Effect of controlling the number of fused rings on polymer photocatalysts for visible-light-driven hydrogen evolution

Li-Yu Ting§ , Jayachandran Jayakumar§ , Chih-Li Chang , Wei-Cheng Lin , Mohamed Hammad Elsayed and Ho-Hsiu Chou *
Department of Chemical Engineering, National Tsing Hua University, No. 101, Sec. 2, Kuang-Fu Rd., Hsinchu 30013, Taiwan. E-mail: hhchou@mx.nthu.edu.tw

Received 15th June 2019 , Accepted 12th August 2019

First published on 14th August 2019

Searching an efficient and robust photocatalyst is important in converting solar energy into chemical energy for clean and renewable fuels. Thiophene and fluorene have been widely used in this field. However, no studies exist on understanding the effect of increasing the number of fused thiophene and fluorene rings. Herein, we demonstrated a series of polymer photocatalysts based on fused rings with fluorene (F) and indenofluorene (IF) units and fused-thiophene rings (thiophene (T), thienothiophene (2T), and dithienothiophene (3T)) in different combinations, denoted as PFnT and PIFnT, where n is the number of fused thiophene rings. We show that the increased number of fused rings on fluorene or thiophene units is important for photocatalytic performance. Particularly, without adding the noble Pt cocatalysts, our PF3T presented an excellent efficiency with a hydrogen evolution rate (HER) of 1095 µmol h−1 g−1 (λ > 420 nm). We show that the photocatalysts are robust, that is, PF2T could be used for over 70 hours. More importantly, even when we stored PF2T in a water/methanol/TEA solution for 20 days, its photocatalytic performance remained constant. This contribution documents the first systematic study on the construction of efficient and robust polymer photocatalysts, allowing the researcher to potentially target the number of fused rings on polymers and provides an important impact in this field.


Inspired by natural photosynthesis, photocatalysts for the conversion of solar energy into chemical energy are crucial for the generation of clean and renewable future fuels. Hydrogen has been recognized as the ideal clean energy source because its burning product is water and it does not cause secondary pollution.1 Fujishima and Honda first reported the concept of the photocatalytic splitting of water into H2 and O2 using titanium dioxide (TiO2) as an electrode.2 The development of photocatalytic H2 production technology based on semiconductors is considered one of the best methods for the solar energy to hydrogen (STH) energy conversion. Although a large number of inorganic semiconductors have been studied for photocatalytic hydrogen evolution,3–5 recently, there has been a surge of interest in organic semiconductors for visible-light-driven water splitting. For example, the first example of graphitic carbon nitride (g-C3N4) as an organic photocatalyst for hydrogen production was reported by Antonietti et al., and this catalyst has been widely investigated and found to give high performance.6–15 However, the fixed structure of g-C3N4 limits its light harvesting region as well as the obtainable STH conversion efficiency.16–21 In addition, without a metal catalyst, such as Pt and Ni–P, pure g-C3N4 shows an unsatisfactory photocatalytic ability.22 Recently, many researchers have considered semiconducting polymers as photocatalysts as their optical and electronic properties can be easily tuned by changing the building blocks.6,7 Accordingly, numerous conjugated polymers and polymer dots23 have been investigated as promising photocatalysts, including poly(p-phenylene),24,25 polybenzothiadiazoles,26–31 polyazomethine,32 poly(perylene),33 polymelamine,34 poly(triazine),35–38 polyheptazine,39 and polyhydrazone,40 for the hydrogen evolution reaction.

Recently, Cooper et al. reported a library of conjugated microporous polymers (CMPs) based on pyrene building units for efficient H2 evolution (17.4 ± 0.9 µmol h−1) under visible-light-driven and metal-free conditions. The optical band gap was further fine-tuned over a broad range of 1.94–2.95 eV by varying the monomer composition of the CMPs.41,42 Later, the same group reported a series of planarized units and phenylene copolymers for hydrogen evolution.43 Although these copolymers used various planar units, each comonomer was linked by a phenylene–phenylene connection, which limited the planarization of the polymer backbone, resulting in optical bandgaps of over 2.77 eV. Recently, the same group also reported a series of CMPs and non-porous linear polymers containing dibenzo[b,d]thiophene sulfone and fluorene analogs as photocatalysts for H2 evolution, and these photocatalysts had large optical band gaps in the range of 2.56–3.46 eV.44,45 In particular, Cooper et al. reported a series of copolymers of 1,4-phenylene and 2,5-thiophene for visible-light-driven hydrogen evolution. These reports showed that polymer photocatalysts were constructed using single-bond-linked thiophene units, phenylene units, or both thiophene and phenylene units in different ratios.46,47 However, there has been no study on understanding the effect of increasing the number of fused thiophene and fluorene rings on conjugated polymers for photocatalytic hydrogen evolution.46 The forced planarization arising from the covalent attachment of the adjacent aromatic units in the polymer backbone should enable π-electron delocalization, narrow the bandgap, suppress interannular rotation, and enhance the intrinsic charge mobility.48,49 Therefore, we expected to enhance the photocatalytic performance of the polymers by introducing these fused aromatic rings. In general, the fluorene unit is known to have excellent photoelectronic properties, such as a high photon-to-electron conversion efficiency, and it is widely used in optoelectronics, biosensors, bioimaging, and light-driven water splitting because of its high fluorescence quantum efficiency.50 However, the absorption of fluorene occurs at wavelengths of less than 420 nm, resulting in poor solar energy harvesting. Thus, the introduction of the fused fluorene moiety, indenofluorene, can provide a longer conjugation length, resulting in a more red-shifted absorption band.

In addition, the thiophene unit is one of the most popular and impressive electron donors or (π-bridges) in organic semiconductors because of its small dihedral angle and the ease, with which its electronic structure can be adjusted by substitution or oxidation reactions. However, although thiophene has been widely applied in organic photocatalytic systems, the design strategy typically involves the introduction of only single-bond-linked thiophene units into the polymer backbone.51,52 Unlike thiophenes, fused thiophenes have a more rigid structure, having extended π-conjugated systems and higher electron transport abilities, and these structural and electronic features could be exploited to alter the optical band gap (Eg) of organic materials as well as increase their intermolecular interactions in the solid state. The use of electron-rich structures allows for the construction of conjugated polymer semiconductors with low band gaps.53,54 Recently, we developed low toxicity cycloplatinated polymer dots as efficient photocatalysts for visible-light-driven H2 evolution.55 Our continued interest is in H2 evolution using low-cost and metal-free polymer photocatalysts that function under visible-light irradiation. Thus, we aimed at developing a new type of conjugated copolymer, in which the co-monomers are constructed of both fused fluorene and fused thiophene to study their photocatalytic efficiency and stability under solar irradiation.

Results and discussion

In this study, the fused thiophene units were incorporated with fused fluorene units via a Suzuki–Miyaura coupling polymerization in a one-pot reaction, and the structure and synthetic route of these conjugated copolymers are shown in Fig. 1 and Scheme S1. These two kinds of fused aromatic rings were introduced to extend the conjugation and planarity of the copolymer. The combination of bromothiophenes, such as 5-dibromothiophene (T-Br), 2,5-dibromothieno[3,2-b]thiophene (2T-Br), and 2,6-dibromodithieno[2,3-b:2′,3′-d]thiophene (3T-Br), which are analogous with the comonomer fluorene–boronic ester (F–B), resulted in polymers denoted PFT, PF2T, and PF3T, respectively. For comparison with the PFTs, analogous copolymers prepared from the indenofluorene–boronic ester (IF–B) co-monomer were also prepared by polymerization with these three bromothiophene derivatives, named PIFT, PIF2T, and PIF3T, respectively (Fig. 1). Both the fluorene and indenofluorene co-monomers were modified with octyl groups to provide a large steric hindrance and reduce aggregation when inserting the fused thiophene units, while the indenofluorene moiety had the better solubility of the two because it contained more octyl groups. The good solubility of both polymers at room temperature may have been caused by the average planar polymeric backbone, which resulted in weak interchain interactions and a good solution processability.56,57 Notably, the polymer photocatalysts were further characterized by gel permeation chromatography (GPC) and thermogravimetric analysis (TGA), and all related properties are summarized in Table S1 and Fig. S1.
image file: c9ta06425g-f1.tif
Fig. 1 The molecular design of the conjugated polymer photocatalysts.

We examined the photophysical properties of these copolymers using UV-visible absorption and fluorescence spectrophotometry (Fig. 2a and S2). The absorption and emission spectra of the polymers were measured in dichloromethane (DCM) at room temperature. All six polymers show a strong high-energy absorption band around 418–458 nm, as summarized in Table 1. In addition, we also measured the absorption spectra for F–boronic ester (F–B) and IF–boronic ester (IF–B), revealing maxima at 318 and 358 nm, respectively. The absorption maximum of IF–B showed a 40 nm red-shift compared to that of F–B because of its longer conjugated system (Fig. S3). Similarly, we measured the absorption spectra of bromothiophenes T-Br, 2T-Br, and 3T-Br, which revealed a gradual red-shift of 256, 288, and 312 nm, respectively. Thus, our molecular design strategy should be beneficial for photocatalysis from the viewpoint of visible-light harvesting. The highest occupied molecular orbital (HOMO) energy was determined using a photoelectron spectrometer (Fig. S4), and the lowest unoccupied molecular orbital (LUMO) energy was calculated as EHOMOEg, where Eg was determined from the onset of the absorption spectrum. The resulting values are summarized in Table 1. Accordingly, the Eg values for the resulting polymers are in the range of 2.43–2.64 eV (Fig. S5). This result suggests that we can fine tune the Eg values by attaching multiple fused thiophene ring systems. Interestingly, although the absorption band of the IF–B monomer was more red-shifted (ca. 40 nm) than that of the F–B monomer, their corresponding copolymers actually showed slightly different absorption bands after polymerization with various fused thiophene units, which meant that the fused thiophene units could play a crucial role in affecting the absorption bands of the copolymer.

Table 1 Photophysical properties of six polymer photocatalysts
Polymer Absorption [nm] HOMO/LUMOa,b [eV] Bandgapc [eV] HERd,e [µmol h−1 g−1] AQY at 420 nm (at 460 nm)d,f [%] Lifetimeg [ns]
a HOMO determined by photoelectron spectrometry. b LUMO derived by extracting the EHOMOEg. c Calculated from the onset of the absorption spectrum. d Conditions: 10 mL of triethylamine/methanol/water and 5 mg of photocatalysts (33 vol%), 350 W, xenon light (λ > 420 nm, 1000 W m−2). e Conditions: 10 mL of triethylamine/methanol/water and 1 mg of photocatalysts were used. f The AQY was measured at 420 and 460 nm. g Fluorescence lifetime.
PFT 428 −5.52/−2.90 2.62 42.44 ± 7.87 0.73
PIFT 430, 450 −5.56/−2.92 2.64 1.17 ± 0.59 0.65
PF2T 418 −5.61/−3.05 2.55 628.8 ± 30.1 (655)e 1.17 (1.3) 0.70
PIF2T 438, 458 −5.53/−2.95 2.59 54.4 ± 7.71 0.01 0.64
PF3T 448 −5.47/−2.43 2.43 420.0 ± 46.6 (1095)e 0.43 (0.42) 0.71
PIF3T 444, 470 −5.57/−2.51 2.51 558.9 ± 11.9 (582)e 0.27 (0.69) 0.61

Next, we tested the photocatalytic activities of the resulting conjugated polymers at ambient temperature under sacrificial conditions by a solar simulator with an AM1.5 solar simulation filter, and the irradiance intensity was adjusted to 1 sun (1000 mW m−2) with a cutoff filter (λ > 420 nm). All related properties are summarized in Table 1, and the results from the optimization studies are shown in Tables S2 and S3. Triethylamine (TEA) was used as the sacrificial hole scavenger and methanol was added to avoid the phase separation of water and TEA.58 All of the photocatalysts were tested after purification by the Soxhlet extraction and without the addition any additional noble Pt cocatalysts, only in the presence of Pd-residues in PFnT analogues. To identify and avoid any effects of palladium introduced during polymerization, we used inductively coupled plasma mass spectrometry (ICP-MS) to determine the residual Pd in these polymers. The results suggested that palladium was present in residual amounts ranging from 0.10% to 0.35% in all polymers (Table S1). The control experiments revealed that the kinetic curve of the photocatalytic process promptly decreased under light-off conditions and again increased under light-on conditions, proving that our system was a photocatalytic reaction (Fig. 2b). In addition, no hydrogen was detected in the absence of the photocatalysts or without the TEA sacrificial donor.

image file: c9ta06425g-f2.tif
Fig. 2 (a) Absorption spectra of various photocatalysts. (b) Control experiment for the light-driven hydrogen production from water using PF2T polymers at ambient temperature (determined by GC analysis). (c) HER of the polymer photocatalysts under standard conditions.

Fig. 2c shows the hydrogen evolution rate (HER) of the photocatalytic solution systems (10 mL, water/methanol/TEA) for all the conjugated polymers (5 mg) after the exposure to light as determined by gas chromatography (GC) measurements without adding additional Pt cocatalysts. The photocatalytic performance was enhanced for the copolymers containing fused thiophene rings by the backbone of the conjugated polymers. As shown in Table 1, the HER of PF2T and PF3T exhibited ca. 15 and 10 times higher than that of PFT. Similarly, the HER values of PIF2T and PIF3T showed a significant enhancement in the hydrogen evolution rates compared to that of PIFT under other conditions, suggesting that the insertion of fused thiophene efficiently enhanced the photocatalytic activity. Interestingly, we observed that the photocatalytic efficiency of PF3T (420 µmol h−1 g−1) was lower than that of PIF3T (559 µmol h−1 g−1) and PF2T (629 µmol h−1 g−1). We considered that this was due to the poor solubility of PF3T compared to PF2T or even PIF3T because PF3T showed that many insoluble particles were still inside the solution mixture (water/methanol/TEA) (see Fig. 3a). To not underestimate the photocatalytic activity of the polymers, we reduced the amount of the photocatalysts to 1 mg to let PF3T well-disperse into the standard photocatalytic solution as shown in Fig. 3b, while PF2T and PIF3T were tested under the same condition for comparison. After the optimization, the photocatalytic activity of PF2T (655 µmol h−1 g−1) and PF3T (582 µmol h−1 g−1) still showed a similar HER compared to the above condition. Notably, the performance of PF3T was approximately doubled to reach a HER of 1095 µmol h−1 g−1 (Fig. 3c). Based on this result, undissolved particles in the standard photocatalytic solutions of PF3T were not completely involved in the photocatalytic reaction, resulting in the observed lower photocatalytic efficiencies. In Table S4, we summarized the hydrogen evolution reaction performances of the Pt cocatalysts and the solution conditions. Among them, our design and methods achieved the highest photocatalytic efficiencies. These results demonstrated that the introduction of the fused aromatic rings effectively enhanced the photocatalytic performance and provided a clear design concept to select the number of fused thiophene rings and the solubility of photocatalysts. Furthermore, the apparent quantum yields (AQYs) were calculated using the method of Wrighton, Ginley, and Morse59 (eqn (S1)) from measurements under the standard photocatalytic conditions (Table 1) using a light source with a single wavelength cutoff filter (λ = 420, 460 and 500 nm). Without adding the additional Pt cocatalysts, PF2T, PF3T, and PIF3T showed high AQYs of 1.2%, 0.43% and 0.27% at 420 nm, respectively. Importantly, using a red-shifted irradiation wavelength at 460 nm, PF2T, PF3T, and PIF3T showed similar or higher AQYs of 1.3%, 0.42% and 0.70%, respectively, compared to the AQYs at 420 nm. This finding revealed that the fused thiophene based polymers showed a broad light response region (Fig. S6).

image file: c9ta06425g-f3.tif
Fig. 3 Solubility test for the polymers. Pictorial representation of the polymer photocatalysts ((a) 5 mg and (b) 1 mg) dispersed in a standard solution. HER and stability of the polymer: (c) photocatalytic H2 evolution and (d) transient photocurrent response of PF2T, PF3T, and PIF3T performed with 1 mg of the photocatalysts. (e) Long-term stability tests for PF2T.

To gain insight into the photoresponse ability of these polymers, the transient photocurrent responses were collected under visible-light irradiation at 1.5 V vs. Ag/AgCl (Fig. 3d). Interestingly, a photocurrent response appeared for all the polymers, which was adopted by alternating the switch-on and switch-off methods. For the PFT analogues, PF3T showed a high and stable photocurrent of about 5.8 µA and exhibited a higher sensitivity to light irradiation compared to that of PF2T (3.3 µA) and PIF3T (2.2 µA). The trend of generated photocurrent intensity for these three polymers follow the order of PF3T > PF2T > PIF3T, which was in agreement with the photocatalytic HER. Interestingly, we observed that the fluorene-based PFnT generated a higher HER compared to its PIFnT analogues (Table 1). To find further support for this finding, density functional theory (DFT) calculations and time-resolved photoluminescence spectra were investigated to determine the optimal geometries, HOMO/LUMO electron distribution of the copolymers, and the fluorescent lifetime, respectively (Fig. S7, S8 and Table 1). The dihedral angles were calculated for the PFnTs and PIFnTs between the polyfluorene (PF) or polyindenofluorene (PIF) rings and thiophene (T) rings from DFT optimized structures (see Table S1). The dihedral angles of fluorene-based PFnT (0.01–0.35°) were smaller than those of the indenofluorene-based PIFnT (26.8–27.3°). The PFnT analogues showed longer excited lifetimes of 0.70–0.73 ns compared to that of the PIFnT analogues (0.61–0.65 ns). Both, the results of the dihedral angles and lifetimes, indicated a higher HER for the PFnT analogues compared to that of the PIFnT analogues, which was attributed to the enhancement in the charge carrier mobility and exciton dissociation. Notably, the fused thiophene copolymers not only showed an excellent HER, but also showed a superior photostability characteristic, which was confirmed by running long-term HER experiments. Fig. 3e shows that a stable HER was detected for PF2T for over 70 h without any obvious decay. Even though we stored PF2T in the photocatalytic solution (TEA/water/methanol) for 20 days, its photocatalytic performance remained constant. We further tested the photocatalytic stability of PF3T in the water/methanol/TEA condition as shown in Fig. S9. PF3T also showed a stable photocatalytic reaction in the water/methanol/TEA solution after the cycling test. Such a robust property for polymer photocatalysts has never been reported to date, and could be beneficial for real-world applications.


In summary, we successfully designed and synthesized a series of polymer photocatalysts containing fluorene and indenofluorene with various fused-thiophene rings (thiophene (T), thienothiophene (2T), and dithienothiophene (3T)) in different combinations for visible-light-driven hydrogen evolution. Their photophysical properties and photocatalytic performance were characterized and compared to that of the non-fused aromatic polymers. Without the addition of the noble Pt cocatalysts, PF3T showed the highest efficiency for the HER at 1095 µmol h−1 g−1 (λ > 420 nm). Furthermore, PF2T showed a robust property when stored in a water/methanol/TEA solution for 20 days as its photocatalytic performance remained constant. Based on this study, the first systematic study that controlled the number of fused thiophene and fluorene rings on polymer photocatalysts was demonstrated to give a clear design strategy for photocatalytic hydrogen evolution.

Conflicts of interest

The authors declare no conflict of interest.


The authors gratefully acknowledge the financial support from the Ministry of Science and Technology of Taiwan (MOST 108-2636-E-007-005), and also thank the National Center for High-Performance Computing of Taiwan for providing the computing time. The authors appreciate the Precision Instrument Support Center of National Tsing Hua University for providing the analysis and measurement facilities.


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Optional dedication.
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ta06425g
§ These authors contributed equally.

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