Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 845–851

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Two dinuclear Ru(II) polypyridyl complexes with different photophysical and cation recognition properties Feixiang Cheng ⇑, Chixian He, Mingli Ren, Fan Wang, Yuting Yang College of Chemistry and Chemical Engineering, Qujing Normal University, Qujing 655011, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 We synthesized two analogous Ru(II)

Two analogous Ru(II) complexes with vacant coordination sites have been synthesized. Complex [{Ru(bpy)2}2(l2-L1)]4+ can serve as a fluorescent sensor toward Cu(II), but complex [{Ru(bpy)2}2(l2L2)]4+ exhibits no cation selectivity. 7000

2+

i n f o

Article history: Received 9 June 2014 Received in revised form 21 August 2014 Accepted 11 September 2014 Available online 2 October 2014 Keywords: Analogous Ru(II) complexes UV/Vis absorption Fluorescence Cu(II) recognition Different photophysical properties

+

2+

5000 2+

2+

complex, Ba , Ca , Cd , Co 3+ + 2+ 2+ + + Cr , Li , Mg , Mn , Na , K 2+ 2+ 2+ Ni , Pb , Zn

Cu

2+

Relative Intensity

4000

3000

Cu

2+

Co + Ag 2+ Cd 2+ Pb

3000

2+

2+

complex, Mn , Mg , Ca 2+ + + + Ba , Li , Na , K

2+

1000 1000

0

0 600

700

800

500

600

700

Wavelength (nm)

Wavelength (nm)

[{Ru(bpy)2}2(µ2-L1)]4+

[{Ru(bpy)2}2(µ2-L2)]4+

800

a b s t r a c t Two dinuclear Ru(II) polypyridyl complexes functionalized with vacant coordination sites have been designed and synthesized. Their photophysical properties and interactions with various metal ions have been investigated at room temperature. The two complexes exhibit different UV/Vis absorption and emission intensities. When titrated with various metal ions, complex [{Ru(bpy)2}2(l2-L1)]4+ exhibits a notable fluorescence quenching in the presence of Cu2+ in H2O–CH3CN media (1:1, v/v); its analogous complex [{Ru(bpy)2}2(l2-L2)]4+ exhibits no cation selectivity, the fluorescence intensity of complex [{Ru(bpy)2}2(l2-L2)]4+ has been enhanced by several transition metal ions due to prevention of the photo-induced electron transfer process. The fluorescence titration spectra and Benesi–Hildebrand expression reveal the formation of a 1:1 bonding mode between [{Ru(bpy)2}2(l2-L1)]4+ and Cu2+ ion with the association constant of 5.50  104 M1. Ó 2014 Elsevier B.V. All rights reserved.

The development of selective and efficient chemosensors for transition and heavy metal ions has received increasing attention because these ions play important roles in a wide range of chemical, biological and environmental systems [1–5]. Ru(II) polypyridyl complexes have been proved to be particularly useful in sensor applications, because their abundant photophysical and electrochemical properties are quite sensitive to external inputs. Ru(II) polypyridyl complexes have been applied as luminescent probes for inorganic anions through hydrogen bonding interaction or

⇑ Corresponding author. Tel.: +86 874 8998658; fax: +86 874 8965132.

http://dx.doi.org/10.1016/j.saa.2014.09.103 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

3+

Cr

2000

Introduction

E-mail address: [email protected] (F. Cheng).

Hg 3+ Fe Ni2+ 2+ Zn

4000 2+

2000

2+

2+

5000

500

a r t i c l e

3+

Hg , Fe , Ag

6000

Relative Intensity

complexes with vacant coordination sites.  The two complexes show different photophysical and cation recognition properties. 1 4+  Complex [{Ru(bpy)2}2(l2-L )] can serve as a fluorescent sensor toward Cu(II).

deprotonation of proton [6–8], for neutral molecule through the cleavage of the electron acceptor group [9], for pH through protonation and deprotonation of carboxylic acid, pyridine, amine, phenol, or imidazole groups [10,11], and for biological analytes through electrostatic, groove, classic intercalative, partially intercalative or covalent binding modes [12–14] in recent years, but there are still only a few reports of such sensors available for metal ions, especially for heavy metal ions [15–28]. Copper is the third most abundant essential trace element in human body. It plays an important role in many fundamental physiological processes including iron absorption, elimination of free radicals, development of bone, and so on. Copper imbalance can cause many serious diseases, such as Menkes, Alzheimer’s, and Wilson’s diseases. As a consequence, the detection and quantification of copper have been raised as an important consideration.

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There are many methods for detection of Cu(II), including electrochemical method, inductively coupled plasma mass spectroscopy, inductively coupled plasma-atomic emission spectrometry, atomic absorption spectrometry, colorimetric method, UV/Vis absorption, fluorescence method, etc [29–37]. However, electrochemical method, inductively coupled plasma mass spectroscopy, inductively coupled plasma-atomic emission spectrometry, and atomic absorption spectrometry all require complicated and fine apparatuses, or time consuming pre-treatment procedures to accomplish analytical procedure, making them inappropriate for in-field monitoring and general-purpose detection; UV/Vis absorption spectrometry is less sensitive than the fluorescence spectrometry; the colorimetric method is a simple manner, but its sensitivity is not good enough. The fluorescence method has been developed quickly for its simplicity, high sensitivity, and high-speed spatial analysis. On the other hand, different sensing mechanisms including photoinduced electron transfer [38–41], photo-induced energy transfer [42,43], internal charge transfer [44], fluorescence resonance energy transfer [45], inter-conversion of molecular structure [46– 51], and click chemistry [52] have been researched for Cu(II) fluorescent probes. Although many Cu(II) fluorescent sensors have been synthesized, most of them are based on organic molecules in solution. These organic molecules usually suffer from the water-fast, complicated synthesis, small Stokes’ shifts, and limited photostability. Therefore, the development of new Ru(II) complexes capable of recognizing Cu(II) with high sensitivity and selectivity has attracted our great interests. We design and synthesize two similar Ru(II) complexes containing vacant coordination sites, the vacant coordination sites are accessible to added cations. To our surprise, the two Ru(II) complexes exhibit different photophysical and cation recognition properties, herein, the synthesis, characterization, photophysical and cation recognition properties of the two Ru(II) complexes are presented and discussed. Experimental section Materials 2,20 -Bipyridine, 1,10-phenanthroline, ethylene glycol, tetrabutylammonium perchlorate (TBAP), RuCl33H2O, NH4PF6, NH4OAc, AcOH, CH3CN, EtOH, CHCl3, DMF, ethyl ether, and metal salts were purchased from the Tianjin Chemical Reagent Factory. Furan-2,5-dicarbaldehyde and thiophene-2,5-dicarbaldehyde were purchased from the J&K Scientific Ltd. Solvents and raw materials were of analytical grade and used as received. 1,10-Phenanthroline-5,6-dione [53], Ru(bpy)2Cl22H2O [54], and [{Ru(bpy)2}2(l2-L2)](PF6)4 [55] were synthesized according to literature procedures. Physical measurements 1

H NMR spectra were recorded with a Mercury Plus 400 spectrometer using TMS as internal standard. ESI-MS spectra were obtained with a Bruker Daltonics Esquire 6000 mass spectrometer. Elemental analyses were taken using a Perkin–Elmer 240C analytical instrument. Absorption spectra were obtained with a Varian Cary-100 UV/Vis spectrophotometer and emission spectra with a Hitachi F-4600 spectrophotometer. The emission quantum yields were calculated relative to Ru(bpy)2+ 3 (Ustd = 0.062) in deoxygenated CH3CN solution at room temperature [56]. Electrochemical measurements were carried out using a CHI 660D electrochemical workstation. Cyclic voltammetry and differential pulse voltammetry were performed in CH3CN and DMF solutions using a micro cell equipped with a platinum disk working electrode, a platinum auxiliary electrode, and a saturated potassium chloride

calomel reference electrode with 0.1 mol L1 TBAP as supporting electrolyte. All samples were purged with nitrogen prior to measurements. Synthesis 2,5-Di(1H-imidazo[4,5-f][1,10]phenanthroline-2-yl)furan (L1): Furan-2,5-dicarbaldehyde (142 mg, 1.15 mmol), 1,10-phenanthroline-5,6-dione (711 mg, 3.39 mmol), and ammonium acetate (5.22 g, 67.79 mmol) were stirred in glacial acetic acid (68 mL) and the mixture was heated to 130 °C for 6 h, giving a suspension. The reaction mixture was filtered hot, and the solid was washed successively with hot acetic acid, hot DMF, CHCl3, hot ethanol, and ethyl ether, affording the desired product as a yellow solid. Yield: 193 mg (33.5%). 1H NMR (400 MHz, DMSO-d6): d = 7.51 (s, 4H), 7.86 (dd, J = 8.0, 4.4 Hz, 4H), 8.95 (dd, J = 8.0, 1.6 Hz, 4H), 9.06 (dd, J = 8.0, 1.6 Hz, 4H), 14.18 (s, 2H). ESI-MS: m/z 505.4 (M + H)+. IR mmax (KBr, cm1): 3124s (br), 1623 m, 1562 m, 1488s, 1426s, 1353 m, 1188w, 1125w, 1074 m, 1027w, 993w, 909w, 805s, 738s, 680 m, 641 m, 409w. 2,5-Di(1H-imidazo[4,5-f][1,10]phenanthroline-2-yl)thiophene (L2): Compound L2 was prepared by the same procedure as that described for L1, except thiophene-2,5-dicarbaldehyde (182 mg, 1.40 mmol) was used instead of furan-2,5-dicarbaldehyde to react with 1,10-phenanthroline-5,6-dione (779 mg, 3.71 mmol). Yield: 503 mg (74.4%) of a yellow solid. 1H NMR (400 MHz, DMSO-d6): d = 7.81 (s, 4H), 7.93 (s, 2H), 8.74 (d, J = 7.6 Hz, 2H), 8.86 (d, J = 7.6 Hz, 2H), 9.06 (d, J = 6.0 Hz, 4H), 13.92 (s, 2H). ESI-MS: m/z 521.4 (M + H)+. IR mmax (KBr, cm1): 3076s (br), 1648 m, 1568s, 1507 m, 1477s, 1431s, 1395 m, 1351 m, 1186w, 1073 m, 1032w, 921w, 808s, 737s, 709 m, 628 m, 409w. [{Ru(bpy)2}2(l2-L1)](PF6)4: A mixture of ligand L1 (62 mg, 0.12 mmol) and Ru(bpy)2Cl22H2O (163 mg, 0.31 mmol) in ethylene glycol (30 mL) was heated to 150 °C for 12 h under nitrogen to give a clear deep red solution, then solvent was evaporated under reduced pressure. The residue was purified twice by column chromatography on alumina, eluted first with CH3CN–EtOH (10:1, v/v) to remove impurities, then with CH3CN–EtOH (2:1, v/v) to afford the complex [{Ru(bpy)2}2(l2-L1)]Cl4. This complex was dissolved in a minimum amount of water followed by dropwise addition of saturated aqueous NH4PF6 until no more precipitate formed. The precipitate was recrystallized from CH3CN–Et2O mixture (vapor diffusion method) to afford a red solid. Yield: 109 mg (46.4%). 1H NMR (400 MHz, DMSO-d6): d = 7.37 (s, 4H), 7.61 (t, J = 6.4 Hz, 4H), 7.65 (s, 4H),7.79 (d, J = 2.4 Hz, 2H), 7.86 (d, J = 5.6 Hz, 4H), 7.97 (s, 4H), 8.11-8.14 (t, 8H), 8.23 (t, J = 8.0 Hz, 4H), 8.87 (d, J = 8.4 Hz, 4H), 8.91 (d, J = 8.4 Hz, 4H), 9.12 (s, 2H), 9.58 (s, 2H). ESI-MS: m/z 811.1 (M  2PF6)2+, 738.2 (M  3PF6  H)2+, 665.3 (M  4PF6  2H)2+, 492.2 (M  3PF6)3+, 444.0 (M  4PF6  H)3+, 333.1 (M  4PF6)4+. Found: C, 44.21; H, 2.67; N, 11.92. Calcd for C70H48F24N16OP4Ru2: C, 43.99; H, 2.53; N, 11.73. IR mmax (KBr, cm1): 3424s (br), 1607 m, 1449 m, 1312w, 1196w, 1046w, 845s, 766 m, 729w, 558 m. Results and discussion Synthesis The outline of the synthesis of the ligands L1 and L2, and their Ru(II) complexes is presented in Scheme 1. Both ligands were synthesized on the basis of the method for the imidazole ring preparation established by Steck and Day [57]. Ligands L1 and L2 were prepared through reaction of 1,10-phenanthroline-5,6-dione with furan-2,5-dicarbaldehyde and thiophene-2,5-dicarbaldehyde, respectively, in refluxing glacial acetic acid at a molar ratio of 2:1. Complexes [{Ru(bpy)2}2(l2-L1)]4+ and [{Ru(bpy)2}2(l2-L2)]4+

F. Cheng et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 845–851

847

Scheme 1. Synthesis of ligands and their Ru(II) complexes.

were prepared by refluxing Ru(bpy)2Cl22H2O and ligands in ethylene glycol, and isolated as their PF 6 salts. Both complexes were characterized by 1H NMR, elemental analyses, ESI-MS, and IR spectroscopy. Photophysical properties The UV/Vis absorption spectra of the two ligands and their Ru(II) complexes have been studied in CH3OH/CHCl3 (1:1, v/v) and CH3CN solutions, respectively, at a working concentration of 105 mol L1. The spectra are shown in Fig. 1 with the data summarized in Table 1. Ligand L1 shows intraligand p ? p* or n ? p* transitions at around 406, 383, 368, and 290 nm with one shoulder at around 260 nm; ligand L2 exhibits intraligand p ? p* or n ? p* transitions at around 398, 290, and 251 nm with two shoulder 1.8

1.5

Absorbance

1.2

0.9

0.6

0.3

0.0

250

300

350

400

450

500

550

600

Wavelength (nm) Fig. 1. UV/Vis absorption spectra of ligands L1 (blue) and L2 (green) in CH3OH– CHCl3 (1:1, v/v) solution, of complexes [{Ru(bpy)2}2(l2-L1)]4+ (red) and [{Ru(bpy)2}2(l2-L2)]4+ (black) in CH3CN solution at room temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

bands at around 420 and 379 nm. Assignments of the absorption bands of the two complexes were made on the basis of the welldocumented optical transitions of analogous Ru(II) polypyridyl complexes [58–60]. The band at around 286 nm of both complexes is relatively broad which can be assigned to intraligand p ? p* transitions centered on the 2,20 -bipyridine and bridging ligand. The lowest energy band at around 460 nm of complex [{Ru(bpy)2}2(l2-L1)]4+ is attributed to MLCT, dp ? p* transition, which consists of overlapping dp(Ru) ? p*(bpy) and dp(Ru) ? p*(L1) transitions. The lowered symmetry removes the degeneracy of the p* levels, which results in the appearance of a non-symmetrical MLCT band. The peak observed at around 363 nm is attributed to ligand L1 centered intraligand p ? p* transitions. Complex [{Ru(bpy)2}2(l2-L2)]4+ exhibits the spin-allowed dp ? p* MLCT transition at around 463 nm, which upon intersystem crossing directly produces the triplet MLCT excited state. The dp ? p* transition consists of overlapping dp(Ru) ? p*(bpy) and dp(Ru) ? p*(L2) components. In the higher energy region around 433 nm, the spectra of complex [{Ru(bpy)2}2(l2-L2)]4+ display one band consisting of overlapping dp ? p* transition and intraligand p ? p* transition centered on ligand L2. Complex [{Ru(bpy)2}2(l2-L2)]4+ exhibits the characteristic p ? p* transition of ligand L2 at 402 nm with one shoulder at around 379 nm. The MLCT absorption maxima of both complexes are red-shifted by about 10 nm compared with that of Ru(bpy)2+ 3 [61] suggesting that both bridging ligands have larger p framework. The emission spectra of both complexes have been measured in degassed CH3CN solution at room temperature, the emission band maxima and emission quantum yields are listed in Table 1. Upon excitation into the MLCT band of both complexes (105 mol L1), they show characteristic emission at around 609 nm with excitation wavelength at 450 nm (Fig. 2). The emission intensity of complex [{Ru(bpy)2}2(l2-L1)]4+ is stronger than that of complex [{Ru(bpy)2}2(l2-L2)]4+. This may involve photo-induced electron transfer (PET) process. The fluorescence of complex [{Ru(bpy)2}2(l2-L2)]4+ is partly quenched by the occurrence of a PET process between the lone pair electrons of the sulfur and the appended chromophore [29].

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Table 1 Photophysical and electrochemical data of Ru(II) polypyridyl complexes. Compound

Absorption 4

1

kmax, nm (10 e, M

a b

1

cm

)

Emissiona

E1/2, V (DEp, mV)b

kmax, nm U (298 K)

Oxidation

Reduction

L1

406 383 368 290 260

(3.61) (4.40) (3.44) (3.88) (shoulder)

L2

420 398 379 290 251

(shoulder) (4.47) (shoulder) (3.32) (4.10)

[{Ru(bpy)2}2(l2-L1)]4+

460 363 286 255 245

(4.46) (3.80) (18.01) (8.29) (8.00)

609 (0.053)

1.30 (73)

1.31 (66) 1.50 (67)

[{Ru(bpy)2}2(l2-L2)]4+

463 433 402 379 287 253 245

(5.16) (5.13) (5.74) (shoulder) (14.13) (7.38) (7.57)

609 (0.061)

1.32 (78)

1.30 (63) 1.50 (72)

The emission quantum yields are calculated relative to Ru(bpy)2+ 3 (Ustd = 0.062) in deoxygenated CH3CN solution at 298 K; the uncertainty in quantum yields is 15%. Redox potentials are given vs. SCE, scan rate = 200 mV/s and DEp is the difference between the anodic and cathodic waves.

0.000008

7000

a

0.000006

6000

0.000002

Current / A

Relative Intensity

0.000004 5000 4000 3000

0.000000 1.6

1.5

1.4

1.3

1.2

-0.000002

1.1

1.0

0.9

Potential / V

-0.000004 2000

-0.000006 -0.000008

1000

-0.000010 0 500

550

600

650

700

750

800

850

0.00005

0.000012

0.00004

0.000009

Electrochemical properties The electrochemical properties of both complexes have been studied in CH3CN and DMF solutions with 0.1 mol L1 TBAP as supporting electrolyte. In CH3CN solution, the reduction couples of both complexes are not well-behaved, due to adsorption of the reduced species onto the surface of the platinum electrode. In DMF solution, both complexes display two clear reduction processes, but do not show the oxidative couples due to the insufficient anodic window of this solvent. Therefore, the oxidation potentials are recorded in CH3CN, but the reduction potentials are recorded in DMF (Table 1). Both complexes exhibit a Ru(II)-centered reversible oxidation wave at around 1.31 V for the RuII/III couple (Fig. 3a). This potential is slightly more positive (by about 30 mV) than that of Ru(bpy)2+ 3

0.000006

0.00003

Current / A

Fig. 2. Emission spectra of complexes [{Ru(bpy)2}2(l2-L1)]4+ (black) and [{Ru(bpy)2}2(l2-L2)]4+ (red) in CH3CN solution at room temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

b

Current / A

Wavelength (nm)

0.000003

0.00002 0.000000 -1.0

-1.2

0.00000 -0.6 -0.00001

-1.4

-1.6

-1.8

Potential / V

0.00001

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.0

-2.2

Potential / V

Fig. 3. Electrochemical spectra of complex [{Ru(bpy)2}2(l2-L1)]4+ (5  104 mol/L), (a) oxidation potential (cyclic voltammetry) is recorded in 0.1 mol L1 TBAP CH3CN solution; (b) reduction potentials (cyclic voltammetry and differential pulse voltammetry) are recorded in 0.1 mol L1 TBAP DMF solution.

(+1.28 V vs. SCE) [62], which indicates that ligands L1 and L2 are stronger p-acceptors than 2,20 -bipyridine. In the reduction processes of both complexes, the two reversible reduction couples located at around 1.31 and 1.50 V can be assigned to the reduc-

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tion of the 2,20 -bipyridine ligands on each ruthenium center. The reductions of the 2,20 -bipyridine ligands occur sequential at each metal center prior to reduction of the second 2,20 -bipyridine ligand (Fig. 3b).

The cation binding properties of both complexes have been investigated by UV/Vis absorption and fluorescence spectroscopy. The titration experiments have been carried out in H2O-CH3CN system (1:1, v/v) at room temperature with 0.1 mol L1 TBAP to keep a constant ionic strength. The chloride, nitrate, or perchlorate salts of Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Cr3+, Co2+, Ni2+, Cu2+, Ag+, Zn2+, Cd2+, Hg2+, Mn2+, Fe3+, and Pb2+ (5 equiv.) ions have been used to evaluate the metal ion binding property and selectivity of both  2 complexes. Br, Cl, I, NO 3 , SO4 , and ClO4 do not affect the photophysical properties of both complexes. As shown in Fig. 4, complex [{Ru(bpy)2}2(l2-L1)]4+ shows a selective fluorescence quenching only with Cu2+ among the various metal ions examined in H2O–CH3CN solution, the enhancement of the fluorescence intensities with Ag+, Hg2+, and Fe3+ are significantly small, all other metal ions have no obvious effect on the fluorescence property. The fluorescence intensity of complex [{Ru(bpy)2}2(l2-L2)]4+ is not significantly affected by Li+, Na+, K+, Mg2+, Ca2+, Ba2+, and Mn2+ (Fig. 5). All other transition metal ions can enhance the fluorescence intensity of complex [{Ru(bpy)2}2(l2-L2)]4+, the most dramatic fluorescent enhancement is observed with Cu2+. With the aim of comparison, the mononuclear Ru(II) complex Ru(1) has been prepared by replacing the bridging ligand L1 in complex [{Ru(bpy)2}2(l2-L1)]4+ with ligand 2-phenyl-1H-imidazo[4,5f][1,10]phenanthroline. The fluorescence property of complex Ru(1) has not affected upon addition of Cu2+ ion (50 equiv.) (Fig. S1). These studies indicate that, first, complex [{Ru(bpy)2}2(l2-L1)]4+ can serve as a selective fluorescence sensor toward Cu2+ among 17 metal ions, but complex [{Ru(bpy)2}2(l2-L2)]4+ has no cation-selective property; second, the fluorescence quenching of complex [{Ru(bpy)2}2(l2-L1)]4+ is due to Cu2+ binding, not simply because of the paramagnetic nature of Cu2+ ion. The proposed mechanism for the fluorescence enhancement of complex [{Ru(bpy)2}2(l2-L2)]4+ is that the lone pair electrons of the sulfur atom would no longer be available for the photo-induced electron transfer process owing to its involvement in the coordinative interaction [29]. With regard to the fluorescence quenching mechanism of complex [{Ru(bpy)2}2(l2-L1)]4+, the free energy associated with electron transfer mechanism has been calculated by using the

Cu

2+

2+

3000

Co + Ag 2+ Cd 2+ Pb

3+

Cr

2000

2+

0 500

600

700

Fig. 5. Fluorescence responses of complex [{Ru(bpy)2}2(l2-L2)](PF6)4 (105 mol L1 in H2O–CH3CN = 1:1, v/v) to various metal ions (5 equiv.) at room temperature.

equation DG = Eox  Ered  Es + C [63]. The RuII/III oxidation couple potential is at 1.30 V and the CuII/I reduction couple potential is at around 1.08 V in the environment [64]. Es is the singlet excited energy of the fluorophore and the value is 2.32 eV. C is the Coulombic energy term, which is negligible in polar environments. The result shows a thermodynamically unfavourable DG value of around 0.06 eV for an electron transfer process. So, fluorescence quenching of complex [{Ru(bpy)2}2(l2-L1)]4+ upon Cu2+ binding most likely occurs predominantly by means of energy transfer [23]. To further evaluate the sensitivity and linear relationship of the present sensor with Cu2+, the fluorescence titration experiments of complex [{Ru(bpy)2}2(l2-L1)]4+ with Cu2+ in varying concentrations have been carried out. A tracing of the emission spectra upon the sequential addition of Cu2+ (0–5  105 mol L1) to the H2O-CH3CN solution (1:1, v/v) of complex [{Ru(bpy)2}2(l2-L1)]4+ (105 mol L1) is shown in Fig. 6, the emission intensity of complex [{Ru(bpy)2}2(l2-L1)]4+ decreases dramatically with the increasing concentration of Cu2+ ion, suggesting that Cu2+ ion is an efficient killer for the excited state complex [{Ru(bpy)2}2(l2-L1)]4+. When Cu2+ ion concentration is as high as 1.0 equiv, the emission intensity of complex [{Ru(bpy)2}2(l2-L1)]4+ decreases to 41% of its original value. By plotting the changes in the fluorescence intensity at 616 nm as a function of Cu2+ concentration, sigmoidal curve has been obtained and shown in Fig. 6 inset. The formation of a 1:1 7000

+

2+

Relative Intensity

6000

2+

2+

complex, Ba , Ca , Cd , Co 3+ + 2+ 2+ + + Cr , Li , Mg , Mn , Na , K 2+ 2+ 2+ Ni , Pb , Zn

6000

4000 3000 Cu

5000 4000

4000

3000

2000

3000

0

1

2

3 2+

Cu

4

5

equiv.

2000 1000 0 500

600

5000

2+

1000 0 500

6000

2+

5000

2000

800

Wavelength (nm)

Relative Intensity

3+

2+

1000

Relative Intensity

2+

Hg , Fe , Ag

2+

complex, Mn , Mg , Ca 2+ + + + Ba , Li , Na , K

7000 7000

2+

Hg 3+ Fe Ni2+ 2+ Zn

4000

Relative Intensity

Cation binding

5000

700

800

Wavelength (nm) Fig. 4. Fluorescence responses of complex [{Ru(bpy)2}2(l2-L1)](PF6)4 (105 mol L1 in H2O/CH3CN = 1:1, v/v) to various metal ions (5 equiv.) at room temperature.

550

600

650

700

750

800

850

Wavelength (nm) Fig. 6. Fluorescence titration spectra of complex [{Ru(bpy)2}2(l2-L1)]4+ (105 mol L1) with added Cu(ClO4)2 (0–5  105 mol L1) in H2O–CH3CN solution (1:1, v/v) at room temperature; the inset shows the fluorescence intensity taken at 616 nm vs. the added Cu2+ equivalents.

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0.0018

Y = 9.326 x 10-5 + 1.696 x 10-9X R = 0.9994

0.0016

6000

Fluorescence Intensity

0.0014

1/(Fo - F)

complex + other metal ion 2+ complex + other metal ion + Cu

7000

0.0012 0.0010 0.0008 0.0006 0.0004

5000 4000 3000 2000 1000

0.0002 0

200000

400000

600000

800000

1000000 0

1/[Cu2+]

bonding mode between [{Ru(bpy)2}2(l2-L1)]4+ and Cu2+ ion is preliminary estimated. The bonding mode is supported by linear relationship obtained in the Benesi–Hildebrand expression [65]. In the case of a 1:1 bonding mode, the following equation is applicable: 1/ (Fo  F) = 1/(Fo  F1)Ks [Cu2+] + 1/(Fo  F1). Fo is the fluorescence intensity of complex [{Ru(bpy)2}2(l2-L1)]4+ in the absence of Cu2+, F is the observed fluorescence at each concentration tested, F1 is the fluorescence intensity at saturation, and Ks is the association constant. As shown by Fig. 7, a Plot of 1/(Fo  F) vs. 1/[Cu2+] gives a satisfactory straight line (R = 0.9942) when [Cu2+] varies from 106 mol L1 to 105 mol L1, which indicates that the fluorescence quenching can also be used to determine the concentration of Cu2+. The association constant for Cu2+ is estimated to be 5.50  104 M1. Furthermore, the detection limit of the complex [{Ru(bpy)2}2(l2-L1)]4+ for the determination of Cu2+ is estimated to be 5.78  107 mol L1, which could cause about 5% reduction of the fluorescence intensity. The detection limit is sufficiently low for the detection of the submillimole concentration range of Cu2+ ions found in many chemical and biological systems. In addition, the electrospray ionization mass (ESI-MS) spectrum (Fig. S2) also shows the formation of a 1:1 bonding mode between [{Ru(bpy)2}2(l2-L1)]4+ and Cu2+. The two peaks appeared at m/z 277.32 and 299.33 are assigned to [M + Cu(ClO4)2  4PF6  2ClO4  H]5+ and [M + Cu(ClO4)2  4PF6  ClO4]5+, respectively. It is important to investigate the influence of the fluorescence spectra of complex [{Ru(bpy)2}2(l2-L1)]4+ at different pH range in order to find a suitable pH span in which complex [{Ru(bpy)2}2(l2-L1)]4+ can selectively detect Cu2+ efficiently. The pH effect on the fluorescence of complex [{Ru(bpy)2}2(l2-L1)]4+ has been studied. As shown in Fig. S3, the emission spectra of complex [{Ru(bpy)2}2(l2-L1)]4+ are sensitive to the pH changes. As the pH increases from 1.82 to 6.22, emission intensity increases slightly by about 7% due to the concurrent dissociation of the protons on the two protonated imidazole groups (Fig. S3a). Further raising pH from 6.22 to 11.31 can induce the deprotonation on neutral imidazole groups (Fig. S3b), the emission intensities are found to decrease sharply by about 91% and the emission maxima are obviously red-shifted from 612 to 620 nm. Fluorescence intensity of complex [{Ru(bpy)2}2(l2-L1)]4+ is strong in the pH range from 1.82 to 7.47. To study the practical applicability, the effects of pH on the fluorescence response to Cu2+ (5  105 mol L1) of complex [{Ru(bpy)2}2(l2-L1)]4+ (105 mol L1) has been also investigated at room temperature (Fig. S4). Experimental result shows that for complex [{Ru(bpy)2}2(l2-L1)]4+, in the presence of the Cu2+ ion, there is a fluorescence quenching with different

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16

Fig. 8. Fluorescence response of complex [{Ru(bpy)2}2(l2-L1)]4+ (105 mol L1 in H2O/CH3CN = 1:1, v/v) to various cations and selectivity of [{Ru(bpy)2}2(l2-L1)]4+ for Cu2+ in the presence of other metal ions. The red bars represent the fluorescence intensity of [{Ru(bpy)2}2(l2-L1)]4+ in the presence of 1.0 equiv. of the cation of interest: 1 Li+, 2 Na+, 3 K+, 4 Mg2+, 5 Ca2+, 6 Ba2+, 7 Zn2+, 8 Pb2+, 9 Ni2+, 10 Mn2+, 11 Cr3+, 12 Cd2+, 13 Ag+, 14 Fe3+, 15 Hg2+, 16 Co2+, the green bars represent the fluorescence intensity of the above solution upon further addition of 1.0 equiv. of Cu2+. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

quenching efficiency under different pH values in a pH range from 1.82 to 11.31. Complex [{Ru(bpy)2}2(l2-L1)]4+ exhibits the highest fluorescence response toward the Cu2+ ion in the pH range from 5.48 to 7.47, therefore, neutral solution has been chosen as an optimum experimental condition for the titration experiments. For a sensor of Cu2+ ion, the influence of the other coexisting metal ions should be investigated. Thus, fluorescence competition experiments have been further carried out at room temperature. Complex [{Ru(bpy)2}2(l2-L1)]4+ (105 mol L1) is treated with 1 equiv. of Cu2+ ion in the presence of different background metal ions (1 equiv.). It is found from Fig. 8 that the presence of 1 equivalent of other metal ions has no significant influence on the fluorescence intensity of complex [{Ru(bpy)2}2(l2-L1)]4+. Upon further addition of 1 equivalent of Cu2+ ion, the fluorescence intensity of complex [{Ru(bpy)2}2(l2-L1)]4+ is dramatically quenched. These results indicate that our proposed sensor exhibits high selectivity to Cu2+ over other metal ions.

1.8 1.5

Absorbance

Fig. 7. A Plot of 1/(Fo  F) vs. 1/[Cu2+] gives a satisfactory straight line, indicating a 1:1 bonding mode.

1

1.2 0.9 0.6 0.3 0.0 200

300

400

500

600

Wavelength (nm) Fig. 9. UV/Vis absorption titration spectra of complex [{Ru(bpy)2}2(l2-L1)]4+ (105 mol L1) with added Cu(ClO4)2 (0–5  105 mol L1) in H2O–CH3CN solution (1:1, v/v) at room temperature.

F. Cheng et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 845–851

The Cu2+ binding studies of complex [{Ru(bpy)2}2(l2-L1)]4+ have been investigated further using UV/Vis absorption at room temperature. No naked eye color change for the sensor on adding Cu2+ has been observed. Fig. 9 shows the changes in the absorption spectrum of complex [{Ru(bpy)2}2(l2-L1)]4+ (105 mol L1) with the addition of Cu2+ (0–5  105 mol L1) in the H2O–CH3CN (1:1, v/v) solution. With the increase in concentration of Cu2+ ion, the absorbance at 458, 286, and 211 nm increases 6%, 33%, and 98%, respectively. The bands at 255 and 246 nm are blue-shifted to 242 and 233 nm, respectively, accompanied by increases in their intensities. The band at 363 nm shows slight decrease in the absorption intensity. One isosbestic point at 391 nm has been observed. Conclusion We designed and synthesized two analogous dinuclear Ru(II) polypyridyl complexes containing vacant coordination sites. The two Ru(II) complexes show different photophysical and cation recognition properties due to the slight difference of the structure. The MLCT band of complex [{Ru(bpy)2}2(l2-L2)]4+ is broad than that of complex [{Ru(bpy)2}2(l2-L1)]4+; the fluorescence intensity of complex [{Ru(bpy)2}2(l2-L2)]4+ is weaker than that of complex [{Ru(bpy)2}2(l2-L1)]4+. The fluorescence intensity of complex [{Ru(bpy)2}2(l2-L2)]4+ has been enhanced by several transition metal ions due to prevention of the photo-induced electron transfer process, but complex [{Ru(bpy)2}2(l2-L1)]4+ exhibits a remarkable fluorescence quenching toward Cu2+ in H2O–CH3CN solution (1:1, v/v) with high sensitivity and selectivity by virtue of energy transfer. The fluorescence changes of complex [{Ru(bpy)2}2(l2-L1)]4+ are remarkably specific for Cu2+ in the presence of other heavy and transition metal ions with a detection limit of 5.78  107 mol L1, which meet the selective requirements for biomedical and environmental monitoring application. Hence, complex [{Ru(bpy)2}2(l2-L1)]4+ has potential application value in tracking of Cu2+ in chemical and biological systems. Acknowledgment We are grateful to the National Natural Science Foundation of China (21261019) and the Yunnan Provincial Science and Technology Department (2010ZC148) for financial support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.09.103. References [1] R. Kagit, M. Yildirim, O. Ozay, S. Yesilot, H. Ozay, Inorg. Chem. 53 (2014) 2144– 2151. [2] S. Heng, A.M. Mak, D.B. Stubing, T.M. Monro, A.D. Abell, Anal. Chem. 86 (2014) 3268–3272. [3] T. WeiAuthor Vitae, G. GaoAuthor Vitae, W. QuAuthor Vitae, B. ShiAuthor Vitae, Q. Lin, H. YaoAuthor Vitae, Y. Zhang, Sens Actuators B. 199 (2014) 142–147. [4] E. Woz´nica, K. Maksymiuk, A. Michalska, Anal. Chem. 86 (2014) 411–418. [5] S. Kumari, G.S. Chauhan, ACS Appl. Mater. Interfaces 6 (2014) 5908–5917. [6] S. Das, S. Karmakar, S. Mardanya, S. Baitalik, Dalton Trans. 43 (2014) 3767– 3782. [7] Y. Cui, Y.L. Niu, M.L. Cao, K. Wang, H.J. Mo, Y.R. Zhong, B.H. Ye, Inorg. Chem. 47 (2008) 5616–5624. [8] P. Anzenbacher, J.D.S. Tyson, K. Jursíková, F.N. Castellano, J. Am. Chem. Soc. 124 (2002) 6232–6233. [9] R. Zhang, Z. Ye, Y. Yin, G. Wang, D. Jin, J. Yuan, J.A. Piper, Bioconjugate Chem. 23 (2012) 725–733. [10] A.G. Zhang, Y.Z. Zhang, Z.M. Duan, K.Z. Wang, H.B. Wei, Z.Q. Bian, C.H. Huang, Inorg. Chem. 50 (2011) 6425–6436. [11] H.D. Batey, A.C. Whitwood, A.K. Duhme-Klair, Inorg. Chem. 46 (2007) 6516– 6528.

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