Heteroleptic iridium(III) complex with N-heterocyclic carbene ligand: Synthesis, photophysics, theoretical calculations and electrochemiluminescence

Journal of Organometallic Chemistry 846 (2017) 335e342

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Heteroleptic iridium(III) complex with N-heterocyclic carbene ligand: Synthesis, photophysics, theoretical calculations and electrochemiluminescence Yuyang Zhou*, Lingyan Kong, Kai Xie, Chengbao Liu School of Chemistry, Biology and Material Engineering, Jiangsu Key Laboratory of Environmental Functional Materials, Suzhou University of Science and Technology, Suzhou, Jiangsu, 215009, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 May 2017 Received in revised form 10 July 2017 Accepted 12 July 2017 Available online 13 July 2017

A novel C(carbene)^O bidentate ancillary ligand has been designed and synthesized through an imidazolium bromide with carboxylic acid as a N-heterocyclic carbene (NHC) precursor, and further successfully been coordinated to iridium center for the first time in this work. Accordingly, a NHC-based iridium(III) complex has been synthesized and characterized by chemical structural (NMR, mass, UPLC purity, single crystal structure), photophysical, theoretical calculation, electrochemical and electrochemiluminescent (ECL) properties. Most importantly, five alkyl amines have been selected as co-reactants for ECL generation of this novel ECL luminophores and the relationships between ECL intensity and the co-reactant have been investigate by experimental results and theoretical calculations, which would be very important to exploit novel excellent co-reactants for the emerging iridium-based ECL luminophores in the future. © 2017 Elsevier B.V. All rights reserved.

Keywords: N-heterocyclic carbene Iridium complex Electrochemistry Electrochemiluminescent Co-reactant

1. Introduction Since the detailed ECL mechanism of tri-2,2’-bipyridylruthenium(II) [abbreviated as Ru(bpy)2þ 3 ] revealed by Bard and co-workers in the early 1970s [1,2], electrochemiluminescence (ECL), also named electrogenerated chemiluminescene, has been developed to be an powerful analytical technique in terms of sensitivity, dynamic concentration response range [3,4]. Due to the importance of luminophores in the luminescence-based analytical techniques, exploiting novel excellent ECL dyes is always the research hotspot in the ECL-related technologies [5e7]. Currently, except ruthenium(II) complexes, there has been a large number of non-ruthenium ECL dyes reported so far, such as europium [8], rhenium [9], copper [10], osmium [11], aluminum [12,13], terbium [14], platinum [15] and iridium [15e24]. In the non-ruthenium system, iridium(III) complexes received much attention [25e30] and are identified as the most competent candidates of the alternatives of Ru(bpy)2þ 3 due to their high quantum efficiencies, tunable emission colors. Our goup [23,24,31] and others [32e38] have focused on exploiting novel excellent ECL dyes

* Corresponding author. E-mail address: [email protected] (Y. Zhou). http://dx.doi.org/10.1016/j.jorganchem.2017.07.017 0022-328X/© 2017 Elsevier B.V. All rights reserved.

comprising iridium(III) complexes. After the successful isolation of a crystalline and stable carbene by Arduengo and co-workers in 1991 [39], N-heterocyclic carbene (NHC) derived from the deprotonation of imidazolium salts has attracted much attentions as coordination ligands in organometallic chemistry [40]. Due to NHC's strong coordination abilities to transition metals, a large number of NHC-based transition-metal complexes, such as ruthenium [41], gold [42,43], silver [44], rhenium [45], platinum [46,47], copper [48] etc., have been designed and synthesized. Beyond their traditional applications in catalytic chemistry, luminescence and related functional materials have also become emerging research focuses for NHC-based organometallic complexes [49]. Since the pioneering work about NHC-based iridium(III) complexes as dopants of organic light emitting diodes (OLEDs) reported by Sajoto et al., [50] the synthetic methods [51] and novel applications [52] about luminescent iridium(III) complexes containing NHC ligands have been gradually revealed. From then on, more and more kinds of NHC ligands have been designed and incorporated into iridium complexes, which have displayed excellent properties in related areas, such as OLEDs [53e55] and bio-imaging [52]. However, as a family of relatively new luminescent complexes with increasing importance, the studies about ECL properties of the NHC-based iridium complexes

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are still very rare until now [56]. Herein, N-methyl-N’-(acetic acid)imidazolium bromide as a novel NHC precursor has been designed and successfully synthesized. Furthermore, one NHC-based iridium(III) complexes with C(carbene)^O bidentate ancillary ligand has also been synthesized through metal transfer reaction between silver and iridium(III) in this work (as shown in Scheme 1). Beside the characterization of chemical structure, the photoluminescent and electrochemiluminescent properties of this novel NHC-based iridium(III) complex have been comprehensively investigated in this work. Most importantly, the relationship between ECL performance and co-reactant are also thoroughly investigated based on experimental results and theoretical calculations in this study, which would be very helpful to develop efficient coreactant assisted ECL system for the emerging iridium-based ECL luminophores in the future. 2. Results and discussions

by NMR and mass spectra, the purity is up to 99% according to UPLC spectra (Figure S3 in supplementary material). Furthermore, the single crystal structure of this complex has also been obtained from the mixture solution of methanol and dichloromethane and shown in Fig. 1. The critical bond lengths and angles of Irppyimac have also been listed in Table 1. Notably, the bond length between carbene carbon and iridium atom (Ir(1)-C(3) in Table 1 and Fig. 1) is a little longer than that between general carbon (non-carbene) and iridium atom (such as Ir(1)-C(18) and Ir(1)-C(7) in Table 1 and Fig. 1). According to Fig. 1 and Table 1, two coordinated nitrogen atoms in the main ligands (N(4) and N(3)) are in the trans position and the bond angle of N(4)-Ir(1)-N(3) are nearly to 180 , while the coordinated carbon atoms in the main ligand (C(18) and C(7)) are in the cis position and the bond angle of C(18)-Ir(1)-C(7) is close to 90 . Furthermore, two bond angles between main ligand and central iridium atom (C(18)-Ir(1)-N(4) and C(7)-Ir(1)-N(3) in Table 1 and Fig. 1) are nearly identical to each other, while they are

2.1. Synthesis and characterization Due to the unique nature of carbene, including electronic effect and steric effect [57], it's usually hard to find a general method to synthesize the NHC-based iridium complexes after reviewing the literature reported so far [50,51,53,56,58,59]. In this work, C^O bidentate ligand as a novel NHC-based ancillary ligand is coordinated to iridium(III) center and the corresponding NHC-based iridium(III) complex (abbreviated as Irppyimac in this work) has been successfully synthesized through two major steps as shown in Scheme 1. The first step mainly involved the synthesis of two critical intermediates, i.e. iridium(III) dichlorobridged dimer and silver carbene. For the intermediate of iridium(III) dichlorobridged dimer, the synthetic method is well-established [60,61] and there is no need to discuss further. However, it should be noted the synthesis of Ag carbene. According to Scheme 1, the quaternary ammonium reaction between 1-methyl-1H-imidazole and 2bromoacetic acid was employed to synthesize the imidazolium bromide as NHC precursor. Subsequently, Ag carbene was formed by the reaction between this precursor with silver oxide in dichloromethane solution with nitrogen atmosphere protecting from light [62]. Without purification, Ag carbene was directly used to react with iridium(III) dichlorobridged dimer and the final NHCbased iridium(III) complex was generated through metal transfer reaction in base condition as shown in Scheme 1. The chemical structure of this novel NHC-based iridium complex is characterized

Fig. 1. Crystal structure of Irppyimac. The hydrogen atoms are omitted for clarity.

Scheme 1. The synthetic routes of iridium(III) complex used in this work. i: CH3CN, reflux, 24 h, N2; ii: Ag2O, CH2Cl2, reflux, 12 h, N2, in dark; iii, 2-ethoxyethanol/H2O(v:v ¼ 3:1), reflux, 24 h, N2; iv: xylenes, K2CO3, 24 h, N2, in dark.

Y. Zhou et al. / Journal of Organometallic Chemistry 846 (2017) 335e342 Table 1 Selected bond length (Å) and angles ( ) for Irppyimac. Selected bond length Ir(1)-C(18) Ir(1)-N(4) Ir(1)-N(3) Ir(1)-C(7) Ir(1)-C(3) Ir(1)-O(1) Selected bond angles C(18)-Ir(1)-N(4) C(7)-Ir(1)-N(3) C(3)-Ir(1)-O(1) N(4)-Ir(1)-N(3) C(18)-Ir(1)-C(7)

1.993(6) 2.055(5) 2.052(4) 2.054(6) 2.097(6) 2.185(4) 80.8(2) 79.9(2) 85.50(19) 172.91(19) 86.1(2)

slightly larger than that between NHC-based ancillary ligand and iridium atom (C(3)-Ir(1)-O(1) in Table 1 and Fig. 1). According to single crystal structure, the comparative longer bond length and larger bond angle between NHC ligand and iridium center may be reasonably to explain the difficulty of synthesizing NHC-based iridium complex [50e52].

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properties. In order to further investigate the nature of this novel NHCbased iridium(III) complex, density function theory (DFT) and time dependent density function theory (TD-DFT) have also been used to calculate the ground and excited states of Irppyimac using Gaussian 09 package. The 6-31 þ G(d) basis set was used for C, N, H, O, and the iridium atom employed the LANL2DZ basis set, which have been proven reliable for iridium(III) complexes [66,67]. The electron distributions on frontier orbitals involving the singlet and triplet states of Irppyimac have been shown in Fig. 3, and the calculated transition wavelength and assignment are also listed in Table 2. According to Fig. 3, the HOMO and HOMO-1 of Irppyimac have the mixture electron distributions both on main ligand and NHCbased ancillary ligand, while the electrons of LUMO and LUMOþ1 are mainly located on main ligand. In addition with the assignment of singlet (S1) and triplet (T1) state from the TD-DFT calculation results, the main character natures of S1 and T1 state could be reasonable ascribed to the mixture of metal to ligand charge transfer (MLCT) and ligand to ligand charge transfer (LLCT). 2.3. Electrochemistry and electrochemiluminescence

2.2. Photophysical properties and theoretical analysis Fig. 2 displayed the absorption and photoluminescent spectra of Irppyimac in acetonitrile solution. Notably, this novel NHC-based iridium complex exhibited strong intraligand absorption bands (p-p*) below 300 nm (ε > 10000 M1cm1, ε is molar extinction coefficient) and weaker MLCT (metal to ligand charge transfer) transition bands from 350 to 500 nm, and while there is almost no absorption band above 500 nm. Such a feature is very similar to other reported iridium complexes [52,56]. Under the excitation of 395 nm as the representative wavelength of the MLCT state, the photoluminescent spectrum of Irppyimac has been recorded as shown in Fig. 2B. The major emission peak is centered at 513 nm, which is very close to its analogue of bis(2-phenylpyridine-C2,N0 )(acetoylacetonate)iridium(III) with the same main ligand of 2-phenylpyridine [63]. Such phenomenon revealed that the emission colors of the heteroleptical iridium complexes are intensely related with the nature of main ligand. Furthermore, according to the former reported method and using fac-tri(2-phenylpyridine-C2,N0 ) iridium(III) (abbreviated as facIr(ppy)3) as the reference, [64,65] the quantum efficiency of Irppyimac in deaerated acetonitrile solution have also been calculated to be 17.8%, which demonstrates that this novel NHC-based iridium(III) complex has comparatively excellent photoluminescent

It's well known that the redox properties are highly related with ECL of luminophore, therefore, cyclic voltammetry (CV) was employed to investigate the redox character of Irppyimac. Fig. 4 shows the CV curve of 1 mM Irppyimac in deaerated acetonitrile solution on the surface of Pt working electrode. All the potentials listed in this work are all calibrated to ferrocene/ferrocenium (Fc/ Fcþ). Accordingly, one reversible oxidation peak has been observed at Pt electrode with the scan rate of 0.1 V/s. The half peak oxidative potential (Eox 1/2) is 0.62 V and the DEP is 77 mV. Meanwhile, there is no obvious reduction peaks until scanning negative to 1.8 V except a small peak recorded at ca. 0.75 V. Importantly, compared with the oxidative potential of the widely-used tris(2,2’-bipyridyl) ruthenium(II) (abbreviated as Ru(bpy)2þ 3 ) [24], this novel NHCbased iridium complex has much lower oxidative potential (<0.8 V vs Fc/Fcþ), which has many advantages in the real applications [37]. Co-reactant assisted ECL is one of the most important pathway for ECL generation, which has also been successfully used in the commercialized ECL immunoassay system based on ruthenium complex [68,69]. According to the redox nature of Irppyimac as shown in Fig. 4, the type of oxidative-reduction co-reactant should be our best choice to generate ECL of this novel luminophores. However, as for how to select the best co-reactant for a novel

Fig. 2. The absorption (A) and normalized photoluminescent spectra (B) of Irppyimac in acetonitrile solution. ε is the molar extinction coefficient, excitation wavelength is 395 nm.

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Fig. 3. The electronic distributions on frontier orbitals and corresponding energy level of Irppyimac based on the optimized structure of the ground state.

Table 2 TD-DFT calculation results of Irppyimac. State

Energy [eV]

lmax [nm]

f

Assignments

T1

2.56

484

0.0000

S1

2.80

443

0.0260

HOMO/LUMO (61.23%) HOMO-1/LUMO (11.95%) HOMO/LUMOþ1 (þ23.17%) HOMO/LUMO (68.76%)

Fig. 4. Cyclic voltammograms of 1 mM Irppyimac in acetonitrile solution containing 0.1 M TBAPF6 after pumping N2 atmosphere for 15 min. Scan rate: 0.1 V/s, Pt electrode area: 3.14 mm2.

luminophore, there is still no general answer so far. As we know, though there are a large number of researches focusing on exploiting new ECL luminophores [31,35,37,38,70e72], tri-n-propylamine (TPA) is always widely used as an important co-reactant in the emerging iridium(III)-based luminophores. Though there are some reports focusing on the studies of co-reactants [73e77], it's still very important to optimize the co-reactant especially for the new luminophore. Herein, in this work, five alkyl amines including tri-n-propylamine (TPA), triethylamine (TEA), dimethylethanolamine (DMEA), diisopropylethylamine (DIPEA) and diisopropylamine (DIPA) have all been selected as the co-reactants to assist the ECL generation of this novel NHC-based iridium complex. Due to the importance of oxidation potential of co-reactant for the ECL generation, the oxidative curves of these five alkyl amines are also investigated and listed in Figure S4 in supplementary material. According to Figure S4, all these five alkyl amine display an irreversible oxidation peak on Pt electrode in acetonitrile solution. The sequence of the

oxidation peak potential is TEA (0.67 V) > DMEA (0.64 V) > DIPA (0.62 V) > TPA (0.57 V) > DIPEA (0.44 V). Subsequently, the ECL of Irppyimac assisted by these five alkyl amine have been investigated and listed in Fig. 5AeE, respectively. It's found that an apparent ECL signal has all been observed along with the oxidation of the co-reactant and Irppyimac. Though ECL signals of Irppyimac are all found in these five alkyl amines assisted ECL system, the ECL intensities are much variable with the type of co-reactant in Fig. 5(from A to E). The integrated ECL intensity of Irppyimac follows the sequence: TEA > DIPA > DIPEA > TPA > DMEA. Interestingly, the ECL intensity of Irppyimac with TEA (Fig. 5B) is about 3 times higher than that with TPA (Fig. 5A). Furthermore, based on the former reported method [23], after pulsing the potential from 0 to 100 mV over the peak potential of Irppyimac, the ECL spectrum of Irppyimac with TEA has also been characterized and listed in Fig. 5F. The ECL spectrum is centered at 508 nm and it's very close to the major peak of PL spectra located at 513 nm. It should be noted that compared with the heteroleptic iridium(III) complexes with the traditional O^O or N^O ancillary ligands [22,23,38], the ECL spectra of this novel heteroleptic iridium(III) complex with NHC ancillary ligand showed obviously blue-shift, which may be helpful to design deep-blue ECL dyes and develop multichannel analytical techniques in the future. To the best of our knowledge, theoretical calculations have been employed to try to explain the relationship between ECL intensity and frontier orbital energy levels of the radicals of co-reactant [78], and higher HOMO level of co-reactant radical has been proven to be in favor of increasing the ECL intensity of luminophores [56,67,79]. According to former literature [56,67], the ground state geometries of these final alkyl amines were finally optimized in the absence of solvent with mPW1PW91 [80] functional in conjunction with 6e31G basis set. The calculation results demonstrate that the HOMO energy levels of these co-reactants’ radicals obey the following sequences: DIPEA(-3.2 eV) > TEA(-3.3 eV) ¼ TPA(3.3 eV) > DIPA(-3.4 eV) > DMEA(-3.6 eV). For DMEA, the theoretical result is in consistent with its ECL experimental result (i.e. the lowest HOMO energy level of the co-reactant radical is usually predicated to be the weakest ECL signal in the oxidative-reduction co-reactant assisted ECL pathway according to the former researches [56,67,79]). While for the co-reactants of TPA, TEA, DIPEA and DIPA, due to the similarity of their chemical structures, they have very similar HOMO energy levels and there is some discrepancy order between theoretical results and experimental results, which may be reasonably explained by the complexities of the chemical and electrochemical reactions involved in the ECL generation [36]. In brief, TEA has been proven as the best co-reactant for Irppyimac and this novel NHC-based iridium complex displays green ECL emission centered at 508 nm under the assistance of TEA.

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Fig. 5. (AeE) The CV and ECL curves of 1 mM Irppyimac versus potentials in acetonitrile solution containing 0.1 M TBAPF6 and 10 mM coreactant after pumping N2 atmosphere for 15 min. Scan rate: 0.1 V/s, the coreactant used in figure A to figure E is tri-n-propylamine (TPA), triethylamine (TEA), dimethylethanolamine (DMEA), diisopropylethylamine (DIPEA) and diisopropylamine (DIPA), respectively. (F) The normalized ECL spectra of 1 mM Irppyimac in acetonitrile solution containing 10 mM TEA and 0.1 M TBAPF6. Potentials pulsing from 0 to 100 mV over the oxidative potential of Irppyimac.

3. Conclusion An imidazolium bromide with carboxylic acid has been designed and synthesized as the NHC precursor through quaternary ammonium reaction between 1-methyl-1H-imidazole and 2bromoacetic acid. Subsequently, through metal-transfer reaction between silver and iridium(III), a novel green emissive iridium(III) complex with NHC-based ancillary ligand have been synthesized and thoroughly investigated through chemical structural photophysical, electrochemical and theoretical calculation methods in

this work. Most importantly, based on this novel ECL luminophores, five alkyl amines are selected as the oxidative-reduction co-reactants to comprehensively investigate the relationships between ECL intensity and co-reactant by experimental and theoretical calculation method in this work. Triethylamine (TEA), rather than TPA, has been proven to be the best co-reactant for this novel ECL luminophore. This study not only would pave the road of exploiting excellent co-reactant systems for the emerging iridium-based ECL luminophores, but also could shed light on the design of new type NHC-based bidentate ligand (Ccarbene^O) coordinated to iridium and

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enrich luminescent NHC-based iridium(III) complexes in the future. 4. Experimental section Materials. All chemicals involving the synthesis of iridium complexes were all purchased from commercial sources and used without further purification. Spectrum pure (SP)-grade acetonitrile for absorption and photoluminescent spectra were purchased from Sinopharm Chemical Reagent Co. Ltd, while extra-dry acetonitrile, ferrocene, tri-n-propylamine (TPA), triethylamine (TEA), dimethylethanolamine (DMEA), diisopropylethylamine (DIPEA), diisopropylamine (DIPA) and tetra-n-butylammonium hexafluorophosphate (TBAPF6) for electrochemistry and ECL characterization were obtained from Energy Chemical Inc. (Shanghai, China) without further purification. Apparatus and Methods. 1H NMR and 13C NMR spectra were acquired on a VARIAN-400 magnetic resonance spectrophotometer. Mass spectra were measured on a Varian ProStar LC240 (America). UVevis spectra and photoluminescent spectra were recorded UVevis spectrophotometer (TU-1950, Beijing Purkinje General Instrument Co., Ltd, Beijing, China) and Edinburgh FLS980 type steady-state/transient spectrometer. Three-electrode setup was used for electrochemical and ECL experiments. A Pt disk sealed in polytetrafluoroethylene (PTFE) was used as the working electrode and two Pt wires were for the auxiliary electrode and quasi-reference electrode, respectively. Before each experiment, the working electrode was all polished with alumina (0.05 mm) for several minutes, sonicated in water and in ethanol for 5 min each, and finally dried in an oven at 120  C. Ferrocene/ferrocenium (Fc/Fcþ) was used to calibrate the potentials in this work. Both CV and ECL intensity were all recorded on a MPIEII ECL detector (Xi'an Remax Electronics, China) equipped with a photomultiplier tube (Model: R9880U-20, Hamamatsu, Japan). The ECL spectra were recorded by pulsing from 0 to 100 mV past oxidation potential of iridium(III) complexes with a step time of 0.1 s using TEA as a co-reactant. Theoretical calculations. Density function theory (DFT) and time dependent density function theory (TD-DFT) have also been used to calculate the ground and excited states of Irppyimac using Gaussian 09 package, respectively. The 6-31 þ G(d) basis set was used for C, N, H, O, and the iridium atom employed the LANL2DZ basis set. The ground state geometries of these final alkyl amine radicals are optimized in the absence of solvent with mPW1PW91 [80] functional in conjunction with 6e31G basis set. 4.1. Synthesis and characterization Imidazolium bromide. An acetonitrile solution of 1-methyl1H-imidazole (1.64 g, 20 mmol) and 2-bromo-acetic acid (2.78 g, 20 mmol) were refluxed for 24 h under nitrogen atmosphere. After cooling to room temperature, the solvent was evaporated under reduced pressure. The formed oil was purified by repetitive recrystallization from THF/CH3OH and the colorless crystalline product was finally obtained. Yield: 3.9 g (88%).1H NMR (400 MHz, CD3OD), d (ppm): 9.02 (s, 1 H), 7.65e7.62 (m, 2 H), 5.16 (s, 2 H), 3.99 (s, 3 H). 13C NMR (376 MHz, CD3OD), d (ppm):169.08, 139.22, 125.07, 124.53, 50.78, 36.73. Ag carbene. A little modification procedure reported by Wang was used [62]. Ag2O (232 mg, 1 mmol) was added to the solution of Imidazolium bromide (444 mg, 2 mmol) in dichloromethane (50 mL). The solution was stirred and refluxed for 12 h with nitrogen atmosphere under dark condition. And then cooling to room temperature and filtration, the solvent in filtrate was evaporated under reduced pressure. After drying under vacuum condition, the yellowish-brown crystalline product (620 mg) was obtained and

used in the next step without further purification. Iridium(III) dichlorobridged dimer. A solution of IrCl3$3H2O (706 mg, 2 mmol) and 2-phenylpyridine (620 mg, 4 mmol) in the mixture of 2-ethoxyethanol/H2O (V:V ¼ 3:1) was refluxed for 24 h under nitrogen atmosphere. After cooling to room temperature, the solution was filtrated and the precipitate was successively washed with water, ethanol and n-hexane. The iridium(III) dichlorobridged dimer (980 mg) was obtained after drying and used in the next step without further purification. Irppyimac. Potassium carbonate (685 mg, 5 mmol) was added to the solution of iridium(III) dichlorobridged dimer (536 mg, 0.5 mmol) and Ag carbene (329 mg, 0.5 mmol) in xylenes (100 mL). The mixtures was stirred and refluxed for 24 h under dark condition with nitrogen protection. And then cooling to the room temperature, the solvent was evaporated under reduced pressure. The residues were chromatographed on a silica gel column with CH2Cl2/ CH3OH and the purified product (yellowish-green powder) was obtained after drying under vacuum condition. Yield: 326 mg (51%), UPLC purity: 99%. 1H NMR (400 MHz, CDCl3), d (ppm): 8.94 (m, 1 H), 8.34 (m, 1 H), 7.88 (m, 2 H), 7.77 (m, 2 H), 7.58 (dd, J ¼ 6.4, 1.2 Hz, 1 H), 7.52 (dd, J ¼ 6.4, 1.2 Hz, 1 H), 7.14 (m, 1 H), 7.00 (m, 1 H), 6.89 (d, J ¼ 2 Hz, 2 H), 6.85e6.75 (m, 2 H), 6.65 (td, J ¼ 7.6, 1.2 Hz, 1 H), 6.62 (d, J ¼ 2 Hz, 1 H), 6.36 (dd, J ¼ 8, 1.2 Hz, 1 H), 5.98 (dd, J ¼ 7.2, 0.8 Hz, 1 H), 4.53 (s, 2 H), 2.64 (s, 3 H). 13C NMR (376 MHz, CDCl3), d (ppm):172.441, 171.366, 171.241, 167.267, 165.065, 152.162, 149.290, 147.513, 144.264, 143.209, 136.803, 135.985, 131.120, 130.964, 129.866, 129.144, 124.292, 123.825, 122.230, 122.184, 121.999, 121.804, 121.334, 119.749, 119.696, 118.056, 54.980, 36.516. Tof-Ms: m/z (experimental) 639.1491, m/z (calculated) 639.1500. Acknowledgement The authors are grateful to National Natural Science Foundation of China (NSFC No. 21505097); Natural Science Foundation of Jiangsu Province (BK20150283); University Scientific Research Project of Jiangsu Province (15KJB150027); the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); Excellent Innovation Team in Science and Technology of University in Jiangsu Province and Jiangsu Collaborative Innovation Center of Technology and Material for Water Treatment for financial support. We also thank to Supercomputing Center, CNIC, CAS for the calculation supporting. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jorganchem.2017.07.017. References [1] N.E. Tokel, A.J. Bard, Electrogenerated chemiluminescence. IX. Electrochemistry and emission from systems containing tris(2,2'-bipyridine)ruthenium(II) dichloride, J. Am. Chem. Soc. 94 (1972) 2862e2863. [2] N.E. Tokel-Takvoryan, R.E. Hemingway, A.J. Bard, Electrogenerated chemiluminescence. XIII. Electrochemical and electrogenerated chemiluminescence studies of ruthenium chelates, J. Am. Chem. Soc. 95 (1973) 6582e6589. [3] L. Hu, G. Xu, Applications and trends in electrochemiluminescence, Chem. Soc. Rev. 39 (2010) 3275e3304. [4] W. Miao, Electrogenerated chemiluminescence and its biorelated applications, Chem. Rev. 108 (2008) 2506e2553. [5] L. Hu, G. Xu, Applications and trends in electrochemiluminescence, Chem. Soc. Rev. 39 (2010) 3275e3304. [6] A. Kapturkiewicz, Cyclometalated iridium(III) chelatesda new exceptional class of the electrochemiluminescent luminophores, Anal. Bioanal. Chem. (2016) 1e21. [7] Z. Liu, W. Qi, G. Xu, Recent advances in electrochemiluminescence, Chem. Soc. Rev. 44 (2015) 3117e3142. [8] M.M. Richter, A.J. Bard, Electrogenerated chemiluminescence. 58. Ligandsensitized electrogenerated chemiluminescence in europium labels, Anal.

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