Chinese Chemical Letters  2016, Vol.27 Issue (04): 518-522   PDF    
The magic of integration: Exploring the construction of dithienylethene-based infinite coordination polymers and their synergistic effect for gaseous ammonia probe applications
Yan-Kai Lia, 1, Jun-Ji Zhangb, 1 , Zi-Jun Biana, c, You-Xin Fub, Fei Liua, Chen-Hui Wanga, Xiang Mab, Jun Hua, Hong-Lai Liua    
a Key Laboratory for Advanced Materials and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China;
b Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China;
c Xi'an Thermal Power Research Institute Co., Ltd. (Suzhou Branch), Suzhou 215153, China
Abstract: Infinite coordination polymers are recognized as excellent platform for functionalization. Dithienylethene motifs, which are one of the most attractive functional moieties, were incorporated into an infinite coordination polymer, to deliver a "smart" porous material that can response to external stimuli. The obtained dithienylethene-based infinite coordination polymers (named Cu-DTEDBA) share the advantages of both infinite coordination polymers (porosity and stability) and dithienylethene motifs (photochromism). The physical and chemical properties of Cu-DTEDBA were characterized by FTIR, TEM, SEM, XRD, TGA, UV-vis, EDX and BET. Moreover, the combination of dithienylethene and infinite coordination polymers gives rise to a synergistic effect, which induces functional behaviors of ammonia sensor applications. Both open and closed forms of Cu-DTEDBA exhibit distinct colorimetric change upon exposure to gaseous ammonia, which is not observed in dithienylethene free molecules.
Key words: Dithienylethene     Photochromism     Infinite coordination polymers     Synergistic effect     Gaseous ammonia probe    
1. Introduction

As an emerging class of inorganic-organic hybrid materials, infinite coordination polymers (ICPs) have spurred flourishing research interests in the past decade [1, 2, 3, 4]. The structure of ICPs, in which repeating organic ligands are linked together by metal nodes, are highly appealing because the numerous choices of transitional metal nodes and predefined organic linkers give rise to a high degree of tailorability for ICPs [5, 6, 7, 8, 9]. The merits of tailorablility, together with the presence of porosity that often enjoyed by ICPs provide researchers with versatile network structures which can be deliberately designed for a varieties of applications, such as catalysis [10], gas adsorption [11], biosensing [12] and drug delivery [13]. Apart from the above properties, ICPs still have huge potency to be further exploited to extent their applications. Specifically, the employment of functional ligand precursors can afford ''smart'' materials which can carry out multiple tasks under different external stimuli [14]. However, the fabrication of such functional ICPs have been rarely reported and remains a challenging subject.

Among the external stimuli, light is one of the most attractive way to manipulate molecular functions, due to its superiority in spatial, temporal and energetic resolutions and the minimum of side reactions [15, 16, 17, 18, 19, 20]. One of motifs are known as photochromism, which stars the marvellous photo-isomerizable molecules, including azobenzenes [21, 22, 23, 24], spiropyrans [25, 26] as well as dithienylethenes (DTE) [27, 28, 29]. Among these photochromic molecules, DTE derivatives, which undergo photo-induced isomerization involving ring-opening/closing reactions [30, 31], are one of the most appealing classes of molecules, due to their excellent thermal stability, rapid response, and fatigue resistance [29, 32, 33]. Moreover, the geometry of DTE derivatives undergoes limited change under isomerization, which favors their involvement in the construction of ICPs, for the isomerization exhibits negligible effect to the stability of coordination polymer structure.

In spite of limited reports of functional ''smart'' ICPs, their counterpart inorganic-organic hybrid materials (for example, metal-organic frameworks) have found themselves more and more involved in the construction of functional porous materials [34, 35, 36], which gave us inspiration for designing DTE-based photocontrollable ICPs. The amazing structure of ICPs, that an infinite scaffold being assembled by repeating ligands and metal nodes, gives rise to a synergistic combination of both merits of photoswitchable ligands and coordination polymer network. The synergistic effects, which induces unique performances occurred in the hybrid structures while not available in ''free'' molecules, results in an enhanced efficiency of both functional motifs and coordination polymer structure. Herein, not only have we fabricated photochromic DTE-based ICPs, but also have fully utilized the synergistic effect of DTE motifs and ICPs to deliver unique properties that are not applying to DTE-based small molecules. The synergistic combination of photochromic moieties and porosity would provide more space to transduce the host- guest interaction to a colorimetric probe for gaseous ammonia.

2. Experimental

Materials and methods: DMF, Cu(NO3)2·3H2O were purchased from Sinopharm Chemical Reagent Co., Ltd., China. The preparation of DTEDBA was described in supporting information. 1H NMR spectra were collected on a Bruker Avance DPS-300 spectrometer using CDCl3 or DMSO-d6 as solvent and tetra-methylsilane as an internal reference. Thermogravimetry analysis (TGA) were performed under N2 on a NETZSCH STA449F3, with a heating rate of 10 8C min-1. The morphology of the samples were characterized on a JEOL JEM-2100 transmission electron microscopy (TEM). Scanning electron microscopy (SEM) pictures was conducted on a JEOL JSM-6360LV. Powder X-ray diffraction (XRD) measurements were carried out on a D/Max-2550 VB/PC diffractometer (40 kV, 200 mA), using Cu radiation. Nitrogen adsorption isotherms were measured at 77 K usingMicromeritics Tristar 3020 static volumetric analyzer. Before adsorption measurements the polymer was degassed at 100 8C under vacuum. The Brunauer-Emmett-Teller (BET) surface areawas calculatedwithin the relative pressure range 0.05 to 0.30 and the pore diameters were calculated by non-local DFT method. The adsorption spectra for ethanol suspension of Cu-DTEDBA (0.1mgmL-1 ) were measured on a Shimadzu, UV-2550 spectrophotometer. To examine the absorption spectral change upon the addition ammonia, 2 mL aqua ammonia was added into 2mL above suspension, and then the absorption spectra was measured (Fig. S4 in Supporting information). FT-IR data were obtained using a Nicolet Magna-IR 550 spectrometer.

Synthesis of Cu-DTEDBA: A mixture of DTEDBA (20 mg, 0.04 mmol), Cu(NO3)2·3H2O (9.6 mg, 0.04 mmol), DMF 6 mL and water 2 mL were sealed into a glass vial and heated at 80 8C for 36 h. After cooling to room temperature, the mixture was filtered. The solid was collected, washed with DMF for 3 times and dried at 80 8C to afford Cu-DTEDBA (12.3 mg, yield 57%).

3. Results and discussion

To fabricate DTE-based ICP, the DTE derivative of DTEDBA [4, 40- (4, 40-(cyclopent-1-ene-1, 2-diyl)bis(5-methylthiophene-4, 2-diyl))- dibenzoic acid] that containing carboxylic groups were designed and synthesized to deliver ligand for the coordination with metal ion species (Scheme 1a). As illustrated in Scheme 1b, an ICP named Cu-DTEDBA was prepared from heating the mixture of DTEDBA and Cu(NO3)2·3H2O in DMF/H2O at 80 8C. Cu-DTEDBA was obtained as light green solid. Fourier transform infrared spectroscopy (FTIR) was conducted to confirm the chemical structure of the Cu-DTEDBA (Fig. 1a). The stretching vibration of thiophene rings around 1432 cm-1 certified the presence of DTE moieties in the ICP skeleton. Coordination of the carboxylate groups of DTEDBA to Cu2+ ions was proved by a shift in the CO stretching frequency to 1532 cm-1 and 1608 cm-1. Energy dispersive X-ray (EDX) spectroscopy, in which the peaks of C, O, S, Cu were found, was conducted to confirm the chemical composition of the sample (Fig. S1 in Supporting information). Cu-DTEDBA was found to be thermally stable up to approximate 300 8C by the thermogravimetric analysis (TGA) (Fig. 1e). The DTE moieties began to decompose above 300 8C and degradation of framework occurs at 500 8C. As evident from scanning electron microscopy (SEM) and transmission electron micrographs (TEM) images (Fig. 1b and c), Cu-DTEDBA exhibited the morphology of crystalline structure to some extent rather than completely amorphous morphology. The powder X-ray diffraction (PXRD) pattern (Fig. 1f) also suggested the presence of long range order for Cu-DTEDBA. The adsorption and desorption isotherm of nitrogen gas were collected under 77 K to examine the permanentporosity (Fig. 1d), from whichthe surface area of Cu-DTEDBA was calculated to be 203 m2 g-1 by using the Brunauer-Emmett-Teller (BET) model. Hysteresis loops between adsorption and desorption branches indicated the presence of mesopores of Cu-DTEDBA [37, 38], which is of interest for applications that require rapid mass transfer process through so-called transport pores [39].

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Scheme 1.Photoisomerization and ammonia responsibility of DTEDBA (a) and Cu-DTEDBA (b) under UV and visible light.

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Fig. 1.(a) Fourier transform infrared spectroscopy (FTIR) of DTEDBA and Cu-DTEDBA; (b) scanning electron microscopy (SEM) image of Cu-DTEDBA; (c) transmission electron microscopy (TEM) image of Cu-DTEDBA; (d) the N2 adsorption isotherms of Cu-DTEDBA measured at 77 K. (e) Thermogravimetric analysis (TGA) of DTEDBA and Cu-DTEDBA. (f) Powder X-ray diffraction (PXRD) pattern of Cu-DTEDBA.

Similar as discovered in small molecules of DTE derivatives, the DTE moieties was also able to undergo rapid photochromic reaction in Cu-DTEDBA. When the light green solid of Cu-DTEDBA powder was irradiated by low density ultraviolet (UV) light, it turned to deep blue within 5 min. Whereas the exposure of these blue powder to visible light (>470 nm) led to the recovery of the light green sample in around 30 min (Fig. 2a). The photochromic mechanism for DTE derivatives that the transformation between the strong UV-absorption in the open-ring form and the strong visible light absorption in the closed-ring form is also applicable in Cu-DTEDBA, which results in the distinguishable colors [34] (the open form and the closed form of Cu-DTEDBA are denoted as Cu-DTEDBA(o) and Cu-DTEDBA(c), respectively). The UV-vis absorption spectra of Cu-DTEDBA in EtOH suspension confirmed the reversible ring open/close reaction (Fig. 2b). The absorption peak centered at 580 nm grew while the peak of 330 nm decreased significantly with the inceasing time of UV-irrdiation, compared with similar transformation of the EtOH solution of DTEDBA molecules upon exposure to UV irradiation (Fig. S2 in Supporting information).

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Fig. 2.(a) Visual changes of Cu-DTEDBA upon exposure to low-intensity ultraviolet light and alternate visible light; (b) absorption spectral changes of Cu-DTEDBA (0.1 mg mL-1, EtOH suspension) upon irradiation with 365 nm.

The porosity of Cu-DTEDBA ensures the accommodation of guest molecules, while the functionalization with smart ligands enables the modulation of host-guest interactions with various external stimuli. As mentioned before, the reported hybrid materials are mainly based on the ligand motifs, while the functions originated form inherent synergistic effects between ligands and ICPs structures are rarely discussed. Accordingly, we hypothesized that the host-guest interactions in Cu-DTEDBA would affect the optical properties of DTE moieties in the skeleton, thus making it favorable to be a chemical sensor to transduce intra-molecular interactions to detectable color change. As prove-of-principle, we selected gaseous ammonia as the target guest molecule. In our experiment, gaseous ammonia was produced by heating aqua ammonia. After dried by a pad of soda lime, the generated gaseous ammonia flowed to solid sample of Cu-DTEDBA. Upon exposure to gaseous ammonia, Cu-DTEDBA(o) exhibited a quick colorimetric change from light green to dark cyan (within 3 s), indicating an interaction between ammonia and Cu-DTEDBA(o). Interestingly, after the adsorption of ammonia, the solid sample of Cu-DTEDBA(o)·NH3 maintained its photochromic behavior and underwent photo-induced isomerization to form Cu-DTEDBA(c)·NH3 (vice versa) with an even faster conversion rate than their pristine counterparts (Cu-DTEDBA(o) and Cu-DTEDBA(c)) (Table S3 in Supporting information). Moreover, the ammonia adsorbed Cu-DTEDBA(c)·NH3 also displayed a detectable colorimetric change compared to the pristine Cu-DTEDBA(c) (from dark blue to violet), demonstrating that both open/closed isomer constructed ICPs can be used as gas probes (Fig. 3a). The ability to adsorb gas into the pores allows the guest molecules to be immobilized in intimate proximity to the active adsorption sites, which favors the application of gas probe. To gain some elucidation of the possible mechanism of the current probe for the ammonia detection, FTIR was conducted to exploit the interactions between ammonia and the framework of Cu-DTEDBA (Fig. 3b). A shift in the FTIR absorption from 1608 cm-1 to 1647 cm-1 and a significant decrease for the absorption at 772 cm-1 were detected in Cu-DTEDBA after the exposure to ammonia, which was ascribed to the alternation of the frequency of stretching vibration for carbonyl group as well as the intensity of the vibration for Cu-O bond, respectively. This alternation suggested the occurrence of the complexation between the ammonia and Cu-O clusters or the coordination effect is able to affect the interaction between the carboxylic ligand and Cu nodes. It is worth mentioning that the color change induced by gaseous signal could be difficult to observe for DTEDBA small molecules in solution (Fig. S5 in Supporting information), due to the absence of porosity and metal sites to accommodate and interact with ammonia. This indicates that the sensing behavior of DTE-based ICPs is indeed originated from the synergistic effects of DTE motifs and ICP structures, rather than from the simple free DTE molecules. Furthermore, cycling tests were conducted to examine the stability and reversibility of Cu-DTEDBA as the chemical sensor. Upon the exposure to ammonia, almost the same color change were observed for each cycle. After heating at 120 8C for about 5 min, the regenerated Cu-DTEDBA exhibits recovered color, indicating the removal of adsorbed ammonia from the pores (Fig. 3c). Only minute deterioration of response time and sensing abilities were discovered after 10 cycles of regeneration, due to the excellent stability of the coordination polymer. As a result, the synergistic effect of DTE motifs and ICPs may be a new strategy to design functional materials with novel properties, which may broaden the design and application of functional materials.

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Fig. 3.(a) Color change of Cu-DTEDBA(o) and Cu-DTEDBA(c) upon exposure to gaseous ammonia; (b) FTIR of Cu-DTEDBA and Cu-DTEDBA·NH3 (c) cycling tests of Cu-DTEDBA(o) and Cu-DTEDBA(c) for ammonia sensing. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Conclusion

We used carboxylic DTE derivatives as the organic ligand to construct novel Cu-DTEDBA with rigid structure and porosity. Rapid photochromic reaction successfully occurred for Cu-DTEDBA, in consistent with the obvious change in color. More importantly, Cu-DTEDBAexhibit detectable colorimetric change after exposure to gaseous ammonia, indicating a synergistic effect between ICPs and DTE motifs. This would lead to unique functionalities of resulting materials, thus broadening the design and application of smart functional materials. Currently, the design of more sophisticated synergistic photochromic porous materials based on various interactions are under further investigation in our lab.

Acknowledgments

This work was financially supported by National Basic Research Program of China (No. 2013CB733501), National Natural Science Foundation of China (Nos. 91334203, 21376074, 21402050) and the Fundamental Research Funds for the Central Universities of China (No. WK1314008).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2016.01.056.

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