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Aug 26, 2016 - ABSTRACT: We report a class of multiresponsive colorimetric and fluorescent pH probes based on three different reaction mechanisms ...
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A Class of Multiresponsive Colorimetric and Fluorescent pH Probes via Three Different Reaction Mechanisms of Salen Complexes: A Selective and Accurate pH Measurement Jinghui Cheng, Fei Gou, Xiaohong Zhang, Guangyu Shen, Xiangge Zhou, and Haifeng Xiang* College of Chemistry, Sichuan University, Chengdu 610041, China S Supporting Information *

ABSTRACT: We report a class of multiresponsive colorimetric and fluorescent pH probes based on three different reaction mechanisms including cation exchange, protonation, and hydrolysis reaction of K(I), Ca(II), Zn(II), Cu(II), Al(III), and Pd(II) Salen complexes. Compared with traditional pure organic pH probes, these complex-based pH probes exhibited a much better selectivity due to the shielding function of the filled-in metal ion in the complex. Their pH sensing performances were affected by the ligand structure and the central metal ion. This work is the first report of “off−on−on′− off” colorimetric and fluorescent pH probes that possess three different reaction mechanisms and should inspire the design of multiple-responsive probes for important analytes in biological systems.

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INTRODUCTION Since many biological and geochemical processes that occur in freshwater, seawater, and marine sediments are often related to pH changes, protons have been becoming some of the most important targets of interest. Moreover, intracellular pH, which has a vital role in cell biology, can regulate and control the structure and function of protein. Some organelles including lysosomes and mitochondria depend on proton gradients to function properly.1 Traditionally, pH is measured by a glass electrode, but it is a bulky, invasive, and single-point measurement, which would limit its bioapplications.2 Since fluorescence method3,4 offers some unique advantages, such as high sensitivity, good selectivity, short response time, real-time and parallel monitoring, cell imaging, and in situ and noninvasive measurement, the design and synthesis of organic fluorescent pH probes1 have been attracting more and more attention recently. General fluorescent pH probes possess a pH-sensitive recognition moiety, such as −OH,2,5 −SH,5a −NR2,6 NR,2,6a,7 −CO2H,8 and −SO3H,9 and a fluorophore moiety (Figure 1a). The fluorophore moiety, as a signal transducer, converts the recognition information into a fluorescence signal change through charge transfer (CT), photoinduced electron transfer (PET), monomer−excimer, or electronic energy transfer (EET) mechanism.1 Furthermore, spirolactam ring-opening process (Figure 1b) is also widely used to measure pH.4b,10 An ideal fluorescent pH probe must satisfy some basic requirements, such as good selectivity, high sensitivity, good photostability, high fluorescence quantum yield (Φ), good biocompatibility (hydrosoluble and low toxic), appropriate © XXXX American Chemical Society

Figure 1. Reported approaches to fluorescent pH probes: (a) general probe structure; (b) spirolactam-opening process for pH sensing.

acid-dissociation constant (pKa), and restricted pH range. The selectivity behavior that is the relative sensing response for H+ over other biological species is one of the most important properties for a pH probe. For a chemist, however, it is very difficult to predict the selectivity behavior, because metal ions acting as Lewis acids would be potential interferents. For example, it is well-known that most of the above recognition moieties and spirolactam ring-opening processes (Figure 1) can react or coordinate with many metal ions.3,4,6b,10a Therefore, a lot of tailored pH probes with different recognition and fluorophore moieties must be prepared. In addition, most of the Received: May 18, 2016

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DOI: 10.1021/acs.inorgchem.6b01212 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry fluorescent pH probes containing a single pH-sensitive recognition moiety exhibit one step-type “on−off” fluorescence response under different pH values (Figure 2a). This would

advantages, including facile preparation, reasonable stabilities, rich photophysical properties, and biological activities, have wide applications in catalysts, 12 magnetic and optical materials,13 supramolecular materials,14 and bioapplications.15 Our research group explores a major topic in the design and synthesis of the Salen-based fluorescent probes.16 For the purpose of detecting pH,16c they have many advantages, such as strong and long-wavelength absorption, high fluorescence quantum yield, and multiple pH-sensitive moieties (−OH, NC, −NEt2, and −SO3H).16a,b Furthermore, CN bonds in Salen ligands would be decomposed through hydrolysis in the presence of excess acid.16c The biggest problem, however, is their poor selectivity, because many metal ions can be chelated by Salen ligands to yield stable complexes. For example, the fluorescence of Salen ligands would be enhanced by the chelating of Mg2+, Al3+, and Zn2+ but quenched by the chelating of Cu2+, Co2+, Pd2+, and Ni2+.16c,d Our recent work16d demonstrated that Salen complexes via transmetalation reactions17 have a much better selectivity performance for the detection of Al3+ than the corresponding free Salen ligands, because there is no open coordination site in the Salen complex, which thus forces a transmetalation reaction to occur before it can be utilized to sense an exogenous metal (the shielding function of the filled-in metal ion in the Salen complexes).4h To solve the problem of selectivity and increase sensing mechanisms of pH probes, in the present work we reported a new class of “off−on−on′−off” colorimetric and fluorescent pH probes based on a series of K(I), Ca(II), Zn(II), Cu(II), Al(III), and Pd(II) Salen complexes in an organic solvent (MeCN) and aqueous solvent. For these complexbased pH probes, their selectivity performances and fluorescence responses could be well-tuned by the identity of the central metal ion. To the best of our knowledge, this is the first example of fluorescent pH probes that measure pH through three different reaction mechanisms including cation exchange, protonation, and hydrolysis reaction.

Figure 2. Different fluorescence responses of fluorescent pH probes (a, b) reported and (c) this work.

limit the precise pH measurement in a much narrower region and be harmful for rapid qualitative visual sensing. Furthermore, it is in urgent need to develop window-shaped “off−on− off” fluorescent pH probes (Figure 2b) containing multiple pHsensitive recognition moieties, as many bioprocesses, such as cell metabolism and apoptosis, only work in an ultranarrow pH range, where their function or activity is accurately controlled as being “turn on” or “turn off” by a small pH change. Nonetheless, designing and building the system of off−on− off fluorescent pH probes is complicated and unexpected. Until now, thess kinds of probes are much scarcer,8b,11 because a fluorescent pH probe usually would tend to show only one “off−on” fluorescence response under different pH values even though it has multiple pH-sensitive recognition moieties.5,6c,d,7,10 Salen, N,N′-bis(salicylidene)ethylenediamine, is a particular class of tetradentate [O^N^N^O] chelating bis-Schiff base (Figure 3), which can be prepared by the condensation of salicylaldehyde and diamine. Salen compounds with many

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RESULTS AND DISCUSSION Synthesis and Characterization. Salen compounds are inexpensive and are facile to prepare and tune in terms of fluorescence bands,16 compared with other well-known robust and expensive tetradentate porphyrin compounds.4g,18 Moreover, Salen compounds with reasonable stabilities might be more suitable to detect pH through multiple reaction

Figure 3. Sensing mechanisms (a) and chemical structures (b) of Salen complexes in the present work. B

DOI: 10.1021/acs.inorgchem.6b01212 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Optical Data of the Salen Compounds at Room Temperature λabs, nm (ε, dm3 mol−1 cm−1)

medium L1 L2 L3 Al-L1 Cu-L1 Ca-L1 K-L1 Zn-L1 Pd-L1 Cu-L2 Zn-L2 Cu-L3 Zn-L3 a

MeCN MeCN H2O MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN H2O H2O

374 403 318 390 381 342 386 386 382 310 331 304 234

(1.12 (2.18 (1.20 (1.56 (1.48 (1.01 (1.29 (1.47 (4.54 (3.15 (1.16 (9.64 (1.56

× × × × × × × × × × × × ×

4

10 ); 104) 104); 104); 103); 103); 103); 104); 103); 104); 104); 103); 104);

433 (1.01 × 10 ); 564 (4.63 × 10 ) 4

377 438 434 420 436 436 436 405 409 349 312

4

(7.81 × 103) (6.49 × 103); 570 (1.54 × 104) (7.53 × 103); 571 (1.73 × 104) (4.07 × 103); 560 (1.33 × 104) (8.00 × 103); 556 (1.02 × 104) (7.1 × 103); 582 (1.87 × 104) (3.13 × 103); 578 (1.13 × 104) (1.1 × 104) (3.46 × 104); 431 (3.5 × 104) (1.27 × 104) (1.15 × 104)

λem, nm

Φ

606 501 436 615 b 620 616 615 b b 508 b 433

0.80 0.34 0.45 0.69 0.65 0.63 0.67

0.35 0.49

pKa

3.19 5.88; 3.30 5.92; 6.30; 5.06; 5.43; 6.12; 5.42; 3.77

4.11 4.43 4.15 4.08, 3.69 3.36 3.84

on/offa

160 1082 126 112 153 82 205 96 56 41

On: maximum emission intensity; off: minimum emission intensity. bNonemissive.

mechanisms. For the purpose of examining different sensing performances, three types of Salen ligands, namely, N,N′-bis(4(diethylamino)-2-hydroxybenzylidene)dicyano-1,2-ethenediamine (L1), N,N′-bis(salicylidene)-phenazine-2,3-diamine (L2), and N,N′-bis(5-sulfonatosalicylidene)-1,2-phenylenediamine disodium salt (L3; Figure 3), were synthesized according to our previous reports.16 Direct complexation between the corresponding metal ion salt and ligand or metal ion templated method could be used to prepare their K(I), Ca(II), Zn(II), Al(III), Cu(II), and Pd(II) complexes. The optical data of all the Salen compounds are listed in Table 1 and shown in Figures S1−S3 in the Supporting Information. L2 emitted strong green-yellow fluorescence (λem = 501 nm, Φ = 0.34) in MeCN. L3 having the sulfonate groups showed a good solubility and strong blue fluorescence (λem = 436 nm, Φ = 0.45) in water.16a Remarkably, L1 with a donor (−NEt2)−acceptor (−CN) (DA) system exhibited strong absorption (λabs = 564 nm) and saturated red fluorescence (λem = 606 nm, Φ = 0.80) in MeCN.16b,c Except Pd(II) and Cu(II) Salen complexes, all other Salen complexes exhibited similar strong fluorescence with the corresponding Salen ligands. Cu(II) Salen complexes were nonemissive, due to the fluorescence quenching effect of paramagnetic Cu2+ center. The origin of emission from Pd-L1 is not fluorescence but phosphorescence, due to spin−orbit coupling of heavy Pd element. Pd-L1 had weak near-infrared phosphorescence in degassed solution19 and was almost nonemissive in oxygenstatured MeCN. pH Probes Based on M-L1. The pH sensing application of L1 was reported in our previous work.16c M-L1 could be acting as pH probes as well, because their absorbance and fluorescence intensity highly depend on the pH values of the solution (Figures 4−6 and S4−S8). Under strong acid condition (pH < 2.0 in MeCN), all M-L1 complexes were colorless and nonemissive to form the first “off” platform (the first step of hydrolysis reaction), and then their emission intensities would increase to form the second off−on platform (the second step of protonation reaction) along with the increase of pH values (Figure 6). The pH range of the second platform was 3.3−3.7, 3.7−4.0, 3.3−3.7, 2.2−2.8, and 3.2−3.6 for K-L1, Zn-L1, CuL1, Al-L1, and Pd-L1, respectively. For Ca-L1, however, its first and second platforms might be overlapped. If pH values were further increased, the emission intensities would increase again to form the third off−on−on′ platform (the third step of cation exchange reaction). The pH range of the third platform was

Figure 4. Images of M-L1 (1.0 × 10−5 mol dm−3 in MeCN) under different pH values (top) under sunlight and (bottom) under 360 nm UV light.

5.0−5.6, >4.0, 4.2−6.1, 4.6−5.4, >4.2, and >4.2 for K-L1, CaL1, Zn-L1, Cu-L1, Al-L1, and Pd-L1, respectively. The emission intensities of Ca-L1 and Al-L1 remained constant under neutral and alkaline conditions, but for K-L1, Zn-L1, PdL1, and Cu-L1, their emission intensities reduced ∼18%, 70%, 98.8%, and 99.9%, respectively, and formed the fourth off−on− on′−off platform (the complex itself). The pH range of the fourth platform was >6.4, >6.6, >5.3, and >6.4 for K-L1, Zn-L1, Cu-L1, and Pd-L1, respectively. Since these M-L1 pH probes showed an obvious colorful change in the full region of pH values, they could be used in both the ratiometric detection and naked-eye rapid visual sensing. Moreover, L1 ligand and M-L1 complexes containing a DA system had intensive absorption (λabs = 556−582 nm) and saturated red fluorescence (λem = 606−620 nm, Φ up to 0.80). Therefore, they are advantageous for bioapplications due to the absence of a high-energy UV, blue, or green excitation and less absorption and autofluorescence from proteins. In particular, Cu-L1 possessed a very narrow fluorescence-on window (pH: 4.6−5.4) and a high ratio between turn-on and turn-off fluorescence over 1000, which C

DOI: 10.1021/acs.inorgchem.6b01212 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Absorption (top) and emission (bottom) spectra of Cu-L1 in MeCN under different pH values.

Figure 6. Normalized emission intensities (K-L1: λem = 616 nm, Ca-L1: λem = 620 nm, Zn-L1: λem = 615 nm, Al-L1: λem = 616 nm, Pd-L1: λem = 606 nm; and Cu-L1: λem = 606 nm) vs different pH values (in MeCN).

Figure 7. Normalized emission intensities (Zn-L2: λem = 508 nm and Cu-L2: λem = 501 nm in MeCN; Zn-L3: λem = 433 nm and Cu-L3: λem = 436 nm in B−R buffer solution) vs pH values.

D

DOI: 10.1021/acs.inorgchem.6b01212 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. 1H NMR spectra of Zn-L1 under different pH values in CD3CN.

atoms (NC) were protonated (pKa = 4.15). The 1H NMR signals of NCH− showed a little change. The 1H NMR signals of −OH, however, were shifted from δ = 12.8 into 10.1, accompanyied with a remarkable change from a small broad peak into a sharp peak. These obvious changes might be reasonably attributed to the fact that the intramolecular hydrogen bonds16 (−OH···N; Figure 8) would be destroyed after the protonation of N atoms. This second step of protonation also leaded to a color change from pink into pale pink and emission decrease (Figures 4 and 5). When HCl was added excessively (pH ≈ 2.0), the CN bonds in Salen ligand were decomposed through hydrolysis (the first step), resulting in a rise in the 1H NMR signals of salicylaldehyde,5c color disappearing, and fluorescence quenching. However, the −NEt2 moieties were found to be not pH-sensitive in our case. The first step of hydrolysis was irreversible, but both the second step of protonation and the third step of cation exchange were reversible. As depicted in Figure 9, the fluorescence intensity of Zn-L1 at pH = 5.5 was much higher than that at high pH = 7.0. Switching was highly reversible over at least 10 cycles, with negligible change in emission intensities. The response times of switch were moderately fast (10−30 min; see Figures S13 and S14) for these Salen complexes. For all Salen complexes, the hydrolysis of Salen ligand proceeded in strong acid condition (pH < 3.8). For Ca-L1, ML2, and M-L3, however, the second step of protonation did not appear in their pH titration curves. One possible reason was that their pKa values (4.1−3.5) of the second step of

would help to accurately and sensitively measure pH in a restricted range. pH Probes Based on M-L2 and M-L3. In MeCN, Zn-L2 and Cu-L2 (Figures 7, S9, and S10) had similar pH-sensing performances with the corresponding M-L1 complexes, except that Zn-L2 and Cu-L2 did not have the second platform in their pH titration curves. Since the preferred solvent of metal ion and pH probes should be water, L3 and complexes Zn-L3 and Cu-L3 with sulfonate groups were used to detect pH values in pure water. These sulfonate groups enabled not only good solubility in water but also little change in their optical properties.16a Expectedly, Zn-L3 and Cu-L3 had good pHsensing performances in the pure aqueous Britton−Robinson (B−R) buffer solution (a mixture of 0.04 mol/L H3BO3, H3PO4, and CH3COOH in water; Figures 7, S11, and S12). Possible Sensing Mechanisms. On the basis of the above absorption and fluorescence analysis (Figures 4 and 5) of M-L1 complexes, there might be three different reaction mechanisms under different pH values. And thus 1H nuclear magnetic resonance (1H NMR) analysis was used to investigate the possible reaction mechanisms. Since Cu-L1 did not have 1H NMR signal in CD3CN, Zn-L1 was examined. As depicted in Figure 8, the 1H NMR signals (δ = 12.8 ppm) belonging to the −OH appeared after adding HCl (pH ≈ 5.0), which indicated that Zn2+ in Zn-L1 was exchanged by H+ and was confirmed by the fact of fluorescence enhancement. This cation exchange reaction (pKa = 6.30) was the third step of the pH titration curve (Figure 6). If HCl was further added (pH ≈ 3.9), N E

DOI: 10.1021/acs.inorgchem.6b01212 Inorg. Chem. XXXX, XXX, XXX−XXX

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divided into two types: one for cytosol (pH = 6.8−7.4) and the other for acidic organelles (pH = 4.5−6.0), including lysosomes and endosomes.1 The pKa values (Table 1) of the second step of protonation and the third step of cation exchange for these Salen complexes were in the ranges of 3.19−4.43 and 5.06− 6.30, respectively. In particular, Cu-L1 (pH: 4.6−5.4), Cu-L2 (pH: 4.2−4.8), and Cu-L3 (pH: 4.4−5.1) exhibited a very narrow fluorescence-on window and a high ratio between turnon and turn-off fluorescence, indicating that they might have potential applications in imaging acidic organelles of a cell. Interference of General Metal Ions. For a good chemical probe, one of the basic requirements is its selectivity behavior. Since the conditions of intracellular and other environments are very complicated, an additional experiment on the interference of metal ions was tested. As expected, free Salen ligands acting as metal ion chelating agents exhibited a very poor selectivity for pH sensing.16b,d For the purpose of testing the selectivity of Salen complexes (Figure 10), 13 kinds of metal ions (Na+, Ag+, Ca2+, Zn2+, Mg2+, Pb2+, Mn2+, Cu2+, Ni2+, Cd2+, Pd2+, Fe3+, and Al3+) that probably interfered with pH detection were performed under the similar conditions: the concentrations of Salen complexes were all kept at 1.0 × 10−5 mol dm−3, and 2

Figure 9. pH cycling (emission intensity at 615 nm) of Zn-L1 1.0 × 10−5 mol dm−3 in MeCN upon alternate addition of NaOH and HCl.

protonation were a bit small, which might lead to an overlap of the hydrolysis and protonation. The third step of cation exchange of Al-L1, Ca-L1, and Zn-L3 was not observed in their pH titration curves, because these complexes had similar fluorescence properties with the resultant free Salen ligands after cation exchange. It is well-known that pH value is one of basic parameters of cellular events. pH probes can generally be

Figure 10. Selectivity of Zn-L1 (emission intensity at 615 nm), Cu-L1 (emission intensity at 606 nm), and Al-L1 (emission intensity at 615 nm) in MeCN (1 × 10−5 mol dm−3) toward 2.0 equiv of other interfered metal ions and/or H+ (pH = 5.0). F

DOI: 10.1021/acs.inorgchem.6b01212 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry equiv of metal ions were added. Zn-L1 showed a poor selectivity in fluorescence measurement. Zn-L1 itself emitted strong red fluorescence (λem = 615 nm). Addition of H+, Ag+, or Al3+ into Zn-L1 solution led to enhanced fluorescence, but addition of some other metal ions, such as Cu2+, Fe3+, Ni2+, and Pd2+, resulted in fluorescence quenching, which revealed that these metal ions might partly or fully replace Zn2+ of Zn-L1, and consequently they would interfere pH sensing. Cu-L1 had a much better selectivity than Zn-L1, since it was nonemissive itself. Only when H+ was added, the fluorescence (λem = 606 nm) turned on, and other metal ions did not interfere pH sensing, except that Al3+ had a small interference. Al-L1, like Zn-L1, itself showed strong red fluorescence (λem = 615 nm). This fluorescence was shifted to λem = 606 nm after adding H+, but all other metal ions exhibited little interference. The above experiments revealed that the selectivity of these Salen complexes was in the order of Al-L1 > Cu-L1 > Zn-L1, which was consistent with the order of the stability constant for complex formation (Ks).16d In theory, Ks can evaluate the stability of a complex. The values of Ks indicate the complexation strength: the higher Ks, the more stable the complex is.4h Al(III) Salen complexes had the lowest interference from other metal ions because of their highest Ks.16d At the same time, competition experiments were also examined under the condition of the presence of both Salen complex and 2 equiv of the other metal ions (pH = 5.0; Figure 10) to explore their practical applicability as H+-selective fluorescent probes. Except that of Zn-L1 solutions, the fluorescence of Al-L1 and Cu-L1 solutions containing H+/ other metal ions exhibited little variations compared with those only containing H+. These results indicated that both Al-L1 and Cu-L1 might have potential applications in pH sensing, because they had the minimum interference from metal ions. pH Test Paper. Since the preferred solvent of a pH probe is water, Cu-L1 was used to detect pH in water by the method of test paper. The homemade test papers were prepared by dipping filter papers into the dimethyl sulfoxide (DMSO) solution of Cu-L1. Under room light and 360 nm UV light, the test papers showed obvious color changes after dipping into water with different OH− (pH = 11), H+ (pH = 5.5), or H+ (pH = 2.5), respectively, as shown in Figure 11.

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CONCLUSIONS

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EXPERIMENTAL SECTION

A series of Salen complexes were synthesized for the examination of their photophysical properties and pH-sensing applications. Some of these complexes exhibited sensitive and selective off−on−on′−off pH fluorescence response through three different reaction mechanisms including cation exchange, protonation, and hydrolysis reaction. Both the chemical structures of Salen ligands and central metal ions had a tremendous impact on their pH-sensing performances. Salen complexes had a much better selectivity than the corresponding free ligands because of the shielding function of the fill-in metal ion in the Salen complexes. This work demonstrated a novel platform for the design of multiple-responsive fluorescent probes.

Materials and Instrumentation. All chemicals were purchased from commercial suppliers and used without further purification. UV/ visible absorption spectra were measured by a UV 765 spectrophotometer (Shanghai Jingke) with quartz cuvettes of 1 cm path length. Fluorescence spectra were measured by an F-7000 Fluorescence spectrophotometer (Hitachi) at room temperature (slit width = 5.0 nm, photon multiplier voltage = 400 V). 1H and 13C NMR spectra were recorded on a Brüker Advance 400 spectrometer (1H: 400 MHz, 13 C: 101 MHz). Chemical shifts (δ) for 1H and 13C NMR spectra are given in parts per million relative to tetramethylsilane (Si(CH3)4). The residual solvent signals were used as references for 1H and 13C NMR spectra, and the chemical shifts were converted to the Si(CH3)4 scale (CDCl3: δH = 7.26 ppm, δC = 77.16 ppm). Elemental analysis was recorded on a Euro EA 3000. All the Salen compounds were synthesized by the reported method.16 Hazards. Caution! For the purpose of safety, students should wear nitrile gloves and goggles at ALL TIMES during the experiment. Chemical reactions should be handled in f ume hoods. The starting materials and Salen compounds might be cancer suspect agents. Do not directly touch with hand; in case of touch, rinse directly. Since aldehyde and amine are toxic, they should be avoided of touching the skin. MeCN and EtOH are f lammable. High-energy UV light is harmf ul to the skin and eyes. Unnecessary exposure of skin and eyes to UV light is not allowed. Measurement of Fluorescence Quantum Yield. Fluorescence quantum yield Φ was measured by the optical dilute method with a standard of quinine sulfate (Φr = 0.55, 0.05 mol dm−3 sulfuric acid) calculated by Φs = Φr(Br/Bs)(ns/nr)2(Ds/Dr), where the subscripts s and r refer to the sample and reference standard solution, respectively; n is the refractive index of the solvents; D is the integrated intensity. The excitation intensity B is calculated by B = 1 − 10−AL, where A is the absorbance at the excitation wavelength and L is the optical path length (L = 1 cm in all cases). The refractive indices of the solvents at room temperature are taken from standard source. Errors for Φ values (±10%) are estimated. Measurement of pH Sensing. pH titration experiment was started with Salen complex of known concentration (6 mL of 1.0 × 10−5 mol dm−3 in MeCN or B−R buffer solution). For the titration and interference experiments, various metal nitrate salts, HCl, or NaOH (1.0 × 10−2 mol dm−3 in water) were added by a microsyringe. For the competition experiments, H+ (pH = 5.0) and one metal ion were added to the solution simultaneously. Absorption and fluorescence measurements were done over 1 h after adding the metal salt, HCl, or NaOH to the Salen complex solutions at room temperature. The pH of MeCN was calculated by the amount of HCl or NaOH. The pH of water (the pure aqueous B−R buffer solution) was measured by a pH meter. Synthesis of Al-L1. A 250 mL Schlenk flask was charged with L1 (420 mg, 1.0 mmol) in 150 mL of CH3CN. After then, 50 mL of CH3CN solution containing AlCl3 (133 mg, 1.0 mmol) was added. The mixture was stirred at 70 °C for 12 h, and then the solvent was removed by reduced-pressure distillation. A green precipitate was

Figure 11. Color changes of homemade Cu-L1 test papers before (pH = 7.0) and after dipping into water with pH values of 11, 5.5, and 2.5 (top) under room light and (bottom) under 360 nm UV light. G

DOI: 10.1021/acs.inorgchem.6b01212 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Synthesis of Zn-L3. H2O (10 mL) containing sodium 3-formyl-4hydroxy-5-chlorobenzenesulfonate (520 mg, 2.0 mmol) was added to 15 mL of MeOH containing o-phenylenediamine (108 mg, 1.0 mmol). After the initial suspension quickly turned into a clear solution, 10 mL of H2O containing Zn(OAc)2·2H2O (219 mg, 1.0 mmol) was added. Instantaneously a suspension was obtained and then was filtered to give the product (60% yield). Anal. Calcd (found): C, 36.80 (36.79); H, 1.54 (1.55); N, 4.29 (4.28)%. 1H NMR (in DMSO-d6) 8.95 (s, 2 H), 7.82 (d, J = 2.4 Hz, 2 H), 7.77−7.80 (m, 2 H), 7.34−7.36 (m, 2 H), 6.78 (d, J = 8.9 Hz, 2 H). 13C NMR (DMSO-d6) 173.11, 163.50, 139.89, 134.41, 134.38, 132.98, 127.83, 123.01, 118.10, 117.03. Synthesis of Cu-L3. H2O (10 mL) containing sodium 3-formyl-4hydroxy-5-chlorobenzenesulfonate (520 mg, 2.0 mmol) was added to 15 mL of MeOH containing o-phenylenediamine (108 mg, 1.0 mmol). After the initial suspension quickly turned into a clear solution, 10 mL of H2O containing Cu(CH3COO)2·H2O (200 mg, 1.0 mmol) was added. Instantaneously a suspension was obtained and then was filtered to give the product (54% yield). Anal. Calcd (found): C, 36.91 (36. 90); H, 1.55 (1.56); N, 4.30 (4.29)%.

observed and purified by water and CH3CN wash (67% yield). Anal. Calcd (found) for C26H28AlClN6O2·H2O: C, 58.15 (58.14); H, 5.63 (5.64); N, 15.65 (15.66)%. Synthesis of Ca-L1. Hot absolute ethanol (50 mL) containing 4(diethylamino)salicylaldehyde (386 mg, 2.0 mmol) and diaminomaleonitrile (108 mg, 1.0 mmol) was added to 150 mL of hot absolute ethanol containing Ca(CH3COO)2 (158 mg, 1.0 mmol) and then was stirred for 12 h at 70 °C. The resulting red mixture was filtered hot and cooled in a freezer to give dark green solid (72% yield). Anal. Calcd (found): C, 62.88 (62.87); H, 5.68 (5.69); N, 16.92 (16.93)%.1HNMR (in CDCl3) 1.16 (t, J = 7.1 Hz, 12H), 3.36 (q, J = 7.1 Hz, 8H), 6.23 (s, 2H), 6.26 (d, J = 2.4 Hz, 2H), 7.13 (d, J = 8.7 Hz, 2H), 8.42 (s, 2H). 13 CNMR (CDCl3) 163.42, 160.95, 152.75, 134.52, 120.88, 111.68, 108.52, 104.74, 98.96, 43.99, 11.72. Synthesis of Zn-L1. Hot absolute ethanol (50 mL) containing 4(diethylamino)salicylaldehyde (386 mg, 2.0 mmol) and diaminomaleonitrile (108 mg, 1.0 mmol) was added to 150 mL of hot absolute ethanol containing Zn(CH3COO)2·2H2O (219 mg, 1.0 mmol) and then was stirred for 2 d at 70 °C. The resulting red mixture was filtered hot and cooled in a freezer to give dark blue crystals (78% yield). Anal. Calcd (found): C, 59.21 (59.37); H, 6.03 (6.01); N, 14.80 (14.76)%. 1 H NMR (in CD3CN) 1.88 (t, J = 7.1 Hz, 12H), 3.19 (q, J = 7.1 Hz, 8H), 5.92 (s, 2H), 6.21 (d, J = 7.4 Hz, 2H), 6.99 (d, J = 9.0 Hz, 2H), 7.75 (s, 2H). 13C NMR (CD3CN) 172.90, 169.21, 157.63, 138.28, 130.58, 114.26, 107.93, 103.23, 95.85, 45.85, 12.98. Synthesis of Cu-L1. Hot absolute ethanol (50 mL) containing 4(diethylamino)salicylaldehyde (386 mg, 2.0 mmol) and diaminomaleonitrile (108 mg, 1.0 mmol) was added to 100 mL of hot absolute ethanol containing Cu(CH3COO)2·H2O (200 mg, 1.0 mmol) at 70 °C. Gold-yellow crystals were obtained in a few days and then were filtered and washed with ethanol (65% yield). Anal. Calcd (found): C, 60.04 (59.88); H, 5.43 (5.42); N, 16.16 (16.11)%. Synthesis of K-L1. Ethanol (150 mL) containing L1 (420 mg, 1.0 mmol) and potassium acetate (216 mg, 2.2 mmol) was refluxed at 78 °C for 1 h. After the mixture cooled to room temperature, a green solid (89% yield) was obtained by filtration. Anal. Calcd (found): C, 58.40 (58.38); H, 5.28 (5.29); N, 15.72 (15.73)%. 1H NMR (in CDCl3) 1.23 (t, J = 7.1 Hz, 12H), 3.43 (q, J = 7.1 Hz, 8H), 6.33 (s, 2H), 6.36 (d, J = 2.3 Hz, 2H), 7.21 (d, J = 8.7 Hz, 2H), 8.49 (s, 2H). 13 C NMR (CDCl3) 164.45, 161.96, 153.76, 135.56, 121.90, 112.70, 109.58, 105.81, 97.85, 45.05, 12.74. Synthesis of Pd-L1. Diphenylether (150 mL) containing L1 (210 mg, 0.5 mmol) and Pd(C6H5CN)2Cl2 (290 mg, 0.75 mmol) was stirred at 180 °C for 2 h. After the solution was cooled to room temperature, 200 mL of dichloromethane was added. After the free Pd2+ was removed by centrifugation, the resultant complex was obtained by precipitation with hexane. The above operation was repeated twice to remove the solvent of diphenylether. Finally, the pure complex (30% yield) was obtained on a silica gel column using dichloromethane as an eluent. Anal. Calcd (found): C, 55.47 (55.48); H, 5.01 (5.02); N, 14.93 (14.92)%. 1H NMR (in CDCl3) 1.25 (t, J = 7.1 Hz, 12H), 3.45 (q, J = 7.1 Hz, 8H), 6.35 (d, J = 7.3 Hz, 2H), 6.48 (s, 2H), 7.17 (d, J = 8.9 Hz, 2H), 7.72 (s, 2H). Synthesis of Zn-L2. Hot absolute ethanol (50 mL) containing salicylaldehyde (244 mg, 2.0 mmol) and 2,3-diaminophenazine (210 mg, 1.0 mmol) was added to 150 mL of hot absolute ethanol containing Zn(CH3COO)2·2H2O (219 mg, 1.0 mmol) and then was stirred for 12 h at 70 °C. The resulting brown mixture was filtered hot to give brown solid (65% yield). Anal. Calcd (found): C, 64.81 (64.83); H, 3.35 (3.34); N, 11.63 (11.62)%. 1H NMR (in DMSO-d6) 8.50 (s, 2H), 8.31 (s, 2H), 8.24 (s, 2H) 8.17 (s, 2H), 7.88 (s, 2H), 7.52 (m, 2H), 7.13 (m, 2H), 7.10 (m, 2H). Synthesis of Cu-L2. Hot absolute ethanol (50 mL) containing salicylaldehyde (244 mg, 2.0 mmol) and 2,3-diaminophenazine (210 mg, 1.0 mmol) was added to 150 mL of hot absolute ethanol containing Cu(CH3COO)2·H2O (200 mg, 1.0 mmol) and then was stirred for 12 h at 70 °C. The resulting deep brown mixture was filtered hot to give deep brown solid (60% yield). Anal. Calcd (found): C, 65.06 (65.03); H, 3.36 (3.37); N, 11.67 (11.66)%.

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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.6b01212. Absorption and emission data for Salen ligand and complexes. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21372169). We acknowledge Comprehensive training platform of specialized laboratory, College of Chemistry, Sichuan University, for material analysis.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b01212 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01212 Inorg. Chem. XXXX, XXX, XXX−XXX