Effect of Controlling Thiophene Rings on D-A Polymer Photocatalysts Accessed via Direct Arylation for Hydrogen Production

Conjugated polymer photocatalysts for hydrogen production have the advantages of an adjustable structure, strong response in the visible light region, adjustable energy levels, and easy functionalization. Using an atom- and step-economic direct C–H arylation method, dibromocyanostilbene was polymerized with thiophene, dithiophene, terthiophene, and fused thienothiophene and dithienothiophene, respectively, to produce donor–acceptor (D-A)-type linear conjugated polymers containing different thiophene derivatives with different conjugation lengths. Among them, the D-A polymer photocatalyst constructed from dithienothiophene could significantly broaden the spectral response, with a hydrogen evolution rate up to 12.15 mmol h−1 g−1. The results showed that the increase in the number of fused rings on thiophene building blocks was beneficial to the photocatalytic hydrogen production of cyanostyrylphene-based linear polymers. For the unfused dithiophene and terthiophene, the increase in the number of thiophene rings enabled more rotation freedom between the thiophene rings and reduced the intrinsic charge mobility, resulting in lower hydrogen production performance accordingly. This study provides a suitable process for the design of electron donors for D-A polymer photocatalysts.


Synthesis and Characterization of the CPs
As shown in Scheme 1, DBCS bearing an electron-withdrawing -CN group was facilely synthesized via Knoevenagel condensation [36]. The structure of the DBCS was confirmed by NMR spectra (Figures S1 and S2). D-A linear conjugate polymers of CP-T, CP-BT, CP-BTT, CP-TT, and CP-DTT were obtained, respectively, by using thiophene (T), unfused BT and BTT, and fused TT and DTT as electron-donating building blocks to polymerize with electron-accepting DBCS.
The chemical structures of all CPs were further verified by Fourier-transform infrared (FT-IR) spectra ( Figure 1). All CPs exhibited peaks at a wavelength of ~816 cm −1 , corresponding to the characteristic peak of the stretching vibration of C-S-C. The characteristic signal of the C=C double bonds stretching mode appeared at ~1635 cm −1 , demonstrating the presence of vinylene linkers. Additionally, the signals at ~2215 cm −1 were assigned to the stretching vibration bands of the C≡N triple bond. The CPs displayed peaks at ~3420 cm −1 , which suggested the successful incorporation of thiophene units at the α position into all CPs. To sum up, the FT-IR signals of all the above indicated that the target CPs were successfully synthesized via direct C-H arylation.
With the above structural and electronic features, thiophene derivatives can be used to finely control the optical band gap of the photocatalysts. Furthermore, the linear conjugate polymers possessed the merits of easy synthesis, structural simplicity, and atom-and step-economic and efficient D-A architecture. These polymers containing thiophene-based units with different conjugations (Scheme 1) are expected to be ideal models for structure-property-performance correlation studies via the PHP reaction.  With the above structural and electronic features, thiophene derivatives can be used to finely control the optical band gap of the photocatalysts. Furthermore, the linear conjugate polymers possessed the merits of easy synthesis, structural simplicity, and atom-and step-economic and efficient D-A architecture. These polymers containing thiophene-based units with different conjugations (Scheme 1) are expected to be ideal models for structureproperty-performance correlation studies via the PHP reaction.
The morphologies of the powdery CPs were evaluated by scanning electron microscopy (SEM). Well-defined dimensions and aggregation are displayed for the CPs in Figure 2. Owing to the conjugation effect, unfused thiophene groups can easily adjust their conformation to remain coplanar with the benzene ring; thus, CP-T, CP-BT, and CP-BTT exhibited a lamella-like morphology. Normally, high crystallinity is conducive to obtaining efficient charge transport and separation of carriers for PHP, as organic polymers with high crystallinity can promote the separation and transport of photo-generated charge carriers due to the minimized formation of defects and charge traps. Otherwise, owing to the rigid structure, the fused CP-TT and CP-DTT exhibited nanoparticle morphology, which should have resulted in a higher HER. Furthermore, by involving the thiophene units in the conjugate backbone, the SEM morphologies of unfused CP-T, CP-BT, and CP-BTT were different, suggesting that the variation in the thiophene number had an influence on their morphologies. polymers with high crystallinity can promote the separation and transport of photogenerated charge carriers due to the minimized formation of defects and charge traps. Otherwise, owing to the rigid structure, the fused CP-TT and CP-DTT exhibited nanoparticle morphology, which should have resulted in a higher HER. Furthermore, by involving the thiophene units in the conjugate backbone, the SEM morphologies of unfused CP-T, CP-BT, and CP-BTT were different, suggesting that the variation in the thiophene number had an influence on their morphologies.

Opto-Electronic Properties of the CPs
To shed light on the effect of different thiophene units on the opto-electronic properties, all the CPs were systematically characterized by different measurements, including UV-vis, steady-state photoluminescence (PL), the transient photocurrent response (TPR), and cyclic voltammetry (CV) (Figures 3 and S3). A broader range of light absorption for a photocatalyst will be beneficial to photon capture and utilization [29]. Longer conjugated systems involving larger numbers of thiophene rings will lead to the enhanced delocalization of π-conjugation and result in a red shift of UV-vis absorption [27], which should be conducive to light harvesting by photocatalysts. The UV-vis diffuse reflectance spectroscopy of the CPs exhibited broad absorption between 300 and 600 nm. Compared with CP-T, the other CPs showed significant red-shifts. The range of light absorption for all CPs decreased in the order of CP-DTT > CP-BTT > CP-BT > CP-TT > CP-T. The varied planarity, electron-donation abilities, and conjugation lengths of T, BT, BTT, TT, and DTT endowed their corresponding parent CPs, i.e., CP-T, CP-BT, CP-BTT, CP-TT, and CP-DTT, with different rigidities, light absorptances, and electron separation capacity, and thus finely modulated their opto-electronic properties. As a result, CP-DTT exhibited the broadest absorption range, even with an extension to the near-infrared region, owing to the stronger electron-donation ability of DTT and the enhanced D-A interaction and interchain π-π* transition. The light absorption trends of the CPs were reflected by the evolution of colors, i.e., stronger absorbance toward longer wavelengths corresponded to deeper colors (insets in Figure 3a). CP-DTT exhibited the most redshifted light absorption with the deepest color, which should be conducive to light harvesting. The optical bandgaps (Eg) estimated by plotting the curves of (αhν) 1/2 vs. hν ( Figure S4

Opto-Electronic Properties of the CPs
To shed light on the effect of different thiophene units on the opto-electronic properties, all the CPs were systematically characterized by different measurements, including UV-vis, steady-state photoluminescence (PL), the transient photocurrent response (TPR), and cyclic voltammetry (CV) (Figure 3 and Figure S3). A broader range of light absorption for a photocatalyst will be beneficial to photon capture and utilization [29]. Longer conjugated systems involving larger numbers of thiophene rings will lead to the enhanced delocalization of π-conjugation and result in a red shift of UV-vis absorption [27], which should be conducive to light harvesting by photocatalysts. The UV-vis diffuse reflectance spectroscopy of the CPs exhibited broad absorption between 300 and 600 nm. Compared with CP-T, the other CPs showed significant red-shifts. The range of light absorption for all CPs decreased in the order of CP-DTT > CP-BTT > CP-BT > CP-TT > CP-T. The varied planarity, electron-donation abilities, and conjugation lengths of T, BT, BTT, TT, and DTT endowed their corresponding parent CPs, i.e., CP-T, CP-BT, CP-BTT, CP-TT, and CP-DTT, with different rigidities, light absorptances, and electron separation capacity, and thus finely modulated their opto-electronic properties. As a result, CP-DTT exhibited the broadest absorption range, even with an extension to the near-infrared region, owing to the stronger electron-donation ability of DTT and the enhanced D-A interaction and interchain π-π* transition. The light absorption trends of the CPs were reflected by the evolution of colors, i.e., stronger absorbance toward longer wavelengths corresponded to deeper colors (insets in Figure 3a). CP-DTT exhibited the most red-shifted light absorption with the deepest color, which should be conducive to light harvesting. The optical bandgaps (E g ) estimated by plotting the curves of (αhν) 1/2 vs. hν ( Figure S4) for CP-T, CP-BT, CP-BTT, CP-TT, and CP-DTT were 2.16, 2.03, 1.95, 2.13, and 1.88 eV, respectively, which meant that the introduction of unfused BT and BTT and the fused TT and DTT could finely modulate the E g of the photocatalyst. In theory, the minimum photon energy required to drive the fully decomposed water reaction is 1.23 eV, corresponding to photons with a wavelength of approximately 1000 nm. However, in reality, owing to the influence of semiconductor band bending and the presence of water decomposition overpotential, the requirement for semiconductor E g is greater than the theoretical value, generally believed to be greater than 1.8 eV. Therefore, the E g of CP-T, CP-BT, CP-BTT, CP-TT, and CP-DTT is suitable for PHP reactions.
PL spectroscopy was used to examine the photoinduced electron-hole (e − -h + ) recombination behavior. Photoluminescence is a simple, but useful, index to check the photo-to-photon conversion. A weak photoluminescence intensity usually means lower radiative recombination of e − -h + pairs, indicating charge separation and, thus, photocatalysis [4]. Figure  1.88 eV, respectively, which meant that the introduction of unfused BT and BTT and the fused TT and DTT could finely modulate the Eg of the photocatalyst. In theory, the minimum photon energy required to drive the fully decomposed water reaction is 1.23 eV, corresponding to photons with a wavelength of approximately 1000 nm. However, in reality, owing to the influence of semiconductor band bending and the presence of water decomposition overpotential, the requirement for semiconductor Eg is greater than the theoretical value, generally believed to be greater than 1.8 eV. Therefore, the Eg of CP-T, CP-BT, CP-BTT, CP-TT, and CP-DTT is suitable for PHP reactions. PL spectroscopy was used to examine the photoinduced electron-hole (e − -h + ) recombination behavior. Photoluminescence is a simple, but useful, index to check the photo-to-photon conversion. A weak photoluminescence intensity usually means lower radiative recombination of e − -h + pairs, indicating charge separation and, thus, photocatalysis [4]. Figure 3b shows the PL spectra of the powdery CP-T, CP-BT, CP-BTT, CP-TT, and CP-DTT excited by 437, 445, 460, 477, and 562 nm wavelengths, respectively. The PL intensities of all CPs decreased in the order of CP-BTT > CP-TT > CP-T > CP-BT > CP-DTT. CP-DTT had the lowest PL intensity, meaning that CP-DTT can effectively promote intramolecular charge separation and minimize radiation recombination. On the contrary, CP-BTT displayed the strongest PL emission, indicating a high ratio of radiative charge recombination.
A photocatalytic reaction is initiated by photon capture to generate excited electronhole pairs (i.e., excitons), followed by their separation into free charges, which eventually drive redox reactions. Typically, the driving force for the photocatalytic redox reaction is governed by the FMOs of CPs, i.e., the HOMO levels and the lowest unoccupied molecular orbital (LUMO). HOMO and LUMO, with electron-donating and electron-withdrawing characteristics, are responsible for oxidation and reduction reactions, respectively [29]. Here, the changes in the FMOs with the structural evolution from CP-T, CP-BT, CP-BTT, and CP-TT to CP-DTT involving different thiophene building blocks were investigated A photocatalytic reaction is initiated by photon capture to generate excited electronhole pairs (i.e., excitons), followed by their separation into free charges, which eventually drive redox reactions. Typically, the driving force for the photocatalytic redox reaction is governed by the FMOs of CPs, i.e., the HOMO levels and the lowest unoccupied molecular orbital (LUMO). HOMO and LUMO, with electron-donating and electron-withdrawing characteristics, are responsible for oxidation and reduction reactions, respectively [29]. Here, the changes in the FMOs with the structural evolution from CP-T, CP-BT, CP-BTT, and CP-TT to CP-DTT involving different thiophene building blocks were investigated by CV measurements. The LUMO levels were calculated as −4.80 − (E red − E Fc/Fc+ ), while the HOMO levels were calculated as E LUMO − E g . As shown in Figure 3c, versus the vacuum hydrogen electrode, the LUMO levels of all CPs were greater than −4.8 eV, all of which had sufficient driving forces for proton reduction in thermodynamics. Compared with the single thiophene (T), the unfused thiophenes (BT and BTT) and the fused thiophenes (TT and DTT) had stronger electron donating ability and higher HOMO levels, as predicted by DFT calculation. As a result, the CP-BTT and CP-DTT exhibited higher HOMO levels compared with the other three CPs, i.e., CP-T, CP-BT, and CP-TT. The above results imply that, besides optical properties (Figure 3a), the electrochemical properties (i.e., FMOs) can also be finely tuned by the introduction of different thiophene derivatives.
To check the photo-to-current efficiencies of the CPs, TPR measurements were carried out under visible light irradiation under 1.5 V and Ag/AgCl conditions through the alternating switch method. As shown in Figure 3d, the photo-to-current intensity of the five CPs followed the order of CP-DTT > CP-BT > CP-BTT > CP-TT > CP-T. Among them, CP-DTT (0.46 µA) and CP-BT (0.38 µA) showed higher and more stable photocurrents compared with the other three CPs. The highest TPR response of CP-DTT can be ascribed to the strongest electron-donating ability of DTT, which was conducive to yielding the highest intermolecular charge transport.
To gain deeper insight into the structure-property correlation, we calculated the optimized geometries and the frontier molecular orbitals (FMOs) of the T, BT, BTT, TT, and DTT building blocks by the density functional theory (DFT) to reveal their effects on the opto-electronic properties of the polymers CP-T, CP-BT, CP-BTT, CP-TT, and CP-DTT. The DFT prediction revealed that TT and DTT possessed entirely planar geometries, while BT had rotating σ-bonds between the thiophene rings, and BTT had a twisted geometry with a dihedral angle of 32.9 • (Figure 4). The highest occupied molecular orbital (HOMO) levels of the thiophene-based building blocks predicated by DFT decreased in the order of DTT > TT > BTT > BT > T. The optimized geometries and HOMO levels of the various thiophene derivatives were consistent with the evolutions of the UV-vis spectra. Among them, the DTT building block featured an entirely planar geometry, along with the strongest electron-donating ability (i.e., the most up-shifted HOMO level), which endowed CP-DTT with the strongest D-A interaction, most effective π-conjugation, and, thus, most red-shifted light absorption with the narrowest E g . by DFT calculation. As a result, the CP-BTT and CP-DTT exhibited higher HOMO levels compared with the other three CPs, i.e., CP-T, CP-BT, and CP-TT. The above results imply that, besides optical properties (Figure 3a), the electrochemical properties (i.e., FMOs) can also be finely tuned by the introduction of different thiophene derivatives.
To check the photo-to-current efficiencies of the CPs, TPR measurements were carried out under visible light irradiation under 1.5 V and Ag/AgCl conditions through the alternating switch method. As shown in Figure 3d, the photo-to-current intensity of the five CPs followed the order of CP-DTT > CP-BT > CP-BTT > CP-TT > CP-T. Among them, CP-DTT (0.46 µA) and CP-BT (0.38 µA) showed higher and more stable photocurrents compared with the other three CPs. The highest TPR response of CP-DTT can be ascribed to the strongest electron-donating ability of DTT, which was conducive to yielding the highest intermolecular charge transport.
To gain deeper insight into the structure-property correlation, we calculated the optimized geometries and the frontier molecular orbitals (FMOs) of the T, BT, BTT, TT, and DTT building blocks by the density functional theory (DFT) to reveal their effects on the opto-electronic properties of the polymers CP-T, CP-BT, CP-BTT, CP-TT, and CP-DTT. The DFT prediction revealed that TT and DTT possessed entirely planar geometries, while BT had rotating σ-bonds between the thiophene rings, and BTT had a twisted geometry with a dihedral angle of 32.9° (Figure 4). The highest occupied molecular orbital (HOMO) levels of the thiophene-based building blocks predicated by DFT decreased in the order of DTT > TT > BTT > BT > T. The optimized geometries and HOMO levels of the various thiophene derivatives were consistent with the evolutions of the UV-vis spectra. Among them, the DTT building block featured an entirely planar geometry, along with the strongest electron-donating ability (i.e., the most up-shifted HOMO level), which endowed CP-DTT with the strongest D-A interaction, most effective π-conjugation, and, thus, most red-shifted light absorption with the narrowest Eg.

PHP of the CPs
The above structure-property correlations and FMO levels of CPs indicate that all five CPs were suitable for the PHP reaction, with sufficient driving force for proton reduction. We tested the PHP performances ( Figure 5) of the CPs by using ascorbic acid (AA) as a sacrificial electron donor (SED) under visible light irradiation (λ > 420 nm). All CPs could be well-dispersed in H2O without the aid of a water-soluble organic co-solvent to assist wettability. Furthermore, a co-catalyst was not added to provide redox reaction sites and reduce the activation energy of the reaction. The hydrogen production rates

PHP of the CPs
The above structure-property correlations and FMO levels of CPs indicate that all five CPs were suitable for the PHP reaction, with sufficient driving force for proton reduction. We tested the PHP performances ( Figure 5) of the CPs by using ascorbic acid (AA) as a sacrificial electron donor (SED) under visible light irradiation (λ > 420 nm). All CPs could be well-dispersed in H 2 O without the aid of a water-soluble organic co-solvent to assist wettability. Furthermore, a co-catalyst was not added to provide redox reaction sites and reduce the activation energy of the reaction. The hydrogen production rates  (Figure 5b), which matched well with the trend of the PL intensity and TPR, as mentioned above. The HER of 12.15 mmol h −1 g −1 for CP-DTT was above the average of the abovementioned types of organic photocatalysts and outperformed most of the reported linear CPs (Table 1). Among all CPs, the single-thiophene-based CP-T showed the lowest HER (0.43 mmol h −1 g −1 ). The HERs of the twisted and unfused BT-and BTT-based CPs CP-BT and CP-BTT were nearly equal to each other (4.89 vs. 4.22 mmol h −1 g −1 ), meaning that the increase in the unfused thiophene numbers was not conducive to the enhancement of PHP performance, mainly because of the reduced intrinsic charge mobility. For the fused thiophene TT-and DTT-based CPs, the photocatalytic HERs were 1.78 and 12.15 mmol h −1 g −1 , following the order of CP-DTT > CP-TT > CP-T, showing that increasing the number of fused thiophene in polymers is beneficial for PHP. AA: ascorbic acid, SA: sodium ascorbate, TEOA: triethanolamine, and TEA: trimethylamine. a All light sources are a 300 W Xe lamp.
The stability of the best-performing CP-DTT was further checked by the recycling test. No significant reduction in continuous PHP was observed. After four cycles of CP-DTT for 20 h, about 92% of the initial H2 evolution (0.34 mmol) remained. This means that CP-DTT showed good stability for PHP (Figure 5c).   Ref. AA: ascorbic acid, SA: sodium ascorbate, TEOA: triethanolamine, and TEA: trimethylamine. a All light sources are a 300 W Xe lamp.

CP-DTT
The stability of the best-performing CP-DTT was further checked by the recycling test. No significant reduction in continuous PHP was observed. After four cycles of CP-DTT for 20 h, about 92% of the initial H 2 evolution (0.34 mmol) remained. This means that CP-DTT showed good stability for PHP (Figure 5c).

Materials and Methods
There was no further purification of all of the starting reagents. Anhydrous toluene was distilled freshly with calcium hydride (CaH 2 ). The standard Schlenk techniques were used for all polymerizations.
The Bruker Advance III 400 model 400 MHz NMR spectrometer (Berlin, Germany) was used for NMR measurements (the sample was dissolved with 0.5 mL of deuterium reagent). All theoretical calculations were carried out on Gaussian 09W Packs using the semi-empirical method by PM6 [47][48][49]. The chemical structure was optimized and characterized at 298 K by frequency analysis. An FT-IR spectrometer (Bruker, ALPHA, Berlin, Germany) was used to obtain FT-IR spectra in the range of 4000-500 cm −1 (using the KBr compression method at a pressure of 1.0 T). The morphologies were determined by SEM (MLA650F, FEI, Hillsboro, OR, USA) (Working distance: 5 mm and magnification: 50-200,000 times). A UV-2600 scanning UV-vis spectrophotometer (Schimadzu, Kyoto, Japan) was used to characterize the DRS spectra (reference substance: BaSO 4  and CP-DTT were excited by 437, 445, 460, 477, and 562 nm wavelengths, respectively. The normal three-electrode cell system was used for CV measurement, which was carried out on a CHI660E (Chenhua, Shanghai, China) electrochemical workstation. In the electrode system, platinum wire was the counter electrode, the Ag/AgCl electrode was the reference electrode, and the glassy carbon electrode was the working electrode. The supporting electrolyte was 5 mL acetonitrile used to dissolve tetra-n-butylammonium hexafluorophosphate (TBAPF6, 1.5 g). To characterize the TPR, an electrochemical workstation (CHI650E/700E, Shanghai, China) was used, which was equipped with a conventional three-electrode system. In the electrochemical workstation, the platinum plate was used as the counter electrode and the configuration Ag/AgCl (saturated with KCl) was used as the reference electrode.

Synthesis of Dibromocyanostilbene (DBCS)
DBCS was obtained through the classic Knoevenagel condensation reaction from simple starting chemicals. The specific steps are as follows: 4-bromophenylacetonitrile (389 mg, 2 mmol), 4-bromobenzaldehyde (406 mg, 2.2 mmol), and ethanol (0.7 mL) were added to a round-bottomed flask containing a solution of KOH (aq. 40%, 0.46 mL) and ethanol (0.46 mL) [36]. The reaction mixture was stirred at 30 • C for 1.5 h and the resulting solid was filtered and recrystallized from water/methanol (v:v = 20:1) to produce the compound as a white solid (700.6 mg, 96.5%). 1 3 (3.5 mg, 3 mol %), anhydrous Cs 2 CO 3 (434.4 mg, 2 equiv), PivOH (10.3 mg, 30 mol%), and toluene (5 mL) was added to Schlenk tubes [50][51][52][53]. Each reaction system was deoxidized by a reiterative vacuum and argon filling. To remove dissolved air, the freeze-vacuumthaw cycle methods were performed for the mixture and it was then rigorously stirred at 130 • C for 48 h under an argon atmosphere. When the reaction mixture was cooled to room temperature, the solvent was removed with CH 2 Cl 2 and the reaction residue was cleaned. The soluble impurities and inorganic salts in the undissolved crude CPs product were washed successively with CH 2 Cl 2 , ethanol, and water. Then, the product on the filter paper was moved to a vacuum for 24 h at 65 • C. Eventually, polymeric powders were obtained for CP-T, CP-BT, CP-BTT, CP-TT, and CP-DTT with yields of 19.7%, 88.9%, 83.6%, 66.1%, and 89.2%, respectively.

PHP Tests
For the typical PHP test, a gas chromatograph (GC9790, FuLi, Wenzhou, China) was equipped with a thermal conductive detector, using argon as the carrier gas. It was linked to a photocatalytic online analysis system (LabSolar-III AG, Beijing Perfect Light, Beijing, China). Five grams of AA were dissolved in 30 mL of H 2 O; the mixed aqueous solution was prepared for the ultrasonic dispersal of the photocatalysts (6 mg). The KOH solution was used to adjust the pH to 4.0. To remove the dissolved air from the mixture and maintain the vacuum, an oil pump was used. A 300 W Xe lamp (Beijing Perfect Light, PLS-SXE300, Beijing, China) under full-arc light irradiation was prepared to irradiate the reaction vessel. A flow of cooling water was used to keep the reaction temperature at 25 • C.

Conclusions
In summary, a series of polymer photocatalysts based on thiophene (T), unfused thiophene (BT and BTT), and fused thiophene (TT and DTT) were successfully designed and synthesized by atom-economic direct C-H arylation for visible-light-driven hydrogen evolution. Specifically, without adding any co-catalysts or co-solvents, CP-DTT showed the highest hydrogen evolution efficiency of 12.15 mmol h −1 g −1 (λ > 420 nm light irradiation). The results showed that the fused thiophene ring broadened light absorption, narrowed the band gap, enhanced the photocurrent intensity, and then improved the photocatalytic performance. The photocatalytic performance of the CPs increased with the increase in the amount of fused thiophene amount, but chain thiophene was not affected by the number. For the unfused dithiophene and terthiophene, the increase in the number of thiophene rings enabled more rotation freedom between the thiophene rings and thus decreased the intrinsic charge mobility of polymeric chains, resulting in lower hydrogen production performance. This work is the first systematic study to have been conducted on controlling the number of thiophene rings and its effect on polymer photocatalysts' properties and photocatalytic performance, which provides useful guidance for the design of thiophene-containing CP photocatalysts for PHP applications.