Mercury Ions Complexation with a Series of Heterocyclic Derivatives of

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Mercury Ions Complexation with a Series of Heterocyclic Derivatives of 3-Hydroxychromone: Spectral Effects and Prospects for Ultrasensitive Hg2þ Probing Denis Svechkarev, Bogdan Dereka, and Andrey Doroshenko* Institute for Chemistry, Kharkov V. N. Karazin National University, 4 Svobody Square, 61077 Kharkov, Ukraine ABSTRACT: Complexation of three 3-hydroxychromone derivatives bearing a nitrogen-containing heterocyclic moiety in the position 2 of the chromone bicycle benzimidazole, quinoline, and 2,5-diphenyloxazole, with mercury(II) ions is reported. Formation of chelate complexes with the metal cations coordinated with the cavity formed by 3-OH and 4-CdO groups was shown, as well as the possibility of side moiety heteroatom participation in binding of metal ions. High sensitivity to mercury of 2,5-diphenyloxazole-substituted 3-hydroxychromone was elucidated, allowing to detect Hg2þ below the maximum permissible concentration for drinking water. This makes the above-mentioned compound a prospective basis for development of sensors for ultralow mercury concentration detection in water. Unusual fluorescence ignition of 2-(quinolin-2-yl)-3hydroxychromone at low Hg2þ concentrations, rarely observed for heavy metals ions complexation with organic fluorescent ligands, was discussed.

1. INTRODUCTION Being among the highly toxic chemical elements able to impact ecology and human health, mercury is one of the most studied environmental pollutants. Because of its high bioaccumulation, mercury concentrations increase considerably up to the food chains. Depending on the objective, different ways of mercury content analysis can be applied. But to investigate Hg transformations and transport processes, determination of all the existing mercury species is important using a combination of various analytical methods.1 Selective and sensitive detection of Hg2þ is a question of primary importance, particularly for environmental monitoring. Therefore, numerous analytical strategies have been developed, including electrochemical methods,2,3 optical techniques based on doped inorganic phosphors and their nanoparticles,4 and organic molecular probes, including those on the polymer support.57 In the past few years, a new type of biosensor for inflow mercury monitoring was developed based on luminescent bacteria and yeast cells as biomonitors.8 In the domain of the new molecular fluorescent probes design, particular attention is focused on the compounds having multiple bands in their emission spectra. The example of the most popular molecules demonstrating such a feature are the representatives of 3-hydroxychromone (3HC) series. These substances are characterized with unique spectral properties determined by a fast adiabatic photochemical process occurring in their electronically excited state, the excited-state intramolecular proton transfer reaction (ESIPT, Figure 1).9,10 This ultrafast excited state process (240 fs for unsubstituted flavonol molecule11) results in the appearance of two bands in r 2011 American Chemical Society

the 3HC fluorescence spectrum that are due to the emission of the so-called normal form (N*, with a hydroxy group proton in its initial position) and the phototautomer one (T*, where the hydroxylic proton moves to the carbonyl oxygen in the position 4). Moreover, the ratio of the integral fluorescence intensities of these bands depends neither on fluorescent molecule concentration in the sample nor on the excitation light intensity. The discussed ratio is exclusively determined by the parameters of the probe’s microenvironment: polarity, proton donor ability, presence of the surface active compounds, and so on.1215 This allowed the development of the new analytical approach, ratiometric fluorescent monitoring, where the emission bands intensity ratio serves as the analytical signal.16 Thus, it has become possible to determine characteristics of different media in complex biological and ecological objects and samples, in the cases where application of traditional intensometric techniques can be challenging due to high complexity or principal impossibility to control the probe concentration in the sample. Novel techniques based on the principles of ratiometric fluorescence detection have also been suggested for mercury analysis, and most of them are now based on complex or hybrid probes.17,18 In advantage, molecules of a number of 3HC derivatives demonstrate high affinity to metal ions in solutions and often act as ligands. It is important to stress that spectral properties of 3-hydroxychromones complexes with multivalent metal ions differ significantly from those of unbound dyes molecules. In Received: November 17, 2010 Revised: March 4, 2011 Published: April 04, 2011 4223

dx.doi.org/10.1021/jp110974n | J. Phys. Chem. A 2011, 115, 4223–4230

The Journal of Physical Chemistry A

Figure 1. Excited-state intramolecular proton transfer in the molecule of 3-hydroxychromone.

Figure 2. Structure of possible 3HC complexes with polyvalent metals ions.

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of the proton accepting sites have been described for benzimidazole,32,33 pyridine,34 pyrazole35 and quinoline36 3HC derivatives. Beside the possibility of several hydrogen bonding pathways in these molecules, a particular uncertainty arises upon the interaction with metal ions, as they possess not only one ortho-hydroxy/carbonyl chelating cavity traditional for all 3-hydroxychromones, but several other cavities, potentially able to form chelate complexes, and/or several metal binding centers with high affinity to metal ions. Thus, additional complex types become possible for such compounds, where the metal ion is chelated inside the “alternative” cavity formed with participation of the side heterocycle (type c; Figure 9), and also “outer” complexes with coordination of the metal ion with the lone pair of a heteroatom of the side moiety (type d; Figure 7). From the general point of view, molecules exhibiting H-bonding and metal binding concurrence are interesting not only because of their sensitivity and selectivity to different cations, but also due to the uncertainty in mechanisms of their complex formation and spectral behavior of the formed interaction products. In the present paper, we discuss spectral behavior of three heterocyclic representatives of the 3-hydroxychromone family upon their interaction with mercury ions, Hg2þ, and also consider a possibility of different types of complexes formed for the products of the metal binding.

2. EXPERIMENTAL SECTION

Figure 3. “Flavonol-like” (left) and “alternative” (right) hydrogen bonds in III.

particular, their spectral and fluorescent parameters considerably depend on the nature and concentration of a metal ion, as well as solvent, temperature, and so on. This gives a possibility to apply these compounds as prospective fluoroionophores for qualitative and quantitative detection of metal ions in solutions. Thus, complex formation of 3HC derivatives has been investigated for Sc3þ, Ga3þ, Th3þ,19 Al3þ,20 Pb2þ,21 Sn2þ, Cu2þ,22,23 as well as iron ions,24 other transition metals cations,25 and several anions and neutral molecules.26,27 A separate note should be made about the development of sensors for alkali and alkali-earth metals, which are particularly important for biological and biochemical applications. Several crown-containing 3HC derivatives have been suggested as a base for such sensors.27,2931 There are several types of complexes assumed for flavonol derivatives, which differ by their structure and stoichiometry28,29 (Figure 2): chelate, so-called “outer” and mixed complexes. In the chelate complexes, the metal ion enters the complexation cavity in the dye’s molecule, thus, forming a stable cyclic structure (type a). “Outer” complexes are formed due to weak donor acceptor interaction of the metal ion with the lone pair of the carbonyl group oxygen atom, which is not involved in the intramolecular hydrogen bonding with 3-hydroxy group (type b). Mixed type complexes are polymetallic formations that represent a combination of both above-discussed complex types (type ab). Several 3-hydroxychromone derivatives possessing a heterocyclic moiety in the position 2 of the chromone bicycle are shown to be capable of realizing alternative pathways of intramolecular hydrogen bond formation (Figure 3). In particular, such concurrence

2.1. Subjects of Investigation. Complexation of three 3-hydroxychromone derivatives containing a heterocyclic moiety in the position 2 of chromone bicycle, compounds IIII (Figure 4), with mercury(II) ions in methanol solutions was investigated. In our previous papers, we described synthetic procedures for incorporation of benzimidazole,32,33 quinoline,36 and 2,5-diphenyloxazole37 moieties into the parent chromone system. Due to the sensitivity of 3-hydroxychromones to the presence of water traces, methanol chosen as a solvent for spectral investigations was additionally dried following the procedure described in ref 38. The following commercially available anhydrous metal salts were used: mercury acetate, cadmium chloride, nickel chloride, and lead acetate. Barium perchlorate was obtained from perchloric acid and barium oxide, then purified by recrystallization, and dried under gravimetrical control by roasting a sample of its trihydrate at 215 °C and 0.15 mmHg for 3 h.39 2.2. Spectral Measurements. Absorption spectra of the methanol solutions with the dyes’ concentration in the range of (16) 105 M were recorded with Hitachi U3210 spectrophotometer, and fluorescence spectra were recorded with Hitachi F4010 spectrofluorimeter in the standard 10 mm quartz cell at 20 °C. Spectrophotometric and spectrofluorimetric titrations were performed in the spectrofluorimetric cell with 2 mL of the solution of compounds IIII by adding from 3 to 150 μL of the correspondent metal salt solution (mercury acetate or barium perchlorate) with the calibrated micropipet. Absorption and fluorescence spectra were recorded after every single addition and corrected on the dilution coefficient. Fluorescence spectra were corrected on the changes of absorption at the excitation wavelength. 2.3. Results Processing. Estimation of the apparent complexation equilibrium constants and stoichiometry of the forming complexes was performed with the specially developed program realizing the Fletcher-Powell algorithm for nonlinear least4224

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Figure 4. 3HC heterocyclic derivatives under study.

Figure 5. Absorption spectra changes for compounds I (on the top, 1010 to 103 M Hg2þ) and II (at the bottom, 108 to 104 M Hg2þ).

squares.40 Five wavelengths within the long-wavelength absorption band have been chosen for analysis. Obtained equilibrium constants were averaged by weighted scheme using the inverse standard deviations squares of the estimated Ki values for each analytical wavelength as statistical weights. Quantum-chemical calculations with the unrestricted geometry optimization of the unbound molecules under study and their complexes with Hg(II) ions were performed using the full-valent semiempirical method PM6.41 Calculations of the electron density redistribution upon electronic excitation for compound I were made with Gaussian 03 (release E.01;42 for ground state molecular structure optimization) and NWChem (version 5.143 equipped with special module for ESS analysis,44 “excited state structural analysis”, for electronic spectra calculation) program packages.

3. RESULTS AND DISCUSSION 3.1. Benzimidazole and 2,5-Diphenyloxazole Derivatives. Investigation of the mercury ions effect on the spectral behavior of the molecules under study allowed revealing its similarity for

compounds I and II. Figure 5 depicts absorption spectra changes of compounds I (top) and II (bottom), while the concentration of Hg2þ increases from 1010 to 103 M for the 2,5-diphenyloxazole derivative and from 108 to 104 M for the benzimidazole one. As one can see, at mercury ions addition to the solution, appearance and growth of the new absorption band was observed for both discussed fluorophores. Binding of Hg2þ ions to compounds I and II results in significant bathochromic shifts in their absorption spectra. Such character of the complex absorption band in both cases can be explained by the Hg2þ ion coordination with the electron acceptor center of the ligand molecule, which increases the intensity of intramolecular charge transfer at the electronic excitation. The absorption data for the unbound compounds and their complexes are presented in Table 1. Sufficient complexation-related red shifts and relatively high stability constants allow us to assume the formation of the chelate complexes in both cases.29 Presence of Hg2þ ions in the solution influences also the fluorescent properties of the investigated compounds. Being an effective fluorescence quencher, mercury forms weakly fluorescent complexes Ia and IIc. The intracomplex heavy atom effect connected with the increase of spinorbital interaction probability results in the efficient intersystem crossing regulated depopulation of the S1 state. This is the most probable mechanism of the mercury-induced fluorescence quenching. At the same time, fluorescence intensities of the normal and tautomeric forms of I and II decrease in a different manner. As it is shown in the Figure 6, phototautomer emission intensity lowers quicker than that of the normal form. We consider this fact as an additional argument in favor of the chelate complexes formation, which requires disruption of the intramolecular hydrogen bond making ESIPT reaction impossible. For the case of compound I, which possesses only one chelate metal binding site, it can be assumed without any doubts, that metal ion enters the chelate cavity formed by the 3-hydroxy and the carbonyl groups of the molecule, giving the complex Ia (Figure 7). According to our TDDFT quantum-chemical modeling, the excited state electron density redistribution in I is directed to the electron withdrawing carbonyl group and chromone benzene ring (Figure 8), while the oxazole cycle and terminal benzene ring play the role of the electron donor moieties. This is in good agreement with the fact that binding of the Hg2þ ion into the above-discussed cavity is accompanied with the increase of the charge transfer toward the carbonyl oxygen and causes an expressed bathochromic shift of the longwavelength absorption band of the complex. Total increase of the excited state polarity of I molecule expressed as the vector difference of its dipole moments in S1 and S0 states exceeds 19 D according to our calculations. The excited state 3-hydroxy group acidity and carbonyl oxygen basicity increases owing to 4225



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Table 1. Absorption Spectra Characteristics and Equilibrium Constants for the Investigated Compounds Complexation with Mercury(II) Ions ML stoichiometry complex 1

1

M2L stoichiometry complex 1

compound λmax, nm (νmax, cm ) λmax, nm (νmax, cm ) Δλ, nm (Δνmax, cm ) λmax, nm (νmax, cm1) Δλ, nm (Δνmax, cm1) I

385 (25980)

445 (22460)

60 (3520)

II

370 (27020)

400 (25000)

30 (2020)

III

380 (26320)

415 (24000)

35 (2320)

425 (23660)

40 (2320)

lg K1

lg K2

7.93 ( 0.18 4.18 ( 0.10 5.29 ( 0.12

420 (23720)

40 (2600)

5.70 ( 0.81 3.27 ( 0.65

Figure 8. Excited state electron density redistribution in the I molecule, TD/b3lyp/cc-pvdz. Numbers on the arrows are the charge transfer indices (in % of the electron charge) according to ESSA approach.44.

Figure 9. Calculated geometry and relative energies of the complexes formed by compound II and Hg2þ ions (according to the semiempirical PM6 calculations).

Figure 6. Fluorescence spectra changes for compound I (top, 1010 to 103 M Hg2þ, λex = 385 nm) and II (bottom, 108 to 104 M Hg2þ, λex = 375 nm) upon the addition of mercury ions.

Figure 7. Binding of Hg2þ ions in the ML and M2L complexes formed by compound I.

such a charge redistribution, together they are the reason and driving force of the excited state proton transfer reaction typical to the most of 3-hydroxychromones. In the case of compound II there exist two possible pathways of the metal ions chelating, the cavity a, traditional for 3-hydroxychromones, and the cavity c, formed by the nitrogen atom of the heterocyclic moiety in position 2 and the hydroxy group in

position 3. Using the semiempirical quantum chemical modeling with PM641 method, which usually gives results close to those of more expensive ab initio methods, we estimated the relative energies for the complexes of different structure formed by Hg2þ and II (Figure 9). According to these data, the most energetically favorable is the binding of mercury ion into the cavity c. This fact can be easily clarified due to the higher nucleophilic ability of the heterocycle nitrogen atom in comparison with that of the carbonyl oxygen one. Moreover, such a soft Lewis acid like Hg2þ should be strongly attracted by the softer base, nitrogen atom, comparing with the much harder base, the oxygen one. The same conclusion is also supported by the geometry analysis of the two discussed concurrent chelate cavities, the c one corresponds better to the ionic radius of Hg2þ. Independent experimental evidence of the larger dimensions of the cavity c were obtained from another complexation study of the above-discussed compound with Ba2þ ions (Figure 10). Earlier investigations of our research group showed that barium is not capable to form chelate complexes with the derivatives of 3HC.28 This occurs because of incompatibility of the type a cavity mean radius estimated to be 0.800.90 Å with much larger ionic radius of Ba2þ (1.39 Å38). Our experiments unambiguously point out to the chelate complex formation of compound II with barium ions (high bathochromic shift of the long-wavelength absorption band, considerably higher equilibrium constant, 4226



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Figure 10. Changes in the absorption spectra upon addition of Ba2þ ions to the methanol solution of II (107 to 104 M Ba2þ). Figure 12. Absorption spectra of compound I in pure methanol and upon addition of bivalent heavy metal ions (metal concentrations vary in the range of 23 107 M).

Figure 11. Spectrophotometric titration curve of I at 23440 cm1/ 427 nm.

pK = 5.77 ( 0.02, in comparison with the data of28), which is possible only in the case of side heterocyclic moiety participation in the complexation. An interesting situation was revealed for compound I at its titration by Hg2þ: after reaching a certain concentration of the metal ions in solution (∼105 M), a tendency of blue shift was revealed in the absorption spectra, accompanied also by changes in the long-wavelength band shape (Figure 5, top). These facts indicate the existence of the second complexation equilibrium with the metalligand stoichiometry 2:1. Fluorescence intensity continues its decrease, though, as it is shown in the Figure 6 (top), synchronous emission quenching of both normal and phototautomer bands was clearly observed. The above finding is in good agreement with the hypothesis that the second metal ion binding does not involve the 3-hydroxy group of I. Taking into consideration this phenomenon, relatively low spectral shift value and considerably lesser stability constant of the second interaction stage (Table 1), we supposed that a mixed complex of stoichiometry M2L was formed. The second metal ion coordinates to the nitrogen atom of side oxazole fragment, forming the Iad complex (Figure 7). The principle ability of 2,5-diphenyloxazole to form complexes with several transition metals, and particularly with mercury(II) ions, was reported in.45 Binding of the second Hg2þ cation to the oxazolic nitrogen atom should decrease its electron donor ability in respect to the cation-bound

Figure 13. Fluorescence quenching of I detected on the analytical wavelength 555 nm (in the maximum of the phototautomer emission band) upon the addition of bivalent heavy metal ions (F0 is the initial fluorescence intensity of the methanol solution of I; F is the intensity upon addition of 2 107 M of the specified heavy metal).

chelate formed by two oxygen atoms in positions 3 and 4. This should decrease the intracomplex excited state electron density redistribution and finally determine the hypsochromic shift in the absorption spectra. The spectrophotometric titration curve for I is shown in the Figure 11. The analytical wavelength corresponds to the absorption regions of both the complexes formed (Ia and Iad). Compound I demonstrates high sensitivity to the mercury ions in the model alcohol solutions. The lower detection limit for Hg2þ estimated on the base of our experimental data is of ∼13 1010 M. This value lies rather good below to the maximum permissible concentration (MPC) of mercury in the drinking water.46 Thus, we have all the reasons to propose I as highly sensitive spectrophotometric and fluorimetric chemosensing compound for mercury. To investigate the principle possibility of application of compound I for mercury ions probing, its selectivity to Hg2þ in model methanol solutions was studied with respect to a series of several bivalent heavy metal ions. As it follows from the data in the Figure 12, spectral response to the presence of nickel, cadmium and lead ions in the methanol solution of I is fairly low. Only barium ions have definite influence on the absorption of I at this level of concentration. 4227



Figure 14. Absorption spectra changes for compound III upon addition of mercury ions (107 to 103 M Hg2þ).

Figure 15. Fluorescence spectra changes for compound III upon addition of mercury ions (top 107 to 105 M Hg2þ, bottom 105 to 103 M Hg2þ, λex = 390 nm).

However, addition of mercury ions results in much stronger growth of the absorption in the long-wavelength region. Investigations of the fluorescence quenching showed similar results (Figure 13). Maximum of the phototautomer emission (555 nm in methanol) was chosen as a reference wavelength because it shows the most distinctive changes upon metal ions addition. Presence of mercury leads to the most significant quenching of fluorescence emission, both due to the heavy atom effect and interruption of the ESIPT process at complexation. Compound I also shows noticeable response to Ba2þ ions (however, less than that to Hg2þ), while the other tested metals effect was significantly lower. Compound II has no additional metal binding sites and, thus, could not interact with Hg2þ ions in two steps; this statement is

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Figure 16. Two-stage complexation of III with Hg2þ ions accompanied by subsequent fluorescence ignition and quenching (measured at 20140 cm1/497 nm).

supported also by the relatively good isosbestic point during our complexation experiments (Figure 5). 3.2. Quinoline Derivative. Changes in the absorption spectra for compound III upon its interaction with Hg2þ are similar to those for I. When mercury ions were added to the methanol solution of this dye, a new red-shifted absorption band appearance was observed (Figure 14). When the total mercury concentration in the solution reaches ∼105 M this band underwent a slight hypsochromic shift (Table 1). At the same time, fluorescence of quinoline derivative III is influenced by Hg2þ ions in a rather different manner. Increase of mercury concentration up to ∼105 M leads to a strong integral fluorescence ignition for more than 5 times of magnitude, together with simultaneous change of the shape and position of the emission band (Figure 15, top). According to our X-ray structural analysis data,36 conformation with an intramolecular H-bond to the quinoline nitrogen atom prevails in the crystalline state of compound III. Our quantum chemical modeling and experimental fluorescence measurements reveal conservation of the same intramolecular H-bonded conformation also in the gas phase and in solutions. This major conformation of III is nonfluorescent owing to the high intersystem crossing efficiency (related to its non-H-bonded carbonyl group nπ* states) resulting in the population of triplet levels, where the excitation energy dissipates via radiationless channels. Thus, the registered low intensive emission of III is due to the traces of its conformation with a “flavonol-like” intramolecular H-bond to carbonyl oxygen.36 In this case, we supposed that the mercury ion enters into the cavity a, formed by a heterocyclic substituent and 3-hydroxygroup, but not into the traditional “flavonolic” cavity, formed by a 3-hydroxy and 4-carbonyl groups. This should result in the increase of the fraction of molecules of III that can fluoresce and, thus, explain the luminescence ignition observed experimentally upon binding with Hg2þ ions. At the same time, mercury itself quenches the fluorescence emission of the formed complexes owing to the internal heavy atom effect. As a result of both these concurring antagonistic influences, fluorescence ignition prevails on the first stage of complexation. After reaching a certain content of mercury ions (∼105 M) in solution of compound III, as well as for its above-discussed 2, 5-diphenyloxazole analog, typical changes in character and shape of its absorption spectra are observed, pointing out to the formation of the second successive complex of stoichiometry M2L. Fluorescence emission intensity decreases while the second complex 4228



Figure 17. Suggested structure of the complexes formed by III with Hg2þ ions.

forms. Figure 16 depicts the dependence of the linear emission intensity of III and its complexes at the maximum of the newformed band upon the negative mercury ions concentration logarithm. Linear gain of fluorescence intensity upon the log[c(Hg2þ)] was observed up to a moment when the second complex starts to prevail, after which linear decrease of emission was detected. Considering these findings (Figure 16, Table 1), we suggested that, in the case of compound III, as well as for I, a mixed complex of stoichiometry M2L was formed on the second stage (Figure 17). Coordination of the additional Hg2þ ion to the ML complex should decrease its intramolecular excited state electron density redistribution. That is the reason of the second complex hypsochromic shift in the electronic absorption spectra. Increased heavy atom effect in the M2L complex is the reason of its lower fluorescence quantum yield in comparison with that of ML one.

4. CONCLUSIONS 3-hydroxychromones with nitrogen-containing heterocyclic moieties in the position 2 of the chromone bicycle form two types of complexes with mercury ions having stoichiometry ML and M2L: “outer”, chelate, and polymetal complexes of the mixed type. 3HC derivative with 2,5-diphenyloxazole moiety in the position 2 forms chelate complex of traditional “flavonol-like” type with metal ion coordination in the cavity including oxygen atoms of 3-OH and 4-CdO groups. This complex is characterized by significant bathochromic shift in the absorption spectra and high stability constant. Because of high sensitivity to mercury, this compound could be applied for design of the inflow sensor devices for monitoring of this highly toxic pollutant in water at ultralow concentrations. Benzimidazole- and quinoline-substituted 3-hydroxychromones, in contrary, form chelate complexes with mercury ion coordination into the cavities with participation of the side heterocyclic moiety nitrogen atom and the dissociated 3-hydroxyl group. Quinoline derivative III complexation is characterized by significant fluorescence ignition on the first stage, which is quite unusual for heavy metals complexes with fluorescent organic ligands. At high mercury concentrations derivatives bearing 2,5-diphenyloxazole and quinoline moieties form mixed polymetal complexes which are characterized by significant fluorescence quenching. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], [email protected]. Phone: þ38 (057) 7075335.

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’ ACKNOWLEDGMENT The authors acknowledge partial financial support of this investigation from the Ministry of Education and Science of Ukraine (Ukraine-Israel Research Grant M/83-2009). The authors also express their gratitude to the Ukrainian-American Laboratory of Computational Chemistry (Kharkov, Ukraine; Jackson, U.S.A.) for the possibility to conduct high-level quantum-chemical calculations. ’ REFERENCES (1) Leopold, K.; Foulkes, M.; Worsfold, P. Anal. Chim. Acta 2010, 663, 127–138. (2) Liu, S.-J.; Nie, H.-G.; Jiang, J.-H.; Shen, G.-L.; Yu, R.-Q. Anal. Chem. 2009, 81, 5724–5730. (3) Dalkiran, B.; Ozel, A. D.; Parlayan, S.; Canel, E.; Ocak, U.; Kihc, E. Monatsh. Chem. 2010, 141, 829–839. (4) Fu, J.; Wang, L.; Chen, H.; Bo, L.; Zhou, C.; Chen, J. Spectrochim. Acta, Part A 2010, 77, 625–629. (5) Ensafi, A. A.; Far, A. K.; Meghdadi, S. Sens. Actuators, B 2008, 133, 84–90. (6) Amini, M. K.; Khezri, B.; Firooz, A. R. Sens. Actuators, B 2008, 131, 470–478. (7) Yang, X..-F.; Li, Y.; Bai, Q. Anal. Chim. Acta 2007, 584, 95–100. (8) Eltzov, E.; Marks, R. S.; Voost, S.; Wullings, B. A.; Heringa, M. B. Sens. Actuators, B 2009, 142, 11–18. (9) Sengupta, P. K.; Kasha, M. Chem. Phys. Lett. 1979, 68, 382–385. (10) Kasha, M. J. Chem. Soc., Faraday Trans. 2 1986, 82, 2379–2392. (11) Schwartz, B. J.; Peteanu, L. A.; Harris, C. B. J. Phys. Chem. 1992, 96, 3591–3598. (12) Woolfe, G. J.; Thistlethwaite, P. J. J. Am. Chem. Soc. 1981, 103, 6916–6923. (13) Ormson, S. M.; Brown, R. G. Prog. React. Kinet. 1994, 19, 45–91. (14) Swinney, T. C.; Kelley, D. F. J. Chem. Phys. 1993, 99, 211–221. (15) Chou, P.-T.; Martinez, M. L.; Clements, J.-H. J. Phys. Chem. 1993, 97, 2618–2622. (16) Letard, J.-F.; Delmond, S.; Lapouyade, R.; Braun, D.; Rettig, W.; Kreissler, M. Recl. Trav. Chim. Pays-Bas 1995, 114, 517–523. (17) Tian, M.; Ihmels, H. Chem. Commun. 2009, 3175–3177. (18) Li, C.-Y.; Zhang, X.-B.; Qiao, L.; Zhao, Y.; He, C.-M.; Huan, S.-Y.; Lu, L.-M.; Jian, L.-X.; Shen, G.-L.; Yu, R.-Q. Anal. Chem. 2009, 81, 9993–10001. (19) Bishop, E. Indicators; Pergamon Press: Oxford, 1972. (20) Binbuga, N.; Henry, W. P.; Schulz, T. P. Polyhedron 2007, 26, 6–10. (21) Lapouge, C.; Cornard, J. P. J. Phys. Chem. A 2005, 109, 6752–6761. (22) Thompson, M.; Williams, C. R.; Elliot, G. E. P. Anal. Chim. Acta 1976, 85, 375–381. (23) Vestergaard, M.; Kerman, K.; Tamiya, E. Anal. Chim. Acta 2005, 538, 273–281. (24) Engelmann, M. D.; Hutcheson, R.; Cheng, I. F. J. Agric. Food Chem. 2005, 53, 2953–2960. (25) Zhang, J.; Brodbelt, J. S.; Wang, J. J. Am. Soc. Mass Spectrom. 2005, 16, 139–151. (26) Matsunaga, H.; Kanno, C.; Yamada, H.; Takahashi, Y.; Suzuki, T. M. Talanta 2006, 68, 1000–1004. (27) Roshal, A. D.; Munos, O.; Sakhno, T. V.; Buadon, M. T. Chem. Heterocycl. Compd. 2002, 38, 1412–1418. (28) Roshal, A. D.; Grigorovich, A. V.; Doroshenko, A. O.; Pivovarenko, V. G.; Demchenko, A. P. J. Phys. Chem. A 1998, 102, 5907–5914. (29) Roshal, A. D.; Grigorovich, A. V.; Doroshenko, A. O.; Pivovarenko, V. G.; Demchenko, A. P. J. Photochem. Photobiol., A 1999, 127, 89–100. (30) Douhal, A.; Roshal, A. D.; Organero, J. A. Chem. Phys. Lett. 2003, 519–525. 4229



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