Design and Construction of an Azo-Functionalized POP for Reversibly Stimuli-Responsive CO2 Adsorption

A porous azo-functionalized organic polymer (JJU-2) was designed and prepared via oxidative coupling polymerization promoted by FeCl3. JJU-2 exhibited reversibly stimuli-responsive CO2 adsorption properties as a result of the trans/cis isomerization of the polymer’s azo-functionalized skeleton. Under UV irradiation and heat treatment, this porous material displayed various porous structures and CO2 adsorption properties. The initial Brunauer-Emmett-Teller (BET) surface area of JJU-1 is 888 m2 g−1. After UV irradiation, the BET surface area decreases to 864 m2 g−1, along with the decrease of micropores around 0.50 nm and 1.27 nm during the trans-to-cis isomerization process. In addition, CO2 sorption isotherms demonstrate an 8%t decrease, and the calculated Qst of CO2 has decreased from 29.0 kJ mol−1 to 26.5 kJ mol−1 due to the trans to cis conversion of the azobenzene side group. It is noteworthy that JJU-2′s CO2 uptakes are nearly constant over three cycles of alternating external stimuli. Therefore, this azo-functionalized porous material was a potential carbon capture material that was responsive to stimuli.


Introduction
Modern industrial-era greenhouse gas emissions have a significant impact on global warming and environmental change. Carbon dioxide (CO 2 ) is regarded as one of the most important greenhouse gases, resulting in an increased focus on reducing or capturing CO 2 . Unfortunately, the concentration of CO 2 in the atmosphere continues to rise globally, endangering human health and the global ecosystem. Consequently, the development of materials and technologies for CO 2 capture and utilization is crucial for addressing the intractable global problem [1]. Numerous scientists and researchers have focused on the development of functional porous materials to capture and transform CO 2 under mild conditions [2], including zeolite [3,4], porous carbon [5,6], and metal-organic framework [7][8][9] among others. Porous organic polymers (POPs), as a class of emerging porous materials, are gradually attracting more research efforts due to their excellent physical and chemical stability, large surface area, good designability, and diverse structures, resulting in their wide applications in gas sorption [10][11][12], heterogeneous catalysis [13][14][15], sensor [16][17][18], and drug delivery [19][20][21]. Friedel-Crafts alkylation [22,23], Yamamoto reaction [24,25], Suzuki coupling reaction [26,27], and Sonogashira-Hagihara cross-coupling reaction [28,29] are examples of effective synthetic techniques that have been utilized to create POPs. However, some polymerization techniques have disadvantages that limit their applications in the synthesis of POPs, such as undesirable by-products, high energy consumption, low yield, harsh reaction conditions, and costly catalysts. Among these techniques, oxidative coupling polymerization utilizing FeCl 3 is regarded as one of the most popular approaches to constructing POF due to its low-cost, large-scale preparation, simple operation, and high universality [30][31][32][33]. On the other hand, carbazole and its derivatives are used as organic monomers to prepare POPs via FeCl 3 -promoted oxidative coupling polymerization due to the large-conjugated system, rigid structure, high reactivity, and excellent photoelectric properties of carbazole and its 2 of 11 derivatives. Thus, some carbazole-based POPs have been synthesized and implemented in diverse fields [34][35][36].
In recent years, effective regulation of the interaction between CO 2 and porous skeletons has become an important area of research, as it is believed to be an energy-saving, accessible, and environmentally-friendly method for capturing and utilizing CO 2 . By altering the skeleton configuration, stimuli-responsive porous materials are an effective way to regulate CO 2 sorption. Some porous functional materials have response properties to various external stimuli, including pH value [37], temperature [38], and light [39]. Light is a readily available source of renewable energy on Earth, and the introduction of lightresponsive groups can alter the structures of porous materials [40]. Recently, metal-organic frameworks (MOFs) as a class of porous organic-inorganic hybrid materials have been utilized in a variety of applications [41][42][43], particularly photoresponsive MOFs for CO 2 sorption [44,45]. However, MOFs' poor stability severely restricts their application. POPs are more stable than MOFs and thus provide an excellent platform for light-responsive CO 2 capture. For high-performance CO 2 sorption, photoresponsive functional groups are the most important factor in the construction of POPs. Azobenzene is considered one of the readily available and light-sensitive groups. To date, azobenzene-functionalized light-responsive POPs with good photocontrol performance have attracted considerable interest in CO 2 capture [46,47]. Due to the exceptional photoelectric properties of carbazole and light-responsive azobenzene, carbazole-based POPs with azobenzene fragments serve as a platform for realizing light-responsive CO 2 sorption.
Based on our previous reports [48,49], we design and develop a carbazole-based azo-functionalized POP (JJU-2) by coupling 1,3,5-tris(9H-carbazole-9-yl) benzene (TCB) and azobenzene (AB) (Scheme 1). High stability, a large surface area, and well-developed pores characterize the prepared sample. In the meantime, the porous skeleton contains azo functional groups as a light-responsive sensitive component, resulting in stimuliresponsive CO 2 sorption through cis and trans structure transformations under ultraviolet (UV) irradiation and heat treatments. Furthermore, the controlled CO 2 sorption/release can be recycled at least three times without difficulty. FeCl3-promoted oxidative coupling polymerization due to the large-conjugated system, rigid structure, high reactivity, and excellent photoelectric properties of carbazole and its derivatives. Thus, some carbazole-based POPs have been synthesized and implemented in diverse fields [34][35][36].
In recent years, effective regulation of the interaction between CO2 and porous skeletons has become an important area of research, as it is believed to be an energysaving, accessible, and environmentally-friendly method for capturing and utilizing CO2. By altering the skeleton configuration, stimuli-responsive porous materials are an effective way to regulate CO2 sorption. Some porous functional materials have response properties to various external stimuli, including pH value [37], temperature [38], and light [39]. Light is a readily available source of renewable energy on Earth, and the introduction of light-responsive groups can alter the structures of porous materials [40]. Recently, metal-organic frameworks (MOFs) as a class of porous organic-inorganic hybrid materials have been utilized in a variety of applications [41][42][43], particularly photoresponsive MOFs for CO2 sorption [44,45]. However, MOFs' poor stability severely restricts their application. POPs are more stable than MOFs and thus provide an excellent platform for light-responsive CO2 capture. For high-performance CO2 sorption, photoresponsive functional groups are the most important factor in the construction of POPs. Azobenzene is considered one of the readily available and light-sensitive groups. To date, azobenzenefunctionalized light-responsive POPs with good photocontrol performance have attracted considerable interest in CO2 capture [46,47]. Due to the exceptional photoelectric properties of carbazole and light-responsive azobenzene, carbazole-based POPs with azobenzene fragments serve as a platform for realizing light-responsive CO2 sorption.
Based on our previous reports [48,49], we design and develop a carbazole-based azofunctionalized POP (JJU-2) by coupling 1,3,5-tris(9H-carbazole-9-yl) benzene (TCB) and azobenzene (AB) (Scheme 1). High stability, a large surface area, and well-developed pores characterize the prepared sample. In the meantime, the porous skeleton contains azo functional groups as a light-responsive sensitive component, resulting in stimuliresponsive CO2 sorption through cis and trans structure transformations under ultraviolet (UV) irradiation and heat treatments. Furthermore, the controlled CO2 sorption/release can be recycled at least three times without difficulty.

Materials and General Methods
Unpurified chemicals were purchased and utilized. UV-Vis spectroscopy was carried out using a Mapada V-1200 (Shanghai Meipuda Instrument Co., Ltd., Shanghai, China). Using ZF-1A ultraviolet analyzer, trans-cis transformations were carried out (JIAPENG, Shanghai, China). IFS 66V/S Fourier transform infrared spectrometer was utilized to Scheme 1. The synthetic process of JJU-2.

Synthesis of JJU-2
In a 100 mL round-bottom flask, 1,3,5-tris(9H-carbazole-9-yl) benzene (229 mg, 0.4 mmol, 1 eq), azobenzene (583 mg, 3.2 mmol, 8 eq), and FeCl 3 (583 mg, 3.6 mmol, 9 eq) were added, followed by three cycles of vacuuming and inserting with N 2 . Then, 20 mL of dried CHCl 3 was added to the reaction system using a syringe. The reaction was heated at 80 • C for 24 h in an N 2 atmosphere. After the mixture cooled to room temperature, it was filtered to obtain the crude product. The product was further washed with HCl (0.3 M) aqueous solution, distilled water, methanol, and dichloromethane in order to remove unreacted monomers and catalyst residues. The product was dried under vacuum for 6 h at 60 • C to produce JJU-2 (orange solid powder, 463 mg, yield of 57%).

Structural Description
The UV-visible adsorption spectra of azobenzene in dichloromethane exhibit a typical trans and cis isomerization transformation with obvious π→π* absorption peaks, such as a strong peak at 319 nm and a weak peak at 435 nm ( Figure 1a). To examine the successful polymerization of organic monomers and the structure of POPs, Fourier transform infrared (FT-IR) spectra are measured (Figure 1b). By comparing the FT-IR spectra of the initial monomers and JJU-2, the characteristic absorption peaks are summarized as follows: (a) there is an obvious absorption peak at 3000 cm −1 , which is mainly attributed to the C-H stretching vibration of the hydrogen atom in phenyl ring; (b) the C-H deformation vibration peaks (760-660 cm -1 ) of four adjacent hydrogen atoms on the carbazolyl 1, 2-diXsubstituted phenyl ring is obviously weakened because the number of adjacent hydrogen atoms decreases from four to three; (c) some peaks in the range of 800-690 cm −1 belong to the C-H deformation vibration of ring hydrogen atoms of the 1,2-disubstituted phenyl ring of the carbazole group; (d) the peaks around 1300 cm −1 corresponds to the -C-N-bond between the aromatic ring and the azobenzene group; (e) FTIR bands located at 1458 cm −1 belongs to -N=N-stretching vibration in the azo-functional group in JJU-2, which confirms the successful synthesis of azo-linked POPs [50]. The 13 C solid-state NMR measurement is then used to examine the structure of the POP of interest (Figure 1c). Due to the presence of aromatic carbon atoms in the benzene ring, the 13 C solid-state NMR spectrum of JJU-2 exhibits three main peaks between 100 and 150 ppm. Notably, a high peak is observed at 123 ppm, which is attributable to unsubstituted phenyl carbon atoms, whereas two weak signals at 108 and 139 ppm are primarily attributable to substituted phenyl carbon atoms [51]. It further verifies the preparation of the JJU-2 target. As shown in Figure 1d, the thermogravimetric analysis (TGA) curve was measured in air at a heating rate of 10 • C min −1 . The first weight loss of the as-synthesized JJU-2 is about~3.1% beforẽ 330 • C, which may be attributed to the adsorbed water and other solvent molecules as the previous similar reports [52,53]. At higher temperatures, the skeleton gradually decomposes because of the oxygen reactions of C, N, and H elements in the air. The TGA curve confirms the good thermal stability of as-synthesized samples.
In addition, powder X-ray diffraction (PXRD) of as-synthesized POP reveals no diffraction peaks, indicating that as-synthesized JJU-2 is an amorphous solid material (Figure 2a). Using a scanning electron microscope (SEM) and transmission electron microscope (TEM), it is possible to observe the morphology and microstructure of JJU-2. The SEM image reveals that the prepared POP is an irregular crosslinked solid (Figure 2b). Moreover, the In addition, powder X-ray diffraction (PXRD) of as-synthesized POP reveals no diffraction peaks, indicating that as-synthesized JJU-2 is an amorphous solid material (Figure 2a). Using a scanning electron microscope (SEM) and transmission electron microscope (TEM), it is possible to observe the morphology and microstructure of JJU-2. The SEM image reveals that the prepared POP is an irregular crosslinked solid ( Figure  2b). Moreover, the TEM images of JJU-2 demonstrate that the synthesized POP has a disordered, worm-like porous structure (Figure 2c

Gas Sorption Properties
In order to examine the internal pores, synthesized JJU-2 is immersed in dichloromethane for two days and then activated at 150 °C for ten hours under a vacuum. The N2 sorption isotherms of the initial sample were measured at 77 K, and after 5 h of UV irradiation, they were measured at 77 K once more. As depicted in Figure 3a, the majority of the curves exhibited a type I isotherm with a substantial capillary N2 uptake

Gas Sorption Properties
In order to examine the internal pores, synthesized JJU-2 is immersed in dichloromethane for two days and then activated at 150 • C for ten hours under a vacuum. The N 2 sorption isotherms of the initial sample were measured at 77 K, and after 5 h of UV irradiation, they were measured at 77 K once more. As depicted in Figure 3a, the majority of the curves exhibited a type I isotherm with a substantial capillary N 2 uptake in the low P/P 0 region. It confirms that JJU-1 is microporous. The desorption curves of both materials showed little hysteresis due to the diffusional limitation of adsorbed N 2 molecules in restricted micropores [54]. The sample surface areas are computed using the Langmuir and BET models, which are summarized in Table 1. Figure 3a and Table 2 demonstrate BET surface area plots. After 5 h of UV light irradiation, the surface area of the resultant material is smaller than that of the initial material. The NLDFT is utilized to determine the pore size distribution curves for both samples. After UV irradiation, the pore size distributions are between 0.50 nm and 1.27 nm, with a gradual decrease (Figure 3b). Similarly, the pore volumes of materials also undergo a change. Light-responsive azobenzene in the porous skeleton is primarily responsible for the pore change.    [55][56][57]. The dynamic structural changes of assynthesized POP can be used as a light-responsive CO2 adsorbent under mild conditions; therefore, the CO2 sorption isotherms are carried out at 273 and 298 K, respectively, under 1 atmosphere. The initial POP has a strong capacity for CO2 sorption, with a maximum absorption of 68 cm 3 g −1 at 273 K and 37 cm 3 g −1 at 298 K and 1 atm (Figure 4a). Notably, after 5 h of UV irradiation, JJU-2 exhibits different CO2 sorption behaviors due to the trans/cis isomerization of azobenzene in its porous structure. As shown in Figure 4b, the  Surface areas calculated from BET a and Langmuir b models; c The total pore volume calculated at P/P 0 = 0.97. Based on previous research, the trans/cis isomerization of azobenzene can result in different geometric and dipole variations [55][56][57]. The dynamic structural changes of as-synthesized POP can be used as a light-responsive CO 2 adsorbent under mild conditions; therefore, the CO 2 sorption isotherms are carried out at 273 and 298 K, respectively, under 1 atmosphere. The initial POP has a strong capacity for CO 2 sorption, with a maximum absorption of 68 cm 3 g −1 at 273 K and 37 cm 3 g −1 at 298 K and 1 atm (Figure 4a). Notably, after 5 h of UV irradiation, JJU-2 exhibits different CO 2 sorption behaviors due to the trans/cis isomerization of azobenzene in its porous structure. As shown in Figure 4b, the CO 2 sorption amount decreases to 62 cm 3 g −1 at 273 K and 34 cm 3 g −1 at 298 K. Similar to the previous reports [58][59][60], the hysteresis of the CO 2 desorption is mainly ascribed to the electronic attraction between adsorbed and desorbed CO 2 molecules, and the interaction between the porous skeleton and CO 2 . Meanwhile, it is probably caused by the pore window and pore expansion in POPs.  Table 3 provides samples of representative materials [48,49,[61][62][63][64][65][66][67][68][69][70].   Using CO 2 adsorption at 273 and 298 K, the CO 2 adsorption enthalpy (Qst) can be calculated to determine the interaction force between CO 2 and the skeleton of JJU-2 (Figure 4c,d). The calculated CO 2 Q st value of the initial sample is as high as 29.0 kJ mol −1 , but after 5 h of UV irradiation, the value decreases to 26.5 kJ mol −1 , proving the trans/cis structure transformation of JJU-2 ( Figure 4e). Table 3 provides samples of representative materials [48,49,[61][62][63][64][65][66][67][68][69][70].
According to the porous structure of initial and UV-treated samples, the decreased micropore volume may be the main reason for reducing the CO 2 adsorption capacity of JJU-2 after irradiating by UV light. Reversible circularity is another vital factor for stimulus-responsive CO 2 sorption porous materials. As illustrated in Figure 5a-e and Table 4, the CO 2 adsorption isotherms of JJU-2 are repeatedly measured for three cycles after UV irradiation and thermal regeneration, which exhibits that the as-synthesized JJU-2 has good light-and thermal-responsive circularity for adsorbing CO 2 at 273 K under 1 atm under the controllable external stimulation. micropore volume may be the main reason for reducing the CO2 adsorption capacity of JJU-2 after irradiating by UV light. Reversible circularity is another vital factor for stimulus-responsive CO2 sorption porous materials. As illustrated in Figure 5a-e and Table 4, the CO2 adsorption isotherms of JJU-2 are repeatedly measured for three cycles after UV irradiation and thermal regeneration, which exhibits that the as-synthesized JJU-2 has good light-and thermal-responsive circularity for adsorbing CO2 at 273 K under 1 atm under the controllable external stimulation.

Conclusions
In conclusion, an Azo-Functionalized POP is designed and synthesized via an oxidative coupling method. The porous structure and light-responsive properties of this POP material are investigated in depth. In POPs, UV irradiation and thermal regeneration can specifically affect the trans/cis transition of azo groups. Under UV irradiation, the capacity for CO 2 sorption decreases by 8%. In the meantime, this stimuli-responsive POP can revert to its original structure following thermal regeneration. According to recycling tests, the CO 2 adsorption performance of this synthesized POP can be recycled at least three times via UV irradiation and thermal regeneration. This study aims to provide a method for constructing POPs with CO 2 adsorption that are reversibly stimulated.  Institutional Review Board Statement: Not applicable.

Data Availability Statement:
No new data were created or analyzed in this study. Data sharing is not applicable to this article.