Preview only show first 10 pages with watermark. For full document please download

Development Of A Pyrene Based ‘‘turn On’’ Fluorescent Chemosensor For Hg2+ { Gandhi Sivaraman, Thangaraj Anand And Duraisamy Chellappa

Development of a pyrene based ‘‘turn on’’ fluorescent chemosensor for Hg2+ { Gandhi Sivaraman, Thangaraj Anand and Duraisamy Chellappa

   EMBED


Share

Transcript

  Development of a pyrene based ‘‘ turn on ’’ fluorescent chemosensor for Hg 2+ { Gandhi Sivaraman, Thangaraj Anand and Duraisamy Chellappa * Received 19th June 2012, Accepted 6th September 2012 DOI: 10.1039/c2ra21202a We have designed, synthesized, and evaluated a new pyrene-based ‘‘ turn on ’’ fluorescent chemosensorwhich showed remarkable enhanced fluorescence intensity in the presence of Hg 2+ ions and a highselectivity towards Hg 2+ ions over a wide range of metal ions in aqueous acetonitrile (ACN). Theprobe shows selectivity and sensitivity with a 12-fold enhancement in fluorescence towards Hg 2+ .Upon the addition of Hg 2+ ions, the probe displayed an apparent color change, which could beobserved by the naked eye under a UV lamp. It’s in vitro sensitivity to Hg 2+ was demonstrated in candida albicans cells with the use of confocal microscopy. Introduction The development of artificial receptors for the sensing andrecognition of environmentally and biologically importantspecies has been actively investigated in recent years. 1 In thisregard, chemosensors that can highly sensitively and selectivelydetect heavy and transition metal ions (HTM) such as Hg 2+ ,Pb 2+ , Cd 2+ , and Cu 2+ are especially important. 2 Mercury isconsidered as a prevalent toxic metal in the environment becauseboth elemental and ionic mercury can be converted by bacteria inthe environment to methyl mercury, which subsequently bioaccumulates through the food chain. 3 Mercury can accumulatein the human body and affects a wide variety of diseases even ina low concentration, such as digestive, kidney, and especiallyneurological diseases. 4 As a result, developing new and practicalmulti signaling chemosensors for Hg 2+ is still a challenge. 5 TheU.S. Environmental Protection Agency (EPA) standard for themaximum allowable level of inorganic mercury in drinking wateris 2 ppb. 6 Sensors based on metal ion-induced changes influorescence appear to be particularly attractive and are one of the first choices because of their simplicity and efficiency in evenvery low concentrations. 7 Thus, designing fluorescent sensors formercury 8 has drawn worldwide attention. Generally, mercury isknown to cause fluorescence quenching of fluorophores via thespin–orbit coupling effect. 9 Fluorescence quenching is not onlydisadvantageous for a high signal output upon recognition butalso hampers temporal separation of spectrally similar complexeswith time-resolved fluorometry. 10 However, the sensors thatshow fluorescence enhancement on binding to the cation of interest are preferred because these allow a lower detection limitand high-speed spatial resolution via microscope imaging. 11,12 Turn on fluorescent chemosensors 13 for heavy metal ions,particularly for mercury have remained so far limited becausethese metal ions are known to quench fluorescence 14 emission via enhanced spin–orbit coupling, 15 energy, or electron transfer. Thesimplicity, effectiveness and inexpensive nature spurred many toput continuous effort into developing new selective fluorescentsensors for different analytes. 16 In particular imine based sensorshave drawn attention due to simplicity and sensitivity. 17 Most of the studies about fluorescent chemosensors of Hg 2+ are based ona fluorescence quenching mechanism, 14 only a few reports arepresent in the literature on fluorescence ‘‘ turn on ’’ chemosensorsfor Hg 2+ . 18 Herein we report a new ‘‘ turn on ’’ selectivefluorescent chemosensor. Results and discussion Chemosensor PTE-1 was synthesised by a one step condensationof pyrene-1-carboxaldehyde with 2-(methylthio) aniline in 61%yield (Scheme 1). 1 H and 13 C-NMR spectra and ESI-MS of PTE-1 are given in Fig. S1–S3 { . The molecular framework wasdesigned as a platform for the construction of efficientionophores via its thioether S and imino N atom. 19 This Schiff base was stable in neutral acetonitrile and water–acetonitrilesolution for at least 3 days. The fluorescence emission intensityof the probe PTE-1 is unaffected by pH values between 4.5–12.This observation indicates that the chemosensor PTE-1, which is School of Chemistry, Madurai Kamaraj University, Madurai-625021,Tamilnadu, India. E-mail: [email protected] { Electronic supplementary information (ESI) available: Computationaldetails, NMR, MS spectra. See DOI: 10.1039/c2ra21202a Scheme 1 Synthesis of compounds PTE-1 and PTE-2. RSC Advances Dynamic Article Links Cite this: RSC Advances , 2012, 2 , 10605–10609www.rsc.org/advances PAPER This journal is ß The Royal Society of Chemistry 2012 RSC Adv. , 2012, 2 , 10605–10609 | 10605  pH-stable and also organic solvent-stable, may be useful as apotential chemosensor material.The absorption spectrum of PTE-1 shows the typical pyreneabsorption bands at 230(sh), 290, 375 and also a low energy bandcentred at 404 nm is attributed to the imino bridge. Uponaddition of 1–1.2 equiv. of Hg 2+ ions to the PTE-1, the mostsignificant changes were broadening of the absorption bandsaround 350–450 nm observed with hypochromism (Fig. 1). Thisis also responsible for the change of colour, which is perceptibleto the naked eye, from pale yellow to colorless. The absorptionspectra with several metal cations (Na + , K + , Ca 2+ , Mn 2+ , Fe 3+ ,Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Hg 2+ , Pb 2+ , and Sn 2+ ) using theirchloride salts in H 2 O–CH 3 CN (30 : 70,v/v) are shown in Fig.S4 { .The fluorescence spectra of PTE-1 with several metal cations(Na + , K + , Ca 2+ , Mn 2+ , Fe 3+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ ,Hg 2+ , Pb 2+ , and Sn 2+ ) using their chloride salts in H 2 O–CH 3 CN(30 : 70,v/v) are shown in (Fig. 2). The probe PTE-1 is veryweakly fluorescent compared to pyrene, due to the photoinduced electron transfer from the imine nitrogen to the pyrenefluorophore and/or C L N isomerisation. Both being the primaryprocesses involved in the deactivation of the excited state, whichlead to the fluorescence quenching of PTE-1. By virtue of theligating atoms N and S, PTE-1 readily forms a chelate with theHg 2+ ion thereby both the photo induced electron transfer (PET)and C L N isomerisation are inhibited and the fluorescence fromthe probe is enhanced.The fluorescent properties of PTE-1–Hg 2+ were surveyed intypical organic solvent systems as well as aqueous solutions. Itexhibits weak monomer emission around 390–410 nm with acharacteristic, but a very intense emission of pyrene centered at462 nm. The pyrene excimer region increases as the watercontent is increased partially in the vicinity of 30% watercomposition, and then changes were not so significant up to 40%aqueous solution. This observation implies that the complexa-tion of PTE-1 with Hg 2+ ions presumably leads to stacking of pyrene moieties from PTE-1 + Hg 2+ resulting in the switch of thepyrene monomer emission to an excimer emission in solutionstate but ESI-MS provides information for 1 : 1 complexation.From this we came to understand that there may be a possibilityof intermolecular excimer formation.A 12-fold enhancement of fluorescence was observed uponaddition of Hg 2+ compared to that of PTE-1 in the H 2 O– CH 3 CN mixture. Upon interaction with various metal ions of alkali (Na + , K + ), alkaline earth (Ca 2+ ) and transition-metal ions(Mn 2+ , Fe 3+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Pb 2+ , and Sn 2+ ), amuch weaker response is observed relative to Hg 2+ at the sameconcentration, and the fluorescence signal of PTE-1–Hg 2+ in thepresence or absence of these contrast ions also exhibited only amild difference (Fig. S5 { ).To test the reversibility of the probe, we added Na 2 S (0.1 M)to the solution of PTE-1–Hg 2+ . The addition of 0.52 mL of Na 2 Scould restore the initial value of free probes due to the K  d valueof 10 2 50 for Hg 2+ at standard conditions in the form of [HgS 2 ]. 20,21 Thus the probe could be revived on addition of Hg 2+ (Fig. S6 { ).The titration curves showed a smooth and steady increaseuntil a plateau was reached (15 m M Hg 2+ ) with a 12-fold increaseat the plateau (Fig. 3). The chemosensor PTE-1 exhibited a very Fig. 1 UV–vis spectra of PTE-1 (10 m M) in the presence of Hg 2+ ions(1.0 equiv. and 1.2 equiv.) in H 2 O–CH 3 CN (30 : 70, v/v). Fig. 2 Fluorescence spectra in the presence of different metal chloridesin H 2 O–CH 3 CN (30 : 70, v/v). The inset shows the response of the othermetal ions with PTE-1. Excitation was performed at 365 nm. Fig. 3 Fluorescence titration of PTE-1 (1 m M) in the presence of different amounts of Hg 2+ in H 2 O–CH 3 CN (30 : 70, v/v). Excitation wasperformed at 365 nm. 10606 | RSC Adv. , 2012, 2 , 10605–10609This journal is ß The Royal Society of Chemistry 2012  efficient fluorescent response, and over 70% of the totalfluorescent intensity increase was observed with 1 equivalent of Hg 2+ . The association constant ( K  a ) of PTE-1 with Hg 2+ is 1.02 6 10 4 M 2 1 (error , 10%), obtained by a nonlinear curve fittingof the fluorescent titration results. 22 Fig. S7 { , exhibits thedependence of the intensity ratio of emission at 472 nm ( F  f  / F  i ) onHg 2+ . This curve served as the calibration curve for detectingHg 2+ . The detection limit 23 was calculated from titration resultsand it was found to be 2.2 6 10 2 8 M.To understand the binding of Hg 2+ with PTE-1 a Job plot wasconstructed by measuring the fluorescence at different molefractions of Hg 2+ . It exhibits a maximum at 0.5 mol fraction of Hg 2+ thus indicating formation of a 1 : 1 complex between PTE-1 and the Hg 2+ ion (Fig. S8 { ). It was further supported by thepeak at 655.35 in the ESI-MS spectrum corresponding to amolecular weight of PTE-1 + HgCl 2 + CH 3 OH + H + (Fig. S9 { ).To understand the effect of sulfur in PTE-1 the Schiff basePTE-2 was synthesised (Scheme 1) and its fluorescence behaviourwas investigated. The PTE-2 shows a very weak fluorescencewith the metal ions but the enhancement is not so significantcompared to that of PTE-1 with an excitation of 340 nm (Fig.S10 { ). This ascertains that the chelate formation of PTE-1 withHg 2+ plays a crucial role in the fluorescence and sensing of metalions.To further understand the absorption and fluorescencebehaviour of the probe PTE-1 and PTE-1 + Hg 2+ complex, wecarried out DFT calculations with the 6-31G basis set using theGaussian 03 program. 24 The dihedral angle 2 123.685 for C L N– C–C reveals the s- trans conformation of PTE-1 whereas that forPTE-1 + Hg 2+ is found to be 138.252. The charges on the Natom for PTE-1 and PTE-1 + Hg 2+ are found to be 2 0.430, 2 0.248 respectively thus revealing a significant reduction in theelectron density on the N atom upon coordination of the probewith Hg 2+ . This may inhibit fluorescence quenching by the photoinduced electron transfer from nitrogen to the fluorophorepyrene. From natural bond orbital analysis, it is evident thatnon-bonding orbitals containing a lone pair of electrons localisedon nitrogen are compatible with the HOMO-1 and HOMOrespectively of PTE-1 and PTE-1 + Hg 2+ . The TDDFTcalculations focused on oscillator strength indicate three strongtransitions for the probe and one strong and two weaktransitions for the PTE-1 + Hg 2+ complex. The oscillatorstrengths for the n–  p * transition of the probe and its mercurycomplex are 0.4329, 0.5446 corresponding respectively to thewavelengths 388 and 397.14 nm. As we use excitation source at340 nm, a n–  p * transition is possible for the probe but for thecomplex it is not viable as a wave length of 397 nm is away fromthe excitation wave length. From the DFT/TDDFT results wethus concluded that the fluorescence enhancement in thecomplex is due to the inhibition of photo induced electrontransfer (Fig. S11, 12 and Table. S1 and S2 { ).The sensitivity of probe PTE-1 to Hg 2+ was examined in candida albicans cells by using confocal microscopy. Thequalitatively in vitro results are exhibited in Fig. 4. After thecells were incubated with 10 m M of PTE-1 for 30 min at 37 u C, noobvious fluorescence could be imaged (Fig. 4a). At the sameexperimental conditions, strong fluorescence was imaged 10 minafter introduction of 10 m M of Hg 2+ to the same cells, displayingenhanced blue fluorescence as in Fig. 4b. The cell imagingexperiments demonstrate the good cell-membrane permeabilityof PTE-1 as a probe for imaging Hg 2+ within candida albicanscells and no changes in the cell morphology and cell viability(Fig. S13 { ) were observed. Conclusion In conclusion, we have designed, synthesized, and evaluated anew pyrene based ‘‘ turn on ’’ fluorescent sensor which showedremarkable enhanced fluorescence intensity in the presence of Hg 2+ ions and a high selectivity towards Hg 2+ ions over a widerange of metal ions in aqueous acetonitrile. Background metalions showed only a small interference with the detection of Hg 2+ ions, indicating that the receptors could be used as efficient Hg 2+ selective ‘‘ turn on ’’ fluorescent sensors. This significantlyenhanced fluorescence is due to the formation of a 1 : 1 complexPTE-1–Hg 2+ in which the rotation of acyclic C L N is frozen. Thesensitivity of PTE-1 to Hg 2+ was demonstrated in candidaalbicans cells , indicating its potential application for imaging of Hg 2+ in living cells. Materials and methods Pyrene-1-carboxaldehyde, 2-(methylthio) aniline and anilinewere obtained from Aldrich and used as such. Metal chloridesalts were procured from Merck. Absorption spectra wererecorded using a JASCO V-530 spectrophotometer whilefluorescence analyses were done using a JASCO spectrofluori-meter. The excitation and emission slit widths were kept constantas 5 nm. NMR spectra were recorded on a Bruker (Avance)300 MHz NMR instrument. Electrospray ionisation massspectral (ESI-MS) analysis was performed in the positive ionmode on a liquid chromatography-ion trap mass spectrometer(LCQ Fleet, Thermo Fisher Instruments Limited, US). Micro-analysis (C, H, and N) was performed using a Perkin-Elmer4100 elemental analyzer. Synthesis of PTE-1 Pyrene-1-carboxaldehyde was mixed with 2-(methylthio) anilinein acetonitrile. The reaction mixture was stirred and refluxed for3 h in the presence of a drop of acetic acid. The purification of the crude product obtained after distillation of the solvent bycolumn chromatography using hexane/ethyl acetate yields ayellow solid. (61% yield). 1 H-NMR (300 MHz, CDCl 3 ) 2.56(s,3H), 7.18–7.29(m, 4H), 8.01–8.17(m, 3H), 8.23–8.26(m, 4H),8.75(d, 1H), 9.17(d, 1H), 9.40(s, 1H). 13 C-NMR (75 MHz,CDCl 3 ) 14.94, 117.5, 122.9, 124.6–134.3, 150.04, 158.4. MS Fig. 4 Intracellular Hg 2+ imaged in candida albicans cells at 37 u C withthe use of confocal microscopy. (a) Candida albicans cells incubated withPTE-1 for 30 min. (b) The candida albicans cells in part a 10 min afterbeing treated with 10 m M of Hg 2+ . (c) A bright field image of probetreated candida albicans cells . This journal is ß The Royal Society of Chemistry 2012 RSC Adv. , 2012, 2 , 10605–10609 | 10607  (ESI): 352.17 (M+H + ). Elemental analysis for C 24 H 17 NS:calculated: C, 82.02; H, 4.88; N, 3.99 and found: C: 81.94; H,4.81; N, 3.94. Synthesis of PTE-2 PTE-2 was synthesised using aniline instead of 2-(methylthio)aniline by adopting same procedure as that of PTE-1. 1 H-NMR(300 MHz, CDCl 3 ) 7.33–7.26(m, 1H) 7.40–7.38(m, 2H), 7.51– 7.46(m, 2H), 8.27–8.03(m, 7H), 8.77(d, 1H), 9.04(d, 1H), 9.49(s,1H). MS (ESI): 306.47 (M+H + ). Computational details We carried out density functional theory (DFT) calculationswith the 6-31G* basis set using the Gaussian 03 program inorder to understand the turn on fluorescence behaviour of PTE-1 on complexation with Hg 2+ . Initially the geometries of PTE-1and the PTE-1 + Hg 2+ complex were optimized by DFT-B3LYP using 6-31G and LANL2DZ basis sets respectively. Theground state optimized geometries absorption behaviour andcorresponding transitions of PTE-1 and the PTE-1 + Hg 2+ complex were obtained from TDDFT using the above basissets. In vitro cellular imaging Candida albicans cells were incubated with the probe PTE-1(10 m M) in PBS buffer (1% DMSO) for 30 min, then the cellswere centrifuged and washed with PBS buffer three times. PTE-1treated cells were incubated with Hg 2+ (10 m M) for 10 min andthe cells were imaged using a confocal fluorescence microscope(Zeiss LSM 510 META). Acknowledgements G.S. would like to thank UGC for a senior research fellowship.G.S., T.A. and D.C. acknowledge DST-IRHPA, FIST forinstrumental facilities. References 1 ( a ) V. Amendola, L. Fabbrizzi, F. Forti and M. Licchelli, Coord.Chem. Rev. , 2006, 250 , 273; ( b ) A. Ojida, H. Nonaka, Y. Miyahara, S.Tamaur, K. Sada and I. Hamachi, Angew. Chem., Int. Ed. , 2006, 45 ,5518.2 ( a ) R. Martinez, F. Zapata, A. Caballero, A. Espinosa, A. Tairragaand P. Molina, Org. Lett. , 2006, 8 , 3235; ( b ) R. R. Avirah, K.Jyothish and D. Ramaiah, Org. Lett. , 2007, 9 , 121; ( c ) S. Yoon, E. W.Miller, Q. He, P. H. Do and C. J. Chang, Angew. Chem., Int. Ed. ,2007, 46 , 6658; ( d  ) Y. Shiraishi, H. Maehara, K. Ishizumi and T.Hirai, Org. Lett. , 2007, 9 , 3125.3 H. H. Harris, I. J. Pickering and G. N. George, Science , 2003, 301 ,1203.4 M. Harada, Crit. Rev. Toxicol. , 1995, 25 , 1–24.5 ( a ) E. M. Nolan and S. J. Lippard, J. Am. Chem. Soc. , 2003, 125 ,14270; ( b ) G. Zhang, D. Zhang, S. Yin, X. Yang, Z. Shuai and D.Zhu, Chem. Commun. , 2005, 2161; ( c ) S. Yoon, A. E. Albers, A. P.Wong and C. J. Chang, J. Am. Chem. Soc. , 2005, 127 , 16030; ( d  ) A.Caballero, R. Martinez, V. Lloveras and J. Veciana, J. Am. Chem.Soc. , 2005, 127 , 15666; ( e ) D. Y. Sasaki and B. E. Padilla, Chem.Commun. , 2005, 1581; (  f  ) A. Coskun and E. U. Akkaya, J. Am. Chem.Soc. , 2006, 128 , 14474; (  g  ) R. Guliyev, A. Coskun and E. U. Akkaya, J. Am. Chem. Soc. , 2009, 131 , 9000; ( h ) A. Coskun and E. U. Akkaya, Org. Lett. , 2007, 9 , 607; ( i  ) M. Zhu, M. Yuan, X. Liu and D. Zhu, Org. Lett. , 2008, 10 , 1481.6 Mercury Update: Impact on Fish Advisories; EPA Fact Sheet EPA-823-F-01-001; Environmental Protection Agency, Office of Water:Washington, DC, 2001.7 ( a ) R. Martı´nez-Ma´n˜ez and F. Sancenon, Chem. Rev. , 2003, 103 ,4419; ( b ) J. S. Kim and D. T. Quang, Chem. Rev. , 2007, 107 , 3780.8 B. Valeur and I. Leray, Coord. Chem. Rev. , 2000, 205 , 3.9 D. S. McClure, J. Chem. Phys. , 1952, 20 , 682.10 K. Rurack, U. Resch-Genger and W. Rettig, J. Photochem.Photobiol., A , 1998, 118 , 143.11 ( a ) S. Yoon, A. E. Albers, A. P. Wong and C. J. Chang, J. Am. Chem.Soc. , 2005, 127 , 16030; ( b ) S. Yoon, A. E. Albers, A. P. Wong andC. J. Chang, Angew. Chem., Int. Ed. , 2007, 46 , 6658.12 ( a ) M. Zhang, M. Yu and C. H. Huang, J. Am. Chem. Soc. , 2007, 129 , 10322; ( b ) E. Sasaki, H. Kojima, H. Nishimatsu, Y. Urano, K.Kikuchi and T. Nagano, J. Am. Chem. Soc. , 2005, 127 , 3684; ( c ) D.Yang, Z. N. Sun, N. W. Chung and J. G. Shen, J. Am. Chem. Soc. ,2006, 128 , 6004; ( d  ) N. C. Lim, H. C. Freake and C. Bruckner, Chem.–Eur. J. , 2005, 11 , 38; ( e ) G. Zhang and D. Zhu, Chem.Commun. , 2005 , 2161; (  f  ) L. Liu, G. Zhang and D. Zhu, Org. Lett. ,2008, 10 , 4581; (  g  ) C. Wang, G. Zhang and D. Zhu, Chem.–Eur. J. ,2008, 14 , 5680.13 ( a ) E. M. Nolan and S. J. Lippard, J. Am. Chem. Soc. , 2003, 125 ,14270; ( b ) L. Prodi and M. Montali, J. Am. Chem. Soc. , 2000, 122 ,6769; ( c ) R. Martinez, A. Espinosa, A. Tarraga and P. Molina, Org.Lett. , 2005, 7 , 5869.14 ( a ) A. B. Descalzo, M. M. Ramo´n and R. Radeglia, J. Am. Chem.Soc. , 2003, 125 , 3418; ( b ) S. Y. Moon, N. J. Youn and S. M. Park, J.Org. Chem. , 2005, 70 , 2394.15 ( a ) S. A. Sallam, Transition Met. Chem. , 2006, 31 , 46; ( b ) C. Sousa, C.Freire and B. de Castro, Molecules , 2003, 8 , 894.16 ( a ) J. S.Wu, W. M.Liu, X. Q. Zhuang and S. T. Lee, Org. Lett. , 2007, 9 , 33; ( b ) G. Q. Yang, F. Morlet-Savary, Z. K. Peng, S. K. Wu andJ. P. Fouassier, Chem. Phys. Lett. , 1996, 256 , 536; ( c ) Z. M. Li andS. K. Wu, J. Fluoresc. , 1997, 7 , 237; ( d  ) P. F. Wang and S. K. Wu, J. Photochem. Photobiol., A , 1995, 86 , 109; ( e ) D. Ray and P. K.Bharadwaj, Inorg. Chem. , 2008, 47 , 2252; (  f  ) Z. X. Li, M. M. Yu,L. F. Zhang, M. Yu, J. X. Liu, L. H. Wei and H. Y. Zhang, Chem.Commun. , 2010, 46 , 7169; (  g  ) V. Chandrasekhar, P. Bag and M. D.Pandey, Tetrahedron , 2009, 65 , 9876; ( h ) J. S. Wu, W. M. Liu, J. C.Ge, H. Y. Zhang and P. F. Wang, Chem. Soc. Rev. , 2011, 40 , 3483; ( i  )K. J. Wallace, Inorg. Chim. Acta , 2012, 381 , 15.17 ( a ) G. Farruggia, S. Iotti, L. Prodi, M. Montalti, N. Zaccheroni, P. B.Savage, V. Trapani, P. Sale and F. I. Wolf, J. Am. Chem. Soc. , 2006, 128 , 344; ( b ) H. S. Jung, K. KO, S. H. Kim, S. P. Bhuniya, J. Lee, Y.Kim, S. Jin and J. S. Kim, Inorg. Chem. , 2010, 49 , 8552; ( c ) Y. Xu, J.Meng, L. Meng, Y. Dong, Y. Cheng and C. Zhu, Chem.–Eur. J. ,2010, 16 , 12898; ( d  ) R. Joseph, J. P. Chinta and C. P. Rao, J. Org.Chem. , 2010, 75 , 3387; ( e ) A. Sola, F. Oto´n, A. Espinosa, A. Ta´rragaand P. Molina, Dalton Trans. , 2011, 40 , 12548; (  f  ) B. Pedras, E.Oliveira, H. Santos, L. Rodriguez, R. Grehuet, T. Avile´s, J. L. Capeloand C. Lodeiro, Inorg. Chim. Acta , 2009, 362 , 2627.18 ( a ) A. K. Mahapatra, J. Roy and P. Sahoo, Org. Biomol. Chem. ,2012, 10 , 2231; ( b ) J. Huang, Y. Xu and X. Qian, J. Org. Chem. , 2009, 74 , 2167; ( c ) M. Li, H.-Y. Lu, R.-L. Liu, J.-D. Chen and C.-F. Chen, J. Org. Chem. , 2012, 77 , 3670; ( d  ) V. Bhalla, M. Kumar, P. R.Sharma and T. Kaur, Inorg. Chem. , 2012, 51 , 2150; ( e ) A. Thakur andS. Ghosh, Organometallics , 2012, 31 , 819; (  f  ) D. Wua, C. He, X. Huand C. Duan, Dalton Trans. , 2009, 10457.19 ( a ) M. S. J. Niasari, J. Coord. Chem. , 2009, 62 , 980; ( b ) K. Shanker,R. Rohini, V. Ravinder, P. M. Reddy and Y. P. Ho, Spectrochim.Acta, Part A , 2009, 73 , 205.20 T. Forster and K. Z. Kasper, Electrochemistry , 1955, 59 , 976.21 ( a ) D. M. Findlay and R. A. N. McLean, Environ. Sci. Technol. , 1981, 15 , 1388; ( b ) R. D. Armstrong, D. F. Porter and H. R. Thirsk, J.Phys. Chem. , 1968, 72 , 2300; ( c ) W. Huang, C. Song, C. He and C.Duan, Inorg. Chem. , 2009, 48 , 5061.22 ( a ) K. A. Conners, Binding Constants ; Wiley: New York, 1987; ( b ) B.Valeur, Molecular Fluorescence. Principles and Applications ; Wiley-VCH: Weinheim, 2002.23 M. Shortreed, R. Kopelman, M. Kuhn and B. Hoyland, Anal. Chem. ,1996, 68 , 1414.24 Gaussian 03, Revision C.02, M. J. Frisch, G. W. Trucks, H. B.Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A.Montgomery Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M.Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, 10608 | RSC Adv. , 2012, 2 , 10605–10609This journal is ß The Royal Society of Chemistry 2012  G. Scalmani, N. Rega, G. A. Petersson, H Nakatsuji, M. Hada, M.Ehara, K. Toyota, R. Fukuda, J Hasegawa, M. Ishida, T. Nakajima,Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C.Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D.Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S.Clifford, J. Cioslowski, SB. B. tefanov, G. Liu, A. Liashenko, P.Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W.Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A.Pople, Gaussian, Inc., Wallingford CT, 2004. This journal is ß The Royal Society of Chemistry 2012 RSC Adv. , 2012, 2 , 10605–10609 | 10609