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• 0   Introduction

  The global increase in CO₂ emissions due to uninhibited fossil fuel use has led to explore new renewable and clean energy resources. Photo-induced catalytic splitting of water to non-carbon-contain fuels, i.e. hydrogen, is a promising approach in converting solar energy into storable chemical energy^[1-5]. As a redox reaction, water splitting can be divided into two reactions: (i) water oxidation, to yield O₂ and (ii) water reduction, producing H₂. In an artificial photosynthetic (AP) system, the reductive side of water splitting is the light-driven generation of hydrogen from aqueous protons. A variety of photocatalytic systems have been broadly investigated for solar hydrogen production^[6-13]. Most of the photocatalytic H₂-production systems involve three important components: a light-absorbing photosensitizer (PS), a water-reducing catalyst (WRC), and a sacrificial reductant. While noble metal co-catalysts such as Pt and Pd were widely used for photocatalytic H₂ evolution, but the scarcity and high cost of precious metals pose serious limitations to wide use. Hence, the development of efficient catalysts for water reduction to H₂ made of inexpensive, earth-abundant elements such as Fe, Co, Ni, and Cu has become a prime focus of research on light-driven hydrogen generation.

  In an effort to find stable earth-abundant WRCs, a few recent reviews on catalysts made of earth-abundant elements for water splitting have been published^[14-16]. In particular, thiosemicarbazone ligands have been used as versatile molecules, arising from the possibility to act as N, S-donor systems^[17-18]. And the number of potential donor atoms can be increased by using carbonylic compounds that contain additional donor groups in suitable positions for chelation. Moreover, they provide a conventional proton immi-gration path through the thiolate/thioamide resonance tautomer equilibrium^[19-20], which have potential future for multi-electron transfer chemistry in H₂ reduction. And these cobalt thiosemicarbazone complexes have gained much interest and might be promising candidates for proton reduction, in the case that the redox potential of the complex was well modified^[21-22]. In this article, we introduced a triphenylphosphine moiety as the additional donor to enhance the coordination ability and modify the redox potential of the metal centres (Scheme 1)^[23-24]. Recent advances have reported Ni-bis(diphosphine) complexes act as effective reduction catalysts for electrochemical photocatalytical hydrogen production in homogeneous system^[25-26]. We expected that the presence of S and P donors would possibly facilitate the formation of reducing species and increase the catalytic efficiency for the H₂ evolution^[27]. Herein, we introduced the rhodamine-modified ligands to improve the light absorption efficiency and electron transfer ability between the catalyst 2 and photosensitizer Fl. The photocatalytic activity of catalyst 2 was higher than that of 1 under low catalyst concentration. The photo-induced electron transfer pathway mechanisms for the hydrogen evolution in these cobalt-thiosemicarbazone complexes were also proposed.

  Scheme 1.  Structures of thiosemicarbazone ligands L1 and L2

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  1   Experimental

  1.1   Materials and physical measurements

  All chemicals were of reagent grade quality obtained from commercial sources and used without further purification. The elemental analyses of C, H and N were performed on a Vario EL Ⅲ elemental analyzer. ¹H NMR spectra were measured on a Varian INOVA 400M spectrometer. ESI mass spectra were carried out on a HPLC-Q-Tof MS spectrometer. The fluorescent spectra were measured on a JASCO FP-6500.

  Cyclic Voltammogram measurements: Electro-chemical measurements of catalysts 1 and 2 were performed in CH3CN solutions with 0.1 mol·L^-1 TBAPF6 working on ZAHNER ENNIUM Electro-chemical Workstation with a homemade Ag/AgCl electrode as a reference electrode, a platinum silk with 0.5 mm diameter as a counter electrode, and glassy carbon electrode as a working electrode. The addition of NEt₃HCl (0.1 mol·L^-1 in CH₃CN) was carried out with syringe.

  Fluorescence quenching experiments: A solution (2.0 mL) of Fl at 10 μmol·L^-1 concentration in a EtOH/H₂O solution (1:1, V/V, pH=11.6) was prepared in a quartz cuvette fitted with a septum cap, and the solution was degassed under N₂ for 5 min. Aliquots of 20 μL of NEt₃ and the catalysts (2.5 μmol·L^-1) were added, and the intensity of the fluorescence was monitored by steady state fluorescence exciting at 460 nm on a Spex Fluoromax-P fluorimeter with a photomultiplier tube detector.

  Photocatalytic water splitting experiments: Varying amounts of the catalysts 1 and 2, Fl and NEt₃ in EtOH/H₂O solution (1:1, V/V) were added to obtain a total volume of 5.0 mL in a 20 mL flask. The flask was sealed with a septum and degassed by bubbling argon for 15 min. The pH value of this solution was adjusted to a specific pH value by adding HCl or NaOH and measured with a pH meter. After that, the samples were irradiated by a 500 W Xenon lamp with the 400 nm light filter, and the reaction temperature was maintained at 293 K by using a thermostat water bath. The generated photoproduct of H₂ was chara-cterized by GC 7890T instrument analysis using a 5A molecular sieve column (0.6 m×3 mm), thermal conductivity detector, and argon used as carrier gas. The amount of hydrogen generated was determined by the external standard method.

  1.2   Synthesis of L1 and L2

  The ligand L1 was synthesized by using the reported methods^[28]. Anal. Calcd. for C₂₁H₁₉N₂PS₂(%): H, 4.85; C, 63.9; N, 7.1; Found(%): H, 5.10; C, 63.3; N, 7.2. ¹H NMR (CDCl₃, 400 MHz): 10.2 (s, 1H), 8.6 (d, 1H), 8.2 (m, 1H), 7.1~7.5 (m, 13H), 2.6 (s, 3H). API-MS m/z: 393.07 ([M-H]⁺).

  The ligand L2 was synthesized according to procedures reported previously^[29]. Anal. Calcd. for C₄₅H₄₃N₄SOP(%): H, 6.03; C, 75.2; N, 7.8; Found(%): H, 6.11; C, 74.9; N, 8.0. ¹H NMR (CDCl₃, 400 MHz): 9.37 (s, 1H), 8.4 (t, 1H), 8.11 (d, 1H), 8.08 (m, 1H), 7.6~7.7 (m, 4H), 7.4~7.6 (m, 10H), 7.38 (m, 3H), 7.3 (m, 2H), 6.5 (s, 2H), 6.2 (s, 2H), 3.1 (m, 2H), 1.13 (m, 3H); API-MS m/z: 715.0 ([M-H]⁺).

  1.3   Synthesis of complex 1

  The ligand L1 (0.6 mmol, 0.236 g) and Co(BF₄)₂·6H₂O (0.3 mmol, 0.1 g) was dissolved in 10 mL ethanol. The solution was refluxed for 2 h, and then slowly evaporated at room temperature. The red block crystals were obtained in several days. Yield: ~55%. ESI-MS m/z: 845.23 for the +1 charge cation. Anal. Calcd. for CoC₄₂H₃₇N₄OP₂S₄BF₄·0.5H₂O(%): H, 3.99; C, 52.6; N, 5.84; Found(%): H, 4.02; C, 52.1; N, 5.89. IR (KBr, cm^-1): 3 424 (br), 3 056 (m), 1 630 (s), 1 586 (s), 1 480 (m), 1 460 (m), 1 436 (m), 1 313 (s), 1 127 (m), 1 063 (m), 1 012 (m), 875 (s), 754 (m), 727 (m), 696 (m), 562 (m), 526 (m), 498 (m).

  1.4   Synthesis of complex 2

  Co(BF₄)₂·6H₂O (0.05 mmol, 0.017 g) and the ligand L2 (0.05 mmol, 0.035 g) was dissolved in 15 mL of dichloromethane/ethanol (1:1, V/V). The solution was stirred at boiling temperature for 2 h to obtain a clear black red solution and allowed to stand at room temperature. The red block crystals suitable for single-crystal X-ray diffraction were obtained. Yield: ~40%. ESI-MS m/z: (2L2+Co)³⁺, 497.16; (2L2+Co+BF4)²⁺, 789.76. Anal. Calcd. for CoC₉₁H_85.5N₈O₃P₂S₂B_2.50F₁₀·H₂O ·0.5C₂H₅OH(%): H, 5.15; C, 61.7; N, 6.33; Found(%): H, 5.20; C, 61.1; N, 6.37. IR (KBr, cm^-1): 3 243 (br), 2 973(s), 1 647 (m), 1 606 (m), 1 560 (m), 1 527 (m), 1 499 (m), 1 446 (s), 1 366 (s), 1 306 (m), 1 243 (m), 1 186 (m), 1 124 (m), 1 083 (m), 944 (s), 882 (s), 748 (s), 693 (m), 611 (m), 522 (s).

  1.5   X-ray crystallography

  The data were collected on a Bruker Smart APEX Ⅱ X-diffractometer equipped with graphite monochromated Mo Kα radiation (λ=0.071 073 nm) using the SMART and SAINT^[30] programs at 296 K for complexes 1 and 2. Final unit cell parameters were based on all observed reflections from integration of all frame data. The structures were solved in the space group by direct method and refined by the full-matrix least-squares using SHELXTL-97 fitting on F^{2 [31]}. For complexes 1 and 2, all non-hydrogen atoms were refined anisotropically. Except the solvent water molecules, the hydrogen atoms of organic ligands were located geometrically and fixed isotropic thermal para-meters. The BF₄^- groups in complex 2 were disordered; therefore, large thermal displacement parameters were found for these atoms and refined with partial occupancy. One of the benzene rings was disordered into two parts with the S. O. F. (sites occupied factor) of each part being fixed as 0.5. One ethylamine moiety and ethyl groups in complex 2 were disordered into two parts with the S.O.F. of each part being fixed as 0.5. The crystal data and details of the structure refine-ment of complexes 1 and 2 are summarized in Table 1.

  Table 1.  Crystal data and structure refinements for complexes 1 and 2

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  CCDC: 1523357; 1, 1523358, 2.

  2   Results and discussion

  2.1   Crystal structural description

  Single-crystal structure representation for the complexes 1 and 2 are shown in Fig. 1. In the crystal of complex 1, each central cobalt environment can be described as a distorted octahedron, comprising two N, S atoms from thiosemicarbazone groups, one P atoms from the triphenylphosphine group and the O atom originated from oxidized P atom. One of the P atoms in the ligand L1 was oxidized under the synthesis process. The two ligands bind to a cobalt(Ⅱ) in a mer configuration as found in the related cobalt thiosemicarbazone complexes^[32-33]. The C-S, C-N and N-N bond distances are agreed well with the normal range of single and double bonds, resulting in the extensive electron donation environment over the entire molecular skeleton^[34-35]. Similar to the complex 1, in the crystal of complex 2, the metal center was octahedrally coordinated by two N, S atoms and a P or O chelator atom that originated from the different ligands L2 and L2′. And the distance between two xanthene rings of rhodamine ligands in adjacent complex was 0.36 nm. The presence of intermolecular strong π-π stacking interactions between the rhodamine groups implied the possibility on the formation of π-π interaction between complex 2 and the xanthene rings within the photosensitizer Fl. As a result, the photogenerated electron would be easily immigrated between the photosensitizer and the photocatalyst during the photoreduction processes, which was beneficial to achieve the high activity in the photocatalytic system^[36].

  Figure 1.  Single-crystal structure representations of 1 (a) and 2 (b) showing the coordination geometry of the metal ion

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  2.2   Cyclic voltammetry of the complexes

  The cyclic voltammetry shows that the redox potential of Co(Ⅱ)/Co(Ⅰ) occurs at -1.0 V and -0.87 V for 1 and 2, respectively. And the complex 2 exhibits one inreversible Co(Ⅲ)/Co(Ⅱ) redox process at -0.6 V. To investigate the role of the Co complex as a WRC catalyst, cyclic voltammetry was performed in the presence of increasing amounts of NEt₃HCl. Addition of NEt₃H⁺ with increasing amounts triggers the appe-arance of a new irreversible cathodic wave near the Co(Ⅱ)/Co(Ⅰ) response^[37-38]. Increasing the concentration of NEt₃H⁺ raises the height of the new wave and shifts it to more negative potentials, indicating that complexes 1 and 2 are able to reduce the proton with a catalysis process.This observation indicates direct protonation of the reduced Co(Ⅰ) center and also shows that the protonation process is fast in the overall catalytic rate.

  2.3   Complexes 1 and 2 as efficient quencher for the photosensitizer Fl

  Complexes 1 and 2 also serves as an efficient quencher for the photosensitizer Fl. To verify this hypothesis further, the Stern-Volmer quenching constant K_q in this system was calculated as 1.9×10⁴ L·mol^-1 upon the addition of 1 into Fl solution in EtOH/H₂O (1:1, V/V). The quenching behavior can be considered as a photoinduced electron transfer process from excited state of Fl^* to 1, providing possibilities for Fl to activate complex 1 producing H₂ in solution. Although the K_q for the catalyst 1 was higher than that reported for NEt₃ (0.44 L·mol^-1), the concentration of NEt₃ (1.08 mol·L^-1, 15%) in the actual photocatalytic reactions is much higher than the catalyst 1 (10 μmol·L^-1). A photoinduced electron transfer from the NEt₃ to the excited state of Fl (reduction quenching) dominated the homogeneous photolysis of the reaction mixture instead of direct quenching by 1. As the Stern-Volmer quenching constant of 2 (9.85×104 L·mol^-1) is higher than that of complex 1 (Fig. 3). It indicates that the modified rhodamine group was in favor of the electronic transport between the catalyst 2 and photosensitizer Fl. When the concentration of NEt₃ was fixed at 0.5 mol·L^-1 (7%, V/V) and catalyst 2 was fixed at 10 μmol·L^-1 under the photocatalytic condition, the direct luminescence quenching of Fl by the catalyst 2 should be largely dominated by the oxidative quenching. The initial photochemical step was the formation of Fl⁺ caused by 2^[39-40]. Because both the photosensitizer Fl and the rhodamine-based catalyst contain highly conjugated xanthene moieties, Fl molecules might have strong π-π stacking interact with the catalyst 2, which is beneficial to transfer electrons from the photoexcited Fl^* to 2^[41]. This process led to a more efficient separation of the photogenerated electrons, thus suppressing the combination of electrons that would result in undesired radiative transition. Therefore, the photo-induced electron transfer pathway obtained by the catalyst 2 could potentially be benefit for the construction of efficient photocatalytic systems.

  Figure 2.  Cyclic Voltammogram of 1 (a) and 2 (b) (1 mmol·L^-1) in CH₃CN with 0.1 mol·L^-1 TBAPF₆ upon addition of NEt₃HCl with different concentrations

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  Figure 3.  Family of the emission spectra of fluorescein solution (10 μmol·L^-1) in EtOH/H₂O (1:1, V/V) solution upon addition of catalyst 1 (a) and 2 (b) with various concentration

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  2.4   Complex 1 as a WRC

  Proton reduction catalytic activity of 1 was evaluated by coupling it with Fl as a photosensitizer (PS) upon irradiation with visible light (λ>400 nm) in an EtOH/H₂O (1:1, V/V) solvent mixture containing NEt₃ at room temperature^[42-44]. The optimum pH value of the reaction mixture was maintained at 11.5, decreasing or increasing pH values resulted in both a lower initial rate and shorter system lifetime for hydrogen evolution (Fig. 4a). The efficiency of the visible light induced hydrogen evolution depends on the concentration of sacrificial reagent NEt₃. The optimal concentration is 15%, with a decrease of activity at lower or higher concentration. Furthermore, the addition of two of the three components of the homogeneous systems could not give any further H₂ evolution which indicated that both the 1 and Fl were decomposed during the photolysis.

  Figure 4.  Initial rates of H₂ production in systems containing 1, Fl and 15% (V/V) of NEt₃ with different pH values (a) and containing 1, Fl at pH 11.5 with different concentrations of NEt₃ (b); Photocatalytic hydrogen evolution of systems containing Fl, 15% (V/V) NEt₃ with different concentrations of 1 (c) and containing 1, 15% (V/V) NEt₃ with different concentrations of Fl (d)

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  In the system with Fl and NEt₃ at fixed concen-trations, the initial rate of H₂ generation exhibits a first-order dependence on catalyst 1. At 10.0 μmol·L^-1 catalysts 1 and 2 mmol·L^-1 Fl, this system exhibits a TON of 2 267 mol_H2·mol_cat^-1 after 10 hours and an initial TOF of 220 mol_H2·mol_cat^-1·h^-1. At higher catalyst con-centrations, even though a larger amount of H₂ is evolved, the TON does not scale linearly with catalyst concentration due to the limited lifetime of the system. To determine the limit of this system, an additional experiment was carried out at different Fl concentra-tions shown in Fig. 4d. At higher Fl concentration as 4 mmol·L^-1, there is no continuous increased H₂ genera-tion detected in 10 h. The TOF reaches a maximum at 2.0 mmol·L^-1 Fl, which indicates that the system is limited by the concentration of the catalyst. The system has a longer life time at higher Fl, thus suggesting that Fl decomposes during photolysis. This phenomenon also demonstrates that the reductive quenching path way dominates in the system of catalyst 1.

  2.5   Complex 2 as a WRC

  Through investigating the influence of different pH values and concentration of NEt₃, we found the highest efficiency of H₂ production could be achieved at pH 11.6 and NEt₃ 7%. For initial experiments, the Fl concentration in the reaction vials was kept at 2 mmol·L^-1, while a 5 μmol·L^-1 catalyst concentration was chosen. The initial TOF calculated was 346.5 mol_H2·mol_cat^-1·h^-1 in 5 h with the TON of about 1 732 mol_H2·mol_cat^-1. To further probe the photocatalytic process, concentrations of both the Fl and the catalyst 2 were varied. Details of the effect of reactant concentration on H₂ evolution are depicted in Fig. 5. In terms of catalyst turnovers, it was observed that the photo-catalytic system performs better at high Fl concentration and low catalyst concentration. These results pointed toward high degradation degree of the catalyst 2, which was more notable at higher concentrations. As shown in Table 2, in the same reaction condition, catalyst 2 has a higher TON and TOF than catalyst 1 in the concentration of 5 μmol·L^-1 for catalyst. However, in the higher catalyst concentration of 20 μmol·L^-1, the catalyst 1 showed the better photo-catalytic activity than catalyst 2. It was attribute to the self-stacking interaction of rhodamine group in high concentration which reduced the electronic transfer ability between the catalyst 2 and Fl. And in the same catalyst concentration, catalyst 2 exhibited higher TON and TOF than catalyst 1 in different Fl concentration in 5 h. These results demonstrated that the fast electronic transfer in catalyst 2 was benifit for the photocatalytic system in low catalyst concentration. When the concentration of the catalyst was lowered from 5 μmol·L^-1 to 1 μmol·L^-1, keeping the Fl at the optimal concentration, the hydrogen evolution continued within 3 hours. Although the quantity of hydrogen production decrease, it led to a large increase in turnovers yielding up to 2 800 mol_H2·mol_cat^-1 and the highest TOF 930 mol_H2·mol_cat^-1·h^-1.

  Figure 5.  Photocatalytic hydrogen evolution of systems containing Fl, NEt₃ with different concentrations of 2 (a) and containing 2, NEt₃ with different concentrations of Fl (b)

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  Table 2.  Data of hydrogen production test in 5 h for complexes 1 and 2

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  The light induced H₂ evolution varied on the concentration of Fl, and the increasing of Fl amount caused the increasing of TON. These results suggest both the 2 and Fl were decomposed during the photolysis, but the decompose rate of Fl was much higher than that of the catalyst 2. The photocatalytic activity of catalyst 2 was decreased after 5 h, while catalyst 1 showed better activity in 10 h. The life time of the hydrogen evolution system was shorten, which could be due to the cooperation effect between rhodamine groups and Fl. This fast energy transfer process resulted the quick decompose of Fl and catalyst 2. Relative to catalyst 1, catalyst 2 has a higher hydrogen production rate in the beginning of the reaction process which also demonstrated the fast electron transfer in rhodamine-modified systems. A tentative mechanism proposed for the high activity of catalyst 2 for the production of H₂ has been deduced. Photogenerated electrons in Fl firstly transfer to the rhodamine groups in 2 under illumination with visible light through the possible intermolecular π-π interactions, and then the ligands transfer electrons to the Co center, where H₂ evolution reactions occur. At last, the electron donor NEt₃ restore the excited Fl⁺ to the ground state to complete the catalytic cycle.

  3   Conclusions

  In summary, we reported a simple but effective method to gain cobalt-thiosemicarbazone complexes 1 and 2 as the efficient water reductive catalyst. Structures of these complexes were determined by single crystal X-ray analysis. Electrochemical analysis demonstrated their WRC activity in presence of a proton source of NEt₃HCl. The water reduction properties of the catalyst were evaluated using Fl as PS with NEt₃ as the sacrificial donor. Introducing the rhodamine group which could cooperate with the photosensitizer makes the catalyst 2 more efficient with the TON and initial TOF reaching to 2 800 mol_H2·mol_cat^-1 and 930 mol_H2·mol_cat^-1·h^-1. The higher photo-catalytic activity in lower catalyst concentration and short time indicates the fast electron transfer ability between the catalyst 2 and photosensitizer Fl. The photocatalytic process is dominated by the oxidative quenching with the formation of Fl⁺ caused by 2 which is benefit for the hydrogen evolution. All these work declare the bright future of the thiosemi-carbazone complex in hydrogen evolution.

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      Zhang P, Wang M, Na Y, et al. Dalton Trans., 2010, 39:1204-1206 doi: 10.1039/B923159P

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  Scheme 1  Structures of thiosemicarbazone ligands L1 and L2

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  Figure 1  Single-crystal structure representations of 1 (a) and 2 (b) showing the coordination geometry of the metal ion

  Thermal ellipsoids are drawn at the 30% probability level; Anions and solvent molecules are omitted for clarity; Selected bond distance (nm) in 1: Co(1)-N(1) 0.197 0(3), Co(1)-O(1) 0.200 6(3), Co(1)-N(2) 0.201 4(3), Co(1)-S(2) 0.222 9(1), Co(1)-P(1) 0.222 2(2), Co(1)-S(1) 0.223 1(1); Selected bond distance (nm) in 2: Co(1)-N(10) 0.194 7(4), Co(1)-O(3) 0.201 0(4), Co(1)-N(2) 0.203 0(4), Co(1)-S(2) 0.221 3(2), Co(1)-P(1) 0.221 7(2), Co(1)-S(1) 0.223 0(2)

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  Figure 2  Cyclic Voltammogram of 1 (a) and 2 (b) (1 mmol·L^-1) in CH₃CN with 0.1 mol·L^-1 TBAPF₆ upon addition of NEt₃HCl with different concentrations

  Scan rate: 100 mV·s^-1

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  Figure 3  Family of the emission spectra of fluorescein solution (10 μmol·L^-1) in EtOH/H₂O (1:1, V/V) solution upon addition of catalyst 1 (a) and 2 (b) with various concentration

  Inset: Stern-Volmer plot for the photoluminescence quenching of Fl by catalysts 1 and 2

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  Figure 4  Initial rates of H₂ production in systems containing 1, Fl and 15% (V/V) of NEt₃ with different pH values (a) and containing 1, Fl at pH 11.5 with different concentrations of NEt₃ (b); Photocatalytic hydrogen evolution of systems containing Fl, 15% (V/V) NEt₃ with different concentrations of 1 (c) and containing 1, 15% (V/V) NEt₃ with different concentrations of Fl (d)

  c₁=10.0 μmol·L^-1, c_Fl=2.0 mmol·L^-1 for (a) and (b); c₁=5.0, 10.0, 15.0, 20.0 μmol·L^-1, c_Fl=2.0 mmol·L^-1 for (c); c₁=10.0 μmol·L^-1, c_Fl=1.0, 2.0, 3.0, 4.0 mmol·L^-1 for (d)

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  Figure 5  Photocatalytic hydrogen evolution of systems containing Fl, NEt₃ with different concentrations of 2 (a) and containing 2, NEt₃ with different concentrations of Fl (b)

  Concentration of NEt₃: 7% (V/V); c_Fl=2.0 mmol·L^-1, c₂=1.0, 5.0, 10.0, 20.0 μmol·L^-1 for (a); c₂=10.0 μmol·L^-1, c_Fl=1.0, 2.0, 3.0, 4.0 mmol·L^-1 for (b)

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  Table 1.  Crystal data and structure refinements for complexes 1 and 2

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  Table 2.  Data of hydrogen production test in 5 h for complexes 1 and 2

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