavelength of unknown samples, respectively. Φ_std, I_std, and A_std stand for Ir(ppy)₃. η_unk and η_std represent the pure solvent refractive indices. The Φ_std of Ir(ppy)₃ in room temperature is known to be 0.97 (error: ±10%) [34].

3.2. Synthesis

Cyclometalated ligands 2-phenylpyridine (ppy), 2-(4-fluorophenyl)pyridine (F₄ppy), 2-(2,4,5-trifluorophenyl)pyridine (F_2,4,5ppy) [35], and Ir(III) chloro-bridged dimers were synthesized according to the literature [36].

L-alanine (2.5 equiv.), potassium tert-butoxide (2.5 equiv.), and iridium(III) chloride bridged dimers (1.0 equiv.) were refluxed for 12 h at 140 °C in DMF (30 mL) under nitrogen atmosphere. The solvent was evaporated under pressure and the coarse product was obtained. Then, silica gel column chromatography was used to purify the crude product to obtain the target complexes Ir1–Ir3.

(ppy)₂Ir(L-alanine) (Ir1): 0.15 g (1.7 mmol) L-alanine and 0.47 g (0.68 mmol) [(ppy)₂Ir(μ-Cl)]₂ obtained 0.57 g of Ir1 as a yellow solid with 51% yield. ¹H NMR (400 MHz, DMSO-d₆): δ 9.12 (dd, J = 38.6, 5.8 Hz, 1H), 8.61 (dd, J = 23.4, 5.8 Hz, 1H), 8.16 (q, J = 8.2 Hz, 2H), 7.94 (q, J = 7.0 Hz, 2H), 7.76–7.67 (m, 2H), 7.42 (dt, J = 26.4, 6.3 Hz, 2H), 6.76 (q, J = 8.0 Hz, 2H), 6.60 (t, J = 7.3 Hz, 2H), 6.31–6.20 (m, 1H), 5.95 (t, J = 7.1 Hz, 1H), 3.82 (t, J = 10.9 Hz, 1H), 1.26 (t, J = 7.1 Hz, 1H), 1.18 (d, J = 7.0 Hz, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ 174.12, 173.14, 168.31, 167.68, 150.54, 149.88, 148.57, 148.32, 144.96, 144.48, 138.56, 132.50, 131.97, 129.82, 129.41, 125.09, 124.60, 123.64, 123.38, 121.57, 120.98, 119.75, 119.61, 66.80, 65.89. MS (ESI) m/z: calcd for [C₂₅H₂₂IrN₃O₂Na]⁺, 612.1316; found, 612.1341 [M + Na]⁺.

(F₄ppy)₂Ir(L-alanine) (Ir2): 0.15 g (1.7 mmol) L-alanine and 0.50 g (0.68 mmol) [(F₄ppy)₂Ir(μ-Cl)]₂ obtained 0.44 g of Ir2 as a yellow solid with 52% yield. ¹H NMR (400 MHz, DMSO-d₆) δ 9.08 (dd, J = 39.9, 5.4 Hz, 1H), 8.57 (dd, J = 19.9, 5.5 Hz, 1H), 8.18 (t, J = 8.3 Hz, 2H), 8.03–7.94 (m, 2H), 7.88–7.78 (m, 2H), 7.46 (dd, J = 24.0, 6.1 Hz, 2H), 6.62 (d, J = 6.9 Hz, 2H), 5.83 (d, J = 10.0 Hz, 1H), 5.51 (t, J = 8.1 Hz, 1H), 3.08 (d, J = 8.0 Hz, 1H), 1.22 (dd, J = 36.7, 7.0 Hz, 3H). ¹³C NMR (101 MHz, DMSO-d₆): δ 182.92, 182.20, 167.43, 167.33, 167.00, 166.72, 163.57, 163.35, 161.08, 160.86, 157.26, 157.22, 156.86, 151.10, 151.04, 150.51, 150.14, 147.55, 147.22, 141.43, 141.28, 137.89, 50.46, 48.57, 21.23. ¹⁹F NMR (565 MHz, DMSO-d₆) δ −110.81, −111.48. MS (ESI) m/z: calcd for [C₂₅H₂₀F₂IrN₃O₂Na]⁺, 647.6602; found, 647.6625 [M + Na]⁺.

(F_2,4,5ppy)₂Ir(L-alanine) (Ir3): 0.15 g (1.7 mmol) L-alanine and 0.52 g (0.68 mmol) [(F_2,4,5ppy)₂Ir(μ-Cl)]₂ obtained 0.54 g of Ir3 as a yellow solid with 55% yield. ¹H NMR (400 MHz, DMSO-d₆): δ 9.14 (d, J = 26.3 Hz, 1H), 8.63 (s, 1H), 8.24 (s, 2H), 8.05 (s, 2H), 7.44 (s, 2H), 7.00 (s, 2H), 3.06 (s, 1H), 1.20 (d, J = 46.9 Hz, 3H). ¹³C NMR (101 MHz, DMSO-d₆): δ 183.72, 183.15, 165.69, 165.42, 164.55, 164.53, 157.81, 154.90, 152.46, 152.02, 149.31, 148.33, 139.14, 137.58, 137.04, 132.63, 131.98, 131.08, 130.62, 123.10, 100.44, 99.52, 55.46, 48.41, 40.89, 21.54. ¹⁹F NMR (565 MHz, DMSO-d₆) δ −115.74 (J = 376.5, 18.3 Hz), −133.33 (d, J = 26.8 Hz), −133.74 (t, J = 29.0 Hz), −134.53 (dd, J = 39.2, 18.6 Hz), −137.55 (d, J = 6.1 Hz), −137.77 (d, J = 6.8 Hz). MS (ESI) m/z: calcd for [C₂₅H₁₆F₆IrN₃O₂Na]⁺, 719.6225; found, 719.6234 [M + Na]⁺.

3.3. Cation-Binding Properties

A 0.1 mol/L aqueous solution of Cu²⁺ and other metal ions (K⁺, Na⁺, Ag⁺, Ca²⁺, Cd²⁺, Co²⁺, Zn²⁺, Fe²⁺, Ni²⁺, Hg²⁺, Pb²⁺, Mg²⁺ and Cr³⁺) was prepared using the corresponding metal nitrate. Various metal ions were added into the solution of Ir1–Ir3 (20 μmol/L) for selective experiments by monitoring the changes of photoluminescence intensity. The cation competition was studied by adding 10.0 equiv. Cu²⁺ into the mixed solution of Ir(III) complexes and other metal ions. In titration experiments, different concentrations of Cu²⁺ ions were gradually added to the solution of Ir1–Ir3 and each time the spectral changes were measured.

4. Conclusions

A series of new luminescent cyclometalated Ir(III) complexes with L-alanine as the ancillary ligand were synthesized, and the electronic absorption, photophysical, and electrochemical properties of these complexes were investigated. All complexes showed strong phosphorescence in acetonitrile solution at room temperature. The luminescent color of the complexes Ir1–Ir3 changed from yellow to sky blue and the PLQYs increased from 0.48 to 0.69 with the increase in fluorine substituents on the cyclometalated ligands. DFT calculations show that the energy of HOMOs and LUMOs can be adjusted and controlled by increasing the number of fluorine substituents on the cyclometalated ligands, which provides a basis for designing efficient iridium complexes with different luminescence colors. Complexes Ir1–Ir3 show a rapid photoluminescence quenching response after adding Cu²⁺, and exhibit good selectivity in acetonitrile solution with detection limits of 1.9 × 10⁻⁶ mol/L, 1.1 × 10⁻⁶ mol/L, and 2.0 × 10⁻⁶ mol/L, respectively. In the test of water samples, the complexes Ir1–Ir3 showed a good recovery rate with 83–121%, 90–111%, and 95–101%. Therefore, the iridium complexes reported in this work can be used as efficient chemosensors for the detection of Cu²⁺.