The spirobifluorene-based fluorescent conjugated microporous polymers for reversible adsorbing iodine, fluorescent sensing iodine and nitroaromatic compounds

European Polymer Journal 115 (2019) 37–44

Contents lists available at ScienceDirect

European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

The spirobifluorene-based fluorescent conjugated microporous polymers for reversible adsorbing iodine, fluorescent sensing iodine and nitroaromatic compounds

T

⁎

Tongmou Geng , Guofeng Chen, Lanzhen Ma, Can Zhang, Weiyong Zhang, Heng Xu AnHui Province Key Laboratory of Optoelectronic and Magnetism Functional Materials, School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246011, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Conjugated microporous polymer Spirobifluorene Iodine capture Fluorescence sensor Nitroaromatic compounds

Two new spirobifluorene-based conjugated microporous polymers, TS-TAD and TS-TADP, were constructed via Friedel-Crafts coupling reactions. TS-TAD and TS-TADP possess high BET surface area of 828 and 783 m2 g−1, large pore volume of 1.51 and 0.54 cm3 g−1, good stability, and display excellent guest uptake of 4.15 and 3.65 g g−1 in iodine vapour as well as reversible adsorption of iodine in solution. The specific surface areas of CMPs were increased by the introduction of bispirofluorene rings compared with the corresponding parent polymers. TS-TAD has a similar structure to TS-TADP, but possesses larger specific surface area and pore values than TS-TADP, hence, TS-TAD has a higher adsorption capacity for iodine than TS-TADP. Moreover, the adsorption rate of TS-TAD is slower than that of TS-TADP in I2 vapor, while it is faster than that of TS-TADP in cyclohexane solution. We also highlight that the both resulting CMPs exhibit outstanding performance for fluorescent sensing to iodine and nitroaromatic compounds, thus making the resulting CMPs ideal absorbent materials for reversible iodine capture, sensing iodine and nitroaromatic compounds to address environmental issues.

1. Introduction Microporous materials have shown to be beneficial in a broad variety of both industrial and academic applications, for example, heterogeneous catalysis [1], gas storage [2], chemical separation [3], and sensing [4]. The classical microporous materials such as zeoliths and zeotypes, activated carbons, and metal organic frameworks (MOF) have been widely explored. Lately, increasing research activities have been focused on the generation of microporous, covalent, purely organic materials, which are called porous organic polymers (POPs), including covalent organic frameworks (COFs) [5,6], polymers of intrinsic microporosity (PIMs) [7], hyper-cross-linked polymers (HCPs) [8], covalent triazine-based frameworks (CTFs) [9], and conjugated microporous polymers (CMPs) [10]. The development of various chemical reactions, building blocks, and synthetic methods has generated a wide range of POPs with specific structures and outstanding properties, driving the rapid growth of the field [11]. The structural characteristics of spirobifluorene are that two fluorene units are connected by spiral-carbon atoms. This spatial configuration induces strong rigidity and increases the stability of the

⁎

system, hampering the oxidation process occurring at position 9 of the fluorene ring. Moreover, spirobifluorene can be chemically modified, like fluorene, with donor and/or acceptor groups in order to tune the emission of the system. Because of their attractive properties, spirobifluorenes have been proposed as suitable compounds in the field of optoelectronic devices, including solar cells and organic light-emitting diodes [12]. The 9,9′-spirobifluorene derivatives have been used as wonderful building blocks in preparation of POPs due to stable chargetransport properties [10], the rigid, and contorted structure [13,14]. With the aid of a 90° node in itself structure unit, two of which biphenyl planes are hyper stable or morphologically stable, which prevents the otherwise stiff polymer chains from space efficient packing, a very high, accessible free volume is obtained (PIM principle), the POPs with spirobifluorene possess high Brunauer-Emmett-Teller (BET) specific surface area [10,15]. Considering its easy preparation and intrinsic structure, spirobifluorene is a novel building block for design of CMPs with special properties [15]. As a strong electron donator, the spirobifluorene contributes to the improvement in photoactivity of the resultant polymers, and such highly rigid contorted block would be in favor of the formation of permanent both micropores and mesopores

Corresponding author at: School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246011, China. E-mail address: [email protected] (T. Geng).

https://doi.org/10.1016/j.eurpolymj.2019.02.047 Received 14 December 2018; Received in revised form 21 February 2019; Accepted 28 February 2019 Available online 01 March 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.

European Polymer Journal 115 (2019) 37–44

T. Geng, et al.

Scheme 1. Synthetic scheme of TS-TAD and TS-TADP.

DXR equipped with a 532 nm diode laser. Fluorescence spectra were recorded on a RF-5301PC spectrophotometer. The samples were prepared as follows: dried CMPs powder (10 mg) ground with an agate mortar was added to 10 mL of organic solvents. After the resulting mixture was well dispersed with ultrasound, the dispersion colloid was obtained.

[4]. In recent years, CTFs is one of the most popular materials because of its high physical and chemical stability and permanent porosity [16]. Due to its high nitrogen content and high porosity, CTFs have attracted wide attention in gas storage and separation. Electron deficient triazine is an excellent receptor, and it is also an active cross-linker [4,17]. The electronic conductivity of the conjugated polymer skeleton can be changed by introducing hetero atoms into porous polymers, thus enhancing the dipole-dipole interaction between sorbate molecules and adsorbents [18,19]. In this contribution, we selected helical difluorine as the main building material, and 2,4,6-trichloro-1,3,5-triazine (TCT) as one of the connecting units under the catalysis of low cost methane-sulfonic acid (Abbreviated to MSFA, CH3SO3H) to construct the porous networks (Scheme 1). The effects of the introduction of bispirofluorene units on pore properties, iodine adsorption, and fluorescence sensing properties would be studied.

3. Synthesis of TS-TAD and TS-TADP 3.1. Synthesis of TS-TAD When TCT (1.6 mmol) and S-TAD (0.6 mmol) was dissolved in odichlorobenzene (12 mL) containing 16.8 mmol CH3SO3H, the mixture was refluxed for 48 h at 140 °C in a nitrogen atmosphere. The precipitate filtered off from the hot reaction mixture was soxhlet extracted 24 h with methanol, tetrahydrofuran (THF), and acetone, then dried in vacuum. The target product was obtained as a brownish red solid (TSTAD, Yield: 88.58%). FT-IR (cm−1): 1618, 1480, 1453, 1383, 1267, 814, 789, 691. ss 13C NMR (ppm): 171, 147, 136, 127, 118, 66. Anal. calcd. (%): C 81.20, N 14.03, H 4.77. Found (%): C 82.03, N 12.094, H 4.641.

2. Experimental part 2.1. Materials and measurements

3.2. Synthesis of TS-TADP

2,2′,7,7′-Tetrabromo-9,9′-spirobifluorene (TBSBF), 1,2-dichlorobenzene (o-DCB), methane-sulfonic acid, and 2,4,6-trichloro1,3,5-triazine or cyanuric chloride (TCT or CC) were purchased from Aladdin reagent (Shanghai) Co. Ltd. Tetrakis(-triphenylphosphine) palladium (Pd(PPh3)4) and 4-(diphenylamino)phenylboronic acid were obtained from J & K scientific LTD. 2,2′,7,7′-Tetrakis(diphenylamino)9,9′-spirobifluorene (S-TAD) was bought from Shanghai Macklin Biochemical Co. Ltd. The infrared spectra were recorded on an iS50FT-IR spectrometer by using KBr pellets. Solid-state 13C CP/MAS NMR measurements were recorded on a Bruker AVANCE III 400 WB spectrometer at a MAS rate of 5 kHz and a CP contact time of 2 ms. Elemental analyses were carried out on a VARIO ELIII cube analyzer. UV–Vis spectrophotometer (UV2501PC) runs the UV probe software, version 2.34. All spectra were obtained as absorbance measurements 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. Scanning electron microscopy (SEM) was performed on a S-3400 N microscope. Thermogravimetric analysis (TGA) measurements were performed on a CDR-4P TGA under nitrogen atmosphere, by heating to 800 °C at a rate of 10 °C min−1. Powder X-ray diffraction (PXRD) data were recorded on a XRD600 diffractometer by means of depositing powder on glass substrate. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface area and pore volume, the Saito-Foley (SF) method was applied for the estimation of pore size distribution (pre-conditioned at 150 °C for 24 h in a high vacuum). Raman spectra were acquired using a

TS-TADP was obtained as brown colored solid using the same procedure with yield of 63.77%. FT-IR (cm−1): 1655, 1597, 1490, 1325, 1261, 814, 749, 696. ss 13C NMR (ppm): 172, 147, 140, 127, 119, 66. Anal. calcd. (%): C 83.94, H 4.87, N 11.19. Found (%): C 82.43, H 5.857, N 11.568. 4. Results and discussion Both two monomers S-TAD and S-TADP are electroactive materials and representative hole-transporting materials [20,21]. The former can be either lightly synthesized [22] or directly commercial purchased. The latter was obtained by Suzuki cross-coupling reaction with 2,2′,7,7′-tetrabromo-9,9′-spirobifluorene and 4-(N,N-diphenylamino) phenylboronic acid in the presence of Pd(PPh3)4 as the catalyst and aqueous Na2CO3 as the base in DMF with 62% yield. The purified product was obtained by washing the crude with pentane (see Supplementary material, Section S1, Scheme S1, Figs. S1, and S2) [12]. The chemical identity of TS-TAD and TS-TADP was analyzed by FTIR, solid state CP/MAS 13C NMR spectroscopy, elemental analysis, and UV–Vis spectra. For both TS-TAD and TS-TADP, an obvious consumption of strong stretching vibration bands of CeCl at 847 cm−1 in the spectra of FT-IR (Fig. S3) were observed for the networks compared to the starting TCT, which suggests the polymerization reaction successful occurring. Absorption peaks at 1618 and 1597 cm−1 are geared to the skeleton vibration of aromatic rings in the spirobifluorene blocks [4]. 38

European Polymer Journal 115 (2019) 37–44

150

100

66.45

Intensity

200

TS-TAD TS-TADP 66.22

118.45 126.87 118.75 127.38 139.87 136.44 146.56 146.65 171.56 171.06

250

Quantity Adsorbed (cm3/g STP)

T. Geng, et al.

50

0

-50

1000

(a) TS-TAD Adsorption isothermal curve TS-TAD Desorption isothermal curve TS-TADP Adsorption isothermal curve TS-TADP Desorption isothermal curve

800 600 400 200 0

0.0

Chemical shift (ppm) C NMR spectra of TS-TAD and TS-TADP.

Incremental Pore Volume (cm3g-1)

Fig. 1. ss

13

The band at 813 cm−1 can be ascribed to the bending vibrations of hydrocarbon bonds originating from benzene rings having two neighboured hydrogen atoms, i.e. the phenyl ring connecting two spirobifluorene units. The architecture of TS-TAD and TS-TADP was further confirmed at the molecular level by 13C CP-MAS NMR spectra (Fig. 1). About 127 ppm belongs to the signal peak of unsubstituted phenyl carbons, which is including broad shoulder absorption bands at about 118 ppm, while the resonances at 136, 140 and 147 ppm belong to substituted phenyl carbons adjacent to other aromatic rings [4,15]. The low intensity summit at 171 ppm is assigned to the substituted phenyl carbons bonding to the triazine rings [4]. In addition, the signals of quaternary carbon atoms of the spirobifluorene units in the CMPs were also collected at round 66 ppm [4,15]. Finally, we determined the C, H, and N by elemental analysis. A agreement between the calculated value and the found value was observed [4]. Nevertheless, the difference of elemental N content between the calculated and found values excess 2% for TS-TAD, this higher elemental analytical difference mean that the extent of reaction of S-TAD and TCT is low because of the compact structure of S-TAD. The optical properties of the both CMPs were analyzed by solid state UV–Vis spectroscopy measurements (Fig. S4). The both CMPs exhibit abroad absorption spectra, but with some difference in the absorption peak and edge, even though they have similar chemical structures. They show absorption bands at 410 and 670 nm, which are red-shift 11 and 312 nm compared with the monomers S-TAD and S-TADP. These results indicate that there are the extended conjugation in TS-TAD and TS-TADP. They are insoluble in water and common organic solvents (such as acetone, N-methylpyrrolidone, chloroform, methanol, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and THF, and stable in 3 mol L−1 HCl solution or 3 mol L−1 NaOH solution, implying their excellent chemical stability, which are similarly to previous reports. The thermal stability of TS-TAD and TS-TADP were investigated by TGA under a N2 atmosphere (Fig. S5). Decomposition of the two CMPs occurred above 630 and 495 °C, implying their high thermal stability [3,4]. As expected, these polymers are amorphous and exhibit no longrange crystallographic order, as evidenced by PXRD (Fig. S6) [10]. SEM images reveal the morphologies of the both TS-TAD and TS-TADP consisting of aggregated particles with different morphologies, shapes and sizes (Fig. S7). This morphology can lead to some outer surface area, such as, large meso- and macropores due to interstitial voids [23]. At 77 K, the porosity of the prepared polymers was determined by the adsorption/desorption isotherms of N2 (Fig. 2a). TS-TAD exhibited a combination of type I and type II nitrogen sorption isotherm feature and showed a steep uptake of nitrogen gas at low relative pressure (P/ P0 < 0.01), meaning that the material is microporous. Moreover, the increase of nitrogen adsorption capacity at a relative pressure above 0.97 may be due to the intergranular porosity being related to the

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0 ) 0.30

(b)

0.25 0.20

TS-TAD TS-TADP

0.15 0.10 0.05 0.00 0

2

4

6

8

10

12

14

Average Diameter (nm)

16

18

Fig. 2. N2 adsorption (solid symbols)/desorption (open symbols) isotherms (a) at 77 K and NLDFT pore size distribution (b) of TS-TAD and TS-TADP.

mesoscopic and macroscopic structure and special voids of the samples. It can also be seen from the diagram that the isotherm of TS-TADP rises sharply under low relative pressure (P/P0 < 0.01) and slowly under high relative pressure (P/P0 > 0.01), which is related to the typical Itype isotherm, indicating their microporosity. The listed in Table 1 are the key structural properties derived from the isotherm data such as BET surface area, micropore surface area and volume, total pore volume, and median pore diameter based on slit pore model of the Horvath-Kawazoe (H-K) method [4,15]. TS-TAD and TS-TADP have large the BET surface area of 828 and 783 m2 g−1 and high pore volume of 1.51 and 0.54 cm3 g−1 at P/P0 = 0.9, respectively [3,15]. Notably, these high values were reached without addition of a selected porogen, showing that this microporosity is indeed intrinsic to the structure of the polymer scaffold. As spirobifluorene and 1,3,5-triazine are bulky and rigid structural units, their incorporation into polymer networks is expected to result in nondense packing and a large amount of free volume, here detectable as accessible microporosity. Pore size distributions (PSD) of the two CMPs were calculated by nonlocal density functional theory (NLDFT) and are presented in Fig. 2b. Both the materials show a large number of micropores peaked at 1.88 and 1.22 nm, respectively (Table 1) [4,15,23]. The pore properties of CMPs are related to the structure and preparation methods [24]. The relationship between the structure control performance of monomers has been reported. The general rule is that with the increase of the monomer strut length, the specific surface area, micropore volume, and micropore surface area decrease. Our results are consistent with the Refs. [25,26]. In general, the micropore size distribution is shifted systematically to larger pore diameters as the monomer strut length is increased [25]. With regard to the topological structures of the two resulting CMPs, TSTADP shows longer linker length than TS-TAD. However, TS-TADP shows smaller pore size than TS-TAD. It could be because the extent of reaction for TS-TAD is lower than that for TS-TADP. 39

European Polymer Journal 115 (2019) 37–44

T. Geng, et al.

Table 1 Pore parameters of the TS-TAD and TS-TADP. CMPs

SBETa (m2 g−1)

Vtotal (tpv)b (cm3 g−1)

Vmicroc (cm3 g−1)

Vmicro/Vtotal

Smicroc (m2 g−1)

Dominant pore size (nm)

TS-TAD TS-TADP

828 783

1.51 0.54

1.34 0.30

0.89 0.55

647 273

1.88 1.22

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 = 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 t-plot method.

adsorbent and the high affinity of the host material to iodine will enhance the absorption of iodine by the porous absorbent [18,37,38]. TSTAD and TS-TADP have similar structures. Because the specific surface area and pore value of TS-TAD are larger than that of TS-TADP, TS-TAD has a higher adsorption capacity for iodine than TS-TADP. The evaluated theoretical maximum adsorption capacity of iodine are 7.44 and 2.66 g g−1 based on the pore volumes of TS-TAD and TS-TADP (1.51 and 0.54 cm3 g−1) and density of I2 (4.93 g cm−3) [19,40,41], which means that TS-TAD is only adsorbed in pores, while TS-TADP is adsorbed both in the pore and on the surface. Hence, TS-TADP shows a faster kinetics than TS-TAD in I2 vapor. The thermal stability of iodine-loaded TS-TAD and TS-TADP samples was tested by TGA under nitrogen (Fig. S5). The TGA curves revealed the significant weight loss of the iodine-loaded CMPs in the range of 90 to 300 °C. Since the pristine CMPs are stable under the condition, the mass loss should be mainly attributed to the release of iodine from the iodine-loaded CMPs upon being heated. The mass loss of iodine were thus estimated to be 3.89 g g−1 (93.73%) and 3.31 g g−1 (90.68%) relative to the mass of TS-TAD and TS-TADP, which are close to the saturated adsorption values [29,32]. The subtle differences may be caused by some incomplete release of iodine at the calculated temperature [27]. This high retention capacity can be attributed to the charge transfer complex interactions [31]. The mechanism of the iodine enrichment was preliminarily studied by FT-IR spectra, UV–Vis spectra, PXRD patterns, and Raman spectra. It was found from FT-IR spectral analysis that the characteristic peak position of the materials changed significantly before and after the adsorption (Fig. S3). Such as, the peaks of the hydrocarbon bonds (CeH) on the phenyl rings blue drifted to 9 cm−1/disappear, the peaks of carbon-carbon double bonds (C]C) on the phenyl unities red shifted 8 cm−1/disappear. For TS-TAD, the peaks of the C]N bonds on the triazine ring red shifted 22 and 6 cm−1, while the peaks of the carbon nitrogen single bonds blue shifted 22 cm−1; while for TS-TADP, the peaks of the C]N bonds on the triazine red shifted 37 and 58 cm−1, the peaks of the carbon nitrogen single bonds blue shifted around 5 cm−1 [27,31]. Aforementioned spectral changes of the absorption pesks indicate that the conjugated systems comprising of the triazine rings, phenyl rings and N atoms in triphenylamine groups involve in the formation of charge-transfer complex with iodine. These chargetransfer interactions contributed to the ultrahigh loading of iodine in the pores of CMPs. This is a striking feature compared to the most adsorbent materials that lack accessible interactions with iodine molecules [31,35]. Fig. S8 was showed the ultraviolet visible spectra of the I2@CMPs and the pristine iodine. No additional peaks at about 730 nm resulting from iodine crystals were observed in any of the iodine-loaded sorbents, indicating that the adsorbed iodine molecules were completely transformed into polyiodide anions [35]. Moreover, PXRD measurements (Fig. S6) revealed that iodine-loaded the both CMPs presented amorphous in nature without any prominent crystalline diffraction peaks [35,42]. The species of iodine in TS-TAD and TS-TADP networks were detected by Raman spectroscopy (Fig. S9). The spectra showed that I2-loaded TS-TADP and TS-TAD have strong absorption peaks at around 167 and 165 cm−1 which have been confirmed to be the signature peaks of I5− [27,43]. The absorption peaks at 144 and

According to the designed structures, TS-TAD and TS-TADP are rich in nitrogen and contain an abundant π-conjugated system, including aromatic rings and C]N bonds. Moreover, they have also good thermal stabilities. Therefore, the materials are suitable for the application in adsorption and desorption experiments of iodine vapor [27,28]. Significant amounts of iodine radionuclides (129I and 131I) with half-lives of up to 15.7 million years are generated in nuclear power plants, and these pose a great danger to the environment and human health if released, which need to be removed from exhaust fumes of nuclear power plants regularly. Previous studies showed that nitrogen containing CMPs are promising to address this issue [29,30]. In this work, we have chosen to use a stable 127I instead of the radioactive 129I and 131I. The isotopes of iodine have almost the same chemical behavior [31]. Iodine vapor exposure to 30 mg of TS-TAD and TS-TADP powders can be carried out in a 350 K closed chamber according to the established experimental method of iodine vapor adsorption. This treatment is close to that of typical nuclear fuel [31–33]. As shown in the Fig. 3 (Inserts), during the adsorption process, the colors of TS-TAD and TS-TADP deepened from brown to almost black [27,32]. Iodine uptake was measured by gravimetric method. The adsorption rates of TS-TAD and TS-TADP were very fast, with near linear increments of more than 8 h and saturation after 24 h and 12 h respectively. It is worth noting that the absorption capacities of TS-TAD and TS-TADP run up to 4.15 and 3.65 g g−1, respectively [27–34]. As seen, Table S1, although the specific surface area of CMPs was increased by the introduction of bispirofluorene rings, the contents of nitrogen atoms decreased compared with the corresponding parent polymers (TTPB [18] and TTPPA [19]), the adsorption capacity of TSTAD and TS-TADP decreased slightly. However, comparison with TTDAB and Tm-MTDAB possessing small specific surface area [19], although their nitrogen content is higher than that of TS-TAD and TSTADP, the adsorption capacity of TS-TAD and TS-TADP increased significantly. The iodine adsorption capacity of TS-TAD and TS-TADP was also increased compared with other CMPs with no bispirofluorene units containing nitrogen [35,36], sulfur [37,38], and no heteroatoms [39]. Previous studies have shown that both the porous properties of the

Fig. 3. Gravimetric I2 uptake of TS-TAD and TS-TADP as a function of time at 350 K. Insert: Photographs showing the color change of TS-TAD and TS-TADP before and after iodine sorption. 40

European Polymer Journal 115 (2019) 37–44

T. Geng, et al.

167 cm−1 are attributed to di-iodine units for [I−·(I2)] [44]. Combined with the literature reports, some charge-transfer complexes were formed through charge transfer interaction between iodine guest molecules and the electron-rich TS-TAD and TS-TADP and led to the generation of I3− and I5− [27,43,44]. Iodine capture by TS-TAD and TS-TADP in solution phase were also examined. When TS-TAD and TS-TADP were immersed in cyclohexane (CHA) solution of iodine (100 mg L−1) in a small sealed vial at room temperature, the purple solution faded tardily and became colorless 12 h later (Figs. S10 and S11). This result suggested that the iodine was encapsulated in the two CMPs [28,33,44]. We used the UV–Vis spectra to characterize the adsorption kinetic of I2. It can be seen that two stages of adsorption kinetics were obtained: the adsorption capacity for iodine increased quickly during the first 12 and 24 h for TS-TAD and TSTADP, respectively, and after that low increase was observed until equilibrium was reached [34,43]. In the 100 mg L−1of iodine solution in CHA, TS-TAD and TS-TADP exhibited the removal efficiencies of up to 99.27% and 75.07% [29,45], and possessed low iodine adsorption capacity with 0.993 and 0.751 g g−1. The lower iodine adsorption quantity in solution than that of solid state could be attributed to the coencapsulation of solvent molecules. For the inner cavities of the CMPs, in particular they should be filled with iodine solution, instead of pure I2, which significantly decreases its iodine capture capacity [33]. In addition, TS-TAD shows not only higher adsorb capacity but also faster adsorption rate than those of TS-TADP. This is because that the amount of iodine adsorbed in solution are low, both of them adsorbed iodine are in pores. TS-TAD has large specific surface area and large pore size, which is more favorable to the diffusion and adsorption of iodine. Such results further prove that high surface area, open porous structures and strong affinity of absorbents to iodine molecules would lead to an increase in iodine capture. To detect the structure of iodine adsorbed in TS-TAD and TS-TADP, Raman spectroscopy was performed on these materials prepared via the iodine sorption in solution process. The peak at 213 cm−1 is originated from ν1 symmetric pattern of iodine in gaseous state, while the TS-TAD and TS-TADP show no distinct peaks. However, after the iodine sorption in CHA solution, iodine loaded TS-TAD and iodine loaded TS-TADP exhibit the peaks characterized I3−. We can speculate that the peaks at 142 and 140 cm−1 is pertained to the ν1 of a triiodide, while the peak at 144 cm−1 is attributed to an asymmetric triiodide (Fig. S12) [43]. According to the Raman spectra (Figs. S9 and S12), the ionic species in the iodine-loaded CMPs vary from the I2 states. For example, the iodine-containing specie in the CMPs is triiodide after absorbing I2 from solution. While, the iodine-containing species in the CMPs are I5−, I3− and diiodine molecules after absorbing I2 vapor. We conjectured that this is because the concentration of iodine is high in the gas phase, and iodine is easily to bound I3− to form I5−; while in CHA solution, the concentration of iodine is low, and the triiodide ion is not easily bound to iodine molecule to form I5− [43]. The adsorption selectivity and their characteristics have been carried out with water, ethanol (EtOH), and dioxane (DOX), CHA at 350 K for 60 h (Fig. S13 and Table S2). The uptake capacity of TS-TAD and TSTADP decreased with the increase of a solvent's polarity. To our pleasant surprise, the water sorption values of the both CMPs are very low, which leads to a high iodine–water selectivity. Therefore, TS-TAD and TS-TADP can sorption iodine over water, which is advantageous for the actual industrial application of the iodine sorption process under humid conditions. Moreover, their physicochemical and hydrothermal stability of them leads to the application of capture iodine in the presence of water reversible [43,47]. Reversible iodine uptake by porous adsorbent is vital for their effective use and therefore, I2@TS-TAD and I2@TS-TADP were subjected to such studies. The desorption process of I2@TS-TAD and I2@TS-TADP were achieved with the faster iodine release efficiency, and as high as 93.59% and 97.51% of the adsorbed iodine could be released during 120 min at 125 °C (Fig. S14), showing that I2@TS-TAD and I2@TS-

TADP can be recycled [27,33]. Captured I2 could be readily released by immersing the I2-loaded samples in ethanol at room temperature, during which time the colorless solvent gradually became dark brown as time went on, which clearly shows that the iodine guests were dissociating from the spherical CMPs, as is shown in Fig. S15 [27,28,34]. In addition, we also monitored the release of iodine from ethanol by Ultraviolet visible spectra, which showed two absorbance maxima at 291 and 360 nm (Fig. S16), which support the presence of polyiodide. We observed that as soon as the both loaded CMPs with iodine are immersed in ethanol, iodine is released into ethanol. (Fig. S15) [45]. The released iodine amount increased with time up to 8 and 4 h (Fig. S17), which is different from vapor adsorption time required to reach equilibrium. Especially, although TS-TAD shows slower in the release rate than TSTADP, TS-TAD in EtOH shows more excellent ability to release adsorbed iodine (∼96.89%) than TS-TADP (∼57.52%) [45,46]. To check if iodine adsorption is a reversible process, recycling tests were performed by taking the I2@CMPs and heating it at 398 K under dynamic vacuum for 2 h. After that, the samples were reused for I2 capture under previously described conditions (1.0 bar, 398 K, 22 h). Repeat three times, the iodine uptake capacity and recycling percentage of recovered TS-TAD and TS-TADP were found to be 73.73%, 95.58% and 92.98%, 95.58%, respectively (Fig. S18), indicating that TS-TADP can be more efficiently recycled and reused without significant loss of I2 capture capacity than TS-TAD can be [33,35,47]. In order to study the potential application of TS-TAD and TS-TADP as the fluorescence sensors for detecting iodine and nitroaromatic compounds (NACs), they were dispersed in some common organic solvents (acetonitrile (ACN), acetone, DMF, DOX, chloroform, THF, and EtOH). As shown in Fig. 4a, b, the fluorescence spectra of the CMPs largely depend on the solvent molecules. There are significant

Fig. 4. Fluorescent spectra of TS-TAD (λex = 400 nm) (a) and TS-TADP (λex = 390 nm) (b) in various solvents. Inserts: Photographs of TS-TAD (a) and TS-TADP (b) in various solvents under UV irradiation at 365 nm. 41

European Polymer Journal 115 (2019) 37–44

T. Geng, et al.

1.35

fluorescence emitting behavior when they were dispersed in DOX and THF. While they exhibit the poor fluorescence emitting effects for other solvents [48,49]. Upon excitation under the UV light at 365 nm, another interesting phenomenon is observed. TS-TAD emits yellow luminescence, while the TS-TADP emits bright cyan luminescence in DMF, DOX, and THF, which can be identified clearly with the naked eye, as shown in inserts of Fig. 4 [49,50]. The fluorescence response time of the TS-TAD and TS-TADP suspension in DOX to I2, o-nitrophenol (o-NP), and picric acid (PA) for specific periods of time at 25 °C was measured (Figs. S19 and S20). As we expected, the fluorescence of the TS-TAD and TS-TADP was efficiently quenched by I2, o-NP, and PA in the relatively short time. Once I2, o-NP, or PA are added, the fluorescence of TS-TAD and TS-TADP pronounced decrease immediately [51]. The results indicate that the microporous architecture facilitates the sensing process and improves the response to the NACs [52,53]. The sensitivity of TS-TAD and TS-TADP toward PA, o-NP, and I2 were investigated via the real-time fluorescence response. The solution of PA, o-NP, or I2 were gradually added to the suspension of TS-TAD or TS-TADP in DOX, respectively. The corresponding fluorescence spectra were measured and the peak position of the emission spectra were not changed upon the addition of different concentrations of PA, o-NP, or I2 (Figs. S21, S22, and Fig. 5). Then, we calculated the quenching coefficient by the Stern–Volmer equation: I0/I = 1 + Ksv[A], where I is the fluorescence intensity after adding the concentration of analyte [A], and Ksv is the quenching coefficient. There are good linear Stern–Volmer relationships of TS-TAD and TS-TADP for sensing I2, o-NP, and PA. The values of Ksv were achieved from 103 to 104 L mol−1 for I2, oNP, and PA (Fig. 6 and Table S3), which means that TS-TAD and TSTADP have high sensitivity for I2, o-NP and PA [26,54,55]. This result clearly indicates the amplified fluorescence response for TS-TAD and

100

(a)

I0/I

1.25

40 20

0

20

40

60

80

-4

1.00 0.0

1.20

0.3

0.4

0.5

(b)

I0/I

1.10 I2 o-NP PA

1.00 0.0

0.1

0.2

0.3 -4

0.4

-1

[Ior NACs]*10 mol L 2

TS-TADP in the detection of o-NP, PA or I2 [55]. The detection limits of I2, o-NP, and PA for TS-TAD and TS-TADP are from 10−9 to 10−12 mol L−1 (Table S2). These results demonstrated once again high sensitivity of TS-TAD and TS-TADP for I2, o-NP, and PA detection in DOX solution [56]. We also investigated the quenching mechanism by measuring the absorption spectra of I2 and NACs (such as, o-NP, PA, 2,4-dinitrotoluene (DNT), nitrobenzene (NB), p-nitrotoluene (p-NT), m-dinitrobenzene (mDNB), and p-dinitrobenzene (p-DNB)), as well as phenol (PhOH), and made a comparison with the emission spectra of TS-TAD and TS-TADP. As shown in Fig. S23, no overlap between the two spectra was observed, which means that there is no energy transfer between I2, NACs and TSTAD as well as TS-TADP, suggesting that they are not the energy transfer mechanism [50,54]. The HOMO and LUMO energies of TSTAD, TS-TADP and analytes were calculated to illucidate the mechanism of fluorescence quenching by I2 and the NACs (Fig. S24 and Table S4). The LUMO of TS-TAD and TS-TADP lie at higher energy level than that of I2 and some NACs (p-DNB, DNT, m-DNB, o-NP, and PA), which provide a driving force of the photoinduced electron transfer from TS-TAD and TS-TADP to electron-deficient I2 and aforementioned NACs, hence quenching the fluorescence [50,53]. In sharp contrast, the LUMO energy levels of PhOH, NB and p-NT are higher than that of TSTAD and TS-TADP and prevented the occurrence of electron transfer [57,58]. Besides, the non-linear behaviours for the quenching efficiency indicate that the quenching have been encountered in case of a mix of static and dynamic interaction of chromophore and analyte (Fig. 5) [50]. The selectivity of TS-TAD and TS-TADP toward I2, o-NP, and PA detection were then investigated by testing the fluorescence change in the presence of other NACs or I2. As shown in Fig. 7, among all the NACs or I2 examined, only I2, o-NP, and PA caused the significant

100

I2 NB o-NP p-NT PA DNT p-DNB PhOH m-DNB

12

I0/I

0.2

[Ior NACs]*10 -4 mol L-1 2

1.05

-1

(b)

10 8 6 4 2 0

0.1

1.15

[Ior2 NACs]*10 mol L

0

1.15

1.05

0

14

1.20

1.10

DNT m-DNB p-DNB o-NP PA p-NT NB phOH I2

60

16

I2 o-NP PA

(a)

Fig. 6. The Stern–Volmer plots of (a) TS-TAD (λex = 400 nm) and (b) TS-TADP (λex = 390 nm) for I2 and NACs in DOX.

80

I0/I

1.30

20

40

60

80

-4

100 -1

120

[Ior NACs]*10 mol L 2

Fig. 5. The plots of I0/I of (a) TS-TAD (λex = 400 nm) and (b) TS-TADP (λex = 390 nm) against varying I2 and NACs concentrations in DOX. 42

European Polymer Journal 115 (2019) 37–44

T. Geng, et al.

10 9 8 7 6 5 4 3 2 1 0

(a)

TS-TAD+I or NACs 2 TS-TAD+NACs+I 2

3.0

I0/I

I0/I

2.5 2.0 1.5

(c)

P m- A D N B N B D N T pN p- T D N B Ph O H

Fr

ee oN P

I 2 oN P PA N m B -D N B D N T pN p- T D N Ph B O H

Fr

ee

1.0

3.0

6

TS-TAD+NACs TS-TAD+NACs+PA

(d)

5

I0/I

2.5

I0/I

TS-TAD+NACs TS-TAD+o-NP+NACs

(b)

2.0

TS-TADP+NACs TS-TADP+I +NACs 2

4 3

1.5

2

N B oN P N T PA D N p- T D N Ph B m OH -D N B

ee Fr

I 2

1

oN m P -D N B N B D N T pN p- T D N Ph B O H

Fr

ee PA

1.0

Fig. 7. Selectivities and competition experiments of TS-TAD and TS-TADP for sensing (a) I2, (b) o-NP, (c) PA, and (d) I2. (a), (b), (c) Fluorescence intensity of TS-TAD upon the addition of DOX solutions of different NACs followed by (a) I2, (b) o-NP, and (c) PA ((a) 7.5 × 10−4 mol L−1, (b) (c) 5.0 × 10−4 mol L−1, λex = 400 nm). (d) Fluorescence intensity of TS-TADP upon the addition of DOX solutions of different NACs followed by I2 (5.0 × 10−4 mol L−1, λex = 390 nm).

fluorescence quenching of TS-TAD and TS-TADP. To verify the selectivity of TS-TAD and TS-TADP for the practical detection of o-NP, PA, and I2, the experiments of competition were further conducted by addition of o-NP, PA, or I2 into the DOX solution of the competitive NACs and I2. We can see in Fig. 7 that neither of the competitive NACs showed an appreciable influence on the I2, o-NP, and PA detection except to the influence each other between them for TS-TAD. NB, p-NT, m-DNB, p-DNB, and PhOH have negligible effects on the fluorescence intensity of TS-TADP. But varying degrees of fluorescence quenching are observed with o-NP and PA. Thus, TS-TAD and TS-TADP show extra-high selectivity for I2 over NACs [55]. These results further identified that TS-TAD and TS-TADP exhibit satisfactory selectivity toward o-NP, PA, or I2 detection. To investigate the fluorescent stability of TS-TAD in the solid state, annealing experiments were conducted. The TS-TAD was baked in air environment at 50 °C for 0.5 h, then at 100 °C for another 0.5 h, followed by 150 °C and 200 °C. The fluorescent emission spectra of TSTAD after annealing in air was showed Fig. S25. It was reported that TSTAD was still very stable after baked at 150 °C for 30 min, and the fluorescence spectra were almost the same as those measured before annealing. Accompanying with the further increased temperature, the fluorescent emission intensity was reduced. These results should be ascribed to the conjugated structure and the stable bispirofluorene as well as triazine units introduced [59].

adsorption capacity in vapor with a maximum uptake of 4.15 and 3.65 g g−1, respectively. The absorbed iodine could be recovered. Furthermore, it exhibited extremely high detection sensitivity to NACs and iodine. The present work suggests that the dual functional materials may be a promising high-capacity absorbents for radioactive iodine and the sensitive fluorescence sensors of nitroaromatic compounds and iodine. Acknowledgements This work was supported by Natural Science Foundation of Anhui Education Department (under Grant No. KJ2018A0319) and the open fund of AnHui Province Key Laboratory of Optoelectronic and Magnetism Functional Materials (under Grant No. ZD2017007). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.02.047. References [1] C.Y. Lin, D.T. Zhang, Z.H. Zhao, Z.H. Xia, Covalent organic framework electrocatalysts for clean energy conversion, Adv. Mater. 5 (2017) 1703646. [2] Y.F. Zeng, R.Q. Zou, Y.L. Zhao, Covalent organic frameworks for CO2 capture, Adv. Mater. 28 (2016) 2855–2873. [3] B.G. Hauser, O.K. Farha, J. Exley, J.T. Hupp, Thermally enhancing the surface areas of Yamamoto-derived porous organic polymers, Chem. Mater. 25 (2013) 12–16. [4] S. Gu, J. Guo, Q. Huang, J.Q. He, Y. Fu, G.C. Kuang, C.Y. Pan, G.P. Yu, 1,3,5Triazine-based microporous polymers with tunable porosities for CO2 capture and fluorescent sensing, Macromolecules 21 (2017) 8512–8520. [5] A.P. Cote, A.I. Benin, N.W. Ockwig, M. O’Keeffe, A.J. Matzger, O.M. Yaghi, Porous, crystalline, covalent organic frameworks, Science 310 (2005) 1166–1170. [6] H.M. El-Kaderi, J.R. Hunt, J.L. Mendoza-Cortes, A.P. Cote, R.E. Taylor, M. O’Keeffe, O.M. Yaghi, Designed synthesis of 3D covalent organic frameworks, Science 316 (2007) 268–272.

5. Conclusion Two spirobifluorene-based fluorescent conjugated microporous polymers with amorphous structure (TS-TAD and TS-TADP) were prepared by using of CH3SO3H as a catalyzer, which present developed microporosity, high specific surface area, large pore volume, as a result of the rigid and intrinsic spatial structure of spirobifluorene, and accessible nitrogen groups in the pore walls. They display excellent I2 43

European Polymer Journal 115 (2019) 37–44

T. Geng, et al.

[7] N.B. McKeown, P.M. Budd, Polymers of intrinsic microporosity (PIMs): organic materials for membrane separations, heterogeneous catalysis and hydrogen storage, Chem. Soc. Rev. 35 (2006) 675–683. [8] C.D. Wood, B. Tan, A. Trewin, F. Su, M.J. Rosseinsky, D. Bradshaw, Y. Sun, L. Zhou, A.I. Cooper, Microporous organic polymers for methane storage, Adv. Mater. 20 (2008) 1916–1921. [9] P. Katekomol, J. Roeser, M. Bojdys, J. Weber, A. Thomas, Covalent triazine frameworks prepared from 1,3,5-tricyanobenzene, Chem. Mater. 25 (2013) 1542–1548. [10] J. Weber, A. Thomas, Toward stable interfaces in conjugated polymers: microporous poly(p-phenylene) and poly(phenyleneethynylene) based on a spirobifluorene building block, J.Am, Chem. Soc. 130 (2008) 6334–6335. [11] C. Gu, N. Huang, F. Xu, J. Gao, D.L. Jiang, Cascade exciton-pumping engines with manipulated speed and efficiency in light-harvesting porous π-network films, Sci. Rep. 5 (2015) 8867. [12] F. Polo, F. Rizzo, M.V. Gutierrez, L.D. Cola, S. Quici, Efficient greenish blue electrochemiluminescence from fluorene and spirobifluorene derivatives, J. Am. Chem. Soc. 134 (2012) 15402–15409. [13] P.I.S. Tobat, F.L. Thomas, S. Josef, Comparison of charge-carrier transport in thin films of spiro linked compounds and their corresponding parent compounds, Adv. Funct. Mater. 16 (2006) 966–974. [14] U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weissortel, J. Salbeck, H. Spreitzer, M. Gratzel, Solid-state dye-sensitized mesoporous TiO2 solar cells with high photonto-electron conversion efficiencies, Nature 395 (1998) 583–585. [15] Q. Chen, J.X. Wang, Q. Wang, N. Bian, Z.H. Li, C.G. Yan, B.H. Han, Spiro(fluorene9,9′-xanthene)-based porous organic polymers: preparation, porosity, and exceptional hydrogen uptake at low pressure, Macromolecules 44 (2011) 7987–7993. [16] A.K. Sekizkardes, S. Altarawneh, Z. Kahveci, T. İslamoğlu, H.M. ElKaderi, Highly selective CO2 capture by triazine-based benzimidazole linked polymers, Macromolecules 47 (2014) 8328–8334. [17] S. Hug, L. Stegbauer, H. Oh, M. Hirscher, V. Lotsch, Nitrogen rich covalent triazine frameworks as high-performance platforms for selective carbon capture and storage, Chem. Mater. 27 (2015) 8001–8010. [18] T.M. Geng, Z.M. Zhu, W.Y. Zhang, Y. Wang, A nitrogen-rich fluorescent conjugated microporous polymer with triazine and triphenylamine, units for highiodine capture and nitro aromatic compounds detection, J. Mater. Chem. A 5 (2017) 7612–7617. [19] T.M. Geng, S.N. Ye, Z.M. Zhu, W.Y. Zhang, Triazine-based conjugated microporous polymers with N, N, N′, N′-tetraphenyl-1,4-phenylenediamine, 1,3,5-tris(diphenylamino)benzene and 1,3,5-tris[(3-methylphenyl)- phenylamino]benzene as the core for high iodine capture and fluorescence sensing of o-nitrophenol, J. Mater. Chem. A 6 (2018) 2808–2816. [20] F.Y. Zhang, A. Kahn, Investigation of the high electron affinity molecular dopant F6-TCNNQ for hole-transport materials, Adv. Funct. Mater. 28 (2018) 1703780. [21] C. Isenberg, T.P.I. Saragi, Revealing the origin of magnetoresistance in unipolar amorphous organic field-effect transistors, J. Mater. Chem. C 2 (2014) 8569–8577. [22] T.P.I. Saragi, F.L. Thomas, S. Josef, High ON/OFF ratio and stability of amorphous organic field-effect transistors based on spiro-linked compounds, Synthetic Met. 148 (2005) 267–270. [23] Y.Z. Liao, H.G. Wang, M.F. Zhu, A. Thomas, Efficient supercapacitor energy storage using conjugated microporous polymer networks synthesized from BuchwaldHartwig coupling, Adv. Mater. 12 (2018) 1705710. [24] X.Y. Wang, C. Zhang, Y. Zhao, S.J. Ren, J.X. Jiang, Synthetic control and multifunctional properties of fluorescent covalent triazine based frameworks, Macromol. Rapid Commun. 37 (2016) 323–329. [25] J.X. Jiang, F.B. Su, A. Trewin, C.D. Wood, H.J. Niu, J.T.A. Jones, Y.Z. Khimyak, A.I. Cooper, Synthetic control of the pore dimension and surface area in conjugated microporous polymer and popolymer networks, J. Am. Chem. Soc. 130 (2008) 7710–7720. [26] J.X. Jiang, A. Trewin, F. Su, C.D. Wood, H. Niu, J.T.A. Jones, Y.Z. Khimyak, A.I. Cooper, Microporous poly(tri(4-ethynylphenyl)amine) networks: synthesis, properties, and atomistic simulation, Macromolecules 42 (2009) 2658–2666. [27] X.H. Guo, Y. Tian, M.C. Zhang, Y. Li, R. Wen, X. Li, X.F. Li, Y. Xue, L.J. Ma, C.Q. Xia, S.J. Li, Mechanistic insight into hydrogen-bond-controlled crystallinity and adsorption property of covalent organic frameworks from flexible building blocks, Chem. Mater. 30 (2018) 2299–2308. [28] D. Shetty, J. Raya, D.S. Han, Z. Asfari, J.C. Olsen, A. Trabolsi, Lithiated polycalix[4] arenes for efficient adsorption of iodine from solution and vapor phases, Chem. Mater. 29 (2017) 8968–8972. [29] T. Skorjanc, D. Shetty, S.K. Sharma, J. Raya, H. Traboulsi, D.S. Han, J. Lalla, R. Newlon, R. Jagannathan, S. Kirmizialtin, J.C. Olsen, A. Trabolsi, Redox-responsive covalent organic nanosheets from viologens and calix[4]arene for iodine and toxic dye capture, Chem. Eur. J. 24 (2018) 1–9. [30] Y.Z. Liao, Z.H. Cheng, W.W. Zuo, A. Thomas, C.F.J. Faul, Nitrogen-rich conjugated microporous polymers: facile synthesis, efficient gas storage and heterogeneous catalysis, ACS Appl. Mater. Interfaces 9 (2017) 38390–38400. [31] C. Wang, Y. Wang, R.L. Ge, X.D. Song, X.Q. Xing, Q.K. Jiang, H. Lu, C. Hao, X.W. Guo, Y.N. Gao, D.L. Jiang, A 3D Covalent organic framework with exceptionally high iodine capture capability, Chem. Eur. J. 24 (2018) 585–589. [32] M. Janeta, W. Bury, S. Szafert, Porous silsesquioxane−imine frameworks as highly efficient adsorbents for cooperative iodine capture, ACS Appl. Mater. Interfaces 10 (2018) 19964–19973. [33] Z.J. Yin, S.Q. Xu, T.G. Zhan, Q.Y. Qi, Z.Q. Wu, X. Zhao, Ultrahigh volatile iodine

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42] [43] [44]

[45]

[46]

[47] [48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

[59]

44

uptake by hollow microspheres formed from a heteropore covalent organic framework, Chem. Commun. 53 (2017) 7266–7269. D.Y. Chen, Y. Fu, W.G. Yu, G.P. Yu, C.Y. Pan, Versatile adamantane-based porous polymers with enhanced microporosity for efficient CO2 capture and iodine removal, Chem. Eng. J. 334 (2018) 900–906. Y.Z. Liao, J. Weber, B.M. Mills, Z.H. Ren, C.F.J. Fau, Highly efficient and reversible iodine capture in hexaphenylbenzene-based conjugated microporous polymers, Macromolecules 49 (2016) 6322–6333. X. Qian, B. Wang, Z.Q. Zhu, H.X. Sun, F. Ren, P. Mu, C.H. Ma, W.D. Liang, A. Li, Novel N-rich porous organic polymers with extremely high uptake for capture and reversible storage of volatile iodine, J. Hazard. Mater. 338 (2017) 224–232. X. Qian, Z.Q. Zhu, H.X. Sun, F. Ren, P. Mu, W.D. Liang, L.H. Chen, A. Li, Capture and reversible storage of volatile iodine by novel conjugated microporous polymers containing thiophene units, ACS Appl. Mater. Interfaces 8 (2016) 21063–21069. F. Ren, Z.Q. Zhu, X. Qian, W.D. Liang, P. Mu, H.X. Sun, J.H. Liu, A. Li, Novel thiophene-bearing conjugated microporous polymer honeycomb-like porous spheres with ultrahigh iodine uptake, Chem. Commun. 52 (2016) 9797–9800. Y.F. Chen, H.X. Sun, R.X. Yang, T.T. Wang, C.J. Pei, Z.T. Xiang, Z.Q. Zhu, W.D. Liang, An Li, Weiqiao Deng, Synthesis of conjugated microporous polymer nanotubes with large surface areas as absorbents for iodine and CO2 uptake, J. Mater. Chem. A 3 (2015) 87–91. Q.Q. Dang, X.M. Wang, Y.F. Zhan, X.M. Zhang, An azo-linked porous triptycene network as an absorbent for CO2 and iodine uptake, Polym. Chem. 7 (2016) 643–647. R.X. Yao, X. Cui, X.X. Jia, F.Q. Zhang, X.M. Zhang, A luminescent Zinc(II) metal−organic framework (MOF) with conjugated π-electron ligand for high iodine capture and nitro-explosive detection, Inorg. Chem. 55 (2016) 9270–9275. H. Bildirir, J.P. Paraknowitsch, A. Thomas, A tetrathiafulvalene (TTF)- conjugated microporous polymer network, Chem. Eur. J. 20 (2014) 9543–9548. C.Y. Pei, T. Ben, S.X. Xu, S.L. Qiu, Ultrahigh iodine adsorption in porous organic frameworks, J. Mater. Chem. A 2 (2014) 7179–7187. G. Das, H. Prakasam, S. Nuryyeva, D.S. Han, A.W. Ahmed, J.C. Olsen, K. Polychronopoulou, P.I. Carlos, F. Ravaux, M. Jouiad, L. Trabolsi, Multifunctional redox-tuned viologen-based covalent organic polymers, J. Mater. Chem. A 4 (2016) 15361–15369. G. Das, T. Skorjanc, S.K. Sharma, F. Gá ndara, M. Lusi, D.S.S. Rao, S. Vimala, S.K. Prasad, J. Raya, D.S. Han, R. Jagannathan, J.C. Olsen, A. Trabolsi, Viologenbased conjugated covalent organic networks via zincke reaction, J. Am. Chem. Soc. 139 (2017) 9558–9565. Y. Lin, X. Jiang, S.T. Kim, S.B. Alahakoon, X. Hou, Z. Zhang, C.M. Thompson, R.A. Smaldone, C. Ke, An elastic hydrogen bonded cross-linked organic framework for effective iodine capture in water, J. Am. Chem. Soc. 139 (2017) 7172–7175. S. Gu, J.Q. He, Y.L. Zhu, Z.Q. Wang, D.Y. Chen, G.P. Yu, C.Y. Pan, J.G. Guan, K. Tao, ACS Appl. Mater. Interfaces 8 (2016) 18383–18392. Y. Kai, G.W. Peiyao, T. Hu, L. Shi, R. Zeng, M. Forster, T. Pichler, Y. Chen, U. Scherf, Nanofibrous and graphene-templated conjugated microporous polymer materials for flexible chemosensors and supercapacitors, Chem. Mater. 27 (2015) 7403–7411. H.P. Ma, B. Li, L.M. Zhang, D. Han, G.S. Zhu, Targeted synthesis of core–shell porous aromatic frameworks for selective detection of nitro aromatic explosives via fluorescence two dimensional response, J. Mater. Chem. A 3 (2015) 19346–19352. Y.K. Li, S.M. Bi, F. Liu, S.Y. Wu, J. Hu, L.M. Wang, H.L. Liu, Y. Hu, Porosity-induced emission: exploring color-controllable fluorescence of porous organic polymers and their chemical sensing applications, J. Mater. Chem. C 3 (2015) 6876–6881. X.F. Wu, H.B. Li, Y.X. Xu, B.W. Xu, H. Tong, L.X. Wang, Thin film fabricated from solution-dispersible porous hyperbranched conjugated polymer nanoparticles without surfactants, Nanoscale 6 (2014) 2375–2380. X.M. Liu, Y.H. Xu, D.L. Jiang, Conjugated microporous polymers as molecular sensing devices: microporous architecture enables rapid response and enhances sensitivity in fluorescence-on and fluorescence-off sensing, J. Am. Chem. Soc. 134 (2012) 8738–8741. C. Gu, N. Huang, J. Gao, F. Xu, Y.H. Xu, D.L. Jiang, Controlled synthesis of conjugated microporous polymer films: versatile platforms for highly sensitive and label-free chemo- and biosensing, Angew. Chem. Int. Ed. 126 (2014) 4850–4855. Z. Chen, M. Chen, Y.L. Yu, L.M. Wu, Robust synthesis of free-standing and thickness controllable conjugated microporous polymer nanofilms, Chem. Commun. 53 (2017) 1989–1992. G.Q. Lin, H.M. Ding, D.Q. Yuan, B.S. Wang, C. Wang, A pyrene-based, fluorescent three-dimensional covalent organic framework, J. Am. Chem. Soc. 138 (2016) 3302–3305. W. Zhang, L.G. Qiu, Y.P. Yuan, A.J. Xie, Y.H. Shen, J.F. Zhu, Microwave-assisted synthesis of highly fluorescent nanoparticles of a melamine-based porous covalent organic framework for trace-level detection of nitroaromatic explosives, J. Hazard. Mater. 221–222 (2012) 147–154. C. Gu, N. Huang, Y. Wu, H. Xu, D.L. Jiang, Design of highly photofunctional porous polymer films with controlled thickness and prominent microporosity, Angew. Chem. Int. Ed. 54 (2015) 11540–11544. G. Das, B.P. Biswal, S. Kandambeth, V. Venkatesh, G. Kaur, M. Addicoat, T. Heine, S. Verma, R. Banerjee, Chemical sensing in two dimensional porous covalent organic nanosheets, Chem. Sci. 6 (2015) 3931–3939. W.B. Wu, S.H. Ye, G. Yu, Y.Q. Liu, J.G. Qin, Z. Li, Novel functional conjugative hyperbranched polymers with aggregation-induced emission: synthesis through one-pot “A2+B4” polymerization and application as explosive chemsensors and PLEDs, Macromol. Rapid Commun. 33 (2012) 164–171.