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A spirobifluorene-based supramolecular polymer: Solvent-induced SCSC transformation and fluorescent sensing
Inorganic Chemistry Communications 112 (2020) 107703
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A spirobifluorene-based supramolecular polymer: Solvent-induced SCSC transformation and fluorescent sensing
T
Yanxue Shanga, Lijuan Lva, Juan Dua, Qianqian Yanga, Jianbo Yina, Di Liua, Romgming Wanga,b, , Daofeng Suna,b, Jianzhuang Jiangc ⁎
a
College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, China State Key Laboratory of Heavy Oil Processing, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, China c Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, China b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Supramolecular polymer Single-crystal-to-single-crystal transformation Fluorescent sensing Metal ions NACs
Three crystalline supramolecular polymers (SPs), [(TCPSBF)·(C4H8O2)3]n (intermediate), [(TCPSBF)·(C4H8O2)2]n (UPC-S3) and [(TCPSBF)0.5]n (UPC-S4), were constructed by solvent diffusion method based on 2,2′,7,7′-tetrakis (4- cyanophenyl)-9,9′-spirobi-[9H-fluorene] (TCPSBF). Interestingly, single crystal diffraction and PXRD analyses display the solvent-induced single-crystal-to-single-crystal (SCSC) transformation among three SPs. The unstable intermediate crystal gradually changes to UPC-S3 with the loss of a lattice dioxane molecule at room temperature, and UPC-S3 may be further transformed into UPC-S4 with the loss of the other two lattice dioxane molecules by solvent soaking. Meanwhile, the intermediate crystal can also be directly transformed into UPC-S4 by solvent soaking. As a result, UPC-S4 exhibits highly thermal and chemical stability. The fluorescent spectrum tests show that UPC-S4 possesses potential applications not only in the detecting of small molecules including nhexane and nitroaromatic derivatives, but also in selective sensing of Fe3+.
1. Introduction With the development of social economy and the continuous improvement of the scale of industrialization, water pollution has become an unavoidable topic [1]. The pollutants including metal ions and toxic organic small molecules pose a great threat to the environment and
⁎
human health. Therefore, the trace detection of these compounds has attracted wide attention of researchers [2,3]. In the past decades, various detection devices, such as gas chromatography, inductively coupled plasma emission spectrometer (ICP), and cyclic voltammetry, have been developed to detect these pollutants. Although these methods bear high selectivity and good limit of detection (LOD), they have the
Corresponding author at: College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, China. E-mail address: [email protected] (R. Wang).
https://doi.org/10.1016/j.inoche.2019.107703 Received 4 November 2019; Received in revised form 22 November 2019; Accepted 27 November 2019 Available online 23 December 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
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disadvantages of high cost, inconvenience and operation trouble [4]. Thus, it is very urgent to develop the new technologies with low cost, rapid detect and easy operation. Recently, the fluorescent sensing materials including metal organic frameworks (MOFs) and hydrogenbonded organic frameworks (HOFs) have showed the promising application perspectives in the detection of pollutes due to their portability, cheapness, and low LOD [5–10]. However, it is still difficult to develop new-type fluorescent materials with high selectivity for the detection of the pollutes. Supramolecular polymers (SPs) are a kind of materials formed by non-covalent interactions including hydrogen bonds, π-π and electrostatic interactions. Derived from the structural features and host-guest interactions, these materials show versatile interesting properties, such as stimuli-responsiveness, adaptability, surface selectivity and recyclability [11,12]. In the past two decades, SPs have been widely recognized by researchers for their great application potentials in biomedicine, nanotechnology, molecular recognition, separation, ion capture, catalysis and adhesives [13–18]. Since the weak interactions endow supramolecules with “on demand” capture and release of molecular cargos under external stimuli including solvent, light and heat, a lot of fluorescent SPs exhibit fascinating photochemical performance [19–23]. Therefore, they were widely utilized as fluorescent sensing materials in the detection of metal ions, small molecules and biomacromolecules [24–26]. For instance, Pu group has developed a series of chiral supramolecular polymer platforms to simultaneously detect the chirality and concentration of chiral small molecules [27]. Chu and coauthors reported a novel fluorescent sensing platform based on the cytochrome c-peptide SPs for the detection of protein kinase assay [28]. In 2016, Chen and co-authors reported a supramolecular polymer based nanoprobe for the ratiometric oxygen sensing in living cells [29]. However, most of these reported supramolecular fluorescence sensing systems are based on gel and amorphous materials, which is difficult to clarify the relationship between structure and performance [30,31]. Accordingly, there are few reports on the application of crystalline organic supramolecular polymers in the fluorescent sensing of analytes [32–34]. In this work, we report a crystalline organic supramolecular polymer, [(TCPSBF)0.5]n (UPC-S4, TCPSBF = 2,2′,7,7′-tetrakis(4- cyanophenyl)9,9′-spirobi- [9H-fluorene), which is prepared by the solvent diffusion method or SCSC transformation from the [(TCPSBF)·(C4H8O2)3]n (intermediate crystal) or [(TCPSBF)·(C4H8O2)2]n (UPC-S3). PXRD and TGA analyses show that UPC-S4 possess highly thermal and chemical stability. Importantly, the fluorescent spectrum studies display that UPC-S4 not only may detect small molecules including water, n-hexane and cyclohexane, but also can selectively sense Fe3+ and 4-nitro-o-phenylenediamine.
The phase purity of three SPs was studied by powder X-ray diffraction (PXRD). Compared with the PXRD pattern of intermediate crystals simulated from its single crystal diffraction data, the one of assynthesized samples has some obvious changes, especially in the small angle region, indicating that the lattice dioxane molecules start lose once its crystal leaves the mother liquor, Fig. S2. In fact, after standing 24 h at room temperature, the intermediate crystal was almost completely transformed into UPC-S3, Fig. S3. This is consistent with the result observed from their single-crystal X-ray diffraction structural analyses. However, PXRD patterns of as-synthesized UPC-S3 and UPCS4 matched well with the simulated those from their single-crystal data, showing their high phase purity, Figs. S3 and S4, which were also confirmed by the results of elemental analyses as shown in Supporting Information. Interestingly, UPC-S3 only loses one of three lattice solvents compared with the intermediate crystal, but its structure of UPCH3 can be stable to about 100 °C as proved by the in-situ PXRD test, Fig. S5. Further PXRD studies demonstrated that UPC-S4 could be obtained based on the SCSC transformation by soaking the intermediate crystal and UPC-S3 in different polar and non-polar organic solvents, Fig. 2a and 2b. This illustrates that these two supramolecular polymers easily lose all lattice dioxane molecules under the extraction of outside solvents. Different from the intermediate crystal and UPC-S3, UPC-S4 shows high stability. After soaking UPC-S4 in various organic solvents and water for 24 h, no obvious change are observed from the PXRD patterns, indicating its excellent chemical stability in different solvent, Fig. 2c. Meanwhile, in-situ PXRD analysis show that the structure of UPC-S4 can be well maintained until 220 °C, revealing its high thermal stability, Fig. 2d. The thermal gravimetric (TG) analysis show that UPC-S3 has two weight loss stages, Fig. S6. Before 100 °C, it hardly loses lattice solvents, which is agreement with the result observed from in-situ PXRD. After 100 °C, it slowly loses 9.9% weight until 160 °C, which corresponds to the loss of first lattice dioxane molecules (calculated value 9.8%). Another 10% of weight loss is observed until about 280 °C, which is related to the loss of second lattice dioxane molecules (calculated value 9.8%). Subsequently, the structure keeps stable until 450 °C, and then gets collapsed slowly. In contrast, UPC-S4 does not show any weight loss before 450 °C owing to the absence of lattice solvents. After 450 °C, the structure of UPC-S4 also starts to collapse and decompose with a similar trend with UPC-S3. These results are very identical with the results observed from single-crystal structural and elemental analyses, further demonstrating the SCSC transformation from UPC-S3 to UPC-S4 and high thermal stability of UPC-S4. Considering the high chemical and thermal stability of UPC-S4 and excellent fluorescence properties of spirobifluorene-based compounds, the detailed studies for the fluorescent performance of UPC-S4 were carried out in solid and solution. The solid state luminescent spectra of TCPSBF and UPC-S4 are shown in Fig. S7. It can be clearly seen that UPC-S4 displays a stronger emission peak than TCPSBF at 429 nm (λex = 340 nm). This is probably attributed to the more twisted conformation owing to the restriction of hydrogen bonds in the orderly structure of UPC-S4[20]. However, compared with the spectrum of TCPSBF, no peak shift was observed for the one of UPC-S4, indicating that there was no change of molecular energy level after TCPSBF was assembled into UPC-S4. Correspondingly, the fluorescent spectra of UPC-S4 emulsion in different solvents are displayed in Fig. S8. Interestingly, UPC-S4 exhibits strong fluorescence emission in ethanol and methanol, but very weak fluorescence emission in water, n-hexane and cyclohexane. The results show that water and non-polar solvents have obvious fluorescent quenching effect on UPC-S4, revealing its application potential in solvent sensing based on the fluorescence turn-off detection. Based on the above results, the fluorescent sensing of UPC-S4 to metal ions and nitroaromatic compounds (NACs) was further studied. In a general procedure, 2 mg finely ground samples of UPC-S4 was dispersed in 2 mL ethanol to get a 1 mg mL−1 suspension, then the
2. Results and discussion The very unstable intermediate crystal was first synthesized by the assembly of TCPSBF in 1,4-dioxane. Then UPC-S3 was obtained by the SCSC transformation from the intermediate crystal at room temperature after 24 h. Interestingly, the stable UPC-S4 could be prepared not only through slowly diffuse methanol into the dichloromethane solution of TCPSBF, but also by the SCSC transformation of intermediate crystal or UPC-S3 after soaking them to various solvents. Single-crystal structural analysis shows that intermediate crystal or UPC-S3 crystallizes in the triclinic P-1 space group, while UPC-S4 crystallizes in the orthorhombic Ibca space group, Table S1 (ESI). The asymmetric unit contains one TCPSBF and three lattice dioxane molecules for intermediate crystal, one TCPSBF and two lattice dioxane molecules for UPC-S3, but only half of TCPSBF for UPC-S4, Fig. S1. As shown in Fig. 1, the three-dimensional (3D) structures of three SPs were formed by the connection of the hydrogen bonds and C-H···π interactions among TCPSBF molecules. Meanwhile, there are lattice dioxane molecules residing in the rhombic pores for intermediate crystal and UPC-S3. The related hydrogen-bonded parameters and dihedral angles between the adjacent benzene rings of spirobifluorene for three SPs are listed in Tables S2–S5. 2
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Fig. 1. The structure of organic building block, TCPSBF (a); 3D packing mode of intermediate crystal (b), UPC-S3 (c), and UPC-S4 (d).
Fig. 2. (a) The PXRD patterns showing the change from intermediate to UPC-S4; (b) The PXRD patterns showing the change from UPC-S3 to UPC-S4; (c) The PXRD patterns of UPC-S4 in different solvents; (d) The in-situ PXRD patterns of UPC-S4.
Fe3+) were gradually added into the ethanol emulsions of UPC-S4, and the photoluminescence (PL) spectra were collected. As shown in Fig. 3b and S9, all ions have the influence on the fluorescence intensity of UPCS4 to some extent. Meanwhile, Ba2+, Al3+, Pb2+, Li+, Ca2+, Mg2+, Cr3+, Na+, Zn2+ and Ag+ bear slight fluorescent enhancement effect
sample were treated to obtain an emulsion by ultrasonication for 5 min, into which various analytes in ethanol (10 mM) were gradually added. In order to examine the sensing effect of UPC-S4 on metal ions, ethanol solutions containing the different metal ions (Ba2+, Al3+, Pb2+, Li+, Ca2+, Mg2+, Cr3+, Na+, Zn2+, Ag+, Cd2+, Co2+, Cu2+, Ni2+, K+ and 3
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Fig. 3. (a) Emission intensity of UPC-S4 emulsion upon the gradual addition of Fe(NO3)3 solution (10 mM) in ethanol; (b) Percentage of fluorescence change by introducing different metal ions into the ethanol emulsion of UPC-S4; (c) Stern-Volmer plot of Fe3+; (d) Emission intensity of UPC-S4 emulsion upon the gradual addition of 4-N-O-PDA solution (10 mM) in ethanol; (e) Percentage of fluorescence change by introducing different NACs into the ethanol emulsion of UPC-S4; (e) Stern-Volmer plot of 4-N-O-PDA.
on UPC-S4, but Cd2+, Co2+, Cu2+, Ni2+ and K+ have slight fluorescent quenching effect on UPC-S4. In particular, the PL intensity of UPC-S4 emulsion decreases rapidly upon the addition of Fe3+ solution, which can be observed with the naked eye under UV light, Fig. 3a. When the addition amount reaches 150 μL, the quenching percentage reaches 92.5%, Fig. 3b, which is attributed to the inhibition of the radiative electron-hole recombination via capturing electrons due to the existence five unpaired electrons in d orbital of Fe3+[35]. The fluorescence quenching efficiency was calculated using the Stern-Volmer (SV) equation: (I0/I) = KSV[A] + 1, where I0 and I are the fluorescence intensities before and after the addition of the analytes, respectively, [A] is the molar concentration of the analytes, and Ksv is the quenching coefficient[36]. The value of KSV for Fe3+ was calculated to be 4.3 × 103 M−1, which is much higher than the fluorescence quenching efficiency of others metal ions (Fig. 3c). These results demonstrates that
UPC-S4 has selective sensing to Fe3+ through fluorescence quenching, which can be directly observed by naked eye. As mentioned above, it is very important to conveniently, rapidly and selectively detect the pollutants based on the practical application. To further investigate the sensing property of UPC-S4, the detection for different NACs, including 4-nitro-o-phenylenediamine (4-N-O-PDA), 4-nitrobenzaldehyde (4-NBA), 1,3-dinitrobenzene (1,3-DNB), 4-nitrotoluene (4-NT), 2,4-dinitrotoluene (2,4-DNT), 3-nitroaniline (3-NA), 4-nitrophenol (4-NP), 2-nitrobenzene-1,4-diamine (2-NB-1,4-DA) and 2-nitroaniline (2-NA), were also carried out. As shown in Figs. 3e and S10, all the NACS have some effects on the PL intensity of UPC-S4 emulsions when the addition quantity is 50 μL, but exhibit the big difference in the fluorescence quenching percentage. The quenching efficiency is observed with the following order: 4-N-O-PDA » 2-NA > 2-NB-1,4-DA > 4-NP » 3-NA » 2,4-DNT > 4-NT > 1,3-DNB > 44
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NBA. It can be clearly seen that 4-N-O-PDA has the best quenching effect. When the addition quantity of 4-N-O-PDA is 50 μL, the quenching percentage for 4-N-O-PDA reaches 98.9%. Similar to the sensing of Fe3+, the fluorescent quenching effect can also be observed by the naked eye under UV light, Fig. 3d. The value of KSV for 4-N-OPDA is calculated to be 3.9 × 104 M−1, Fig. 3f. The results indicate that UPC-S4 can high-effectively sense 4-N-O-PDA with a moderate selectivity over the other NACs. According to the analysis of the structural characteristics, NACs with amino and hydroxyl groups possess better fluorescence quenching effect. Therefore, the quenching mechanism is mainly attributed to the hydrogen-bond interactions between the amido or hydroxyl H of analytes and the cyano N of UPC-S4. Because there is no pores in UPC-S4, NACs can not influence its internal structure. Thus, these interactions between UPC-S4 and NACs should occur on the crystal surface.
[4] [5] [6] [7] [8] [9]
3. Conclusion
[10]
In summary, three supramolecular polymers assembled from a fluorescent precursor, 2,2′,7,7′-tetrakis(4-cyanophenyl)-9,9′-spirobi [9H-fluorene], have been successfully constructed by the solvent diffusion method. Single-crystal structure and PXRD analyses demonstrate that the solvent-induced SCSC transformation between three supramolecular polymers. TG and PXRD analyses display that UPC-S4 bears highly chemical and thermal stability. Fluorescent spectrum studies illustrate that UPC-S4 not only can detect small molecules including water and hexane, but also may sense Fe3+ with a high selectivity, 4nitro-o-phenylenediamine with a moderate selectivity, which can be observed through naked eye under UV light. These results show that UPC-S4 has application potential in detecting metal ions and NACs.
[11] [12] [13] [14] [15] [16] [17]
Author contributions
[18]
R. Wang and Y. Shang put forward the ideas and design the experiments. Y. Shang conducted most of the experiments. R. Wang and Y. Shang analyzed most of the data. L. Lv and J. Du participated in the fluorescent measurement. Q. Yang, J. Yin and D. Liu participated in the structural determination and PXRD analyses. R. Wang, D. Sun, J. Jiang, and Y. Shang discussed and co-wrote this paper. All authors approved the final version of the manuscript.
[19] [20] [21] [22]
Declaration of Competing Interest
[23]
The authors declared that there is no conflict of interest.
[24]
Acknowledgements [25]
This work was financially supported by the National Natural Science Foundation of China (Nos. 21631003, 21771193, and 21571187), Taishan Scholar Foundation (No. ts201511019), and the Fundamental Research Funds for the Central Universities (No. 18CX05020A).
[26] [27]
Appendix A. Supplementary material
[28]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.107703.
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A spirobifluorene-based supramolecular polymer: Solvent-induced SCSC transformation and fluorescent sensing
T
Yanxue Shanga, Lijuan Lva, Juan Dua, Qianqian Yanga, Jianbo Yina, Di Liua, Romgming Wanga,b, , Daofeng Suna,b, Jianzhuang Jiangc ⁎
a
College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, China State Key Laboratory of Heavy Oil Processing, School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, China c Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, China b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Supramolecular polymer Single-crystal-to-single-crystal transformation Fluorescent sensing Metal ions NACs
Three crystalline supramolecular polymers (SPs), [(TCPSBF)·(C4H8O2)3]n (intermediate), [(TCPSBF)·(C4H8O2)2]n (UPC-S3) and [(TCPSBF)0.5]n (UPC-S4), were constructed by solvent diffusion method based on 2,2′,7,7′-tetrakis (4- cyanophenyl)-9,9′-spirobi-[9H-fluorene] (TCPSBF). Interestingly, single crystal diffraction and PXRD analyses display the solvent-induced single-crystal-to-single-crystal (SCSC) transformation among three SPs. The unstable intermediate crystal gradually changes to UPC-S3 with the loss of a lattice dioxane molecule at room temperature, and UPC-S3 may be further transformed into UPC-S4 with the loss of the other two lattice dioxane molecules by solvent soaking. Meanwhile, the intermediate crystal can also be directly transformed into UPC-S4 by solvent soaking. As a result, UPC-S4 exhibits highly thermal and chemical stability. The fluorescent spectrum tests show that UPC-S4 possesses potential applications not only in the detecting of small molecules including nhexane and nitroaromatic derivatives, but also in selective sensing of Fe3+.
1. Introduction With the development of social economy and the continuous improvement of the scale of industrialization, water pollution has become an unavoidable topic [1]. The pollutants including metal ions and toxic organic small molecules pose a great threat to the environment and
⁎
human health. Therefore, the trace detection of these compounds has attracted wide attention of researchers [2,3]. In the past decades, various detection devices, such as gas chromatography, inductively coupled plasma emission spectrometer (ICP), and cyclic voltammetry, have been developed to detect these pollutants. Although these methods bear high selectivity and good limit of detection (LOD), they have the
Corresponding author at: College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, China. E-mail address: [email protected] (R. Wang).
https://doi.org/10.1016/j.inoche.2019.107703 Received 4 November 2019; Received in revised form 22 November 2019; Accepted 27 November 2019 Available online 23 December 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.
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disadvantages of high cost, inconvenience and operation trouble [4]. Thus, it is very urgent to develop the new technologies with low cost, rapid detect and easy operation. Recently, the fluorescent sensing materials including metal organic frameworks (MOFs) and hydrogenbonded organic frameworks (HOFs) have showed the promising application perspectives in the detection of pollutes due to their portability, cheapness, and low LOD [5–10]. However, it is still difficult to develop new-type fluorescent materials with high selectivity for the detection of the pollutes. Supramolecular polymers (SPs) are a kind of materials formed by non-covalent interactions including hydrogen bonds, π-π and electrostatic interactions. Derived from the structural features and host-guest interactions, these materials show versatile interesting properties, such as stimuli-responsiveness, adaptability, surface selectivity and recyclability [11,12]. In the past two decades, SPs have been widely recognized by researchers for their great application potentials in biomedicine, nanotechnology, molecular recognition, separation, ion capture, catalysis and adhesives [13–18]. Since the weak interactions endow supramolecules with “on demand” capture and release of molecular cargos under external stimuli including solvent, light and heat, a lot of fluorescent SPs exhibit fascinating photochemical performance [19–23]. Therefore, they were widely utilized as fluorescent sensing materials in the detection of metal ions, small molecules and biomacromolecules [24–26]. For instance, Pu group has developed a series of chiral supramolecular polymer platforms to simultaneously detect the chirality and concentration of chiral small molecules [27]. Chu and coauthors reported a novel fluorescent sensing platform based on the cytochrome c-peptide SPs for the detection of protein kinase assay [28]. In 2016, Chen and co-authors reported a supramolecular polymer based nanoprobe for the ratiometric oxygen sensing in living cells [29]. However, most of these reported supramolecular fluorescence sensing systems are based on gel and amorphous materials, which is difficult to clarify the relationship between structure and performance [30,31]. Accordingly, there are few reports on the application of crystalline organic supramolecular polymers in the fluorescent sensing of analytes [32–34]. In this work, we report a crystalline organic supramolecular polymer, [(TCPSBF)0.5]n (UPC-S4, TCPSBF = 2,2′,7,7′-tetrakis(4- cyanophenyl)9,9′-spirobi- [9H-fluorene), which is prepared by the solvent diffusion method or SCSC transformation from the [(TCPSBF)·(C4H8O2)3]n (intermediate crystal) or [(TCPSBF)·(C4H8O2)2]n (UPC-S3). PXRD and TGA analyses show that UPC-S4 possess highly thermal and chemical stability. Importantly, the fluorescent spectrum studies display that UPC-S4 not only may detect small molecules including water, n-hexane and cyclohexane, but also can selectively sense Fe3+ and 4-nitro-o-phenylenediamine.
The phase purity of three SPs was studied by powder X-ray diffraction (PXRD). Compared with the PXRD pattern of intermediate crystals simulated from its single crystal diffraction data, the one of assynthesized samples has some obvious changes, especially in the small angle region, indicating that the lattice dioxane molecules start lose once its crystal leaves the mother liquor, Fig. S2. In fact, after standing 24 h at room temperature, the intermediate crystal was almost completely transformed into UPC-S3, Fig. S3. This is consistent with the result observed from their single-crystal X-ray diffraction structural analyses. However, PXRD patterns of as-synthesized UPC-S3 and UPCS4 matched well with the simulated those from their single-crystal data, showing their high phase purity, Figs. S3 and S4, which were also confirmed by the results of elemental analyses as shown in Supporting Information. Interestingly, UPC-S3 only loses one of three lattice solvents compared with the intermediate crystal, but its structure of UPCH3 can be stable to about 100 °C as proved by the in-situ PXRD test, Fig. S5. Further PXRD studies demonstrated that UPC-S4 could be obtained based on the SCSC transformation by soaking the intermediate crystal and UPC-S3 in different polar and non-polar organic solvents, Fig. 2a and 2b. This illustrates that these two supramolecular polymers easily lose all lattice dioxane molecules under the extraction of outside solvents. Different from the intermediate crystal and UPC-S3, UPC-S4 shows high stability. After soaking UPC-S4 in various organic solvents and water for 24 h, no obvious change are observed from the PXRD patterns, indicating its excellent chemical stability in different solvent, Fig. 2c. Meanwhile, in-situ PXRD analysis show that the structure of UPC-S4 can be well maintained until 220 °C, revealing its high thermal stability, Fig. 2d. The thermal gravimetric (TG) analysis show that UPC-S3 has two weight loss stages, Fig. S6. Before 100 °C, it hardly loses lattice solvents, which is agreement with the result observed from in-situ PXRD. After 100 °C, it slowly loses 9.9% weight until 160 °C, which corresponds to the loss of first lattice dioxane molecules (calculated value 9.8%). Another 10% of weight loss is observed until about 280 °C, which is related to the loss of second lattice dioxane molecules (calculated value 9.8%). Subsequently, the structure keeps stable until 450 °C, and then gets collapsed slowly. In contrast, UPC-S4 does not show any weight loss before 450 °C owing to the absence of lattice solvents. After 450 °C, the structure of UPC-S4 also starts to collapse and decompose with a similar trend with UPC-S3. These results are very identical with the results observed from single-crystal structural and elemental analyses, further demonstrating the SCSC transformation from UPC-S3 to UPC-S4 and high thermal stability of UPC-S4. Considering the high chemical and thermal stability of UPC-S4 and excellent fluorescence properties of spirobifluorene-based compounds, the detailed studies for the fluorescent performance of UPC-S4 were carried out in solid and solution. The solid state luminescent spectra of TCPSBF and UPC-S4 are shown in Fig. S7. It can be clearly seen that UPC-S4 displays a stronger emission peak than TCPSBF at 429 nm (λex = 340 nm). This is probably attributed to the more twisted conformation owing to the restriction of hydrogen bonds in the orderly structure of UPC-S4[20]. However, compared with the spectrum of TCPSBF, no peak shift was observed for the one of UPC-S4, indicating that there was no change of molecular energy level after TCPSBF was assembled into UPC-S4. Correspondingly, the fluorescent spectra of UPC-S4 emulsion in different solvents are displayed in Fig. S8. Interestingly, UPC-S4 exhibits strong fluorescence emission in ethanol and methanol, but very weak fluorescence emission in water, n-hexane and cyclohexane. The results show that water and non-polar solvents have obvious fluorescent quenching effect on UPC-S4, revealing its application potential in solvent sensing based on the fluorescence turn-off detection. Based on the above results, the fluorescent sensing of UPC-S4 to metal ions and nitroaromatic compounds (NACs) was further studied. In a general procedure, 2 mg finely ground samples of UPC-S4 was dispersed in 2 mL ethanol to get a 1 mg mL−1 suspension, then the
2. Results and discussion The very unstable intermediate crystal was first synthesized by the assembly of TCPSBF in 1,4-dioxane. Then UPC-S3 was obtained by the SCSC transformation from the intermediate crystal at room temperature after 24 h. Interestingly, the stable UPC-S4 could be prepared not only through slowly diffuse methanol into the dichloromethane solution of TCPSBF, but also by the SCSC transformation of intermediate crystal or UPC-S3 after soaking them to various solvents. Single-crystal structural analysis shows that intermediate crystal or UPC-S3 crystallizes in the triclinic P-1 space group, while UPC-S4 crystallizes in the orthorhombic Ibca space group, Table S1 (ESI). The asymmetric unit contains one TCPSBF and three lattice dioxane molecules for intermediate crystal, one TCPSBF and two lattice dioxane molecules for UPC-S3, but only half of TCPSBF for UPC-S4, Fig. S1. As shown in Fig. 1, the three-dimensional (3D) structures of three SPs were formed by the connection of the hydrogen bonds and C-H···π interactions among TCPSBF molecules. Meanwhile, there are lattice dioxane molecules residing in the rhombic pores for intermediate crystal and UPC-S3. The related hydrogen-bonded parameters and dihedral angles between the adjacent benzene rings of spirobifluorene for three SPs are listed in Tables S2–S5. 2
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Fig. 1. The structure of organic building block, TCPSBF (a); 3D packing mode of intermediate crystal (b), UPC-S3 (c), and UPC-S4 (d).
Fig. 2. (a) The PXRD patterns showing the change from intermediate to UPC-S4; (b) The PXRD patterns showing the change from UPC-S3 to UPC-S4; (c) The PXRD patterns of UPC-S4 in different solvents; (d) The in-situ PXRD patterns of UPC-S4.
Fe3+) were gradually added into the ethanol emulsions of UPC-S4, and the photoluminescence (PL) spectra were collected. As shown in Fig. 3b and S9, all ions have the influence on the fluorescence intensity of UPCS4 to some extent. Meanwhile, Ba2+, Al3+, Pb2+, Li+, Ca2+, Mg2+, Cr3+, Na+, Zn2+ and Ag+ bear slight fluorescent enhancement effect
sample were treated to obtain an emulsion by ultrasonication for 5 min, into which various analytes in ethanol (10 mM) were gradually added. In order to examine the sensing effect of UPC-S4 on metal ions, ethanol solutions containing the different metal ions (Ba2+, Al3+, Pb2+, Li+, Ca2+, Mg2+, Cr3+, Na+, Zn2+, Ag+, Cd2+, Co2+, Cu2+, Ni2+, K+ and 3
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Fig. 3. (a) Emission intensity of UPC-S4 emulsion upon the gradual addition of Fe(NO3)3 solution (10 mM) in ethanol; (b) Percentage of fluorescence change by introducing different metal ions into the ethanol emulsion of UPC-S4; (c) Stern-Volmer plot of Fe3+; (d) Emission intensity of UPC-S4 emulsion upon the gradual addition of 4-N-O-PDA solution (10 mM) in ethanol; (e) Percentage of fluorescence change by introducing different NACs into the ethanol emulsion of UPC-S4; (e) Stern-Volmer plot of 4-N-O-PDA.
on UPC-S4, but Cd2+, Co2+, Cu2+, Ni2+ and K+ have slight fluorescent quenching effect on UPC-S4. In particular, the PL intensity of UPC-S4 emulsion decreases rapidly upon the addition of Fe3+ solution, which can be observed with the naked eye under UV light, Fig. 3a. When the addition amount reaches 150 μL, the quenching percentage reaches 92.5%, Fig. 3b, which is attributed to the inhibition of the radiative electron-hole recombination via capturing electrons due to the existence five unpaired electrons in d orbital of Fe3+[35]. The fluorescence quenching efficiency was calculated using the Stern-Volmer (SV) equation: (I0/I) = KSV[A] + 1, where I0 and I are the fluorescence intensities before and after the addition of the analytes, respectively, [A] is the molar concentration of the analytes, and Ksv is the quenching coefficient[36]. The value of KSV for Fe3+ was calculated to be 4.3 × 103 M−1, which is much higher than the fluorescence quenching efficiency of others metal ions (Fig. 3c). These results demonstrates that
UPC-S4 has selective sensing to Fe3+ through fluorescence quenching, which can be directly observed by naked eye. As mentioned above, it is very important to conveniently, rapidly and selectively detect the pollutants based on the practical application. To further investigate the sensing property of UPC-S4, the detection for different NACs, including 4-nitro-o-phenylenediamine (4-N-O-PDA), 4-nitrobenzaldehyde (4-NBA), 1,3-dinitrobenzene (1,3-DNB), 4-nitrotoluene (4-NT), 2,4-dinitrotoluene (2,4-DNT), 3-nitroaniline (3-NA), 4-nitrophenol (4-NP), 2-nitrobenzene-1,4-diamine (2-NB-1,4-DA) and 2-nitroaniline (2-NA), were also carried out. As shown in Figs. 3e and S10, all the NACS have some effects on the PL intensity of UPC-S4 emulsions when the addition quantity is 50 μL, but exhibit the big difference in the fluorescence quenching percentage. The quenching efficiency is observed with the following order: 4-N-O-PDA » 2-NA > 2-NB-1,4-DA > 4-NP » 3-NA » 2,4-DNT > 4-NT > 1,3-DNB > 44
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NBA. It can be clearly seen that 4-N-O-PDA has the best quenching effect. When the addition quantity of 4-N-O-PDA is 50 μL, the quenching percentage for 4-N-O-PDA reaches 98.9%. Similar to the sensing of Fe3+, the fluorescent quenching effect can also be observed by the naked eye under UV light, Fig. 3d. The value of KSV for 4-N-OPDA is calculated to be 3.9 × 104 M−1, Fig. 3f. The results indicate that UPC-S4 can high-effectively sense 4-N-O-PDA with a moderate selectivity over the other NACs. According to the analysis of the structural characteristics, NACs with amino and hydroxyl groups possess better fluorescence quenching effect. Therefore, the quenching mechanism is mainly attributed to the hydrogen-bond interactions between the amido or hydroxyl H of analytes and the cyano N of UPC-S4. Because there is no pores in UPC-S4, NACs can not influence its internal structure. Thus, these interactions between UPC-S4 and NACs should occur on the crystal surface.
[4] [5] [6] [7] [8] [9]
3. Conclusion
[10]
In summary, three supramolecular polymers assembled from a fluorescent precursor, 2,2′,7,7′-tetrakis(4-cyanophenyl)-9,9′-spirobi [9H-fluorene], have been successfully constructed by the solvent diffusion method. Single-crystal structure and PXRD analyses demonstrate that the solvent-induced SCSC transformation between three supramolecular polymers. TG and PXRD analyses display that UPC-S4 bears highly chemical and thermal stability. Fluorescent spectrum studies illustrate that UPC-S4 not only can detect small molecules including water and hexane, but also may sense Fe3+ with a high selectivity, 4nitro-o-phenylenediamine with a moderate selectivity, which can be observed through naked eye under UV light. These results show that UPC-S4 has application potential in detecting metal ions and NACs.
[11] [12] [13] [14] [15] [16] [17]
Author contributions
[18]
R. Wang and Y. Shang put forward the ideas and design the experiments. Y. Shang conducted most of the experiments. R. Wang and Y. Shang analyzed most of the data. L. Lv and J. Du participated in the fluorescent measurement. Q. Yang, J. Yin and D. Liu participated in the structural determination and PXRD analyses. R. Wang, D. Sun, J. Jiang, and Y. Shang discussed and co-wrote this paper. All authors approved the final version of the manuscript.
[19] [20] [21] [22]
Declaration of Competing Interest
[23]
The authors declared that there is no conflict of interest.
[24]
Acknowledgements [25]
This work was financially supported by the National Natural Science Foundation of China (Nos. 21631003, 21771193, and 21571187), Taishan Scholar Foundation (No. ts201511019), and the Fundamental Research Funds for the Central Universities (No. 18CX05020A).
[26] [27]
Appendix A. Supplementary material
[28]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.inoche.2019.107703.
[29]
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