Effects of Thiophene and Benzene Ring Accumulation on the Photocatalytic Performance of Polymers.

Six polymers were prepared with 4,4',4″-(1,3,5-triazine-2,4,6-triyl)triphenylamine as the amine unit, and six different aldehyde units as substrates. The effects of the number of thiophene and benzene rings on the degradation of tetracycline (TC) in water were studied using polymer photocatalysts, and the reaction mechanism was discussed. The results indicate that ThTA-3 containing three thiophene group monomers and BATA-1 with one benzene ring unit monomer have higher absorption and utilization of visible light. In addition, ThTA-3 and BATA-1 have stronger charge separation and transfer capabilities and better morphology and thermal stability.

Introduction

Humans consume a lot of energy and produce a great deal of pollutants such as wastewater and exhaust gas. When they are discharged into the environment, they cause serious environmental pollution, which is now endangering the survival of humans and other organisms.[1−3] Environmental pollution, especially water pollution, is one of the largest problems humans currently face. This pollution mainly comes from industrial production wastewater, urban domestic sewage, and the discharge of solid and gas waste into the environment.[4] It is conceivable that the pollutants in water bodies will enter the human body through biological chains or other means if they are not treated, and they may even cause many unavoidable ecological issues.[5−7] Biological, physical, and chemical treatment methods can alleviate this phenomenon to a certain extent, but the effect is often unsatisfactory.[8,9] With the continuous development of technology, semiconductor photocatalytic technology has slowly attracted more attention.[10−13]

In the past few decades, researchers have mainly focused on inorganic semiconductor materials (such as TiO2 and Bi2WO6),[10,14] but they have deficiencies such as a low utilization rate of visible light absorption, and their structures cannot be completely controlled.[15,16] Therefore, organic conjugated polymer materials are promising new materials because of their excellent inherent properties.[17] The application prospects of porous organic conjugated materials in various industries and fields are very broad.[18] Most of them are connected by covalent bonds, their specific surface area is relatively large, and they have crystalline or amorphous state structural characteristics. Organic conjugated polymer materials can be designed, with certain groups introduced or removed as needed, to adjust the photoelectric properties and pore properties of the materials to obtain the target product. Because of this, organic conjugated polymer materials can be applied in a wide array of fields, such as photocatalytic degradation,[19,20] hydrogen production,[21] adsorption,[22] fluorescence detection,[23] and heterogeneous catalysis.[24] Designing and synthesizing effective applications or novel molecular structures has become a hot issue in this field.

Here, we studied the effect of thiophene and benzene ring accumulation on the photocatalytic performance of the polymerization products using several different polymerization monomers. After mixing the six aldehyde monomers with 4,4′,4″-(1,3,5-triazine-2,4,6-triyl) in a certain ratio and degassing with three freeze–thaw cycles, six kinds of organic conjugated polymer photocatalysts (ThTA series and BATA series) were prepared. By comparing the degradation experiment results of tetracycline (TC) and a norfloxacin aqueous solution, the efficiency difference between the two series was obtained, and the reason for this difference was studied through diffuse reflection, photocurrent response, AC impedance, fluorescence, and other analyses.

Results and Discussion

Thiophene-2,5-dicarbaldehyde (TD), thieno[3,2-b]thiophene-2,5-dicarbaldehyde (TTD), dithieno[3,2-b:2′,3′-d]thiophene-2,6-dicarbaldehyde (DTD), 1,4-phthalaldehyde (PD), [1,1′-biphenyl]-4,4′-dicarbaldehyde (BDD), and [1,1′:4′,1″-terphenyl]-4,4″-dicarbaldehyde (TDD) were used as the aldehyde synthetic monomers, and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)triphenylamine (TAT) was used as the amine synthetic monomer. TD, TTD, DTD, PD, BDD, and TDD were placed in 25 mL Schlenk tubes with TAT using a mixture of mesitylene/1,4-dioxane/6 M AcOH as the reaction solvent and were sealed and heated at 120 °C for 72 h. Its shape and structure are shown by Heine and co-workers’ speculation[25] (Scheme ). The performance of the organic conjugated polymer photocatalysts was studied using Fourier transform infrared spectroscopy (FTIR), diffuse reflection spectrum (DRS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), thermogravimetric analysis (TGA), photocurrent effect, electrochemical impedance (EIS), and photoluminescence spectrum (PL).

Scheme 1

Synthetic Routes of the ThTA and BATA Series

The Fourier transform infrared (FTIR) spectra of the ThTA series polymers were studied first (Figure a). In Figure S1a, the carbonyl group (−C=O−) in TD is around 1682 cm–1, and the highest intensity peak disappeared in the polymer ThTA-1. The two sharp intermediate bands at 3463 and 3347 cm–1 of TAT also disappeared in the spectrum of ThTA-1. At the same time, in the spectrum of ThTA-1, an imine bond peak at 1588 cm–1 was generated. In summary, TD and TAT successfully reacted to form the polymer ThTA-1.[26,27] In Figure S1b,c, both the carbonyl peaks of TTD and DTD and the peak of the primary amino group of TAT disappeared in the polymer. Moreover, the polymers ThTA-2 and ThTA-3 generated characteristic imine bond peaks at 1584 and 1595 cm–1, respectively, indicating the successful syntheses of ThTA-2 and ThTA-3. Then, while studying the FTIR of the BATA series polymers (Figure S1d–f), it was found that imine bonds similar to those in the ThTA series polymers were successfully generated. This proves that BATA-1, BATA-2, and BATA-3 were successfully synthesized.

Figure 1

(a) Infrared spectra of the polymers and (b) XRD patterns of the polymers.

(a) Infrared spectra of the polymers and (b) XRD patterns of the n class="Chemical">polymers.

To study the crystallinity of the organic conjugated polymers, we studied the XRD of the ThTA and BATA series materials. As can be seen from Figure b, the intensities of the diffraction peaks of the six polymers of the ThTA and BATA series were small, that is, no obvious crystal diffraction peak signals appeared. This shows that the arrangement of atoms in the polymer does not have long-range order, indicating they are amorphous structure polymers. However, because the ThTA series polymers have better crystallinity, there will be fewer lattice defects, which is more conducive to the conduction of the charge carrier. So, we predict that this series of polymers will have a better performance. Thermogravimetric analysis (Figure S2a,b) was used to study the thermal stability of the ThTA and BATA series of polymer photocatalysts. The thermal stability of the ThTA series was slightly worse than that of the BATA series. The ThTA series began to decompose at about 150 °C, the 5% thermal weight loss temperature was around 200 °C, and the final remaining mass accounted for about 50% of the total mass. For the BATA series, the obvious quality decline only occurred when the temperature exceeded 480 °C. Even so, their thermal stability was still very good.

The surface morphologies of the two series of six substances were characterized by SEM, as shown in Figure . Among the ThTA series, ThTA-3 had the best morphological characteristics, showing a nanorod-like morphology with uniform thickness. ThTA-2 was composed of relatively uniform round nanosheet particles, and the morphology of ThTA-1 was messier and more irregular. As the number of polythiophene in the polymerized monomer increased, the morphology became more characteristic. In the BATA series, BATA-1 had a three-dimensional structure formed by the agglomeration of regular and uniform-sized nanospherical particles, BATA-2 was composed of flaky particles, and BATA-3 had a fragmented, disordered shape, that is, the morphology of the polymer became worse as the number of benzene rings in the monomer increased. The morphology of the BATA series became messier with the increased number of benzene rings, while the morphology of the ThTA series was more regular and even as thiophene accumulated. We think this is because the benzene rings of the BATA series are connected by carbon–carbon single bonds, and the degree of morphological disorder becomes larger as the number of single bonds increases. On the contrary, the ThTA series is a thiophene stacked polymer, and its morphology becomes more regular as the amount of stacking increases.

Figure 2

SEM images of ThTA-1 (a), ThTA-2 (b), ThTA-3 (c), BATA-1 (d), BATA-2 (e), and BATA-3 (f).

SEM images of ThTA-1 (a), n class="Chemical">ThTA-2 (b), ThTA-3 (c), BATA-1 (d), BATA-2 (e), and BATA-3 (f).

The visible light absorption efficiency of semiconductor photocatalysts has a crucial influence on its catalytic effect. Therefore, we analyzed the light absorption characteristics of the organic conjugated polymer samples with different numbers of thiophenes and benzene rings using the UV–vis diffuse reflection pattern (Figure ). From the diffuse reflectance spectrum of the ThTA series in Figure a, the organic conjugated polymers ThTA-1, ThTA-2, and ThTA-3 had a similar absorption response range to visible light. ThTA-3 containing trithiophene had the strongest absorption intensity and appeared red-shifted, showing better light absorption characteristics. The band gap energy spectrum was calculated by the following formulawhere α, h, υ, A, Eg, and n are the absorption coefficient, Planck’s constant, optical frequency, characteristic constant, band gap energy, and direct or indirect optical transition constant, respectively.[28] A study of the abscissa of the tangent of the three materials intersecting the x-axis revealed that the band gaps Eg of ThTA-1, ThTA-2 and, ThTA-3 were 2.15, 2.07, and 1.99 eV, respectively (Figure b). Therefore, increasing the number of thiophene units in synthetic polymer monomer molecules can reduce the energy band gap of the material. The low energy band gap of ThTA-3 shows that it has good absorption and utilization of visible light.

Figure 3

(a) UV–vis diffuse reflectance spectra of ThTAs, (b) band gap energy spectrum of ThTAs, (c) UV–vis diffuse reflectance spectra of BATAs, and (d) band gap energy spectrum of BATAs.

(a) UV–vis diffuse reflectance spectra of ThTAs, (b) band gap energy spectrum of n class="Chemical">ThTAs, (c) UV–vis diffuse reflectance spectra of BATAs, and (d) band gap energy spectrum of BATAs.

Then, the UV–vis diffuse reflection spectrum of the BATA series were studied. As can be seen from Figure c, the difference in the light absorption intensity of the three materials was not very large, but there was a difference in the absorption range. The position of the edge of the light absorption band of BATA-3 was about 513 nm, and the light absorption ranges of BATA-1 and BATA-2 were significantly wider than that of BATA-3. The band gap diagram shown in Figure d was also obtained through the above formula, and the band gaps of the three polymer materials were 2.46, 2.48, and 2.55 eV, respectively. In short, from the energy band gaps of the three different materials, BATA-1, with a low energy band gap, had a high utilization rate for visible light.

Under visible light, electrons transition and migrate on the surface of the photocatalyst, which produces a transient photocurrent response.[29] The magnitude of the photocurrent response can reflect the separation ability of the photogenerated electrons and holes in the photocatalyst, that is, the greater the intensity, the better the charge separation effect. Therefore, to study the charge generation, separation, and transport capabilities of the ThTA and BATA series as photocatalyst materials, we conducted a photocurrent study (Figure a,b). It can be clearly seen in Figure a that the photocurrent intensity produced by the polymer ThTA-3 is greater than that of ThTA-1 and ThTA-2. Unlike in the ThTA series, as the number of benzene rings increases, the intensity of the transient photocurrent response decreases steadily in the BATA series (Figure b), and BATA-3 has the smallest photocurrent response. EIS can reflect the electron–hole separation effect of the photocatalyst. The smaller the radius of the EIS spectrum, the better the electron–hole separation effect and the better the photocatalytic effect. Therefore, we tested the EIS of the ThTA and BATA series (Figure c,d). In the ThTA series, ThTA-3, with the most thiophene groups in the polymerized monomer, had the smallest EIS radius, followed by ThTA-2, and ThTA-1 had the largest. In the BATA series, the radius increased as the number of benzene rings increased, and the radius of BATA-1, containing three benzene rings, was the largest. This indicates that the ThTA-1 and BATA-3 surfaces have large resistance values, which hinder the transfer of photogenerated electrons on the surface of semiconductor materials.[30]

Figure 4

(a) Transient photocurrent–time curves of ThTAs, (b) transient photocurrent–time curves of BATAs, (c) EIS curves of ThTAs, and (d) EIS curves of BATAs.

(a) Transient photocurrent–time curves of n class="Chemical">ThTAs, (b) transient photocurrent–time curves of BATAs, (c) EIS curves of ThTAs, and (d) EIS curves of BATAs.

The photoluminescence spectrum (PL) can reflect the electron–hole recombination rate. When the PL peak of the photocatalytic material is weak, there is a lower electron–hole recombination rate and a higher photocatalytic efficiency. In the two series of photocatalysts synthesized in this experiment, there were significant differences (Figure S3). In the ThTA series, the absorption peaks of the three polymers are similar, with two characteristic peaks around 428 nm and 453 nm. As the amount of thiophene in the polymerized monomer increased, the peak intensity decreased, and the peak intensity of ThTA-3 was the lowest. In the BATA series, the peak intensity of BATA-3 was the lowest. Therefore, we predict that ThTA-3 and BATA-1 have the best photocatalytic efficiency within the same series.[31]

X-ray photoelectron spectroscopy (XPS) was used to explore the elements and bonding capabilities of the ThTA and BATA series. The C, N, O, and S in the ThTA series polymers were all derived from the aldehyde-based unit substances TD, TTD, DTD, and the amino unit TAT, and no other impurities entered during the reaction (Figure a). Similarly, in the BATA series, C, N, and O were derived from the aldehyde-based unit substances PD, BDD, TDD, and the amino unit TAT (Figure b). In Figure S4, there are two main characteristic peaks for C 1s of ThTA-1 at 284.8 and 285.8 eV, corresponding to the characteristic peaks of −C=C– and −C=N–, respectively.[32,33] At the same time, at 398.7 eV, ThTA-1 shows the characteristic peak for N 1s −C=N– and the characteristic peak generated by the vibration process in the highly conjugated system at 399.8 eV. There are similar performance characteristics in the C 1s and N 1s spectra of ThTA-2, ThTA-3, BATA-1, BATA-3, and BATA-3. This shows that the photocatalytic materials were successfully synthesized.[31]

Figure 5

Full XPS spectrum of the ThTA series (a) and BATA series (b).

Full XPS spectrum of the ThTA series (a) and n class="Chemical">BATA series (b).

To understand the photocatalytic effect of the ThTA and BATA series polymer materials, we studied the six polymers (50 mg) degrading tetracycline (TC, 10 mg/L) in water (Figure a,b). For the ThTA series, the degradation efficiency went from 50.1% for ThTA-1 containing one thiophene monomer to 68.8% for ThTA-2 containing two thiophene monomers, and the degradation efficiency of the final trithiophene monomer polymer ThTA-3 reached 91.6%. As the number of thiophenes in the monomer increased, the degradation efficiency also increased. For the BATA series polymers, as the number of benzene rings in the polymer main chain increased, the degradation efficiency reduced from 74.2 to 49.7%, and to 29.6%, indicating that the polymer degradation efficiency in this series decreases as the number of benzene rings in the monomer increases. The degradation experiments of these two materials confirmed our previous conclusions.

Figure 6

(a) Degradation curves of ThTAs for TC under visible light, (b) degradation curves of BATAs for TC under visible light, (c) photocatalytic stability tests of the ThTA-3 sample for the degradation of TC, (d) photocatalytic stability tests of the BATA-1 sample for the degradation of TC, (e) ion trapping experiment of ThTA-3 degrading the TC system, and (f) ion trapping experiment of TABA-1 degrading the TC system.

(a) Degradation curves of n class="Chemical">ThTAs for TC under visible light, (b) degradation curves of BATAs for TC under visible light, (c) photocatalytic stability tests of the ThTA-3 sample for the degradation of TC, (d) photocatalytic stability tests of the BATA-1 sample for the degradation of TC, (e) ion trapping experiment of ThTA-3 degrading the TC system, and (f) ion trapping experiment of TABA-1 degrading the TC system.

Later, the degradation effects of these six polymer photocatalysts on norfloxacin were studied. It was found that the ThTA series had a certain catalytic degradation effect, and the degradation efficiency increased as the number of thiophenes in the monomer increased, as shown in Figure S5. However, the BATA series did not have an effective degradation effect. This shows that the ThTA series photocatalysts can degrade a wider range of pollutant types than the BATA series photocatalysts. TC degradation cycle experiments were conducted on the six photocatalysts to evaluate their stability (Figure c,d). In general, in the ThTA series, the degradation efficiency of ThTA-3 was more than 80% after four cycles. ThTA-2 fell from 68.8 to 52.3%, which is still more than half of the efficiency. But for ThTA-1, after four cycles, its efficiency was only 28.9%. This shows that the stabilities of ThTA-3 and ThTA-2 are better than that of ThTA-1. However, since the S atom in the ThTA series of thiophene monomers is easily combined with tetracycline (TC), a small amount of contaminants will remain during repeated use and washing. Therefore, the stability of the ThTA series was poor when the cycle experiment was performed. In the BATA series, the degradation efficiency hardly decreased, indicating that the synthesized material can be effectively recycled.

Based on the four cyclic degradations of tetracycline, a series of tests were conducted on the recovered ThTA-3 and BATA-1 catalysts (Figures S6 and S7). The peak intensity and position of the peaks in the XRD, infrared, and diffuse reflectance spectra did not change significantly. There were no obvious changes in the SEM morphology, with only minor morphology damage, and the overall damage was not large. This shows that the ThTA-3 and BATA-1 polymers have very good stability as photocatalysts for degrading tetracycline.

There are a variety of active free radicals in the degradation system, and the groups that play a major role in each degradation system are different. We studied the main active groups in the catalytic degradation process of ThTA-3 and BATA-1, as they had the highest degradation efficiency of tetracycline in the ThTA and BATA series. Here, we used K2S2O8, EDTA-2Na, 1,4-p-benzoquinone (BQ), and IPA as the capture agents for the photogenerated electrons (e–), photogenerated holes (h+), superoxide radicals (O2–), and hydroxyl radicals (•OH)[34] (Figure e,f). For ThTA-3, the degradation rate of the system with EDTA-2Na added was most affected, from the original 91.6 to 35.5%. After adding isopropanol, the rate dropped to 78.1%, while adding p-benzoquinone and K2S2O8 showed no significant change to the system. This means that h+ plays a major role in degrading TC in ThTA-3, •OH has a small effect, and O2– and e– have little to no effect. For BATA-1, after adding BQ, the degradation efficiency dropped from 74.2 to 30.8%. The degree of change to the system with EDTA-2Na added was the next highest, dropping the efficiency to 51.9%. As for the addition of the two remaining active free radical scavengers, the effect on the degradation efficiency was not significant, only dropping to 72.8 and 71.9%, respectively. This shows that in the process of BATA-1 degrading TC, O2– plays a major role, followed by h+, and •OH and e– have almost no effect.

We tested the VB-XPS for the ThTA and BATA series (Figure S8a,b). The test results show that the VBs of ThTA-1, ThTA-2, ThTA-3, BATA-1, BATA-2, and BATA-3 are 2.17, 1.95, 1.64, 2.39, 2.39, and 2.39 eV, respectively. Using the following empirical formula ECB = EVB – Eg, the conduction band positions (CB) of ThTA-1, ThTA-2, ThTA-3, BATA-1, BATA-2, and BATA-3 are 0.02, −0.12, −0.35, −0.07, −0.09, and −0.16 eV, respectively. We then proposed the mechanism of ThTA-3 photocatalytic degradation of TC (Figure ). Under visible light, the polymer is excited by light, so that the electrons in the ground state are excited to the CB, forming photogenerated holes (h+) and electrons (e–). Among them, h+ played a major role in the process, and the generated •OH also played a small role.

Figure 7

Mechanism of ThTA-3 degrading the TC system.

Mechanism of ThTA-3 degrading the n class="Chemical">TC system.

Conclusions

In summary, six organic conjugated polymer photocatalysts, the ThTA and BATA series, were prepared and subjected to freeze–vacuum–thaw cycles three times, smoothly synthesizing the materials. For the materials of the ThTA series, increasing the number of thiophenes in the polymerized monomer effectively improved the degradation of the polymer under visible light in the degradation experiment of a tetracycline aqueous solution, and a better morphology was obtained. For the BATA series materials, increasing the number of benzene rings in the polymerized monomer reduced the catalytic efficiency. A series of tests were conducted to explore the effect of the increase in the number of thiophene and benzene ring groups on the photocatalytic performance of the polymer. Specifically, ThTA-3, containing three thiophene group monomers, and BATA-1, with one benzene ring unit monomer, had relatively strong photogenerated charge carrier separation and transport capabilities under the stimulation of light. The two have higher absorption and utilization of visible light, better morphology, and better thermal stability, highlighting a way to improve the catalytic effect of conjugated organic polymers.

Experimental Section

Chemicals and Instrumentation

Thiophene-2,5-dicarbaldehyde (TD), thieno[3,2-b]thiophene-2,5-dicarbaldehyde (TTD), dithieno[3,2-b:2′,3′-d]thiophene-2,6-dicarbaldehyde (DTD), 1,4-phthalaldehyde (PD), [1,1′-biphenyl]-4,4′-dicarbaldehyde (BDD), [1,1′:4′,1″-terphenyl]-4,4″-dicarbaldehyde (TDD), and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)triphenylamine (TAT) were all purchased from SunaTech Inc. 1,4-dioxane and mesitylene were purchased from Meryer (Shanghai) Chemical Technology Co., Ltd. Fourier transform infrared spectroscopy (FTIR) was measured in the range of 4000–400 cm–1 using KBr pellets on an infrared spectrometer (Vertex, Swiss Bruker Company). A UV–vis diffuse reflectance spectrometer (UV–vis DRS) (Cary 300, American Varian Corporation), a thermogravimetric analyzer (TGA) (SDT Q600, American TA Corporation), an electrochemical workstation (CHI 660e, Shanghai Chenhua Instrument Co. Ltd.), and a xenon lamp (PLS–SXE300C, Pofillai Beijing) were also used in the study.

Synthesis of ThTA-1

Thiophene-2,5-dicarbaldehyde (63.27 mg, 0.45 mmol) and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)triphenylamine (106 mg, 0.3 mmol) were weighed and placed in a dry 25 mL Schlenk tube. The reaction system used a mixed solution of mesitylene (1.5 mL), 1,4-dioxane (1.5 mL), and 6 M acetic acid (0.5 mL) as the solvent. The Schlenk tube was sealed with a rubber stopper wrapped in a sealing film to increase the tightness of the entire system. The device was then sonicated for 30 min to evenly disperse the mixture. Finally, after placing the Schlenk tube in liquid nitrogen to freeze the liquid in the tube, it was subjected to three freeze–thaw cycles to remove the gas. To ensure an oxygen-free environment in the experimental system, it was sealed under vacuum and reacted at 120 °C for 72 h. At the end of the reaction, the device was cooled to normal temperature, treated by suction filtration, and the solid material was washed repeatedly with anhydrous CHCl3, anhydrous methanol, and anhydrous acetone several times. The obtained solid substance was subjected to Soxhlet extraction with anhydrous THF for 24 h. Finally, the solid material was obtained by filtration and dried in a vacuum drying oven at 80 °C. The reaction yielded 135.9 mg (80.29%) of solid with a dark yellow color.

Synthesis of ThTA-2

Similar to the ThTA-1 synthesis procedure, thieno[3,2-b]n class="Chemical">thiophene-2,5-dicarbaldehyde (89 mg, 0.45 mmol), 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)triphenylamine (106 mg, 0.3 mmol), mesitylene (1.5 mL), 1,4-dioxane (1.5 mL), and 6 M acetic acid (0.5 mL) were reacted. The reaction yielded 186.1 mg (95.44%) of solid that was orange in color.

Synthesis of ThTA-3

Similar to the ThTA-1 synthesis procedure, dithieno[3,2-b:2′,3′-d]n class="Chemical">thiophene-2,6-dicarbaldehyde (113.55 mg, 0.45 mmol), 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)triphenylamine (106 mg, 0.3 mmol), mesitylene (1.5 mL), 1,4-dioxane (1.5 mL), and 6 M acetic acid (0.5 mL) were reacted. The reaction yielded 198.2 mg (90.28%) of solid that was orange-red in color.

Synthesis of BATA-1

Similar to the ThTA-1 synthesis procedure, n class="Chemical">terephthalaldehyde (60.32 mg, 0.45 mmol), 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)triphenylamine (106 mg, 0.3 mmol), mesitylene (1.5 mL), 1,4-dioxane (1.5 mL), and 6 M acetic acid (0.5 mL) were reacted. The reaction gave 140.1 mg (84.24%) of solid with a golden yellow color.

Synthesis of BATA-2

Similar to the ThTA-1 synthesis procedure, [1,1′-biphenyl]-4,4′-dicarbn class="Chemical">aldehyde (94.61 mg, 0.45 mmol), 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)triphenylamine (106 mg, 0.3 mmol), mesitylene (1.5 mL), 1,4-dioxane (1.5 mL), and 6 M acetic acid (0.5 mL) were reacted. The reaction gave 183.4 mg (91.42%) of solid that was pale yellow in color.

Synthesis of BATA-3

Similar to the ThTA-1 synthesis procedure, [1,1′:4′,1″-terphenyl]-4,4″-dicarbn class="Chemical">aldehyde (128.84 mg, 0.45 mmol), 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)triphenylamine (106 mg, 0.3 mmol), mesitylene (1.5 mL), 1,4-dioxane (1.5 mL), and 6 M acetic acid (0.5 mL) were reacted. The reaction gave 199.5 mg (84.95%) of solid with a golden yellow color.

Photocatalytic Activity Measurements

Fifty milligrams of photocatalysts were dispersed in 50 mL of 10 mg/L tetracycline solution and stirred for 30 min under dark conditions to achieve adsorption–desorption equilibrium. A 300 W xenon lamp with a 420 nm cutoff filter was used as the visible light source. The reactor was moved toward the visible light source to achieve adsorption–desorption in the dark. Within 60 min, 3 mL of the reaction solution was taken every 10 min, and a sample of water was taken from a clarifier with a 45 nm filtration head. Then, a UV–visible spectrophotometer was used to measure the residual amount of TC in the clarifier. The maximum absorption wavelength of TC was 357 nm.