- Email: [email protected]
A novel thiourea–hydrazone-based switch-on fluorescent chemosensor for acetate
Journal of Luminescence 131 (2011) 592–596
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A novel thiourea–hydrazone-based switch-on fluorescent chemosensor for acetate Weiwei Huang a, Zheng Chen a, Hai Lin b, Huakuan Lin a,n a b
Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China Key Laboratory of Functional Polymer Materials of Ministry of Education, Nankai University, Tianjin 300071, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2010 Received in revised form 19 October 2010 Accepted 26 October 2010 Available online 3 November 2010
A novel sensor for acetate containing thiourea and hydrazone structure has been synthesized. The anion recognition via hydrogen-bonding interactions could be monitored by anion complexation, which induced changes in UV–vis and fluorescence spectra. In particular, the recognition process could be easily detected by the ‘naked-eye’. Further insights to the nature of interactions between receptor 1 and AcO were obtained by 1H NMR titration experiments. & 2010 Elsevier B.V. All rights reserved.
Keywords: Acetate Anion recognition Switch-on Naked-eye
1. Introduction Anions play an important role in the real world. Acetate and dicarboxylates are critical components of numerous metabolic processes [1]. Without them, many enzymes and antibodies are unable to function properly [2]. In industry, dicarboxylates are important raw materials for synthesis of many common daily necessities such as nylon, paper and paints [3]. In this sense, designing receptors that can recognize acetate is of great importance. During the past few years, considerable efforts have been devoted to the development of artificial acetate and carboxylate receptors, for example, Doo Ok Jang had designed a benzimidazolebased receptor that showed ratiometric fluorescent changes only with acetate [4], and Huang et al. had synthesized a calyx [4] arene fluorescent receptor that exhibited selective recognition towards AcO [5]. Despite these progresses, sensing acetate is achieved mainly by observing the fluorescence intensity changes or fluorescence redshift of receptors, because fluorescence techniques can measure concentrations even one million times smaller than those through absorbance techniques [6]. But compared with naked-eye sensing, fluorescence detection is really troublesome as it requires an expensive fluorescence spectrometer. Consequently, there is a need to develop receptors that show color changes when binding with acetate.
n
Corresponding author. Tel.: + 86 22 23502624. E-mail address: [email protected] (H. Lin).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.10.036
In this paper, we designed and synthesized a new cleft-shaped thiourea–hydrazone-based receptor, which was a charge-neutral colorimetric sensor. The receptor could form hydrogen bond with acetate and its spectral responses changed obviously in UV–vis and fluorescent titrations. In addition, we could also observe a notable color change when adding acetate, giving naked-eye anion sensing possible.
2. Experimental 2.1. Materials All reagents used for synthesis were obtained commercially and used without further purification. In the titration experiments, all the anions were added in the form of tetrabutylammonium (TBA) salts, which were purchased from Sigma-Aldrich Chemical, stored in a vacuum desiccator containing self-indicating silica and dried fully before using. DMSO was dried using CaH2 and then distilled in reduced pressure.
2.2. General method 1
H NMR spectra were recorded on a Varian UNITY Plus-400 MHz spectrometer at the Key Laboratory of Functional Polymer Materials of Ministry of Education, Nankai University and UV–vis spectroscopy titrations were performed on a Shimadzu UV2450 spectrophotometer at 298 K. Elemental analysis for C, H and N were carried out on a
W. Huang et al. / Journal of Luminescence 131 (2011) 592–596
593
Then the precipitate was filtered while it was still hot and washed with hot ethanol for several times. Yellow solid was obtained at 80% yield. 1H NMR (400 MHz, DMSO-d6) 12.33 (s, 2H, N–H), 10.43 (s, 2H, N–H), 8.87 (d, 2H, phen-H), 8.54 (d, 4H, phen-H), 8.03 (s, 2H, C–H), 7.56 (d, 4H, phenyl-H), 7.41 (t, 4H, phenyl-H), 7.26 (t, 2H, phenyl-H). ESI-MS (m/z): calculated for C28H22N8O6S2 [M] + : 533, found: 533. Elemental analysis calculated for C28H22N8O6S2: C, 62.92; H, 4.12; N, 20.97. Found: C, 62.69; H, 4.26; N, 20.91.
PerkinElmer 240C element analyzer at the Institute of ElementoOrganic Chemistry, Nankai University. A series of DMSO solutions having same host concentration and different anion concentrations were prepared . The affinity constants Ks were obtained by the determination of absorption of the series of solutions and analysis of obtained absorption values with non-linear least square calculation method for data fitting. 2.3. Synthesis of receptor 1, 1, 10-phenanthroline-2, 9-dialdehyde bis-(phenylthioureasemicarbo-hydrazone)
3. Results and discussion The receptor 1 was synthesized according to the procedure reported in Scheme 1. To a solution of 1, 10-phenanthroline-2, 9-dicarbaldehyde (236 mg, 1 mmol) in ethanol, 4- phenylthioureasemi- carbazide (334 mg, 2 mmol) was added. After adding catalytic amounts of acetic acid, the resulting solution was refluxed for 2 h.
3.1. UV–vis anion titration studies The anion binding ability of receptor 1 was evaluated through UV–vis titrations by adding standard solutions of tetrabutylammonium
S NH NH
NH2
N Ethanol, 2h
N
N
N
+
NH
HN
S
S NH
N
HN
N
OHC
CHO
1
Scheme 1. General synthetic routes to the target receptor 1 .
1.2 0.25
1.0 Absorbance
0.20
Absorbance
0.8
0.15 0.10 0.05
0.6 0.00 -0.00002 0.00000 0.00002 0.00004 0.00006 0.00008 0.00010 0.00012 0.00014 0.00016
0.4
[AcO]
7.2 equiv 0.2 0 equiv 0.0
300
400 Wavelength/nm
500
Fig. 1. Evolution of the UV–vis spectrum of receptor 1 (2.0 10 5 M) during titration with AcO in DMSO; inset: titration plots (observed binding profiles and corresponding non-linear fit plots monitored by the absorption increase at 457 nm).
594
W. Huang et al. / Journal of Luminescence 131 (2011) 592–596
analyses of the titration curves gave Kass ¼9997 (the association constant of the receptor with AcO ). Similar changes were observed in UV–vis spectra of receptor on the addition of H2PO4 ions, but the spectral responses were somewhat smaller. In particular, receptor 1 was insensitive to excess equivalents of the weak basic anions such as F , Cl , Br and I (see Fig. 2).
salt of anions to a dry DMSO solution of the receptor at 298.270.1 K. Fig. 1 shows the UV–vis spectral changes of 1 (2 10 5 M) when acetate ions are added. It is evident from Fig. 1 that on the addition of AcO the absorption band at 373 nm decreased gradually and a new absorption band appeared at 457 nm, accompanied with a color change in the solution from light-yellow to orange (see Fig. 2). Complex formation through the hydrogen bond interactions between receptor 1 and acetate caused intramolecular charge-transfer (ICT) between the electron rich urea unit and the electron deficient benzene moiety. It induced blueshift in absorption spectrum, so the distinct color changes in the solution were observed. In addition, there was one well defined isosbestic point at 405 nm, indicating that the stable complex was obtained with a certain stoichiometric ratio between receptor 1 and AcO . A similar phenomenon was actually observed when H2PO4 was added (see Fig. 3). With the Origin program, non-linear fitting
3.2. Fluorescent anion titration studies For a better understanding of the binding mode, the fluorescence titration experiments were carried out (see Fig. 4). There was a strong emission band centered at 405 nm, when excited at 392 nm. In literature published before, most of the anion chemosensors, especially the urea- and thiourea-based sensors, are switch-off fluorescent chemosensors, or non-fluorescent sensors.
1.2
Absorbance
1.0 0.8 0.6
A B C D E F G 1+AcO-
0.4
1,1+ F-, C1-, Br-
0.2 0.0
1+ H2PO4--
300
400 500 Wavelength/nm
600
Fig. 2. Left: absorption spectra of receptor 1 (2 10 5 M) on the addition of 5 equivalent of various anions such as AcO , H2PO4 , F , Cl , Br and I in DMSO. Right: color changes of receptor 1 in DMSO; (1) ¼1.0 10 5 M, (anion) ¼ 5.0 10 3 M, A ¼ free receptor, B ¼ AcO , C ¼ H2PO4 , D ¼F , E ¼Cl , F ¼ Br and G¼I .
1.2
1 + H2PO40.20
0.15
Absorbance
Absorbance
1.0
0.8
0.10
0.05
0.6
0.00 -0.000020.00000 0.00002 0.00004 0.00006 0.00008 0.00010 0.00012 0.00014 0.00016
[H2PO4-]
0.4
83 equiv 0.2 0 equiv 0.0 300
400 Wavelength/nm
500
Fig. 3. Evolution of UV–vis spectrum of receptor 1 (2.0 10 5 M) during titration with H2PO4 in DMSO; inset: titration plots (observed binding profiles and corresponding non-linear fit plots monitored by the absorption increase at 457 nm).
W. Huang et al. / Journal of Luminescence 131 (2011) 592–596
This may be interpreted by the photo-induced electron transfer (PET) quenching mechanism [7,8], or the heavy atom effect of the sulfur atom. But considering the sensitivity of chemosensing, a switch-on, rather than a switch-off, fluorescent sensor would be more preferred [9]. It is evident from Fig. 4 that receptor 1 displays the switch-on reaction towards AcO . This phenomenon may be due to bindinginduced rigidity of the host molecule [10,11]. The configuration of the free receptor 1 was flexible and could rotate freely. On complexation with AcO ion, the host molecule 1 was rigidified, which gave rise to a large increase in emission intensity because of
3.3. Determination of binding constant and stoichiometry In Fig. 5, Job’s plot [12] of receptor 1 and AcO in DMSO shows the maximum at a molar fraction of 0.5. This result indicates that receptor 1 binds acetate anion guest at a 1:1 ratio. For a complex of 1:1 stoichiometry, the following relation could be derived easily, where X is the absorption intensity, and CH or CG is the concentration of the host or the anion guest, respectively, and Kass the affinity constant of host–guest complexation [13]: ½ðCH þ CG þ1=Kass Þ2 4CH CG 1=2 g=2CH
140 120 8 equiv
100
Intensity
inhibition as well as vibrational and rotational relaxation modes of non-radiative decay.
X ¼ X0 þ ðXlim X0 ÞfCH þCG þ 1=Kass
160
80
0 equiv
60 40 20 0 350
400 450 Wavelength/nm
500
0.25
0.20
0.15
ð1Þ
The affinity constants of receptor 1 for anionic species are calculated and listed in Table 1 below. According to the results from Job’s plot, the proposed modes of host–guest bonding are depicted in Scheme 2. In the structure, acetate is located on one side of receptor 1 via N–Hyanion hydrogen bonds, on the same plane. As clearly shown in Table 1, the order of binding affinity of receptor 1 with anions is AcO 4H2PO4 bF Cl Br I in DMSO. This shows that receptor 1 has high selectivity for anions with two- or threedimensional spatial structure, rather than globular. The reason may be that the distance between two oxygen atoms of AcO might be a better fit for the distance between the two –NH of receptor 1, that is to say the configuration of AcO and receptor 1 are better matched. 3.4.
Fig. 4. Fluorescence spectra of receptor 1 in DMSO in the presence of increasing concentration of AcO ; lex ¼ 392 nm; (1) ¼ 2.0 10 5 M.
Absorbance / 457 nm
595
1
H NMR titrations
To further investigate the nature of the interaction between anions and the receptor, 1H NMR titrations were carried out in DMSO-d6. Fig. 6 shows the 1H NMR spectral changes of receptor 1 (1 10 2 M) in DMSO-d6 in the absence and presence of different equivalents of acetate. Obviously, the proton signals at 12.35 and 10.43 ppm, which have been assigned to Ha and Hb (marked in Scheme 2), respectively, can be observed in the absence of AcO . On addition of 0.5 equivalent of AcO , the signals of Ha and Hb are broadened, and benzene rings exhibited an upfield shift slightly, being ascribed to electron transfer from anions to electron-withdrawing benzene ring. This also supported the hypothesis that hydrogen bonding was formed when acetate bonded with receptor 1.
0.10 4. Conclusions
0.05 0.0
0.1
0.2
0.3
0.4 0.5 0.6 0.7 [1]/([1]+[AcO- ])
0.8
0.9
1.0
Fig. 5. Job’s plot for complexation of receptor 1 with AcO determined by UV–vis in DMSO, (1) + (anions)¼ 2.0 10 2 M.
In summary, we have synthesized a new sensor for acetate based on thiourea and phenathroline structure, and its binding properties towards a group of anions are investigated. The study shows that receptor 1 is a good sensor in the selective recognition of AcO with obvious color changes, which can be easily detected by the ‘naked-eye’. The selectivity of this new sensor towards acetate
Table 1 Affinity constants of receptor 1 with anions in DMSO at 298.2 7 0.1 K. Anions (M 1)
AcO
H2PO4
F
Cl
Br
I
Kassa
9997
382
NDb
ND
ND
ND
a b
Affinity constants determined by UV–vis in DMSO. ND¼ cannot be determined.
596
W. Huang et al. / Journal of Luminescence 131 (2011) 592–596
N
N
N
N Α cO-
N
NH
HN
NH
HN
S
S
Hb
N N
N
H
S
H O
N
N N S
O
H
H
N
Ha
Scheme 2. Proposed 1–AcO binding mode in solution .
ability towards acetate. We are currently working towards modifying our system for use in more competitive environments such as in aqueous solution as well as developing sensors for other biologically important anions. References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 6. 1H NMR titration of a 1.0 10 2 M solution of receptor 1 in DMSO-d6 with [Bu4N]AcO.
offers us the opportunity of monitoring biologically important acetate ion. It is also expected to be applied for the detection of acetate in analytical chemistry because of its good recognition
[9] [10] [11] [12] [13]
D. Voet, J.G. Voet, Biochemistry, 2nd ed., Wiley, New York, 1995. T. Gunnlaugsson, A.P. Davis, J.E. O’Brien, M. Glynn. Org. Lett. 4 (2002) 2449. S. Carvalho, R. Delgado, N. Fonseca, V. Felix, New J. Chem. 30 (2006) 247. T.Y. Joo, N. Singh, G.W. Lee, D.O. Jang, Tetrahedron Lett. 48 (2007) 8846. R. Miao, Q.Y. Zheng, C.F. Chen, Z.T. Huang, Tetrahedron Lett. 46 (2005) 2155. R. Martinez-Manez, F. Sancenon, Chem. Rev. 103 (2003) 4419. T. Gunnlaugsson, A.P. Davis, M. Glynn, Chem. Commun. (2001) 2556. T. Gunnlaugsson, P.E. Kruger, T.C. Lee, R. Parkesh, F.M. Pfeffer, G.M. Hussey, Tetrahedron Lett. 44 (2003) 6575. F. Han, Y.H. Bao, Y.H. Yan., T.M. Fyles, J.Z. Hao, X.J. Peng, J.L. Fan, Y.K. Wu, S.G. Sun, Chem. Eur. J. 13 (2007) 2880. D.H. Lee, J.H. Im, J.H. Lee, J.I. Hong, Tetrahedron Lett. 43 (2002) 9637. S. Watanabe, O. Onogawa, Y. Komatsu, K. Yoshida, J. Am. Chem. Soc. 120 (1998) 229. Y. Liu, C.C. You, H.Y. Zhang, Supramolecular Chemistry, Nankai University Press, Tianjin, 2001, pp. 453–455. J. Bourson, J. Pouget, B. Valeur, J. Phys. Chem. 97 (1993) 4552.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A novel thiourea–hydrazone-based switch-on fluorescent chemosensor for acetate Weiwei Huang a, Zheng Chen a, Hai Lin b, Huakuan Lin a,n a b
Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China Key Laboratory of Functional Polymer Materials of Ministry of Education, Nankai University, Tianjin 300071, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2010 Received in revised form 19 October 2010 Accepted 26 October 2010 Available online 3 November 2010
A novel sensor for acetate containing thiourea and hydrazone structure has been synthesized. The anion recognition via hydrogen-bonding interactions could be monitored by anion complexation, which induced changes in UV–vis and fluorescence spectra. In particular, the recognition process could be easily detected by the ‘naked-eye’. Further insights to the nature of interactions between receptor 1 and AcO were obtained by 1H NMR titration experiments. & 2010 Elsevier B.V. All rights reserved.
Keywords: Acetate Anion recognition Switch-on Naked-eye
1. Introduction Anions play an important role in the real world. Acetate and dicarboxylates are critical components of numerous metabolic processes [1]. Without them, many enzymes and antibodies are unable to function properly [2]. In industry, dicarboxylates are important raw materials for synthesis of many common daily necessities such as nylon, paper and paints [3]. In this sense, designing receptors that can recognize acetate is of great importance. During the past few years, considerable efforts have been devoted to the development of artificial acetate and carboxylate receptors, for example, Doo Ok Jang had designed a benzimidazolebased receptor that showed ratiometric fluorescent changes only with acetate [4], and Huang et al. had synthesized a calyx [4] arene fluorescent receptor that exhibited selective recognition towards AcO [5]. Despite these progresses, sensing acetate is achieved mainly by observing the fluorescence intensity changes or fluorescence redshift of receptors, because fluorescence techniques can measure concentrations even one million times smaller than those through absorbance techniques [6]. But compared with naked-eye sensing, fluorescence detection is really troublesome as it requires an expensive fluorescence spectrometer. Consequently, there is a need to develop receptors that show color changes when binding with acetate.
n
Corresponding author. Tel.: + 86 22 23502624. E-mail address: [email protected] (H. Lin).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.10.036
In this paper, we designed and synthesized a new cleft-shaped thiourea–hydrazone-based receptor, which was a charge-neutral colorimetric sensor. The receptor could form hydrogen bond with acetate and its spectral responses changed obviously in UV–vis and fluorescent titrations. In addition, we could also observe a notable color change when adding acetate, giving naked-eye anion sensing possible.
2. Experimental 2.1. Materials All reagents used for synthesis were obtained commercially and used without further purification. In the titration experiments, all the anions were added in the form of tetrabutylammonium (TBA) salts, which were purchased from Sigma-Aldrich Chemical, stored in a vacuum desiccator containing self-indicating silica and dried fully before using. DMSO was dried using CaH2 and then distilled in reduced pressure.
2.2. General method 1
H NMR spectra were recorded on a Varian UNITY Plus-400 MHz spectrometer at the Key Laboratory of Functional Polymer Materials of Ministry of Education, Nankai University and UV–vis spectroscopy titrations were performed on a Shimadzu UV2450 spectrophotometer at 298 K. Elemental analysis for C, H and N were carried out on a
W. Huang et al. / Journal of Luminescence 131 (2011) 592–596
593
Then the precipitate was filtered while it was still hot and washed with hot ethanol for several times. Yellow solid was obtained at 80% yield. 1H NMR (400 MHz, DMSO-d6) 12.33 (s, 2H, N–H), 10.43 (s, 2H, N–H), 8.87 (d, 2H, phen-H), 8.54 (d, 4H, phen-H), 8.03 (s, 2H, C–H), 7.56 (d, 4H, phenyl-H), 7.41 (t, 4H, phenyl-H), 7.26 (t, 2H, phenyl-H). ESI-MS (m/z): calculated for C28H22N8O6S2 [M] + : 533, found: 533. Elemental analysis calculated for C28H22N8O6S2: C, 62.92; H, 4.12; N, 20.97. Found: C, 62.69; H, 4.26; N, 20.91.
PerkinElmer 240C element analyzer at the Institute of ElementoOrganic Chemistry, Nankai University. A series of DMSO solutions having same host concentration and different anion concentrations were prepared . The affinity constants Ks were obtained by the determination of absorption of the series of solutions and analysis of obtained absorption values with non-linear least square calculation method for data fitting. 2.3. Synthesis of receptor 1, 1, 10-phenanthroline-2, 9-dialdehyde bis-(phenylthioureasemicarbo-hydrazone)
3. Results and discussion The receptor 1 was synthesized according to the procedure reported in Scheme 1. To a solution of 1, 10-phenanthroline-2, 9-dicarbaldehyde (236 mg, 1 mmol) in ethanol, 4- phenylthioureasemi- carbazide (334 mg, 2 mmol) was added. After adding catalytic amounts of acetic acid, the resulting solution was refluxed for 2 h.
3.1. UV–vis anion titration studies The anion binding ability of receptor 1 was evaluated through UV–vis titrations by adding standard solutions of tetrabutylammonium
S NH NH
NH2
N Ethanol, 2h
N
N
N
+
NH
HN
S
S NH
N
HN
N
OHC
CHO
1
Scheme 1. General synthetic routes to the target receptor 1 .
1.2 0.25
1.0 Absorbance
0.20
Absorbance
0.8
0.15 0.10 0.05
0.6 0.00 -0.00002 0.00000 0.00002 0.00004 0.00006 0.00008 0.00010 0.00012 0.00014 0.00016
0.4
[AcO]
7.2 equiv 0.2 0 equiv 0.0
300
400 Wavelength/nm
500
Fig. 1. Evolution of the UV–vis spectrum of receptor 1 (2.0 10 5 M) during titration with AcO in DMSO; inset: titration plots (observed binding profiles and corresponding non-linear fit plots monitored by the absorption increase at 457 nm).
594
W. Huang et al. / Journal of Luminescence 131 (2011) 592–596
analyses of the titration curves gave Kass ¼9997 (the association constant of the receptor with AcO ). Similar changes were observed in UV–vis spectra of receptor on the addition of H2PO4 ions, but the spectral responses were somewhat smaller. In particular, receptor 1 was insensitive to excess equivalents of the weak basic anions such as F , Cl , Br and I (see Fig. 2).
salt of anions to a dry DMSO solution of the receptor at 298.270.1 K. Fig. 1 shows the UV–vis spectral changes of 1 (2 10 5 M) when acetate ions are added. It is evident from Fig. 1 that on the addition of AcO the absorption band at 373 nm decreased gradually and a new absorption band appeared at 457 nm, accompanied with a color change in the solution from light-yellow to orange (see Fig. 2). Complex formation through the hydrogen bond interactions between receptor 1 and acetate caused intramolecular charge-transfer (ICT) between the electron rich urea unit and the electron deficient benzene moiety. It induced blueshift in absorption spectrum, so the distinct color changes in the solution were observed. In addition, there was one well defined isosbestic point at 405 nm, indicating that the stable complex was obtained with a certain stoichiometric ratio between receptor 1 and AcO . A similar phenomenon was actually observed when H2PO4 was added (see Fig. 3). With the Origin program, non-linear fitting
3.2. Fluorescent anion titration studies For a better understanding of the binding mode, the fluorescence titration experiments were carried out (see Fig. 4). There was a strong emission band centered at 405 nm, when excited at 392 nm. In literature published before, most of the anion chemosensors, especially the urea- and thiourea-based sensors, are switch-off fluorescent chemosensors, or non-fluorescent sensors.
1.2
Absorbance
1.0 0.8 0.6
A B C D E F G 1+AcO-
0.4
1,1+ F-, C1-, Br-
0.2 0.0
1+ H2PO4--
300
400 500 Wavelength/nm
600
Fig. 2. Left: absorption spectra of receptor 1 (2 10 5 M) on the addition of 5 equivalent of various anions such as AcO , H2PO4 , F , Cl , Br and I in DMSO. Right: color changes of receptor 1 in DMSO; (1) ¼1.0 10 5 M, (anion) ¼ 5.0 10 3 M, A ¼ free receptor, B ¼ AcO , C ¼ H2PO4 , D ¼F , E ¼Cl , F ¼ Br and G¼I .
1.2
1 + H2PO40.20
0.15
Absorbance
Absorbance
1.0
0.8
0.10
0.05
0.6
0.00 -0.000020.00000 0.00002 0.00004 0.00006 0.00008 0.00010 0.00012 0.00014 0.00016
[H2PO4-]
0.4
83 equiv 0.2 0 equiv 0.0 300
400 Wavelength/nm
500
Fig. 3. Evolution of UV–vis spectrum of receptor 1 (2.0 10 5 M) during titration with H2PO4 in DMSO; inset: titration plots (observed binding profiles and corresponding non-linear fit plots monitored by the absorption increase at 457 nm).
W. Huang et al. / Journal of Luminescence 131 (2011) 592–596
This may be interpreted by the photo-induced electron transfer (PET) quenching mechanism [7,8], or the heavy atom effect of the sulfur atom. But considering the sensitivity of chemosensing, a switch-on, rather than a switch-off, fluorescent sensor would be more preferred [9]. It is evident from Fig. 4 that receptor 1 displays the switch-on reaction towards AcO . This phenomenon may be due to bindinginduced rigidity of the host molecule [10,11]. The configuration of the free receptor 1 was flexible and could rotate freely. On complexation with AcO ion, the host molecule 1 was rigidified, which gave rise to a large increase in emission intensity because of
3.3. Determination of binding constant and stoichiometry In Fig. 5, Job’s plot [12] of receptor 1 and AcO in DMSO shows the maximum at a molar fraction of 0.5. This result indicates that receptor 1 binds acetate anion guest at a 1:1 ratio. For a complex of 1:1 stoichiometry, the following relation could be derived easily, where X is the absorption intensity, and CH or CG is the concentration of the host or the anion guest, respectively, and Kass the affinity constant of host–guest complexation [13]: ½ðCH þ CG þ1=Kass Þ2 4CH CG 1=2 g=2CH
140 120 8 equiv
100
Intensity
inhibition as well as vibrational and rotational relaxation modes of non-radiative decay.
X ¼ X0 þ ðXlim X0 ÞfCH þCG þ 1=Kass
160
80
0 equiv
60 40 20 0 350
400 450 Wavelength/nm
500
0.25
0.20
0.15
ð1Þ
The affinity constants of receptor 1 for anionic species are calculated and listed in Table 1 below. According to the results from Job’s plot, the proposed modes of host–guest bonding are depicted in Scheme 2. In the structure, acetate is located on one side of receptor 1 via N–Hyanion hydrogen bonds, on the same plane. As clearly shown in Table 1, the order of binding affinity of receptor 1 with anions is AcO 4H2PO4 bF Cl Br I in DMSO. This shows that receptor 1 has high selectivity for anions with two- or threedimensional spatial structure, rather than globular. The reason may be that the distance between two oxygen atoms of AcO might be a better fit for the distance between the two –NH of receptor 1, that is to say the configuration of AcO and receptor 1 are better matched. 3.4.
Fig. 4. Fluorescence spectra of receptor 1 in DMSO in the presence of increasing concentration of AcO ; lex ¼ 392 nm; (1) ¼ 2.0 10 5 M.
Absorbance / 457 nm
595
1
H NMR titrations
To further investigate the nature of the interaction between anions and the receptor, 1H NMR titrations were carried out in DMSO-d6. Fig. 6 shows the 1H NMR spectral changes of receptor 1 (1 10 2 M) in DMSO-d6 in the absence and presence of different equivalents of acetate. Obviously, the proton signals at 12.35 and 10.43 ppm, which have been assigned to Ha and Hb (marked in Scheme 2), respectively, can be observed in the absence of AcO . On addition of 0.5 equivalent of AcO , the signals of Ha and Hb are broadened, and benzene rings exhibited an upfield shift slightly, being ascribed to electron transfer from anions to electron-withdrawing benzene ring. This also supported the hypothesis that hydrogen bonding was formed when acetate bonded with receptor 1.
0.10 4. Conclusions
0.05 0.0
0.1
0.2
0.3
0.4 0.5 0.6 0.7 [1]/([1]+[AcO- ])
0.8
0.9
1.0
Fig. 5. Job’s plot for complexation of receptor 1 with AcO determined by UV–vis in DMSO, (1) + (anions)¼ 2.0 10 2 M.
In summary, we have synthesized a new sensor for acetate based on thiourea and phenathroline structure, and its binding properties towards a group of anions are investigated. The study shows that receptor 1 is a good sensor in the selective recognition of AcO with obvious color changes, which can be easily detected by the ‘naked-eye’. The selectivity of this new sensor towards acetate
Table 1 Affinity constants of receptor 1 with anions in DMSO at 298.2 7 0.1 K. Anions (M 1)
AcO
H2PO4
F
Cl
Br
I
Kassa
9997
382
NDb
ND
ND
ND
a b
Affinity constants determined by UV–vis in DMSO. ND¼ cannot be determined.
596
W. Huang et al. / Journal of Luminescence 131 (2011) 592–596
N
N
N
N Α cO-
N
NH
HN
NH
HN
S
S
Hb
N N
N
H
S
H O
N
N N S
O
H
H
N
Ha
Scheme 2. Proposed 1–AcO binding mode in solution .
ability towards acetate. We are currently working towards modifying our system for use in more competitive environments such as in aqueous solution as well as developing sensors for other biologically important anions. References [1] [2] [3] [4] [5] [6] [7] [8] Fig. 6. 1H NMR titration of a 1.0 10 2 M solution of receptor 1 in DMSO-d6 with [Bu4N]AcO.
offers us the opportunity of monitoring biologically important acetate ion. It is also expected to be applied for the detection of acetate in analytical chemistry because of its good recognition
[9] [10] [11] [12] [13]
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