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Robust and Long-Lived Excited State Ru(II) Polyimine Photosensitizers Boost Hydrogen Production Song Guo, Kai-Kai Chen, Ru Dong, Zhi-Ming Zhang, Jianzhang Zhao, and Tong-Bu Lu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02226 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018
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ACS Catalysis
Robust and Long-Lived Excited State Ru(II) Polyimine Photosensitizers Boost Hydrogen Production Song Guo,†,§ Kai-Kai Chen,†,§ Ru Dong,† Zhi-Ming Zhang,† * Jianzhang Zhao,‡ Tong-Bu Lu†* †
Institute for New Energy Materials and Low Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China ‡ State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024 China Supporting Information Placeholder ABSTRACT: The prototypical [Ru(bpy)3]2+ (bpy = 2,2´-bipyridine, Ru-1) with 3MLCT state (metal-to-ligand charge-transfer, πM →πL*) is one of the most widely used photosensitizers (PSs) for photocatalytic hydrogen production. However, its photostability and excited state lifetime (< 1 µs) are eagerly to be improved to further enhance the performance of hydrogen production. Herein, [Ru(bpy)2(3-pyrenyl-1,10-phenanthroline)]2+ (Ru-3) with 3IL/3MLCT equilibrated state and [Ru(bpy)2(3-pyrenyl ethynylene-1,10phenanthroline)]2+ (Ru-4) with 3IL state (intraligand charge transfer, πL→πL*) as lowest excited state were firstly introduced into the photocatalytic hydrogen evolution system. Photophysical and photocatalytic characteristics manifest that 3IL state complex (Ru4) shows a long-lived excited state (up to 120 µs) and much enhanced photostability with no photobleaching over 13 h in stark contrast to Ru-1 and [Ru(bpy)2(1,10-phenanthroline)]2+ (Ru-2). Photocatalytic reactions with these Ru(II) complexes as PSs, Co(dmgH)2pyCl (C-1) as a catalyst, N, N-dimethyl-p-toluidine (DMT) as an electron donor indicate that the catalytic performance of Ru-4 and Ru-3 is dramatically enhanced compared to that of Ru-2 and Ru-1, and the TON and TOF towards Ru-4 can reach up to 9140 and 6.3 min-1 under the optimized condition. Photoluminescence studies reveal that the stern-volmer quenching constant of excited state Ru-4 by DMT is determined as 2.8 × 104 M-1, which is 4.5, 42 and 44-fold higher than those of Ru-3 (6.2 × 103 M-1), Ru-2 (6.7 × 102 M-1), and Ru-1 (6.3 × 102 M-1), respectively. Transient absorption spectra confirmed that the reductive quenching mechanism is the dominated process, and the quenching constant of electron transfer from reduced PSs of Ru-1 – Ru-4 to C-1 catalyst has the same order of magnitude (~105 M-1). The increased photocatalytic activity of Ru-3 and Ru-4 is due to their prominent photostability and efficient electron transfer from DMT to PSs. This work not only contributes to a deep understanding in photocatalytic process with the PSs of three different excited state types, but also opens up an avenue to explore robust and longlived PSs with 3IL state for efficient hydrogen production. KEYWORDS: Photosensitizer, 3IL state, robust, long-lived, photocatalytic hydrogen evolution
INTRODUCTION Photocatalytic hydrogen production, as an ideal way to produce clean and renewable hydrogen energy, has attracted considerable attention in the past decades.1-10 Throughout the molecular systems for photocatalytic hydrogen production, a three-component system composed of PS, catalyst, and electron donor, has been developed and widely used.2,11-19 As well known, constructing effective hydrogen production system not only requires highly active catalysts, but also needs robust potential matching PSs with preferable visible-light absorption ability and long excited state lifetimes, which can provide enough time for electron transfer from DMT to PSs and from PSs to catalysts in the photocatalytic process.20 In the past decades, much efforts have been focused on the design and synthesis of efficient catalysts,21-38 and a great progress has been achieved, in which a series of molecular catalysts including Rh,21-23 Pd,24,25 Pt,26-28 Fe,29 Co,30-34 and Ni35 have been prepared36 and used for catalytic hydrogen production. For examples, Hammarström and coworkers employed [Fe,Fe]-
.
dinuclear complex as a catalyst, with a turnover number (TON) of 200 towards catalyst.37 Eisenberg et al. reported a high efficient cobalt-based dithiolene complex as a catalyst, reaching a TON of as high as 9000.38 Additionally, a series of cobalt glyoximes, especially for Co(dmgH)2pyCl, were prepared and used as catalysts due to their low overpotentials for hydrogen production.33 In above studies, the prototypical [Ru(bpy)3]2+ was selected as PS to evaluate the photocatalytic performance and mechanism of the catalysts, and it has been widely used for more than 50 years due to its commercial availability, favorable visible-light harvesting ability, and appropriate potentials to drive both water oxidation and proton reduction reactions.2,33,37-45 However, its poor photostability and relatively short excited state lifetime (< 1 µs) are eagerly to be improved to further enhance the performance of hydrogen production.46
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production. Photophysical studies indicate that the excited state lifetimes of Ru-3 and Ru-4 are dramatically enhanced to 64.5 and 120.0 µs, respectively, being 107 and 200 times longer than that of parent Ru-2 PS (0.6 µs) with a 3MLCT excited state. In addition, Ru-3 and Ru-4 possess an efficient electron transfer efficiency from sacrificial agent to excited PSs and much improved photostability, in which Ru-4 can be stable for over 13 h under the visible-light irradiation, while Ru-1 and Ru-2 totally decompose within 2 h under the same condition (Figure 1c). Subsequently, the photocatalytic activity of Ru-3 and Ru-4 was significantly enhanced in comparison with that of traditional PSs (Ru-1 and Ru-2).
Scheme 1. Schematic show of energy level diagram of PSs with (a) 3 MLCT excited state, (b) 3IL excited state and (c) triplet excited state equilibrium between 3MLCT excited state and 3IL excited state.
As well known, PS, as a mediator of electron transfer between electron donor and catalyst, and a main body of light harvesting, have played an important role in the construction of efficient photocatalytic systems.10 However, compared to catalysts, only limited numbers of literatures demonstrated photocatalytic process of new PSs for hydrogen production, due to their complex photochemical process, unpredictable excited state and redox potentials in the water splitting process.17,47,48 Currently, organometallic PSs with 3MLCT state are widely used, including Ru(II), Ir(III),13, 49,50 Pt(II),14,51 and Re(I) -based complexes34,52. Though Ir(III), Re(I), and Pt(II)based PSs show much more photostability than [Ru(bpy)3]2+ PS, they are still unable to meet the requirement for hydrogen production due to their small extinction coefficients in the visible region. In contrast, Ru(II)-based PSs, especially for the archetypical [Ru(bpy)3]2+, have a wider application in hydrogen evolution system due to their favorable visible light harvesting ability and redox potential.53,54 Along this line, to improve the photostability of [Ru(bpy)3]2+ PS, Khnayzer and coworkers synthesized a new PS of [Ru(dpp)3]2+ [dpp = 4,7diphenyl-1,10-phenanthroline].47 and Natali et al. tried to modify [Ru(bpy)3]2+ by suitable functionalization of 4,4’positions of bipyridine ligands with methyl and tertiary butyl for unveiling the key factors to improve photocatalytic efficiencies.48 However, their hydrogen evolution efficiencies are all comparable to that of archetypical [Ru(bpy)3]2+ as their excited state lifetime and stability were not obviously improved. As a result, a promising opportunity, but a considerable challenge remains for chemists to explore stable and long lived excited state PSs that can efficiently drive the hydrogen production. In this work, the pyrenyl and pyrenylethynylene were introduced into [Ru(bpy)3]2+ according the previous investigations (Scheme S1),55-61 resulting in the Ru complexes with 3IL excit3 3 ed state and MLCT/ IL equilibrated excited state (Scheme 1, Scheme 2, Figure S1–S10). Herein, the Ru(II) polypyridine complexes (Ru-3 and Ru-4) with 3IL state and 3MLCT/3IL equilibrated state were firstly used for photocatalytic hydrogen
Scheme 2. The structures for Ru-1~Ru-4, C-1, and DMT.
EXPERIMENTAL SECTION Materials. All the reactions were carried out under an argon atmosphere unless otherwise mentioned. All the solvents were analytical grade and distilled before use. The [RuCl2(cymene)]2, Pd(PPh3)4 and Pd(PPh3)2Cl2 were purchased from Sigma-Aldrich. The diisopropylamine, NH4PF6 and CuI were purchased from Alfa Aesar. Chromatographic grade acetonitrile was purchased from Adamas Reagent. Ru-2 ~ Ru-4 were synthesized according to the literature method,57 and the synthetic scheme for Ru(II) complexes is presented in Scheme S1. The synthetic intermediates and target complexes were evidenced by 1H NMR and MS spectroscopy. Instrumentation. Electrochemical measurements were carried out on a CHI 760E electrochemical workstation at room temperature. The amount of the hydrogen product was analyzed by gas chromatography (Shimadzu GC-2014+AT 230C, TDX-01 column, TCD, argon carrier). UV-vis absorption spectra were recorded on a LAMBDA750 UV-vis spectrophotometer (Figure S11a). Fluorescence spectra were taken on Hitachi F4500 spectrofluorometer (Figure S11b). Transient absorption spectra were measured on the LP980 laser flash photolysis instrument (Edinburgh, U.K.). Photocatlytic experiments were conducted with a blue LED light (Zolix, MLED4). Photocatalytic Hydrogen Production. Photocatalytic hydrogen evolution was conducted under 1 atm of Ar at 25 °C in 16 mL reactor containing PS (5.0 × 10-6 M), catalyst (7.8 × 10-5 M), DMT (5.5 × 10-3 M), H2O (2.2 × 10-3 M) and 5 mL CH3CN. The mixture was continuously stirred and irradiated under a blue LED light (λ = 450 nm, 122 mW·cm-2, irradiation area of 0.8 cm2). Spectrum Measurement. All the solvents were chromatographically pure for spectrum measurement, and the measurements were performed under an argon atmosphere unless oth-
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ACS Catalysis
erwise stated. In order to ensure the accuracy of the test results, [Ru(bpy)3]2+ was used as standard sample to correct the instrument and the method of measurement. The moles of photons absorbed by the PS (np). np = ptλ/NAhc Where np represents moles of photons absorbed by the PS, p is the absorbed power of the light, t is the irradiation time (4 h). λ is the irradiation wavelength number (450 nm), NA is the Avogadro constant (6.022 × 1023 mol-1 ), h is the Planck constant (6.63 × 10-34 J˙s), c is the speed of the light (3 × 108 m˙ s-1 ). Quantum effciency of photocatalytic hydrogen production (Φ Φ). Φ = 2nH2 /np Where Φ represents quantum effciency of photocatalytic hydrogen production, nH2 is the moles of H2 detected by the GC. RERSULTS AND DISCUSSION Photocatalytic Hydrogen Production. Until now, most of the PSs used for hydrogen production are the complexes with 3 MLCT excited state in the three-component system.13,14,38,52,6264 Herein, Ru-3 with 3MLCT/3IL equilibrated excited state and Ru-4 with 3IL excited state were introduced into the photocatalytic hydrogen evolution system (Scheme 2). In a typical process, C-1 was used as the proton reduction catalyst to investigate the performance of different PSs, due to its low overpotential for hydrogen evolution and well-known photocatalytic mechanism, although it often suffers the loss of catalytic activity caused by the hydrogenation reduction of dmgH ligand.65 As shown in Figure 1, H2 is rapidly produced upon the visible light illumination, and its evolution rate tends to zero at around 2 h using Ru-1 and Ru-2 as the PSs. Such a quick termination reaction can be ascribed to complete degradation of PSs (Figure 2a and 2b). In contrast, Ru-3 and Ru-4containing photocatalytic systems are more durable, and the photocatalytic reactions ceased at around 7 h, which could be
largely attributed to the improved photostability of Ru-3 and Ru-4 (Figure 2c and 2d) and the inactivation of C-1 as the loss of dmgH ligand during the hydrogenation reduction.65 Further, the recycle photocatalytic experiments were performed by adding catalyst or PS after 7 h photocatalytic reaction (Figure S12). It could be observed that photocatalytic activity of Ru-4containing system partially restored when the catalyst was added. In contrast, trace amount of H2 was detected with the addition of PS, indicating the decomposition of the catalyst in this photocatalytic system. To further confirm this proposal, the hydrogen evolution capacity of Ru-1-Ru-4 with different pre-illumination times was detected to directly compare their photostability (Figure 1c). Remarkably, Ru-4 shows a constant ability of hydrogen production with a 12 h pre-illumination. For Ru-3, there was no obvious loss for the hydrogen evolution capacity with a 2 h pre-illumination, however, the photocatalytic activity decreased gradually after a 2 h preillumination. Ultimately, its photocatalytic ability was almost exhausted within 12 h. For Ru-1 and Ru-2, the hydrogen evolution capacity drastically decreases at the beginning of preillumination, and completely loses the hydrogen evolution ability after a 2 h pre-illumination. It could be obviously observed that Ru-4 possesses a distinguished photostability, thus highlighting the potential application of PSs with 3IL state for photocatalytic water splitting. The H2 evolution amount corresponds to a TON (defined as n(H2)/n(catalyst)) of 376 and 395 for Ru-1 and Ru-2containing hydrogen evolution systems (Table 1). Under the same condition, the TONs of Ru-3 and Ru-4-containing system were up to 1080 and 1160, respectively. The turnover frequency (TOF) of Ru-3 and Ru-4 reached 2.6 min-1 and 2.8 min-1 within 6 h, which was ca. three times higher than that of Ru-1 (0.9 min-1) and Ru-2 (0.9 min-1). The quantum efficiency of Ru-3 and Ru-4 was up to 5.2 % and 5.9 %, respectively, over two times higher than that of Ru-1 and Ru-2.
Figure 1. (a) Photocatalytic hydrogen production of Ru-1-Ru-4; (b) TON (red) and TOF (blue) within 6 h; (c) photocatalytic ability of Ru-1-Ru-4-containing systems after different pre-illumination time with PS alone. Photocatalytic conditions: 5.0 µM PSs, 77.5 µM C-1, and 5.5 mM DMT in CH3CN/H2O.
Table 1. The results for photocatalytic hydrogen production.a
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PSs
H2 (µmol)
TONb
TOF (min-1)c
Ru-1
9.4
376
0.9
2.5%
Ru-2
9.9
395
0.9
2.4%
Ru-3
27.3
1080
2.6
5.2%
Ru-4
29.0
1160
2.8
5.9%
Ru-4e
45.7
9140
6.3
8.8%
Φd
5.0 µM PSs, 77.5 µM C-1, and 5.5 mM DMT in CH3CN/H2O; TON and TOF values within 6 h; d quantum efficiency of hydrogen production; e 1.0 µM PSs, 0.25 mM C-1, and 70.0 mM DMT in CH3CN/H2O. All the results are averaged over three reactions with deviations below 5%. a
b,c
Figure 2. UV-vis absorption spectra of 5.0 µM (a) Ru-1, (b) Ru2, (c) Ru-3, (d) Ru-4 in the mixed solvent of CH3CN/H2O with different irradiation time; (e) the proposed mechanism of photostability of PSs with different excited state types (triplet state energy levels: 2.58 eV for Ru-2, 1.98 eV for Ru-3, 1.16 eV for Ru-4).
Further, photocatalytic performance of these PSs was optimized with various experiments via changing the concentrations of PSs, DMT and C-1. It could be found that the TON and TOF towards Ru-4 can reach up to 9140 and 6.3 min-1 with a hydrogen yields of 45.7 µmol (Table 1). Otherwise, no or trace amount of H2 was detected for the control experiments in the absence of sensitizer, water, catalyst, electron donor or light, demonstrating that the above factors all played important roles in the photocatalytic hydrogen production. In order to
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unveil the key factors for the enhancement of photocatalytic hydrogen evolution ability for Ru-3 and Ru-4, the photostability, excited state lifetime, redox potential, photocatalytic hydrogen evolution mechanism, and the efficiency of electron transfer were systemically investigated by steady-state spectrum, electrochemistry, and transient-state spectrum, respectively. Steady-state spectrum for photostability and electron transfer. The photostability of Ru-1 – Ru-4 was investigated through monitoring the change of UV-vis absorption spectra in the mixed solution of CH3CN/H2O upon the irradiation of 450 nm LED light (Figure 2). The absorption peak around 450 nm for Ru-1 shows a significant decline, and a new absorption peak at around 480 nm gradually rose as the illumination time went on. Accordingly, Ru-1 shows a poor photostability and almost decomposed completely within 2 h. In this process, a new species formed, determined as [Ru(bpy)2(OH2)2]2+ according to previous reports,63,66,67 which possesses of poor photochemical activity in the photocatalytic system.68 Ru-2 exhibits a similar process of photodecomposition with that of Ru-1, which also decomposed completely within 2 h upon the irradiation of 450 nm LED light. Compared with Ru-1 and Ru-2, the photostability of Ru-3 is much improved, although it gradually decomposes within 3.5 h. Remarkably, the absorption peaks around 417 nm from the absorption of the Phen ligand with large π-conjugation and around 480 nm from the 3 MLCT transition for Ru-4 are both almost unchanged after 13 h illumination, indicating that Ru-4 is ultrastable under the irradiation. The photostability of these PSs is in the order of Ru-4 > Ru-3 > Ru-2 ≈ Ru-1, which is in accordance with persistent time of hydrogen production for these four photocatalytic systems (Figure 1a). Additionally, the UV-vis absorption spectra of these PSs was also detected in the dark (Figure S13) to confirm that the photodecomposition process happened in the excited state of the PSs. As shown in Figure S13, there was no change in UV-vis absorption spectra of Ru-1 – Ru-4 after aging for 12 h in the dark, indicating that these PSs were stable in the ground state. All the above results reveal that the photodecomposition of Ru-1 - Ru-3 happened in the excited state, not in their ground state, and Ru-4 is a stable PS under the illumination. According to the molecular structure and excited state characteristics of these Ru(II) complexes, it can be partly attributed to two possibilities for the improved photostability of Ru-3 and Ru-4. Firstly, the large conjugated system of the ligands (Py-C≡C-phenanthroline for Ru-4 and Py-phenanthroline for Ru-3) have bulky steric hindrance and strong hydrophobicity, which could effectively prevent the attack from the nucleophilic species (e.g. H2O). Secondly, as well known, 3MC state of Ru (II) complexes is labile owing to the distinct distortions along the Ru-N bonding axes. The energy gap between 3MC state and 3IL state was larger than the gap between 3MC state and 3MLCT state (Figure 2e). As a result, a transition from 3IL state to 3MC state may be prohibited, which could contribute to the photostability of Ru-4.63,64,69 We performed the UV-vis spectra of PSs (Ru-1 and Ru-4) in the presence of DMT under different irradiation time (Figure S14). The results show that the system containing Ru-4 and DMT is more instable than that of Ru-1 and DMT, as the faster electron transfer from DMT to Ru-4 than that from DMT to Ru-1, resulting in a higher concentration of reduced Ru-4 compared to that of re-
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ACS Catalysis
duced Ru-1 at the same time. Under this condition, the reduced Ru-4 could decompose before back to the ground state by a backward electron transfer from reduced Ru-4 to DMT.+. However, it could be obviously observed that the quenching constant of reduced PSs by C-1 is more than 10 times larger than that of excited PSs by DMT (Table 2), providing the possibility that the reduced PSs could efficiently donor electrons to C-1 to avoiding its decomposition (Figure S15). UV-vis spectra of PSs (Ru-1
– Ru-4) in the presence of DMT and C-1 were also performed to track the photostability of photocatalytic system under 450 nm illumination (Figure S15). For Ru-1 and Ru-4, the absorption band between 430 nm – 460 nm rapidly rose and reached vertex within 5 min, indicating an equilibrium between generation and consumption of Co2+. And then, these absorption peaks decreased with prolonging the illumination time, indicating the
Figure 3. Phosphorescence quenching of (a) Ru-1, (b) Ru-2, (c) Ru-3, (d) Ru-4 with DMT as the quencher in acetonitrile; (e) SternVolmer plot of Ru-1 – Ru-4. λex = 475 nm, cPS = 5.0 µM at 25 oC. Table 2. Photophysical properties of these Ru(II) complexes (Ru-1–Ru-4). λabs/nm
λem /nm
ε/(M-1cm-1)
τT/µs a
τT/µs b
K1 / (M-1) c
K2 / (M-1) d
K3 / (M-1) e
Ru-1
446
594
17866
0.8
99.4
6.3 × 102
1.8 × 105
2.2 × 103
Ru-2
450
626
8778
0.6
100.6
6.7 × 102
4.1 × 105
2.4 × 103
Ru-3
450
620
14077
64.5
81.0
6.2 × 103
3.4 × 105
4.3 × 104
Ru-4
418
626/667/726
38777
120.0
119.0
2.8 × 104
3.5 × 105
2.3 × 105
a
Triplet excited state lifetime measured by transient absorption. b Lifetime of the reduced PSs. c Stern-Volmer quenching constant of PS with DMT as the quencher. d Stern-Volmer quenching constant of the reduced PS quenched by C-1, e Stern-Volmer quenching constant of PS quenched by C-1.
decrease of the generation rate of Co2+. The decrease rate of Co2+ concentration in Ru-1-containing system was up to 0.35 h-1, 4.4 times faster than that of Ru-4-containing system (0.08 h-1). Obviously, the PSs is the only difference between Ru-1 and Ru-4-containing systems. As a result, it can be concluded that Ru-4 is more robust under this photocatalytic condition in comparison with Ru-1 – Ru-3 (Figure S15). In order to study the electron transfer efficiency in the photocatalytic process, the phosphorescence titration of PSs with electronic sacrificial agent or catalyst was performed. As shown in Figure 3, the phosphorescence of Ru-1 at 594 nm could be gradually quenched with increasing the concentration of DMT with a Ksv value of 6.3 × 102 M-1. For the Ru-2, a similar quenched profile with that of Ru-1 was observed with a quenching constant of 6.7 × 102 M-1. Compared with that of Ru-1 and Ru-2, Ru-3 and Ru-4 show a much enhanced quenching effect with the Ksv values of 6.2 × 103 M-1 and 2.8 × 104 M-1, respectively. It could be concluded that Ru-4 has more efficient electronic capture ability from DMT than other three PSs. In this process, it should be pointed out that the energy transfer from PSs to DMT can be excluded by transient spectra and unmatched energy levels between them (Figure
S16). Further, a similar phosphorescence quenching profile was observed for these PSs with C-1 as the quencher instead of DMT, and the order of Ksv value was Ru-4 (2.3 × 105 M-1) > Ru-3 (4.3 × 104 M-1) > Ru-2 (2.4 × 103 M-1) = Ru-1 (2.2 × 103 M-1) (Figure S17). Nanosecond transient absorption spectroscopy reveals that the phosphorescence quenching of PSs by the catalyst might be caused by the electron transfer or energy transfer from PSs to C-1 (see the transient absorption spectrum section). The bimolecular quenching constant of Ru-4 by C-1 is determined as 1.9 × 109 M-1s-1, which is very close to diffusion controlled process. Nevertheless, the driving force for electron transfer from Ru-4 to C-1 is very small (only 0.08 eV, see Table 3), suggesting that the energy transfer from Ru4 to C-1 may be the dominated process. It is worth mentioning that the order of excited state lifetimes of Ru-1 – Ru-4 matched well with the order of phosphorescent quenching constants with DMT (or C-1) as the quencher. These results reveal that the long-lived excited states of PSs are beneficial to intermolecular electron/energy transfer (Table 2), which could further promote photocatalytic hydrogen evolution (Figure S18). Cyclic voltammograms. Different excited state population of Ru-3, Ru-4 and Ru-1 indicates that they might possess differ-
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ent redox properties and could supply different driving forces for the electron transfer. Here, cyclic voltammograms (CVs) of all the molecules, including Ru-1 – Ru-4, C-1, and DMT, were performed in the acetonetrile solution to evaluate the thermodynamic feasibility of the electron transfer process among the three-component systems (Figure 4 and Table 3). Both Ru-1 and Ru-2 shows very similar CV curves. They give an oxidation potential (vs SCE) of Ru2+/3+ at 1.29 V and three
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reduction potentials (vs SCE) at -1.33 V, -1.52 V, and -1.77 V in the negative region, corresponding to L0/-1, L-1/-2, and L-2/-3 (L = ligand), respectively. The oxidation potentials (vs SCE) of Ru-3 and Ru-4 are at 1.35 V and 1.34 V, respectively, attributed to the oxidation process from Ru2+ to Ru3+. The CV curve of Ru-3 gave four reduction peaks in the negative region due to the non-conjugated connection between pyrene and phenanthroline.
Figure 4. CVs of Ru-1 - Ru-4, DMT and C-1. (a) Ru-1, (b) Ru-2, (c) Ru-3, (d) Ru-4, (e) DMT, and (f) C-1. The CVs were determined in the deaerated CH3CN solution, containing 0.5 mM photosensitizer, ferrocene, and 0.10 M Bu4NPF6 as the supporting electrolyte, with a scan rate of 0.05 V/s-1 and a positive initial scan direction. Glassy carbon electrode, Ag/AgNO3 and Pt silk was used as the working electrode, reference electrode and counter electrode, respectively. Table 3. Redox potentials of Ru-1 - Ru-4 and the ∆GCS for the intermolecular electron transfers (with Ru(II) complexes as electron donor and the C-1 as electron acceptor). The potential values are given with respect to SCE (Fc as internal reference, E1/2(Fc+/Fc) = +0.40 V vs. SCE ).a Eox
Ered
E0,0
∆GCS b
∆GCS c
∆GCS d
∆GCS e
Ru-1
1.29
-1.33,
-1.52,
-1.77
2.28
-0.23
-0.32
-0.56
-0.23
Ru-2
1.29
-1.33,
-1.52,
-1.78
2.32
-0.27
-0.36
-0.56
-0.23
Ru-3
1.35
-1.28,
-1.48,
-1.73,
2.28
-0.28
-0.26
-0.61
-0.28
Ru-4
1.34
-1.13,
-1.46,
-1.80
2.25
-0.40
-0.08
-0.46
-0.03
DMT
0.72
-
-
-
-
-
C-1
-
-
-
-
-
-
-1.92
-0.67,
-1.10
a
Cyclic voltammetry was carried out in the deaerated acetonitrile containing 0.10 M Bu4NPF6 supporting electrolyte; glassy carbon electrode, Ag/AgNO3 and Pt silk was used as the working electrode, reference electrode and counter electrode, respectively. [Ag+] = 0.1 M, 0.5 mM PSs and 0.5 mM ferrocene, at 293 K. The values of the change in Gibbs free energy for the electron transfer process: b from DMT to PS, c from the excited PS to C-1, d from the reduced PS to Co3+ (C-1), e from the reduced PS to Co2+(C-1). The first three peaks located at -1.28 V, -1.48 V, and -1.73 V (vs SCE), could be ascribed to the reduction processes of L0/-1,
L-1/-2 and L-2/-3, respectively, and the last reduction peak at 1.92 V (vs SCE) corresponds to the reduction process of Py0/-1.
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For Ru-4, the first reduction peak in the native region located at -1.13 V (vs SCE) can be assigned to one-electron reduction of Py-C≡C-phenanthroline, it is more positive than that in other complexes due to the existence of large conjugated ligand. DMT as a strong electron donor shows an oxidation potentials at 0.72 V (vs SCE). CV curve of C-1 gives two reversible reduction waves at −0.67 V and −1.10 V (vs SCE), corresponding to Co3+/2+ and Co2+/1+, respectively (Figure S19). Based on above results, the Gibbs free energy changes for the electron transfer in the photocatalytic reaction could be estimated by eq 1 and 2.70,71
∆G 0 CS = e[ EOX − E RED ] − E00 + ∆GS ∆GS = −
e2 4πε Sε 0 RCC
−
1 1 1 e2 1 + − 8πε 0 RD RA ε REF ε S
where e is the electronic charge, EOX is the oxidation potential of electron-donor unit, ERED is the reduction potential of the electron-acceptor unit, E00 is the approximate energy level obtained from the onset of phosphorescence emission (5% relative intensity).72,73 ∆GS is the static coulombic energy, which was estimated from eq 2. εS is the static dielectric constant of the solvent, RCC is the center-to-center separation distance, estimated as +∞ due to it is an intermolecular distance.74 RD is the radius of the electron donor, RA is the radius of the electron acceptor, εREF is the static dielectric constant of the solvent used for electrochemical study, and ε0 is the permittivity of free space. Herein, ∆GS was determined to be zero as RCC = +∞ and εREF = εS. The Gibbs free energy changes for the electron transfer 75 from reduced PS to catalyst could be evaluated with eq 3.
∆G 0 CS = e[ E 1 RED − E 2 RED ]
where e is the electronic charge, E1RED is the reduction potential of electron-donor unit, E2RED is the reduction potential of the electron-acceptor unit. The Gibbs free energy changes (∆GCS) of the electron transfer are worked out by Weller Equation (eq 1, eq 2, and eq 3). As a consequence, values of ∆GCS are negative at all tested points, indicating that the related electron transfer processes between DMT and excited PS, excited PS and C-1, as well as reduced PS and C-1 are thermodynamic feasible. For the electron transfer process from DMT to PSs, the absolute value of ∆GCS is in the order of Ru-4 > Ru-3 > Ru-2 > Ru-1, which is positive correlation with that of the electron transfer efficiency (Figure 3e). Given that the values of Gibbs free energy change from reduced PSs to Co3+ and Co2+ were both negative, thus it could be concluded that the reduced PSs can consecutively donate two electrons to C-1, producing Co2+ and Co+. In this process, there are sufficient driving forces (-0.56 eV for Ru-1, -0.56 eV for Ru-2, -0.61 eV for Ru-3, and -0.46 eV for Ru-4) to drive the electron transfer from the reduced PSs (Ru-1 – Ru-4) to Co3+. As well known, the reduction potential of Co2+/Co+ (-1.10 V vs SCE) was more negative than that of Co3+/Co2+ (-0.67 V vs SCE). Thus, it requires a stronger driving force to transfer electron from the reduced PSs to Co2+. The driving forces between reduced Ru-3/Co2+ and Ru-4/Co2+
were determined as -0.28 eV and -0.03 eV, respectively, which are qualified to drive the electron transfer process. In the photocatalytic process, the initial hydrogen evolution rate with Ru-4 as PS was a little slower than that of Ru-3, which may result from the smaller driving force of Ru-4 compared to that of Ru-3 (Figure S18, Table 2). However, the superior photostability and long-lived excited state Ru-4 can redeem the defect of small driving force, as a result, Ru-4 was determined to be the most efficient PSs among these four Ru(II) complexes for photocatalytic hydrogen evolution. Transient absorption spectrum. To unveil the photocatalytic mechanism, transient absorption spectrum of these photocatalytic systems was detailly studied. Upon pulsed laser excitation at 443 nm, both a bleaching peak around 450 nm and a positive peak around 380 nm were observed for Ru-1, which could be regarded as the typical 3MLCT absorption (Figure 5a). The excited state lifetime of Ru-1 was determined as 0.8 µs (Figure 5b). When DMT was added into the solution of Ru-1, a new absorption peak around 500 nm appeared with a longlived decay (99.4µs). This new peak could be assigned to the absorption of [Ru(bpy)3]+, which derived from the reductive electron transfer from DMT to the excited Ru-1 (Figure 5c and 5d).76,77 The long-lived reduced state of Ru-1 could be efficiently oxidized by C-1 with a stern-volmer constant of 1.8 × 105 M-1 (Table 2). The transient absorption spectra of Ru-1 in the presence of different concentrations of C-1 was further performed to verify the energy or electron transfer process from PS to the catalyst. It is worth noting that the characteristic absorption peaks of [Ru(bpy)3]3+ centered at approximately 410 nm and 680 nm fail to be detected for Ru-1 in the presence of C-1,78 and the transient profile of Ru-1 with C-1 was almost in line with that of the isolated Ru-1, although the excited state lifetime of Ru-1 was becoming shorter and shorter with increasing the concentration of C-1. Moreover, the transient spectra of Ru-1 with different concentrations of C-1 took on a single exponential decay without forming additional transient signatures. Therefore, the excited Ru-1 quenched by C-1 could be attributed to two possibilities: i) the energy transfer from excited PSs to C-1 was possible as the spectral overlap between the phosphorescence spectrum of Ru-1 and the absorption spectrum of C-1, which exhibits visible-light absorption ability in the region of 550 nm – 650 nm as evidenced by UV-vis spectrum (Figure S11 and S16).20,79 However, it is well known that the excited state of C-1 can’t be detected by nanosecond transient absorption spectrum due to its ultra-shortlived excited state.80 ii) an oxidative photoinduced electron transfer process maybe happen, which could be confirmed by CV. However, the charge separated state could not be observed due to the negligible cage escape yields.48 Above all, the reductive quenching mechanism should be a dominated process in the photocatalytic system of Ru-1.
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Figure 5. Nanosecond transient absorption spectra of (a) Ru-1, (b) the decay of Ru-1 at 450 nm, (c) Ru-1 in the presence of 5.2 mM of DMT, (d) Kinetic traces of the reduced Ru-1 followed at 500 nm, (e) Ru-1 in the presence of 77.5 µM of C-1, (f) Kinetic traces of Ru-1 with different concentration of C-1 followed at 450 nm. These spectra were recorded in acetonitrile after pulsed excitation at 443 nm under N2 atmosphere at 25 oC.
Figure 6. Nanosecond transient absorption spectra of (a) Ru-4, (b) the decay of Ru-4 at 418 nm, (c) Ru-4 in the presence of 5.2 mM of DMT, (d) Kinetic traces of the reduced Ru-4 followed at 430 nm and 600 nm, (e) Ru-4 in the presence of 77.5 µM of C-1, (f) Kinetic traces of Ru-4 with different concentration of C-1 followed at 418 nm. All the spectra were recorded in acetonitrile after pulsed excitation at 405 nm under N2 atmosphere at 25 oC.
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Ru-2 and Ru-1 both show a 3MLCT excited state, thus the transient spectra of Ru-2 with DMT or C-1 exhibit a similar evolution process with that of Ru-1 (Figure S20). As shown in Figure S20, Ru-2 also gave a bleaching peak at 445 nm and a positive peak at 364 nm with a decay state of Ru-2 was obtained by fitting the decay at 500 nm af ter the addition of DMT. The electron transfer route of photocatalytic system of DMT/Ru-2/C-1 was also a reductive quenching mechanism, which was confirmed by the same method of Ru-1. Upon photoexcitation at 405 nm, the transient absorption spectra of Ru-4 shows two bleaching peaks approximately located at 380 nm and 420 nm (Figure 6), corresponding to its steady-state absorption spectra. A positive absorption band in the region of 450 nm – 700 nm was observed, which could be attributed to the absorption of triplet state of Py-C ≡ Cphenanthroline. Thus, it can be concluded that the excited state of Ru-4 populated on the 3IL state with a long-lived decay at 418 nm (120.0 µs), which was strikingly different from that of Ru-1 and Ru-2 with a short-lived 3MLCT state (0.8 µs for Ru-1 and 0.6 µs for Ru-2). On addition of DMT into the solution of Ru-4, three new peaks centered at 425 nm, 500 nm, and 600 nm appeared in the transient absorption spectra of Ru-4 (Figure 6c). The absorption peaks at 425 nm and 500 nm in the transient absorption spectra of Ru-4 were consistent with those of reduced Ru-1. The kinetic trace followed at 600 nm was performed to reveal its affiliation with the absorption peak at 425 nm. It was found that the new peak at 600 nm shows a same dynamics decay (119.0 µs) with that at 425 nm, indicating that all the absorption peaks at 425 nm, 500 nm, and 600 nm could be assigned to the same transient species of
DMT 120.0 µs
2.8 x 104 M-1
hν ν
DMT 119.0 µs -
Ru-4
Ru-4*
the reduced Ru-4. The long-lived reduced state of Ru-4 could be efficiently oxidized by C-1 with a stern-volmer quenching constant of 1.8 × 105 M-1 and bimolecular quenching constant of 2.9 × 109 M-1s-1, which was near the diffusion-controlled limits (~1010 M-1s-1) (Table S1).81 As shown in Figure 3e and 6f, both phosphorescence and excited state lifetime of Ru-4 could be efficiently quenched by C-1. However, the transient signal of Ru-4 remained unchanged before and after adding C-1, meanwhile its excited state with different concentration of C-1 could decay monotonically to the baseline, and no additional transient signal was observed. All these results are the same as those of Ru-1, and support the reduction mechanism as a dominated process for photocatalytic hydrogen production system of DMT/Ru-4/C-1. For Ru-3, an equilibrium between 3MLCT and 3IL exited states has been established.57 The transient spectra of Ru-3 shows a bleaching band at around 380 nm and a positive absorption peak in the range of 410 nm – 800 nm, which corresponds to the ground state absorption of the pyrenyl ligand and triplet state of pyrenyl ligand, respectively (Figure S21). The triplet state lifetime of Ru-3 was up to 64.5 µs due to an energy equilibrium between 3IL state and 3MLCT state. The absorption peaks of the reduced Ru-3 at 413 nm, 499 nm, and 595 nm were observed for the mixture solution of Ru-3 and DMT, which were similar with those of the reduced Ru-4. The lifetime of the reduced Ru-3 was determined as 81.0 µs, which could be efficiently quenched by C-1 with a quenching constant of 3.4 × 105 M-1. Accordingly, the
120.0 µs
Ru-4
Ru-4*
Co(II)
DMT
3.5 x 105 M-1
-
Ru-4
Ru-4 hν ν
Co(III)
H2O
2.8 x 104 M-1
Co(I)
119.0 µs
DMT
H2
Scheme 3. Proposed photoredox cycle for visible-light induced hydrogen evolution with Ru-4.
Transient absorption spectroscopy of PSs with DMT and C1 were investigated to uncover the evolution pro cess of reduced PSs (Figure S22 and s23). As shown in Figure S23, the decay from reduced PS to catalyst was becoming faster and faster with increasing the concentration of C-1, indicating an efficient electron transfer from the reduced PS to C-1. In this process, the spectra of reduced Ru-1 didn’t come back to the
baseline, but produced a new peak around 480 nm in the presence of C-1, which matches well with the absorption spectrum of [Ru(bpy)2(OH2)2]2+ (Figure S22e). A similar trend was also observed in the transient absorption spectra of Ru-2. For Ru-3, it can be obviously seen that the stability of Ru-3 was improved compared to that of Ru-1 and Ru-2 as a much weaker peak appeared around 480 nm. By contrast, the transient ab-
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sorption spectra of reduced Ru-4 almost returned to the baseline and no obvious peak around 480 nm was detected. These results demonstrate that Ru-4 is more stable than the traditional Ru(II) complexes (Ru-1 and Ru-2) in the three-component system under illumination. The second order rate constants of reduced Ru(II) complexes by C-1 and bimolecular rate constant of excited Ru(II) complexes by DMT were supplied in Figure S24 and Table S1. The rate constant of Ru-4 with 3IL state seems to react with DMT at most 3 times smaller than that for Ru-1 with 3MLCT state, which may support that the Ru-4 could be viewed as Ru-sensitized pyrene triplet states. Above all, the photoredox cycle was illuminated and some important parameters were labeled in scheme 3. Upon photoexcitation, the excited states of PSs (Ru-1 – Ru-4) were populated, then an electron transfer from DMT to PS occurred, afterwards, the reduced PSs successively donated two electrons to C-1 (Scheme 3). Accordingly, the excited state lifetimes of the PSs and reduced PSs all play an important role in the electron transfer process. On one hand, phosphorescence quenching constant of Ru-4 by DMT was determined as 2.8 × -1 104 M , 4.5, 42 and 44-fold higher than those of Ru-3, Ru-2, and Ru-1, respectively, which was due to the longer lifetime of Ru-4 (up to 120 µs) compared to the shorter lifetime of other Ru(II) complexes. On the other hand, the mentioned PSs all show long-lived reduced states (99.4 µs for Ru-1, 100.6 µs for Ru-2, 81.0 µs for Ru-3 and 119.0 µs for Ru-4) in the presence of DMT, which could be oxidized by C-1 with efficient quenching constant of 1.8 × 105 M-1 for Ru-1, 4.1 × 105 M-1 for Ru-2, 3.4 × 105 M-1 for Ru-3, 3.5 × 105 M-1 for Ru-4, respectively. These constants of reduced PSs by C-1 show a same order of magnitude, and they are 10 times higher than those of excited PSs quenched by DMT. Accordingly, it could be concluded that the efficiency of electron transfer from DMT to excited PSs plays a decisive role in improving the photocatalytic hydrogen evolution activity of Ru-1 – Ru-4containing systems. Hence, Ru-4 and Ru-3 with the long-lived excited state possess a higher efficiency for capturing electron from DMT than that of typical PSs (Ru-1 and Ru-2), and then promote photocatalytic hydrogen production. CONCLUSIONS This is the first time to systematically explore the photocatalytic process and activity of PSs with different nature of their excited states for hydrogen production. Photocatalytic activity of 3IL state complexes (Ru-4 and Ru-3) was much enhanced compared to that of the 3MLCT complexes (Ru-1 and Ru-2), which can be predominantly attributed to their remarkable photostability and long-lived excited state (up to 120 µs for Ru-4). The reductive mechanism was determined as a dominated process for photocatalytic hydrogen production and the electron transfer process from DMT to PS was identified as the efficiency determining step in the three-component system. This study not only reveals that Ru-4 with 3IL state could be used as the efficient PS for hydrogen production, but also open an avenue for design and synthesis of stable and longlived excited state PSs to further improve the efficiency for photolysis of water and photocatalytic CO2 reduction. ASSOCIATED CONTENT
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Supporting Information. Experimental procedures, nanosecond transient absorption experimental details, molecular structure characterization, additional spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *
[email protected] *
[email protected] § S. Guo and K.-K. Chen contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21703155/21790052/21671032/21331007), the 973 program of China (2014CB845602). REFERENCES (1) Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37–38. (2) Gueret, R.; Poulard, L.; Oshinowo, M.; Chauvin, J.; Dahmane,
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