Inorganic Chemistry Communications 73 (2016) 147–151
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Short communication
A highly selective turn-on sensor for Hg2 + based on a phosphorescent iridium (III) complex Qunbo Mei a,b,⁎, Yujie Shi a,b, Chen Chen a,b, Qingfang Hua a,b, Bihai Tong c,⁎ a Key Laboratory for Organic Electronics and Information Displays, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China b Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China c College of Metallurgy and Resources, Anhui University of Technology, Ma'anshan, Anhui 243002, China
a r t i c l e
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Article history: Received 28 July 2016 Received in revised form 16 September 2016 Accepted 1 October 2016 Available online 5 October 2016 Keywords: Iridium (III) complex Benzothiazole derivatives. Phosphorescent chemosensor Hg2+
a b s t r a c t A highly selective phosphorescent chemosensor for Hg2+ based on the iridium (III) complex Ir(DTBT)2(acac) (DTBT = 2-(5-(1,2 dihydroacenaphthylen-5-yl)thiophen-2-yl)benzothiazole, acac = acetylacetone) was synthesized and characterized. Ir(DTBT)2(acac) exhibited relatively weak fluorescenceat at about 700 nm. Ir(DTBT)2(acac) displayed a dramatic color change from near-infrared to yellow-green with the addition of Hg2+. More significantly, the chemosensor performed “turn-on” phosphorescent responses toward Hg2+. © 2016 Elsevier B.V. All rights reserved.
Mercury is currently used in a wide range of products, such as industrial chemicals, electrical or electronic devices, dental amalgam, energyefficient fluorescent light bulbs and batteries, which is one of the most dangerous and ubiquitous pollutants [1–3]. Among various heavy and soft cations, Hg2+ is one of the most toxic heavy mental ions. Once mercury ions enter the environment, bacteria can convert the inorganic Hg2+ into methylmercury or dimethyl mercury. Some research implicated that mercury pollution can cause some serious irreversible neurological damage [4]. Thus, the development of methods for the Hg2 + determination is significant for environment and human health. Several techniques for the determination of Hg2+ have been devised by utilizing electrochemical, chromogenic, and fluorogenic properties as output signals over the past few years [5–7]. Heavy metal complexes, which have more advantages with long excited state lifetime, high photoluminescence efficiency and excellent color tuning, have been received more and more attention in OLEDs and chemosensors [8–11]. Phosphorescent iridium (III) complex is also widely used to detect Hg2 + for its excellent properties [12–14]. In our previous work [15], we have synthesized iridium (III) complex Ir(TBT)2(acac) (TBT = thiophen-2-yl-benzothiazole) which can be used as a phosphorescent chemosensor with high sensitivity and selectivity for detecting of Hg2+. This chemosensor was easily prepared and found to be possible to detect the Hg2 + with dramatic color change of the solution. ⁎ Corresponding authors. E-mail addresses:
[email protected] (Q. Mei),
[email protected] (B. Tong).
http://dx.doi.org/10.1016/j.inoche.2016.10.002 1387-7003/© 2016 Elsevier B.V. All rights reserved.
Nevertheless, this chemosensor performed the “turn-off” response to Hg2 +. We also introduced a weak electron-drawing group acenaphthene into TBT unit to synthesize DTBT which can be used as Hg2+ probe simultaneously [16]. Upon addition of Hg2+ into dichloromethane (DCM) solution of DTBT, the solution was changed from colorless to yellow. Most of all, DTBT shown a “turn-on” response to Hg2+. Therefore, we hope to synthesize a phosphorescent iridium (III) complex chemosensor employing DTBT as cyclometalated ligand which can exhibited “turn-on” phosphorescent response toward Hg2 +. In this work, we report a novel iridium (III) complex Ir(DTBT)2(acac), which can also serve as a “naked-eye” detector for Hg2 +, but with “turn-on” response to Hg2 +. Furthermore, complex Ir(DTBT)2(acac) displayed a near-infrared emission, which can be also used in biological detection. The synthesize method of Ir(DTBT)2(acac) was according to literature [17–19]. As shown in Scheme 1, Ir(DTBT)2(acac) was acquired successfully in 58.96% yield. The structure of Ir(DTBT)2(acac) was confirmed by 1H NMR, 13C NMR, and MALDI-TOF. The detail descriptions of these characterizations for Ir(DTBT)2(acac) are available in supporting information. Iridium (III) complex Ir(DTBT)2(acac) was further identified using single crystal X-ray analysis to establish their exact solid state structure. As shown in Fig.1, the complex has distorted octahedral coordination geometry around iridium atom with two cyclometalated ligands DTBT and one ancillary ligand acetylacetone. The DTBT with the nitrogen atoms N1 and N2 residing at the trans locations, and the Ir–N distances
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Scheme 1. Synthesis of phosphorescent sensor Ir(DTBT)2(acac).
lie between 2.0636 and 2.0741Å. The cyclometalated carbon atoms C9 and C32 are mutually cis on the iridium and show marginally shorter distances 1.9830 and 1.9950Å. It implied that there is a stronger trans influence of the phenyl group. The bond lengths between Ir center and the O1, O2 of ancillary ligand are ranging from 2.1187 to 2.1419Å, which are longer than that between the Ir center and the cyclometalated ligands. The detail single crystal data of Ir(DTBT)2(acac) are showed in Tables S1 and S2. The UV–vis absorption spectra of Ir(DTBT)2(acac) (c = 2.0 × 10−5 M) in DCM at room temperature was shown in Fig.1. The absorption spectra of Ir(DTBT)2(acac) can be divided into two regions which was in keeping with most iridium (III) complexes. The intense absorption bands in the region of 250–370 nm are due to spin-allowed singlet interligand (π–π*) transitions of cyclometalated ligand. The weak absorption bands in the region of 370–660 nm are owing to the single spin-allowed metal to ligand charge transfer (1MLCT) and triplet spinforbidden metal to ligand charge transfer (3MLCT) [20]. To a probe, selectivity is a very important parameter. We were carried out detailed optical tests to investigate the identification ability of probe Ir(DTBT)2(acac) toward Hg2 + in the presence of acetonitrile (MeCN). As shown in Fig.2, the behavior of Ir(DTBT)2(acac) toward different cations (Ag+, Cd2 +, Co2 +, Cr3+, Cu2+, Fe3 +, Hg2+, K+, Mg2 +, Na+, Ni2+, Pb2+, Zn2+) was investigated by UV–vis absorption. Upon
addition of Hg2+ to the solution of Ir(DTBT)2(acac), it appeared a response in absorption spectra. Although, with addition of Cu2 + and Fe3+ the absorption band at 365 nm and 500 nm reduced, the band at 425 nm increased likewise with addition of Hg2+, they have difference in the spectra. As displayed in Fig. S5, with addition of different cations (Ag+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, K+, Mg2+, Na+, Ni2+, Pb2+, Zn2+), the emission peaks at 520 nm were decreased a little, the peak at 700 nm remain unchanged. Whereas, upon addition of Hg2 +, the peak at 520 nm increased rapidly, the peak at 700 nm fell down. Changes in the color of Ir(DTBT)2(acac) (c = 2.0 × 10−5 M in DCM) with the presence of 2.0 eq. different metal ions (Ag+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, K+, Mg2+, Na+, Ni2+, Pb2+, Zn2+) shown in Fig. S6. Only when mixed Hg2 + with Ir(DTBT)2(acac) changed the color. Above all, it shown high selectivity of Ir(DTBT)2(acac) toward Hg2+. Fig.3 showed the UV–vis absorption spectral changes of Ir(DTBT)2(acac) (c = 2.0 × 10−5 M in DCM) in the presence of increasing amount of Hg2+ (dissolved in MeCN). The absorption spectra was no longer changing when adding more than 0.7 equiv. Hg2 + in Ir(DTBT)2(acac) (c = 2.0 × 10−5 M in DCM). Upon addition of Hg2 + to Ir(DTBT)2(acac), new absorption bands appeared at 266 nm and 424 nm, the absorption bands at 368 nm and 498 nm disappeared
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Fig. 2. UV–vis spectra of Ir(DTBT)2(acac) (c = 2.0 × 10−5 M in DCM) in the presence of 2 equiv. of different metal ions at 298 K.
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Wavelength (nm) Fig. 3. Changes in the UV–vis absorption spectra of Ir(DTBT)2(acac) (c = 2.0 × 10−5 M in DCM) on addition of Hg2+ at 298 K.
Fig. 5. UV–vis spectra of Ir(DTBT)2(acac) (c = 2.0 × 10−5 M in DCM) in the presence of 2 equiv. of Hg2+ and 2 equiv. of other metal ions in dichloromethane. Ag+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+, K+, Mg2+, Na+, Ni2+, Pb2+ and Zn2+ were added, respectively, at 298 K.
progressively. Meanwhile, the well-defined isosbestic points at 335 nm, 400 nm and 475 nm clearly indicated that there are two phase in equilibrium. Just as we expected, it shown obvious color change from orange to yellow green, which indicating that Ir(DTBT)2(acac) can serve as a sensitive “naked-eye” detection for Hg2+. As shown in Fig.4, the solution of Ir(DTBT)2(acac) (c = 2.0 × 10−5 M) in DCM exhibits a weak emission at 520 nm and 700 nm when excited at 440 nm. With the addition of Hg2+ (0–1 equiv.), the peak at 520 nm gradually increased. There was an obvious “turn-on” process with the appearance and enhancement of a new luminescence. Whereas the peak at 700 nm have a gradual decrement trend. Moreover, a competition experiment was done to examine anti-interference ability of Ir(DTBT)2(acac) as a Hg2+ selective chemosensor. According to Fig.5 only slight UV–vis changes of Ir(DTBT)2(acac) (c = 2.0 × 10−5 M in DCM) with Hg2+ were observed in the presence of 2 equiv. of different metal ions (Ag+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+, K+, Mg2+, Na+, Ni2+, Pb2+, Zn2+). Except for Cu2+ and Fe3+, other coexistent metal ions had a negligible interfering effect on the absorption of Ir(DTBT)2(acac) upon addition of Hg2+. Generally, all of these showed
that the iridium complex Ir(DTBT)2(acac) exhibited excellent competitive ability for Hg2+ over other cations. Furthermore, the electrochemical behavior of Ir(DTBT)2(acac) to Hg2 + was investigated by cyclic voltammetry (CV), showed in Fig.6. The onset oxidation and reduction potentials of each complex were then used to calculate the HOMO and LUMO levels based on HOMO/ LUMO = −(4.8 + Eonset) eV. As a result, the HOMO level of Ir(DTBT)2(acac) was − 5.21 eV, while the Ir(DTBT)2(acac) with Hg2+ was −5.27 eV. Consistent with the phenomenon of colour changed by adding Hg2 + in Fig. 3 and Fig. 4, the HOMO energy level is lower, which leading to the energy gap between the HOMO and LUMO level increased, resulting in the remarkable blue-shift in the absorption and luminescence spectra. In order to seek the detection mechanism of probe Ir(DTBT)2(acac) to Hg2 +, a comparison experiment of 1H NMR titration of Ir(DTBT)2(acac) with different concentration of Hg2+ was putting into execution. As shown in Fig.7, with the addition of Hg2 +, the H (δ = 5.18 ppm) which belong to the methylene of ancillary ligand disappeared, indicating that auxiliary ligand was removed from complex
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Fig. 7. 1H NMR spectral of Ir(DTBT)2(acac) in CDCl3 in the presence of Hg2+.
Ir(DTBT)2(acac). Meanwhile, we also carried out a MALDI-TOF analysis to confirm the detection mechanism, which showed in Fig. S7. When 1 equiv. of Hg2+ was added to the Ir(DTBT)2(acac) solution, the peak appeared at 929.392 (m/z), which corresponding to [Ir(DTBT)2]+. This phenomenon was different from Ir(TBT)2(acac) with Hg2+. The fast decomposition of Ir(TBT)2(acac) with departure of acac from the complex to form two new complexes [Ir(TBT)2]+ and [Ir(TBT)2(H2O)2Hg]+ [15]. Such a process results in the evident changes of absorption and emission properties. The reason for the different response mechanism with Hg2+ between the Ir(TBT)2(acac) and Ir(DTBT)2(acac) was that the electron withdrawing substituent effect of acenaphthene on the thiophene ring. Different electronic cloud distribution which caused by the different substituents would result in different binding mechanism. The electron cloud density of the thiophene ring decreased due to the substitution of electron withdrawing group. So the sulfur atom of the thiophene ring of Ir(DTBT)2(acac) can not coordinate with Hg2 +. Based on the result of the 1H NMR titration and MALDI-TOF analysis, the possible binding mechanism of Ir(DTBT)2(acac) with Hg2+ was described in Fig.8. In conclusion, we have demonstrated a “turn-on” phosphorescent chemosensor for Hg2 + with high sensitivity and selectivity based on Ir(DTBT)2(acac). Meanwhile, the dramatic color change of the solution made the detection of Hg2+ possible naked-eye. With 1H NMR titration and MALDI-TOF experiments the sensing mechanisms of Ir(DTBT)2(acac) have been analysed in detail. It is believed that the present strategy may offer a new approach for developing high sensitive and selective Hg2+ sensors in environmental applications. Acknowledgements The authors acknowledge financial support from the National Basic Research Program of China (973 Program, 2012CB933301,
2012CB723402), the Ministry of Education of China (IRT1148), the National Natural Science Foundation of China (BZ2010043, 20974046, 20774043, 51173081, 50428303, 61106017, 61136003), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, YX03001) and the Research Fund for Nanjing University of Posts and Telecommunications (NY215153). Appendix A. Supplementary material CCDC 1435617 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/ data_request/cif. Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.inoche. 2016.10.002. References [1] X. Ma, J. Wang, Q. Shan, Z. Tan, G. Wei, D. Wei, Y. Du, Org. Lett. 14 (2012) 820. [2] I. Samb, J. Bell, P.Y. Toullec, V. Michelet, I. Leray, Org. Lett. 13 (2011) 1182. [3] Z.Z. Deris, J.D. Swarbrick, K.D. Roberts, M.A.K. Azad, J. Akter, A.S. Horne, R.L. Nation, K.L. Rogers, P.E. Thompson, T. Velkov, J. Li, Bioconjug. Chem. 25 (2014) 750. [4] E.M. Nolan, S.J. Lippard, Chem. Rev. 108 (2008) 3443. [5] X. Zhang, Y. Xiao, X. Qian, Angew. Chem. Int. Ed. 47 (2008) 8025. [6] J.V. Ros-Lis, R. Martínez-Máñez, K. Rurack, F. Sancenón, J. Soto, M. Spieles, Inorg. Chem. 43 (2004) 5183. [7] H. Yang, Z. Zhou, K. Huang, M. Yu, F. Li, T. Yi, C. Huang, Org. Lett. 9 (2007) 4729. [8] Y. Zhang, Y.H. Xu, Q.L. Niu, J.B. Peng, W. Yang, X.H. Zhu, Y. Cao, J. Mater. Chem. 17 (2007) 992. [9] L. Fu, M. Pan, Y. Li, H. Wu, H. Wang, C. Yan, K. Li, S. Wei, Z. Wang, C. Su, J. Mater. Chem. 22 (2012) 22496. [10] W.S. Tang, X.X. Lu, K.M.C. Wong, V.W.W. Yam, J. Mater. Chem. 15 (2005) 2714. [11] K.K.W. Lo, M.W. Sze, K.S. Lau, Inorg. Chem. 47 (2008) 602. [12] H. Zeng, F. Yu, J. Dai, H. Sun, Z. Lu, M. Li, Q. Jiang, Y. Huang, Dalton Trans. 41 (2012) 4878. [13] Q. Zhao, S. Liu, F. Li, T. Yi, C. Huang, Dalton Trans. (2008) 3836.
Fig. 8. The possible binding mechanism of Ir(DTBT)2(acac) with Hg2+.
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