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Conjugated mac

aDepartment of Chemistry, Sungkyunkwan

sson@skku.edubInstitute of Advanced Composite Materials,

Jeolabuk-do 55324, KoreacLaboratory of Nuclear Magnetic Resonan

Research Facilities (NCIRF), Seoul NationaldKorea Basic Science Institute, Daejeon 3413eDepartment of Chemistry Education, Chonn

Korea. E-mail: kcko1982@jnu.ac.kr

† Electronic supplementary information (ESEM images of the assembled silica spherecontrol CMP-F, the Stern–Volmer plots othe MA-CMP-F and control CMP-F, the sisubstrates, emission quenching behavioperchlorate, and nitrate, the SEM imageh recycle, and characterization dataSee DOI: 10.1039/c8ta06744a

‡ These authors contributed equally.

Cite this: J. Mater. Chem. A, 2018, 6,17312

Received 12th July 2018Accepted 22nd August 2018

DOI: 10.1039/c8ta06744a

rsc.li/materials-a

17312 | J. Mater. Chem. A, 2018, 6, 173

ro–microporous polymer filmsbearing tetraphenylethylenes for the enhancedsensing of nitrotoluenes†

Chang Wan Kang,‡a Doo Hun Lee,‡a Young Jun Shin,a Jaewon Choi,b Yoon-Joo Ko,c

Sang Moon Lee,d Hae Jin Kim,d Kyoung Chul Ko *e and Seung Uk Son *a

Conjugated microporous polymer films (CMP-Fs) bearing tetraphe-

nylethylenemoieties showed aggregation-induced emission behavior,

which can be utilized for the emission quenching-based sensing of

nitrotoluenes. In this work, macroporous CMP-Fs (MA-CMP-Fs) were

engineered using assembled silica spheres as templates. The MA-

CMP-Fs showed much better sensing performance toward nitro-

toluenes with Stern–Volmer constants (KSV) of up to 1.39 � 105 M�1,

compared with CMP-Fs (KSV up to 5.57 � 104 M�1). The sensing

performance of the MA-CMP-F is superior to those of recent emissive

CMP materials (KSV in the range of 2.08 � 103 to 6.78 � 104 M�1 for

nitroarenes) in the literature, which is attributable to the additional

macroporosity of the CMP-F. Also, we demonstrated that various

functional CMP films with macroporous inner structures can be

developed using tailored building blocks.

Recently, the macroporous engineering of microporous mate-rials has attracted the special attention of scientists.1,2 Micro-porous polymer materials have been prepared by the transitionmetal catalyzed cross-coupling of organic building blocks.3–9 Forexample, the Sonogashira coupling of multi-ethynyl arenes withmulti-haloarenes results in conjugated microporous polymer

University, Suwon 16419, Korea. E-mail:

Korea Institute of Science and Technology,

ce, National Center for Inter-University

University, Seoul 08826, Korea

3, Korea

am National University, Gwangju 61186,

SI) available: Experimental procedures,s, PXRD patterns of the MA-CMP-F andf the emission quenching behaviors ofmulation results of the MA-CMP-F andrs of the MA-CMP-F toward O2, NO2,of the MA-CMP-F recovered aer the

of MA-CMP-Fs containing porphyrins.

12–17317

(CMP) materials.6–8 Although the unique microporosity of CMPsenables the diffusion of guest molecules into inner CMPs, theguest molecules can hinder each other's diffusion pathways,which can limit the interaction of substrates with inner func-tional moieties of CMPs. The introduction of secondary mac-roporosity into the CMP materials can further facilitate thediffusion of substrates. Thus, it is noteworthy that binaryporosity materials have been synthesized to enhance the func-tionalities of organic/inorganic microporous materials.1,2

The efficient sensing of dangerous compounds has been animportant research subject.10 During the last decade, variousfunctional materials including emissive polymers have beendeveloped for the sensing of explosive compounds such asnitrotoluenes.11–13 Emission-based optical methods for thesensing of explosives are attractive due to their low cost,portability, and high sensitivity.11 Recently, the phenomenon ofaggregation-induced emission (AIE) has been utilized as a newprinciple in emission-based sensing materials.14,15 For example,CMPs bearing tetraphenylethylene (TPE) moieties showedemission through the aggregation-induced suppression ofintramolecular motions16 and were applied for the sensing ofharmful compounds.17–19 Moreover, the lm form of emissiveCMP materials can be engineered to facilitate sensingprocesses.20–23 One can expect that the introduction of macro-pores into the TPE-based CMPs can further enhance the emis-sion quenching-based sensing performance.

Our research group has studied the morphological engi-neering of CMPmaterials.24,25 Using various template materials,the morphological structures of CMPs could be controlled. Wehave demonstrated that the performance of CMP materials iscritically dependent on the morphological structures of CMPmaterials.26–28 We have tried to introduce secondary macro-porosity into the CMP lm based on a template method.

In this work, we report the preparation of macroporous CMPlms (MA-CMP-Fs) having TPE moieties and their AIE-basedsensing performance toward nitrotoluenes, compared with thecontrol CMP lm. Fig. 1 shows synthetic schemes for the MA-CMP-F and control CMP-F.

This journal is © The Royal Society of Chemistry 2018

Fig. 1 Synthetic schemes for MA-CMP-F and control CMP-F. Fig. 2 SEM images of the (a–c) MA-CMP-F and (f–h) CMP-F. (d and e)TEM images of a piece of MA-CMP-F.

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For use as template materials, silica spheres with an averagediameter of 400 nm were assembled on a glass plate (Fig. S1 inthe ESI†). When we used silica spheres with diameters of �200and >750 nm, the silica lms were too thick and too thin,respectively, to use as templates (Fig. S2 in the ESI†). Throughthe Sonogashira coupling of tetra(4-ethynylphenyl)ethylenewith tetra(4-bromophenyl)ethylene, the silica spheres werecoated with CMP layers. The etching of silica spheres resulted inthe MA-CMP-F which was transferable to another support suchas a polymer lm. As a control material, the CMP layer wasformed on the silica of a conventional thin layer chromatog-raphy (TLC) plate. The etching of silica resulted in a CMP-F witha solid and dense inner structure.

The MA-CMP-F and CMP-F were investigated by scanning(SEM) and transmission electron microscopy (TEM). Fig. 2ashows a side view of the MA-CMP-F, revealing a macroporousstructure with spherical void space and indicating that the CMPmaterials perfectly replicated the assembled silica spheres onthe glass. Top views of the MA-CMP-F in Fig. 2b and c show theuniform and closest packing of hollow spheres. The TEMimages of the MA-CMP-F in Fig. 2d and e show the overlappingof hollow CMP spheres with a uniform shell thickness of 15–20 nm. The lm thickness of the MA-CMP-F was measured to be�2.9 mm with �10 layers of hollow spheres. Considering thediameter (400 nm) of the templating silica spheres, theobserved lm thickness indicates the closest packing of silicaspheres. The void portion induced by the silica templates wascalculated to be �84% of the MA-CMP-F (Fig. S3 in the ESI†). Incontrast, the side view of the control CMP-F in Fig. 2f showsa solid and dense inner structure. We engineered the lm

This journal is © The Royal Society of Chemistry 2018

thickness of the control CMP-F to 2.9 mm through scanning theamount of building blocks. Top views of the control CMP-F inFig. 2g–h show a slightly undulating at shape.

According to the analysis of N2 sorption isotherm curves, theMA-CMP-F and CMP-F showed similar surface areas of 676 and647 m2 g�1, respectively (Fig. 3a). The microporous nature (poresizes < 2 nm) of the MA-CMP-F and CMP-F was conrmed withmicropore volumes of 0.22 and 0.21 cm3 g�1, respectively (insetof Fig. 3a). Both the MA-CMP-F and CMP-F showed amorphouscharacteristics, which is the conventional property of CMPs inthe literature29,30 (Fig. S4 in the ESI†). Thermogravimetricanalysis (TGA) showed that the MA-CMP-F and CMP-F are stableup to �290 �C (Fig. 3b).

The chemical structures of the MA-CMP-F and CMP-F wereinvestigated by infrared absorption spectroscopy (IR). Both theMA-CMP and CMP-F showed vibration peaks at 1598, 1501, and1437 cm�1, corresponding to the C]C vibrations of aromaticand alkene groups, respectively (Fig. 3c). The optical propertiesof the MA-CMP-F and CMP-F were characterized by UV/visabsorption and emission spectroscopy. The MA-CMP-F andCMP-F showed a yellow color with maximum absorption peaksat 365 and 409 nm, respectively (Fig. 3d). The blue-shiedabsorption of the MA-CMP-F, compared with the CMP-F, indi-cates that the macroporous structures induced smaller conju-gation domains of CMP materials. The MA-CMP-F and CMP-Fshowed emission at 517 and 523 nm, respectively. In the solidstate 13C nuclear magnetic resonance (NMR) spectra of the MA-CMP-F and CMP-F, aromatic 13C peaks appeared at 143.7, 130.4,and 120.7 ppm (Fig. 3e). The 13C peaks of ethylene moieties

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Fig. 3 (a) N2 adsorption–desorption isotherm curves obtained at 77 Kand pore size distribution diagrams based on the density functionaltheory (DFT) method, (b) TGA curves, (c) IR absorption spectra, (d) UV/vis absorption (solid line) and emission spectra (dotted line, excitationwavelength of 410 nm), and (e) solid state 13C NMR spectra of the MA-CMP-F and CMP-F.

Fig. 4 Emission-quenching based sensing behaviors of the (a) MA-CMP-F and (b) CMP-F toward 4NT. Emission quenching of the (c) MA-CMP-F and (d) CMP-F by DNT, 4NT, 2NT, 4-chlorotoluene (4CT), andtoluene (T) (average results of five tests). (e) Recycling tests of the MA-CMP-F for the sensing of 4NT (0.5 mM). (f) SEM images of the MA-CMP-F recovered after five successive recycles for the sensing of 4NT.

Table 1 Emission-based sensing parameters of the MA-CMP-F andCMP-F for nitrotoluenes (average results of five sets)a

Entry Nitrotoluenes

KSVb (M�1)

MA-CMP-F CMP-F

1 2,4-Dinitrotoluene (DNT) 63 700 21 5002 4-Nitrotoluene (4NT) 138 600 55 7003 2-Nitrotoluene (2NT) 87 700 52 300

a Conditions: ethanol, lex: 410 nm. b KSV values were obtained throughplotting Io/I vs. [M] based on the Stern–Volmer equation (Io/I ¼ KSV[M] +1, Io: the original emission intensity, I: the intensity of emission in thepresence of nitrotoluenes, and [M]: the concentration of nitrotoluenes).

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overlapped with the peak at 143.7 ppm. The 13C peaks ofinternal alkynes appeared at 85–92 ppm, indicating that theCMPmaterials were formed via the Sonogashira coupling of theused building blocks.

Based on emission-quenching behaviors, emissive materialscan be applied for the sensing of nitrotoluenes.11 Thus,considering the AIE-based emission properties, we studied thesensing performance of the MA-CMP-F and control CMP-Ftowards nitrotoluenes. Although 2,4,6-trinitrotoluene (TNT) isone of the most famous explosive compounds, we could not usethe TNT as a substrate because of its limited commercialavailability and safety regulations in synthetic trials. Instead, westudied 2,4-dinitrotoluene (DNT), 4-nitrotoluene (4NT), and 2-nitrotoluene (2NT) as substrates.

Fig. 4 and S5–S13 in the ESI† and Table 1 summarize thesensing results. Overall, the MA-CMP-F showed much superior

17314 | J. Mater. Chem. A, 2018, 6, 17312–17317

sensing performance to the control CMP-F, indicating that themacroporous inner structure is benecial in the sensing ofnitrotoluenes (Fig. 4a–d and Table 1). The sensing efficiencies ofthe MA-CMP-F and CMP-F towards nitrotoluenes were observed

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in the increasing order of DNT < 2NT < 4NT (Fig. 4c and d). TheMA-CMP-F was sensitive to nitrotoluenes and not effective for 4-chlorotoluene (4CT) and toluene (T) (Fig. 4c). The MA-CMP-Fcould be reused through simple washing in ve successivesensing tests of 4NT. The relative quenching degrees wereretained as 101, 100, 101, 103, and 100% in the rst, second,third, fourth, and h recycles, compared with the originalquenching degree (Fig. 4e). The MA-CMP-F recovered aer theh recycle showed no signicant structural changes in SEManalysis, indicating its chemical stability (Fig. 4f and S9 in theESI†).

The Stern–Volmer constants (KSV) of the MA-CMP-F weremeasured to be 1.39 � 105, 8.77 � 104, and 6.37 � 104 M�1 for4NT, 2NT, and DNT, respectively. In comparison, the KSV valuesof the CMP-F were measured to be 5.57 � 104, 5.23 � 104, and2.15 � 104 M�1 for 4NT, 2NT, and DNT, respectively (Table 1and Fig. S8 in the ESI†). Considering the recent emissive CMPs,the sensing performance of the MA-CMP-F is superior to those(2.08 � 103 to 6.78 � 104 M�1 for nitroarenes) in the litera-ture17–23,31–41 (Table S1 in the ESI†). The limit of detection (LOD)of the MA-CMP-F toward 4NT was measured to be 0.0731 ppm,which is superior to values (0.096–0.19 ppm) in the litera-ture.17,38,41 Moreover, the emission quenching of the MA-CMP-Fby 4NT could be visualized, which is a benet of the lm natureof the MA-CMP-F (Fig. S10 in the ESI†).

The LUMO levels of the MA-CMP-F were computationallysimulated at �2.31 to �2.89 eV (Fig. 5a, b, S5 and S6 in theESI†). The LUMO levels of DNT, 4NT, 2NT, 4CT and T weresimulated at �3.44, �2.82, �2.77, �0.78, and �0.39 eV,respectively, indicating that the emission quenching of the MA-CMP-F by 4CT and T is not favorable. However, it can be ex-pected that basically, the electron decient molecules havinglower LUMO levels than the LUMO of the MA-CMP-F canquench the emission of the MA-CMP-F. In this regard, theLUMO levels of toluene derivatives having systematic functional

Fig. 5 (a) Model systems (CMP-1, CMP-4, and CMP-P) of the MA-CMP-F and (b) computationally simulated HOMO (black)–LUMO (red)energy levels of model systems of theMA-CMP-F and substrates (DNT,4NT, 2NT, 4CT, and T).

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groups were calculated. According to the DFT calculations, mosttoluene derivatives having a mono-substituent at the paraposition showed higher LUMO levels than the LUMO level of theMA-CMP-F (Fig. S11 in the ESI†). Among conventional redoxgases, O2 and NO2 having the LUMO levels at �3.51 and�2.90 eV, respectively, were expected to quench the emission ofthe MA-CMP-F. However, these gases did not interfere with thesensing performance of the MA-CMP-F toward 4NT, possiblydue to their low concentrations in ethanol solution (Fig. S12 inthe ESI†). In addition, interestingly, we found that the emissionof the MA-CMP-F could be quenched by perchlorate and nitrate,common anions of inorganic explosives,42 possibly through theelectron transfer from the HOMOs of perchlorate (�2.71 eV)and nitrate (�1.10 eV) to the LUMO of the MA-CMP-F (Fig. S13in the ESI†).

The sizes of DNT, 4NT, and 2NT were simulated as 5.63 A �6.76 A, 4.28 A � 6.75 A, and 5.53 A � 6.03 A with molar volumesof 124, 101, and 99 cm3 mol�1, respectively (Fig. S7 in the ESI†).Thus, the size effect was critical in the sensing of DNT and thebest sensing performance of 4NT is the combined result of thesize and the electronic effects.

The synthetic strategy used for the MA-CMP-F could beextended to other building blocks such as tetra(4-ethynylphenyl) porphyrins and tetra(4-ethynylphenyl) metalporphyrins43 (Fig. 6a and S14 and refer to the synthetic proce-dures in the ESI†).

Fig. 6 (a) Synthetic scheme of CMP bearing porphyrins. SEM imagesof MA-CMP-Fs containing (b–d) metal-free porphyrin and (e–g) Cr–Fporphyrin moieties.

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Unfortunately, although we have failed to nd appropriateapplications for the last four years, the expected MA-CMP-Fswith metal free porphyrins (Fig. 6b–d) and Cr–F porphyrins(Fig. 6e–g) could be obtained.43

In conclusion, this work shows that the functional perfor-mance of CMP-Fs can be enhanced by the engineering of theinner morphological structure. Using the assembled silicaspheres as templates, the MA-CMP-F could be synthesized.Because of the facilitated diffusion of substrates into the innerMA-CMP-F through macropores, the MA-CMP-F showed muchenhanced sensing performance toward nitrotoluenes,compared with the CMP-F. We believe that more various func-tional CMP lms with macroporous inner structures can bedeveloped by the synthetic strategy in this work.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This work was supported by the Basic Science ResearchProgram (2016R1E1A1A01941074) through the NationalResearch Foundation of Korea (NRF) funded by the Ministry ofScience, ICT and Future Planning. K. C. K. acknowledges thenancial support by the Basic Science Research Program(2016R1A6A3A11933303) through the NRF of Korea funded bythe Ministry of Education.

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