Gelation of Metalloporphyrin-Based Conjugated Microporous

Oct 20, 2014 - By following the same synthesis route, for the first time, the interpenetrating polymer network organogels were prepared by forming two...
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Gelation of Metalloporphyrin-Based Conjugated Microporous Polymers by Oxidative Homocoupling of Terminal Alkynes Keyi Wu, Jia Guo,* and Changchun Wang State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, P. R. China S Supporting Information *

ABSTRACT: A Pd(II)/Cu(I) cocatalyzed homocoupling reaction of terminal alkynes to diynes was used to synthesize conjugated polymer organogels with tetragonal topological frameworks consisting of Zn-porphyrin units as nodes and diynes as struts. This material appears fibrous with a micrometer length, possesses outstanding elastic properties, and could be organized into desired modules. Upon drying, the transformed xerogels afford superior thermal stability and microporosity, implying that they are supported by conjugated microporous polymer (CMP) skeletons at the molecular level. The microporosity of CMP-structured xerogels could be adjusted by varying the monomer concentrations, reaction temperatures and solvent species. The notable narrowed pore size distribution is achieved under optimal conditions, which results in CMP-supported xerogels outperforming the most reported CMPs, although the networks are still amorphous in nature. By following the same synthesis route, for the first time, the interpenetrating polymer network organogels were prepared by forming two CMP components sequentially in a temperature-controlled manner and in one pot. This provides an unprecedented combination of multiple interwoven CMP modules whose functions could be assembled synergistically for prospective broader applications.

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INTRODUCTION Conjugated microporous polymers (CMPs), which are a class of amorphous porous organic polymers, have gained continuous appealing attractiveness because of the self-supporting microporous networks in their structure and their fully conjugated π-electron skeletons. Tremendous efforts have been devoted to design novel CMPs with diverse applicability.1−10 As reported in recent years, CMPs promise great potential in a wide spectrum of fields such as gas storage/ separation,11−19 light emission,20−25 heterogeneous catalysis,26−34 supercapacitance,35,36 chemical sensing,36−39 and so forth. Thus, CMPs are a subject of continued interest and encourage progress in the field of organic porous materials. Since Cooper et al. first reported on the poly(aryleneethynylene) (PAE) CMPs in 2007,1 the challenge has remained to generate soluble CMPs that can be assembled into colloids, films, coatings or other prototypes for state-of-the-art utilization. Thus, the development of a synthetic technique has been pursued that enables CMPs to be shaped directly into desired morphologies at the microscopic level. For instance, spherical CMP nanoparticles were synthesized using the oil-inwater miniemulsion system in our group.30,39,40 In contrast to the solution refluxing method for bulky CMPs, the template technique is conducive to the confined evolution of CMPs and is amenable to the homogeneous modification of surface © 2014 American Chemical Society

chemistry to improve CMP dispersibility in common solvents. It is thus feasible that the CMP film could be fabricated via a self-assembly or spin-coating technique, without additive addition. An inevitable issue that is encountered is that highsurface-area CMPs are often subjected to harsh reaction conditions in polar solvents, while current emulsion conditions are mild. This restrains the evolution of CMP networks and reduces their porosity. In this context, Cooper’s group adopted the hyperbranching synthetic strategy to constitute soluble CMPs by the incorporation of alkyl end groups to limit molecular weight and enhance solubility.41 Although discrete hyperbranched chains were formed to provide a combination of porosity and processability, the resultant CMPs resemble dendrimer structures without extended networks and interpenetrating pore channels, and result in the unpredictability of porosity, which depends considerably on processing conditions. Nonconjugated polyurethane networks prepared by a sol−gel process were of equal microporosity and could be processed into elastic films,42 but with a large limitation on pore size as a result of the flexible carbamate linkage that provides the powerful H-bonding interaction between proximate chains. A Received: August 22, 2014 Revised: October 19, 2014 Published: October 20, 2014 6241

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For the fabrication of freeze-drying xerogels, the organogel, which was synthesized as described above, soaked in 50 mL of 1,4-dioxane for 12 h to replace original solvent, THF. After that, the organogel was quickly frozen by liquid N2 and freeze-dried under −56 °C in a freezedryer. For the rapid gelation of ZnTEPP, 5,10,15,20-Tetrakis(4-(ethynyl)phenyl)porphyrin-zinc(II) (38.7 mg, 0.05 mmol), bis(triphenylphosphine)palladium(II) dichloride (35.1 mg, 0.05 mmol), and copper iodide (9.5 mg, 0.05 mmol) were added into the purified THF (2 mL). After the reagents were all dissolved under ultrasonication (SONICS-750 with a tip operated at 75 W), TEA (420 μL) was added to start the reaction. The gel was formed in 20 s. Synthesis of PAE-Based Xerogels. 1,3,5-triethynylbenzene (60.1 mg, 0.4 mmol), bis(triphenylphosphine)palladium(II) dichloride (21.0 mg, 0.03 mmol), and copper iodide (5.7 mg, 0.03 mmol) were added into the purified THF (16 mL). After the reagents were all dissolved under ultrasonication (SONICS-750 with a tip operated at 75 W), TEA (250 μL) was added to start the reaction. The reaction was kept at 60 °C for 36 h without any stirring. The resultant organogel was submerged into another 40 mL of purified THF for 24 h. Finally, the organogel was dried in the oven at 80 °C with air flowing. Synthesis of ZnP-PAE IPN Xerogels. 5,10,15,20-Tetrakis(4(ethynyl)phenyl)porphyrin-zinc(II) (46.5 mg, 0.06 mmol), 1,3,5triethynylbenzene (4.5 mg, 0.03 mmol), bis(triphenylphosphine)palladium(II) dichloride (5.8 mg, 0.00825 mmol), and copper iodide (1.6 mg, 0.00825 mmol) were added into the purified THF (3.6 mL). After the reagents were all dissolved, TEA (70 μL) was added to start the reaction. The reaction was hold at room temperature for 24 h and then heated to 60 °C for another 24 h without any stirring. The resultant organogel was submerged into 15 mL purified THF for 24 h. Finally, it was dried in the oven at 80 °C with air flowing. Characterizations. 1H NMR spectra were recorded on a Bruker DRX 400 (400 MHz) spectrometer. Chemical shifts were reported in ppm downfield from tetramethylsilane (TMS) as the internal standard (Bruker Germany). UV−vis spectra of ZnP-based organogels were measured in a quartz cell of 0.1 cm-path length on a Shimadzu UV3600 PC spectrometer (Shimadzu Japan). For comparison, CHCl3 solution of porphyrin was measured in a quartz cell of 1 cm-path length. Matrix Assisted Laser Desorption Ionization-Time of Flight Mass (MALDI-TOF MS) spectra were performed on a 5800 spectrometer in reflector mode using dithranol as a matrix (AB SCIEX USA). Solid-state 13C CP/MAS NMR measurements were performed on a Bruker 400 MHz NMR spectrometer at a MAS rate of 12 kHz and a CP contact time of 2 ms (Bruker Germany). Elemental analyses were carried out on a VARIO EL3 analyzer (Elementar Germany). FE SEM was carried on an S-4800 scanning electron microscope at an acceleration voltage of 200 kV (Hitachi, Japan). SEM was carried on a VEGA TS 5136MM scanning electron microscope at an acceleration voltage of 20 kV (TESCAN, Czech). The energydispersive X-ray spectroscopy (EDS) was performed on the same scanning electron microscope with a QUANTAX 400 energydispersive spectrometer (Bruker, Germany). Thermogravimetric analysis (TGA) data was obtained with a Pyris 1 by heating the sample from 100 to 800 °C at a heating rate of 20 °C min−1 (PerkinElmer, USA). Nitrogen sorption measurements were collected at 77 K by an ASAP2020 volumetric adsorption analyzer (Micromeritics, USA). The samples were treated at 130 °C for 24 h before measurement. Powder X-ray diffraction (PXRD) patterns were collected on an X-ray diffraction spectrometer (Bruker D8 Advance, Germany) with Cu Kα radiation at λ = 0.154 nm operating at 40 kV and 40 mA.

new strategy has been developed to take into account the direct fabrication of CMP-based device blocks. For instance, conglutination of powdered CMPs with polymer adhesives is ease of operation,43 but π-electron delocalization and mobility is blocked over the conjugated skeleton within the entire matter. Hence, some new synthetic techniques such as surface initiation,44 layer-by-layer cross-linking,45 and electrochemical polymerization46,25 have been used in the synthesis of CMP membranes on the matrix, but product yields and resultant porosities are unsatisfactory. The high-throughput synthesis of solvated CMP monoliths with moldable prototypes on the macroscopic scale is of paramount importance, in particular to engineer CMPs as functional modules into electrical or optical devices. In this study, we used an oxidative homocoupling of terminal alkyne groups 4 7 to polymerize 5,10,15,20-tetra(4ethynylphenyl)porphyrin-Zn(II) (ZnTEPP) into the nodes of topological frameworks with cocatalysis of a Pd(II)/Cu(I) system at room temperature in air. Typical THF organogels could be formed in a controllable manner, and their microscopic morphologies, growth kinetics, and elastic properties were studied to prove the performance of the chemical organogels. As we know, the reported CMPs have often been synthesized under severe reaction conditions via the C−C cross-coupling reactions, e.g. Suzuki,6,41 Sonogashira,1,3 and Yamamoto reactions,20,21 and the formed products precipitated out of solvents after reaction. It is therefore exciting to observe the gelation of conjugated polymer networks by oxidative homocoupling in an open vessel at ambient temperature, as this promises great potential for industrial production at a large scale. Upon heat drying, ZnP xerogels with given shapes and shrunken dimensions were obtained. This not only entails organogel formability, but also allows for an extended πconjugation network and thus microporosity because of the intrinsic CMP networks. Systematic studies were then conducted to tune the micropore properties. We have envisaged the formation of an unprecedented interpenetrating network gel comprising two CMP components by sequential polymerization in one pot.

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EXPERIMENTAL SECTION

Materials. Unless otherwise stated, all reagents were purchased from commercial suppliers and used without further purification. Double-distilled water was used for all the experiments. THF was freshly distilled from K/benzophenone ketyl under nitrogen. Copper iodide and tetrabutylammonium fluoride (TBAF) were purchased from Adamas (Swiss). Trimethylsilylacetylene, bis(triphenylphosphine)palladium(II) dichloride were purchased from Energy (China). All the solvents used were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Synthesis of ZnP-Based Xerogels. 5,10,15,20-Tetrakis(4(ethynyl)phenyl)porphyrin-zinc(II) (38.7 mg, 0.05 mmol), bis(triphenylphosphine)palladium(II) dichloride (3.5 mg, 0.005 mmol), and copper iodide (1.0 mg, 0.005 mmol) were added into the purified THF (2 mL). After the reagents were all dissolved under ultrasonication (SONICS-750 with a tip operated at 75 W), TEA (42 μL) was added to start the reaction. The reaction proceeded at room temperature for 48 h without any stirring. The resultant organogel was submerged into another 10 mL purified THF for 24 h to remove the catalyst residues and the trace amount of unreacted monomers. Finally, the organogel was placed in the oven at 80 °C with air flowing to form xerogels. Since the catalyst residues were very hard to wash out of a gel completely, the product yield was somewhat beyond 100% (usually 102−105%).

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RESULTS AND DISCUSSION Synthesis of ZnP-Based Organogels. Metalloporphyrins tend to align into multiple dimensional geometries by π−π stacking and/or van der Waals interactions. Interest has therefore been generated with regards the creation of novel supramolecular architectures such as nanowires, discotic liquid crystals, helical ribbons, and sheet-like platelets.48−55 These 6242

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Figure 1. (a) Synthesis of ZnII-porphyrin-based (ZnP) framework by dimerization reaction of terminal alkyne groups at room temperature in air. SEM images of (b) 5,10,15,20-tetra(4-ethynylphenyl) porphyrin-ZnII and (c) fibrous ZnP framework xerogel. The insets in b and c show the THF solution of (A) reaction mixtures and the (B) ZnP organogel after the alkyne homocoupling reaction, respectively.

plates (see SEM image, Figure 1c). According to the reaction vessel, the different shaped organogels were removed in integrated form, and, in turn, original shapes could be fixed in xerogel albeit with largely reduced sizes (see Figure S1 in the Supporting Information). The gelation of the ZnP-based network was studied using a variety of experimental conditions including solvent species, monomer concentrations, and reaction temperatures (see Table S1 in the Supporting Information). Polar solvents, lower temperatures, and concentrated precursors seemingly all had a pronounced effect on gelation in the catalytic polymerization of ZnTEPP. Based on the results, we gained insight into the gel forming dynamics as the reaction proceeded. Figure 2 shows the time-dependent change in transmittance detected at 700 nm. The transmittance is relatively constant initially, decreases rapidly, and then levels off. This means that when the ZnTEPP concentration was 25 mM in THF, the solution turbidity had a pronounced increase around 2000 min. This implies that the polymerization reaches the gel point and the gelation occurs. We found that the ZnP organogel could form instantly within 20 s by adding 10 times the amounts of catalyst and base.

superstructures are assembled or covalently linked with one another resulting in the formation of a soft organogel with three-dimensional networks. Through direct polymerization, however, polymeric metalloporphyrin has rarely been found to form organogel thus far. Herein, we proved that the dimerization of terminal alkyne groups of ZnTEPP at ambient temperature in air could result in an extended topological ZnPbased network (Figure 1a). The building blocks might not be perfectly connected by diyne links. However, in contrast to the cross-coupling of terminal halides and alkynes as used for synthesis of known CMPs,56 the oxidative dimerization of terminal alkynes is an extremely efficient homocoupling reaction, wherein the common side products are enynes produced in trace amounts.47 Prior to the terminal alkyne dimerization, one can see that the ZnTEPP monomers tend to crystallize out of THF and display sheet-like plates, mostly because of the strong π−π interaction-induced assembly (Figure 1b). Using the standard Pd(II)/Cu(I) catalysis system, the oxidative coupling of terminal alkyne groups proceeded slowly without any stirring and the organogel was formed within ∼48 h. The freeze-drying xerogel was interwoven with fibrous nanowire, without showing any crystalline monomer 6243

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To investigate the mechanical properties of organogels, we determined the rigidity and flow behaviors of the native organogels by measuring their rheological parameters. In a typical frequency sweep experiment, the variation of storage (G′) and loss (G″) moduli were monitored as a function of applied frequency under a constant strain (1%) at room temperature for the different organogels with given concentrations. G′ represents the ability of the deformed material to restore its original geometry, and G″ symbolizes the tendency of a material to flow. For an ideal liquid, G′ = 0, and for an ideal solid, G″ = 0. For viscoelastic materials like gels, G′ is greater than G″, which suggests the dominant elastic behavior of the system. It is observed from Figure 4 that with an increase in Figure 2. Transmittance change against reaction time as 25 mM of ZnTEPP was used for the formation of ZnP organogels.

To confirm the formation of fewer structural defects in the network, the conversion of ZnTEPP was estimated in real time by 1H NMR spectroscopy. Since the homocoupling of terminal alkyne groups leads to a loss of protons, the condensation progress in the stock solutions could be monitored by estimating the signal intensity of the protons from the unreacted ZnTEPP in THF-d8. The signal of the terminal protons of ZnTEPP (δ = 3.79 ppm) disappears gradually relative to that of the solvent peak (δ = 2.47 ppm) as a reference while the reaction is in progress (see Figure S2 in the Supporting Information). In Figure 3, the conversions, Figure 4. Storage (G′) and loss moduli (G″) of two THF gels prepared using 25 and 75 mM of ZnTEPP as a function of oscillatory shear frequency.

applied frequency, G′ and G″ remain almost invariant. For the two cases that are the different cross-linking gels prepared using 25 mM and 75 mM ZnTEPP monomers, G′ > G″, which indicates solid-like behavior. The stable and wide plateaus for G′ and G″ are observed in two cases, implying that the formed gel is a stable three-dimensional network. The cross-linking densities of the two gels are different dependent on the ZnTEPP monomer concentration used. The high cross-linking gel results in a G′ value as much as ∼10 times that of the low cross-linking gel. This suggests that the organogel network with higher density is more rigid. The difference in these two moduli (G′/G″ = DG) is considered to be a measure of the dominance of the elastic behavior of the material over its viscous properties. The DG value is higher for the low-density gel (13.34) than for the high-density gel (6.60) at 100 rad/s. This indicates that the elastic property is more dominant than the viscous nature in the low-density organogel relative to the high one. Owing to the relatively high solid content of our ZnPbased organogels, it is also observed that the moduli of ZnPbased organogels are 2 orders of magnitude larger than those of other reported organogels formed by small molecular porphyrins52 and diblock copolymers.57 ZnP-Based CMPs Transformed from the Corresponding Organogels. CMP has a typical microporous structure. Analogous to the characteristics of crystalline metal−organic frameworks (MOFs)58−60 and covalent organic frameworks (COFs),61,62 the micropore size distribution can be controlled finely in the CMP networks by the rigid node-strut topology.3 The high surface area of the CMP is shown because of the three-dimensionally extended networks with self-supporting rigid skeletons. Compared with the CMP stiffness, the

Figure 3. Conversion vs reaction time during the gel formation. The conversion was estimated by the 1H NMR integration of the lines at δ = 3.79 ppm with respect to δ = 2.47 ppm.

estimated by 1H NMR integration at δ = 3.79 ppm, are plotted as a function of reaction time. When 25 mM of ZnTEPP was used, the conversion increased approximately in two stages: in the initial stage there was a rapid increase to ∼70%, and in the second stage a plateau of ∼90% was reached even after stirring for a prolonged period. When the ZnTEPP concentration was reduced to 7.5 mM, the gelation was shortened and the reaction could be completed beyond the gel point (50%), and reached ∼100% conversion. These results prove that almost all of the ZnTEPP molecules are imposed covalently into the topological networks as nodes, and formation of extended three-dimensional networks will be one of the main premises for the transformation of gel to CMP with inherent micropores and high surface areas. 6244

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= 120.2, 130.8, 143.2, and 149.2 ppm, which can be assigned to Cpyrrole, Car-H, Car-C, and Car-Cporphyrin sites, respectively. Compared to the solid-state 13C NMR spectrum of the ZnTEPP monomer (see Figure S3 in the Supporting Information), the lines at 80 ppm ascribed to the terminal alkyne groups are very low. This indicates that the skeleton conformation is easily accessible to the free ending groups, thereby allowing for more efficient linking for the resulting complete network. Thermogravimetric analysis (TGA) revealed the exceptional thermal stability of ZnP-based xerogels, which are relatively stable below 500 °C in air. This is in contrast to the ZnTEPP molecules that are decomposed completely at 450 °C (Figure 7).

covalently cross-linking networks in earlier known xerogels are soft. Thus, they shrunk extensively upon removal of the solvent molecules, giving rise to a rearrangement of polymeric skeletons and loss of interstices within the microscopic structure. Even though the supramolecular architectures remain, only macroporous structures and relatively low surface areas may exist. In our case, the fully conjugated organogel consists of rigid and dense networks composed of metalloporphyrin units as nodes and diynes as struts, all of which are connected topologically in a highly efficient polymerization. It is therefore likely that the xerogel will be featured distinctly with the CMP-like microscopic porous architectures as well as control of desired monolith shapes. The as-prepared organogels were dried at 80 °C for 24 h, and the obtained xerogels were characterized by a series of measurements to determine their CMP structures. As displayed in Figure 5, the macroporous structures hedged by the crossed

Figure 7. TGA plots of ZnTEPP monomer (dash line) and ZnP-based xerogel (solid line). Figure 5. FE SEM image of ZnP-based xerogel drying at 80 °C for 24 h.

Because a wide scope of conditions has been adapted to form a gel, their effect on microporous structure was studied as the organogels dried at 80 °C, and the optimized microporosity was determined as the reaction temperatures, monomer concentrations or solvent species were varied. Through measurements of the N2 isotherm sorption at 77 K, the results, which include surface areas and pore volumes, were compiled in Table 1. The Brunauer−Emmett−Teller (BET) model was applied to calculate the apparent surface area (SBET) and pore volume (Vtot), and de Boer statistical thickness (t-plot) analysis was used to evaluate the specific surface areas contributed by the micro- (Smicro) and external (Sext) pores. As the feed concentration of ZnTEPP monomer was increased from 7.5 to 75 mM under otherwise identical conditions, the xerogel surface area was enhanced until a maximum value of 906.43 m2 g−1. Thereafter, the surface area declined somewhat but was still in the range of 800−900 m2 g−1. The dominant surface areas are attributed to the micropores, and their changing trend is almost synchronized with the total surface areas and pore volumes. We reason that the xerogel molecular structure is featured with a CMP-like three-dimensional topological framework, which is responsible for the intrinsic voids and channels. The giant microscopic interfaces created by the wholly conjugated network skeletons are permanent and steady without interference of solvent molecule removal. However, the low monomer concentrations affect the topological network density so that it may collapse partially upon elimination of the filling solvent molecules. Subsequently, we varied the reaction temperatures and solvent species to examine whether the CMP

nanofibers disappear in the drying xerogel, and flat and smooth surfaces are observed. Although the captured THF solvents were extracted out of the organogel, the xerogel could still retain its original shape but with a largely contracted size. The elaborate structures were characterized at the molecular level by solid-state 13C cross-polarization magic angle spinning (CP/ MAS) NMR in Figure 6. The xerogel produces four signals at δ

Figure 6. Solid-state 13C CP/MAS NMR spectrum of ZnP-based xerogels at a CP contact time of 2 ms and a MAS rate of 12 kHz. Asterisks denote spinning sidebands. 6245

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Table 1. Porosity of a Series of ZnP-Based Xerogels Prepared under Different Conditionsa entry

ZnTEPP (mM)

T (°C)

solvent

C1 C2 C3 C4 C5 T6 T7 S8 S9 S10 S11 S12

75 25 12.5 10 7.5 25 25 25 25 25 25 25

25 25 25 25 25 −10 60 25 25 25 25 25

THF THF THF THF THF THF THF toluene dioxane acetone DMF DMSO

SBET (m2 g−1)b 799.28 874.61 906.43 479.96 295.24 757.34 323.13 590.04 296.44 271.18 170.45 583.35

(1039.33) (957.35) (1120.01) (586.25) (304.41) (862.24) (420.49) (797.30) (364.88) (436.76) (221.58) (807.12)

Smicro (m2 g−1)c

Sext (m2 g−1)c

Vtot (cm3 g−1)d

716.07 817.48 801.02 424.78 292.79 711.52 279.65 515.68 271.68 204.33 153.00 486.74

83.21 57.14 105.41 55.17 2.45 45.82 43.48 74.36 24.76 66.85 17.45 96.61

0.3860 0.3736 0.4346 0.2245 0.1075 0.3252 0.1523 0.3131 0.1478 0.1648 0.0834 0.3771

PdCl2(PPh3)2, CuI, Et3N, 2 days; drying at 80 °C bSurface area is calculated from the N2 adsorption isotherm using the Brunauer−Emmett−Teller method and the value in parentheses is the Langmuir surface area. cThe external and micropore surface areas are obtained using the t-plot method based on the Halsey thickness equation. dTotal pore volume at P/P0 = 0.99. a

structure was formed under a wide scope of reaction conditions. It is surprising that gelation was observed to occur at −10 °C and that the resultant xerogel also had a high surface area. The effect of the solvents is also notable; some lead to the impaired porosity. The primary reason may lie in that the polymer growth in an appropriate solvent is inclined to generate a greater degree of overall conformational freedom, which allows for the efficient linking along the threedimensional directions. The drying temperature has a remarkable influence on the pore parameters (see Table S2 in the Supporting Information), which as far as we are concerned, is caused by the extent of segment stacking. According to the SEM image of the freeze-drying xerogel (Figure 1c), the organogel segments do not stack tightly and the majority of the micropores are filled by the adsorbed solvent molecules. Although the rough ZnP framework in an organogel remained in freeze-drying, the as-produced xerogel could not yet form more micropores. When the organogel was dried at 25 °C, all solvent molecules could be removed to encourage segment stacking. As a result, the micropore surface area was increased from 92.33 to 225.64 m2 g−1. When the drying temperature increased to 80 °C, the thermal segment motion in the organogel was enhanced sufficiently to perfect the segment stacking and consequently, the micropore surface area increased to 817.48 m2 g−1. In comparison with the significant increase in micropore surface area because of a change of the drying methods and temperature, the increase in external surface area was tiny and negligible. We therefore demonstrated the feasibility and rationale of a gel-to-CMP transformation and found the optimized conditions for the assynthesized xerogels that afford excellent microporosity because of the CMP-like topological networks that exist at the molecular level. We studied the pore size distribution of xerogels that were synthesized in THF under favorable reaction conditions. Of those members, we compared Xerogel-C2, -C5, and -T6 to evaluate the effect of temperature and monomer concentration on pore properties (pore type, size, and size distribution), respectively. Figure 8a shows the N2 adsorption and desorption isotherms of Xerogel-C2, -C5, and -T6, all of which give rise to typical type I gas sorption isotherms, in accordance with the IUPAC classifications, which is indicative of a microporous character. The N2 sorption isotherm shapes are almost similar, and show a significant hysteresis at low relative pressures. This

Figure 8. (a) N2 sorption isotherms and (b) NLDFT pore-size distribution histograms of Xerogel-C2, -C5, and -T6, respectively.

probably results from a remarkable swelling of the ZnP-based xerogels upon gas adsorption. This is frequently observed in a soft network as increasing numbers of adsorbate molecules condense in the pores. The pore size distributions were calculated by the nonlocal density functional theory (NLDFT) model and their populations were found to be centered at ∼0.8 nm and ∼1.3 nm for all three xerogels (Figure 8b). Interestingly, the 6246

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Scheme 1. (a) Synthesis of PAE-CMP-Structured Xerogels by the Dimerization of Terminal Alkyne groups of 1,3,5Triethynylbenzene under the Modified Conditions; (b) One-Pot Protocol to Preparing the Dual-CMP-Knitting Xerogels with the IPN Structure

populations of the pore size distributions are dependent on the gel-forming conditions. Xerogel-C2 has a dual distribution of micropore sizes, which are equally populated, whereas XerogelT6 and -C5 have a predominant pore size distribution at 1.3 nm. This is almost comparable to the controlled pore sizes in crystalline COFs and MOFs, although their powder X-ray diffraction (PXRD) pattern still reveals an amorphous nature in structure (see Figure S4 in the Supporting Information). To the best of our knowledge, most known CMPs have rather broad pore size distributions, which spread nearly from micropore to large mesopore. The accurate control of pore properties is hard to achieve as a result of the amorphous framework structure that involves a great deal of rotation or bending skeletons to cause a flawed pore size distribution. The closest studies toward the synthetic control of CMP pores and surface areas have been reported by the Cooper group.3 They presented an ingenious route to tune the cumulative pore volume and surface area by varying the monomer strut lengths or by conducting statistical copolymerization of monomer struts with differing lengths. However, the broad pore size distributions remain unchanged, and to date, there is no powerful way to obtain narrowly distributed pores in the amorphous structure. In contrast to the earlier reports, our results prove unprecedentedly that a monodisperse micropore size could be formed in the xerogel by the homocoupling of terminal alkynes with appropriate reaction conditions. We speculate that the harsh conditions,

such as the concentrated solutions and elevated temperatures, would yield an open network structure with freely outstretching hyperbranched chains so that ultramicropores (∼0.5−1 nm) in the xerogel are created by entangling or interpenetrating chains. However, we also note that because structural changes in measurements are not considered in the evaluation of pore size, typical methods like the NLDFT model may not be suited to determine the reliable or comparable pore size distribution. Thus, a more detailed analysis for the dynamic and structural heterogeneities of these materials is in progress using wideangle X-ray scattering methodology. Dual-CMP-Knitting Xerogels with IPN Structure. The combination of multiple components within the CMPs has been performed by the statistical copolymerization of relative compositions with differing molecular size. This could allow for the continuous control over CMP microporosity such as surface area, cumulative pore volume and pore size. Moreover, because of the excellent porosity and versatile applicability of CMP, fusion of two or more CMP networks would be so attractive that they enable the creation of synergistic properties on multiple CMP ingredients and can perform multifunctions for wider application. However, the copolymerization of more molecular species often results in random chemical compositions and amorphous networks without an ability to “weave” CMP into the desired architecture. Although the physical blending of component CMP networks is thought to be one 6247

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solution, electronic communication would be blocked on the molecular scale and thus the cooperative effect provided by the CMP components is impaired or eliminated. We envisioned the construction of a CMP-interwoven interpenetrating polymer network (IPN) comprising two or more CMP networks that are at least partially interlaced on a molecular scale but are not covalently bonded to each other. From a synthetic point of view, the reported conventional harsh conditions for the synthesis of CMPs may not be suited to form IPN structures since multiple components may be polymerized into one complicated framework. Also, we are aware that growing CMPs precipitate out of the reaction mixture. It is unlikely that a sequential IPN will form by a process wherein the second component CMP is formed following the formation of the first CMP. Nevertheless, the gel-to-CMP transition method is remarkably advantageous from a technical point of view. As shown in Scheme 1a, we found that PAE could also be gelated by the dimerization of the terminal alkyne groups of 1,3,5triethynylbenzene (TEB) (see Table S3 in the Supporting Information), but that elevated temperature was required to accelerate gelation of the PAE networks. If the reaction were conducted at 25 °C, PAE gelation would be completed within 1 week or more depending on the added monomer concentrations. As a result, we can adopt a one-pot approach to form CMP-knitted IPN organogels by a temperature-controlled sequential polymerization, as depicted in Scheme 1b. ZnTEPP monomers mixed with TEB and Pd(II)/Cu(I) catalysts were polymerized at 25 °C for 24 h into ZnP-based CMP networks. Then the reaction was heated to 60 °C to allow the immobilized TEB monomers to polymerize within the ZnPbased CMP organogels, thus resulting in a dual-CMP-network interwoven IPN. This is the first report to propose the idea of constituting a totally rigid, conjugated, and CMP-interlaced IPN monolith. FE SEM images in Figure 9 show a freeze-drying xerogel consisting of ZnP, PAE and IPN networks. The PAE gel is composed of roughly grained particles with domain sizes less than 500 nm, and a morphology that differs from that of the fibrous ZnP gel. When TEB and ZnTEPP monomers were added in a 1:2 molar ratio, the formed sequential IPN gel seems to involve two separated phases throughout the xerogel, where the continuous fiber-interlocked network is assumed to be ZnPbased CMP, and the coarse PAE-CMP grains are distributed discontinuously. EDS spectra provide a quantitative microanalysis of elements in the vicinity of the xerogel surface. Apart from the abundant carbon, we compared the Pd, Cu, I, P, and Zn contents in the three samples (Table S4 and Figure S5 in the Supporting Information). For the two homopolymerizing xerogels, no Zn exists in the PAE network gel but ∼7% Zn exists in the ZnP-based network gel. For the IPN-1 xerogel, the Zn content is reduced to 5.9%, which implies that the two component networks may exist simultaneously. Less than 2% catalyst residues are found in the xerogels, with minimal influence on the opening networks. The porosity of the IPN xerogel was measured by N2 isotherm sorption at 77 K, and the results were summarized in Table 2. The PAE xerogel has a high surface area of 850.96 m2 g−1 and a pore volume of 0.5991 cm3 g−1, both of which are comparable to those of reported PAE-CMP powders.1 A type IV isotherm was observed in the PAE xerogel, which evolves to a mesoporous character, and, in addition to the micropores, a significant population of mesopores was observed (see Figure S6 in the Supporting Information). The mesopores may be

Figure 9. FE SEM images of freeze-drying PAE, ZnP, IPN-2 xerogels, respectively.

Table 2. Porosity of PAE, ZnP, and IPN Xerogels entry

TEB (mol %)

PAE IPN-1 IPN-2 ZnP

100 80 33.3 0

SBET (m2 g−1)a 850.96 637.49 494.59 874.61

(1461.68) (968.33) (631.17) (957.35)

Smicro (m2 g−1)b

Sext (m2 g−1)b

Vtot (cm3 g−1)c

382.71 496.38 444.43 817.48

468.25 141.11 50.16 57.14

0.5991 0.3540 0.2765 0.3736

a Surface area is calculated from the N2 adsorption isotherm using the Brunauer−Emmett−Teller method and the value in parentheses is the Langmuir surface area. bThe external and micropore surface areas are obtained using the t-plot method based on the Halsey thickness equation. cTotal pore volume at P/P0 = 0.99.

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created by tightly packed grains in the gel network, and this is also in line with observations in the FE SEM image (Figure 9a). As the sequential polymerization in one pot was carried out by varying the molar ratios of TEB and ZnTEPP, the BET surface areas decreased from 637.49 to 494.59 m2 g−1 with increasing ZnTEPP monomer content. This reveals the occurrence of intense polymer interpenetration. Also, the N2 sorption isotherm shape changes completely, and a typical type I isotherm is found for IPN-2 (see Figure S7 in the Supporting Information). This proves that it turns into a pure microporous material, accompanied by the disappearance of nanoparticles as viewed in the FE SEM images.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (21004012 and 21474015) and STCSM (13520720200 and 14ZR1402300).

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REFERENCES

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CONCLUSIONS We have synthesized a fiber-knitted conjugated polymer organogel consisting of ZnP units positioned in a topological framework as nodes and exhibiting a distinct CMP character in the microscopic structure. The highly efficient reaction for gel formation is the dimerization of terminal alkyne groups at ambient temperature in air, without any harsh conditions. Many common organic solvents and low monomer concentrations and reaction temperatures were adapted to produce organogels with a characteristic elastic property. Heat-drying organogels could be transformed into intact xerogels and they allowed for the fixing of monolith shapes in the molds. Extensive studies on the microporosity of a series of xerogels, for the first time, demonstrated that they are of typical CMP-like microporous structure, with high surface area and micropore size domain. A modulation in gel formation plays an essential role in control over CMP porosity in the form of a xerogel. Unprecedentedly, a narrow pore size distribution comparable to those of crystalline porous materials could be attained by lowering the monomer concentrations or reaction temperatures. This equates to significant progress in the adjustment of CMP pores in a controllable fashion with the aim of bridging the gap between long-distance ordering COFs and amorphous organic porous materials. In light of the flexibility and controllability in the preparation of organogels, we synthesized sequential IPN xerogels by temperature-tuned successive polymerization in one pot, which resulted in two different CMP interpenetrating networks at the microscopic scale. Following the polymerization of ZnTEPP at room temperature, the PAE network was produced at elevated temperature with the same reaction in the earlier formed ZnP-based gel. The porosity of two-component IPN xerogels could be tailored by varying the content of PAE and ZnP networks. To date, this is the first report to present two-component CMP monoliths with inherent IPN structures. We envisage great promise in terms of the fantastic advantages that may arise from a convergence in versatile function, CMP block engineering, and an extension of moldable CMP varieties by the novel gel-to-CMP route. This product may be broadly applicable, particularly in optoelectric devices.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed procedure of monomer synthesis, gelation conditions, photographs of organogels and xerogels, 1H NMR spectra, solid-state 13C CP/MAS NMR spectra, PXRD, pore properties of varying xerogels, and EDS results. This material is available free of charge via the Internet at http://pubs.acs.org. 6249

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