1,3,5-Triazine-Based Microporous Polymers with Tunable Porosities

Nov 1, 2017 - College of Chemistry and Chemical Engineering, State Key Laboratory of Power Metallurgy, Hunan Provincial Key Laboratory of Efficient an...
1 downloads 9 Views 5MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX-XXX

pubs.acs.org/Macromolecules

1,3,5-Triazine-Based Microporous Polymers with Tunable Porosities for CO2 Capture and Fluorescent Sensing Shuai Gu,† Jun Guo,‡ Qiao Huang,† Jianqiao He,† Yu Fu,† Guichao Kuang,† Chunyue Pan,† and Guipeng Yu*,†,§ †

College of Chemistry and Chemical Engineering, State Key Laboratory of Power Metallurgy, Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, China ‡ School of Chemistry and Material Science, Guizhou Normal University, Guiyang 550000, China § State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 110762, China S Supporting Information *

ABSTRACT: The synthetic control over pore structure remains highly desirable for porous organic frameworks. Here, we present a competitive chemistry strategy, i.e., a systematical regulation on Friedel−Crafts reaction and Scholl coupling reaction through tuning the ratios of monomers. This leads to a series of spirobifluorene-based microporous polymers (SbfTMPs) with systematically tuned porosities and N content. Unlike the existing copolymerization strategy by which the synthesized polymers exhibit a monotonic change tendency in the porosities, our networks demonstrate an unusually different trend where the porosity increases first and then decreases with the increasing Ph/Cl ratios for the monomers. This is mainly ascribed to the completion of coexisting reaction routines and the different “internal molecular free volumes” of the repeating units. The as-made networks feature tunable capacities for CO2 adsorption over a wide range and attractive CO2/N2 selectivities. Moreover, these donor− acceptor type frameworks exhibit selective and highly sensitive fluorescence-on or fluorescence-off properties toward volatile organic compounds, which implies their great potential in fluorescent sensors.

■

INTRODUCTION Porous materials are proving to be valuable paltforms over a wide range of fields like chemical separation, catalysis, sensing, and gas storage.1,2 Recently, triazine-based porous polymers (TPPs) with high physicochemical stabilities and permanent porosity are one of the most popular materials.3 TPPs featuring high nitrogen content and excellent porosity have attracted considerable attention for gas storage and separation.4 The πstacked triazine units and aromatic units make TPPs attractive for serving as high-performance semiconductors in energy devices and catalysis.5−7 TPPs have been currently reported as novel nanocarriers for cancer therapy and imaging.8,9 More interesting, TPPs with donor−acceptor structures can also be developed as fluorescence sensors for detecting volatile organic compounds.10 Functions of nanoporous organic polymers (NOPs) including TPPs mainly depend on their porous properties, and thus many efforts have focused on pore engineering. In general, surface areas and pore dimensions of NOPs could be regulated by varying the nature of building blocks or topology optimization.11−15 Based on the design strategy of building blocks, NOPs with periodically heterogeneous pore structures were synthesized.16,17 The task-specific pore-wall engineering via anchoring selected functional groups on the channel also © XXXX American Chemical Society

enables NOPs with systematically tuned porosities and functionalities.18 Nevertheless, the porosities of NOPs in the previous works overly rely upon the meticulous selection of the monomers and precise control of the topology, which usually requires tedious synthetic hurdles and high-cost and unsustainable mass production procedures, impeding their large-scale production. Recently, copolymerization strategies were advocated for the development of NOPs with tunable porosities.19 With the statistical copolymerization strategy, the control of the porosities and the optical gaps of NOPs were facilely achieved.20−22 More recently, the porosities of organic copolymers were found to be tailorable and closely associated with internal molecular free volumes of the building blocks.23 However, such copolymerization strategies are still limited to single chemical reactions, such as Suzuki coupling21 or Ullmann cross-coupling reactions.22 Conjugated microporous polymers (CMPs) which combine high porosity with unique fluorescent characteristics have attracted intensive attention of researchers for addressing energy and environmental issues.24 They have emerged as Received: August 28, 2017 Revised: October 21, 2017

A

DOI: 10.1021/acs.macromol.7b01857 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Diagram of the Preparation of Sbf-TMPs from the Concurrent Chemical Reactions and Its Applications in CO2 Capture and Fluorescent Sensing

useful platforms for light harvesting,25 light emitting,26 chemosensors,10 and photocatalysts.20,27 However, most of the fluorescent conjugated microporous polymers are synthesized via Suzuki cross-coupling reaction,21 Yamamoto reaction,19 or Sonogashira−Hagihara reaction,28 which rely on specific monomers with certain reactive groups (such as bromoarenes, iodoarenes, aromatic boronic acids, and ethynyl-substituted arenes) and noble metal catalysts (such as palladium and nickel compounds). These setbacks undoubtedly limit the access to explore new structures, new functions, and mass manufacturing. The convenient, simple operation and easy-handling methods for the production of channel structurecontrollable NOPs remain an aspect that needs more contribution and insight. This work demonstrates that the heteroatom doping content, porosities, and gas sorption properties of TPPs could be welladjusted by facilely controlling the relative ratios of the concurrent Friedel−Crafts and Scholl coupling reactions. A series of spirobifluorene and 1,3,5-triazine-based microporous polymers (Sbf-TMPs) with conjugated backbones were synthesized under the catalysis of low-cost anhydrous AlCl3. Their competition was preferentially controlled by adjusting the ratios of cross-linking agent and building blocks. The resulting polymers exhibit regular changes in the Brunauer−Emmett− Teller (BET) surface areas, pore widths, pore volume, and hence a well-tunable adsorption capacity toward CO2. Moreover, strong fluorescent activity enables the as-made polymers a selectively fluorescence turn-on or fluorescence turn-off behavior toward a large scale of organic vapors, demonstrating their great potentials as fluorescence sensors for addressing environmental issues. To the best of our knowledge, this is the first report of pore engineering using functionality-based competitive polymerization chemistry.

as well as others30,34 reported that the Scholl dehydrogenative coupling polymerization and Friedel−Crafts polymerization reaction could certainly coexist in the same blending system under the catalysis of anhydrous AlCl3. Herein, we have selected spirobifluorene as the main building block and cyanic chloride as one of the linker units for the construction of porous networks (Scheme 1). First, the electron-deficient 1,3,5triazine acts a excellent acceptor and also a active cross-linker. Second, 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 micropores/ mesopores. The competitive reaction triggered by the blocklinker ratios resulted in a series of spirobifluorene and 1,3,5triazine-based microporous polymers (Sbf-TMP@x:y) with systematically variable backbone formulations (Scheme 2), Scheme 2. Preparation Routes and the Structures of SbfTMPs

where x/y represents the Ph/Cl ratios of the monomers. All samples were obtained in high yields under similar reaction conditions. They are insoluble in water and common organic solvents (such as methanol, acetone, chloroform, N-methylpyrrolidone, and tetrahydrofuran) and stable in 3 M HCl solution or 3 M NaOH solution, implying their excellent chemical stability. Except for Sbf-TMP@4:0, an obvious depletion of intense C−Cl stretching vibration bands at 850 cm−1 in the FT-IR

■

RESULTS AND DISCUSSION Friedel−Crafts and Scholl polymerization reactions have been widely used to prepare NOPs for their advantages including cost-effective and mild synthesis conditions, simple purification, and excellent reproducibilities.29,30 Previously, our group31−33 B

DOI: 10.1021/acs.macromol.7b01857 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. N2 adsorption (solid symbols)/desorption (open symbols) isotherms (a) at 77 K and NLDFT pore size distribution (b) of Sbf-TMPs. High-resolution transmission electron microscopy image of Sbf-TMP@4:2 (inset in panel a). 13

spectra (Figure S1) was observed for the networks compared to the starting cyanuric chloride (CC), which suggests the successful occurrence of the Friedel−Crafts polymerization. It has been demonstrated that the Scholl polymerization reaction could occur under the same experimental conditions according to our previous work.33 Hence, the reduction in the intensity of the 746 cm−1 bands in the FT-IR spectra of Sbf-TMPs correlates to the fact that the aromatic rings of spirobifluorene were coupled with 1,3,5-triazine rings and/or other aromatic rings, indicative of Friedel−Crafts and/or Scholl polymerization reaction. The exhausted intensity of the C−Cl bands was observed due to the occurrence of Friedel−Crafts polymerization reaction. Peaks at about 1700 cm−1 for the FT-IR spectra of Sbf-TMPs are characteristic CN stretching modes for 1,3,5-triazine rings, which are in good accordance with that of the monomer CC. Absorption peaks at 1610 cm−1 were assigned to the skeleton vibration of aromatic rings in the spirobifluorene blocks. These results (Figure S1) implied that the overwhelming majority of C−Cl in cyanuric chloride (CC) was involved in the polymerization reaction. The ratios of peak areas of the C−Cl absorption band (about 850 cm−1) against the CN absorption band (about 1700 cm−1) were probed and are 34.5%, 33.4%, 37.2%, and 32.0% respectively for SbfTMP@4:4, Sbf-TMP@4:3, Sbf-TMP@4:2, and Sbf-TMP@4:1, while for the starting material cyanuric chloride the value reached 264.8%. This indicates that 87.0%, 87.4%, 86.0%, and 87.9% of C−Cl functional group in the 4:4, 4:3, 4:2, and 4:1 Ph/Cl system was eliminated during the polymerization processes, respectively. Considering the onefold conversion reaction of C−Cl functional group and the deactivation of triazine rings, each triazine is expected to be linked with spirobifluorenes, whereas spirobifluorene may be linked to triazines (from the Friedel−Crafts reaction) or spirobifluorenes (by the Scholl reaction). The proportions of aromatic rings from spirobifluorene involved in the Friedel−Crafts coupling are calculated to be 87.0%, 65.6%, 43.0%, and 22.0% in the 4:4, 4:3, 4:2, and 4:1 Ph/Cl system, respectively. It is clear that the Scholl polymerization reaction possesses much inferior activity than the Friedel−Crafts polymerization reaction. Moreover, the mono- and disubstitution of chlorine in the monomer CC could occur at low temperatures (0 and 25 °C),35,36 which is sufficiently lower than the temperature usually required to initiate the Scholl polymerization reaction.30 This further suggests that Friedel−Crafts polymerization reaction is dominant in our blending systems. The structural integrity of Sbf-TMP@4:4 and Sbf-TMP@4:0 was further confirmed by

C CP-MAS NMR spectra (Figures S3 and S4). Signals at 126 ppm are attributed to the unsubstituted phenyl carbons, while the distinct resonances at 139 and 140 ppm belong to substituted phenyl carbons adjacent to other aromatic rings. As shown in the spectrum of Sbf-TMP@4:4, the signal at 148 ppm is assigned to the substituted phenyl carbons bonding to the triazine rings. In addition, the signals of quaternary carbon atoms of spirobifluorene-based units in these polymers were also collected at 65 ppm. The thermal stability of Sbf-TMPs was studied by thermogravimetric analysis (TGA) under a N2 atmosphere (Figure S5). Decomposition of these polymers occurred above 300 °C, and they retained a weight residue above 60% at 800 °C, implying their high thermal stability. High-resolution transmission electron microscopy (HR-TEM) (inset in Figure 1a) demonstrates that the polymer Sbf-TMP@4:2 has an alternately dark and bright microstructure, implying its porous structure. Elemental analysis (Table S1) shows that the nitrogen content of these materials could be systematically adjusted by varying the ratios of Ph/Cl. For example, SbfTMP@4:4 with low Ph/Cl ratio shows the highest nitrogen content of 10.86%, while Sbf-TMP@4:1 with high Ph/Cl ratio exhibits the lowest nitrogen content of 3.01%. The high H content may result from the adsorbed H2O in the polymers. The deviations between the calculated and found values were attributed to the incomplete combustion and trapped adsorbates in the polymer networks.37 Inductively coupled plasma (ICP) analysis shows only 0.08 and 0.11 wt % Al contents for Sbf-TMP@4:2 and Sbf-TMP@4:0, respectively (Table S1). Porosity of the as-made polymers was assessed by the N2 adsorption/desorption isotherms at 77 K (Figure 1a). A steep rise at low relative pressures (P/P0 < 0.01) followed by a slow rise at high relative pressures (P/P0 > 0.01) was observed for the isotherms of Sbf-TMPs, which were attributed to the typical type I isotherm, suggesting their microporosity. The isotherms of Sbf-TMP@4:2 exhibit a hysteresis phenomenon, which was resulted from the mesopores or the interparticulate voids of the samples. The Brunauer−Emmett−Teller (BET) model was used to calculate the surface areas (Table 1). Our results demonstrate that the porosities could be facilely regulated by controlling the ratio of Ph/Cl in the monomers in the competitive chemical reactions system. For Sbf-TMPs, with the increasing ratios of Ph/Cl, which results in the predominant reaction transforms from Friedel−Crafts to Scholl chemistry, the BET surface areas and pore volumes increase from SbfC

DOI: 10.1021/acs.macromol.7b01857 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

distributed at 2−4 nm. The dominant pore sizes increase from Sbf-TMP@4:4 to Sbf-TMP@4:2 and then decrease in the cases of Sbf-TMP@4:1 and Sbf-TMP@4:0, implying that the pore diameters could be well tunable using the competitive reaction system. The variation trend of the porosities in our work is significantly different from that of the reported statistical copolymerization strategy by which a monotonic change tendency was presented.19,38,39 A possible explanation could be the different ratio of Ph/Cl in the monomers mixtures, and the disparate reaction activity between Friedel−Crafts and Scholl chemistry results in a regular change in the polymer structure. Under the Ph/Cl ratio of 4:4, the Friedel−Crafts reaction could be the dominant reaction in the polymerization system, and the resultant polymer Sbf-TMP@4:4 consists of predominant repeating units A and B (Figure S8). With the decreasing Ph/Cl ratio in the monomer mixtures, most of the spirobifluorene afforded the polymers via the Friedel−Crafts routine, and the residual aromatic monomer follows the Scholl polymerization chemistry. Hence, representative repeating units B, C, D, and E (Figure S9) are embedded in the chain of SbfTMP@4:3 and Sbf-TMP@4:2, while for Sbf-TMP@4:1 the most possible repeating units are D, C, E, and F (Figure S10).

Table 1. Pore Parameters of the Polymers sample

Ph/Cl

SBETa

VTotalb

VMicro

dominant pore sizec

Sbf-TMP@4:4 Sbf-TMP@4:3 Sbf-TMP@4:2 Sbf-TMP@4:1 Sbf-TMP@4:0

4:4 4:3 4:2 4:1 4:0

479 596 715 646 635

0.27 0.34 0.41 0.37 0.37

0.16 0.19 0.23 0.20 0.23

0.57, 0.57, 0.57, 0.57, 0.57,

0.99 0.99 0.99, 1.55 0.99, 1.59 1.59

a Brunauer−Emmett−Teller surface area in m2 g−1. bPore volume determined from the N2 isotherm at P/P0 = 0.99 in cm3 g−1. cPore size (in nm) derived from N2 isotherm with the NLDFT approach.

TMP@4:4 (479 m2 g−1, 0.27 cm3 g−1) to Sbf-TMP@4:2 (715 m2 g−1, 0.41 cm3 g−1) and subsequently decrease from SbfTMP@4:2 to Sbf-TMP@4:0 (635 m2 g−1, 0.37 cm3 g−1). SbfTMP@4:2 exhibits the highest BET surface area of 715 m2 g−1 and the largest pore volume of 0.41 cm3 g−1. Pore size distributions (PSDs) of Sbf-TMPs were calculated by nonlocal density functional theory (NLDFT) and are presented in Figure 1b. All the materials show abundant ultramicropores centered at 0.57 nm and some micropores peaked at 0.99 and 1.59 nm (Table 1). Sbf-TMP@4:2 displays spot mesopores

Figure 2. Structures, IMFV diagrams, and values of representative repeating units of Sbf-TMPs. D

DOI: 10.1021/acs.macromol.7b01857 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 3. (a) CO2 adsorption isotherms of Sbf-TMPs. (b) Gas adsorption of CO2 and N2 for the Sbf-TMPs at 273 K and 1 bar.

capture capacities (92−133 mg g−1) for Sbf-TMPs are comparable to those of the known networks39 (84−160 mg g−1) and COFs19 (38−157 mg g−1) synthesized via the statistical copolymerization strategy. To investigate the binding affinity of Sbf-TMPs for CO2, isosteric heats of adsorption (Qst) for CO2 were calculated from the adsorption isotherms at 273 and 298 K using the Clausius− Clapeyron equation (Figure S6). As shown in Table 2, SbfTMP@4:4 displays the highest Qst of 30.7 kJ mol−1, indicating its strong interaction with CO2 guest molecules. The Qst values for these materials are in the range of 27.9−30.7 kJ mol−1 at low loading, which fall in the desirable range for the regenerability of CO2 sorbents according to the literature.43 As depicted in Figure 3c, these materials exhibit a CO2 uptake of 92−133 mg g−1 at 1.0 bar and 273 K. In a sharp contrast, the values for N2 are 3.1−3.9 mg g−1 under the same conditions (Table 2), implying their excellent CO2 /N 2 separation capability. To assess the potential applications of Sbf-TMPs for the selective separation of CO2 in the postcombustion flue gas, the selectivities of CO2 over N2 for the mixture compositions (CO2/N2 = 15/85) were calculated by utilizing the prevalent ideal adsorbed solution theory (IAST) model at 1.0 bar and 273 K (Figure S7). As summarized in Table 2, Sbf-TMPs reveal the IAST CO2/N2 selectivities from 31 to 74. Sbf-TMP@4:2 exhibits the highest CO2/N2 selectivity of 74, which could be comparable to that of other 1,3,5-triazinebased polymers, such as CTF-FL (48),34 NOPs (81),44 NPTNs (45),45 and PCTFs (56),46 further indicating their potential applications of the CO2 separation from gas mixtures. UV−vis absorption spectra of Sbf-TMPs in the solid state were measured at room temperature (Figure 4a). The as-made networks exhibit a broad absorption band centered at around 400 nm, which can be attributed to the π−π* transitions of the conjugated segments in the polymer frameworks, consistent with other reported networks.47 Sbf-TMPs exhibit bright fluorescence under UV light (Figure 4). Upon excitation at 400 nm, they display their maximum emission wavelengths in the range 587−619 nm (Figure 4b). The presence of a slight amount of 1,3,5-triazine bridge acceptor in the framework produced a red-shift in the absorbance onset, which endows Sbf-TMP@4:1 the highest peak absorbance (642 nm). Timecorrelated fluorescence spectra were performed at 400 nm for Sbf-TMPs under solid state to investigate the excited-state dynamics (Figure 5). The average fluorescence lifetimes estimated by a double-exponential fitting are 1.76, 1.43, 1.63, 0.82, and 2.60 ns for Sbf-TMP@4:4, Sbf-TMP@4:3, SbfTMP@4:2, Sbf-TMP@4:1, and Sbf-TMP@4:0, respectively

According to the literature,30 in the case of Sbf-TMP@4:0 only the Scholl reaction occurred, leading to a single repeating unit (F, Figure S11) composed of the neat spirobifluorene moiety. Previous studies have demonstrated that rigid molecules with large internal molecular free volumes (IMFVs)40,41 are beneficial to develop highly porous solids.42 With the increasing Ph/Cl ratios, IMFVs of the representative repeating units of our polymers in this work increase first and then decrease (Figure 2 and Table S2). The porosity of these materials exhibits a similar variation tendency and appears to be mainly determined by the IMFVs of the polymer repeating units, which falls in line with the reported works.23,42 Moreover, the size of the dominant repeating units increases from Sbf-TMP@4:4 to Sbf-TMP@ 4:2, while it decreases from Sbf-TMP@4:2 to Sbf-TMP@4:0 (Figure S12). Previous studies have demonstrated that polymers with longer repeating units tend to deliver higher specific surface areas and larger pore sizes than those with a short repeating unit.13 Hence, with the increasing Ph/Cl ratios, BET surface areas of these polymers increased first and then decreased, reaching a peak value of 715 m2 g−1 for Sbf-TMP@ 4:2. CO2 uptakes of Sbf-TMPs were probed at 273 and 298 K under 1.0 bar (Figure 3a). With the variable ratios of Ph/Cl, the CO2 uptake capacity of Sbf-TMPs (Table 2) increases from 107 Table 2. CO2 and N2 Sorption Data of the Polymers and Carbons sample

CO2 uptakea

CO2 uptakeb

N2 uptakea

Qstc

SCO2/N2d

Sbf-TMP@4:4 Sbf-TMP@4:3 Sbf-TMP@4:2 Sbf-TMP@4:1 Sbf-TMP@4:0

107 124 133 114 92

63 67 76 64 61

3.1 3.3 3.1 3.5 3.9

30.7 28.1 28.6 29.4 27.9

55 62 74 35 31

Gas sorption in mg g−1 at 1 bar/273 K. bGas sorption in mg g−1 at 1 bar/298 K. cIsosteric adsorption enthalpies (Qst) of CO2 in kJ mol−1 calculated by the Clausius−Clapeyron equation at low uptake. d SCO2−N2 is calculated by the IAST model from 85% N2 and 15% CO2, at 1 bar/273 K. a

mg g−1 for Sbf-TMP@4:4 to 133 mg g−1 for Sbf-TMP@4:2 and then decreases to 92 mg g−1 for Sbf-TMP@4:0 at 1.0 bar and 273 K. At an elevated temperature of 298 K, these polymers exhibit a tunable CO2 uptake capacity ranges from 61 to 115 mg g−1. As a whole, the variation trend in CO2 capacity is in good agreement with that of the pore volume, indicating the pore volume plays an crucial role in the CO2 capture. The CO2 E

DOI: 10.1021/acs.macromol.7b01857 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. (top) Photographs under ambient light and (middle) fluorescence pictures of Sbf-TMPs. Absorbance (a) and fluorescence (b) spectra of Sbf-TMPs.

exposure of Sbf-TMP@4:2 to nitrobenzene resulted in the fluorescence quenching, which decreased its original emission peak intensity 0.3-fold (Figure 6c). Notable, the presence of nitro compounds such as nitromethane and 4-fluoronitrobenzene significantly induced the fluorescence quenching for SbfTMP@4:2 while other volatile organic solvents enhanced its fluorescence intensity (Figure 6c,d). For electron-deficient and electron-rich organic compounds, the fluorescence-on and -off mechanisms can be explained based on the electron transfer between the polymer materials and the volatile organic compounds.50,51 As estimated from cyclic voltammetry, the bottom of the conduction band of Sbf-TMP@4:2 is −3.45 eV (Table S4), higher than the lowest unoccupied molecular orbitals (LUMOs) of electron-deficient analytes (such as −3.65 eV for nitrobenzene), which drives electron transfer from the polymer to the nitro compounds and causes fluorescence quenching for the polymer (Figure S13). Alternatively, the electrons of the electron-rich organic compounds with higher LUMO (such as −1.38 eV for benzene) than that of the polymer Sbf-TMP@4:2 flow to the conduction band of SbfTMP@4:2, which enhances the fluorescence intensity. For nonelectron-deficient organic solvents such as ethanol or hexane, the enhancement in emission peak intensity may be attributed to a swelling effect of the solvent onto the polymer network, consistent with other reports on porous polymers.52 A thin layer sample of Sbf-TMP@4:2 was also prepared to conduct the fluorescence sensing experiments. Upon exposure to nitrobenzene vapor for only 2 min, the fluorescence intensity of the thin layer sample of Sbf-TMP@4:2 decreased drastically, and only 55% of the intensity of pristine Sbf-TMP@4:2 remained (Figure S14). Upon exposure to ethyl ether vapor, Sbf-TMP@4:2 exhibits an increasing fluorescence intensity with the increasing exposure time, and the value reaches its maximum at 60 s (Figure S15). While prolonging the exposure time to 70 s, the intensity is alomost the same as that at 60 s. Such quick response demonstrated that our Sbf-TMPs are quite sensitive to volatile organic compounds. Note that the

Figure 5. Time-correlated fluorescence spectroscopy of Sbf-TMPs.

(Table S3). A possible explanation for the fluorescence lifetime trend is that the triazine exerts an important effect on the intermolecular and intramolecular charge transfer. On the one hand, there were strong C−H···N hydrogen bonds in the adjacent triazine-based molecule chains, which promoted their aggregation and resulted in the self-quenching of the excited states. Hence, the polymer Sbf-TMP@4:0 without triazines exhibits longer fluorescence lifetime than the other polymers. On the other hand, the electron-rich spirobifluorene and the electron-deficient triazine rings could form donor−acceptor (D−A) structure, which could facilitate intramolecular charge transfer and the exciton migration.48,49 As a result, the polymers with high triazine content on the whole can retain the excited state for a longer time. The potential application of our Sbf-TMPs as fluorescence sensors for volatile organic compounds was investigated. Typically, upon exposure to 20 different volatile organic solvents, Sbf-TMP@4:2 exhibited significant changes in the fluorescence spectra (Figure 6a,b). For example, after exposure to benzene (Figure 6c), the fluorescence emission peak intensity of Sbf-TMP@4:2 increased 2.2-fold. However, the F

DOI: 10.1021/acs.macromol.7b01857 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 6. (a, b) Fluorescence of Sbf-TMP@4:2 upon exposure to different volatile organic compounds. (c, d) Bar diagram of solvents based on the emission response of Sbf-TMP@4:2.

■

fluorescence intensity can be almost recovered to its original level when the ethyl ether vapor was removed. This indicated that these polymer sensors could be easily regenenerated after exposure to organicr vapor.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01857. Additional details such as materials and instruments, synthesis procedures, FT-IR spectroscopy, NMR spectra, thermogravimetric analysis, elemental analysis, structure analysis, time-correlated fluorescence spectroscopy, redox potential and LUMO of compouds, fluorescence spectroscopy of the thin layer sample (PDF)

CONCLUSIONS

In summary, we have developed a new strategy to adjust the porosities and gas sorption properties of NOPs by regulating the competition of two coexisted chemical reactions (i.e., Friedel−Crafts and Scholl polymerization reaction). The competition was preferentially controlled by adjusting the ratios of knitting agent and building block. The as-made networks delivers an unprecedented variation trend of the porosities, i.e., monotonic change tendency, which is in good accordance with the completion of coexisted coupling chemistries and different “internal molecular free volumes” of repeating units. Such facile competitive system provides a simple way to tailor the element content and the porosity of the resultant networks, such as surface areas, pore volumes, pore diameters, and also photophysical properties. We think this strategy is not restricted to the present competitive system and is widely applicable to many other systems and also provides a model for the systematic study of the functionalization of porous organic polymers. Moreover, the fluorescence-on and -off properties toward aromatic vapors may endow the as-made materials with potential application as fluorescence sensors for volatile organic compounds.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph 86-731-88836961 (G.Y.). ORCID

Shuai Gu: 0000-0001-5322-6976 Guichao Kuang: 0000-0002-6682-8260 Guipeng Yu: 0000-0001-8712-0512 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the financially support from the National Science Foundation of China (Nos. 21674129, 21376272, and 21636010), Hunan Nature Science-Zhuzhou Joint Foundation (2015JJ5015), the Hunan Provincial Science and Technology Plan Project (No. 2016TP1007), and the Open-End Fund for the Valuable and Precision Instrments of Central South G

DOI: 10.1021/acs.macromol.7b01857 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

(17) Zhu, Y.; Wan, S.; Jin, Y.; Zhang, W. Desymmetrized Vertex Design for the Synthesis of Covalent Organic Frameworks with Periodically Heterogeneous Pore Structures. J. Am. Chem. Soc. 2015, 137, 13772−13775. (18) Huang, N.; Krishna, R.; Jiang, D. Tailor-Made Pore Surface Engineering in Covalent Organic Frameworks: Systematic Functionalization for Performance Screening. J. Am. Chem. Soc. 2015, 137, 7079−7082. (19) Pei, C.; Ben, T.; Li, Y.; Qiu, S. Synthesis of copolymerized porous organic frameworks with high gas storage capabilities at both high and low pressures. Chem. Commun. 2014, 50, 6134−6136. (20) Sprick, R. S.; Jiang, J.-X.; Bonillo, B.; Ren, S.; Ratvijitvech, T.; Guiglion, P.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution. J. Am. Chem. Soc. 2015, 137, 3265−3270. (21) Yu, M.; Wang, X.; Yang, X.; Zhao, Y.; Jiang, J.-X. Conjugated microporous copolymer networks with enhanced gas adsorption. Polym. Chem. 2015, 6, 3217−3223. (22) Xiang, Z.; Mercado, R.; Huck, J. M.; Wang, H.; Guo, Z.; Wang, W.; Cao, D.; Smit, M. H. B. Systematic Tuning and Multifunctionalization of Covalent Organic Polymers for Enhanced Carbon Capture. J. Am. Chem. Soc. 2015, 137, 13301−13307. (23) Zhang, Y.; Zhu, Y.; Guo, J.; Gu, S.; Wang, Y.; Fu, Y.; Chen, D.; Lin, Y.; Yu, G.; Pan, C. The role of the internal molecular free volume in defining organic porous copolymer properties: tunable porosity and highly selective CO2 adsorption. Phys. Chem. Chem. Phys. 2016, 18, 11323−11329. (24) Pallavi, P.; Bandyopadhyay, S.; Louis, J.; Deshmukh, A.; Patra, A. A soluble conjugated porous organic polymer: efficient white light emission in solution, nanoparticles, gel and transparent thin film. Chem. Commun. 2017, 53, 1257−1260. (25) Bonillo, B.; Sprick, R. S.; Cooper, A. I. Tuning Photophysical Properties in Conjugated Microporous Polymers by Comonomer Doping Strategies. Chem. Mater. 2016, 28, 3469−3480. (26) Swager, T. M. 50th Anniversary Perspective: Conducting/ Semiconducting Conjugated Polymers. A Personal Perspective on the Past and the Future. Macromolecules 2017, 50, 4867−4886. (27) Li, R.; Ma, B. C.; Huang, W.; Wang, L.; Wang, D.; Lu, H.; Landfester, K.; Zhang, K. A. I. Photocatalytic Regioselective and Stereoselective [2 + 2] Cycloaddition of Styrene Derivatives Using a Heterogeneous Organic Photocatalyst. ACS Catal. 2017, 7, 3097− 3101. (28) Zhou, Y.-B.; Wang, Y.-Q.; Ning, L.-C.; Ding, Z.-C.; Wang, W.L.; Ding, C.-K.; Li, R.-H.; Chen, J.-J.; Lu, X.; Ding, Y.-J.; Zhan, Z.-P. Conjugated Microporous Polymer as Heterogeneous Ligand for Highly Selective Oxidative Heck Reaction. J. Am. Chem. Soc. 2017, 139, 3966−3969. (29) Puthiaraj, P.; Cho, S.-M.; Lee, Y.-R.; Ahn, W.-S. Microporous covalent triazine polymers: efficient Friedel-Crafts synthesis and adsorption/storage of CO2 and CH4. J. Mater. Chem. A 2015, 3, 6792−6797. (30) Li, L.; Ren, H.; Yuan, Y.; Yu, G.; Zhu, G. Construction and adsorption properties of porous aromatic frameworks via AlCl3triggered coupling polymerization. J. Mater. Chem. A 2014, 2, 11091− 11098. (31) Xiong, S.; Fu, X.; Xiang, L.; Yu, G.; Guan, J.; Wang, Z.; Du, Y.; Xiong, X.; Pan, C. Liquid acid-catalysed fabrication of nanoporous 1,3,5-triazine frameworks with efficient and selective CO2 uptake. Polym. Chem. 2014, 5, 3424−3431. (32) Fu, X.; Zhang, Y.; Gu, S.; Zhu, Y.; Yu, G.; Pan, C.; Wang, Z.; Hu, Y. Metal Microporous Aromatic Polymers with Improved Performance for Small Gas Storage. Chem. - Eur. J. 2015, 21, 13357−13363. (33) Gu, S.; He, J.; Zhu, Y.; Wang, Z.; Chen, D.; Yu, G.; Pan, C.; Guan, J.; Tao, K. Facile Carbonization of Microporous Organic Polymers into Hierarchically Porous Carbons Targeted for Effective CO2 Uptake at Low Pressures. ACS Appl. Mater. Interfaces 2016, 8, 18383−18392.

University. The authors thank Prof. Bien Tan of Huazhong University of Science and Technology for helpful discussions on synthesis methods and Dr. Guigang Zhang of Max-PlanckInstitut fur Kolloid und Grenzflachenforschung for useful comments on optical properties.

■

REFERENCES

(1) Pulido, A.; Chen, L.; Kaczorowski, T.; Holden, D.; Little, M. A.; Chong, S. Y.; Slater, B. J.; McMahon, D. P.; Bonillo, B.; Stackhouse, C. J.; Stephenson, A.; Kane, C. M.; Clowes, R.; Hasell, T.; Cooper, A. I.; Day, G. M. Functional materials discovery using energy-structurefunction maps. Nature 2017, 543, 657−664. (2) Yang, J.-S.; Swager, T. M. Fluorescent Porous Polymer Films as TNT Chemosensors: Electronic and Structural Effects. J. Am. Chem. Soc. 1998, 120, 11864−11873. (3) Sekizkardes, A. K.; Altarawneh, S.; Kahveci, Z.; Iṡ lamoğlu, T.; ElKaderi, H. M. Highly Selective CO2 Capture by Triazine-Based BenzimidazoleLinked Polymers. Macromolecules 2014, 47, 8328−8334. (4) Hug, S.; Stegbauer, L.; Oh, H.; Hirscher, M.; Lotsch, V. NitrogenRich Covalent Triazine Frameworks as High-Performance Platforms for Selective Carbon Capture and Storage. Chem. Mater. 2015, 27, 8001−8010. (5) Katekomol, P.; Roeser, J.; Bojdys, M.; Weber, J.; Thomas, A. Covalent Triazine Frameworks Prepared from 1,3,5-Tricyanobenzene. Chem. Mater. 2013, 25, 1542−1548. (6) Soorholtz, M.; Jones, L. C.; Samuelis, D.; Weidenthaler, C.; White, R. J.; Titirici, M.-M.; Cullen, D. A.; Zimmermann, T.; Antonietti, M.; Maier, J.; Palkovits, R.; Chmelka, B. F.; Schüth, F. Local Platinum Environments in a Solid Analogue of the Molecular Periana Catalyst. ACS Catal. 2016, 6, 2332−2340. (7) Hao, L.; Zhang, S.; Liu, R.; Ning, J.; Zhang, G.; Zhi, L. BottomUp Construction of Triazine-Based Frameworks as Metal-Free Electrocatalysts for Oxygen Reduction Reaction. Adv. Mater. 2015, 27, 3190−3195. (8) Rengaraj, A.; Puthiaraj, P.; Haldorai, Y.; Heo, N. Su.; Hwang, S.K.; Han, Y.-K.; Kwon, S.; Ahn, W.-S.; Huh, Y. S. Porous Covalent Triazine Polymer as a Potential Nanocargo for Cancer Therapy and Imaging. ACS Appl. Mater. Interfaces 2016, 8, 8947−8955. (9) Bai, L.; Phua, S. Z. F.; Lim, W. Q.; Jana, A.; Luo, Z.; Tham, H. P.; Zhao, L.; Gao, Q.; Zhao, Y. Nanoscale covalent organic frameworks as smart carriers for drug delivery. Chem. Commun. 2016, 52, 4128−4131. (10) Sang, N.; Zhan, C.; Cao, D. Highly sensitive and selective detection of 2,4,6-trinitrophenol using covalent-organic polymer luminescent probes. J. Mater. Chem. A 2015, 3, 92−96. (11) Jin, Y.; Hu, Y.; Zhang, W. Tessellated multiporous twodimensional covalent organic frameworks. Nat. Rev. Chem. 2017, 1, 0056. (12) Spitler, E. L.; Koo, T. B.; Novotney, L. J.; Colson, W. J.; UribeRomo, J. F.; Gutierrez, D. G.; Clancy, P.; Dichtel, R. W. A 2D Covalent Organic Framework with 4.7-nm Pores and Insight into Its Interlayer Stacking. J. Am. Chem. Soc. 2011, 133, 19416−19421. (13) Chen, Q.; Liu, D.-P.; Zhu, J.-H.; Han, B.-H. Mesoporous Conjugated Polycarbazole with High Porosity via Structure Tuning. Macromolecules 2014, 47, 5926−5931. (14) Keller, N.; Bessinger, D.; Reuter, S.; Calik, M.; Ascherl, L.; Hanusch, F. C.; Auras, F.; Bein, T. Oligothiophene-Bridged Conjugated Covalent Organic Frameworks. J. Am. Chem. Soc. 2017, 139, 8194−8199. (15) Halder, A.; Kandambeth, S.; Biswal, B. P.; Kaur, G.; Roy, N. C.; Addicoat, M.; Salunke, J. K.; Banerjee, S.; Vanka, K.; Heine, T.; Verma, S.; Banerjee, R. Decoding the Morphological Diversity in Two Dimensional Crystalline Porous Polymers by Core Planarity Modulation. Angew. Chem. 2016, 128, 7937−7941. (16) Pang, Z.-F.; Xu, S.-Q.; Zhou, T.-Y.; Liang, R.-R.; Zhan, T.-G.; Zhao, X. Construction of Covalent Organic Frameworks Bearing Three Different Kinds of Pores through the Heterostructural Mixed Linker Strategy. J. Am. Chem. Soc. 2016, 138, 4710−4713. H

DOI: 10.1021/acs.macromol.7b01857 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (34) Dey, S.; Bhunia, A.; Esquivel, D.; Janiak, C. Covalent triazinebased frameworks (CTFs) from triptycene and fluorene motifs for CO2 adsorption. J. Mater. Chem. A 2016, 4, 6259−6263. (35) Blotny, G. Recent applications of 2,4,6-trichloro-1,3,5-triazine and its derivatives in organic synthesis. Tetrahedron 2006, 62, 9507− 9522. (36) Patel, H. A.; Karadas, F.; Canlier, A.; Park, J.; Deniz, E.; Jung, Y.; Atilhan, M.; Yavuz, C. T. High capacity carbon dioxide adsorption by inexpensive covalent organic polymers. J. Mater. Chem. 2012, 22, 8431−8437. (37) Bhunia, A.; Vasylyeva, V.; Janiak, C. From a supramolecular tetranitrile to a porous covalent triazine-based framework with high gas uptake capacities. Chem. Commun. 2013, 49, 3961−3963. (38) Jiang, J.-X.; Su, F.; Trewin, A.; Wood, C. D.; Niu, H.; Jones, J. T. A.; Khimyak, Y. Z.; Cooper, A. I. Synthetic Control of the Pore Dimension and Surface Area in Conjugated Microporous Polymer and Copolymer Networks. J. Am. Chem. Soc. 2008, 130, 7710−7720. (39) Yao, S.; Yang, X.; Yu, M.; Zhang, Y.; Jiang, J.-X. High surface area hypercrosslinked microporous organic polymer networks based on tetraphenylethylene for CO2 capture. J. Mater. Chem. A 2014, 2, 8054−8059. (40) Tsui, N. T.; Paraskos, A. J.; Torun, L.; Swager, T. M.; Thomas, E. L. Minimization of Internal Molecular Free Volume: A Mechanism for the Simultaneous Enhancement of Polymer Stiffness, Strength, and Ductility. Macromolecules 2006, 39, 3350−3358. (41) Swager, T. M. Iptycenes in the Design of High Performance Polymers. Acc. Chem. Res. 2008, 41, 1181−1189. (42) Chong, H. J.; Ardakani, J. S.; Smith, J. K.; MacLachlan, J. M. Triptycene-Based Metal Salphens-Exploiting Intrinsic Molecular Porosity for Gas Storage. Chem. - Eur. J. 2009, 15, 11824−11828. (43) Wilmer, C. E.; Farha, O. K.; Bae, Y.-S.; Hupp, J. T.; Snurr, R. Q. Structure−property relationships of porous materials for carbon dioxide separation and capture. Energy Environ. Sci. 2012, 5, 9849− 9856. (44) Liu, Y.; Wu, S.; Wang, G.; Yu, G.; Guan, J.; Pan, C.; Wang, Z. Control of porosity of novel carbazole-modified polytriazine frameworks for highly selective separation of CO2-N2. J. Mater. Chem. A 2014, 2, 7795−7801. (45) Wu, S.; Liu, Y.; Yu, G.; Guan, J.; Pan, C.; Du, Y.; Xiong, X.; Wang, Z. Facile Preparation of Dibenzoheterocycle-Functional Nanoporous Polymeric Networks with High Gas Uptake Capacities. Macromolecules 2014, 47, 2875−2882. (46) Gu, C.; Liu, D.; Huang, W.; Liu, J.; Yang, R. Synthesis of covalent triazine-based frameworks with high CO2 adsorption and selectivity. Polym. Chem. 2015, 6, 7410−7417. (47) Efrem, A.; Wang, K.; Amaniampong, P. N.; Yang, C.; Gupta, S.; Bohra, H.; Mushrif, S. H.; Wang, M. Direct arylation polymerization towards narrow bandgap conjugated microporous polymers with hierarchical porosity. Polym. Chem. 2016, 7, 4862−4866. (48) Xu, Y.; Nagai, A.; Jiang, D. Core-shell conjugated microporous polymers: a new strategy for exploring color-tunable and -controllable light emissions. Chem. Commun. 2013, 49, 1591−1593. (49) Dai, P.; Yang, L.; Liang, M.; Dong, H.; Wang, P.; Zhang, C.; Sun, Z.; Xue, S. Influence of the Terminal Electron Donor in D-D-π-A Organic Dye-Sensitized Solar Cells: Dithieno[3,2-b:2′,3′-d]pyrrole versus Bis(amine). ACS Appl. Mater. Interfaces 2015, 7, 22436−22447. (50) Pramanik, S.; Zheng, C.; Zhang, X.; Emge, T. J.; Li, J. New Microporous Metal-Organic Framework Demonstrating Unique Selectivity for Detection of High Explosives and Aromatic Compounds. J. Am. Chem. Soc. 2011, 133, 4153−4155. (51) Liu, X.; Xu, Y.; Jiang, D. 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. 2012, 134, 8738−8741. (52) Bonillo, B.; Sprick, R. S.; Cooper, A. I. Tuning Photophysical Properties in Conjugated Microporous Polymers by Comonomer Doping Strategies. Chem. Mater. 2016, 28, 3469−3480.

I

DOI: 10.1021/acs.macromol.7b01857 Macromolecules XXXX, XXX, XXX−XXX