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Article

Spirobifluorene-Based D-A Type Conjugated Polymer Photocatalysts for Water Splitting

by
Hao Zhao
1,*,
Pengyao Sun
2,
Hui Xu
2,
Xinyi Xiao
1,
Zhiyuan Kong
1,
Shige Song
1,
Weihao Li
1,
Luzun Liu
1,
Jiadong Wang
1 and
Xiaobo Pan
2,*
1
School of Physics and Electronic Information, Yantai University, Yantai 264005, China
2
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(10), 717; https://doi.org/10.3390/catal14100717
Submission received: 26 September 2024 / Revised: 10 October 2024 / Accepted: 11 October 2024 / Published: 14 October 2024
(This article belongs to the Special Issue Exclusive Papers in Green Photocatalysis from China)
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Abstract

:
Exploring synthetic pathways for efficient photocatalysts has always been a major goal in catalysis. The performance of organic photocatalysts is affected by a variety of complex factors, and how to understand the structure–effect relationship is the key to designing efficient photocatalysts. This work explored the feasibility of constructing large-specific-surface-area conjugated microporous polymers (CMPs) based on stereoscopic units like spirobifluorene and achieving efficient photocatalytic activity by modulating the donor–acceptor (D-A) ratio with dibenzothiophene sulfone. Crosslinked pore structures were successfully constructed, and the specific surface area increased with the ratio of spirobifluorene. When the molar ratio of D-A was 1:20, polymer Spso-3 showed the highest photocatalytic hydrogen production activity, at 22.4 mmol h–1 g–1. The findings indicate that constructing D-A type CMPs should be a promising approach to improving the performance of photocatalytic water separation. The appropriate push–pull effect of the D-A structure promotes the photo-induced separation of electron–hole pairs, and the porous structure built on steric units offers ample space for catalytic reactions. This work could provide case references for structural design and the structure–effect relationship of efficient polymer photocatalysts.

Graphical Abstract

1. Introduction

Hydrogen, a secondary energy resource with a wide range of clean and carbon-free sources and rich application scenarios, is the ideal medium to substitute traditional fossil energy sources and support the large-scale development of renewable energy [1,2,3]. Using solar energy for direct photocatalysis-induced water decomposition for hydrogen production is a tempting and challenging energy conversion approach [4,5]. A 100 m2 scale prototype photocatalytic solar hydrogen production system constructed by Domen has operated stably for many years and enables autonomous hydrogen recovery, further demonstrating the feasibility of large-scale photocatalytic hydrogen production in the future [6]. However, the lower hydrogen production efficiency is still the main challenge limiting the use of this solution in reality, and the development of efficient, inexpensive, and environmentally friendly photocatalyst materials has become a research focus in catalytic chemistry and energy chemistry [7].
In recent times, organic conjugated polymers have gained significant interest owing to their distinctive advantages. These encompass the capacity for tunable light absorption and electronic properties, the ease of modifying the skeletal architecture and porosity, superior light stability, and robust economic viability; they mainly include the family of g-C3N4 [8,9,10,11], linear conjugated polymers [12,13,14,15], conjugated microporous polymers (CMPs) [16,17,18,19,20], covalent organic frameworks (COFs) [21,22,23], and covalent triazine-based frameworks (CTFs) [24,25,26]. However, the performance of polymer photocatalysts is affected by a variety of complex factors, and how to understand the structure–effect relationship is the key to designing efficient photocatalysts [20,27]. In our previous studies, the influence mechanisms of numerous parameters in organic photocatalysts were investigated, including the effect of bandgap modulation on the light absorption range, the effect of the donor–acceptor (D-A) structure and ratio on the separation efficiency of electron–hole pairs, and a preliminary mechanism for the promotion of the catalytic efficiency by strong dipole units like B ← N [28,29,30,31]. It is very important to consider the interconnectivity of different factors, among which the optimization of the D-A ratio and the increment in specific surface area are beneficial for photocatalytic reactions. As a molecule with a tetragonal–disphenoid shape, spirobifluorene not only functions as tetrahedral nodes for three-dimensional structures to construct CMPs boasting a large specific surface area but also exhibits remarkable potential in optoelectronic devices on account of its distinctive conjugated structure [32,33,34,35]. Furthermore, the orthogonal conformation of two planar intermolecular fluorene units possesses significant rigidity and can enhance the stability of the resulting CMPs [36]. Building organic photocatalysts around spirobifluorene units would be expected for good photocatalytic performance.
Herein, this work attempted to realize large-specific-surface-area CMP photocatalysts using spirobifluorene as the core unit, matching a wide-bandgap dibenzothiophene sulfone electron acceptor to achieve D-A structure construction (Scheme 1), and developed conjugated polymer series with various strengths of push–pull electron effects by copolymerizing them in various proportions. Crosslinked pore structures were successfully constructed, and the specific surface area increased with the ratio of spirobifluorene. The highest photocatalytic hydrogen production activity of polymer Spso-3, 22.4 mmol h−1 g−1, was observed when the molar ratio of D-A was 1:20, which could be attributed to the increase in the photo-induced carrier separation efficiency, suggesting that the photocatalytic performance is dependent on the molar ratio of D-A in the backbone. The molar ratio of D-A serves as a significant variable in achieving a balance between the separation and transport of photogenerated electrons and holes, further influencing the polymers’ photocatalytic performance. All indications showed that the establishment of D-A type CMPs constitutes a useful approach to enhancing photocatalytic performance for water splitting. The conjugated D-A structure has the ability to facilitate the separation of light-induced electron–hole pairs. Meanwhile, the porous structure constructed on the basis of stereoscopic units offers ample space for catalytic reactions.

2. Results and Discussion

2.1. Structure and Morphology Analyses

Six random polymers were prepared by adjusting the molar feed ratio of 2,2′,7,7′-tetrabromo-9,9′-spirobifluorene, 3,7-dibromo-dibenzothiophene sulfone and 3,7-diborate-dibenzothiophene sulfone using Pd(0)-catalyzed Suzuki–Miyaura coupling polymerization (Scheme 1). The molar ratio of D-A in the polymer structure was controlled between 0:1 and 1:2. The D-A ratio of 0:1 represents the chain polymer of polydibenzothiophene sulfone, which is still named Spso-1 for uniformity, and the other CMPs were named Spso-2 to Spso-5. The polymers obtained were processed through Soxhlet extraction using tetrahydrofuran (THF) to eliminate unwanted impurities. No solubilizing side chains were incorporated into the polymer framework. Consequently, all the obtained polymers were insoluble in common organic solvents like methanol, hexanes, THF, acetone, and so on. This phenomenon might be attributed to the rigid backbone of the polymer, which further impedes the determination of its molecular weight.
The functional group structures of the six polymers were determined by Fourier transform infrared (FT-IR) spectroscopy. As shown in Figure 1a, the infrared spectra of all polymers showed the symmetric and asymmetric stretching vibration peaks of the O=S=O bond of the sulfone group at 1300 and 1155 cm−1, as well as the stretching vibration peak of the aromatic ring –C=C– at around 1600 cm−1 [37,38,39]. The presence of the sulfone group and aromatic ring in the polymers was confirmed by FT-IR spectroscopy. Since the photocatalytic water splitting reaction needs to be carried out under strong light irradiation, which must lead to an inevitable long-term temperature increase in the photoreactor, the polymers should be thermally stable [40]. The thermal stability of the polymers was studied by thermogravimetric analysis (TGA). The decomposition temperatures at which the polymer mass loss was 5% were 469 °C (Spso-1), 449.1 °C (Spso-2), 460.8 °C (Spso-3), 470.1 °C (Spso-4), 445.8 °C (Spso-5), and 441.6 °C (Spso-6), all above 400 °C, indicating that the introduction of spirobifluorene did not affect the thermal stability of the material (Figure 1b). In the low-temperature range below 430 °C, the weight loss was attributed to physically adsorbed water inside and outside the pores, pyrolysis of certain functional groups on the surface of the material, and volatilization of trace amounts of residual solvent [41]. All the polymers met the thermal stability requirements for the photocatalytic reaction. The powder X-ray diffraction (PXRD) patterns of the six conjugated polymers, Spso-1 to Spso-6, are shown in Figure 1c. Polymers Spso-1 to Spso-4 had a distinct sharp diffraction peak at around 2θ = 12.7° and 24.5°, respectively. The diffraction peak at around 24.5° indicated that the π-π stacking distance of the polymer skeleton was in the range of 3.8–4 Å. There was a low-intensity broad peak at around 2θ = 21.8°. Polymers with a molar ratio of spirobifluorene to dibenzothiophene sulfone in the range of (0–1:8) had more distinct peaks, indicating that they had more crystalline structures, which was more conducive to the improvement of photocatalytic activity [42]. However, the PXRD patterns of polymers Spso-5 and Spso-6 did not show obvious diffraction peaks, only a low-intensity broad peak in the shape of a bun, indicating that the crystallinity of the polymers with a molar ratio of spirobifluorene to dibenzothiophene sulfone in the range of (1:4–1:2) was poor and they were amorphous. With the ratio increase in spirodifluorene in the polymer skeleton, the diffraction peak intensity varies from a peak with a certain intensity to a steamed bun-like large package peak, and the structure gradually changes from order to disorder, that is, from semi-crystalline to amorphous. The outcomes revealed that a higher proportion of spirobifluorene in the polymer skeleton would significantly reduce the crystallinity of the material, while a lower proportion of spirobifluorene had little effect on the crystallinity of the material. The unique rigid three-dimensional structure of spirobifluorene will produce a large spatial site resistance in the polymer. As the spirobifluorene ratio increases, the spatial site resistance effect becomes obvious, causing the alignment and regular stacking of polymer molecular chains to become difficult, and the freedom of molecular chains decreases, and the crystallization process is inhibited, which leads to a decrease in crystallinity [43,44,45,46]. The microscopic morphology of the polymer has a significant influence on the photocatalytic activity [47]. By observing the microscopic morphology of the six polymers using a scanning electron microscope (SEM), as shown in Figure 1d, we observed that polymer Spso-1 had an irregular flake-like morphology. When a small amount of spirobifluorene was introduced into the polymer skeleton, that is, when the ratio of spirobifluorene to dibenzothiophene sulfone was in the range of 1:40 to 1:8, polymers Spso-2, Spso-3, and Spso-4 exhibited a thin flake-like morphology. With the further increase in the ratio of spirobifluorene to dibenzothiophene sulfone, polymers Spso-5 and Spso-6 exhibited an aggregated granular morphology. Results of SEM demonstrated that the variation in the D-A ratio changes the polymers’ micromorphology, with a smaller ratio of spirobifluorene in the polymer backbone favoring the formation of flake-like morphology. Different morphologies of the photocatalyst would result in differences in the specific surface area and the number of reactive sites, thus having an important influence on the carrier transport and photocatalytic activity [48]. The nanosheet structure has better carrier transport efficiency than bulk materials [49,50].

2.2. Energy Band and Photophysical Property

The polymers Spso-1~Spso-6 showed almost no difference in color under natural light and were all yellow. Under the irradiation of a 365 nm UV lamp, the fluorescence color of the polymers shifted from yellow-green to blue-green, and a slight blue shift phenomenon occurred (Figure 2a). The blue shift in the polymer color could be attributed to the highly distorted structure of the spirobifluorene, which prevents the departure of electrons from the domain [51].
The ability of the polymer to absorb light is an important element for achieving high-efficiency photocatalytic hydrogen production activity. The solid UV–visible diffuse reflectance spectra (DRS) showed a series of absorption edge bands, ranging from 469 to 495 nm (Figure 2b). The light absorption of all polymers extended to the visible light region. By analyzing the DRS spectra of the six polymers, the maximum absorption wavelengths of polymers Spso-1 to Spso-6 were 479 nm, 480 nm, 495 nm, 482 nm, 474 nm, and 469 nm, respectively. That is, with the increase in the spirobifluorene content in the polymer skeleton, the light absorption of the polymer did not change significantly. There was a slight red shift at first, and then a slight blue shift. From the light absorption edge band, we have the following formula [52]:
Eg = 1240/λ
The optical bandgaps of polymers Spso-1 to Spso-6 were 2.59 eV, 2.58 eV, 2.51 eV, 2.57 eV, 2.62 eV, and 2.64 eV, respectively, decreasing from 2.59 eV to 2.51 eV and then gradually increasing to 2.64 eV. Among the six polymers, Spso-3 had the smallest bandgap, and Spso-6 had the largest bandgap. Compared with polymer Spso-1, Spso-3 had a red shift of 16 nm, and Spso-6 had a blue shift of 10 nm. These results indicated that changing the molar ratio of D-A in the conjugated polymer skeleton could fine-tune the light absorption ability of the polymer, thereby affecting the photocatalytic hydrogen evolution activity of the material. The Mott–Schottky curve was obtained by the Mott–Schottky method at an electrochemical workstation [53]. The intersection of the longest tangent line with the X-axis of curves is the flat band potential of the polymer. A positive slope of the curve indicates an n-type semiconductor and a negative slope indicates a p-type semiconductor. For n-type semiconductors, the flat band potential is negative 0.1–0.3 V compared to the conduction band potential. The conductor position of the semiconductor is calculated according to the following formula:
ELUMO (NHE) = E (Ag/AgCl) + 1.976 V − 0.3V
Combined with UV–Vis absorption spectra, the corresponding valence band positions are obtained (Figure 2c and detailed energy level data in Table 1). The results show that the lowest unoccupied molecular orbital (LUMO) level of all polymers is above the reduction potential of water, which has enough driving force for proton reduction half-reaction. The LUMO energy level of the six polymers did not change significantly, indicating that changing the molar ratio of D-A did not have a significant effect on the energy level of the materials. The Brunauer–Emmett–Teller (BET) specific surface areas of the polymers were determined using nitrogen adsorption–desorption experiments. The specific surface areas of the six polymers, Spso-1~Spso-6, are listed in Table 1, at 54.3, 177.3, 131.8, 267.4, 384.8, and 681.7 m2 g−1, respectively. Due to the highly distorted spatial structure of spirodifluorene, increasing the ratio of spirodifluorene significantly improved the specific surface area of the materials. The specific surface area of the polymers generally shows an increasing trend, while the specific surface area of Spso-3 slightly decreases, which may be related to its unique lamellar structure.
The fluorescence emission peaks of Spso-1 to Spso-6 are 511 nm, 507 nm, 505 nm, 503 nm, 492 nm, and 485 nm, respectively, with a trend of gradual blue shift (Figure 3a), suggest that the photophysical properties and electronic structures of the polymers can be finely adjusted within a wide range by altering the molar ratio of D-A. Figure 3b shows a comparison chart of the fluorescence intensity of the polymer. The fluorescence emission peak intensity shows a trend of decreasing first and then increasing, indicating that the probability of photogenerated electron and hole recombination decreases first and then increases. Spso-6, the polymer with the highest ratio of spirodifluorene and dibenzothiophene sulfone units, has the strongest fluorescence emission intensity, indicating that the structure of polymer Spso-6 may limit the migration of photogenerated electrons and holes under light irradiation so that the photogenerated electrons and holes can quickly recombine. Polymers Spso-2, Spso-3, and Spso-4 all have relatively low fluorescence emission intensities, indicating that the photogenerated electrons and holes generated by these three polymers can be better separated and migrated, which is helpful for the improvement of photocatalytic hydrogen production performance.
The fluorescence lifetime data of the six polymers are listed in Table 2, and the average fluorescence lifetime is calculated as follows [54]:
τAVG = A1 × τ1 + A2 × τ2 + A3 × τ3
As shown in Figure S1, the fluorescence lifetimes of the polymers are shorter, and all of them are in the nanosecond scale, at 2.95 ns, 2.42 ns, 2.45 ns, 0.93 ns, 1.85 ns, and 1.33 ns, respectively. Among all the polymers, the shorter lifetimes of the polymers with higher proportions of spirobifluorene, Spso-4~Spso-6, indicate that the probability of photogenerated electrons and holes complexing in these polymers is higher, and they will be deactivated very quickly, which is unfavorable for the transport of the electrons to the surface of the materials and will affect the photocatalytic hydrogen-producing half-reaction. When the ratio of spirobifluorene to dibenzothiophene sulfone unit is less than 1:8, the polymer has a relatively long lifetime. This indicates that the D-A molar ratio in the polymer main chain has a substantial impact on the fluorescence lifetime of the polymers. When the D-A molar ratio is relatively low, the fluorescence lifetime of the material is relatively long. Conversely, when the molar ratio of D-A is relatively high, the fluorescence lifetime is relatively short. Longer fluorescence lifetimes would result in sufficient time for photogenerated electron/hole transfer, which in turn affects the performance of photocatalytic hydrogen production [55]. The D-A ratio affects the fluorescence lifetime of polymers as well. Transient photocurrent response testing provides further insight into the ability of polymers to transport photoinduced charge carriers. This measurement was carried out in four on/off cycles. The illumination used had wavelengths greater than 420 nm and an intensity of 100 mW·cm–2. As shown in Figure 3c, first, the i-t curves of all polymers had a good correlation with the process of switching the light on and off, indicating that the polymers all had good photocatalytic activity. Secondly, there were significant differences in the photocurrent intensity, and the photocurrent density from high to low was Spso-3, Spso-2, Spso-4, Spso-5, Spso-1, and Spso-6. Electrochemical impedance spectroscopy (EIS) characterizes the charge transport properties (Figure 3d), and Spso-3 has the smallest Nyquist circle radius, suggesting that interfacial charge transfer is more efficient, which is consistent with the photocurrent response. These results combined with fluorescence emission spectroscopy and fluorescence lifetime results indicate that polymer Spso-3 has better carrier separation efficiency.

2.3. Photocatalytic Performance

The photocatalytic activity for hydrogen production was estimated for all the samples under visible light irradiation with wavelengths greater than 420 nm. The curves of H2 evolution vs. irradiation time are shown in Figure 4a, with average hydrogen evolution rates (HERs) of 12, 21.2, 22.4, 18.6, 14.4, and 7.5 mmol g–1 h–1 for Spso-1 to Spso-6, respectively (Figure 4b). The HER initially increases and then decreases as the pyrene content increases. When Spso-3 is used as a photocatalyst, the maximum HER of 22.4 mmol g–1 h–1 is obtained. Under visible light irradiation, this value is markedly higher than numerous other reported values of materials containing thiophene units [56,57,58,59,60,61,62,63,64,65]. These results indicate that the D-A ratio is a crucial variable in balancing photogenerated electron and hole segregation and transportation, which has an important impact on optimizing the photocatalytic performance of polymers [66]. These remarkable HER values imply that constructing D-A type CMPs could be an effective tactic for enhancing the photocatalytic water-splitting ability. The D-A structure can promote the separation of photogenerated electron/hole pairs, and the porous structure built on steric units offers sufficient space for the catalytic reaction. To study the relationship between light absorption and hydrogen production activity, polymer Spso-3 was selected for the apparent quantum yield (AQY) test. AQY represents the ratio of the number of photons effectively utilized in the photochemical reaction to the total number of photons absorbed [67,68]. Monochromatic light filters with wavelengths of 420 nm, 450 nm, and 500 nm were used to conduct the photocatalytic experiment on polymer Spso-3 for 3 h, under the reaction conditions of 40 mL of deionized water, 10 mL of triethanolamine, and 10 mg of the photocatalyst. The hydrogen production amounts of polymer Spso-3 at 420 nm, 450 nm, and 500 nm for 1 h were 179, 140, and 30.3 mol g–1 h−1, respectively. The AQY values of polymer Spso-3 at 420 nm, 450 nm, and 500 nm were 35.6%, 12.7%, and 1.82%, respectively. The AQY of polymer Spso-3 at 420 nm was the highest, indicating that it had the highest utilization rate of 420 nm monochromatic light. This value is remarkably higher than the previously reported AQY value for comparable organic photocatalysts [57,58,62,63,64,65,69,70,71]. Consistent with the absorption spectrum, the HER of Spso-3 decreased significantly at 500 nm, suggesting that the main hydrogen-producing photons are below 500 nm.
To prevent the palladium catalyst residue from causing photocatalytic testing errors, the palladium residue was determined by inductively coupled plasma mass spectrometry (ICP-MS). There was no obvious correlation between the hydrogen production activity of the polymer and the content of the palladium catalyst (R2 = 0.24, Figure 4d). The lower residual concentration (0.23–0.68 wt%) is much lower than the addition of palladium in other organic photocatalytic systems as an adjunct (ca. 3 wt%). So while Pd residues were detected in the polymers, the low concentrations did not influence the regularity of the photocatalytic test results.
The stability of this organic photocatalyst was evaluated by repeating experiments on Spso-3. It was subjected to five consecutive photocatalytic cycles of 15 h (with each cycle being 5 h) under visible light irradiation. Using triethanolamine as the sacrificial agent, every 5 h was taken as a hydrogen production cycle, and the sacrificial agent was replenished and degassed. As shown in Figure 5a, we observed that after 15 h of continuous visible light catalytic testing, the rate of hydrogen production by polymer Spso-3 hardly decreased, indicating that there was no harmful photochemical degradation of Spso-3 during illumination, and it was a stable photocatalyst. Following a 15 h photocatalytic reaction, the recovered Spso-3 was analyzed using UV–vis adsorption spectra (Figure 5b), PXRD (Figure 5c), and FT-IR (Figure 5d). Spso-3 retains its original structure and crystallinity well after the photocatalytic reaction, resulting in excellent photocatalytic cycling stability. Moreover, by comparing the peak PXRD intensity before and after photocatalysis, a certain degree of decrease in the crystallinity of Spso-3 is observed. After a 15 h photocatalytic reaction under visible light (λ > 420 nm), the flake morphology of Spso-3 became smooth, which may have caused a partial decrease in the photocatalytic performance (Figure 5e,f). The above results indicate that Spso-3 possesses outstanding HER performance and remarkable long-time stability, possessing the potential to be a good polymer photocatalytic material.

3. Materials and Methods

3.1. Materials and Reagents

In this study, Dibenzothiophene sulfone (purity 99.28%), Pinacol bis(boronic acid) ester (purity 98%), 1,1′-bis (diphenylphosphine) ferrocene] palladium dichloride (purity 98%), Tetri (triphenylphosphine) palladium (purity 98%), 1,3,6,8-tetrabromopyrene (purity of 98%), 2,2′,7,7′-tetrabromo-9,9′-spirodifluorene (M1, purity greater than 97%), 1,4-dioxane (AR), anhydrous potassium carbonate (AR), potassium acetate (AR), anhydrous magnesium sulfate (AR), barium sulfate (AR), sodium bicarbonate (AR), sulfuric acid (purity 98%), nitric acid (AR), hydrochloric acid (AR), N,N-Dimethylformamide (AR), methanol (AR), dichloromethane (AR), ethanol (AR), acetone (AR) and tetrahydrofuran (THF, AR) were used. All reactions and manipulations were carried out under an argon atmosphere by using standard Schlenk techniques or an inter-atmosphere glovebox. Before use, all the solvents were dried by refluxing with sodium and benzophenone and degassed by applying three freeze–pump–thaw cycles. Chloroform-d was dried by using a 4 Å molecular sieve (2–3 days). All chemicals (reagents and solvents) were obtained from commercial suppliers (Energy Chemical, Heowns, Tianjin, China) and directly used without further purification.

3.2. Polymer Synthesis Methods

3.2.1. Synthesis of 3,7-Dibromo-dibenzothiophene Sulfone (M2)

Dibenzothiophene sulfone monomer (7.0 g, 32.4 mmol) was dissolved in 240 mL concentrated sulfuric acid in an atmosphere of argon, and n-bromosuccinimide (NBS) (12 g, 67.4 mmol) was added slowly in three batches. The mixture was then stirred at room temperature for 24 h. After the reaction was complete, the reaction solution was slowly poured into a large amount of ice water for dilution. The solids were filtered and then washed with water and a solution of saturated sodium bicarbonate until the pH was neutral. After drying in a vacuum drying oven, the pure white solid was obtained by repeated recrystallization with trichloromethane (150 mL × 3) three times (5.5 g, 45%). 1H NMR (400 MHz, CDCl3): δ 7.93 (s, 2H), 7.76–7.78 (d, J = 8.0 Hz, 2H), 7.62–7.64 (d, J = 8.0 Hz, 2H).

3.2.2. Synthesis of 3,7-Diborate Ester-dibenzothiophene Sulfone (M3)

3,7-Dibromo-dibenzothiophene sulfone (4.1 g, 10.7 mmol), pinanol borate (8.4 g, 33.0 mmol), and potassium acetate (6.3 g, 6.3 mmol) were mixed in a 250 mL round-bottom flask under the protection of argon. [1,1′-bis(diphenylphosphonyl)ferrocene] palladium dichloride (0.40 g, 0.5 mmol) was dissolved in 120 mL dried 1, 4-dioxane, and reacted at 106 °C for 24 h. After the reaction, about 150 mL water was added to the reaction mixture, and then 100 mL dichloromethane was extracted 3 times. The organic phase was combined and dried with anhydrous magnesium sulfate, and the organic solvent was removed by rotary evaporation. The solid crude product was purified by a silica gel column (petroleum ether: dichloromethane = 2:1), and the white solid M3 was obtained (2.6 g, 50%). 1H NMR (400 MHz, CDCl3): δ 8.27 (s, 2H), 8.04–8.06 (d, J = 8.0 Hz, 2H), 7.79–7.81 (d, J = 8.0 Hz, 2H), 1.36 (s, 24H).

3.2.3. Typical Procedure of Suzuki–Miyaura Coupling Polymerization

Shown as Scheme 1, under anaerobic and anhydrous conditions, 2,2′,7,7′-tetrabromo-9,9′-spirodifluorene (M1), 3,7-dibromo-dibenzothiophene sulfone (M2), 3,7-diborate ester dibenzothiophene sulfone (M3), Pd(PPh3)4 (5%) and K2CO3 were dissolved in mixed solvent of degassed DMF and degassed H2O. Then, the reaction was refluxed at 130 °C for 48 h. After cooling to room temperature, the reaction mixture was filtered through the funnel to gain crude product, the polymer was further purified and washed with H2O, MeOH, CH2Cl2, THF, and acetone. Then, the resulting polymers were subjected to Soxhlet extraction using THF for 48 h to remove the undesirable impurities.

3.2.4. Synthesis of Spso-1

M2 (0.187 g, 0.50 mmol), M3 (0.234 g, 0.50 mmol), anhydrous potassium carbonate (0.830 g, 6.0 mmol), tetraphenylphosphine) palladium (0.0125 g, 8.7 μmol), 16 mL of N,N-dimethylformamide, and 3 mL of water were used for Suzuki–Miyaura coupling polymerization. The yellow-green solid was obtained, the yield was 0.213 g, and the yield was 82% (Pd: 0.22 wt%).

3.2.5. Synthesis of Spso-2

M1 (0.0152 g, 0.024 mmol), M2 (0.169 g, 0.45 mmol), M3 (0.234 g, 0.50 mmol), anhydrous potassium carbonate (0.830 g, 6.0 mmol), tetraphenylphosphine) palladium (0.0125 g, 8.7 μmol), 16 mL of N,N-dimethylformamide, and 3 mL of water were used for Suzuki–Miyaura coupling polymerization. The yellow-green solid was obtained, the yield was 0.163 g, and the yield was 78% (Pd: 0.38 wt%).

3.2.6. Synthesis of Spso-3

M1 (0.0284 g, 0.045 mmol), M2 (0.153 g, 0.41 mmol), M3 (0.234 g, 0.50 mmol), anhydrous potassium carbonate (0.830 g, 6.0 mmol), tetraphenylphosphine) palladium (0.0125 g, 8.7 μmol), 16 mL of N,N-dimethylformamide, and 3 mL of water were used for Suzuki–Miyaura coupling polymerization. The yellow-green solid was obtained, the yield was 0.162 g, and the yield was 77% (Pd: 0.22 wt%).

3.2.7. Synthesis of Spso-4

M1 (0.0632 g, 0.10 mmol), M2 (0.112 g, 0.30 mmol), M3 (0.234 g, 0.50 mmol), anhydrous potassium carbonate (0.830 g, 6.0 mmol), tetraphenylphosphine) palladium (0.0125 g, 8.7 μmol), 16 mL of N,N-dimethylformamide, and 3 mL of water were used for Suzuki–Miyaura coupling polymerization. The yellow-green solid was obtained, the yield was 0.202 g, and the yield was 98% (Pd: 0.32 wt%).

3.2.8. Synthesis of Spso-5

M1 (0.105 g, 0.17 mmol), M2 (0.0625 g, 0.17 mmol), M3 (0.234 g, 0.50 mmol), anhydrous potassium carbonate (0.830 g, 6.0 mmol), tetraphenylphosphine) palladium (0.0125 g, 8.7 μmol), 16 mL of N,N-dimethylformamide, and 3 mL of water were used for Suzuki–Miyaura coupling polymerization. The green solid was obtained, the yield was 0.171 g, and the yield was 83% (Pd: 0.39 wt%).

3.2.9. Synthesis of Spso-6

M1 (0.158 g, 0.25 mmol), M3 (0.234 g, 0.50 mmol), anhydrous potassium carbonate (0.830 g, 6.0 mmol), tetraphenylphosphine) palladium (0.0125 g, 8.7 μmol), 16 mL of N,N-dimethylformamide, and 3 mL of water were used for Suzuki–Miyaura coupling polymerization. The green solid was obtained, the yield was 0.183 g, and the yield was 99% (Pd: 0.68 wt%).

3.3. Instrumentation/General Methods

The Fourier transform infrared (FTIR) spectra of the samples were recorded with a Bruker Vertex 70 (Billerica, MA, USA) at room temperature by using the conventional KBr pellet method. Thermogravimetric analysis was performed on a NETZSCH STA449C (Netzsch GmbH, Selb, Germany) instrument under nitrogen flow at a heating rate of 10 °C min−1 up to 800 °C. The surface morphology of the polymers was evaluated with a Hitachi S4800 Cold Field Emission SEM (Tokyo, Japan). The Powder X-ray diffraction (PXRD) measurements were performed using X-ray diffraction (X’pert pro, PANalytical, Almelo, The Netherlands), equipped with Cu Kα radiation (λ = 1.54056 Å) at the scattering angle 2θ between 3 and 50°. The UV–visible absorption spectra of the polymers were recorded for the dry-pressed disk samples on a Carry 5000 UV–Vis–NIR spectrometer (Agilent, Santa Clara, CA, USA). The fluorescence data were measured on an FLS920 fluorescence spectrophotometer (Edinburgh Instruments, Livingston, UK) with the polymer powders and optically dilute solutions (A < 0.1). Time-correlated single-photon counting experiments were performed on an Edinburgh FLS920 fluorescence spectrophotometer with picosecond pulsed LED excitation sources and an R928 detector (Agilent, Santa Clara, CA, USA). An EPLED-330 diode (λ = 334.6 nm, instrument response 881.6 ps, bandwidth 9.6 nm, Edinburgh Instruments, UK) was used as the light source. Decay times were fitted in the F900 software (ver. 7.1.3) using suggested lifetime estimates. Photocurrent response was recorded using a CHI-660E electrochemical workstation (CH Instruments, Shanghai, China) in a standard three-electrode configuration. The polymers (2.0 mg) were dispersed in ethanol (0.5 mL) containing 1% Nafion by ultrasound for 1 h. The work electrodes were prepared via drop-casting the mixture of 200 μL onto the surface of the FTO glass substrate electrode and then filmed at 80 °C in a vacuum oven. The Pt plate served as the counter electrode, and a saturated Ag/AgCl electrode as a reference electrode. A 0.5 M Na2SO4 solution was used as the electrolyte. Sunlight was simulated with a 300 W Xenon lamp (Hefan Instrument, Shanghai, China) and a 420 nm cut-off filter.

3.4. Photocatalytic Activity Measurements

A flask was charged with the polymer powder (10.0 mg), a 4:1 vol. mixture of water and triethanolamine. The resulting suspension was ultrasonicated until the photocatalyst was dispersed before degassing by N2 bubbling for 30 min. The accumulated number of evolved gases was monitored every 60 min using a gas chromatograph (450-GC, Varian, Palo Alto, CA, USA) equipped with a thermal conductive detector (TCD). The light source was a 300 W Xe lamp with a cut-off filter applied to generate visible light (λ > 420 nm). Hydrogen was detected with a TCD detector, referencing standard gases with known concentrations of hydrogen. Hydrogen dissolved in the reaction mixture was not measured, and the pressure increase generated by the evolved hydrogen was neglected in the calculations.

3.5. AQY Measurements

The apparent quantum yield (AQY) for H2 evolution was measured using monochromatic visible light at 420 nm. The energy (E) of irradiation was determined to be 30 mW cm−1 by a calibrated power meter. The irradiation area was controlled at 9.8 cm2. Depending on the amount of hydrogen produced by the photocatalytic reaction in an average of 3 h, the AQY was calculated with the following equation:
A Q Y = N e N p × 100 %
= 2 × M × N A × h × c S × P × t × λ × 100 %
where M is the amount of H2 (mol), NA is the Avogadro constant (6.022 × 1023/mol), h is the Planck constant (6.626 × 10–34 J·s), c is the speed of light (3 × 108 m/s), S is the irradiation area (cm2), P is the intensity of irradiation light (W/cm2), t is the photoreaction time (s), λ is the wavelength of the monochromatic light (m).

4. Conclusions

In conclusion, the D-A type CMP photocatalysts with spirobifluorene as the core unit and dibenzothiophene sulfone as the electron acceptor were successfully synthesized. Crosslinked pore structures were successfully constructed and the specific surface area increased with the ratio of spirobifluorene. When the molar ratio of D-A was 1:20, the polymer Spso-3 showed the highest photocatalytic hydrogen production activity of 22.4 mmol h–1 g–1, which was probably caused by the increase in the photo-induced carrier separation efficiency, indicating that the photocatalytic performance depended on the molar ratio of D-A in the skeleton. These results indicate that the D-A ratio is a crucial variable in balancing photogenerated electron and hole segregation and transportation, which has an important impact on optimizing the photocatalytic performance of polymers. The D-A structure can promote the separation of photogenerated electron/hole pairs, and the porous structure built on steric units offers sufficient space for the catalytic reaction. All the findings suggest that the construction of D-A type CMPs could be an efficient approach to improve the photocatalytic performance for water splitting.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14100717/s1, Figure S1: The average fluorescence lifetimes of polymers.

Author Contributions

Conceptualization, H.Z. and X.P.; methodology, P.S., H.X., L.L., J.W. and X.X.; validation, X.X. and P.S.; formal analysis, S.S. and W.L.; investigation, Z.K.; resources, X.P.; data curation, H.Z.; writing—original draft preparation, H.Z.; writing—review and editing, X.P.; supervision, X.P.; project administration, H.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Shandong Provincial Natural Science Foundation, grant number ZR2023QE014; and National Natural Science Foundation of China, grant number 22171111.

Data Availability Statement

Dataset available on request from the authors.

Acknowledgments

The authors thank the Supercomputing Center of Lanzhou University for the DFT calculations.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 14 00717 sch001
Scheme 1. Synthetic route of Spso-1~Spso-6.
Scheme 1. Synthetic route of Spso-1~Spso-6.
Catalysts 14 00717 sch001
Catalysts 14 00717 g001
Figure 1. (a) FT-IR spectra of the polymers. (b) TGA traces for the polymers were measured under a nitrogen atmosphere. (c) PXRD patterns and (d) SEM images of all polymers.
Figure 1. (a) FT-IR spectra of the polymers. (b) TGA traces for the polymers were measured under a nitrogen atmosphere. (c) PXRD patterns and (d) SEM images of all polymers.
Catalysts 14 00717 g001
Catalysts 14 00717 g002
Figure 2. (a) Digital photographs of polymers in natural light (top) and 365 nm UV light (bottom). (b) UV–vis absorption spectra and (c) energy level diagrams of polymers.
Figure 2. (a) Digital photographs of polymers in natural light (top) and 365 nm UV light (bottom). (b) UV–vis absorption spectra and (c) energy level diagrams of polymers.
Catalysts 14 00717 g002
Catalysts 14 00717 g003
Figure 3. (a) Fluorescence emission spectra and (b) comparison diagram of fluorescence emission intensity of polymers. (c) Comparison of photocurrent behaviors of the polymers under visible light (λ > 420 nm, 100 mW cm−2). (d) Electrochemical impedance spectrum of the polymers.
Figure 3. (a) Fluorescence emission spectra and (b) comparison diagram of fluorescence emission intensity of polymers. (c) Comparison of photocurrent behaviors of the polymers under visible light (λ > 420 nm, 100 mW cm−2). (d) Electrochemical impedance spectrum of the polymers.
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Figure 4. (a) Time-dependent hydrogen evolution with polymers lacking a metal cocatalyst under visible light illumination (wavelength > 420 nm, dosage of each photocatalyst is 10 mg). (b) Comparison of visible-light-driven H2 production activity among the polymers (wavelength > 420 nm). (c) UV–vis diffuse reflectance spectrum and apparent quantum yields (at 420, 450, and 500 nm) of Spso-3. (d) Correlation between photocatalytic hydrogen production activity and palladium residue.
Figure 4. (a) Time-dependent hydrogen evolution with polymers lacking a metal cocatalyst under visible light illumination (wavelength > 420 nm, dosage of each photocatalyst is 10 mg). (b) Comparison of visible-light-driven H2 production activity among the polymers (wavelength > 420 nm). (c) UV–vis diffuse reflectance spectrum and apparent quantum yields (at 420, 450, and 500 nm) of Spso-3. (d) Correlation between photocatalytic hydrogen production activity and palladium residue.
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Figure 5. (a) Stability test using Spso-3 in water over a time period of 25 h (λ > 420 nm). (b) UV–vis adsorption spectra, (c) PXRD spectra, (d) FT–IR spectra, and (e,f) SEM images of Spso-3 before and after the photocatalytic cycling experiments.
Figure 5. (a) Stability test using Spso-3 in water over a time period of 25 h (λ > 420 nm). (b) UV–vis adsorption spectra, (c) PXRD spectra, (d) FT–IR spectra, and (e,f) SEM images of Spso-3 before and after the photocatalytic cycling experiments.
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Table 1. Energy level and specific surface area data of polymers.
Table 1. Energy level and specific surface area data of polymers.
Sampleλonset
(nm)		λem
(nm)		Eg
(eV)		BET Specific Surface Area
(m2 g−1)		LUMO
(eV)		HOMO
(eV)
Spso-14795112.5954.3−0.601.99
Spso-24805072.58177.3−0.601.98
Spso-34955052.51131.8−0.641.87
Spso-44825032.57267.4−0.621.95
Spso-54744922.62384.8−0.631.99
Spso-64694852.64681.7−0.592.05
Table 2. Fluorescence lifetime data of polymers.
Table 2. Fluorescence lifetime data of polymers.
Sampleτ1 [ns]
(Rel.%)		τ2 [ns]
(Rel.%)		τ3 [ns]
(Rel.%)		τ [ns]χ2
Spso-11.12 (44.93)3.34 (48.89)13.30 (6.18)2.951.155
Spso-20.61 (33.38)2.31 (54.79)8.05 (11.83)2.421.070
Spso-30.91 (53.22)2.88 (39.30)11.24 (7.39)2.451.139
Spso-40.39 (79.74)1.71 (16.66)9.18 (3.61)0.931.140
Spso-50.59 (61.36)2.18 (30.68)10.32 (7.96)1.851.287
Spso-60.75 (55.59)1.88 (42.73)6.62 (1.68)1.331.130
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Zhao, H.; Sun, P.; Xu, H.; Xiao, X.; Kong, Z.; Song, S.; Li, W.; Liu, L.; Wang, J.; Pan, X. Spirobifluorene-Based D-A Type Conjugated Polymer Photocatalysts for Water Splitting. Catalysts 2024, 14, 717. https://doi.org/10.3390/catal14100717

AMA Style

Zhao H, Sun P, Xu H, Xiao X, Kong Z, Song S, Li W, Liu L, Wang J, Pan X. Spirobifluorene-Based D-A Type Conjugated Polymer Photocatalysts for Water Splitting. Catalysts. 2024; 14(10):717. https://doi.org/10.3390/catal14100717

Chicago/Turabian Style

Zhao, Hao, Pengyao Sun, Hui Xu, Xinyi Xiao, Zhiyuan Kong, Shige Song, Weihao Li, Luzun Liu, Jiadong Wang, and Xiaobo Pan. 2024. "Spirobifluorene-Based D-A Type Conjugated Polymer Photocatalysts for Water Splitting" Catalysts 14, no. 10: 717. https://doi.org/10.3390/catal14100717

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