Coordination-Directed Stacking and Aggregation-Induced Emission

Jan 5, 2017 - The fluorescence emission of ZnL2 enhances in THF/H2O as H2O% (volume %) is above 60% and aggregates particles with several hundred nano...
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Coordination-Directed Stacking and Aggregation-Induced Emission Enhancement of the Zn(II) Schiff Base Complex Dan Wang,† Shu-Mu Li,‡ Jian-Quan Zheng,§ Duan-Yang Kong,† Xiang-Jun Zheng,*,† De-Cai Fang,*,† and Lin-Pei Jin† †

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China ‡ Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § Beijing Key Laboratory of Bioactive Substances and Functional Foods, Beijing Union University, Beijing 100191, P. R. China S Supporting Information *

ABSTRACT: 2-(Trityliminomethyl)-quinolin-8-ol (HL) and its Zn(II) complex were synthesized and characterized by single-crystal X-ray diffraction. HL is an unsymmetrical molecule and coordinated with Zn(II) ion to form ZnL2 in the antiparallel-mode arrangement via ZnO (hydroxyl group) and ZnN (quinoline ring) of HL. A high degree of ZnL 2 molecules ordering stacking is formed by the coordination bonds and intermolecular π−π interactions, in which head-to-tail arrangement (J-mode stacking) for L− is found. HL is nonfluorescent and ZnL2 is weakly fluorescent in THF. The fluorescence emission of ZnL2 enhances in THF/ H2O as H2O% (volume %) is above 60% and aggregates particles with several hundred nanometers are formed, which is confirmed by DLS data and TEM images. The J-aggregates stacking for L− in ZnL2 results in aggregation-induced emission enhancement (AIEE) for ZnL2 in THF/H2O. Theoretical computations based on B3LYP/6-31G(d, p) and TD-B3LYP/6-31G(d, p) methods were carried out. ESIPT is the supposed mechanism for fluorescent silence of HL, and fluorescence emission of ZnL2 is attributed to the restriction of ESIPT process. The oscillator strength of ZnL2 increases from 0.017 for monomer to 0.032 for trimer. It indicates that a high degree of ZnL2 molecules ordering stacking in THF/H2O is of benefit to fluorescence enhancement. HL is an ESIPT-coupled AIEE chemosensor for Zn(II) with high selectivity and sensitivity in aqueous medium. HL can efficiently detect intracellular Zn(II) ions because of ESIPT-coupled AIEE property of ZnL2 in mixed solvent.

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INTRODUCTION

In recent decades, many sensing mechanisms have been reported, including photoinduced electron transfer (PET),8 intermolecular charge transfer (ICT),9 fluorescence resonance energy transfer (FRET),10 ligand to metal charge transfer (LMCT),11 excimer and exciplex formation,12 and excited state intramolecular proton transfer (ESIPT).13 While an ESIPT chemosensor has a significant feature, large Stokes shift, which can effectively prevent self-absorption, or the inner filter effect happening.14 Hence a great deal of effort is put into chemosensors based on ESIPT. While when the species formed in the detection system is AIE-active, the fluorescence emission based on inhibition of ESIPT can be favored by forming aggregates. This could be used to design AIE-active fluorescence chemosensor for metal ions. Thereby, we report that a newly synthesized compound, HL, can be used to detect Zn2+ and forms the ZnL2 complex which shows fluorescence

Aggregation-induced emission (AIE) and aggregation-induced emission enhancement (AIEE) are unusual phenomena, i.e., some non/weakly emissive compounds in dilute solutions can become highly emissive in their solid/aggregated states. This was first reported by Tang’s group in 2001.1 Several mechanisms have been put forward to explicate AIE and AIEE effects. The restricted intramolecular rotational motion (RIR) is the most popular mechanism that was first raised by Tang and co-workers. 2 Apart from this, some other mechanisms that have been suggested include restriction of intramolecular vibration (RIV),3 and restriction of intramolecular motion (RIM),4 twisted intramolecular charge transfer (TICT),5 hydrogen bond formation,6 and J-aggregate formation.7 On the basis of these mechanisms, a large variety of new AIEgens have been developed, and their utilities as functional materials, especially as chemosensors, bioprobes, and solid state emitters have been explored. AIEgens for the detection of metal ions have high sensitivity and rapid response. © XXXX American Chemical Society

Received: November 17, 2016

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DOI: 10.1021/acs.inorgchem.6b02784 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry emission based on inhibition of ESIPT. The fluorescence emission of ZnL2 was enhanced in certain ratios of THF/H2O mixture because of the formation of J-aggregates. This is the first report on coordination-directed stacking of the ligands in the Zn(II) complexes to form J-aggregates, resulting in aggregation-induced emission enhancement.

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method and refined by full matrix least-squares based on F2 using the SHELX 97 program.15 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions. Crystal data for HL and ZnL2 are summarized in Table S1. Selected bond lengths and bond angles for HL and ZnL2 are tabulated in Table S2. The molecular structure of HL is shown in Figure S4. CCDC nos. 1491860 and 1491861 contain the supplementary crystallographic data for HL and ZnL2, respectively. Calculation Methods. In order to explore the ESIPT process of HL to HL′ and fluorescent property of ZnL2, B3LYP/6-31G(d,p) and TD-B3LYP/6-31G(d,p) methods16 have been used to characterize the energies, structures, and frequencies at ground state and excited state, respectively (Tables S3 and S4). Herein, G-31(d, p) basis set has been employed for all of elements in molecules, including C, H, O, N, and Zn. Methods for Cell Imaging. There are two groups of SH-SY5Y cell line which was cultured in DMEM (Dulbecco’s Modified Eagle Medium). Cells were incubated with 20 μM of HL at 37 ◦C for 16 h. After washing with PBS three times to remove the remaining HL, the cells were then incubated with 20 μM Zn(ClO4)2 for 30 min at room temperature. These incubated cells were washed with PBS and mounted onto a glass slide. The fluorescent images of the mounted cells were obtained using a confocal laser scanning microscope with 405 nm excitation.

EXPERIMENTAL SECTION

General Information and Materials. All solvents and reagents (analytical grade) were used as-received. Elemental analyses were conducted using a Vario EL elemental analyzer. Fourier transform infrared (FT-IR) spectra were measured on an IRAffinity-1 FT-IR spectrometer using KBr pellets. The solutions of metal ions were prepared from LiCl, NaCl, KCl, MgCl2·6H2O, CaCl2, CrCl3·6H2O, Mn(ClO4)2·6H2O, Fe(ClO4)2·xH2O, Co(ClO4)2·6H2O, Ni(ClO4)2· 6H2O, Cu(ClO4)2·6H2O, Cd(ClO4)2·H2O, Zn(ClO4)2·6H2O, Al(ClO4)3·9H2O, Pb(ClO4)2·3H2O, and Hg(ClO4)2·3H2O, respectively. UV−vis absorption spectra were recorded by a spectrophotometer UV-2600 and fluorescence spectra were recorded on a FS5 fluorescence spectrophotometer, with a quartz cuvette (path length =1 cm). 1H NMR spectra were obtained using a Bruker Avance III 400 MHz spectrometer. DLS (dynamic light scattering) results were obtained on Brookhaven Zeta Plus Zeta Potential Analyzer. TEM images were obtained on JEM-2100F TEM. Mass spectra (ESI) were obtained on AB SCIEX Triple TOF 5600+ mass spectrometer. Synthesis of Chemosensor HL (C29H22N2O). A mixture of triphenylmethylamine (0.0518 g, 0.2 mmol), 8-hydroxyquinoline-2carboxaldehyde (0.0346 g, 0.2 mmol) and 2 mL ethanol in a closed 25 mL Teflon-lined autoclave was heated at 80 ◦C for 24h and cooled to room temperature. The solution was evaporated at room temperature. After 2−3 days, reddish brown block-shaped crystals of HL were obtained and filtered, then washed with ethanol, and air-dried in a yield (0.0540 g, 62.5%). 1H NMR (400 MHz, DMSO-d6) (Figure S1 of the Supporting Information): δ = 9.91 (s, 1H), 8.45 (s, 2H), 8.11 (s, 1H), 7.46 (m, 2H), 7.38 (d, J = 4.0 Hz, 6H), 7.33 (d, J = 4.0 Hz, 3H), 7.24 (d, J = 4.0 Hz, 6H), 7.11 (d, J = 4.0 Hz, 1H) ppm. Anal. Calcd. for C29H22N2O: C, 84.03; N, 6.76; H, 5.35. Found: C, 83.88; N, 6.73; H, 5.40. IR (KBr pellet, cm−1): 3428 vs, 3055 w, 1647 m, 1628 m, 1595 w, 1506 s, 1489 s, 1445 s, 1385 m, 1252 m, 1233 m, 1198 m, 1084 w, 841 s, 748 vs, 704 vs, 557 w. Synthesis of L2 (8-Methoxy-quinolin-2-ylmethylene)-tritylamine, C30H24N2O). A mixture of triphenylmethylamine (0.0207 g, 0.08 mmol), 8-methoxyquinoline-2-carboxaldehyde (0.0150 g, 0.08 mmol) and 2 mL isopropanol in a closed 25 mL Teflon-lined autoclave was heated at 80 ◦C for 24 h and cooled to room temperature. Yellow plate-shaped crystals of L2 were obtained and filtered, then washed with isopropanol, and air-dried in a yield (0.0182 g, 51.0%). 1H NMR (400 MHz, DMSO-d6) (Figure S2): δ = 8.47 (s, 2H), 7.93 (s, 1H), 7.57 (m, 2H), 7.38 (m, 6H), 7.33 (m, 3H), 7.25 (m, 7H), 3.93 (s, 3H) ppm. Anal. Calcd for C30H24N2O: C, 84.08; N, 6.54; H, 5.65. Found: C, 83.93; N, 6.57; H, 5.63. IR (KBr pellet, cm−1): 3442 vs, 1641 s, 1612 m, 1563 m, 1470 s, 1445 w, 1384 vs, 1372 m, 1264 m, 1251 m, 1102 vs, 1002 w, 836 m, 804 w, 762 s, 750 s, 699 vs, 634 m, 544 w. L2 ESI-MS (m/z) (Figure S3): found 429.1965, [L2+H]+, calcd. 429.1961). Synthesis of the Complex ZnL2 (C58H42N4O2Zn). A mixture of triphenylmethylamine (0.0518 g, 0.2 mmol), 8-hydroxyquinoline-2carboxaldehyde (0.0346 g, 0.2 mmol), Zn(Ac)2·2H2O (0.0220 g, 0.1 mmol) and 5 mL isopropanol in a closed 25 mL Teflon-lined autoclave was heated at 75 °C for 72h and cooled to room temperature. Orange rod-shaped crystals of the Zn(II) complex were picked out and washed with isopropanol, and air-dried in a yield (0.0235 g, 21.7%). Anal. Calcd for C58H42N4O2Zn: C, 78.06; N, 6.28; H; 4.74. Found: C, 77.90; N, 6.22; H, 4.79. IR (KBr pellet, cm−1): 3431 vs, 1597 s, 1560 s, 1431 m, 1375 m, 1342 w, 1277 w, 1107 m, 746 m, 698 m, 527 w. Structural Determination. Single-crystal data were collected on a Bruker APEX IICCD diffractometer with graphite monochromated Mo−Kα radiation (λ) at 293 K. The structure was solved by the direct

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RESULTS AND DISCUSSION Aggregation-Induced Emission Enhancement. Both HL and its Zn(II) complex (ZnL2) are soluble in THF and insoluble in H2O. HL is very weakly fluorescent in THF and in the solid state. The crystal structure of HL shows that there is an intramolecular OH···N hydrogen bond between the hydroxyl group and the nitrogen atom from the quinolone ring, forming a five-numbered ring in HL. As shown in Scheme 1, Scheme 1. Enol Form (HL) and the Zwitterionic Form (HL′)

the hydrogen transfer from OH to the N atom of quinolone ring may occur when excited, i.e., the excited-state intramolecular proton transfer takes place. Thus, no significant fluorescence emission could be observed. To further explain the fluorescent property of HL, simple modification of OH group in HL to OCH3 group was carried out and compound L2 was obtained. L2 is fluorescent in THF and in the solid state, as shown in Figures S5 and S6. Compared with the fluorescence intensity of HL, that of L2 is much stronger. The stronger fluorescence emission of L2 could be attributed to the restriction of ESIPT. When Zn2+ was added to HL in THF, the solution was fluorescent, as shown in Figure S7. Similarly, ZnL2 is fluorescent in the solid state (Figure S8). This is because Zn2+ is coordinated with HL to form Zn(II) complex and the hydroxyl proton is removed. The coordination of Zn2+ with HL inhibits the ESIPT process.17 HL is an ESIPT-active compound. The fluorescence spectra of complex ZnL2 in THF/ H2O mixture with different water fractions were determined. As shown in Figure 1, we can see the fluorescence intensity of ZnL2 enhanced obviously in THF/H2O (HEPES buffer B

DOI: 10.1021/acs.inorgchem.6b02784 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Emission spectra of ZnL2 (20 μM) in THF/H2O (HEPES buffer solution, 20 mM, pH = 7.4) with different H2O fractions and (b) emission intensity of ZnL2 (20 μM) at 580 nm in THF/H2O (HEPES buffer solution, 20 mM, pH = 7.4). λex = 440 nm.

75% the aggregates further became smaller and the emission intensity decreased. This could be supposed to “concentration quenching” because the aggregates with smaller size indicate the increase of the “concentration” in THF/H2O and the increase of particles collisions as well, which results in fluorescence emission weakening. To date, the most reported AIE/AIEE effects are based on the restriction of the intramolecular rotation (RIR) mechanism. But HL has no AIE or AIEE phenomenon in THF/H2O mixtures although HL has three phenyl groups, as shown in Figure S4. In the ZnL2 complex, there is one ZnL2 molecule in unit cell, as shown in Figure 4a. The two HL molecules are arranged in head-to-tail mode via the coordination bonds between the hydroxyl oxygen atom and quinoline nitrogen atom with Zn2+ to form ZnL2. In addition, as shown in Figure 4b, the distance between the parallel quinoline rings in ZnL2 molecules is 3.5629 Å, which suggests that intermolecular π−π interactions exist in ZnL2, resulting in further enhancement of the head-to-tail stacking in the solid state of ZnL2. A high degree of ZnL2 molecules ordering in J-mode stacking formed. Since THF is a good solvent for ZnL2 while ZnL2 is insoluble in water. J-aggregates are formed in THF/H2O mixtures. The Jaggregates are highly emissive, not only due to high radioactive rates but also due to moderately slow excitation diffusion caused by rather weak J-type exciton coupling.18 Thus, the fluorescence emission was enhanced. The UV−vis absorptions of the Zn2+ complex in THF/H2O mixtures with different H2O % were also measured. As shown in Figure S10, ZnL2 in THF/ H2O has an absorption band at 269 nm when H2O was below 60%. This absorption band is assigned to π−π* transition. As H2O% continued to increase, the band at 269 nm was redshifted to 297 nm. A high degree of ZnL2 molecules ordering in J-aggregates results in the appearance of an excitonic narrow absorption band, which is bathochromatically shifted with respect to a monomer band in the solution.19 Theoretical Exploration of Excited-State Intramolecular Proton Transfer in HL and Fluorescence Property of ZnL2. In order to explore the probable ESIPT of HL → HL′ (Scheme 1), theoretical studies were carried out. The optimized Cartesian coordinates of the species, and the total energies, free energies, and frequencies for the stationary points are listed in Tables S3 and S4, respectively. The probable reaction pathway and relative energy is schematically described in Figure 5. In the ground state, the hydrogen transfer from OH group to N atom in quinoline ring is an endothermic reaction with increasing energy of 15.9 kcal/mol, indicating that there exists in the form of HL not HL′ in general. The obtained transition state is ca. 20.2 kcal/mol above HL, which seems that it could be

solution, 20 mM, pH = 7.4) mixture. As H2O% increased, the fluorescence intensity of ZnL2 in the THF/H2O mixture changed in 3 stages. When H2O% was less than 60%, the fluorescence intensity was nearly unchanged. As H2O% was above 60%, the aqueous mixture gradually turned to be turbid and the aggregates were formed. The fluorescence intensity of ZnL2 in THF/H2O mixture increased by 178 folds from water fraction 60% to 75%. Correspondingly, the quantum yield reached 32.8%, as tabulated in Table 1. The ΦF value of ZnL2 Table 1. Quantum Yields of ZnL2 in THF/H2O (HEPES Buffer Solution, 20 mM, pH = 7.4)a

a

H2O%

0%

65%

70%

75%

ΦF(%) of ZnL2 τ of ZnL2 (10−9 s) kr of ZnL2 (107 s−1) knr of ZnL2 (107 s−1)

2.9 1.95 1.49 49.85

5.3 10.93 0.48 8.66

32.5 11.63 2.80 5.81

32.8 11.66 2.81 5.77

Quantum yields were determined by a calibrated integrating sphere.

in THF is obviously lower than that of ZnL2 in THF/H2O mixture. To further explain fluorescence property of ZnL2 in THF/H2O, the fluorescence lifetimes of the ZnL2 complex in THF and THF/H2O mixture were measured (Figure S9), the lifetime values were listed in Table 1. We found that as the water fraction increased, the lifetime of ZnL2 increased. The radiative rate constant kr (kr = ΦF/τ) and nonradiative rate constant knr [knr = (1 − ΦF)/τ] of ZnL2 in THF and THF/ H2O mixture were also calculated (Table 1). Obviously the knr of ZnL2 in THF is larger than that of ZnL2 in THF/H2O mixture. This indicates that lower ΦF of ZnL2 in THF can be attributed to larger nonradiative decay. As a whole, the fluorescence intensity of ZnL2 in THF/H2O increased when H2O% was above 60%. However, the emission intensity of ZnL2 in THF/H2O decreased significantly when H2O was above 75%. The aggregates of ZnL2 formed in THF/H2O mixtures can be directly confirmed by DLS and TEM. Aggregates were rarely found under the diluted THF/H2O of ZnL2 below 60% H2O. While when H2O% reached 70%, the mixture obviously turned to be a suspension and 500−700 nm particles were confirmed by DLS and TEM (Figures 2 and 3), and the fluorescence intensity of ZnL2 in THF/H2O mixture was enhanced greatly at the same time. The size of ZnL2 aggregates decreased as the water fraction increased (Table S5), and this regularity can be further affirmed by TEM images (Figure 3). This may arise from rapid precipitation of the Zn2+ complex in THF/H2O mixture as H2O % increases. Once the water fraction was above C

DOI: 10.1021/acs.inorgchem.6b02784 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Particle size distribution of ZnL2 (20 μM) in THF/H2O (HEPES buffer solution, 20 mM, pH = 7.4): (a) 70% H2O, (b) 75% H2O, (c) 80% H2O, and (d) 90% H2O.

1011 s−1, much larger than the generate rate of fluorescence (in general 107 to 109). Once the geometric relaxation to HL′(ex) is finished, the reaction might be stop here since the HOMO and LUMO of HL′ are not matched. The calculated wavelength and oscillator strength are 1184 nm and 0.017, respectively, indicating that no fluorescence could be observed. The coordination of Zn2+ with two molecules of HL would form complex ZnL2, in which the ESIPT process is prohibited due to the absence of hydrogen atom in OH or in NH. The calculated wavelength and oscillator strength for ZnL2 are 678 nm and 0.029, respectively, The calculated wavelength and oscillator strength for dimer and trimer of ZnL2 are 678 nm and 0.031, 674 nm and 0.032, indicating that a high degree of HL molecules ordering stacking via coordinated bonds and intermolecular π−π interactions would slightly increase the oscillator strength, but be blue-shifted slightly. Fluorescence Response of HL to Cations. Considering the fluorescence properties of HL and ZnL2 in THF/H2O, we tried to utilize HL to detect Zn(II) ion in aqueous medium and found that HL has response to Zn2+. To explore the selectivity of chemosensor HL, HL was mixed with various metal cations, Li+, Na+, K+, Ca2+, Al3+, Cr3+, Cd2+, Co2+, Cu2+, Fe2+, Hg2+, Mg2+, Mn2+, Ni2+, Pb2+, and Zn2+ in THF/H2O (1:1, v/v, HEPES buffer solution, 20 mM, pH = 7.4), respectively. The compound HL in THF/H2O showed weak fluorescence upon excitation at 440 nm. While only upon addition of Zn2+ to HL

Figure 3. . TEM images of ZnL2 in THF/H2O (HEPES buffer solution, 20 mM, pH = 7.4): (a) 70% H2O and (b) 80% H2O.

overcome under room temperature. However, the reverse process is much easier with only 4.3 kcal/mol activation energy. Once HL absorbs photon(366 nm), it could be excited to an excited state with 78.2 kcal/mol in energy above the ground state, and then HL could be relaxed into HL(ex) with releasing 7.3 kcal/mol energy. From HL(ex), there are two competition pathways to proceed, one is fluorescence emission to return to ground state; another is to take place hydrogen transfer reaction of HL(ex) →HL′(ex) (ESIPT). The exact location of such transition state in excited state with TD-DFT method is very difficult, however, it should be only 1−2 kcal/mol of activation barrier based on the estimation from the excitation energies along the ground Intrinsic Reaction Coordinate (IRC). Such low energy barrier would prevent the fluorescence emission since the rate of ESIPT is estimated to be ca. 2.1 × D

DOI: 10.1021/acs.inorgchem.6b02784 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Molecular structure with an atom labeling for the complex ZnL2 at the 50% probability level (a) and stacking of ZnL2 in the solid state (b).

Figure 6. Bar diagram of fluorescence emission intensity of HL (20 μM) in the presence of 1.0 equiv of Al3+, Zn2+, Cd2+, Co2+, Ca2+, Cr3+, Mn2+, Ni2+, Fe2+, Ca2+, Cu2+, Pb2+, Li+, Na+, K+, and Mg2+ in THF/ H2O (1:1, v/v, HEPES buffer solution, 20 mM, pH = 7.4) at 630 nm, respectively. λex = 440 nm.

based on the standard deviation and linear fitting (3σ/slop) (Figure S12).20 To investigate the binding stoichiometry between HL and Zn2+, the Job’s plot was also carried out (Figure S13), which was based on the changes of fluorescence emission at 630 nm. The result showed that the binding stoichiometry of HL with Zn2+ was 2:1. ESI mass spectrum was also measured (Figure S14) to further illustrate the coordination of HL with Zn2+. The mass spectrum exhibits that a peak at m/z = 891.2671 corresponds to [ZnL2]H+ (calcd m/z 891.2672). This can

Figure 5. Energy profile of the hydrogen-transfer reaction of HL → HL′ in ground state and excited state.

the fluorescence emission was obviously enhanced (Figure 6), which is attributed to both the inhibition of the excited state intramolecular proton transfer (ESIPT) and AIEE effect. In the fluorescence titration profiles (Figure S11), the emission intensity at 630 nm was gradually increased with the increasing of Zn2+. The detection limit was determined to be 8.3 × 10−7 M E

DOI: 10.1021/acs.inorgchem.6b02784 Inorg. Chem. XXXX, XXX, XXX−XXX

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further prove the 2:1 binding stoichiometry between HL and Zn2+. In addition, from Figure S15, we can see that the fluorescence spectrum of HL upon addition of 0.5 equiv. Zn2+ in the detection system is in agreement with that of ZnL2. These results show that HL coordinates with Zn2+ in a 2:1 stoichiometry. The association constant (Ka) of the ZnL2 complex was determined to be 2.23 × 108 M−2 based on the eq 121 (Figure S16). (F − F0)2 /[(F1 − F0)(F1 − F)] = 1/[2K aCF(M)]

Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02784. Comprehensive experimental details, information on Xray crystallographic data, Figure S1−S17, and Tables S1−S5 (PDF) X-ray crystallographic data of compound HL and complex ZnL2 (CCDC nos. 1491860−1491861) (CIF)

(1)

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Other metal ions have no significant influence on the selective recognition of HL toward Zn2+ except Cu2+ and Cr3+ (Figure S17). Cell Imaging. To prove the ability of HL to detect Zn2+ in cells, bioimaging experiments were also carried out. SH-SY5Y cells were first incubated with 20 μM HL for 16 h, and then treated with 20 μM Zn(ClO4)2 for 30 min. As shown in Figure 7, fluorescence could not be observed when cells were only

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-J.Z.). *E-mail: [email protected] (D.-C.F.). ORCID

Xiang-Jun Zheng: 0000-0002-7720-9000 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21671022). REFERENCES

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Figure 7. Cell imaging of SH-SY5Y cells treated with HL (A) before and (B) after incubation with Zn(ClO4)2. Parts (a) and (d) represent the bright-field images, (b) and (e) represent the fluorescence images, and (c) and (f) represent the overlay images, λex = 405 nm.

exposed to HL. While after SH-SY5Y cells were treated with 20 μM Zn(ClO4)2 for 30 min, fluorescence could be observed. These results indicate that HL is cell-permeable and can be used to recognize intracellular Zn2+ ions. AIEE of the Zn2+ complex formed in the response system is of benefit to the detection of Zn(II) ions in cells.

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CONCLUSIONS In summary, the Schiff base HL and its Zn(II) complex (ZnL2) were synthesized, and their structures were determined. The intermolecular hydrogen bond OH···N in HL and a high degree of ZnL2 molecules ordering stacking in ZnL2 are of great impact on fluorescence properties of HL and ZnL 2 , respectively. The experimental results and theoretical studies allowed us to ascribe HL as an ESIPT compound and HL probes Zn2+ to form the Zn(II) complex (ZnL2). The structure of ZnL2 shows that the ligands (L−) are in head-to-tail arrangement via coordination bonds ZnO and ZnN. The J-aggregates for L− in ZnL2 are formed by coordinationdirected stacking and the intermolecular π−π interactions in THF/H2O mixture, resulting in the fluorescence emission enhancement. This suggests design strategy of ESIPT-coupled AIE/AIEE chemosensors for metal ions and their applications in bioimaging to detect intracellular metal ions. F

DOI: 10.1021/acs.inorgchem.6b02784 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02784 Inorg. Chem. XXXX, XXX, XXX−XXX