Triazatriangulenium-based porous organic polymers for carbon dioxide capture

Abstract

Porous organic polymers from triazatriangulenium salts (TAPOPs) were developed via oxidative polymerization. FeCl₃-caused chlorination on the triazatriangulenium core was observed, which has a pronounced impact on the photophysical properties and porosity of the polymers. Optimization of reaction conditions affords TAPOPs with a Brunauer–Emmett–Teller specific surface area as high as 940 m² g⁻¹ and a carbon dioxide uptake capacity up to 15.4 wt% at 273 K and 1.0 bar. Furthermore, the reversible and preferred adsorption of TAPOPs toward carbon dioxide over nitrogen is not only demonstrated by gas sorption experiments but also gas–sorbent interaction measurements through recording the emission change of TAPOPs under different gas atmospheres using fluorescence microscopy.

---

Introduction

The emission of carbon dioxide due to the combustion of fossil fuels is the major cause for global warming and related environmental issues. To reduce carbon dioxide concentration in the atmosphere, it is necessary to develop carbon dioxide capture and separation technologies. Traditionally, carbon dioxide capture is achieved using an amine-based “wet-scrubbing” approach.^1,2 In this way, carbon dioxide is adsorbed through reaction with amine (namely, chemisorption) and liberated by heating; meanwhile, the amine is regenerated. While this method shows considerable uptake capacity and is highly selective for carbon dioxide, it suffers from high regeneration energy, large equipment size, solvent degradation, and equipment corrosion.¹ Physisorption on porous materials is thus considered more promising for carbon dioxide capture and separation, owing to their advantages of fast kinetics, complete reversibility, and excellent uptake capacity.^3–5

Of various porous materials, porous organic polymers (POPs) are of particular interest thanks to their low skeleton density, diverse synthesis, variable functionality, and easy processability.^6–9 The performance of carbon dioxide capture and separation on POPs primarily depends on their porosity and functionality, which are essentially determined by their precursors.^4,5,10 Thus, selection of appropriate building blocks is a critical premise for POPs fabrication and application. It is worth noting that most POPs reported so far are made up of neutral building blocks.^6–9 Exploration of ionic tectons could provide a supplement for the diversity of POPs, as well as improved performance. It has been proved recently that polymerization of ionic building blocks (e.g. phosphonium, pyridinium, and borate ions) is able to yield polymers with permanent porosity.^11–15 Furthermore, the charge in the polymer network is found beneficial for carbon dioxide uptake, due to the interaction between the charge and the carbon dioxide quadrupole moment.¹⁵ As a matter of fact, enhanced carbon dioxide adsorption induced by charge–quadrupole interaction has been reported even earlier on zeolites.¹⁶ These findings point to a fact that the introduction of charge into porous network is favorable for carbon dioxide capture.

Triazatriangulenium salts (TATA) are a class of fluorescent dyes with cationic organic skeleton (Scheme 1).^17–19 The positive charge is distributed over the large conjugated TATA framework including the three electron-donating nitrogen bridges, endowing TATA with excellent stability.^17,18 The high symmetry, triangular nature, and unique cation stability of TATAs allow them to form various materials and supramolecular assemblies with the segregation of cations and anions.^20–22 When TATAs are functionalized with polymerizable groups, cationic TATA-based POPs (TAPOPs) along with the carefully selected counterions are available via suitable reactions. More intriguingly, incorporation of luminescent TATA units into POPs has the potential to spectroscopically probe the interaction between the porous network and gaseous molecules.

Scheme 1 Synthesis of TAPOPs.

Motivated by these considerations, we synthesized two TAPOPs with cationic backbone via FeCl₃-promoted oxidative polymerization of thiophene- /carbazole-functionalized TATA derivatives (Scheme 1) and tested their potential for carbon dioxide capture. These TAPOPs exhibit considerable porosity and high carbon dioxide uptake capacity. Further recording the emission change while purging TAPOPs with different gases using fluorescence microscopy, together with the gas sorption measurement, demonstrate the reversible and preferred adsorption of TAPOPs toward carbon dioxide over nitrogen. Another noteworthy phenomenon is the FeCl₃-caused chlorination of TATA, which affects both the photophysical property and porosity of the resulting TAPOPs.

Experimental

Materials

All chemicals were used as received from Sigma-Aldrich unless otherwise stated. HPLC grade solvents from Sigma-Aldrich and technical solvents from VWR were used as received for reactions and column chromatography, respectively. N,N′,N′′-Triphenyl-triazatriangulenium tetrafluoroborate (Ph₃TATA, Scheme 1), N,N′,N′′-tris(4-bromophenyl)-triazatriangulenium tetrafluoroborate ((4-BrPh)₃TATA, Scheme 1) were synthesized according to reported procedures.¹⁹ Ultra-high-purity grade gases were used for all gas sorption experiments.

Synthesis of TATA monomers and TAPOPs

N,N′,N′′-Tris(4-(2-thienyl)phenyl)-TATA⁺ BF₄⁻ (TATA-1). To a round-bottom flask were added (4-BrPh)₃TATA (1.0 g, 1.2 mmol), 2-thienylboronic acid (650 mg, 5.08 mmol), and N,N-dimethylformamide (20 mL). Potassium carbonate (2.76 g, 20.0 mmol) and water (5 mL) were added, and the mixture was degassed by bubbling argon for 15 min via a syringe through a rubber septum. Tetrakis(triphenylphosphine)palladium (60 mg) was added and the mixture was heated at 100 °C under argon atmosphere for 12 h. The reaction mixture was cooled down to room temperature and poured into deionized water with stirring. The precipitate was dissolved in dichloromethane and washed with deionized water and aqueous sodium tetrafluoroborate solution (0.2 M). The crude product was purified by dry column vacuum chromatography (heptane/toluene/ethanol with 2% increment of ethanol in toluene) to give a red compound. Further precipitation in dichloromethane/toluene yielded red powder (0.76 g, 75%). ¹H NMR (500 MHz, CD₃CN): δ (ppm) 8.08 (d, J = 5.0 Hz, 6H), 7.63 (t, J = 10.0 Hz, 3H), 7.59 (d, J = 5.0 Hz, 3H), 7.52–7.50 (m, 9H), 7.18 (dd, J = 5.0, 5.0 Hz, 3H), 6.49 (d, J = 10.0 Hz, 6H). ¹³C NMR (125 MHz, CD₃CN): δ (ppm) 143.12, 142.93, 138.31, 137.69, 137.28, 130.28, 129.95, 129.70, 127.71, 125.96, 111.14, 108.05. ¹⁹F NMR (282 MHz, CD₃CN): δ (ppm) −151.00, −151.06. MS (MALDI-TOF) m/z: calcd for C₄₉H₃₀N₃S₃⁺ 756.16, found 756.18.

N,N′,N′′-Tris(4-(carbazol-9-yl)phenyl)-TATA⁺ BF₄⁻ (TATA-2). To a round-bottom flask were added (4-BrPh)₃TATA (1.0 g, 1.2 mmol), carbazole (802 mg, 4.8 mmol), copper(I) iodide (92 mg, 0.48 mmol), 1,10-phenanthroline (87 mg, 0.48 mmol), and potassium carbonate (663 mg, 4.8 mmol). The mixture was degassed by bubbling argon for 15 min via a syringe through the rubber septum. The degassed N,N-dimethylformamide (5 mL) was then added. The mixture was refluxed under an argon atmosphere for 18 h and poured into deionized water with stirring when it cooled down to room temperature. The precipitate was dissolved in dichloromethane and washed with deionized water and aqueous sodium tetrafluoroborate solution (0.2 M). The crude product was purified by dry column vacuum chromatography (heptane/toluene/acetone with 5% increment of acetone in toluene) to give a red compound. Further precipitation in dichloromethane/toluene yielded red powder (0.93 g, 71%). ¹H NMR (500 MHz, CD₂Cl₂): δ (ppm) 8.23 (d, J = 10.0 Hz, 6H), 8.14 (d, J = 5.0 Hz, 6H), 7.85 (t, J = 5.0 Hz, 3H), 7.80 (d, J = 10.0 Hz, 6H), 7.70 (d, J = 5.0 Hz, 6H), 7.54 (t, J = 5.0 Hz, 6H), 7.40 (t, J = 5.0 Hz, 6H), 6.74 (d, J = 5.0 Hz, 6H). ¹³C NMR (125 MHz, CD₂Cl₂): δ (ppm) 143.06, 142.66, 140.94, 140.64, 138.21, 136.51, 131.02, 130.95, 126.88, 124.42, 121.36, 121.11, 110.98, 110.19, 108.23. ¹⁹F NMR (282 MHz, CD₂Cl₂): δ (ppm) −153.59, −153.64. MS (MALDI-TOF) m/z: calcd for C₇₃H₄₅N₆⁺ 1005.37, found 1005.45.

TAPOP-1. FeCl₃ (154 mg, 0.95 mmol) was suspended in the mixture of dichloromethane (5 mL) and trifluoroacetic acid (0.5 mL). The solution of TATA-1 (100 mg, 0.12 mmol) in dichloromethane (5 mL) was then added dropwise. The mixture was stirred at room temperature overnight under nitrogen atmosphere and quenched by addition of methanol (20 mL). The resulting precipitate was filtered, washed with methanol, and stirred in HBF₄ (5%, multiple portions) overnight to remove residual FeCl₃ and keep BF₄⁻ as the counterion. The solid was filtered and washed with deionized water, methanol, and dichloromethane, before the polymer was further purified by Soxhlet extraction for one day with water, methanol, and dichloromethane as the solvent, respectively. Finally, reddish TAPOP-1 (96 mg) was obtained in a quantitative yield.

TAPOP-2 was synthesized under the same condition as TAPOP-1 when TATA-2 was used as the monomer. Finally, reddish TAPOP-2 (98 mg) was obtained in a quantitative yield.

To do the control reaction, Ph₃TATA was used as the reactant under the same condition as the polymerization.

Characterization and measurement

¹H and ¹³C NMR spectra were recorded on a Bruker Avance III 500 NMR spectrometer (Bruker, Germany) using the residual solvent as internal reference. ¹⁹F NMR spectra were recorded on a Bruker Avance 300 NMR spectrometer (Bruker, Germany) using α,α,α-trifluorotoluene as external reference (δ: −63.72 ppm). MALDI-TOF mass spectra were recorded on a Bruker Autoflex vertical reflector instrument (Bruker, Germany). Solid-state cross-polarization magic angle spinning (CP/MAS) ¹³C NMR spectra were recorded on a Bruker Avance III 400 NMR spectrometer (Bruker, Germany). X-ray photoelectron spectroscopy (XPS) experiments were carried out in a Kratos AXIS Ultra DLD fitted with a monochromated Al Kα X-ray source and a charge neutralizer. C 1s at 285.0 eV was used for binding energy calibration and the casaXPS software for fitting and quantification. Scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis, and gas sorption measurement were performed according to previously reported procedures.²³ The samples were degassed in vacuo at 120 °C for 12 h before measurements. The specific surface area was calculated according to Brunauer–Emmett–Teller (BET) theory in the relative pressure range P/P₀ = 0.01–0.10 and total pore volume was calculated at P/P₀ = 0.97. Pore size distribution (PSD) profile was estimated from nitrogen adsorption branch by non-local density functional theory (NLDFT).

UV-vis-NIR diffuse reflectance spectra of powder samples were recorded on a Perkin-Elmer Lamda 950 UV-vis spectrometer (Perkin-Elmer Inc., USA) equipped with an integrating sphere. Barium sulfate powder was used as the 100% reflectance standard. The reflectance data were converted to absorption according to the Kubelka–Munk equation.^24,25 Emission images and spectra of the TATA monomers and TAPOPs were collected on fluorescence microscope (Zeiss, Germany) at excitation wavelength of 475 nm. To study gas–adsorbent interaction spectroscopically, integrated fluorescence intensity between 650 and 750 nm was collected while purging TAPOP-2 with different gases.

Results and discussion

Synthesis and characterization

The oxidative polymerization of thiophene and carbazole derivatives has been proved highly effective and already afforded a number of POPs.^10,26–28 Such polymerization can proceed at room temperature at the cost of cheap FeCl₃ in a quantitative yield, which is essential for scale-up preparation of materials. Thus, a series of TAPOPs are prepared via FeCl₃-promoted oxidative polymerization of thiophene-/carbazole-functionalized TATA derivatives (TATA-1 and TATA-2), as shown in Scheme 1. These two monomers were derived from a triply-brominated triphenyl-TATA ((4-BrPh)₃TATA) via palladium(0)-catalyzed Suzuki coupling and copper(I)-catalyzed Ullmann-type reaction, respectively.

Initially, the oxidative polymerization of the above monomers was conducted in dry chloroform using a large excess of anhydrous FeCl₃ (36 equivalents of the monomer), according to reported procedures.^10,27,28 The resulting polymers are bluish, quite different color from the corresponding red monomers. As reported in literature, the absorption properties of TATA derivatives primarily originate from the TATA core structure and are hardly affected by peripheral functional groups.^17–19 Oxidative polymerization is expected to yield C–C coupling at the 5-position and 3,6-positions for thiophene and carbazole moieties, respectively.^10,26–28 Thus, the absorption spectra would be retained if no additional reaction occurred on the TATA core. The notable color change implies the disruption of the TATA core structure during the polymerization.

In order to ascertain the side reactions, control reaction under the same condition was carried out using Ph₃TATA that has no reactive site for oxidative polymerization as the reactant (Scheme 1). Mass spectra of the reaction mixture reveal that the chlorination of TATA took place and the reaction ended up with a dominant product containing three chlorine atoms (Fig. S1a, ESI†). Monochlorinated TATA was isolated and the chlorine atom is found to be located at the TATA core in the position meta-to the carbenium center (Scheme 1), as proved by the NMR spectra (ESI†).

Subsequent attempts were focused on the suppression of chlorination. The solvent was replaced with dichloromethane, as chloroform usually contains chlorine after a certain period of storage. The stoichiometry of FeCl₃ was reduced drastically (eight equivalents of the monomer); meanwhile, trifluoroacetic acid was added to accelerate the polymerization and suppress the possible side reactions. Polymerization under this condition yielded two reddish polymers (TAPOP-1 and TAPOP-2) that have color resembling their precursors. Control reaction reveals that the chlorinated products were formed as well, but to a much lower degree (Fig. S1b, ESI†). Under this condition, monochlorinated TATA was the predominant product; and the formation of chlorinated products suggests that FeCl₃ is responsible for the chlorination. While FeCl₃-caused chlorination has been reported previously,²⁹ such phenomenon has not been observed or has until now been ignored for the neutral thiophene- or carbazole-based POPs prepared under similar conditions.^10,26–28

The as-prepared TAPOPs are insoluble in water and common organic solvents, owing to the hyper-crosslinked polymer network. Thus, the solid-state ¹³C CP/MAS NMR spectroscopy was employed to characterize their structures at the molecular level (Fig. 1). Typically, there are mainly four peaks located in the aromatic region in both spectra. The peaks with chemical shift above 140 ppm are assigned to the central carbon and carbons bound with nitrogen, and the peaks located around 110 ppm mainly originate from the carbon atoms at the position either ipso- or meta- toward the carbenium. With regard to TAPOP-1, the peaks around 136 and 130 ppm are ascribed to the rest of the substituted and unsubstituted carbon atoms in the peripheral motifs, respectively. For TAPOP-2, the peaks around 130 and 125 ppm correspond to the other unascribed carbon atoms in the phenylene linker and carbazole moieties, respectively.

Fig. 1 Solid-state ¹³C CP/MAS NMR spectra of TAPOPs.

The chemical composition of the TAPOPs, as well as the chlorination, was verified by XPS analysis (Fig. 2a). TAPOP-1 shows a quite similar spectrum to TAPOP-2 except that it contains S signal coming from the thiophene moiety. The presence of B and F signals suggests that the counterions in both polymers are still tetrafluoroborate. Apparently, Cl signals, though very weak, can be observed for both polymers. The quantitative analysis by XPS reveals that chlorine in both TAPOPs accounts for approximately 1 wt%, which means that one out of three or four TATA units on average contains one chlorine atom (Table S1, ESI†). Further deconvolution of Cl 2p core-level spectra presents two peaks around 200.9 (Cl 2p_3/2) and 202.5 eV (Cl 2p_1/2) for both TAPOPs (Fig. 2b and c). Both peaks arise from the organic chlorine atoms covalently bound to aromatic rings.³⁰ This also excludes the presence of inorganic chloride ions, typically around 199.0 eV,³¹ which, together with the absence of Fe signal, indicates the complete removal of salt residues.

Fig. 2 Survey (a) and Cl 2p core-level XPS spectra of TAPOP-1 (b) and TAPOP-2 (c).

SEM images of the two polymers reveal a similar morphology of interconnected polydisperse microspheres with particle sizes ranging from 50 to 100 nm (Fig. S2, ESI†). XRD patterns show amorphous feature of these polymers (Fig. S3, ESI†), as commonly observed for thiophene- and carbazole-based polymers prepared by oxidative polymerization.^10,32 Note that a broad peak around 20 degree is observed for both TAPOPs, corresponding to a d-spacing of ∼4.5 Å, which is typical for densely packed amorphous polymers and usually attributed to the average chain–chain distance.³³ TGA suggests that both polymers are stable up to 300 °C under nitrogen atmosphere (Fig. S4, ESI†). The initial weight loss below 120 °C is attributed to adsorbed moisture, as often observed for porous materials.⁵ TAPOPs are also chemically stable, even when exposed overnight to dilute solutions of acid or base, or boiling water.

Unlike most reported POPs, which are either yellowish or brownish and absorb light generally in ultraviolet region, both TAPOPs are reddish and absorb strongly in visible range with absorption maximum around 540 nm (Fig. S5a, ESI†), similar to their precursors. While TATA derivatives are known as highly fluorescent dyes, the emission images collected by fluorescence microscopy show that the as-prepared TAPOPs emit weakly (Fig. S6, ESI†). The emission is even weaker and cannot be detected for the more severely chlorinated polymers (prepared in the early experiments using reported procedures). Thus, it can be inferred that chlorination quenches the emission dramatically and should be avoided for fabrication of fluorescent POPs. Fig. S5b (ESI†) presents the emission spectra of TATA monomers in solution and powder state and the corresponding polymer powder. In acetonitrile solution, TATA-2 exhibits maximum emission around 570 nm, while around 650 nm in powder state. A red-shift of ca. 25 nm for aliphatic TATAs from solution to solid film was found previously by our group and attributed to the formation of a co-facial TATA dimer in film state.^21,34 In powder state, a more pronounced red-shift (ca. 80 nm) was observed for both TATA-1 and TATA-2, which presumably results from different packing mode of the TATA skeleton. Even more red-shifted emission was observed for both TAPOPs, with maximum emission at 760 and 690 nm, respectively. This can be explained by energy migration in the polymer to few chlorinated sites from which emission predominantly occurs.³⁵

Porosities and carbon dioxide uptake

The porosity properties of TAPOPs were characterized by nitrogen sorption isotherms measured at 77 K. As shown in Fig. 3a, both polymers exhibit a combination of type I and II isotherms according to the IUPAC classification.³⁶ A steep nitrogen uptake at very low relative pressures (P/P₀ < 0.01) is indicative of the permanent microporous nature of these TAPOPs. The gradual increase in nitrogen adsorption in the medium relative pressure range comes in part from the outer surface area of the network particles³⁷ and from mesopores in the polymers.⁵ The significantly increasing uptake at high relative pressures above 0.90 is attributed to the nitrogen condensation in interparticular voids formed by the aggregation of polymer microspheres observed in the SEM images in Fig. S2.†^38,39 Hysteresis can be observed apparently in the whole range of relative pressures for all isotherms, which is partly attributed to the swelling in a flexible polymer network, as well as mesopore contribution.³⁸

Fig. 3 (a) Nitrogen adsorption–desorption isotherms of TAPOPs measured at 77 K (the adsorption and desorption isotherms are labeled with solid and open symbols, respectively); (b) PSD profiles of TAPOPs calculated by NLDFT.

The BET specific surface areas of the two TAPOPs calculated in the relative pressure range from 0.01 to 0.10 are quite similar (930 and 940 m² g⁻¹), comparable to most reported neutral thiophene-/carbazole-based POPs^26,27,32 and other ionic POPs.^11–15 The similar nitrogen sorption isotherms and surface areas suggest that the TATA core structure plays a major role in the porosity of the polymers. It has to be mentioned that the more severely chlorinated TAPOPs exhibit much lower specific surface area (ca. 650 m² g⁻¹). This point should be considered for future synthesis of POPs using FeCl₃-mediated polymerization. The pore size distribution profile was analyzed using NLDFT model (Fig. 3b). TAPOP-1 and TAPOP-2 exhibit dominant pore size of 0.59 and 0.63 nm, respectively, which are attributed to the different polymerizable groups and in line with those reported for the neutral thiophene- and carbazole-derived polymers.^10,26 As a result, TAPOP-1 shows a slightly lower total pore volume (0.56 cm³ g⁻¹) than TAPOP-2 (0.71 cm³ g⁻¹). Detailed porosity parameters derived from the isotherms are summarized in Table 1. The level of microporosity in TAPOPs was assessed by the ratio of the micropore volume to the total pore volume (V_micro/V_total), along with the ratio of the micropore surface area to the apparent BET specific surface area (S_micro/S_BET). As shown in Table 1, TAPOP-1 possesses a higher V_micro/V_total and S_micro/S_BET value than TAPOP-2, and thus higher microporosity, as a result of its smaller dominant pore size.

Table 1 Porosity parameters of TAPOPs

Polymers	SBET^a (m^2 g^−1)	Smicro^b (m^2 g^−1)	Smicro/SBET	Vtotal^c (cm^3 g^−1)	Vmicro^d (cm^3 g^−1)	Vmicro/Vtotal	Dpore^e (nm)
a BET specific surface area calculated at the relative pressure ranging from 0.01 to 0.10.b Micropore surface area calculated using the t-plot method.c Total pore volume at P/P0 = 0.97.d Micropore volume calculated using the t-plot method.e Dominant pore size determined by NLDFT model.
TAPOP-1	940	560	0.60	0.56	0.23	0.41	0.59
TAPOP-2	930	460	0.49	0.71	0.19	0.27	0.63

---

To test the potential for greenhouse gas capture, carbon dioxide sorption isotherms of TAPOPs were collected at 273 K and displayed in Fig. 4a. The two isotherms are completely reversible and no significant hysteresis is observed, indicating that the interactions between carbon dioxide and TAPOPs are weak enough to allow for the easy regeneration of these materials. At 273 K and 1.0 bar, TAPOP-1 exhibits a slightly higher carbon dioxide uptake capacity (15.4 wt%, 3.5 mmol g⁻¹) than TAPOP-2 (13.6 wt%, 3.1 mmol g⁻¹), as a result of its higher microporosity. Both polymers, though their specific surface areas are moderate, show comparable carbon dioxide uptake to some other porous polymers possessing higher specific surface areas (>1000 m² g⁻¹) under the same conditions, such as CPOP-7 (3.0 mmol g⁻¹, S_BET = 1430 m² g⁻¹),²⁷ MOP-B (3.3 mmol g⁻¹, S_BET = 1847 m² g⁻¹),⁴⁰ and PAF-3 (3.5 mmol g⁻¹, S_BET = 2932 m² g⁻¹),⁴¹ owing to the presence of charges that interact with carbon dioxide quadrupole moment,^15,16 as well as nitrogen sites that interact with carbon dioxide through Lewis acid–base interaction.⁴

Fig. 4 (a) Carbon dioxide adsorption–desorption isotherms of TAPOPs measured at 273 K (the adsorption and desorption isotherms are labeled with solid and open symbols, respectively); (b) isosteric heat of carbon dioxide adsorption on TAPOPs.

In order to understand the interaction of TAPOPs with carbon dioxide, the isosteric heat (Q_st) of carbon dioxide adsorption was calculated from the adsorption isotherms measured at two different temperatures according to Clausius–Clapeyron equation.⁴² The plot of Q_st as a function of carbon dioxide uptake is presented in Fig. 4b. The heat of adsorption at zero coverage for TAPOP-1 and TAPOP-2 was calculated to be 27.8 and 34.7 kJ mol⁻¹, respectively. The larger initial Q_st value for TAPOP-2 is attributed to more nitrogen sites that come from the carbazole moieties and facilitate more favorably the interaction with carbon dioxide molecules through Lewis acid–base interaction.⁴ The Q_st value initially decreases and then remains approximately constant with increasing carbon dioxide loading as the high-energy nitrogen sites get occupied and become saturated after a certain amount of carbon dioxide adsorption. For TAPOP-1, the Q_st decreases slightly only at the beginning of carbon dioxide adsorption and remains constant and very close to the Q_st value of TAPOP-2. These results reveal that the pore surface nature plays the major role in carbon dioxide adsorption at the beginning. After the pore surface is completely occupied, the surface area and pore volume of micropores play the dominating role in carbon dioxide uptake. This means that a higher carbon dioxide uptake would probably be achieved if the micropore volumes of the polymers can be further increased. Further comparison of TAPOP-2 to neutral carbazole-derived POPs with similar structure reveals that the heat of adsorption of TAPOP-2 is the highest, which indicates the enhanced TAPOPs–carbon dioxide interaction arising from the charge (Table S2, ESI†). In addition, the Q_st values for both TAPOPs are lower than the covalent bond energy, implying that the uptake of carbon dioxide is essentially via physisorption.

To evaluate the carbon dioxide selective adsorption behavior of TAPOPs, the single component adsorption isotherms for carbon dioxide and nitrogen were collected and compared at 273 K (Fig. S7, ESI†). It is clearly shown that the amount of nitrogen adsorbed (0.44 and 0.39 mmol g⁻¹) on both TAPOPs is much lower than that of carbon dioxide (3.5 and 3.1 mmol g⁻¹) at 273 K and 1.0 bar. At this temperature and pressure, the calculated carbon dioxide/nitrogen adsorption ratios are both eight, indicative of preferred adsorption toward carbon dioxide for both TAPOPs, though the selectivity is not very high.

While the emission of TAPOPs is weaker than their precursors, the combination of emission with porosity inspired us to investigate the interaction between the sorbent and gaseous molecules spectroscopically. Fig. 5 presents the evolution of normalized fluorescence intensity of TAPOP-2 under different gas atmosphere. The emission intensity was recorded while purging the polymer with different kinds of gases (oxygen, carbon dioxide, and nitrogen) and normalized against the initial value. It can be seen that TAPOP-2 exhibits the weakest emission intensity under oxygen atmosphere, which is attributed to the oxygen-caused fluorescence quenching.⁴³ When oxygen is replaced with carbon dioxide, the emission is recovered by around 55%. Repeated purging TAPOP-2 with oxygen and carbon dioxide gives reversible emission change that takes place mostly within half a minute. Such reversible and rapid emission change points to a fact that the interaction between TAPOP-2 and carbon dioxide is weak physisorption, in agreement with the analysis of heat of adsorption. Further purging TAPOP-2 with carbon dioxide and nitrogen provides reversible and rapid emission change as well. While the change is small, it reveals that carbon dioxide can recover the emission more significantly than nitrogen. This could be a result of the higher affinity of TAPOP-2 toward carbon dioxide over nitrogen.

Fig. 5 Evolution of emission intensity of TAPOP-2 when purged with oxygen, carbon dioxide, and nitrogen. All curves are normalized at the initial intensity of TAPOP-2 under oxygen atmosphere.

Conclusions

Two TAPOPs with cationic backbone have been synthesized via FeCl₃-promoted oxidative polymerization of thiophene-/carbazole-functionalized TATA salts. The considerable porosity and presence of nitrogen sites and charges endow TAPOPs with comparatively high carbon dioxide uptake capacity. Gas sorption, together with a spectroscopic method, demonstrates the reversible and preferred adsorption of TAPOPs toward carbon dioxide over nitrogen. Moreover, FeCl₃-caused chlorination of TATA is disclosed and proved detrimental for both photophysical property and porosity of the resulting polymers and should be taken into account while designing and preparing other POPs using FeCl₃ as the reactants.

Acknowledgements

This work was supported by Chinese-Danish Center for Molecular Nanoelectronics funded by the National Science Foundation of China (No. 61261130092) and the Danish National Research Foundation. The financial support of the National Science Foundation of China (Grant no. 21374024) is also acknowledged.

Notes and references

1. D. M. D'Alessandro, B. Smit and J. R. Long, Carbon dioxide capture: prospects for new materials, Angew. Chem., Int. Ed., 2010, 49(35), 6058–6082.
2. F. Li and L. S. Fan, Clean coal conversion processes – progress and challenges, Energy Environ. Sci., 2008, 1(2), 248–267.
3. M. Sevilla, P. Valle-Vigón and A. B. Fuertes, N-Doped polypyrrole-based porous carbons for CO₂ capture, Adv. Funct. Mater., 2011, 21(14), 2781–2787.
4. P. Mohanty, L. D. Kull and K. Landskron, Porous covalent electron-rich organonitridic frameworks as highly selective sorbents for methane and carbon dioxide, Nat. Commun., 2011, 2, 4011–4016.
5. P. Arab, M. G. Rabbani, A. K. Sekizkardes, T. İslamoğlu and H. M. El-Kaderi, Copper(I)-catalyzed synthesis of nanoporous azo-linked polymers: impact of textural properties on gas storage and selective carbon dioxide capture, Chem. Mater., 2014, 26(3), 1385–1392.
6. N. B. McKeown and P. M. Budd, Polymers of intrinsic microporosity (PIMs): organic materials for membrane separations, heterogeneous catalysis and hydrogen storage, Chem. Soc. Rev., 2006, 35(8), 675–683.
7. D. C. Wu, F. Xu, B. Sun, R. W. Fu, H. K. He and K. Matyjaszewski, Design and preparation of porous polymers, Chem. Rev., 2012, 112(7), 3959–4015.
8. S. J. Xu, Y. L. Luo and B. E. Tan, Recent development of hypercrosslinked microporous organic polymers, Macromol. Rapid Commun., 2013, 34(6), 471–484.
9. Y. H. Xu, S. B. Jin, H. Xu, A. Nagai and D. L. Jiang, Conjugated microporous polymers: design, synthesis and application, Chem. Soc. Rev., 2013, 42(20), 8012–8031.
10. Q. Chen, M. Luo, P. Hammershøj, D. Zhou, Y. Han, B. W. Laursen, C.-G. Yan and B.-H. Han, Microporous polycarbazole with high specific surface area for gas storage and separation, J. Am. Chem. Soc., 2012, 134(14), 6084–6087.
11. Q. Zhang, S. B. Zhang and S. H. Li, Novel functional organic network containing quaternary phosphonium and tertiary phosphorus, Macromolecules, 2012, 45(7), 2981–2988.
12. Y. Yuan, F. Sun, F. Zhang, H. Ren, M. Guo, K. Cai, X. Jing, X. Gao and G. Zhu, Targeted synthesis of porous aromatic frameworks and their composites for versatile, facile, efficacious, and durable antibacterial polymer coatings, Adv. Mater., 2013, 25(45), 6619–6624.
13. Y. Yuan, F. Sun, L. Li, P. Cui and G. Zhu, Porous aromatic frameworks with anion-templated pore apertures serving as polymeric sieves, Nat. Commun., 2014, 5, 42601–42608.
14. S. Fischer, J. Schmidt, P. Strauch and A. Thomas, An anionic microporous polymer network prepared by the polymerization of weakly coordinating anions, Angew. Chem., Int. Ed., 2013, 52(46), 12174–12178.
15. S. Fischer, A. Schimanowitz, R. Dawson, I. Senkovska, S. Kaskel and A. Thomas, Cationic microporous polymer networks by polymerisation of weakly coordinating cations with CO₂-storage ability, J. Mater. Chem. A, 2014, 2(30), 11825–11829.
16. K. S. Walton, M. B. Abney and M. D. LeVan, CO₂ adsorption in Y and X zeolites modified by alkali metal cation exchange, Micropor. Mesopor. Mater., 2006, 91(1−3), 78–84.
17. B. W. Laursen and F. C. Krebs, Synthesis of a triazatriangulenium salt, Angew. Chem., Int. Ed., 2000, 39(19), 3432–3434.
18. B. W. Laursen and F. C. Krebs, Synthesis, structure, and properties of azatriangulenium salts, Chem.–Eur. J., 2001, 7(8), 1773–1783.
19. P. Hammershøj, T. J. Sørensen, B.-H. Han and B. W. Laursen, Base-assisted one-pot synthesis of N,N′,N′′-triaryltriazatriangulenium dyes: enhanced fluorescence efficiency by steric constraints, J. Org. Chem., 2012, 77(13), 5606–5612.
20. Y. Haketa, S. Sasaki, N. Ohta, H. Masunaga, H. Ogawa, N. Mizuno, F. Araoka, H. Takezoe and H. Maeda, Oriented salts: dimension-controlled charge-by-charge assemblies from planar receptor-anion complexes, Angew. Chem., Int. Ed., 2010, 49(52), 10079–10083.
21. T. J. Sørensen, C. B. Hildebrandt, J. Elm, J. W. Andreasen, A. Ø. Madsen, F. Westerlund and B. W. Laursen, Large area, soft crystalline thin films of N,N′,N′′-trialkyltriazatriangulenium salts with homeotropic alignment of the discotic cores in a lamellar lattice, J. Mater. Chem., 2012, 22(11), 4797–4805.
22. Y. Haketa, M. Takayama and H. Maeda, Solid-state supramolecular assemblies consisting of planar charged species, Org. Biomol. Chem., 2012, 10(13), 2603–2606.
23. X.-M. Hu, Q. Chen, Y.-C. Zhao, B. W. Laursen and B.-H. Han, Straightforward synthesis of a triazine-based porous carbon with high gas-uptake capacities, J. Mater. Chem. A, 2014, 2(34), 14201–14208.
24. J. H. Liao, C. Varotsis and M. G. Kanatzidis, Syntheses, structures, and properties of six novel alkali metal tin sulfides, Inorg. Chem., 1993, 32(11), 2453–2462.
25. A. P. Katsoulidis and M. G. Kanatzidis, Phloroglucinol based microporous polymeric organic frameworks with −OH functional groups and high CO₂ capture capacity, Chem. Mater., 2011, 23(7), 1818–1824.
26. J. Schmidt, J. Weber, J. D. Epping, M. Antonietti and A. Thomas, Microporous conjugated poly(thienylenearylene) networks, Adv. Mater., 2009, 21(6), 702–705.
27. Q. Chen, D.-P. Liu, M. Luo, L.-J. Feng, Y.-C. Zhao and B.-H. Han, Nitrogen-containing microporous conjugated polymers via carbazole-based oxidative coupling polymerization: preparation, porosity, and gas uptake, Small, 2014, 10(2), 308–315.
28. L. J. Feng, Q. Chen, J. H. Zhu, D. P. Liu, Y. C. Zhao and B. H. Han, Adsorption performance and catalytic activity of porous conjugated polyporphyrins via carbazole-based oxidative coupling polymerization, Polym. Chem., 2014, 5(8), 3081–3088.
29. T. D. McCarley, C. O. Noble, C. J. DuBois and R. L. McCarley, MALDI-MS evaluation of poly(3-hexylthiophene) synthesized by chemical oxidation with FeCl₃, Macromolecules, 2001, 34(23), 7999–8004.
30. D. T. Clark, D. Kilcast and W. K. R. Musgrave, Molecular core binding energies for some monosubstituted benzenes, as determined by X-ray photoelectron spectroscopy, J. Chem. Soc., Chem. Commun., 1971,(10), 516–518.
31. K. Kishi and S. Ikeda, X-ray photoelectron spectroscopic study of the reaction of evaporated metal films with chlorine gas, J. Phys. Chem., 1974, 78(2), 107–112.
32. Q. Chen, J.-X. Wang, F. Yang, D. Zhou, N. Bian, X.-J. Zhang, C.-G. Yan and B.-H. Han, Tetraphenylethylene-based fluorescent porous organic polymers: preparation, gas sorption properties and photoluminescence properties, J. Mater. Chem., 2011, 21(35), 13554–13560.
33. N. Ritter, M. Antonietti, A. Thomas, I. Senkovska, S. Kaskel and J. Weber, Binaphthalene-based, soluble polyimides: the limits of intrinsic microporosity, Macromolecules, 2009, 42(21), 8017–8020.
34. T. J. Sørensen, C. B. Hildebrandt, M. Glyvradal and B. W. Laursen, Synthesis, optical properties and lamellar self-organization of new N,N′,N′′-trialkyl-triazatriangulenium tetrafluoroborate salts, Dyes Pigm., 2013, 98(2), 297–303.
35. F. Westerlund, H. T. Lemke, T. Hassenkam, J. B. Simonsen and B. W. Laursen, Self-assembly and near perfect macroscopic alignment of fluorescent triangulenium salt in spin-cast thin films on PTFE, Langmuir, 2013, 29(22), 6728–6736.
36. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure Appl. Chem., 1985, 57(4), 603–619.
37. M. Rose, W. Böhlmann, M. Sabo and S. Kaskel, Element–organic frameworks with high permanent porosity, Chem. Commun., 2008,(21), 2462–2464.
38. J. Weber, J. Schmidt, A. Thomas and W. Böhlmann, Micropore analysis of polymer networks by gas sorption and ¹²⁹Xe NMR spectroscopy: toward a better understanding of intrinsic microporosity, Langmuir, 2010, 26(19), 15650–15656.
39. M. Rose, N. Klein, W. Böhlmann, B. Böhringer, S. Fichtner and S. Kaskel, New element organic frameworks via Suzuki coupling with high adsorption capacity for hydrophobic molecules, Soft Matter, 2010, 6(16), 3918–3923.
40. R. Dawson, E. Stockel, J. R. Holst, D. J. Adams and A. I. Cooper, Microporous organic polymers for carbon dioxide capture, Energy Environ. Sci., 2011, 4(10), 4239–4245.
41. T. Ben, C. Y. Pei, D. L. Zhang, J. Xu, F. Deng, X. F. Jing and S. L. Qiu, Gas storage in porous aromatic frameworks (PAFs), Energy Environ. Sci., 2011, 4(10), 3991–3999.
42. J. A. Dunne, R. Mariwals, M. Rao, S. Sircar, R. J. Gorte and A. L. Myers, Calorimetric heats of adsorption and adsorption isotherms. O₂, N₂, Ar, CO₂, CH₄, C₂H₆ and SF₆ on silicalite, Langmuir, 1996, 12(24), 5888–5895.
43. A. Gilbert and J. Baggott, Essentials of molecular photochemistry, Blackwell Scientific Publications, Oxford, UK, 1991.

---

Footnote

† Electronic supplementary information (ESI) available: NMR and mass spectra, XPS data, XRD pattern, TGA curve, absorption and emission spectra, emission images, comparison of carbon dioxide and nitrogen adsorption. See DOI: 10.1039/c5ra18047c

---