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A_new_colorimetric_and_fluorescent_sensor_for_Hg2+_based_on_4-pyren-1-yl-pyrimidine

A new colorimetric and ?uorescent ratiometric sensor for Hg 2tbased on 4-pyren-1-yl-pyrimidine

Jiena Weng,Qunbo Mei *,Qidan Ling,Quli Fan,Wei Huang *

Key Laboratory for Organic Electronics and Information Displays (KLOEID),Institute of Advanced Materials (IAM),Nanjing University of Posts and Telecommunications,Nanjing 210046,China

a r t i c l e i n f o

Article history:

Received 24October 2011

Received in revised form 15December 2011Accepted 23December 2011

Available online 28December 2011Keywords:Chemosensor Hg 2tPyrene Pyrimidine

a b s t r a c t

A novel ?uorescent ratiometric chemosensor based on 4-pyren-1-yl-pyrimidine (PPM )has been de-signed and prepared for the detection of Hg 2tin the presence of other competing metal ions in ace-tonitrile.The photo exhibits ?uorescence color change of PPM from blue to green without and with Hg 2t,which red shift of wavelength about 105nm in ?uorescence emission spectra.It can serve as a highly selective chemodosimeter for Hg 2twith ratiometric and naked-eye detection.The photophysical properties of PPM con ?rmed a 2:1(PPM e Hg 2t)binding model and the spectral response toward Hg 2twas proved to be reversible.

1.Introduction

Currently,there is great interest in the development of highly se-lective and sensitive ?uorescent sensors for quantifying and exploring the heavy and transition metal ions in biology and in the environ-ment.1Among these metal ions,mercury(II)has attracted consider-able attention due to its toxicity to environment and biological systems.2For the past few years,many studies have been reported on the design and synthesis of chemosensors for detecting Hg 2t.3However,most of these chemosensors molecular to monitor metal ions are often structurally complicated and require sophisticated synthetic process.As a result,considerable efforts have been devoted to design new and practical chemosensors for Hg 2tdetection.

As ?uorophores,pyrene moiety is one of the most useful scaf-folds for the construction of ?uorogenic chemosensors for a variety of important chemical species.4Depending on the relative prox-imity between pyrene moieties,ef ?cient,and sensitive monomer emission at 370e 430nm and excimer emission around 480nm are observed.5Upon coordination with a speci ?c guest ion,the host molecule could be ?ne-tuned to yield monomer and/or excimer emissions depending on the orientation of the two pyrene moie-ties.6Many investigations have been conducted to fabricate ratio-metric ?uorescent sensors for Hg 2t,Cu 2t,Zn 2t,Cd 2t,and Ag tetc.,by introducing two pyrene moieties and utilizing the pyrene

moiety of monomer versus excimer.7However,as far as we know,only a few ratiometric ?uorescent probes with one pyrene moiety for metal ions have been found in the literature until now.8

Theoretically,?uorescent chemosensors consist of ion recogni-tion unit attached with a ?uorogenic unit.The recognition unit is responsible for the selectivity and binding ef ?ciency of the che-mosensor.9As a result,while designing sensors the recognition unit linked to ?uorophore should be carefully examined.Many hetero-cycle,such as pyridine,quinoline,triazole,thiazole,pyrrole,or imidazole etc.,and their derivatives are utilized as recognition unit.10Pyrimidine is one of the important heterocycle with a variety of biological and medicinal activities,11and some of pyrimidine derivatives are applied as photoelectric materials recently.12How-ever,the research using pyrimidine as recognition unit in chemo-sensor has not yet been reported.Herein,we describe a new ratiometric ?uorescent probe with one pyrene moiety for metal ions,using pyrimidine as recognition unit.The new pyr-imidine e pyrene derivative chemosensor PPM (Scheme 1)shows a selective,sensitive,and reversible ?uorescence change response to the Hg 2t.

CH 3

100o

N Triethyl orthoformate AP

PPM

Scheme 1.Synthetic route for PPM .

*Corresponding authors.Tel.:t862585866008;fax:t862585866999;e-mail addresses:iamqbmei@https://www.wendangku.net/doc/098221757.html, (Q.

Mei),iamwhuang@https://www.wendangku.net/doc/098221757.html, ,iamdirector@https://www.wendangku.net/doc/098221757.html, (W.Huang).

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Tetrahedron 68(2012)3129e 3134

2.Results and discussion

The synthetic route for PPM is shown in Scheme 1.The in-termediate compound AP was synthesized from pyrene by classical Friedel e Crafts reaction in 62%yield.PPM was synthesized by ZnCl 2-catalyzed three-component coupling reaction in 21%yield.13The structure of the products was identi ?ed by using 1H NMR,13C NMR,and GC e MS.

The chemosensor PPM displayed sensitive ?uorescence change via pyrimidine ring chelating with Hg 2t.Addition of 2equiv of various alkali,alkaline earth,and transition metal ions (Na t,K t,Mg 2t,Ag t,Cd 2t,Co 2t,Cr 3t,Cu 2t,Fe 3t,Hg 2t,Ni 2t,Pb 2t,Zn 2t)to a solution of PPM (c ?1.25?10à4M)in acetonitrile showed a selec-tive ?uorescence change from blue to green only in case of Hg 2t(Fig.1).This selective ?uorescence change with PPM can be used for the ?uorescence detection of Hg 2tin solution (Fig.1).Similar ?uorescence change was also observed with Hg 2tin the presence of other ions in this study.

Detailed optical studies were carried out to establish the se-lective sensing between Hg 2tand PPM in acetonitrile.Because this small organic molecule displayed not only absorption but also emission variations depending on the metal ion present,so UV e vis and ?uorescence measurements were investigated.

The absorption spectra of PPM in acetonitrile was typical of those for other N -heterocycles with covalently bound pyrenyl groups.14As showed in Fig.2,a long-wavelength absorption for a pyrenyl-based p e p *transition was found centered at 350nm,while the heterocyclic (pyrimidine)-based p e p *transition was centered at 279nm.The chemosensor behavior in the presence of 2equiv of different metal ions (Na t,K t,Mg 2t,Ag t,Cd 2t,Co 2t,Cr 3t,Cu 2t,Fe 3t,Hg 2t,Ni 2t,Pb 2t,Zn 2t)indicated that only Hg 2tpromoted a notable response in its absorption spectra (Fig.3).Upon addition of Cr 3tand Fe 3t,although a weak new peak at 430nm was observed,the typical absorption of PPM at 244nm,279nm,and 350nm showed slight change in UV e vis spectra.

Upon addition of Hg 2t,two strong new peaks at 300nm and 415nm were observed,with the peaks at 279nm and 350nm disappeared.The peak at 415nm in the case of Hg 2tcould be at-tributed to the formation of PPM e Hg 2tchelating complex.The Job ’s plot (Fig.4)for PPM titrated with Hg 2trevealed a 2:1stoi-chiometry of the complexation species.

Systematic spectrophotometric titration with increasing [Hg 2t]revealed a gradual decrease of absorption band centered at 350nm with the concomitant increase in a new low-energy band centered at 415nm (Fig.5).The spectra remained constant after adding approximately 3equiv of Hg 2t.The well-de ?ned isosbestic points at 322nm and 369nm clearly indicated the presence of a unique complex in equilibrium with the free ligand.The new low-energy band,which was red-shifted 65nm,was responsible for the change of color from colorless to yellow (inset of Fig.5).In order to con ?rm the stoichiometry of the formed complex,another titration was carried out with increasing PPM (Fig.S2).From the analysis of the absorption spectral data (Fig.6),the binding of Hg 2talso con ?rmed the formation of 2:1(PPM e Hg 2t)chelating complex,and occurred in a ratiometric manner through a weakened

PPM

Fig.1.Changes in the color of PPM (c ?5.0?10à5M)with 2equiv of various metal ions in acetonitrile.Top:color changes by exciting at 360nm;bottom:color changes for ‘naked-eye ’.

F l u o r e s c e n c e I n t e n s i t y (a .u .)

A b s o r b a n c e (a .u .)

Wavelength (nm)

Fig. 2.UV e vis and ?uorescence spectra (l ex ?370nm)of PPM in acetonitrile (c ?5.0?10à5M).

0.0

0.10.20.30.40.50.60.70.80.91.01.11.21.31.41.5A b s o r b a n c e

Wavelength (nm)

Fig.3.UV e vis spectra of PPM (c ?

5.0?10à5M)in the presence of metal ions in ace-tonitrile.Ag t,Cd 2t,Co 2t,Cr 3t,Cu 2t,Fe 3t,Hg 2t,Mg 2t,Ni 2t,Na t,Pb 2t,and Zn 2tions (2.0equiv)were added,respectively.

A (415n m )-A 0(415n m )

[PPM]/[PPM]+[Hg 2+

]

Fig.4.Job ’s plot for evaluation of the 2:1binding stoichiometry between PPM and Hg(ClO 4)2in acetonitrile medium;the total [PPM ]t[Hg 2t]?5.0?10à5M.

J.Weng et al./Tetrahedron 68(2012)3129e 3134

3130

and ascending complex emission,which made it possible to ratio-metrically detect Hg 2t.

PPM showed very strong ?uorescence in acetonitrile (Fig.1).The emission spectrum displayed typical emission bands centered

at 440nm with a high quantum yield (V ?1.00).It was interesting that the emission spectrum of PPM recorded at different concen-tration in acetonitrile did not show any red-shifted emission (Fig.S3).Sharp concentration quenching of emission at high con-centration (c ?1.0?10à3M)for PPM but with no concomitant red shift in emission spectra,which indicated that PPM was mono-meric in dilute acetonitrile solution rather than being associated via inter-molecular p -stacking interactions.

The ?uorescence behavior of PPM in the presence of the Hg 2twas examined.Upon gradual addition of Hg 2t(0e 0.5equiv)to the solution of PPM in acetonitrile (c ?5.0?10à5M),the intensity of the emission band centered at 440nm decreased gradually and that of a new ?uorescent band centered at 545nm increased gradually,which was attributed to the formation of PPM e Hg 2tcomplex (Fig.7).As a result,an obvious change in ?uorescent color from blue to green was observed (inset of Fig.7).The more addition of Hg 2t(0.6e 3.0equiv),caused ?uorescent quenching and showed a steady and smooth decrease both at 410nm and 545nm (inset of Fig.7).Fitting of the titration curve also suggested a 2:1stoichiometry PPM e Hg 2tcomplexation species.According to the literature,15the detection limit was also estimated from the titration results and was 4.69?10à6M.

To explore practical applicability of PPM as a Hg 2tselective chemosensor,a competition experiment was done.As showed in Fig.8,under the same condition slight ?uorescence changes of PPM were observed in the presence of 2equiv of different metal ions (Na t,K t,Mg 2t,Ag t,Cd 2t,Co 2t,Ni 2t,and Pb 2t).By addition of transition metal ions (Cu 2t,Cr 3t,Fe 3t),the ?uorescent intensity of PPM decreased to varying degrees,especially the addition of 2equiv of Cr 3tor Fe 3tcaused about 43%or 53%?uorescence de-crease of PPM ,respectively,but no any new emission band was observed.And the addition of 2equiv of Zn 2tcaused a shoulder band centered at 491nm (Fig.S4);however,the intensity of the peak at 440nm decreased only 3%.A great difference between addition of 2equiv of Hg 2tand other metal ions was not only the emission band centered at 440nm disappeared almost,but also a new band centered at 545nm appeared.Thus PPM was more sensitively to detect Hg 2tthan other metal ions mentioned above.Further more,upon addition of 2equiv of Hg 2tto the solution,which containing PPM and 2equiv of competing metal ion,the change of the emission was similar to that only 2equiv of Hg 2twas

0.0

0.10.20.30.40.50.60.70.8

0.91.01.11.2A b s o r b a n c e

Wavelength (nm)

(a)(b)

Fig.5.UV e vis spectra of PPM in acetonitrile (c ?5.0?10à5M)in the presence of in-creasing amount of Hg(ClO 4)2(0e 3.0equiv)predissolved in acetonitrile.Arrows in-dicate the absorptions that increase (up)and decrease (down)during the titration experiments.Inset:change in the color of PPM after addition of 2equiv of Hg(ClO 4)2,PPM in acetonitrile (a)and PPM plus 2equiv of Hg(ClO 4)2in acetonitrile (b).

100200300400500

600

700

A 350n m /A 415n m

[Hg 2+

]/[PPM]

(a)

A b s o r b a n c e o f 415n m

[PPM]/[Hg 2+

]

(b) Fig.6.(a):The ratios of absorption at 350nm to those at 415nm (A 350nm /A 415nm )de-pended on the ratios of [Hg 2t]/[PPM ]upon addition Hg(ClO 4)2into a solution of PPM (c ?5.0?10à5M);(b):The variation of absorption at 415nm depended on the ratios of [PPM ]/[Hg 2t]upon addition PPM into a solution of Hg(ClO 4)2(c ?5.0?10à5M).

100200300400500

600700

I n t e n s i t y

Wavelength (nm)

Fig.7.Fluorescence spectra of PPM in acetonitrile (5.0?10à5M,l ex ?400nm)in the presence of increasing amount of Hg(ClO 4)2(0e 0.5equiv)predissolved in acetonitrile.Insets:(a)?uorescence spectra of PPM in the presence of increasing amount of Hg(ClO 4)2(0.5e 3.0equiv).(b)Change in the ?uorescence of PPM after addition of 2equiv of Hg(ClO 4)2.From left to right,PPM in acetonitrile and PPM plus 2equiv of Hg(ClO 4)2in acetonitrile.

J.Weng et al./Tetrahedron 68(2012)3129e 31343131

added to the PPM solution.As a result,PPM displayed selectivity to detect Hg2t.

Reversibility and regeneration are important factors for che-mosensor in practical applications.16Because Iàhas strong binding ability toward Hg2t,KI was added to the solution of PPM e Hg2tspecies to test whether the proposed complex could be reversed. The introduction of KI(2equiv to Hg2t)could immediately restore the initial?uorescence of PPM.When Hg2t(1equiv to PPM)was added to system again,the?uorescence of PPM was quenched (Fig.S6e S8).The regeneration indicated that the chemosensor PPM could be revived with proper treatment.This process could be repeated at least ten times(Fig.9),which signi?ed that PPM can function as a?uorescent switch modulated by Hg2t/KI(Fig.9).

For our system,there were two potential binding sites in the pyrimidine ring,however,all the analysis mentioned-above con-?rmed a2:1(PPM e Hg2t)binding model,thus it might be sug-gested that only one of the nitrogen atoms in PPM operated.In order to study the complexation mode of Hg2twith PPM,a com-parison of1H NMR experiments of PPM with those of TPAPM (diphenyl-(4-pyrimidin-4-yl-phenyl)-amine)17and PPy(2-phenyl-pyridine)were carried out.We performed1H NMR experiments in (DMSO-d6),which were shown in Fig.10and Fig.S9.Upon addition

of excessive Hg2t,considerable change was noted in the chemical shift of H c in the pyrimidine group of PPM,which shifted to down?eld by0.19ppm.The proton signal of H d in the pyrene ring was also shifted to down?eld slightly.In the case of TPAPM,the changes of chemical shift were almost the same as those of PPM. However,treatment of excessive Hg2t,the proton signal of H b in the pyridine group of PPy has nearly no change.Another difference between PPy and PPM in the1H NMR experiments was that the proton signal of H c in the phenyl of PPy shifted to up?eld obviously. These spectral changes might be suggested that Hg2twas bound to the1-position nitrogen atom rather than the3-position nitrogen atom.Based on the result of the1H NMR experiments,the possible binding mechanism of PPM with Hg2twas schematically depicted in Scheme2.Moreover,the formation of new chelating complex between PPM and Hg2tcan be proved by the appearance of new absorption band as well as red-shift emission band in UV e vis spectra and?uorescence emission spectra,respectively.

Fig.9.Fluorescence intensity changes of PPM in acetonitrile(5.0?10à5M,l ex?

400nm)upon alternate addition of Hg(ClO4)2and KI.

Fig.10.(a)Molecular structure formulas of PPM,TPAPM,and PPy;(b)The1H NMR

spectra of(1)PPM in DMSO-d6;(2)e(6)upon gradual addition of solid Hg(ClO4)2to the

solution of PPM in DMSO-d6;(c)The1H NMR spectra of(1)PPy in DMSO-d6;(2)e(3)

upon gradual addition of solid Hg(ClO4)2to the solution of PPy in DMSO-d6

(

)

(

)

Fig.8.Fluorescence responses of PPM in acetonitrile(5.0?10à5M,l ex?400nm)to

various2equiv of metal ions.Bars represent the?nal(I(440nm))over the initial

(I0(440nm))emission intensity.The black bars represent the addition of the competing

metal ion to the solution of PPM.The red bars represent the change of the emission

that occurs upon the subsequent addition of2equiv of Hg2tto the above solution.

N N

MeCN

Scheme2.The possible binding mechanism of PPM with Hg2t.

J.Weng et al./Tetrahedron68(2012)3129e3134

3132

3.Conclusion

In summary,we have developed a novel?uorescent sensor for Hg2tbased on the4-pyren-1-yl-pyrimidine(PPM)with high sen-sitivity and selectivity.This chemosensor was easily prepared and found to be possible to detect the Hg2tratiometrically.The re-markable photophysical properties of PPM con?rmed a2:1 (PPM e Hg2t)binding model and the spectral response toward Hg2twas established to be reversible.More interestingly,the Hg2twas bound to the1-position nitrogen atom of PPM,which has been proposed on the basis of1H NMR experiments.This work opens up the possibility of a family of highly sensitive chemosensors for Hg2tbased on4-aryl-pyrimidine.

4.Experimental

4.1.Materials and instrumentations

Acetonitrile was of high performance liquid chromatography purity and used without further treatment.All other solvents and reagents were of analytical purity and used without further puri?cation.The salts solutions of metal ions were NaNO3, KNO3,Mg(ClO4)2,AgNO3,Cd(NO3)2$4H2O,Co(NO3)2$6H2O, Cr(NO3)3$9H2O,Cu(NO3)2$3H2O,Fe(NO3)3$9H2O,Hg(ClO4)2$3H2O, Ni(NO3)2$6H2O,Pb(NO3)2,Zn(NO3)2$6H2O.1H NMR and13C NMR were measured on a Bruker Ultra Shield Plus400MHz spectrom-eter with TMS as an internal standard and CDCl3/DMSO-d6as sol-vent.Mass spectrometric data were obtained with a Shimadzu GC-MS-QP2010Plus spectrometry.UV e vis spectra were recorded on a Shimadzu UV-3600UV e vis-NIR spectrophotometer.Fluores-cence spectra were determined with Shimadzu RF-5301PC lumi-nescence spectrometer.

4.2.General procedures of spectra detection

Solutions of Fe3t,Cr3t,Pb2twere prepared in mixed solvent of acetonitrile and deionized water(V acetonitrile/V deionized water?9:1). Solutions of all other metal ions were prepared in acetonitrile.The solutions of PPM and KI were also prepared in acetonitrile.In ti-tration experiments,each time a3mL solution of PPM was?lled in a quartz optical cell of1cm optical path length,and the Hg2tso-lution was added into quartz optical cell gradually by using a micro-pipette.Spectral data were recorded at3min after the addition. 4.3.Quantum yield measurement

Fluorescence quantum yield was determined using optically matching solutions of9,10-diphenylanthracene(F f?0.9in cyclo-hexane)as standards at an excitation wavelength of360nm and the quantum yield is calculated using Eq.1.18

F unk?F stdeI unk=A unkT

eI std=A stdT

h unk

h std

(1)

Where F unk and F std are the?uorescence quantum yields of the sample and the standard,respectively.I unk and I std are the in-tegrated emission intensities of the sample and the standard,re-spectively.A unk and A std are the absorbance of the sample and the standard,respectively.And h unk and h std are the refractive indexes of the corresponding solutions.

4.4.Synthesis

4.4.1.1-Pyren-1-yl-ethanone(AP).A mixture of AlCl3(2.95g, 22mmol),acetyl chloride(1.6mL,22mmol)in CH2Cl2(50mL)was added dropwise to a solution of pyrene(4.04g,20mmol)in CH2Cl2(100mL)at0 C.The mixture was stirred at room temperature over night.The mixture was poured into1L deionized water slowly and stirred for2h.And then,the mixture was separated,the organic layer was collected,dried over anhydrous MgSO4,?ltrated,and concentrated.The residue was puri?ed by silica gel column chro-matography to give the compound AP(3.02g,62%)as light yellow solid.1H NMR(400MHz,CDCl3):d9.08e9.05(1H,d,ArH), 8.39e8.37(1H,d,ArH),8.26e8.21(3H,m,ArH),8.17e8.14(2H,m, ArH),8.07e8.03(2H,m,ArH),2.91(3H,s,CH3);13C NMR(100MHz, CDCl3):d202.13,133.99,131.89,131.87,131.05,130.49,129.72, 129.61,129.47,127.11,127.05,126.38,126.32,126.08,124.98,124.26, 123.96,30.46.GC e MS:M,found244.C18H12O requires244.29. 4.4.2.4-Pyren-1-yl-pyrimidine(PPM).To a toluene(20mL)solu-tion of ZnCl2(0.14g,1mmol),and triethyl ortho-formate(5mL, 30mmol)were added AP(2.44g,10mmol),and ammonium ace-tate(1.54g,20mmol).The mixture was heated at100 C under a nitrogen atmosphere for48h.A saturated aqueous solution of NaHCO3(100mL)was added to the mixture to quench the reaction. The mixture was extracted with CHCl3,and the organic extracts were dried over anhydrous MgSO4,?ltrated,and concentrated.The crude product was puri?ed by silica gel column chromatography to give the compound PPM(0.59g,21%)as light yellow powder.1H NMR(400MHz,CDCl3):d9.49(1H,s,pyrimidine e H),8.93e8.92 (1H,d,pyrimidine e H),8.51e8.49(1H,d,pyrene e H),8.21e8.04 (8H,m,pyrene e H),7.80e7.79(1H,d,pyrimidine e H);13C NMR (100MHz,CDCl3):d166.84,159.09,157.04,132.49,132.46,131.30, 130.74,128.87,128.77,128.64,127.51,127.30,126.34,12

5.95,125.63, 125.05,124.91,124.64,123.95,122.76;GC e MS:M,found281. C20H12N2requires280.32.

Acknowledgements

This work was?nancially supported by the National Basic Re-search Program of China(973Program,2009CB930601),the Na-tional Natural Science Foundation of China(Project No.50803027, No.50903001,No.20905038),and the Natural Science Fund for Colleges and Universities in Jiangsu Province(Grant No. 08KJD430020).

Supplementary data

Supplementary data related to this article can be found online at doi:10.1016/j.tet.2011.12.071.

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