Aggregation-Induced Electrochemiluminescence from a

Apr 2, 2018 - Aggregation-induced emission has been extensively found in organic compounds and metal complexes. In contrast, aggregation-induced elect...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Aggregation-Induced Electrochemiluminescence from a Cyclometalated Iridium(III) Complex Tai-Bao Gao,†,§ Jing-Jing Zhang,‡,§ Run-Qi Yan,‡ Deng-Ke Cao,*,† Dechen Jiang,*,‡ and Deju Ye*,‡ †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, and ‡State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *

ABSTRACT: Aggregation-induced emission has been extensively found in organic compounds and metal complexes. In contrast, aggregation-induced electrochemiluminescence (AI-ECL) is rarely observed. Here, we employ two tridentate ligands [2,2′:6′,2″-terpyridine (tpy) and 1,3-bis(1Hbenzimidazol-2-yl)benzene (bbbiH3)] to construct a cyclometalated iridium(III) complex, [Ir(tpy)(bbbi)] (1), showing strong AI-ECL. Its crystal structure indicates that neighboring [Ir(tpy)(bbbi)] molecules are connected through both π−π-stacking interactions and hydrogen bonds. These supramolecular interactions can facilitate the self-assembly of complex 1 into nanoparticles in an aqueous solution. The efficient restriction of molecular vibration in these nanoparticles leads to strong AI-ECL emission of complex 1. In a dimethyl sulfoxide−water (H2O) mixture with a gradual increase in the H2O fraction from 20% to 98%, complex 1 showed a ∼39fold increase in the electrochemiluminescence (ECL) intensity, which was ∼4.04 times as high as that of [Ru(bpy)3]2+ under the same experimental conditions. Moreover, the binding of bovine serum albumin to the nanoparticles of complex 1 can improve the ECL emission of this complex, facilitating the understanding of the mechanism of AI-ECL for future applications.

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INTRODUCTION Aggregation-induced emission (AIE) is a novel photophysical phenomenon reported by Tang and co-workers in 2001,1 in which luminogenic compounds are weakly emissive or nonemissive in a molecule-isolation state but reveal strong emissions in a molecule-aggregation state.2 The occurrence of the AIE process offers new insight for the design and synthesis of highly efficient luminogens and also for the in-depth understanding of the photophysical processes and working mechanisms of these luminogens.2a,3 On the other hand, electrochemiluminescence (ECL) is optical emission arising from a high-energy electron-transfer reaction between electrogenerated species 4 and has wide applications such as biosensors,5 organic light-emitting diodes,6 and food analysis.7 The combination of AIE and ECL can lead to a new physicochemical phenomenon, i.e., aggregation-Induced electrochemiluminescence (AI-ECL),8 in which molecular aggregation of a luminogenic compound through supramolecular interactions (e.g., hydrogen bonds and π−π-stacking interactions) can result in significant enhancement of the ECL intensity of this compound. AI-ECL can not only provide better elucidation of the AIE process but also open a new avenue for the design of novel ECL-based sensors. For a compound showing AI-ECL, its molecule-aggregation state should reveal a strong ECL signal, but there is no signal or a weak signal for its molecule-isolation state. Therefore, molecular self-assembly through supramolecular interactions is essential to trigger © XXXX American Chemical Society

efficient aggregation and the generation of a strong AI-ECL signal. Very recently, Cola and co-workers reported the first observation of the AI-ECL phenomenon from a square-planar platinum(II) complex, Pt-PEG2 (Scheme S1).8 They proposed that the AI-ECL behavior was mainly due to intermolecular Pt···Pt interactions in the resulting supramolecular nanostructures in an aqueous solution. This intriguing result will offer a new tool for applications in biosensing and immunoassay. Considering that some iridium(III) complexes show either prominent AIE2d,3a,b,e,9 or ECL behavior,10 we envisioned that the self-assembly of a well-designed iridium(III) complex could also lead to the generation of AI-ECL for biosensing. In this paper, we designed and synthesized a cyclometalated complex, [Ir(tpy)(bbbi)] [1, where tpy = 2,2′:6′,2″-terpyridine and bbbiH3 = 1,3-bis(1H-benzimidazol-2-yl)benzene; Scheme 1], in which two tridentate ligands, i.e., tpy and bbbi3− chelate an IrIII ion through N^N^N and N^C^N coordination modes, respectively. From the viewpoint of the structure, both tpy and bbbi3− are planar and aromatic ligands, thus facilitating the formation of intermolecular π−π-stacking interactions. Moreover, complex 1 contains additional proton acceptors, i.e., noncoordinated imidazole N atoms (see the N4 and N6 atoms in Figure 1a), which could be involved in intermolecular hydrogen-bonding interactions. Therefore, neighboring [IrReceived: December 13, 2017

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DOI: 10.1021/acs.inorgchem.7b03093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of Complex 1, with Labels a−f and a′−f′ Showing the Proton Types of This Complexa

Reaction conditions: (a) (1) Ir(tpy)Cl3, EtOEtOH, 190 °C, 24 h, argon, and (2) KPF6. Yield: 34%. (b) NaOH, H2O−CH2Cl2, room temperature, 2 days. Yield: 97%.

a

diffraction (XRD), IR, and elemental analysis (Figures S2−S5). It should be noted that complex 1 is difficult to prepare through a one-step reaction of bbbiH3, [Ir(tpy)Cl3], and K2CO3 in ethylene glycol because of the difficulty of removing ethylene glycol (with a high boiling point of 197.4 °C). In order to clarify the molecular stacking mode in complex 1, a single crystal of 1·2H2O·CH3OH was grown in a CH3OH− CH2Cl2 mixture and measured by X-ray crystallography. Its crystallographic data and selected bond lengths and bond angles were summarized in Tables S1 and S2. The perspective view of complex 1 is depicted in Figure 1a. The IrIII ion adopts a distorted octahedral coordination geometry and is chelated by tridentate ligands tpy and bbbi3− through N1, N2, N3, C16, N5, and N7 atoms. The bond lengths of Ir−N(C) are in the range of 1.983(10)−2.062(8) Å and the bond angles of N−Ir− N(C) are in the range of 77.5(3)−177.2(4)°, which are similar to those in other reported cyclometalated iridium(III) complexes.11 Around the IrIII ion in Figure 1a, the C16 atom of the ligand bbbi3− is located at the position trans to the N2 atom of the ligand tpy. The plane of the ligand bbbi3− is almost orthogonal to that of the ligand tpy, with an interplanar angle of 85.4(1)°, which can minimize the steric hindrance. As shown in Figure 1b, neighboring [Ir(tpy)(bbbi)] molecules are linked to a supramolecular chain along the b axis through two types of π−π-stacking interactions, including (1) slipped π−π stacking between two pyridine rings from two adjacent tpy ligands [plan···plan distance = ∼3.75 Å; magenta point segment line]12 and (2) edge-on C−H···π interaction between C13−H from the tpy ligand and the benzene ring from the bbbi3− ligand [C13−H···π = 3.254(1) Å; orange point segment line].12 These chains are further connected by lattice water (H2O) molecules through hydrogen bonds [O1W−H···N4a = 2.745(1) Å; O2W−H···O1Wb = 2.798(1) Å; symmetry codes a = x, y, z + 1/2 and b = x + 1, −y + 1/2, z − 1/2; cyan point segment line], forming a layer structure (Figure 1c) in which lattice CH3OH molecules are attached to H2O molecules through hydrogenbonding interactions [O1−H···O2Wc = 2.840(1) Å; symmetry code c = x − 1, y, z]. These intermolecular π−π-stacking interactions and hydrogen bonds could facilitate the selfassembly of complex 1 in an aqueous solution, thus forming supramolecular nanostructures. The self-assembly of complex 1 into nanoaggregates was studied by dissolving it in an aqueous solution. Dynamic light scattering (DLS) analysis showed that complex 1 could form monodisperse nanoparticles, with an average hydrodynamic size of 120 nm in a dimethyl sulfoxide (DMSO)−H2O (10/90, v/v) mixture, which was also verified by transmission electron microscopy (TEM) analysis (Figure 2). The average nano-

Figure 1. (a) Molecular structure of complex 1 (50% probability). (b and c) Supramolecular chain and layer structures in 1·2H2O·CH3OH, respectively. The big and small red balls are O atoms from lattice H2O and CH3OH molecules, respectively. All H atoms are omitted for clarity.

(tpy)(bbbi)] molecules in complex 1 could efficiently stack through π−π-stacking interactions and hydrogen bonds, resulting in self-assembly into nanoparticles that emit strong ECL signals. To the best of our knowledge, complex 1 is the first example of an iridium(III) complex showing the AI-ECL property. Herein, we discuss the crystal structure of complex 1 and its related self-assembly into nanoparticles in an aqueous solution and demonstrate the AI-ECL behavior of this complex. Moreover, AI-ECL of complex 1 can be enhanced through the addition of bovine serum albumin (BSA).

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RESULTS AND DISCUSSION Complex 1 was synthesized according to the approach shown in Scheme 1. First, complex [Ir(tpy)(bbbiH2)][PF6]2 was synthesized in 34% yield through the reaction of bbbiH3 and [Ir(tpy)Cl3] in ethylene glycol at 190 °C for 24 h, followed by the anion exchange of Cl− with KPF6. Then, this complex was deprotonated by NaOH, obtaining complex 1 in 97% yield. The chemical structure of 1 was characterized by 1H NMR, X-ray B

DOI: 10.1021/acs.inorgchem.7b03093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Moreover, compared to the 1H NMR spectra measured in DMSO-d6 containing 0−30% D2O, the spectrum measured in DMSO-d6 containing 40% D2O shows broader signals (Figures 3 and S4), indicating the formation of nanoparticles. This result is in agreement with the feature of the mixture of 1 and DMSOd6−D2O, with clear solutions involving 0−30% D2O and cloudy solutions involving 40% D2O. The UV−vis spectrum of complex 1 was measured in CH2Cl2 at room temperature (Figure S7), showing the absorption bands at 283, 314, 348, and 365 nm and a weak absorption tail toward 446 nm. The bands at 283 and 314 nm could be assigned to ligand-centered (1LC) transitions (bbbi3− and tpy ligands), which is confirmed by the absorption bands of free ligands tpy (279 nm) and bbbiH3 (309 nm) (Figure S7). The others bands (348−446 nm) of 1 could be mainly attributed to metal-to-ligand charge transfer (1MLCT and 3 MLCT).14 Complex 1 shows weak luminescence in pure DMSO at room temperature. However, upon the addition of H2O from 65% to 90%, complex 1 revealed a gradual increase in the luminescence intensity at 643 nm under excitation with 397 nm light (Figure S8) because of its self-assembly into nanoparticles. This indicates that complex 1 has AIE behavior. We further measured the cyclic voltammetry (CV) and ECL of complex 1 in a DMSO−H2O mixture (20/80, v/v) containing 1 mM tripropylamine (TPA) as the coreactant and 100 mM NaCl and 10 mM phosphate-buffered saline (PBS; pH 7.4) as supporting electrolytes. The obtained CV curve (Figure 4a, black curve) displays an irreversible oxidative peak at a potential of about 1.23 V (vs Ag/AgCl), which could be due to the oxidation of [Ir(tpy)(bbbi)]0 to [Ir(tpy)(bbbi)]+. This value is slightly lower than that measured in the absence of TPA (1.30 V; Figure 4a, inset) but is similar to those of other

Figure 2. TEM (left) and DLS (right) analyses of the nanoparticle size of complex 1 in a DMSO−H2O (10/90, v/v) mixture.

particle size of complex 1 gradually increases to 160 nm with increasing H2O fraction from 50% to 98% (Figure S6). We further used 1H NMR spectra to study the molecular aggregation process of complex 1. As shown in Scheme 1, complex 1 has 12 kinds of protons, including protons a−f in the ligand bbbi3− and protons a′−f′ in the ligand tpy. The chemical shifts from these protons were assigned using 1H−1H COSY NMR (Figure S3). Complex 1 is present as a free molecule when dissolved in a pure DMSO-d6 solvent, while the addition of D2O can tune complex 1 into nanoaggregates, resulting from enhanced intermolecular interactions including π−π-stacking interactions and hydrogen bonds. The formation process of these nanoaggregates was confirmed by the 1H NMR spectra of complex 1 in DMSO-d6 containing different D2O fractions. As shown in Figure 3, upon the addition of 10−40% D2O into the

Figure 3. 1H NMR spectra of 1 in DMSO-d6 containing 0−40% D2O. Labels a−f and a′−f′ of Scheme 1show the proton types of 1. Red and blue dotted lines indicate upfield and downfield chemical shifts, respectively.

DMSO-d6 solution of complex 1, the chemical shifts of protons a′−f′ showed a gradual upfield shift and a downfield shift for those of protons a−f (see also Figures S2 and S4). The upfield shift of protons a′−f′ in tpy could be ascribed to the slipped π−π-stacking interactions between two pyridine rings from two adjacent tpy ligands (Figure 1b), resulting in a strong anisotropic shielding effect.13a,b The downfield shift of protons a−f in the ligand bbbi3− could be due to the electron delocalization effect from the ligand bbbi3− arising from both the edge-on C−H···π interaction between C13−H from the ligand tpy and the phenyl ring from the ligand bbbi3− and the hydrogen bond between imidazole N (N−) and a lattice H2O molecule [O1W−H···N4a = 2.745(1) Å; Figure 1c].13c

Figure 4. (a) Cyclic voltammogram (dark line) and ECL trace (blue line) of complex 1 (c = 200 μM) in a DMSO−H2O (20/80, v/v) mixture containing 1 mM TPA, 100 mM NaCl, and 10 mM PBS (pH 7.4). Inset: CV curves of a blank solution (excluding 1 and TPA) and complex 1 in the absence of TPA. (b) ECL intensity changes of complex 1 (c = 200 μM) upon variation of the H2O fraction of a DMSO−H2O mixture. The scan rate was 0.1 V/s. C

DOI: 10.1021/acs.inorgchem.7b03093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry reported iridium(III) complexes.10c A peak at −0.87 V (see the black CV curve in Figure 4a) from the reduction of [Ir(tpy)(bbbi)]0 to [Ir(tpy)(bbbi)]− was observed, which became a clear peak at −0.88 V in the CV curve of complex 1 in the absence of TPA (see the green CV curve in the inset of Figure 4a). During the potential scanning process, complex 1 showed a strong ECL emission, with a peak at a potential of 1.23 V (Figure 4a, blue curve). In order to clarify the ECL emission of complex 1, we further measured its ECL spectra in a series of aqueous electrolytes containing different amounts of DMSO and H2O. As shown in Figure 4b, complex 1 revealed a ∼39-fold enhanced luminescence when the H2O fraction increased from 20% to 98%. This indicates that complex 1 has obvious AI-ECL behavior, which is in agreement with the result that no ECL was observed for complex 1 in DMSO (Figure S9) because of the absence of molecular aggregation. Moreover, the CV curve measured in DMSO (Figure S9) shows an oxidative peak at 1.26 V and a reduction peak at −0.80 V (vs Ag/AgCl), which are comparable to the corresponding peaks at +1.23 and −0.87 V measured in a DMSO−H2O (20/80, v/v) mixture (Figure 4a). Upon an increase in the H2O fraction from 20% to 98%, the ECL peak potential of complex 1 revealed an obvious decrease from 1.30 to 1.19 V (Figure 4b) because of two possible factors. One is the increase in the solution conductivity, which provided less iR drop in the solution.15 A similar shift of the ECL peak potential was also observed in [Ru(bpy)3]2+ upon an increase in the H2O fraction in a DMSO−H2O mixture (Figure S10). The other is the change of the reference electrode potential. The ECL intensity of complex 1 shows significant enhancement with an increase in the H2O fraction in a DMSO−H2O mixture, although the diffusion of the resulting >100 nm nanoparticles to the electrode surface becomes much slower, leading to slower ECL reactions. This indicates that molecular aggregation of complex 1 makes a dominant contribution to enhancement of the ECL intensity; thus, the diffusion effect from the big nanoparticle size cannot be observed. In addition, in the presence of TPA, we found that the ECL intensity of complex 1 is approximately 4.04 times as high as that of [Ru(bpy)3]2+ (Figure S11), which could offer sensitive ECL signals for biosensing applications. Both photoluminescence and ECL of complex 1 are very weak when it is present in the free molecule state, which could be assigned to the molecular vibration of the complex, thus leading to emission quenching. When complex 1 is present in the aggregation state [e.g., in a DMSO−H2O (10/90, v/v) mixture], the intermolecular interactions (e.g., π−π stacking among [Ir(tpy)(bbbi)] molecules and hydrogen bonds between [Ir(tpy)(bbbi)] and H2O molecules) could efficiently restrict molecular vibration of complex 1, thus enhancing photoluminescence and ECL. As shown in Figure S12, both ECL and photoluminescence of complex 1 revealed a rapid increase in the intensity when the H2O fraction is above ∼70%. Therefore, the AI-ECL property of complex 1 arises from an aggregationinduced restriction of molecular vibration, revealing a different mechanism from that of the platinum(II) complex Pt-PEG2 reported by Cola and co-workers.8 It was found that, in PtPEG2, the aggregation-induced Pt···Pt interactions played key roles in enhancing the ECL intensity. Moreover, the ECL spectrum of 1 shows a broad and featureless emission band around 640 nm (Figure S13), which is similar to the photoluminescence emission (643 nm; see Figure S8), indicating that both ECL and photoluminescence from 1

have the same excited state.16 This excited state has predominantly 3MLCT character,17 which was confirmed by measurement of the photoluminescence spectrum at 77 K (Figure S14), exhibiting a significantly blue-shifted emission at 506 nm compared to the emission of 643 nm at room temperature. Having demonstrated the AI-ECL property of complex 1, we then investigate its interaction with protein, such as BSA, which can restrict molecular vibration, thus leading to AIE.18 Upon the addition of BSA with a concentration range of 0.001−0.5 mg/mL, complex 1 not only showed a gradual increase in the ECL intensity (Figure 5) but also revealed a significant increase

Figure 5. ECL intensity changes of complex 1 in a DMSO−H2O (2/ 98, v/v) solution with different amounts of BSA. 1 mM TPA was added as the coreactant, and 100 mM NaCl and 10 mM PBS were used as supporting electrolytes.

in the hydrodynamic size (see the DLS analysis shown in Figure S15). These increases in both the ECL intensity and nanoparticle size could be due to the binding of BSA to the surface of nanoaggregates from complex 1, which could further reduce molecular vibration and thus improve ECL emission.18 In addition, BSA probably also enhances the adsorption of complex nanoparticles on the electrode; however, this adsorption was not significant. The electrode after exposure to BSA and the complex was taken into a fresh DMSO−H2O solution. No obvious ECL was observed from the electrode, suggesting minor adsorption of the complex on the electrode. These experimental results indicate existing binding interactions between BSA and the nanoparticles of complex 1, which should facilitate the understanding of the mechanism of AIECL for future applications.

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CONCLUSIONS In summary, we have designed and synthesized the first example of cyclometalated iridium(III) complex 1 showing AIECL. The AI-ECL property of complex 1 could be due to the aggrgation-induced restriction of molecular vibration through both π−π-stacking interactions and hydrogen bonds in the resulting nanoparticles. The ECL intensity of complex 1 (using TPA as a coreactant) in the aggregation state was found to be ∼39-fold higher than that in the free molecule state, which was ∼4 times as high as that of the clinically used ECL agent [Ru(bpy)3]2+/TPA. Upon the addition of BSA, complex 1 showed a gradual increase in the ECL intensity because of the binding of BSA to the surface of nanoaggregates from this complex. The discovery of iridium(III) complexes exhibiting strong AI-ECL signals will open the way for the development of D

DOI: 10.1021/acs.inorgchem.7b03093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

MHz, DMSO-d6; see Figure S2): δ 5.53 (d, J = 8 Hz, 2Ha), 6.46 (t, J = 8 Hz, 2Hb), 6.67 (t, J = 8 Hz, 2Hc), 7.27−7.32 (m, 2Hd and 2Ha′), 7.36 (t, J = 8 Hz, 2Hb′), 7.47 (t, J = 8 Hz, 1Hf), 7.83 (d, J = 8 Hz, 2He), 7.94 (t, J = 8 Hz, 2Hc′), 8.74 (d, J = 8 Hz, 2Hd′), 8.82 (t, J = 8 Hz, 1Hf′), 9.22 (d, J = 8 Hz, 2He′). Single-Crystal X-ray Crystallography. The CH3OH−CH2Cl2 solution of 1 was allowed to slowly evaporate, obtaining reddish-brown blocky single crystals of 1·2H2O·CH3OH (CCDC 1582619). A single crystal with dimensions of 0.24 × 0.21 × 0.18 mm3 was used for structural determination on a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. A hemisphere of data was collected in the θ range of 2.23−25.00° using a narrow-frame method with a scan width of 0.30° in ω and an exposure time of 10 s/frame. The numbers of observed and unique reflections are 37386 and 5428 (Rint = 0.0905), respectively. The data were integrated using the Siemens SAINT program,20 with the intensities corrected for the Lorentz factor, polarization, air absorption, and absorption due to variations in the path length through the detector faceplate. Multiscan absorption corrections were applied. The structures were solved by direct methods and refined on F2 by full-matrix least squares using SHELXTL.21 All of the non-H atoms were located from the Fourier maps and refined anisotropically. All H atoms were refined isotropically, with the isotropic vibration parameters related to the non-H atom to which they are bonded. In the final Fourier difference map, the deepest hole and highest peak are −3.429 and +3.137, respectively, which are in the vicinity of the IrIII ion. In the structural refinements of 1·2H2O·CH3OH, ISOR was used because of slight disorder of some C, N, and O atoms. In addition, some disagreeable reflections were omitted. These refinement details can be found in the CIF files containing the information on the hkl and res files. The crystallographic data for complex 1·2H2O·CH3OH are listed in Table S1, and selected bond lengths and angles are given in Table S2.

AI-ECL-based biosensors, organic light-emitting diodes, and food analysis methods. Our work indicates that molecular selfassembly through supramolecular interactions is one of key factors that restricts molecular vibration and thus induces the AI-ECL behavior of complex 1. We will further design and synthesize some analogues of complex 1 to obtain insight into the structure−property relationship that guides the theoretical research and practical application of AI-ECL.

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EXPERIMENTAL SECTION

Materials and Methods. 1,3-Bis(1H-benzimidazol-2-yl)benzene (bbbiH3) and [Ir(tpy)Cl3] (tpy = 2,2′:6′,2″-terpyridine) were prepared according to the literature.19 All other reagents were commercially available and were used without further purification. Elemental analyses were performed on a PerkinElmer 240C elemental analyzer. IR spectra were obtained as KBr disks on a Vector 22 spectrometer. 1H NMR spectra were recorded at room temperature with a 400 MHz Bruker spectrometer. Powder XRD patterns were recorded on a Bruker D8 Advance X-ray diffractometer. UV−vis absorption spectra were measured on a Cary 100 spectrophotometer. Luminescence spectra were measured using a Hitachi F-4600 fluorescence spectrometer. DLS and TEM analyses were recorded on a Brookhaven 90Plus particle size analyzer and a high-resolution transmission electron microscope (JEM-2100), respectively. ECL. An ECL experiment was performed using a three-electrode system including an indium−tin oxide (ITO) slide (5 mm in diameter) as the working electrode, an Ag/AgCl electrode as the reference electrode, and a platinum wire as the counter electrode. Luminescence was collected using a MPI-A ECL analyzer (Ruimai, Xian) in 200 μL of a DMSO solution with different amounts of H2O. 100 mM NaCl and 10 mM PBS (pH 7.4) were added to the solution as supporting electrolytes. In the resulting mixture, the concentration of complex 1 was 200 μM. The potential was scanned from −1.0 to −1.45 V with a scan rate of 0.1 V/s. The luminescence spectra were measured using band-pass filters with a bandwidth of 25 nm from 375 to 625 nm between the ITO electrode and photomultiplier tube. Synthesis of [Ir(tpy)(bbbiH2)][PF6]2. A mixture of Ir(tpy)Cl3 (0.2 mmol, 0.1064 g) and bbbiH3 (0.25 mmol, 0.0775 g) in ethylene glycol (25 mL) was heated under argon at 190 °C for 1 day. After the addition of a saturated KPF6 aqueous solution (25 mL), a yellow solid was obtained. This solid was filtered and purified through silica column chromatography using CH3OH−CH2Cl2 (0−2/100, v/v), obtaining a yellow solid with a yield of 70 mg [34% based on Ir(tpy)Cl3]. Anal. Found (calcd) for C35H24N7F12P2Ir: C, 41.21 (41.06); H, 2.43 (2.36); N, 9.70 (9.58). IR (KBr, cm−1): 3651(w), 3383(m), 1634(w), 1596(w), 1553(w), 1454(m), 1397(w), 1314(w), 1290(w), 1231(w), 1197(w), 1163(w), 1056(w), 839(s), 773(w), 745(w), 641(w), 559(m). 1H NMR (400 MHz, DMSO-d6; see Figure S1): δ 5.82 (d, J = 8 Hz, 2H from the phenyl ring of the bbibH2− ligand), 7.01 (t, J = 8 Hz, 2H from the typ ligand), 7.24 (t, J = 8 Hz, 2H), 7.30 and 7.32 (d, J = 8 Hz, 2H), 7.39 (t, J = 8 Hz, 2H), 7.61 (d, J = 8 Hz, 2H) [7.24−7.61 ppm: total of 8H from two benzoimidazole units of the bbibH2− ligand], 7.87 (t, J = 8 Hz, 1H from the phenyl ring of the bbibH2− ligand), 8.08 (t, J = 8 Hz, 2H), 8.28 (d, J = 8 Hz, 2H), 8.83 (d, J = 8 Hz, 2H), 9.02 (t, J = 8 Hz, 1H), 9.32 (d, J = 8 Hz, 2H) [8.08−9.32 ppm: total of 9H from the typ ligand]. Synthesis of [Ir(tpy)(bbbi)] (1). To a mixture of complex [Ir(tpy)(bbbiH2)][PF6]2 (0.024 mmol, 25 mg) and CH2Cl2 (25 mL) was added an aqueous solution of NaOH [7 mL, containing NaOH (0.48 mmol, 19 mg)]. This mixture was vigorously stirred at room temperature for 2 days. After removal of all solvents under vacuum, ethanol (10 mL) was added, forming a solid. This solid was filtered, washed with ethanol, dried in air, and crystallized in a CH3OH− CH2Cl2 mixture. Yield: 17 mg (97% based on the compound [Ir(tpy)(bbbiH2)][PF6]2). Anal. Found (calcd) for C35H22N7Ir: C, 57.52 (57.36); H, 3.23 (3.03); N, 13.42 (13.38). IR (KBr, cm−1): 3406(m), 3070(m), 1636(w), 1602(w), 1502(w), 1470(w), 1435(s), 1409(w), 1391(w), 1304(w), 1276(m), 1227(w), 1182(w), 1026(w), 915(w), 808(w), 773(m), 746(s), 645(w), 442(w). 1H NMR (400

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03093. Molecular structure of complex Pt-PEG2, crystallographic data, selected bond lengths/angles, NMR spectra, XRD patterns, nanoparticle size changes, UV−vis spectra, luminescence spectra, cyclic voltammogram, ECL spectra, and DLS analysis (PDF) Accession Codes

CCDC 1582619 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.-K.C.). *E-mail: [email protected] (D.J.). *E-mail: [email protected] (D.Y.). ORCID

Deng-Ke Cao: 0000-0002-8256-5413 Dechen Jiang: 0000-0002-2845-3621 Deju Ye: 0000-0002-9887-0914 Author Contributions §

These authors contributed equally.

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DOI: 10.1021/acs.inorgchem.7b03093 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support by the National Science Foundation of Jiangsu Province China (Grant BK 20141314, BK20150567), and from State Key Laboratory of Analytical Chemistry for Life Science (5431ZZXM 1703).

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DOI: 10.1021/acs.inorgchem.7b03093 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b03093 Inorg. Chem. XXXX, XXX, XXX−XXX