1(2Pyridyl)3ferrocenylpyrazolineBased Multichannel

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DOI:10.1002/ejic.201300944

1-(2-Pyridyl)-3-ferrocenylpyrazoline-Based Multichannel Signaling Receptors for Co2+, Cu2+, and Zn2+ Ions Chakka Kiran Kumar,[a] Rajiv Trivedi,*[a] Kanaparthi Ravi Kumar,[a] Lingamallu Giribabu,[a] and Balasubramanian Sridhar[b] Keywords: Ferrocene / Pyridylpyrazoline / Sensors / Fluorescence / Electrochemistry A simple one-pot synthesis, the characterization, optoelectronic, and cation sensing properties of 1-(2-pyridyl)-3-ferrocenylpyrazolines 4–6 are described in this article. Reaction of ferrocenyl chalcones with 2-hydrazinopyridine gave the target compounds in good yield. These compounds were characterized by general spectroscopic techniques and the structure of 4 was determined by means of X-ray crystallography. These ferrocene compounds behave as selective multichannel chemosensors (redox, chromogenic, and fluorescent) in the presence of Co2+, Cu2+, and Zn2+ ions. The binding assay and recognition ability of these receptors towards the metal ions were explained by electrochemical and optical studies. A maximum cathodic shift in the redox potential of the ferro-

cenium couple was observed towards the Co2+ ion (ΔE1/2 = 99–156 mV), while a minimum shift was observed with the Zn2+ ion (ΔE1/2 = 72–129 mV) on complexation with these receptors. Disappearance of the high-energy (HE) band and a red shift (Δλ = 7–13 nm) of the low-energy (LE) band in the absorption spectra of the receptors 4 and 5 was observed upon complexation with these metal ions. This change in absorption was accompanied by a color change from yellow to red/brown, which enabled potential “naked eye” detection. The emission spectra (λex = 350 nm) of receptor 4, in the presence of these cations, showed a 2–7-fold increment in the chelation-enhancement fluorescence (CHEF) and a 4–9-fold increase in the quantum yield.

Introduction

On the other hand, ferrocene-based compounds have proven themselves to be a simple and efficient building block for studying the mixed-valence behavior,[7] allowing attachment to a wide variety of functional groups, high stability in both oxidized and neutral states, and charge transport ability.[8] Although ferrocene behaves as an emission quencher in the intramolecular processes,[9] there are instances where this effect does not interfere in the luminescent processes.[10] Moreover, metal complexation of ferrocene derivatives often leads to a cathodic shift in the redox potential of the FeII/FeIII couple and a perturbation in the lowest energy metal-to-ligand transition.[11] On the basis of these facts, Molina and coworkers have extensively explored the role of different types of ferrocenecontaining molecular materials for selective sensing of cations or anions, and more recently heteroditopic molecules have been designed to detect cations and anions simultaneously.[12] Metal complexes of ferrocene pyridine as well as ferrocenylpyrazoline moieties have also been reported.[13] With these features in mind, we initiated our exploration towards the design of multichannel receptors by combining a redox unit (ferrocene), a fluorophore (aromatic ring), and a binding system (two nitrogen atoms on the pyridylpyrazoline) capable of detecting and hosting the cationic species. These features allowed the investigation of their sensing behavior towards different metal ions by electrochemical and optical methods.

Transition and post-transition metal ions play a significant role in a wide range of chemical, biological, and environmental processes and hence a considerable interest has grown in their efficient and effective trace detections.[1] Over the past decades, numerous groups have reported on the design and development of several types of ligand scaffolds for specific detection of trace metal ions.[2] Among them, cation receptors having cyclic and acyclic rings consisting of several functional groups have also been reported.[3] Receptors having nitrogen binding sites for selective recognition of metal ions constitute an important class of compounds, among them 2-pyridylpyrazolines are expected to influence the absorption and emission spectra by inducing intramolecular charge transfer (ICT) to the metal ion.[4] However, the inherent properties of many transition and heavy metals to act as fluorescence quenchers through enhanced spin-orbital coupling[5] and electron- or energytransfer processes[6] becomes an impediment for the development of efficient fluorescent sensors. [a] Inorganic and Physical Chemistry Division, CSIR-IICT, Hyderabad 500007, Andhra Pradesh, India E-mail: [email protected], [email protected] http://www.iictindia.org [b] X-ray Crystallography Division, CSIR-IICT, Hyderabad 500007, Andhra Pradesh, India Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201300944. Eur. J. Inorg. Chem. 2013, 6019–6027

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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org Recently, we have reported the synthesis, electrochemistry, and optical properties of several ferrocene-linked pyrazolines and -pyrazoles.[14b] In this regard, we describe herein a simple one-pot synthesis of ferrocene substituted with 2-pyridylpyrazolines 4–6 that were characterized by all the fundamental spectral studies: 1H NMR spectroscopy, 13 C NMR spectroscopy, FTIR spectroscopy, elemental analysis, ESI-MS, single-crystal X-ray crystallography (4), electrochemical, and optical (both absorption and emission properties) studies. We also describe the application of these compounds as a multichannel (redox/chromogenic/fluorogenic) sensor towards Co2+, Cu2+, and Zn2+ ions.

Results and Discussion Synthesis and Characterization The target compounds 4–6 were prepared by a one-pot synthetic route using a slightly modified literature report.[14] The reaction of acetyl ferrocene (1) with the appropriate aromatic aldehyde in the presence of KOH in ethanol formed the corresponding chalcone. Subsequent addition of 2-hydrazinopyridine under reflux conditions gave the desired product (Scheme 1). Analytically pure products were obtained by column chromatography and were stable at room temperature for months. All the compounds were characterized by general spectroscopic (FTIR, 1H NMR and13C NMR), electrospray ionization techniques (ESIMS), and elemental analysis.

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with the respective ferrocene and aromatic ring. The ESIMS spectral values and elemental analysis data were also in agreement with the proposed structure (see Experimental Section). The molecular structure of compound 4 was further confirmed by means of X-ray diffraction studies (for details see Supporting Information). Red crystals of compound 4 were obtained through slow evaporation of the acetonitrile solution. Compound 4 was crystallized in the triclinic space group P1¯ with two molecules in the asymmetric unit. The molecule with its numbering scheme is given in Figure 1, which shows a perspective view of compound 4. The pertinent bond length and bond angle data are listed in the Supporting Information. The structure consists of a pyrazoline ring having a ferrocene unit on the C11 atom, naphthalene ring on C13 atom, and the pyridine ring attached to the nitrogen atom N1. It was found that two molecules of compound 4 adopted different conformations (Figure S10 of the Supporting Information).

Figure 1. ORTEP diagram for compound 4 with thermal ellipsoids drawn at 30 % probability level and H atoms are represented by circles of arbitrary radii. (The unit cell contains two molecules, same numbering was suffixed to A and B. Only molecule A is shown for clarity). Scheme 1. Synthesis of ferrocenyl 2-pyridylpyrazolines (4–6).

Both FTIR and NMR spectra of the compounds (4–6) indicated the presence of a pyrazoline ring. 1H NMR spectra of these compounds displayed a set of three signals with an ABX pattern at δ = 3.0–3.4 ppm (H4trans), δ = 3.9– 4.1 ppm (H4cis), and δ = 6.4–6.6 ppm (H5) corresponding to the C4 and C5 hydrogen atoms on the pyrazoline ring. This was further confirmed by the appearance of three signals at δ = 148.7–151.5 ppm (C3), δ = 42.7–43.9 ppm (C4), and δ = 55.7–61.7 ppm (C5) in their respective 13C NMR spectra.[14] In addition, the FTIR spectra showed strong characteristic bands at 1610–1590 cm–1 and 1530–1510 cm–1 corresponding to the ν(C=C) and ν(C=N) stretching frequencies, respectively, indicating the formation of the pyrazoline ring.[15] All the additional peaks and bands observed in the NMR and FTIR spectra were in agreement Eur. J. Inorg. Chem. 2013, 6019–6027

The pyrazoline ring (C11A/C12A/C13A/N1A/N2A) in molecule A was found to adopt an envelope conformation with puckering parameters q2 = 0.1713(4) Å and φ2 = 135.24(1)°. Atom C13A was displaced from the plane by –0.274(4) Å [asymmetric parameter, ΔCs(C13A) = 0.021(2)] with respect to the atoms N1A/N2A/C11A/C12A. The two cyclopentadienyl rings (C1A–C5A) and (C6A–C10A) of the ferrocene in molecule A are eclipsed with a tilt angle of 2.0(3)°. Whereas the pyrazoline ring (C11B/C12B/C13B/ N1B/N2B) in molecule B adopts a twisted conformation with puckering parameters q2 = 0.0912(3) Å and φ2 = –50.01(2)°. Atoms C12B–C13B are twisted from the plane by 0.143(4) Å [asymmetric parameter, ΔC2(N1B) = 0.004(1)] with respect to the atoms N1B/N2B/C11B. The two cyclopentadienyl rings (C1B–C5B) and (C6B–C10B) of the ferrocene in molecule B are eclipsed with a tilt angle of 2.9(2)° (Figure S10a of the Supporting Information).

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www.eurjic.org Redox and Optical Studies The reversibility and the oxidation potential of the free receptors 4–6 were determined by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in a CH2Cl2 solution. Each free receptor exhibited a reversible one-electron redox wave nature (ipc/ipa = 1.02–1.05) over the range 0–1.1 V, typical of a ferrocene derivative, at the halfwave potential E1/2 = 552, 553, and 548 mV versus the FeII/ FeIII couple for 4, 5, and 6, respectively (Figure S12 of the Supporting Information). Because of the lack of conjugation between the aromatic ring and ferrocene moiety, relatively close oxidation potentials were obtained for these receptors. The UV/Vis data for 4–6 were recorded in a CH2Cl2 solution and were consistent with both ferrocene and aromatic chromophores, wherein they exhibited one intense absorption band in the UV region and a relatively weak band in the visible region (Figure 2). The high energy (HE) band was from the π–π* electronic transition of the ligand and the lower energy (LE) band originated from the localized excitation either from an FeII d–d transition[16] or a metalto-ligand charge-transfer (MLCT) process (dπ–π*). Such spectral characteristics (presence of LE band) confer the yellow color for the receptors 4–6. Receptor 4, which has a naphthalene chromophore, showed an intense band in the 290–420 nm region having a maximum (λmax) at 331 nm and a weak absorption band in the 400–560 nm region centered at 451 nm. Receptor 5, with an anthracene unit, showed an intense band centered at (λmax) 335 nm and a relatively weak absorption band centered at 472 nm. In addition to the above bands, receptor 5 exhibited additional peaks or shoulders at 350, 370, and 391 nm. The pyrene moiety bearing receptor 6 exhibited two intense structured bands between 260–395 nm having maxima (λmax) at 278, 346 nm and a weak band at 452 nm. In addition, receptor 6 showed three shoulder bands at 278, 316, and 330 nm.

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ence of ferrocene.[9] All the receptors, 4–6, showed a single fluorescence emission band, which was characteristic of the corresponding fluorophore moiety.[12b,12g,12h] The spectra exhibited well-resolved vibronic features having maxima at 431–440 nm and a shoulder at 407–416 nm. Moreover, in the case of the pyrene receptor 6, in addition to the emission band maxima at 406 nm, another band centered at 503 nm was observed from the formation of a pyrene-excimer.[12h]

Cation Sensing Properties Interaction of receptors 4–6 with different metal ions (Li+, Na+, K+, Mg2+, Ca2+, Cr2+,Mn2+, Fe2+, Co2+, Cu2+, Zn2+, Cd2+, Ni2+, Pb2+, and Hg2+) for displaying binding and complexation ability, was systematically investigated by recording redox (CV and DPV) and optical properties (absorption and emission). No significant change was observed in the redox and optical properties of the receptors 4–6 on stoichiometric addition of monovalent metal ions in CH2Cl2 solution. However, interestingly, similar addition of the divalent metal ions Co2+, Cu2+, Hg2+, and Zn2+ to the receptors significantly altered these properties (Figure 3). No significant changes in redox as well as optical properties were observed for other divalent metal ions, which indicated no complexation.

Figure 3. (a) A shift in the redox potential of receptor 4 in CH2Cl2 solution (1.0 ⫻ 10–3 m) by the addition of 1 equiv. of different metal ions in methanol (1.0 ⫻ 10–2 m) in DPV. (b) Changes in absorption spectra of receptor 4 in CH2Cl2 solution (3.2 ⫻ 10–5 m) on addition of 1 equiv. of different metal cations in methanol (1.0 ⫻ 10–2 m). (c) Visual features observed for receptor 4 in CH2Cl2 solution after addition of 2 equiv. of different metal ions as their acetate salts. Figure 2. UV/Vis spectra of receptors 4 (black, 3.2 ⫻ 10–5 m), 5 (red, 2.4 ⫻ 10–5 m), and 6 (blue, 1.6 ⫻ 10–5 m) in CH2Cl2 solution.

It was expected that with the presence of fluorophore moieties like naphthalene, anthracene, and pyrene, receptors 4–6 should display interesting fluorescence behavior. A weak fluorescence was observed in CH2Cl2 solution (Φ4 = 1.98 ⫻ 10–3) when excited at λexc = 350 nm (Figure S28 of the Supporting Information), which may be due to the presEur. J. Inorg. Chem. 2013, 6019–6027

Some of the interesting and prominent features observed during the titration experiments were: (i) the appearance of two well-defined curves in the DPV indicated the presence of two species corresponding to the free receptor (R) and receptor–cation complex (R·M2+) in the solution (Figure 3a and Figure S13 of the Supporting Information), (ii) “naked eye” colorimetric detection (yellow to red/brown) after addition of metal ions (Figure 3c and Figure S11 of the Sup-

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www.eurjic.org porting Information), and (iii) formation of isosbestic points and a significant perturbation of the absorption bands in the UV/Vis spectra upon the addition of metal ions (Figure 7–Figure 9 and Figure S24 of the Supporting Information). The binding and coordination nature of these divalent cations towards receptors 4–6 were studied by using redox and optical spectral studies. The electrochemical detection of the above mentioned divalent cations by the receptors 4–6 was demonstrated by CV and DPV studies. DPV studies for receptor 4 were carried out by titration with Co2+, Cu2+, Hg2+, and Zn2+ ions, and it exhibited a typical two-wave behavior[17] (in the case of the Hg2+ ion no significant changes were observed) corresponding to the free receptor and cation–receptor-complexed species at a more positive potential (Figure 4). By increasing the amount of guest (cation) in the test solution, the free receptor current peak gradually disappeared and the corresponding cation–receptor complex peak gradually increased (Figure 3). A maximum shift in the redox potential was observed for Co2+ (ΔE = 156 mV) and a minimum shift was observed for Zn2+ (ΔE = 129 mV). The shift in the redox potential was due to the complex formation of the ligand with metal ions. The small shift of redox potential for Zn2+ was probably due to the high electropositive nature of the metal.

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Figure 5. Evolution of the LSV of receptor 4 (red line) in CH2Cl2 (1.0 ⫻ 10–3 m) using [(nBu)4N]ClO4 (0.1 m) as the supporting electrolyte by the sequential addition of 0–1.0 equiv. of (a) Cu2+ (blue line) and (b) Hg2+ (blue line) in methanol (1.0 ⫻ 10–2 m).

Figure 4. Evolution of (a) CV and (b) DPV of receptor 4 (red line) in CH2Cl2 (1.0 ⫻ 10–3 m), using [(nBu)4N]ClO4 (0.1 m) as the supporting electrolyte, by the sequential addition of 0–1.0 equiv. of Cu2+ (blue line) in methanol (1.0 ⫻ 10–2 m).

A similar behavior was observed for receptor 5 on the titrimetric addition of Co2+, Cu2+, and Zn2+ ions (Figure 6 and Figure S17–19 of the Supporting Information). It was observed from the DPV studies that from an increase in the amount of cation in the test solution of receptor 5 a new redox wave was generated at a more positive potential (Co2+, 680 mV; Cu2+, 665 mV; Zn2+, 656 mV). However, when these cations were added to receptor 6, the two wave behavior was not observed. Instead, a remarkable shift in the redox potential was observed after complexation with these metal ions (Table 1). The changes observed in the redox potential due to the addition of stoichiometric amounts of metal ions are mentioned in Table 1. From Table 1 it is evident that there is a marked decrease in the shift of redox potentials from receptors 4 to 6 after complexation (4, 156– 129 mV; 5, 126–103 mV; 6, 99–71 mV) with these divalent cations, and the complexation affinity is in the order 4 ⬎ 5 ⬎ 6. In order to understand the receptor–cation binding ability, the binding enhancement factor[18] has been calculated and the corresponding values are shown in Table 1. The BEF values for the Co2+ receptor–cation complex was found to be greater than those for the respective Cu2+ and Zn2+ receptor–cation complexes, whereas the BEF values for Cu2+ were slightly higher than those for the Zn2+ recep-

The detection of Cu2+ and Hg2+ by receptor 4 was further confirmed by conducting LSV (linear sweep voltammetry). From Figure 5a, it was observed that by stoichiometric addition of Cu2+ to the test solution of receptor 4 a significant shift of the redox voltammetric wave took place towards the higher positive potential. This was in agreement with the redox potential shift observed in CV and DPV (ΔE = 131 mV). These electrochemistry results indicated that, upon sequential addition of the Cu2+ ion, no oxidation of ferrocene took place. This suggests the formation of a Cu2+ complex with the pyridylpyrazoline unit of the free receptor 4. When the same experiment was carried out by the addition of the Hg2+ cation to receptor 4, the LSV curve showed a significant shift towards the cathodic current (Figure 5b). This indicates that the added Hg2+ ion promotes the oxidation of receptor 4 with its concomitant reduction to Hg+. This was supported by the absence of a shift in the redox potential of the CV and DPV studies.

Figure 6. Evolution of DPV of receptor 5 (red line) in CH2Cl2 (1.0 ⫻ 10–3 m) using [(nBu)4N]ClO4 (0.1 m) as the supporting electrolyte by the sequential addition of 0–0.5 equiv. of Co2+ (blue line) in methanol (1.0 ⫻ 10–2 m).

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tor complex. From Table 1, it is evident that among the three divalent cations the binding affinity towards the receptors is in the order Co2+ ⬎ Cu2+ ⬎ Zn2+. Table 1. Voltammetric data of receptors 4–6 and their metal complexes in CH2Cl2 solution. Compound 4 [4·Cu]2+ [4·Zn]2+ [42·Co]2+ 5 [5·Cu]2+ [5·Zn]2+ [52·Co]2+ 6 [6·Cu]2+ [62·Zn]2+ [62·Co]2+

E1/2 (free)[a] E1/2 (complex)[a] ΔE1/2 [mV][b] 552 553 552 549 553 552 553 554 548 549 550 549

BEF[c]

684 681 708

131 129 156

189 168 492

665 656 680

113 103 126

86 62 168

619 622 648

71 72 99

19 21 58

[a] Redox potential from DPV in CH2Cl2 (1.0 ⫻ 10–3 m) using [(nBu)4N]ClO4 (0.1 m) as the supporting electrolyte. In mV. [b] ΔE1/2 = E1/2 (complex) – E1/2 (free receptor). [c] See ref.[18]

The divalent metal ion recognition behavior of these receptors was also monitored by UV/Vis absorption spectroscopy. On sequential stepwise addition of Co2+, Cu2+, and Zn2+ ions, in methanol, to receptor 4 in CH2Cl2 solution, the HE band centered at 331 nm gradually decreased with a small red shift (Δλ = 2–4 nm) and disappeared after a stoichiometric amount of the corresponding cations were added (Figure 7 and Figure S24 of the Supporting Information). A similar behavior was observed for receptor 5 with a relatively larger red shift (Δλ = 2–6 nm) in the HE band centered at 335 nm (Figure 8 and Figure S25 of the Supporting Information) on sequential addition of metal ions. This was accompanied by an increase in the absorption intensity of shoulder bands at 350, 370, and 391 nm with a small red shift (Δλ = 2–4 nm). However, in the case of receptor 6, the HE band centered at 330 nm decreased (Figure 8 and Figure S27 of the Supporting Information) along with a small increment in the absorption of other shoulder bands. Slight but prominent changes in the absorption spectra were observed in the case of receptor 4 towards the Hg2+ ion. However, from the electrochemical experiments it was found that receptor 4 does not form a complex with the Hg2+ ion (Figure 5). This might be because of the weak binding of the Hg2+ ion to the ligand. It is worth mentioning that all the receptors show a red shift (Δλ = 4–13 nm) in their characteristic LE band after complexation (4, 462 nm; 5, 483 nm and 6, 471 nm) with the metal ion at the expense of their HE bands (Table 2). These facts were responsible for a change of color from orange to red/brown, which can be used for “naked eye” detection of these divalent metal ions. A special behavior was observed in the case of receptor 6, where, upon the sequential addition of Co2+, Cu2+, and Zn2+ ions, a marked bathochromic shift was observed at the HE band. On each addition of metal ion to the test solution, a new weak absorption band was formed in the region of 360–410 nm having maxima at 377 nm (Δλ = 47 nm) until the formation of Eur. J. Inorg. Chem. 2013, 6019–6027

Figure 7. UV/Vis spectral changes of receptor 4 (red line) in CH2Cl2 solution (3.2 ⫻ 10–5 m) upon the sequential addition of 0– 1.0 equiv. of Cu2+ (blue line) in methanol (1.0 ⫻ 10–2 m). Arrows indicate the absorption increases or decreases during the titration. Inset: Job’s plot indicates a 1:1 receptor–cation complex formation.

Figure 8. UV/Vis spectral changes of receptor 5 (red line) in CH2Cl2 solution (2.4 ⫻ 10–5 m) upon the sequential addition of 0– 0.5 equiv. of Co2+ (blue line) in methanol (1.0 ⫻ 10–2 m). Arrows indicate the absorption increases or decreases during the titration. Inset: Job’s plot indicates a 2:1 receptor–cation complex formation.

a receptor-cation complex[12h] (Figure 9 and Figure S26 in the Supporting Information). This weak absorption band might be due to the oxidized ferrocene derivatives and was assigned to a Cp–FeIII ligand-to-metal transfer (LMCT) transition.[12h] A Job’s plot analysis was carried out to find the exact stoichiometry of the metal ion with the corresponding receptor. From the Job’s plot analysis, it was found that receptors 4–6 formed complexes with Cu2+ and Zn2+ ions in a 1:1 (receptor/cation) stoichiometric ratio (for receptor 6, Zn2+ formed a 2:1 complex). While for Co2+, complex formation occurred with a 2:1 (receptor/cation) stoichiometric ratio. Formation of isosbestic points (4, 306-310 nm; 5 306311 nm, and 6, 302–310 nm) during titration analysis clearly indicated the formation of complexes without any intermediates. The absorption data observed during the formation of the receptor–cation complex from titration data,

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Table 2. UV/Vis data of receptors 4–6 in the presence of metal ions. λmax[a] (ε, 103 mol–1 cm–1)

Compound 4 [4·Cu]2+ [4·Zn]2+ [42·Co]2+ 5 [5·Cu]2+ [5·Zn]2+ [52·Co]2+ 6 [6·Cu]2+ [62·Zn]2+ [62·Co]2+

331 (72.38), 451 (3.2) 333 (46.22), 458 (5.6) 334 (58.14), 462 (4.4) 334 (47.12), 457 (5.1) 335 (61.0), 350 (50.9), 370 (36.2), 391 (26.7), 472 (2.3) 339 (41.1), 354 (50.7), 372 (49.3), 392 (33.9), 483 (5.1) 338 (57.0), 353 (62.8), 371 (59.7), 391 (37.5), 479 (4.1) 337 (42.0), 353 (50.3), 372 (50.8), 392 (34.7), 478 (4.8) 267 (41.3), 278 (57.6), 316 (42.7), 330 (65.9), 346 (74.1), 452 (5.2) 267 (48.5), 278 (62.5), 317 (36.3), 331 (58.5), 347 (72.6), 377 (11.9), 463 (6.4) 269 (35.9), 279 (45.8), 317 (25.3), 332 (42.5), 348 (55.9), 377 (17.1), 471 (6.3) 267 (45.9), 278 (60.8), 317 (38.5), 332 (43.0), 348 (58.5), 377 (13.9), 463 (6.0)

I.P.[b]

β[c]

310, 353 306, 348 308, 352

β1 = 9.55 ⫻ 105 β1 = 2.63 ⫻ 106 β1 = 1.08 ⫻ 106, β2 = 4.78 ⫻ 105

306, 348 311, 342 299, 353

β1 = 5.54 ⫻ 105 β1 = 1.39 ⫻ 106 β1 = 1.30 ⫻ 106, β2 = 6.51 ⫻ 105

310 301 302, 431

β1 = 1.08 ⫻ 105 β1 = 1.02 ⫻ 105 β2 = 5.51 ⫻ 104 β1 = 1.30 ⫻ 105 β2 = 4.32 ⫻ 104

[a] λmax in nm. [b] Isosbestic points in nm. [c] Binding constants are as follows: β1 in m–1 and β2 in m–2. The calculated values were accurate within ⫾6 %.

resulting titrations, shown in Figure S31 (Supporting Information), small or no obvious changes were observed in the detection of these cations (Zn2+, Cu2+, and Co2+). These results clearly indicate the selectivity of these receptors over the other metal ions. The presence of fluorophore moieties like naphthalene, anthracene, and pyrene within the structure of receptors 4– 6 prompted us to study their recognition properties by fluorometry (Figure 10 and Figure 11 as well as Figures S28– S30 of the Supporting Information). From the fluorescence titration experiments (Figure 10), it was observed that re-

Figure 9. UV/Vis spectral changes in receptor 6 (red line) in CH2Cl2 solution (1.6 ⫻ 10–5 m) upon the sequential addition of 0– 0.5 equiv. of Zn2+ (blue line) in methanol (1.0 ⫻ 10–2 m). Arrows indicate the absorption increases or decreases during the titration. Inset: Job’s plot indicates a 2:1 receptor–cation complex formation.

stoichiometries, and stability constants[19] are displayed in Table 2. The stability constants indicate that among the divalent cations the affinity towards the receptors is Co2+ ⬎ Cu2+ ⬎ Zn2+. However, the stoichiometries proposed from the absorption data (Job’s plot analysis) was further confirmed by ESI-MS studies on receptors 4 and 5 in the presence of metal ions. From the ESI-MS spectra of receptor 4 in the presence of the Co2+ metal ion it was found that m/z = 996 (42 + Co + Na)+, whereas for receptor 5 m/z = 1075 (52 + Co)+ [Figures S32 and S33 of the Supporting Information]. Similarly, for receptor 4 in the presence of the Zn2+ metal ion m/z = 545 (4 + Zn + Na)+ and for receptor 5 m/z = 660 (5 + Zn + CH2Cl2)+ (Figures S34 and S35 of the Supporting Information). To evaluate the further importance of receptors 4–6 as a selective chemosensor towards the aforementioned cations, competitive experiments were carried out. Thus, a solution of 4 and 5 (3.2–2.4 ⫻ 10–5 m) was treated separately with 1.0 equiv. of Zn2+ and Cu2+ and 0.5 equiv. of Co2+ in the presence of 1.0 equiv. each of the interfering metal ions (Mg2+, Ca2+, Mn2+, Fe2+, Cd2+, Ni2+, and Pb2+). From the Eur. J. Inorg. Chem. 2013, 6019–6027

Figure 10. Fluorescence spectral changes of receptor 4 (red line) in CH2Cl2 (3.2 ⫻ 10–5 m) upon the sequential addition of 0–1.0 equiv. of Zn2+ (blue line) in methanol (1.0 ⫻ 10–2 m). Arrows indicate the emission intensity increases during the titration.

Figure 11. Fluorescence spectral changes of receptor 6 (red line) in CH2Cl2 (1.6 ⫻ 10–5 m) upon the sequential addition of 0–0.5 equiv. of Co2+ (blue line) in methanol (1.0 ⫻ 10–2 m). Arrows indicate that the pyrene-excimer intensity decreases during the titration.

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www.eurjic.org ceptor 4 promoted an increase in the emission band intensity upon the sequential addition of Co2+, Cu2+, and Zn2+ ions. On addition of these divalent metal ions to receptor 4 in CH2Cl2 solution (Figure 10), a red shift (Δλ = 15 nm) was observed for the emission-band maximum at 430 nm. This was accompanied by an increase in the chelation-enhanced fluorescence[20] (CHEF, 7 for Zn2+, 4 for Cu2+, and 2 for Co2+) with a 4–9-fold increment in the quantum yield (Φf = 8.98–19.68 ⫻ 10–3). The lack of conjugation of the fluorophore with the binding site leads to lower emission enhancement after the complex formation. The binding constants of receptor 4 with the corresponding cations were calculated from the emission data using the modified Benesi–Hildebrand equation[21] (Figure S36 of the Supporting Information) and was found to be ([4·Zn]2+ = 2.71 ⫻ 106 m–1, [4·Cu]2+ = 8.59 ⫻ 105 m–1 with ⫾10 % error) similar to those observed in the absorption data. The sensitivity of receptor 4 towards these metal ions was determined by calculating the detection limit[22] from the emission data (Figure S37 of the Supporting Information) and was found to be 1.43 μm for the Zn2+ metal ion whereas for Cu2+ ion it was 1.79 μm. The anthracene-based receptor 5 did not show any considerable change in the emission spectra with the addition of these cations (Figure S29 of the Supporting Information). It can be anticipated that, in the excited state, there is no significant effect on the anthracene fluorescence due to the metal complexation. This is perhaps because of the interaction of the ortho-hydrogen atoms of the aromatic substituents. Similar behavior was also observed in the case of the pyrene-based receptor 6, the emission band centered at 430 nm was not altered even at higher concentrations of metal ions. However, a gradual decrease of the pyrene-excimer band centered at 503 nm was observed upon sequential addition of metal ions to receptor 6 (Figure 12 and Figure S30 of the Supporting Information). This indicates that, with an increase in the metal ion concentration, the emission band of the pyrene-excimer for the free receptor decreases leading to the formation of the cation–receptor complex.[23]

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The most significant spectral changes observed, upon the addition of Zn2+ (as triflate salt in a mixture of CD3CN and CDCl3 solution) to a solution of free receptor 4, was the simultaneous downfield shift of the following signals: (i) the protons corresponding to both the unsubstituted and substituted cyclopentadienyl unit of the ferrocene moiety (ΔδHcp = 0.24 ppm, ΔδHα = 0.17–0.38 ppm, and ΔδHβ = 0.21–0.23 ppm), (ii) the protons associated with the C4 atom of the pyrazoline ring (ΔδHtrans = 0.33 ppm, ΔδHcis = 0.64 ppm), and (iii) the protons associated with the 2-pyridine ring (ΔδH6 = 0.11 ppm). A similar behavior was observed when Cu2+ (as triflate salt) was added to the solution of free receptor 4, which indicated that the cation interacts with the nitrogen atoms of the 2-pyridine unit and 1Hpyrazoline ring.

Conclusions The ferrocenyl-pyridylpyrazoline fluorophore triads 4–6 were prepared by a one-pot method using simple and easily available commercial compounds. The compounds were fully characterized by all the spectroscopic techniques and the full structure of compound 4 was established by means of X-ray diffraction analysis. The interaction of the pyridylpyrazoline moiety with the divalent cations has been successfully examined using electrochemical and optical studies. Electrochemical studies showed that receptors 4–6 induced clear perturbation in the redox couple (Fc/Fc+) towards Cu2+, Co2+, and Zn2+ ions with a cathodic shift in redox potential (ΔE = 71–156 mV). The disappearance of the HE band accompanied by a red shift of the LE band (Δλ = 7–13 nm) was observed in the absorption spectra of these receptors 4–6 upon complexation with the cations. This effect was responsible for the change of color from yellow to red/brown, making it facile for “naked eye” detection of these cations. The emission spectrum (λex = 350 nm) of 4 undergoes a 2–7-fold increase in CHEF in the presence of these metal ions. The metal-ligand binding was observed by a shift in the 1H NMR spectra of certain signals of the pyridyl, pyrazoline, and cyclopentadienyl protons. In summary, among the three receptors, 4 could be considered as a valuable receptor for sensing these cations by three different channels; electrochemical, colorimetric, and fluorescent.

Experimental Section

Figure 12. Evolution of the 1H NMR spectrum of 4 upon addition of 1 equiv. Cu2+ and Zn2+ in a mixture of CD3CN and CDCl3. Inset: proposed binding of the metal ion with receptor 4.

Furthermore, 1H NMR spectroscopic analyses were performed to study the binding of the metal ions to the receptors 4–6 by observing the shift in the signals (Figure 12). Eur. J. Inorg. Chem. 2013, 6019–6027

General Information: All the reactions were carried out under a nitrogen atmosphere by using high purity commercial solvents without any further purification. Acetate salts of Li+, Na+, K+, Mg2+,Ca2+, Cr2+, Mn2+, Fe2+, Co2+, Cu2+, Zn2+, Cd2+, Ni2+, Pb2+, and Hg2+, acetyl ferrocene, aromatic aldehydes, and 2-hydrazinopyridine were purchased from Sigma–Aldrich and used without further purification. Chromatography was carried out using neutral alumina. Melting points were determined using a Toshniwal apparatus. FTIR spectra were recorded with a Thermo Nicolet Nexus 670 spectrophotometer using KBr discs. 1H NMR and 13C NMR spectra were recorded at room temperature with a Bruker Avance 300 or Varian Inova 500 spectrometer in CDCl3 or [D6]DMSO as

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www.eurjic.org the solvent; chemical shifts (δ) were reported in ppm (parts per million) with TMS as the internal standard. Coupling constants (J) were given in Hertz (Hz). For ESI-MS, m/z values were expressed in atomic mass units. Synthesis of Ferrocenylpyrazolines: The appropriate aromatic aldehyde (1 mmol) was slowly added to a solution of acetyl ferrocene (1) (228 mg, 1 mmol) and KOH (3 mmol) in ethyl alcohol (15 mL) and stirred at room temperature for 5–6 h. After complete consumption of acetyl ferrocene (monitored by TLC, a red to purple color solid was formed), 2-hydrazino pyridine (120 mg, 1.1 mmol) in ethyl alcohol (5 mL) was slowly added and the resulting mixture was refluxed overnight. After completion of the reaction (monitored by TLC, a yellow color solid was formed), the reaction mixture was cooled, the solid formed was filtered, washed with a cold ethanol/water mixture, and air dried. The resulting solid was purified by column chromatography using an n-hexane/ethyl acetate mixture (9:1 to 7:3) to give the appropriate ferrocene derivative. 1-(2-Pyridyl)-3-ferrocenyl-5-(1-naphthyl)-4,5-dihydro-1H-pyrazoline (4): According to the general procedure, starting from 1-naphthaldehyde (156 mg, 1 mmol), 4 (339 mg, 74 %) was obtained as an orange solid. M.p. = 140–141 °C. 1H NMR (500 MHz, CDCl3): δ = 3.00 [dd, J = 12.0, 4.5 Hz, 1 H, H4 (trans), pyrazoline], 3.90 [dd, J = 12.0, 4.5 Hz, 1 H, H4 (cis), pyrazoline], 3.99 (s, 5 H, Cp), 4.31 1s, β-, 1 H, (Cp), 4.34 (1 H, 1s, β-Cp), 4.49 (1 H, 1s, α-Cp), 4.72 (1 H, 1s, α-Cp), 6.42 (dd, 1 H, J = 11.5, 5.3 Hz, H5, pyrazoline), 6.59 (t, 1 H, J = 6.0 Hz, H5, pyridine), 7.30 (s, 1 H, H3, pyridine), 7.34–7.40 (m, 2 H, aromatic), 7.48–7.58 (m, 2 H, aromatic), 7.63 (m, 1 H, H4, pyridine), 7.75 (d, 1 H, J = 8.3 Hz, aromatic), 7.92 (d, 1 H, J = 8.3 Hz, aromatic), 7.98 (m, 1 H, aromatic), 8.15 (d, 1 H, J = 8.3 Hz, H6, pyridine) ppm. 13C NMR (75 MHz, CDCl3): δ = 43.5, 55.5, 151.5 (pyrazoline), 69.2 (Cp), 66.6, 67.1 (α-Cp), 69.2, 69.7 (β-Cp), 108.5, 113.7, 122.3, 123.4, 125.5, 125.6, 126.0, 127.6, 129.1, 129.9, 134.4, 136.9, 137.5, 147.9, 155.5 (aromatic) ppm. FTIR (KBr): ν˜ = 3081 ν(C–H)Ar, 2927 ν(C–H)Al, 1590 ν(C=C)Ar, 1506 ν(C=N)Ar, 1093 νs(C–N), 539 and 477 νas(Cp–Fe–Cp) cm–1. ESI-MS: m/z = 458 [M + H]+. C28H23FeN3 (457.36): calcd. C 73.53, H 5.07, N 9.79; found C 73.84, H 5.12, N 9.01. 1-(2-Pyridyl)-3-ferrocenyl-5-(9-anthryl)-4,5-dihydro-1H-pyrazoline (5): According to the general procedure, starting from 9-anthraldehyde (206 mg, 1 mmol), 5 (298 mg, 59 %) was obtained as an orange solid. M.p. = 197–198 °C. 1H NMR (500 MHz, CDCl3): δ = 3.49 [dd, J = 12.0, 4.8 Hz, 1 H, H4 (trans), pyrazoline], 3.90 [dd, J = 12.0, 4.8 Hz, 1 H, H4 (cis), pyrazoline], 4.23 (s, 5 H, Cp), 4.39 (1 H, 1s, β-Cp), 4.43 (1 H, 1s, β-Cp), 4.56 (1 H, 1s, α-Cp), 4.90 (1 H, 1s, α-Cp), 6.46 (m, 1 H, H5, pyridine), 6.86 (dd, J = 10.5, 2.3 Hz, 1 H, H5, pyrazoline), 7.22 (d, J = 8.3 Hz, 1 H, H3, pyridine), 7.32– 7.47 (m, 2 H, aromatic), 7.50–7.64 (m, 2 H, aromatic), 7.73 (d, J = 8.3 Hz, 1 H, H4, pyridine), 8.00 (d, J = 9.0 Hz, 1 H, aromatic), 8.07 (d, J = 8.3 Hz, 1 H, aromatic), 8.07 (d, J = 8.3 Hz, 1 H, H6, pyridine), 8.37 (d, J = 9.0 Hz, 1 H, aromatic),8.43 (s, 1 H, aromatic), 8.72 (d, J = 9.0 Hz, 1 H, aromatic) ppm. 13 C NMR (75 MHz, CDCl3): δ = 43.9, 60.7, 148.7 (pyrazoline), 69.4 (Cp), 66.6, 67.1 (α-Cp), 69.8, 77.9 (β-Cp), 113.5, 119.0, 122.3, 124.5, 124.9, 125.2, 126.1, 126.8, 128.5, 128.7, 129.2, 129.3, 129.5, 129.8, 131.3, 132.2, 132.5, 134.0, 146.5 (aromatic) ppm. FTIR (KBr) ν˜ = 3069 ν(C–H)Ar, 2901 ν(C–H)Al, 1579 ν(C=C)Ar, 1513 ν(C=N)Ar, 1105 νs(C–N), 487 νas(Cp–Fe–Cp) cm–1. ESI-MS: m/z = 508 [M + H]+. C32H25FeN3 (507.42): calcd. C 75.75, H 4.97, N 8.28; found C 75.29, H 5.03, N 8.41. 1-(2-Pyridyl)-3-ferrocenyl-5-(1-pyrenyl)-4,5-dihydro-1H-pyrazoline (6): According to the general procedure, starting from 1-pyrenecarboxaldehyde (230 mg, 1 mmol), 6 (292 mg, 52 %) was obtained as Eur. J. Inorg. Chem. 2013, 6019–6027

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an orange solid. M.p. = 241–242 °C. 1H NMR (500 MHz, CDCl3): δ = 3.12 [dd, J = 12.0, 4.5 Hz, 1 H, H4 (trans), pyrazoline], 4.07 [dd, J = 12.0, 4.5 Hz, 1 H, H4 (cis), pyrazoline], 4.00 (s, 5 H, Cp), 4.32 (1 H, 1s, β-Cp), 4.35 (1 H, 1s, β-Cp), 4.51 (1 H, 1s, α-Cp), 4.75 (1 H, 1s, α-Cp), 6.57 (t, J = 5.3 Hz, 1 H, H5, pyridine), 6.70 (dd, J = 6.8, 5.3 Hz, 1 H, H5, pyrazoline), 7.43 (d, J = 8.3 Hz, 1 H, H3, pyridine), 7.51 (t, J = 6.8 Hz, 1 H, aromatic), 7.86 (d, J = 8.3 Hz, 1 H, H4, pyridine),7.91 (d, J = 6.8 Hz, 1 H, aromatic), 7.97–8.03 (m, 3 H, aromatic), 8.04–8.10 (m, 2 H, aromatic), 8.19 (d, J = 6.8 Hz, 1 H, aromatic), 8.24 (d, J = 8.3 Hz, 2 H, aromatic), 8.45 (d, J = 9.8 Hz, 1 H, H6, pyridine) ppm. 13C NMR (75 MHz, CDCl3 + [D6]DMSO): δ = 42.7, 57.7, 151.5 (pyrazoline), 68.5 (Cp), 65.4, 66.6 (α-Cp), 69.1, 69.3 (β-Cp), 107.9, 113.0, 121.4, 122.8, 124.8, 125.0, 125.6, 126.9, 128.4, 129.2, 133.6, 136.3, 146.9, 151.0, 154.7 (aromatic) ppm. FTIR (KBr) ν˜ = 3099 ν(C–H)Ar, 2941 ν(C– H)Al, 1599 ν(C=C)Ar, 1533 ν(C=N)Ar, 1088 νs(C–N), 507 νas(Cp– Fe–Cp) cm–1. ESI-MS: m/z = 531 [M + H]+. C34H25FeN3 (531.44): calcd. C 76.84, H 4.74, N 7.91; found C 77.02, H 4.68, N 8.01. X-Ray Structural Analysis: A suitable crystal of compound 4 was selected for collecting X-ray data at room temperature using a Bruker Smart Apex CCD diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) with the ω-scan method.[24] Preliminary lattice parameters and orientation matrices were obtained from four sets of frames. Unit cell dimensions were determined using 3326 reflections over the range 2.27° ⬍ θ ⬍ 20.56°. Integration and scaling of intensity data was accomplished using the SAINT program.[24] The structure was solved by direct methods using SHELXS97[25] and refinement was carried out by a full-matrix least-squares technique using SHELXL97.[25] Anisotropic displacement parameters were included for all non-hydrogen atoms. H atoms were positioned geometrically and treated as riding on their parent C atoms [C–H = 0.93–0.97 Å and Uiso(H) = 1.2Ueq(C) for H atoms]. CCDC-935804 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Electrochemistry: Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed with CH instruments from the 620C series, using a conventional three-electrode cell consisting of a glassy carbon working electrode, a platinum wire auxiliary electrode, and standard calomel electrode (SCE) as the reference electrode with a scan rate of 100 mV s–1. All the experiments were carried out in a CH2Cl2 (1.0 ⫻ 10–3 m) solution containing 0.1 m [(nBu)4N]ClO4 as the supporting electrolyte. All potential values reported were relative to a ferrocene couple. Deoxygenation of solution was achieved by bubbling nitrogen for at least 10 min, and the working electrode was cleaned after each run. The cation under investigation was added in methanol (1.0 ⫻ 10–2 m). Optical Properties: Absorption and emission spectra of receptors 4–6 were recorded with a UV 3600 (Shimadzu) and spectrofluorimeter (FluoroLog-3, Horiba Jobin Yvon), respectively, in a CH2Cl2 solution ([3] = 3.2 ⫻ 10–5 m, [4] = 2.4 ⫻ 10–5 m and [6] = 1.6 ⫻ 10–5 m). The cation under investigation was added as a methanol solution ([M2+] = 1.0 ⫻ 10–2 m). For fluorescence quantum yield measurements, the test solutions in CH2Cl2 were optically matched at the excitation wavelength (350 nm). The quantum yield was calculated by comparing the integrated areas under the emission curves using quinine sulfate in 0.1 m H2SO4 (Φf = 0.57, at 22 °C) as reference.[9b] The measured Φf values were accurate within ⫾10 %.

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www.eurjic.org Supporting Information (see footnote on the first page of this article): NMR and ESI mass spectra of receptors 4–6, crystallographic data and structural refinement parameters for receptor 4, electrochemical, absorption, and fluorescence spectral changes by titrimetric addition of metal ions to receptors 4–6.

Acknowledgments This work is financially supported by the CSIR-IICT in-house project MLP-0008. Ch. K. K. is grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi for the award of a research fellowship. [1] a) A. P. De Silva, H. Q. N. Gunaratne, T. Gunnalaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Rademacher, T. E. Rice, Chem. Rev. 1997, 97, 1515–1566; b) K. Szacilowski, W. Macyk, A. D. Matuszek, M. Brindell, G. Stochel, Chem. Rev. 2005, 105, 2647–2694; c) V. Amendola, L. Fabbrizzi, F. Forti, M. Licchelli, C. Mangano, P. Pallavicini, A. Poggi, D. Sacchi, A. Taglieti, Coord. Chem. Rev. 2006, 250, 273–299; d) E. L. Que, D. W. Domaille, E. J. Chang, Chem. Rev. 2008, 108, 1517–1549; e) J. Du, M. Hu, J. Fan, X. Peng, Chem. Soc. Rev. 2012, 41, 4511–4535. [2] a) Fluorescent Chemosensors for Ion and Molecule Recognition (Ed.: A. W. Czarnik), ACS Symposium series, Washington, DC, 1993, vol. 538; b) B. Valeur, I. Leray, Coord. Chem. Rev. 2000, 205, 3–40; c) A. P. De Silva, D. B. Fox, A. J. M. Huxley, T. S. Moody, Coord. Chem. Rev. 2000, 205, 41–57; d) L. Prodi, F. Bolletta, I. Leray, Coord. Chem. Rev. 2000, 205, 59–83; e) K. Rurack, Spectrochim. Acta Part A 2001, 57, 2161–2195; f) A. P. De Silva, B. McCaughan, B. O. F. McKinney, M. Querol, Dalton Trans. 2003, 1902–1913; g) J. F. Callan, A. P. De Silva, D. C. Magri, Tetrahedron 2005, 61, 8551–8588; h) D. T. Quang, J. S. Kim, Chem. Rev. 2010, 110, 6280–6301; i) E. B. Veale, T. Gunnlaugsson, Annu. Rep. Prog. Chem. Sect. B 2010, 106, 376– 406; j) R. K. Pathak, J. Dessingou, C. P. Rao, Ana. Chem. 2012, 84, 8294–8300; k) M. T. Albelda, J. C. Frias, E. Garsia-Espana, H.-J. Schneider, Chem. Soc. Rev. 2012, 41, 3859–3877. [3] a) M. Samerio, T. Gonclaves, Chem. Rev. 2009, 109, 190–212; b) X. Chen, X. I. Tian, I. Shin, J. Yoon, Chem. Rev. 2011, 111, 7941–7980; c) X. Chen, T. Pradhan, F. Wang, J. S. Kim, J. Yoon, Chem. Rev. 2012, 112, 1910–1956. [4] a) M. Verma, A. F. Chaudhary, M. T. Morgan, C. J. Fahrni, Org. Biomol. Chem. 2010, 8, 363–370; b) R. D. Hancock, Chem. Soc. Rev. 2013, 42, 1500–1524. [5] D. S. McClure, J. Chem. Phys. 1952, 20, 682–686. [6] A. W. Varnes, R. B. Dodson, E. L. Whery, J. Am. Chem. Soc. 1972, 94, 946–950. [7] N. S. Hush, Coord. Chem. Rev. 1985, 64, 135–157. [8] a) P. D. Beer, P. A. Gale, G. Z. Chen, Coord. Chem. Rev. 1999, 185–186, 3–36; b) P. D. Beer, J. Cadman, Coord. Chem. Rev. 2000, 205, 131–155; c) P. D. Beer, E. J. Hayes, Coord. Chem. Rev. 2003, 240, 167–189. [9] a) R. Giasson, E. J. Lee, X. Zhao, M. Wrighton, J. Phys. Chem. 1993, 97, 2596–2601; b) C. D. Geddes, J. R. Lakowicz in Advanced concepts in fluorescence spectroscopy: Small molecule sensing, Part A; Springer, USA, 2005, Vol. 9. [10] a) S. Fery-Forgues, B. Delavaux-Nicot, J. Photochem. Photobiol. A: Chem. 2000, 132, 137–159; b) R. Zhang, Z. Wang, Y. Wu, H. Fu, J. Yao, Org. Lett. 2008, 10, 3065–3068. [11] a) J. L. Lopez, A. Tarraga, A. Espinosa, M. D. Velasco, P. Molina, V. Lloveras, J. Vidal-Gancedo, C. Rovira, J. Veciana, D. J. Evans, K. Wurst, Chem. Eur. J. 2004, 10, 1815–1826; b) A. Caballero, V. Lloveras, D. Curiel, A. Tarraga, A. Espinosa, R. Garcia, J. Vidal-Gancedo, C. Rovira, K. Wurst, P. Molina, J. Veciana, Inorg. Chem. 2007, 46, 825–838; c) F. Zapata, A. Caballero, A. Espinosa, A. Tarraga, P. Molina, Org. Lett. 2007, 9, 2385–2388.

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