Luminescent probe based on photochromic cyclometalated iridium(III) complex for high selectivity detection of thiophenol
Graphical abstract
A cyclometalated iridium(III) complex (probe 1oo) comprised 2,4-dinitrobenzenesulfonamide (DNBS) as the recognition unit and photochromic diarylethene as photoswitchable unit was synthesized and its fluorescent behavior with thiophenols and UV/Vis lights in 20% DMSO/PBS solution was investigated systematically. The results demonstrated that probe 1oo could serve as a sensitive and selective fluorescent probe to detect thiophenols and could not be disturbed by aliphatic thiols.
Introduction
Organic thiols, including aliphatic thiols and aromatic thiols, play pivotal roles in biological and chemical processes [[1], [2], [3]]. For example: intracellular aliphatic thiol cysteine (Cys) often participates in enzymatic reactions as a nucleophile and stabilizes protein structure [4]; glutathione (GSH) is an antioxidant in organisms, which can prevent damage to important cellular components caused by reactive oxygen species such as peroxides, lipid peroxides, free radicals, and heavy metals [5]; homocysteine (Hcy) is essentially involved in metabolic correlation with the amino acid methionine [6]. In addition, aromatic thiols (thiophenol and its derivatives) are a class of vital chemical reagents, which are widely utilized as raw materials of pesticides, pharmaceuticals, polymers, and dyes [[7], [8], [9]]. Meanwhile, aromatic thiols are also highly toxic pollutants with exceptional volatility. Either liquid or gaseous thiophenol can be absorbed by human body through respiratory tract and finally causes a series of health problems, such as central nervous system damage, muscle weakness and even death [10,11]. Therefore, research and development of effective methods, such as high performance liquid chromatography, gas chromatography and fluorescence assays for detecting thiophenol have aroused extensive attention [[12], [13], [14]]. Among them, fluorescence analysis has become most probable approach for its high sensitivity, selectivity, easy of operation and real-time detection [15,16].
Because thiophenol (pKa = 6.5) exhibited lower pKa than aliphatic thiols (pKa = 8.5) in the neutral conditions (pH = 7.3), thiophenol can generate more nucleophilic thiolates and display stronger nucleophilic ability (Scheme 1). Thus various fluorescent probes have been designed to selectively discriminate thiophenol from aliphatic thiols based on that obviously different nucleophilic ability. We noted that most of the probes are organic molecular probes [[17], [18], [19], [20], [21], [22], [23], [24], [25], [26]] and few organometallic probes have been reported [[27], [28], [29], [30]]. It is well known that transition metal complexes as optical probes generally exhibit a relatively large Stokes shift, easy regulation of excitation and emission wavelength, high luminescence quantum yield and much longer luminescence lifetime. Furthermore, complex-based probes can effectively eliminate the interference of background fluorescence and intracellular auto-fluorescence so as to be better applied to biological imaging [[31], [32], [33], [34], [35], [36], [37], [38], [39]]. For example, Zhang and co-workers [27] reported the first Red-Emissive Ruthenium(II) complex-based luminescent thiophenol probes for cellular imaging. This probes showed relatively low LOD of 32.9 nM, but their response time was as long as 30 min.
Meanwhile, photochromic diarylethenes are special chemical species that can transform between two isomeric states (open and closed states) upon photo-irradiation and lead to distinct changes in absorption or/and emission spectra, and then have been widely used as optical memory devices and switches due to their outstanding photo-reactivity, thermal stability and fatigue resistance [[40], [41], [42]]. Herein, we used 2-phenyl pyridine with fluorescent switchable molecular diarylethene as C^N main ligand, 1,10-phenanthroline as N^N ancillary ligand and then finally designed and synthesized cyclometalated iridium(III) complex-based fluorescent probe 1oo to successfully selectively recognize thiophenol (Scheme 2). In the our designed probe, cyclometalated iridium(III) compound block was selected as fluorophore, strong electron-withdrawing group 2,4-dinitrobenzenesulfonyl (DNBS) was introduced to the N^N ancillary ligand as recognition site and luminescence quenching moiety through the intramolecular electron transfer from cyclometalated block to DNBS group (ICT). The probe displayed virtually no fluorescence initially, fluorophore was released by SNAr elimination reaction upon addition with thiophenol, resulting in the turn-on fluorescence and a distinct emission band appeared at ca. 450 nm. Probe 1oo exhibits excellent selectivity, sensitivity (LOD: 2.43 μM) and fast response to thiophenol in 20% DMSO/PBS (0.01 M, pH = 7.4).
Section snippets
Synthesis and characterization
The synthetic route of probe 1oo was displayed in Scheme 2. Compounds 3, 4 was synthesized according to reported methods [[43], [44], [45], [46]]. Compound 5 was synthesized by classic Suzuki cross coupling reaction between 2-(4-(4-bromo-5-methylthiophen-2-yl)phenyl)pyridine and compound 4. Chloro-bridged dimer 6 was obtained by the reaction of anhydrous IrCl3 with compound 5. Then in mixed solvent of dichloromethane and methanol, the dimer reacted with 5-amino-1,10-phenanthroline to obtain
Conclusions
In summary, a novel cyclometalated iridium(III) complex 1oo based on diarylethenes was successfully designed and synthesized as a high-efficiency probe for detection of thiophenol, in which cyclometalated iridium(III) moiety as the fluorophore and strong electron-withdrawing 2,4-dinitrobenzenesulfonyl (DNBS) as the recognition site and fluorescence quenching moiety. The special photochromic characteristics of the fluorescent switch enable the probe showing different properties to thiophenol
General methods
All chemicals were purchased through commercial suppliers and without further purified before use. The reaction processes were carried out under the protection of argon gas. 1H and 13C NMR spectra were recorded on a Bruker AV400 (400 MHz) NMR spectrometer with tetramethylsilane (TMS) as the internal standard. UV–vis absorption spectra were performed using an Agilent 8454 UV/Vis spectrometer. Infrared spectra (IR) were collected on a Bruker Vertex-70 spectrometer. The fluorescent measurement was
CRediT authorship contribution statement
Yaqian Guo: Investigation, Writing - original draft. Daobin Zhang: Conceptualization, Methodology, Writing - review & editing. Junyun Wang: Software. Shouzhi Pu: Supervision, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We are grateful for the financial support from the National Natural Science Foundation of China (21701065, 41867053), the Project of Science Funds of Jiangxi Education Office (GJJ170690), the Young Talents Project of Jiangxi Science and Technology Normal University (2019QNBJRC005).
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