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A new phosphorescent chemosensor bearing Zn-DPA sites for H2PO4−
Dyes and Pigments 106 (2014) 20e24
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
A new phosphorescent chemosensor bearing Zn-DPA sites for H2PO 4 Hye-bin Kim a,1, Yifan Liu b,1, Dayoung Nam c, Yinan Li a, Sungnam Park c, Juyoung Yoon b, *, Myung Ho Hyun a, ** a
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 690-735, Republic of Korea Department of Chemistry and Nano Science, Ewha Womans University, Global Top5 Research Program, Seoul 120-750, Republic of Korea c Department of Chemistry, Korea University, Seoul 136-701, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2013 Received in revised form 13 February 2014 Accepted 16 February 2014 Available online 25 February 2014
A phosphorescence chemosensor bearing an iridium (III) complex as a communicating source and two Zn-DPA units acting as a preorganized binding site for H2PO 4 is presented. The phosphorescent chemosensor displayed a selective phosphorescence enhancement with H2PO 4 in aqueous solution and at neutral pH. On the other hand, F, Cl, Br, I, NO 3 , CH3COO , and ClO4 did not induce significant changes in phosphorescence. Furthermore, the phosphorescent chemosensor shows selectivity for H2PO 4 over the related phosphate species, such as ATP and pyrophosphate. In addition, there was a distinct lifetime change only for H2PO 4 . The association constant was calculated to be 9.68 104 M1 for H2PO 4. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Phosphorescence chemosensor Anion recognition Dihydrogen phosphate sensing Phosphate sensor Iridium complex Zinc complex
1. Introduction Since anions play important roles in many chemical and biological processes, recognition and detection of anions has attracted much attention [1]. In particular, phosphate is present in the forms of inorganic phosphate, lipid phosphate, or phosphate esters, and these play key roles in cellular signaling [2] and bone mineralization [3]. Accordingly, sensing these phosphate species is an active research goal. Even though fluorescent chemosensors for H2PO 4 have been extensively studied [4], there have only been a couple of reports that evaluated phosphorescent chemosensors [5]. Compared to fluorescent chemosensors, phosphorescent chemosensors show clear advantages, such as relatively large Stokes shifts and long lifetimes, which allow discrimination from the background fluorescence in biological samples [6]. Among the various types of phosphors, highly phosphorescent iridium (III) complexes have been actively developed as phosphorescent chemosensors [6,7].
* Corresponding author. Department of Bioinspired Science, Ewha Womans University, Global Top5 Research Program, Seoul 120-750, Republic of Korea. Fax: þ82 2 3277 3419. ** Corresponding author. E-mail addresses: [email protected] (J. Yoon), [email protected] (M.H. Hyun). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.dyepig.2014.02.015 0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.
In the current work, we report the first example of an H2PO 4selective phosphorescence chemosensor, which works in 100% aqueous solution at neutral pH. Two Zn-DPA (dipicolylamine) units [8,9] cooperatively bind with H2PO 4 resulting in selective phosphorescence enhancement. Simple anions, such as F, Cl, Br, I, NO 3 , CH3COO , and ClO4 , did not induce any significant phosphorescence change. In addition, phosphorescence chemosensor 1Zn displayed selectivity for H2PO 4 over related phosphate species, such as ATP and pyrophosphate, which was also confirmed by lifetime experiments. 2. Experimental 2.1. Materials and equipment Unless otherwise noted, materials were obtained from commercial suppliers and were used without further purification. Flash chromatography was carried out on silica gel (230e400 mesh). 1H NMR and 13C NMR spectra were recorded at 300 MHz and 75 MHz, respectively. Chemical shifts were expressed in ppm and coupling constants (J) are in Hz. UVevis absorption spectra were measured with an Evolution 201 UVevisible spectrophotometer. Phosphorescence spectra were recorded on a Shimadzu RF-5301 pc spectrofluorometer.
H.-b. Kim et al. / Dyes and Pigments 106 (2014) 20e24
Fig. 1. (a) The phosphorescence spectra of [iridium (III) complex 1 (10 mM) þ Zn2þ (20 mM)] upon addition of different anions (100 eq) in CH3CN-HEPES (1 mM, pH 7.4) (1:1). Excitation wavelength: 365 nm.
2.5 2.0
21
mixture was stirred at room temperature overnight. After removing the solvent, the residue was dissolved in methylene chloride (20 mL), which was washed with water (20 mL). The organic layer was separated and dried over anhydrous Na2SO4. The solvent was removed and the residue was purified by silica gel chromatography using methylene chloride and methanol (9:1, v/v) as an eluent. After the product was dissolved into the mixed solvent of methylene chloride and methanol (100 mL, 1:1, v/v), NH4PF6 (0.42 g, 2.58 mmol) was added, and the reaction mixture was stirred at room temperature for 12 h. The solvent was removed and the residue was dissolved in methylene chloride, which was washed with water (20 mL). Again, the organic layer was separated and dried over anhydrous Na2SO4. The residue was dissolved in methylene chloride (10 mL). Diethyl ether (100 mL) was added dropwise to induce precipitation. The solid product was filtered and dried under high vacuum to afford iridium (III) complex 1 as a yellow solid material in a yield of 66% (0.25 g). Mp 145e146 C; IR (KBr pellet) cm1 3052, 1608, 1478; 1H NMR (300 MHz, CDCl3) d (ppm) 3.93 (s, 8H), 4.03 (s, 4H), 6.24 (d, 2H, J ¼ 6.4 Hz), 6.87 (t, 2H, J ¼ 12.0 Hz), 6.99e7.16 (m, 8H), 7.44e7.53 (m, 8H), 7.62e7.78 (m, 12H), 7.87 (d, 2H, J ¼ 6.4 Hz), 8.48e8.66 (m, 4H); 13C NMR (75 MHz, CDCl3) d (ppm) 57.2, 60.2, 119.6, 122.7, 122.9, 123.7, 124.2, 124.7, 124.9, 127.8, 130.9, 131.9, 137.7, 138.1, 143.8, 148.8, 149.1, 151.0, 150.7, 153.2, 155.8, 158.1, 167.9; HRFAB-MS m/z [Mþ] calc’d for C58H50IrN10 1079.3849. Found: 1079.3848. 2.3. Phosphorescence measurements
I / I0
1.5 1.0 0.5 0.0 1-Zn
1-Zn+PPi
1-Zn+ATP
-
1-Zn+H2PO4
Fig. 2. Comparison for phosphorescence intensities of 1-Zn (10 mM) and 1-Zn with PPi, ATP, and H2PO 4 (10 eq.) in HEPES (1 mM, pH 7.4).
A stock solution of compound 1 (0.1 mM) was prepared in CH3CNe H2O (1:1, v/v). A stock solution of Zn(ClO4)2 was prepared in the solvent system. Two equivalents of Zn(ClO4)2 were added, and the final concentrations of 1 and Zn2þ were 1.0 105 M and 2.0 105 M, respectively. The stock solutions (10 mM) of various anions, such as Br, Cl, F, H2PO 4 , NO3 , I , ClO4 and OAc , were prepared using their tetrabutyl ammonium salts in CH3CNeH2O (1:1, v/v). The test solutions were prepared by diluting these stock solutions with HEPES buffer (1 mM, pH 7.4) and CH3CN (see Fig. 1). All of the phosphorescence measurements were carried out in deaerated solution.
2.2. Synthesis 2.4. Phosphorescence lifetime measurements Iridium (III) complex 1 [10] Dipicolylamine (DPA) (0.63 mL, 3.51 mmol) in dry THF (15 mL) was added to the NaH solution (0.093 g, 3.86 mmol) in dry tetrahydrofuran (THF, 10 mL) through a cannula under an argon atmosphere. After stirring the solution for 1 h, the resulting solution was transferred through a cannula into the solution of iridium (III) complex 2 [11] (1.48 g, 1.76 mmol) in dry THF (15 mL). The reaction
The decay of photoluminescence in the samples was measured using our nanosecond photoluminescence spectrometer (the detailed experimental setup is presented in the Supporting Information.) Phosphorescence lifetimes are obtained by a single exponential fit to the photoluminescence decays, as shown in Fig. 4. The lifetime of 1-Zn-H2PO 4 is significantly longer than those of
2þ Fig. 3. (a) Phosphorescence titrations of iridium (III) complex 1-Zn with H2PO 4 in HEPES (1 mM, pH 7.4), which was prepared in situ with iridium (III) complex 1 (10 mM) and Zn (20 mM). (b) BenesieHildebrand plot of PL spectra of complex 1-Zn with H2PO 4.
22
H.-b. Kim et al. / Dyes and Pigments 106 (2014) 20e24
prepared in three steps [11]. A chloride-bridged dimeric iridium complex, [(ppy)2IrCl]2, was obtained using 2-phenylpyridine following the reported procedure [12]. Then, [(ppy)2IrCl]2 was reacted with 4,40 -bis(bromomethyl)-2,20 -bipyridine [13] to produce iridium (III) complex 1. Scheme 1
Intensity (Norm.)
1.0
1-Zn-ATP 1-Zn-H2PO4
0.8
1-Zn-PPi 1-Zn 1 only
0.6 0.4
3.2. Spectral studies of iridium (III) complex 1-Zn
0.2 0.0 0
200
400
600
800
Time ns Fig. 4. The phosphorescence lifetime of iridium (III) complex 1 (10 mM), 1-Zn (10 mM for 1 and 20 mM) and 1-Zn with ATP, H2PO 4 and PPi (10 eq.) in HEPES (1 mM, pH 7.4). Table 1 Phosphorescence lifetimes measured by a nanosecond photoluminescence spectrometer. Sample
Lifetime (ns)
1-Zn-ATP 1-Zn-H2PO 4 1-Zn-PPi 1-Zn 1 only
83 142 93 80 77
other samples, as shown in Table 1. Phosphorescence lifetime measurments were also carried out in deaerated solution. 3. Results and discussion 3.1. Synthesis Iridium complex 1 was prepared by modifying the reported procedure [10]. For the synthesis of 1, bisbromomethyl adduct 2 was first
The photoluminescence changes of complex 1-Zn (10 mM) were first examined towards various anions, including F, Cl, Br, I, NO 3 , CH3COO , ClO4 , and H2PO4 , in acetonitrile-HEPES (1 mM, pH 7.4) (1:1). The UV absorption of compound 1 is displayed in Fig. S1, which shows a maximum absorption at 261 nm and a weak additional absorption at 365 nm. To this iridium complex 1, 2 eq. of Zn(ClO4)2 were added so that complex 1-Zn was formed in situ. The UV absorption spectrum of this complex is also displayed in Fig. S1. The 1-Zn complex was excited at 365 nm, and absorption at 365 nm was attributed to the spin-allowed metal-to-ligand charge transfer (1MLCT) transitions. A broad and featureless region in the photoluminescence spectrum was observed with a maximum wavelength of 580e590 nm, which also originated from the metal-toligand excited state, 3MLCT [14]. Only H2PO 4 induced phosphorescence enhancement; on the other hand, the other anions did not induce any significant phosphorescence change. This was due to the strong interaction between Zn2þ and H2PO 4 . Furthermore, this interaction inhibited the ILCT (Intraligand charge transfer) (from DPA to bpy), thereby increasing the MLCT contribution [15]. Other phosphate species, such as ATP and pyrophosphate (PPi), were also examined with H2PO 4 . It is quite a challenging task to distinguish one of these species from the others. There are some reports for selective fluorescent chemosensors for ATP and PPi in an aqueous solution; however, it is rare for H2PO 4 , and almost no examples exist for phosphorescent chemosensors in a 100% aqueous solution at neutral pH. As shown in Fig. 2, complex 1-Zn showed selective phosphorescence enhancement (over 2-fold) with H2PO 4 , while there is much less change for ATP and PPi in HEPES (pH 7.4). Fig. 3 provides the phosphorescence titration data for H2PO 4 and a BenesieHildebrand plot for this experiment. The association
Scheme 1. Synthesis of iridium (III) complex 1 and proposed binding mode of 1-Zn2þ with H2PO 4.
H.-b. Kim et al. / Dyes and Pigments 106 (2014) 20e24
constant (Ka) of 1-Zn with H2PO 4 was calculated to be 9.68 104 M1 based on the phosphorescence titrations. On the other hand, the association constant for ATP was calculated to be 1.73 104 M1 (Fig. S2). Thus, the association constant for H2PO 4 is about 6 times larger than that for ATP. Phosphorescence lifetime measurements (Fig. 4) also support the trend that was observed by phosphorescence emission changes. Phosphorescence lifetime data were obtained using nanosecond phosphorescence lifetime spectroscopy, as illustrated in Fig. S3. Similar phosphorescence lifetimes for 1 (0.077 ms) and 1-Zn (0.080 ms) were observed. The phosphorescence lifetime of complex 1-Zn (0.080 ms) increased upon the addition H2PO 4 (0.14 ms). On the other hand, there was much less change when ATP or PPi was added.
[2] [3] [4]
4. Conclusion In conclusion, a phosphorescent chemosensor 1-Zn bearing an iridium (III) complex as a communicating source and two Zn-DPA units as a preorganized binding site was prepared as a selective phosphorescence chemosensor for H2PO 4 . Phosphorescent chemosensor 1-Zn displayed a selective phosphorescence enhancement with H2PO 4 in 100% aqueous solution and at neutral pH. On the contrary, other simple anions including F, Cl, Br, I, NO 3, CH3COO, and ClO 4 did not induce a significant phosphorescence change. Furthermore, phosphorescence chemosensor 1-Zn showed selectivity for H2PO 4 over other related phosphate species, i.e., ATP and pyrophosphate. The association constant in aqueous solution was calculated to be 9.68 104 M1 for H2PO 4 , which is 6-times higher than that of ATP. Finally, the phosphorescence lifetime of complex 1-Zn changed from 0.08 ms to 0.14 ms upon the addition of H2PO 4.
[5]
[6]
Acknowledgments This research was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2012-0002571 for M.H.H., No.2010-0020209 for S.P. and National Creative Research Initiative: No. 2012R1A3A2 048814 for J.Y.) Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2014.02.015.
[7]
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Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
A new phosphorescent chemosensor bearing Zn-DPA sites for H2PO 4 Hye-bin Kim a,1, Yifan Liu b,1, Dayoung Nam c, Yinan Li a, Sungnam Park c, Juyoung Yoon b, *, Myung Ho Hyun a, ** a
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 690-735, Republic of Korea Department of Chemistry and Nano Science, Ewha Womans University, Global Top5 Research Program, Seoul 120-750, Republic of Korea c Department of Chemistry, Korea University, Seoul 136-701, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2013 Received in revised form 13 February 2014 Accepted 16 February 2014 Available online 25 February 2014
A phosphorescence chemosensor bearing an iridium (III) complex as a communicating source and two Zn-DPA units acting as a preorganized binding site for H2PO 4 is presented. The phosphorescent chemosensor displayed a selective phosphorescence enhancement with H2PO 4 in aqueous solution and at neutral pH. On the other hand, F, Cl, Br, I, NO 3 , CH3COO , and ClO4 did not induce significant changes in phosphorescence. Furthermore, the phosphorescent chemosensor shows selectivity for H2PO 4 over the related phosphate species, such as ATP and pyrophosphate. In addition, there was a distinct lifetime change only for H2PO 4 . The association constant was calculated to be 9.68 104 M1 for H2PO 4. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Phosphorescence chemosensor Anion recognition Dihydrogen phosphate sensing Phosphate sensor Iridium complex Zinc complex
1. Introduction Since anions play important roles in many chemical and biological processes, recognition and detection of anions has attracted much attention [1]. In particular, phosphate is present in the forms of inorganic phosphate, lipid phosphate, or phosphate esters, and these play key roles in cellular signaling [2] and bone mineralization [3]. Accordingly, sensing these phosphate species is an active research goal. Even though fluorescent chemosensors for H2PO 4 have been extensively studied [4], there have only been a couple of reports that evaluated phosphorescent chemosensors [5]. Compared to fluorescent chemosensors, phosphorescent chemosensors show clear advantages, such as relatively large Stokes shifts and long lifetimes, which allow discrimination from the background fluorescence in biological samples [6]. Among the various types of phosphors, highly phosphorescent iridium (III) complexes have been actively developed as phosphorescent chemosensors [6,7].
* Corresponding author. Department of Bioinspired Science, Ewha Womans University, Global Top5 Research Program, Seoul 120-750, Republic of Korea. Fax: þ82 2 3277 3419. ** Corresponding author. E-mail addresses: [email protected] (J. Yoon), [email protected] (M.H. Hyun). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.dyepig.2014.02.015 0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.
In the current work, we report the first example of an H2PO 4selective phosphorescence chemosensor, which works in 100% aqueous solution at neutral pH. Two Zn-DPA (dipicolylamine) units [8,9] cooperatively bind with H2PO 4 resulting in selective phosphorescence enhancement. Simple anions, such as F, Cl, Br, I, NO 3 , CH3COO , and ClO4 , did not induce any significant phosphorescence change. In addition, phosphorescence chemosensor 1Zn displayed selectivity for H2PO 4 over related phosphate species, such as ATP and pyrophosphate, which was also confirmed by lifetime experiments. 2. Experimental 2.1. Materials and equipment Unless otherwise noted, materials were obtained from commercial suppliers and were used without further purification. Flash chromatography was carried out on silica gel (230e400 mesh). 1H NMR and 13C NMR spectra were recorded at 300 MHz and 75 MHz, respectively. Chemical shifts were expressed in ppm and coupling constants (J) are in Hz. UVevis absorption spectra were measured with an Evolution 201 UVevisible spectrophotometer. Phosphorescence spectra were recorded on a Shimadzu RF-5301 pc spectrofluorometer.
H.-b. Kim et al. / Dyes and Pigments 106 (2014) 20e24
Fig. 1. (a) The phosphorescence spectra of [iridium (III) complex 1 (10 mM) þ Zn2þ (20 mM)] upon addition of different anions (100 eq) in CH3CN-HEPES (1 mM, pH 7.4) (1:1). Excitation wavelength: 365 nm.
2.5 2.0
21
mixture was stirred at room temperature overnight. After removing the solvent, the residue was dissolved in methylene chloride (20 mL), which was washed with water (20 mL). The organic layer was separated and dried over anhydrous Na2SO4. The solvent was removed and the residue was purified by silica gel chromatography using methylene chloride and methanol (9:1, v/v) as an eluent. After the product was dissolved into the mixed solvent of methylene chloride and methanol (100 mL, 1:1, v/v), NH4PF6 (0.42 g, 2.58 mmol) was added, and the reaction mixture was stirred at room temperature for 12 h. The solvent was removed and the residue was dissolved in methylene chloride, which was washed with water (20 mL). Again, the organic layer was separated and dried over anhydrous Na2SO4. The residue was dissolved in methylene chloride (10 mL). Diethyl ether (100 mL) was added dropwise to induce precipitation. The solid product was filtered and dried under high vacuum to afford iridium (III) complex 1 as a yellow solid material in a yield of 66% (0.25 g). Mp 145e146 C; IR (KBr pellet) cm1 3052, 1608, 1478; 1H NMR (300 MHz, CDCl3) d (ppm) 3.93 (s, 8H), 4.03 (s, 4H), 6.24 (d, 2H, J ¼ 6.4 Hz), 6.87 (t, 2H, J ¼ 12.0 Hz), 6.99e7.16 (m, 8H), 7.44e7.53 (m, 8H), 7.62e7.78 (m, 12H), 7.87 (d, 2H, J ¼ 6.4 Hz), 8.48e8.66 (m, 4H); 13C NMR (75 MHz, CDCl3) d (ppm) 57.2, 60.2, 119.6, 122.7, 122.9, 123.7, 124.2, 124.7, 124.9, 127.8, 130.9, 131.9, 137.7, 138.1, 143.8, 148.8, 149.1, 151.0, 150.7, 153.2, 155.8, 158.1, 167.9; HRFAB-MS m/z [Mþ] calc’d for C58H50IrN10 1079.3849. Found: 1079.3848. 2.3. Phosphorescence measurements
I / I0
1.5 1.0 0.5 0.0 1-Zn
1-Zn+PPi
1-Zn+ATP
-
1-Zn+H2PO4
Fig. 2. Comparison for phosphorescence intensities of 1-Zn (10 mM) and 1-Zn with PPi, ATP, and H2PO 4 (10 eq.) in HEPES (1 mM, pH 7.4).
A stock solution of compound 1 (0.1 mM) was prepared in CH3CNe H2O (1:1, v/v). A stock solution of Zn(ClO4)2 was prepared in the solvent system. Two equivalents of Zn(ClO4)2 were added, and the final concentrations of 1 and Zn2þ were 1.0 105 M and 2.0 105 M, respectively. The stock solutions (10 mM) of various anions, such as Br, Cl, F, H2PO 4 , NO3 , I , ClO4 and OAc , were prepared using their tetrabutyl ammonium salts in CH3CNeH2O (1:1, v/v). The test solutions were prepared by diluting these stock solutions with HEPES buffer (1 mM, pH 7.4) and CH3CN (see Fig. 1). All of the phosphorescence measurements were carried out in deaerated solution.
2.2. Synthesis 2.4. Phosphorescence lifetime measurements Iridium (III) complex 1 [10] Dipicolylamine (DPA) (0.63 mL, 3.51 mmol) in dry THF (15 mL) was added to the NaH solution (0.093 g, 3.86 mmol) in dry tetrahydrofuran (THF, 10 mL) through a cannula under an argon atmosphere. After stirring the solution for 1 h, the resulting solution was transferred through a cannula into the solution of iridium (III) complex 2 [11] (1.48 g, 1.76 mmol) in dry THF (15 mL). The reaction
The decay of photoluminescence in the samples was measured using our nanosecond photoluminescence spectrometer (the detailed experimental setup is presented in the Supporting Information.) Phosphorescence lifetimes are obtained by a single exponential fit to the photoluminescence decays, as shown in Fig. 4. The lifetime of 1-Zn-H2PO 4 is significantly longer than those of
2þ Fig. 3. (a) Phosphorescence titrations of iridium (III) complex 1-Zn with H2PO 4 in HEPES (1 mM, pH 7.4), which was prepared in situ with iridium (III) complex 1 (10 mM) and Zn (20 mM). (b) BenesieHildebrand plot of PL spectra of complex 1-Zn with H2PO 4.
22
H.-b. Kim et al. / Dyes and Pigments 106 (2014) 20e24
prepared in three steps [11]. A chloride-bridged dimeric iridium complex, [(ppy)2IrCl]2, was obtained using 2-phenylpyridine following the reported procedure [12]. Then, [(ppy)2IrCl]2 was reacted with 4,40 -bis(bromomethyl)-2,20 -bipyridine [13] to produce iridium (III) complex 1. Scheme 1
Intensity (Norm.)
1.0
1-Zn-ATP 1-Zn-H2PO4
0.8
1-Zn-PPi 1-Zn 1 only
0.6 0.4
3.2. Spectral studies of iridium (III) complex 1-Zn
0.2 0.0 0
200
400
600
800
Time ns Fig. 4. The phosphorescence lifetime of iridium (III) complex 1 (10 mM), 1-Zn (10 mM for 1 and 20 mM) and 1-Zn with ATP, H2PO 4 and PPi (10 eq.) in HEPES (1 mM, pH 7.4). Table 1 Phosphorescence lifetimes measured by a nanosecond photoluminescence spectrometer. Sample
Lifetime (ns)
1-Zn-ATP 1-Zn-H2PO 4 1-Zn-PPi 1-Zn 1 only
83 142 93 80 77
other samples, as shown in Table 1. Phosphorescence lifetime measurments were also carried out in deaerated solution. 3. Results and discussion 3.1. Synthesis Iridium complex 1 was prepared by modifying the reported procedure [10]. For the synthesis of 1, bisbromomethyl adduct 2 was first
The photoluminescence changes of complex 1-Zn (10 mM) were first examined towards various anions, including F, Cl, Br, I, NO 3 , CH3COO , ClO4 , and H2PO4 , in acetonitrile-HEPES (1 mM, pH 7.4) (1:1). The UV absorption of compound 1 is displayed in Fig. S1, which shows a maximum absorption at 261 nm and a weak additional absorption at 365 nm. To this iridium complex 1, 2 eq. of Zn(ClO4)2 were added so that complex 1-Zn was formed in situ. The UV absorption spectrum of this complex is also displayed in Fig. S1. The 1-Zn complex was excited at 365 nm, and absorption at 365 nm was attributed to the spin-allowed metal-to-ligand charge transfer (1MLCT) transitions. A broad and featureless region in the photoluminescence spectrum was observed with a maximum wavelength of 580e590 nm, which also originated from the metal-toligand excited state, 3MLCT [14]. Only H2PO 4 induced phosphorescence enhancement; on the other hand, the other anions did not induce any significant phosphorescence change. This was due to the strong interaction between Zn2þ and H2PO 4 . Furthermore, this interaction inhibited the ILCT (Intraligand charge transfer) (from DPA to bpy), thereby increasing the MLCT contribution [15]. Other phosphate species, such as ATP and pyrophosphate (PPi), were also examined with H2PO 4 . It is quite a challenging task to distinguish one of these species from the others. There are some reports for selective fluorescent chemosensors for ATP and PPi in an aqueous solution; however, it is rare for H2PO 4 , and almost no examples exist for phosphorescent chemosensors in a 100% aqueous solution at neutral pH. As shown in Fig. 2, complex 1-Zn showed selective phosphorescence enhancement (over 2-fold) with H2PO 4 , while there is much less change for ATP and PPi in HEPES (pH 7.4). Fig. 3 provides the phosphorescence titration data for H2PO 4 and a BenesieHildebrand plot for this experiment. The association
Scheme 1. Synthesis of iridium (III) complex 1 and proposed binding mode of 1-Zn2þ with H2PO 4.
H.-b. Kim et al. / Dyes and Pigments 106 (2014) 20e24
constant (Ka) of 1-Zn with H2PO 4 was calculated to be 9.68 104 M1 based on the phosphorescence titrations. On the other hand, the association constant for ATP was calculated to be 1.73 104 M1 (Fig. S2). Thus, the association constant for H2PO 4 is about 6 times larger than that for ATP. Phosphorescence lifetime measurements (Fig. 4) also support the trend that was observed by phosphorescence emission changes. Phosphorescence lifetime data were obtained using nanosecond phosphorescence lifetime spectroscopy, as illustrated in Fig. S3. Similar phosphorescence lifetimes for 1 (0.077 ms) and 1-Zn (0.080 ms) were observed. The phosphorescence lifetime of complex 1-Zn (0.080 ms) increased upon the addition H2PO 4 (0.14 ms). On the other hand, there was much less change when ATP or PPi was added.
[2] [3] [4]
4. Conclusion In conclusion, a phosphorescent chemosensor 1-Zn bearing an iridium (III) complex as a communicating source and two Zn-DPA units as a preorganized binding site was prepared as a selective phosphorescence chemosensor for H2PO 4 . Phosphorescent chemosensor 1-Zn displayed a selective phosphorescence enhancement with H2PO 4 in 100% aqueous solution and at neutral pH. On the contrary, other simple anions including F, Cl, Br, I, NO 3, CH3COO, and ClO 4 did not induce a significant phosphorescence change. Furthermore, phosphorescence chemosensor 1-Zn showed selectivity for H2PO 4 over other related phosphate species, i.e., ATP and pyrophosphate. The association constant in aqueous solution was calculated to be 9.68 104 M1 for H2PO 4 , which is 6-times higher than that of ATP. Finally, the phosphorescence lifetime of complex 1-Zn changed from 0.08 ms to 0.14 ms upon the addition of H2PO 4.
[5]
[6]
Acknowledgments This research was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2012-0002571 for M.H.H., No.2010-0020209 for S.P. and National Creative Research Initiative: No. 2012R1A3A2 048814 for J.Y.) Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2014.02.015.
[7]
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