mesoporous polymers

Microporous and Mesoporous Materials 231 (2016) 92e99

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The synthesis and fluorescence detection properties of benzoquinonebased conjugated microporous/mesoporous polymers Tong-Mou Geng a, *, Deng-Kun Li a, Zong-Ming Zhu a, Ye-Bin Guan a, Yu Wang b a

Collaborative Innovation Center for Petrochemical New Materials, Anhui Key Laboratory of Functional Coordination Compounds, School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246011, PR China b School of Resource and Environmental Science, Anqing Normal University, Anqing 246133, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 January 2016 Received in revised form 1 May 2016 Accepted 15 May 2016 Available online 17 May 2016

The benzoquinone-based conjugated microporous/mesoporous polymers were synthesized through SonogashiraeHagihara cross-coupling with tetrabromo-1,4-benzoquinone (TBrBQ) and 1,4diethynylbenzene (DEB) by both solution polymerization (DBQP) and miniemulsion polymerization (DBQN). The BrunauereEmmetteTeller (BET) surface areas of these polymers were 356 and 25 m2 g
1 and pore average diameter were concentrated at around 2.12 and 3.84 nm for DBQP and DBQN, respectively. The two dimensional p-conjugated polymer frameworks could be combined with permanent microporous/mesoporous, luminescent properties and abundant oxygen atoms in the skeleton. Fluorescence studies found that both DBQP and DPQN could be utilized as fluorescent chemosensors for picronitric acid (PA) in acetone suspension. © 2016 Published by Elsevier Inc.

Keywords: Conjugated microporous/mesoporous polymer Chemosensor Miniemulsion polymerization Picronitric acid Benzoquinone

1. Introduction Conjugated polymers are attractive materials for the detection of chemicals, especially nitroaromatic explosives, because of their remarkable p-conjugation and their enhanced sensitivity through fluorescent signal amplification [1e3]. However, owing to their rigid conformation, they have a high tendency to aggregate in solution and the solid state. To resolve this issue, molecular approaches based on site isolations with bulky polymeric matrices have been developed to prevent the aggregation of conjugated polymers. These approaches provide highly luminescent polymers, but at the price of a loss in interchain electronic communications [4e7]. Conjugated microporous polymers (CMPs) are a class of porous organic polymers (POPs) with a conjugated network structure [8e10], which display a lot of advantages, such as their skeletons are strong covalent bonds between organic units, display higher stability to air and atmospheric moisture and much more severe conditions, for example, high temperature, strong acid and base.

* Corresponding author. School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246011, PR China. E-mail address: [email protected] (T.-M. Geng). http://dx.doi.org/10.1016/j.micromeso.2016.05.022 1387-1811/© 2016 Published by Elsevier Inc.

Furthermore, one of the most attractive aspects is the promise of tuning structures, properties and functionalities through rational chemical design and intensive selection of organic building blocks [8,9]. In contrast to the site isolation approach, the porous network strategy is unique in that it not only enhances luminescence but also promotes p-electronic interactions via the conjugated network scaffolds [10e13]. Moreover, they possess large surface areas, three dimensional p-conjugated polymer framework and provide a broad interface for analytes interaction [8,9,14]. Therefore, CMPs are attractive candidates for sensing materials. Nevertheless, most studies on CMPs to date have focused on the development of synthetic approaches for the control of pore size, surface area and gas storage [15]. Until 2011 there are no reports for sensing property of CMPs [8,16e18]. In recent years, the most of the research on sensing of performance of CMPs are inclined to nitroaromatic explosives [8,19e26], other organic compounds, such as dopamine [26], chiral amino alcohols [27], fluoride anions [28], hypochloroic acid [26], and transition metal ions, such as Fe3þ [24,26], Co3þ [26], Agþ [26], and Hg2þ [29]. The 2,4,6-trinitrophenol (PA, i.e. 2,4,6-trinitrophenol, TNP) is one of nitroaromatic derivatives, which is widely used in dyes, fireworks, glass and leather industries. During commercial production and use, PA is released into the environment, leading to the

T.-M. Geng et al. / Microporous and Mesoporous Materials 231 (2016) 92e99

contamination of soil and aquatic biosystems. Thus, there is an urgent need to develop an efficient and reliable sensor for detection of PA, owing to their serious pollution and potential threats to security [30]. Recently, there are several forms of CMPs emerging in fluorescent sensing field as PA chemosensors. Most of them are insoluble bulk powders, which are synthesized by solution polymerization [17,18,20e22,24,30e37]. The nanoparticles as chemosensors are easily dispersed in organic solvents, which are fabricated in miniemulsion polymerization [36]. The soluble conjugated hyperbranched polymers may be soluble in common organic solvents, which are prepared with both solution and miniemulsion polymerization [25]. The film as a chemosensor [19] is the desired form, which can be prepared by not only miniemulsion and the soluble conjugated hyperbranched polymers [38] but also by direct way, such as electrochemical method [23]. In some case, despite starting from the same monomer and comonomer, the insoluble bulk powders and nanoparticles are obvious difference in optical and fluorescent sensing property [25]. Moreover, the most conjugated POPs are microporous materials, such as CMPs, while the investigation for conjugated microporous/mesoporous polymers are rather few [16]. In general, the building units play a crucial role in designing and controlling the structures and properties of CMPs. Synthesis of novel polymers is very interesting in the field using new building blocks. Quinones are provided with both conjugated structure and oxygen atoms, hence, they are fit for as the building units of CMPs used for sensing [39,40]. In this contribution, we designed and synthesized benzoquinone-based conjugated microporous/mesoporous polymers through the Pd-catalyzed Sonogashira-Hagihara cross-coupling reactions of tetrabromo-1,4-benzoquinone (TBrBQ) and 1,4-diethynylbenzene (DEB) by both solution polymerization in DMF and miniemulsion polymerization technique. Then, we comparatively investigated the porous, fluorescent, and sensing performance for picronitric acid (PA) in acetone suspension of obtained both conjugated microporous/mesoporous polymers (see Scheme 1). 2. Experimental 2.1. Materials and nonoptical characterization Tetrabromo-1,4-benzoquinone (TBrBQ), 1,4-diethynylbenzene (DEB), copper (I) iodide (CuI) (99.5%), tetrakis(triphenylphosphine)palladium (Pd(PPh3)4) and sodium dodecylsulfate (SDS, 98%) were purchased from Aladdin Chemistry Co. Ltd. N,N-dimethylformamide (DMF, 99.0%), methanol, tetrahydrofunan (THF), chloroform, acetone, toluene, hydrochloric acid, sodium hydrate, triethylamine (TEA), picronitric acid (PA), 4nitrotoluene (NT), phenol and p-dichlorobenzene were obtained commercially, and used without further purification. Reactions were carried out under a nitrogen atmosphere. The solid-state 13C NMR spectra were collected from a 9.4 T Bruker DSX NMR 400 MHz spectrometer equipped with a 4 mm HXY triple-resonance MAS probe (in double resonance mode) [8]. Fourier transform infrared (FTIR) spectra were recorded on a FTIR spectrophotometer (model Nicolet Neus 8700) with KBr

Scheme 1. Synthesis of the DBQP and DBQN by SonogashiraeHagihara cross-coupling.

93

compressing tablet. Elemental analyses (C, H and N) were carried out on an analyzer (model VarioELIII). Wide angle X-ray diffraction (XRD) data were recorded on a Rigaku model RINT Ultima III diffractometer by depositing powder on glass substrate, from 2q ¼ 5 up to 60 with 0.02 increment. Scanning electron microscopy (SEM) was performed on a model JEOL-3400LV (Japan) operating at an accelerating voltage of 5.0 kV. The sample was prepared by drop-casting a THF suspension onto mica substrate and then coated with gold. Nitrogen sorption isotherms were measured at 77 K with a Bel Japan Inc. model BELSORP-mini II analyzer. Before measurement, the samples were degassed in vacuum at 150  C for more than 10 h. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas and pore volume. The Saito-Foley (SF) method was applied for the estimation of pore size and pore size distribution. Thermogravimetric analysis (TGA) data were obtained on a ATA409PC instrument (Germany), with a heating rate of 10  C min
1 under flowing N2. UVevis spectra were obtained using a PerkinElmer Lambda 950 UVevis spectrophotometer running the UV probe software, version 2.34. All spectra were obtained as absorbance measurements from 200 to 800 nm, with scan speed set to fast and using a slit width of 5 nm. Solid powdered samples were analyzed using the ISR-2200 integrating sphere attachment with a quartz solid sample holder as diffuse reflection measurement. Emission spectra were obtained on a Shimadzu RF-5301PC spectra fluorophotometer. Spectra were obtained using a fast scan speed and with sensitivity set to high. Slit widths were adjusted so as to maximize the signal-to-noise for each sample. Suspension liquid was analyzed in a quartz cuvette with the standard cell holder attachment [13]. 2.2. Detection of arene derivatives All Fluorescence studies were done using Shimadzu Spectrofluorimeter (model RF 5301PC) with 1 cm quartz cuvettes. A standard stock solution of PA, NT, phenol and p-dichlorobenzene (1.00 mol L
1) was prepared by dissolving an appropriate amount of the arene derivatives in tetrahydrofunan (THF), chloroform, acetone, or absolute ethyl alcohol and adjusting the volume to 100.00 mL in a volumetric flask. This was further diluted to 0.10 mol L
1. For all measurements of fluorescence spectra, excitation was fixed at 365 nm with the emission recorded over the wavelength range of 375e600 nm. The excitation and the emission slit widths were 10.0 nm and 5.0 nm, respectively. The detection limit was calculated with the equation: detection limit ¼ 3S/r, where S is the standard deviation of blank measurements and r is the slope between relative fluorescent intensity versus sample concentration [24,37]. 2.3. General synthetic procedure of DBQP and DBQN All of the networks were synthesized by Sonogashira-Hagihara cross coupling reaction of arylhalides and arylethynylenes [12,41]. The molar ratio of ethynyl to bromo functionalities in the monomer feed was set at 1.5:1 [9,42e45]. 2.4. Polymerization in a solution state A mixture of TBrBQ (212 mg, 0.50 mmol), DEB (189 mg, 1.5 mmol), Pd(PPh3)4 (20 mg, 0.0173 mmol) and CuI (10 mg, 0.053 mmol) was placed in a round bottom flask equipped with a magnetic stirring bar and a reflux condenser. The solids were dissolved in a mixture of anhydrous DMF (8.4 mL) and anhydrous TEA (4.2 mL). After degassing the mixture for 30 min, the reaction was carried out at 90  C for 72 h under a nitrogen atmosphere with

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stirring. The reaction mixture was cooled to room temperature. The brown solid was collected by filtration and washed four times with chloroform, water, methanol and acetone (25 mL  4 for each) to remove any unreacted monomers or catalyst residues. Then, the solid was further washed with methanol for 24 h and THF for 24 h using a Soxhlet extractor. The solid was dried at 50  C in a vacuum oven for 24 h to afford dark brown powder (Abbreviation as DBQP) to (297 mg, 88%) [9,12,41,46]. FTIR (KBr, cm
1): 3418 (m), nH2O; 3298 (m), n≡CeH; 3069, 3021 (m), nAr-H; 2196 (m), nC≡C; 1678 (m), nC]O; 1596(s), 1504, 1444, 1378, nAr. Solid state CP/MAS 13C NMR of DBQP (ppm): eC^CeH (78.3, 84.6); eC^Ce (91.0); C]C (benzene with linking benzene-triple bond; 163.0); C]O (185); C]C (benzene without linking eC^Ce, 130.29); C]C (benzene with linking benzene-triple bond; 116.36) [9,42e45]. Anal. calcd. for DBQP, C26H8O2: C, 88.63; H, 2.29. Found: C, 85.14; H, 2.95. 2.5. Polymerization in a miniemulsion state Direct polymerization to nanoparticles was achieved following minor modification of the procedure reported for the fabrication of conjugated polymer nanoparticles by miniemulsion polymerization [47]. Briefly, in a 150 mL round-bottom flask, 500 mg SDS was dissolved in 50 mL of degassed water. TBrBQ (84.7 mg, 0.2 mmol), DEB (75.7 mg, 0.6 mmol), Pd(PPh3)4 (2.3 mg, 0.002 mmol) and CuI (~0.10 mg) were taken in a round-bottom flask and sealed under N2 atmosphere. Freshly distilled toluene (1.4 mL) and TEA (0.7 mL) were added to the reactant mixture; solids were dissolved by stirring and sonication using an ultrasound bath until the solid dissolved completely. Then the mixture was added to the surfactant solution under vigorously stirring. Stirring at 400 rpm of the miniemulsion at 50  C under N2 atmosphere for 72 h produced a colloidally stable dispersion. After reaction, the mixture was cooled to room temperature and was stirred in an open vessel at room temperature for 24 h to remove the toluene. Then filtered over sintered glass crucible [48,49]. An aliquot of nanoparticle dispersion was precipitated by addition to excess methanol and collected after washing with water, methanol, acetone, and chloroform to remove the surfactants and any unreacted monomer or catalyst residues. The further purification was carried out by Soxhlet extraction from methanol and chloroform for 24 h each. The affording yellow brown powder was dried in vacuum for 24 h at 50  C (Abbreviation as DBQN) [48,49]. FTIR of DBQN (KBr, cm
1): 3435 (m), nH2O; 3298 (m), 3271, n≡CeH; 3036 (m), nAr-H; 2202 (m), nC≡C; 1672 (m), nC]O; 1603 (s), 1493, 1405, nAr; 1183, 1106 (m), dAr-H. Solid state CP/MAS 13C NMR of DBQN (ppm): eC^CeH (77.46, 82.86); C]O (187); C]C (benzene without linking eC^Ce, 138.93, 132.19); C]C (benzene with linking benzene-triple bond; 120.97); eC^Ce (93.0); C]C (benzene with linking benzene-triple bond; 158) [9,42e45]. Anal. calcd. for DBQN, C26H8O2: C, 88.63; H, 2.29. Found: C, 87.56; H, 2.975.

solution polymerization, because of its cross-linked nature, the polymer was precipitated during the reaction [28,50,54,55]. DMF as a solvent might give rise to higher surface area materials [44,45,56,57]. The polymer networks were recovered as brown powders in yields of greater than 88% [53]. A similar polymer network was synthesized by miniemulsion polymerization with toluene and TEA as organic phase and water as continuous phase. Vigorously stirring of the miniemulsion at 50  C under an N2 atmosphere for 3 days produced a colloidally stable nanoparticle dispersion. Residual amounts of organic solvents were removed from the dispersion by stirring it in an open vessel. A very small amount of CuI (~0.10 mg) was employed to avoid any quenching of fluorescence in the resulting miniemulsion [48]. Over an observation period of more than half year, colloidal stability was also fully retained at relatively high concentrations exceeding 1.0 mg mL
1 solids content [47,48,50,51,57,58]. Dispersible and discrete nanoscale conjugated mesoporous polymers have been prepared after complete removal of the surfactants and the morphologies and sizes could be modulated in the nanodroplets of miniemulsion. In contrast to the dendrimer-like soluble CMP, the conjugated mesoporous polymers retain the permanent intrinsic mesopores without change upon removal of solvent molecules [59]. To investigate the chemical stability of the DBQP and DBQN, we dispersed the samples in different organic solvents such as hexane, THF, MeOH, aqueous HCl (1 M) and NaOH (1 M) solutions at 25  C for 24 h. We found that no decomposition occurs to the samples in these conditions [21,22,32,34]. The thermal behavior of the polymers was studied by TGA under an atmosphere of nitrogen with a heating rate of 10  C min
1. As shown in Fig. S1, DBQP and DBQN are stable up to about 190  C and 230  C (with 5% weight loss), respectively [32,34]. In order to confirm the chemical identity of the prepared conjugated microporous/mesoporous polymer samples in this work, FTIR, elemental analysis and solid state 13C CP-MAS NMR spectroscopy had been used to perform characterization on the sample [64]. Infrared spectra for these polymers were shown in Fig. 1. The characteristic terminal C^C triple bond vibration peaks at about 3298 cm
1 had very lower intensity for DBQP than that for DBQN, while the peaks at around 2200 cm
1, which were characteristic of substituted acetylene, were easily detected and were larger for DBQP than that for DBQN, indicated that DBQP had a higher extent of reaction than DBQN [18,19,52]. From 2900 to 3298 cm
1 CeH stretching bands appear, revealed that there was indeed a true polycondensation [55]. The strong absorption band located at 834 cm
1 was assigned to the substituted phenylene rings, and the bands at 1504/1493 and 1412/1405 cm
1 were attributed to the

DBQP

2202.39

3500

834.29

4000

834.29

2196.02

DBQN 3271.41 3298.45 3435.26

In this contribution, we comparatively explored the sensing property of benzoquinone-based conjugated microporous/mesoporous polymers with oxygen atoms for picronitric acid (PA). The monomer tetrabromo-1,4-benzoquinone as a connected building unit was employed because of its oxygen atom [46]. As shown in Scheme 1, the network structures of the benzoquinone-based networks conjugated microporous/mesoporous polymers were prepared by using palladium catalyzed (A4þB2) SonogashiraHagihara cross-coupling reactions using a 1.5:1 M ratio of ethynyl to bromo functionalities [50e53], since this was found to maximize surface areas in the polymers [9,42e45]. The polymerization was carried out in either a solution or a miniemulsion state. For the

3298.45 3417.76

3. Results and discussion

3000

2500

2000

1500 -1

Wavenumber (cm ) Fig. 1. FTIR spectrums of DBQP and DBQN.

1000

500

T.-M. Geng et al. / Microporous and Mesoporous Materials 231 (2016) 92e99

Fig. 2. Solid state CP/MAS

95

13

C NMR spectra of DBQP (a) and DBQN (b).

benzene ring in the prepared benzoquinone-based polymers [60]. The solid-state 13C CP-MAS NMR measurements had been performed to further reveal the structure of the benzene polymers at the molecular level. As shown in Fig. 2(a), Solid-state 13C NMR of the DBQP showed strong peaks at 131 and 116 ppm corresponding to the two types of benzene without linking eC^Ce and with linking benzene-triple bond carbon atoms, respectively, implying that there were less terminal group of ethynyl in DBQP which possessed high extent of reaction. The small signals of internal ethynyls CAreC^CeCAr and terminal ethynyls CAreC^CeH units appeared at 89, 93 and 78, 83 ppm, respectively [19,20,54,60,61]. The signals of terminal ethynyl groups in the region 78 and 83 ppm were stronger in DBQN (Fig. 2 (b)) than that in DBQP (Fig. 2 (a)). Moreover, there was a big peak in 121 ppm which attribute to unsubstituted phenyl carbon atoms and nearly no peaks of internal ethynyl groups at about 90 ppm in DBQN, implying that the benzoquinone units were terminated by 4-ethynylphenyl groups in DBQN and most probably the extent of reaction was not high [21,28,53]. A powder X-ray diffraction (PXRD) pattern shown in Fig. S2 revealed that the prepared DBQP and DBQN were non-ordered, amorphous features [54], which were typical for porous polymers built up from metal-catalyzed coupling reactions as found for other SonogashiraeHagihara cross-coupled networks [28,54] and most other reported CMP networks [43,49,56,58,59]. This characteristic was assignable to the kinetic control of this kind of reaction [56,62]. The morphological information of DBQP and DBQN were obtained

by scanning electron microscopy (SEM), which showed that the conjugated microporous/mesoporous polymer networks comprised small beads with an average diameter from 25 to 30 nm (Fig. 3(a)) and 15 nm (Fig. 3(b)) for DBQP and DBQN, respectively [20,21,57,63], and no one-dimensional morphologies such as wires, belts and fibers exist [28,54]. The porous properties of DBQP and DBQN were then investigated by cryogenic nitrogen adsorption/desorption experiments at 77 K, from which the Brunauer- Emmet-Teller (BET) surface areas of the polymers were calculated (Fig. 4(a)) [28,55]. In spite of their similar chemical structures, the polymer samples showed dramatic differences in accessible surface area, as measured by their N2 adsorption behavior [19]. According to the IUPAC classification, the nitrogen adsorption and desorption isotherms revealed that both porous polymers exhibited type II isotherms. For DBQP, the distinct hysteresis phenomena between the adsorption and desorption cycles at relatively low pressure probably stemmed from the deformation and swelling of the polymer networks [27,55]. High uptake of N2 at low pressure, observed from the N2 adsorption isotherms of DBQP, indicated that the material had some mesoporous. In contrast, the hysteresis phenomena of DBQN was not obvious, low uptake of N2 at low pressure, suggested that DBQN was mainly mesoporous [28,55]. Assuming that the N2 gas was only adsorbed in monolayers, the surface areas were also calculated using the Langmuir method [55]. Remarkably, the high specific surface areas of 356 m2 g
1 (SBET) and 514 m2 g
1 (SLangmuir) were achieved for DBQP. The lower SBET (25.5 m2 g
1) and SLangmuir

Fig. 3. Field emission microscopic images with a scale bar of 500 nm of DBQP (a) and DBQN (b).

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160

Quantity Adsorbed (cm g STP)

Fluorescence Intensity / a.u.

(a)

140

3 -1

120 100

DBQP

80 60

DBQN

40 20 0 0.0

0.2

0.4

0.6

0.8

(a)DBQP

40

chloroform tetrahydrofuran ethanol acetone water

20

0

1.0

400

Relative Pressure (P/P0)

500

600

Wavelength (nm)

Fluorescence Intensity / a.u.

3

-1

Incremental Pore Volume (cm g )

60

0.06 0.05

(b) DBQP DBQN

0.04 0.03 0.02 0.01 0.00 0

2

4

6

8

10

12

Average Diameter (nm)

(b)DBQN and miniemulsion 50

acetone water miniemulsion

40

30

20

10

0 400

450

500

550

600

Wavelength (nm)

Fig. 4. (a) Nitrogen adsorption/desorption isotherms for DBQP and DBQN (circles), (b) Pore width distribution profiles of DBQP and DBQN.

Fig. 5. Fluorescence spectra of DBQP (a) and DBQN (b) in varying polar solvents (1.0 mg mL
1, excited with the same wavelength of 365 nm).

(35.9 m2 g
1) for DBQN were probably due to toluene employed as the solvent and low extent of reaction (Table 1) [21,28,50]. The pore size distribution curves of both polymers were calculated by the HorvatheKawazoe method. As shown in Fig. 4(b), the pores were mainly located in the mesopore range (2.12e4.32 nm and 1.95e3.84 nm) and that the mesopore distribution were centered around 2.12 nm and 3.84 nm for DBQP and DBQN, respectively. DBQP and DBQN at a relative pressure of 0.97 are calculated to be 0.225 and 0.0373 cm3 g
1, and their micropore volumes are 0.0561 and 0.000222 cm3 g
1 calculated using the t-plot method. Thus, the

microporosities are around 24.93% and 0.60%, further indicating that both DBQP and DBQN were belong to mesoporous material and were in agreement with the shape of the N2 isotherms (Fig. 4(a)) [21,28,53,54,59]. The UVevis spectra of monomer TBr4BQ, the conjugated microporous/mesoporous polymer networks of DBQP and DBQN in the solid state and miniemulsion (which is comprised of DBQN, SDS and catalyzers. Content of DBQN is about 1.0 mg mL
1) were recorded and were shown in Fig. S3. Both DBQP and DBQN showed a broad absorption across the wavelength range 200e800 nm, which was consistent with other reported networks [63]. Although the shapes of the spectra were similar, they did show differences in terms of the onset of the absorption. The DBQP showed an absorption onset at around 461 nm with shoulder peaks at 499 nm, while DBQN showed absorption onsets at 316 nm with shoulder peaks at 481 nm. Compared with the monomer TBr4BQ whose absorption onset at 260 nm with shoulder peaks at 440 nm showed a significantly red-shifted [19,21,48,53,54,60], indicating that the conjugated microporous/mesoporous polymer networks could absorb light across a wide range of the UVevis spectra (Fig. S3, parts b and c) [20,59]. Compared to the UVevis spectra of the starting compound TBr4BQ with an absorption maximum at 260 nm, bathochromic shifts between 201 and 56 nm are observed, indicated that both DBQP and DBQN are the extension of the p-

Table 1 Pore and surface properties of DBQP and DBQN. CMPs

SBETa (m2 g
1)

SLangmuira (m2 g
1)

Vtotal (tpv)b (cm3 g
1)

Vmicroc (cm3 g
1)

Smicroc (m2 g
1)

Sexternalc (m2 g
1)

DBQP DBQN

355.76 25.48

513.55 35.94

0.225 0.0737

0.0561 0.000222

2.816 0.273

458.04 25.21

a Specific surface area calculated from the adsorption branch of the nitrogen isotherm using the BET method in the relative pressure (P/P0) range from 0.01 to 0.10. b Total pore volume is obtained from BET data up to P/P0 ¼ 0.97 and is defined as the sum of micropore volume and volumes of larger pores. c Micropore volume calculated from nitrogen adsorption isotherm using the tplot method.

T.-M. Geng et al. / Microporous and Mesoporous Materials 231 (2016) 92e99

97

0

(a)

40

(c)

20

[PA]

35

15 -4

I0/I

30 -1

7.5*10 mol L

25 20 15

10

DBQP-PA DBQN-PA

5

10 0

5 0

Fluorescence Intensity / a.u.

25

45

50

400

450

500

550

Wavelength (nm)

0

600

-1

1.25*10 mol L

10

10

5

400

450

10

12

14

-1

15

20

0

8

500

(d)

20

-3

30

6

25

[PA]

40

4

[PA]*10 mol L

0

(b)

2

-4

I0/I

Fluorescence Intensity / a.u.

50

550

600

Wavelength (nm)

0 0.0

DBQP-PA DBQN-PA

0.5

1.0

1.5

2.0

-4

2.5

3.0

3.5

4.0

-1

[PA]*10 mol L

Fig. 6. Fluorescence spectral changes of DBQP (a) and DBQN (b) suspension in acetone with increasing concentration of PA. The relative fluorescence intensity (I0/I) for acetone € lmer plots of DBQP and DBQN suspension in acetone quenched by PA (d) (lex ¼ 365 nm). dispersion of DBQP and DBQN (1.0 mg mL
1) versus the concentration of PA (c). Stern-Vo

25

20

I0/I

15

10

5

0

Free

PA

NT

Phenol p-Dichlorobenzene

Fig. 7. Degree of fluorescence quenching of DBQP (1.0 mg mL
1) after addition of PA, NT, phenol and p-dichlorobenzene (3.0  10
4 mol L
1) in acetone suspension solution. Excitation wavelength ¼ 365 nm.

conjugated system [55]. However, the absorption spectra of miniemulsion showed narrow absorption and blue shifted maxima at 257 nm and 271 nm with shoulder peaks at 440 nm, which could be

attributed to the influence of the surfactant SDS [28,59] and was in accordance with other literature [48]. In addition, when the solidstates and suspensions of DBQP and DBQN were excited at UV light at 365 nm, there were no color changes occurred, while the color of dilute miniemulsion (content of DBQN: 0.05 mg mL
1) was changed from brown yellow to cyan (Fig. S4) [64]. The fluorescent stability of the DBQP were tested by the DBQP powders baking experiments in air dry oven. The DBQP powders were first baked at 50  C for 30 min in air, then at 100  C for another 30 min, followed by 150  C and so on. After cooling, they were dispersed in acetone (1.0 mg mL
1). It was found from Fig. S5 that DBQP was very stable after baked at 50  C for half an hour, and the fluorescence spectra were almost the same as those tested before heating. Accompanying with the further increased temperature, the fluorescence emission peak was only enhanced a little but no red-shifted occurred [32]. To investigate the emission property, the DBQP and DBQN were dispersed into various solvents and then emission spectra were recorded (Fig. 5). As expected, solvatochromic behavior were neither observed in fluorescence spectra, indicating a negligible dipole moment within DBQP and DBQN both in the ground and excited states, and thus no charge-transfer transition occurs [28]. Moreover, although the maximum emission wavelength were at round 385 nm and 421 nm for DBQP and DPQN, respectively, the

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fluorescence intensity were very distinct. The maximum fluorescence enhancement both DBQP and DBQN were observed for acetone, whereas the DBQP showed mediocre emission enhancement in chloroform and THF, the minimum fluorescence enhancement was in ethanol, water and miniemulsion [65]. To gain insight into the fluorescence quenching process, the titration experiments for testing the dispersion of DBQP and DBQN sensing behavior were thus performed using the 0.10 mol L
1 acetone solution of PA and excited at 365 nm [23]. Upon increasing the molar ratio of PA in an acetone dispersion of DBQP and DBQN, the intensity of the fluorescence emission spectra gradually decreased (Fig. 6 ((a) and (b)), and 95.92%, and 92.34% of the fluorescence were quenched while the PA concentration reaching to 7.5  10
4 and 1.25  10
3 mol L
1, respectively (Fig. 6 (c)) [19,27]. However, there were no the visible color changes occurred for acetone dispersion of both DBQP and DBQN [44]. The results showed that although DBQN is easier and stabler disperse than DBQP, the DBQN is lower sensitive to PA than DBQP. It may be because the degree of reaction for DBQN was lower than that of DBQP [33]. As shown in Fig. 6 (d), the fluorescence intensity decreased linearly with an increase in the PA concentration in the 0.5  10
4
7.5  10
4 and 0e3.5  10
4 mol L
1 concentration ranges for PA in acetone dispersion of DBQP and DBQN, respectively, because of photoinduced electron transfer from DBQP and DBQN to PA. The Stern
Volmer constant, KSV, of DBQP and DBQN for PA were 9.02  104 and 1.79  104 L mol
1. The detection limit, which was calculated as three times the standard deviation of the background noise from the calibration curve, for the determination of PA in acetone was found to be 3.33  10
18 and 2.48  10
13 mol L
1 [24,37]. The results indicated that though the sensitivity of DBQN was lower than that of DBQP as a result of the specific surface area of DBQN being smaller than that of DBQP [16,20,52], the KSV of DBQN almost belongs to the same order of magnitude with DBQP. In consideration of the emulsion stabilization and the effect of metal ions of catalysts on fluorescence of miniemulsion, the synthesis of DBQN is adopted a low temperature, a nonpolar solvent as droplet phase and small catalysts; thereby, the resulting surface areas are impaired and the pore size distributions are flawed due to the low degree of reaction. In addition, since many conjugated molecules tend to dissolve in polar solvents, they are plagued by the poor solubility in the oil-phase droplets of miniemulsion [59]. Thus, it is necessary to higher reaction temperature, more catalysts [66] or combining miniemulsion and solvothermal techniques [59] for increasing the degree of reaction. The selectivity studies of DBQP were performed in the presence of different aromatic derivatives such as PA, NT, phenol and pdichlorobenzene. Fig. 7 shows the corresponding difference of fluorescence intensity of DBQP suspension in acetone (1.0 mg mL
1) after addition of different aromatic derivatives (3.0  10
4 mol L
1). The PA was only observed significant loss of intensity, but little the degree of fluorescence quenching was observed for the other aromatic derivatives, indicating that DBQP exhibited an excellent selectivity for PA over other aromatic derivatives [22,23,67,68]. The quenching mechanism of aromatic derivatives for the CMPs matrices the donoreacceptor electron transfer mechanism with CMPs as the donor and aromatic derivatives as the acceptor rather than the energy transfer mechanism [24,30]. The chemosensing is related to the level of the lowest unoccupied molecular orbital (LUMO) of the nitroarenes. The calculated LUMO energy level of PA was low (
3.92 eV), which provided a strong driving force for the photoinduced electron transfer. In sharp contrast, the LUMO energy levels of other nitroaromatic derivatives are high (such as these of NP and NT are
2.19

and
2.31 eV, respectively), which prevented the occurrence of electron transfer [18,23]. 4. Conclusions In summary, we have described the strategies for the synthesis of the luminescent benzoquinone-based conjugated microporous/ mesoporous polymers via SonogashiraeHagihara cross-coupling reaction by both solution polymerization and miniemulsion polymerization. The conjugated microporous/mesoporous polymer networks with three-dimensionally interlocked skeleton suppresses the aggregation of building blocks, promotes p-electronic conjugation, facilitates exciton migration, and enhances luminescence. The DBQP and DBQN exhibit very high sensitive fluorescence quench upon PA with the SV constant of 9.02  104 and 1.79  104 L mol
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