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Photoelectrochemical Oxygen Reduction Reactions using Phthalocyanine-Based Thin Films on an ITO Electrode Ngo Thi Hong Trang, and Kazuyuki Ishii J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10201 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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The Journal of Physical Chemistry

Photoelectrochemical Oxygen Reduction Reactions using Phthalocyanine-Based Thin Films on an ITO Electrode Ngo Thi Hong Trang & Kazuyuki Ishii* Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 1538505, Japan. *(K. I.) E-mail: [email protected]. Telephone: +81-3-5452-6306. Fax: +81-3-5452-6306

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ABSTRACT

The oxygen reduction reaction (ORR) has attracted much interest, not only with respect to biological processes such as cellular respiration, but also in terms of practical energy conversion, i.e., cathodic reactions in fuel cells. Because the use of light energy is a promising alternative to various kinds of efficient catalysts such as platinum, we have investigated the photoelectrochemical ORR using various phthalocyanines (Pcs). On the irradiation by visible light of an indium tin oxide (ITO) electrode coated by thin films consisting of Pc and polyvinylidene difluoride (PVDF) polymer, the cathodic current corresponding to the ORR increased significantly. A previous study suggested that the high photocurrent might originate from the initial electron transfer between the photoexcited Pc and O2 (Pc* + O2 → Pc+• + O2-•) followed by the hole-transport between Pcs. However, here we propose a new mechanism that can also explain the efficient photoelectrochemical ORR in Pc/PVDF thin films. The dependence on the reduction potential of the Pcs indicates that the electrochemical reduction of the photoexcited Pc is the initial process. Subsequently, electron transfer from Pc-• to O2 occurs, i.e. Pc-• + O2 → Pc + O2-•. Based on the comparison between ZnPc and MgPc derivatives, which have different quantum yields for the lowest excited triplet (T1) state, the high photocurrent mainly originates from the electrochemical reduction of Pcs in the lowest excited singlet (S1) state. Although, the reaction can also occur via the T1 state, the T1 state contributes to an increase in photocurrent only when the electrode potential is below +0.2 V. This study will be useful for the development and design of new catalysts for photoelectrochemical ORR.


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Introduction The oxygen reduction reaction (ORR) attracts broad interest, not only with respect to biological processes such as cellular respiration,1,2 but also in terms of practical energy conversion, i.e., cathodic reactions in various fuel cells.3-25 Platinum and its alloys are utilized as efficient catalysts for the ORR in low temperature fuel cells. This type of fuel cell generates electricity by both the removal of electrons from electron donors (for example, H2, methanol and microbes) and the transfer of these electrons to O2. However, because platinum is an expensive and rare metal, alternative, non-precious metal-based materials that show high catalytic activity for ORR are required for the development of a sustainable society. Transition metal complexes such as cobalt or iron porphyrins (Pors) have been extensively investigated for this purpose.13-25 The use of light energy is also one of promising alternatives to various kinds of catalysts. Electrodes modified with a thin film containing a photosensitizer have been shown to dramatically increase the cathodic current corresponding to the ORR when irradiated with visible light.26-31 Various photosensitizers, such as Pors and phthalocyanines (Pcs), have been investigated until now, but the thin film consisting of ZnPc and polyvinylidene difluoride (PVDF) polymers has been known to show the highest current due to the photoinduced ORR,29 whose reaction product in the electrode is H2O2.32-37 Because the electronic absorption spectrum of the ZnPc/PVDF thin film indicates the presence of strong π－π interactions, which can result in molecular semiconducting properties, Schlettwein et al. proposed that the high photocurrent might originate from the initial electron transfer between the photoexcited ZnPc and O2 (i.e., ZnPc* + O2 → ZnPc+• + O2-•), followed by hole-transport between the ZnPc complexes.29,30 Although the photoelectrochemical ORR is a promising alternative to the high-cost platinumbased catalysts, the photochemistry of molecular semiconductors is complex compared to the


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conventional photochemistry of monomeric photosensitizers observed in solutions.38-51 This complexity has prevented us from developing a full understanding of the mechanisms involved. Thus, clarification of the mechanism is of significant importance in order to develop photoelectrochemical catalysts not only for ORR, but also for several other important reactions. In this work, we propose a new mechanism to explain the efficient photoelectrochemical ORR that occurs at ITO electrodes coated with Pc/PVDF thin films. Here, we have compared various Pcs (Figure 1), and analyzed some of their photochemical properties such as the electronic absorption, singlet oxygen (1∆g) luminescence, and ORR-induced photocurrent. The important features are as follows: (1) We found that the photocurrent of the PVDF-based thin film containing both ZnPc and triphenylphosphine (PPh3) was higher than that comprising ZnPc only. In the ZnPc/PVDF/PPh3 system, PPh3 coordinates to the Zn ion and thus acts as a bulky axial ligand, weakening the intermolecular π － π interactions. Because of the weak intermolecular interactions between the Pc complexes, we can use the well-characterized photochemical properties of monomeric Pcs to analyze the origin of the ORR-induced photocurrent.52-53 (2) The ORR-induced photocurrent shows a dependence on the reduction potentials of the Pcs. This indicates that the electrochemical reduction of the photoexcited Pc (Pc* + electrode → Pc-•) is an initial process. Subsequently, electron transfer from Pc-• to O2 occurs, i.e., Pc-• + O2 → Pc + O2-•. These processes are clearly different from the previously proposed mechanism, i.e., Pc* + O2 → Pc+• + O2-•. (3) Based on the comparison between ZnPc and MgPc derivatives, whose excited triplet quantum yields (ΦT) are different, the electrochemical reduction mainly occurs via the lowest excited singlet (S1) state of the Pcs. However, the reaction via the lowest excited triplet (T1) state can also contribute to the increase in the photocurrent only when the electrode potential is < +0.2 V. Thus, the proposed mechanism


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will be useful in developing new photoelectrochemical catalysts for the ORR.

Figure 1. Molecular structures of phthalocyanine derivatives.


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Experimental section Materials

1Zn (Tokyo Chemical Industry Co, Ltd.), 1Mg (Sigma-Aldrich Co. LLC.), 3Zn

(Sigma-Aldrich Co. LLC.), and meso-tetrakis(sulfonatophenyl)porphyrin (H2TPPS4, SigmaAldrich Co. LLC.) were commercially purchased, and then purified by either TLC or the removal of salts before measurements.54 2Zn, 2Mg, 2Si, and tetraphenylporphinatozinc (ZnTPP) were synthesized by methods previously reported.55,56 PVDF (Sigma-Aldrich Co. LLC.), PPh3 (Wako Pure Chemical Industries, Ltd.), N,N-dimethylacetamide (DMA, Wako Pure Chemical Industries, Ltd. special grade), N,N-dimethylformamide (DMF, Wako Pure Chemical Industries, Ltd. special grade), purified water (Kyoei Pharmaceutical Co., Ltd.), deuterium oxide (D2O, Wako Pure Chemical Industries, Ltd. 99.9% D), potassium nitrate (KNO3, Wako Pure Chemical Industries, Ltd.), and tetrabutylammonium hexafluorophosphate (TBAPF6, Sigma-Aldrich Co. LLC. electrochemical analysis grade) were commercially purchased, and used without further purification. Thin films of Pc derivatives were fabricated by casting the DMA solutions containing Pcs (1×10-4 M) and PVDF (1 g/L) onto either micro cover glass substrates (Matsunami Glass Ind., Ltd.) or an indium tin oxide (ITO) layer (100 nm thick) deposited on quartz glass substrates (ALS Co., Ltd).26-31 40 µL of the Pc/DMA solution was dropped onto the substrate and the solvent was removed by heating in vacuo (~10-3 mbar) at 70 °C for > 2 hours. The substrates were cleaned with purified water, DMA and methanol before use. For the fabrication of PPh3containing thin films, PPh3 (2 g/L) was added to the above-mentioned Pc/DMA solution. The average thickness of the thin films without PPh3 was roughly evaluated to be approximately 100 nm based on the amount of PVDF (40 µg), the density of PVDF (1.78 g cm-3), and the geometrical surface area of the thin films (~2 cm2).


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Instrumental Techniques

Electronic absorption spectra were recorded using a JASCO V-570

spectrophotometer. In the case of thin films, an integral sphere accessory was employed in order to measure the scattered light, and thus, stable baselines were observed.56,57 Singlet oxygen (1∆g) luminescence spectra in the near-infrared (NIR) region were measured using a monochromator (JASCO CT-25CP) with the photomultiplier (Hamamatsu Photonics R5509-42) cooled at 193 K.58 The photon signals amplified by a fast preamplifier (Stanford Research SR445) were measured by the single photon counting method using a photon counter (Stanford Research SR400). The samples were irradiated using a dye laser (630 nm, Sirah CSTR-LG532-TRI-T) pumped with an Nd:YAG laser (Spectra Physics INDI-40, 532 nm, FWHM 7 ns). These measurements were carried out for the Pc-based thin films on the glass substrates that were immersing into D2O. Here, the absorbance at 630 nm owing to Pcs was set to be 0.1, and the relative quantum yields were evaluated using the singlet oxygen yield (Φ∆) of H2TPPS4 as a standard.59 All electrochemical measurements were performed using a VersaSTAT 3 potentiostat (Princeton Applied Research). Cyclic voltammetry (CV) was used to measure the redox potentials of the Pcs and was carried out using a gas-tight BAS SVC-3 electrochemical cell (ALS Co., Ltd).60 For the CV measurements, TBAPF6 (0.1 M) was used as the electrolyte in DMF, which also contained the targeted Pc. To eliminate dissolved O2, the electrolyte solution was bubbled with nitrogen gas for 20 min prior to each measurement. Three conventional electrodes were used: A 3 mm-diameter glassy carbon electrode and a Pt wire were used as the working electrode and auxiliary electrode, respectively, while the reference electrode (Ag/AgNO3) was corrected by reference to the ferrocenium/ferrocene (Fc+/Fc) couple. In DMF, the Fc+/Fc couple


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was observed at approximately +0.34 V vs. NHE. Photoelectrochemical or electrochemical measurements of the ORR were performed using a self-designed electrochemical cell in which an optical window (diameter = 6 mm) is located at the bottom of the cell.61 Here, an aerated aqueous solution containing 0.5 M of KNO3 was used as the electrolyte. CV measurements were performed between +0.58 V and -0.12 V vs. NHE at a scan rate of 20 mV s-1. The ITO electrode coated by Pc-based thin films, a Pt wire, and Ag/AgCl (+0.20 V vs. NHE) were used as the working electrode, auxiliary electrode, and reference electrode, respectively. The samples were irradiated from the bottom of the electrochemical cell using a 150 W xenon lamp (Model C2577, Hamamatsu Photonics K. K.). Here, a sharp cut-filter (< 500 nm, SCF-50S-50Y, Sigma Koki Co., Ltd.) and a water-based NIR filter (1 cm length) were set between the light source and the ITO electrode in order to selectively excite the Q-band of the Pcs. The power of the light at the electrode surface was 210 mW cm-2. Current densities were calculated using the geometrical surface area (0.36 cm2) of the electrode in contact with the electrolyte.


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Results and interpretations Electronic properties of Pc derivatives

In order to obtain information on the redox

properties, CV measurements were carried out for the Pc derivatives in DMF (Supporting Information). The electrochemical data is summarized in Table 1. The redox potentials of Pc complexes obtained in our experiments are similar to those reported previously.62-66 Because the central metal ions, such as Mg2+, Si4+, and Zn2+, are redox-inactive, the observed redox couples are related to the HOMO and the LUMO of the Pc ligand, i.e., the a1u (π) and the eg (π*) orbitals, respectively.52,53,60 In 1Zn, 1Mg, 2Zn, and 2Mg, the first oxidation and reduction potentials are seen at around 0.59 ~ 0.77 and -0.94 ~ -1.02 V, respectively. Thus, neither the introduction of tert-butyl substituents nor the change from Zn →Mg affects the redox potentials significantly. In contrast, compared with the complexes of Zn and Mg, the first oxidation (0.99 V) and the first reduction (-0.83 V) potentials of 2Si are found at less negative potentials. These shifts in the oxidation and reduction potentials can be explained by the highly positive charge (+4) of the silicon ion that results in both the stabilization of the Pc3-• ligand and the destabilization of the Pc-• ligand.62 In the case of fluoro-substituted ZnPc, 3Zn, although the oxidation peak could not be observed even at +1.35 V (vs. NHE) of the electrode potential, the most positive reduction potential was seen at -0.71 V: This is due to the electron-withdrawing effect of fluorosubstituents on the Pc ring. The electronic absorption spectra of the Pc derivatives in DMF are shown in Figure 2. In the electronic absorption spectra of all of the examined Pcs, an intense Q0-0 band is seen at around 670 nm (1Zn: 669 nm, 1Mg: 670 nm, 2Zn: 675 nm, 2Mg: 676 nm, 2Si: 676 nm, 3Zn: 675 nm), which corresponds to the transition from the singlet ground (S0) state to the first excited singlet state (S1) state, which mainly consists of the 1(a1ueg) configuration, i.e., 1(π,π*).52,53 This


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indicates that the excitation energies of these Pcs are influenced little by the central metal and substituents, whereas the redox potentials are dependent on them. Since 3Zn tends to form aggregates compared with 1Zn, the very low concentration of 3Zn in DMA was employed in order to evaluate monomeric Pc.

Table 1. The first oxidation (Eox) and reduction (Ered) potentials of Pcs in DMF (vs. NHE). Compounds

Ered/V

Eox/V

1Zn

-0.94

0.77

2Zn

-0.98

0.62

1Mg

-0.97

0.61

2Mg

-1.02

0.59

2Si

-0.83

0.99

3Zn

-0.71

-


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Figure 2. Electronic absorption spectra of Pcs in DMA and their thin films. In the case of thin films, the dashed lines show the PPh3-free thin films, the solid lines show the PPh3-added thin films. Since the concentrations of Pcs in DMA which were appropriate for evaluating monomeric Pcs were employed, the absorbance of 3Zn that tends to form aggregates is very low.


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Electronic absorption spectra of Pc-based thin films

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In order to investigate the π―π

interactions between Pc rings, electronic absorption spectra of Pc-based thin films were measured (Figure 2). The thin films of 1Zn, 1Mg, and 3Zn showed a very broad Q absorption band (i.e., fwhm = 3.9 × 103 cm-1 for 1Mg), which strikingly indicates the strong intermolecular π―π interactions between Pc rings in the solid state.38-51 On the other hand, in the case of 2Zn, 2Mg, and 2Si, the electronic absorption spectra of the thin films show a relatively sharp Q0-0 band and an observable vibronic band at around 620 nm. These spectral features indicate the existence of monomeric electronic properties even in the solid state. This is reasonably explained by the steric hindrance of the surrounding substituents that weaken the intermolecular π―π interactions between Pc rings. The bandwidth (i.e., fwhm = 1.0 × 103 cm-1 for 1Mg) of the Q0-0 band in the thin films is broader than that (i.e., fwhm = 3.8 × 102 cm-1 for 1Mg) in solution (Figure 2), which indicates the presence of some remaining, weak intermolecular π―π interactions. To further test the effect of π―π interactions, PPh3, which can coordinate to the central metal ions such as Zn2+ and Mg2+, was added to the DMA solutions prior to casting in order to weaken the intermolecular π―π interactions between Pc rings. When PPh3 was added, a relatively sharp Q0-0 band and an observable vibronic band appeared even in the electronic absorption spectra of the thin films of 1Zn and 1Mg. The appearance of monomeric electronic properties indicates that PPh3 reduced the π–π interactions. However, the spectra of 2Zn, 2Mg, 2Si, and 3Zn were unchanged after the addition of PPh3. The spectral changes in 1Zn or 1Mg can be explained by the axial coordination of the bulky PPh3 ligand to the central metal ion, which weakens the π―π interactions between Pcs in the thin films. In the case of 3Zn, the addition of PPh3 did not affect the strong π―π interactions. This is consistent with the fact that


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compared with 1Zn, 3Zn tends to form aggregates even in DMA. Photoelectrochemical ORR

In order to investigate the ORR, CVs of the Pc-based thin

films were measured. The CVs, with and without irradiation, are shown in Figure 3. In all cases, when the electrode potential was negatively scanned to -0.12 V (vs. NHE) in the dark, no significant cathodic current, which would correspond to the ORR, was observed.67 In order to investigate the effects of light irradiation, the thin films were irradiated from the bottom of the electrochemical cell while CV measurements were carried out. Here, the Q absorption band of Pcs was selectively excited by employing both a sharp cut-filter and a water-based filter. The cathodic current, Idark, and the current upon light irradiation, Iirra, for the different thin films are summarized in Table 2. Here, the light irradiation effect (LE) was evaluated by the ratio of photocurrent, Iirra. – Idark, to the integral of the Q band absorbance,

∫

20000

11110

1 /ν~Abs(ν~ ) dν~ .68 The

significant ORR-induced photocurrent was observed for 1Zn, which is consistent with the previous report.26-31,69 In a similar manner, the photocurrents corresponding to the ORR were observed for 1Mg, 2Zn and 2Mg, but were negligibly small for both 2Si and 3Zn. In order to investigate the effect of the addition of PPh3, the CV measurements were carried out for the thin films containing PPh3 while they were irradiated by light. In the case of 2Zn, 2Mg, 2Si, and 3Zn, whose electronic absorption spectra were found to be similar before and after the addition of PPh3, the cathodic current and the ORR-induced photocurrent were unchanged by the addition of PPh3. On the other hand, surprisingly, in the case of 1Zn and 1Mg, the photocurrents were higher in the PPh3-added thin films than those in the corresponding PPh3-free thin films, although, the electronic absorption spectra indicated that the axial coordination of bulky PPh3 decreased the intermolecular π － π interactions between Pcs. In contrast to the previously proposed mechanism, strong intermolecular π－π interactions do not appear to be necessary for


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increasing the photoelectrochemical ORR, while some intermolecular interactions are essential for the hole- or electron-transport. Moreover, the ORR-induced photocurrent is higher in the Zn complexes than that in the corresponding Mg complexes. Thus, we found that the ORR-induced photocurrent is dependent on the substituents, the central metal, and the axial ligand. These results will be analyzed in detail in the Discussion section.

Figure 3. Cyclic voltammograms for measuring the cathodic current owing to the ORR using various Pc-based thin films. The blue and red lines denotes the CVs with and without visible light irradiation, respectively.


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Table 2. The cathodic current (Idark), current upon light irradiation (Iirra) and light irradiation effect (LE) owing to the ORR using the Pc-based thin films. Idark/µA cm-2

Iirra/µA cm-2

(Iirra. – Idark) /µA cm-2

LE/102

1Zn

5.9

20.3

14.4

1.7

1Zn-PPh3

5.6

32.2

26.6

5.0

1Mg

3.6

12.9

9.3

1.3

1Mg-PPh3

3.8

19.2

15.4

3.3

2Zn

1.7

17.2

15.5

2.6

2Zn-PPh3

2.0

16.5

14.5

2.8

2Mg

3.9

11.6

7.7

1.7

2Mg-PPh3

2.7

12.1

9.4

1.8

2Si

4.1

6.0

2.0

0.3

2Si-PPh3

3.9

6.5

2.7

0.4

3Zn

6.3

8.0

1.7

0.2

3Zn-PPh3

4.1

5.8

1.7

0.2

Thin films

a

LE denotes the ratio of photocurrent (Iirra. – Idark) and the integral of the Q band absorbance

between 500 nm and 900 nm, i.e.,

∫

20000

11110

1 /ν~Abs(ν~ ) dν~ .68


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Discussion Reconsideration of ORR-induced photocurrent generation mechanism

The thin films

containing 1M (M = Mg or Zn) and PPh3 (1M-PPh3) show the highest photocurrent; nevertheless their electronic absorption spectra indicate that the axial coordination of the bulky PPh3 ligand to Mg or Zn weakens the intermolecular π―π interactions and provides the monomeric electronic properties. This result is consistent with the relatively high photocurrent observed in the thin film of 2Zn or 2Mg whose intermolecular π―π interactions are weakened by the steric hindrance of the surrounding t-butyl substituents. This experimental result is reasonably explained by the fact that the excited-state lifetime of the monomeric species which is longer than the species with the strong π―π interactions in the thin-film can increase the photochemical reaction yields. In addition, the relatively weak intermolecular π―π interactions are considered to be sufficient to generate a high photocurrent via the hole- or electron-transport. Thus, we reconsidered the ORR-induced photocurrent generation mechanism on the basis of the knowledge of well-characterized photochemical properties of monomeric Pcs: four possible photochemical reactions of monomeric Pcs at an electrode upon the visible light irradiation will be discussed (Scheme 1).

Scheme 1. Possible reactions after the visible light irradiation of Pcs on the electrode.


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Possibilities of direct reactions of Pc* with O2

The direct reactions of the photoexcited Pc,

Pc*, with O2 are divided into two reaction routes, i.e., reactions (1) and (2). The reaction (1) corresponds to the previously proposed mechanism: the electron transfer from Pc* to O2. The reaction (2) involves the formation of singlet oxygen (1∆g) (1O2*, 0.97 eV), and is the energy transfer from Pc in the T1 state (3Pc*) to O2.58,59,70 The energy diagram including the reactions (1) and (2) is typically shown for 1Zn (Figure 4). Here, the energy (1.1 eV) of a charge separate (CS) state, Pc+•-O2-•, was evaluated from the oxidation potential of 1Zn (0.77 V vs. NHE) and the reduction potential of O2/O2-• in an aqueous solution (-0.33 V vs. NHE).32,71,72 According to this energy diagram, the reaction (2) should be more favorable than the reaction (1). In fact, in the absence of the electrode, the fluorescence quantum yield (ΦF) and the singlet oxygen yield (Φ∆) of 1Zn were previously reported to be 0.20 and 0.67 in DMSO, respectively.52,53,73 Thus, in the case of the monomeric ZnPc, the reaction (2) is a major route, while the reaction (1) is a minor, almost negligible route. The reaction (1) is inappropriate for explaining the high photocurrent observed in 1Zn-PPh3.

Figure 4. Energy diagram of 1Zn.


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Next, the evidence for the reaction (2) is considered. Singlet oxygen (1∆g) is produced by the energy transfer from 3Pc* to triplet molecular oxygen (3Σg). Since singlet oxygen (1∆g) is the excited-state of O2,70 the electrochemical reduction of singlet oxygen (1∆g) should occur at the more positive electrode potential than that of O2. Thus, we tried to measure singlet oxygen luminescence at 1275 nm for the thin films of 1Zn and 1Zn-PPh3 on the glass substrates in an aerated D2O solution. However, singlet oxygen (1∆g) luminescence was not observed from the thin films of 1Zn and 1Zn-PPh3 on the glass substrates even in the absence of the electrode (Supporting information). Using H2TPPS4 in D2O as a standard (Φ∆ = 0.43),59 the Φ∆ values for the thin films were roughly evaluated to be less than 0.03, which were much smaller than that of ZnPc in solutions. This can be reasonably interpreted by inefficient diffusion of O2 in the thin films, which inhibits the energy transfer from 3Pc* to O2 (3Σg). Thus, the reaction (2) cannot be the main route for the large photocurrent generation.

Contribution of electrochemical reduction of Pc*

Because the contribution of the reactions (1)

and (2) is negligible, we focused on electrochemical reaction of Pc*, i.e., the reactions (3) and (4). The reaction (3) is the formation of Pc+• due to the electrochemical oxidation of Pc*. We can eliminate the reaction (3) immediately because Pc+• cannot reduce O2. On the other hand, the reaction (4) is the plausible mechanism for the generation of the large ORR-induced photocurrent for the following reasons: (1) based on the S1 energy (1.9 eV) and the T1 energy (1.1 eV), the electrochemical reduction of the photoexcited Pc can occur at the more positive electrode potentials than that of the corresponding Pc in the ground state; and (2) the Gibbs free energy difference, ∆G, of the electron transfer reaction, i.e., Pc-• + O2 → Pc + O2-•, is negative when the reduction potential of Pc lies at a more negative potential than that of O2. Thus, we analyzed the


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relationship between the photocurrent and the reduction potential of Pc (Figure 5).

1Zn 500 Light irradiation effect

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 300 200

1Mg 2Zn 2Mg

100 0 -1.1

2Si -1

3Zn

-0.9 -0.8 -0.7 Reduction potentials of Pcs/V

-0.6

Figure 5. Dependence of light irradiation effects (LEs) on the reduction potentials of Pcs (red: PPh3-added, blue: PPh3-free). The LEs were measured at -0.12 V.

Firstly, we will consider the difference between 3Zn and 1M (M = Mg or Zn), because the thin films of these complexes show very broad Q absorption bands (i.e., fwhm = 3.9 × 103 cm-1 for 1Mg) due to the strong intermolecular π―π interactions between Pc rings. The light irradiation effect of 3Zn is much lesser than that of 1M. This can be reasonably explained by the difference in the reduction potentials. Because the reduction potential (-0.94 and -0.98 V for Zn and Mg complexes, respectively) of 1M is much more negative than that (-0.33 V) of O2,32,71,72 the electron transfer from the anion of 1M, produced by photoelectrochemical reduction, to O2 is favorable. On the other hand, the difference in the reduction potential between 3Zn (-0.71 V) and O2 is within 0.4 V. Therefore, despite the fact that the ∆G of this reaction is negative, the driving force of the electron transfer reaction, i.e., Pc-• + O2 →Pc + O2-•, is small. This analysis is strongly supported by the comparison between 2Si, 2M, and 2M-PPh3 (M = Mg or Zn). The


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electronic absorption spectra of these thin films show the existence of monomeric electronic properties because of the steric hindrance of the surrounding substituents and the bulky axial ligands. The light irradiation effect (LE) dramatically increased on changing the central ion from Si4+ to Mg2+ or Zn2+. Thus, the change in the LE could be also understood by considering the reduction potential. For example, the electron transfer to O2 (-0.33 V) is more preferable in 2M (0.98 and -1.02 V for Zn and Mg complexes, respectively) than that in 2Si (-0.83 V). That is, in order to generate the efficient photocurrent, the Gibbs free energy difference, ∆G, of the electron transfer reaction, i.e., Pc-• + O2 → Pc + O2-•, should be more than 48 kJ/mol (this was calculated using the difference between the reduction potential of 2Si and that of O2). This is because the electron transfer from the Pc-based thin film to O2 occurs only at around a solid-liquid interface. Thus, it is concluded that the major route that can explain the large ORR-induced photocurrent is the reaction (4) that includes the initial electrochemical reduction of Pc* and the subsequent electron transfer from Pc-• to O2.

Difference between the S1 state and the T1 state

Because

the

intermolecular

π―π

interactions between Pc rings and the reduction potentials are similar between the Zn and Mg complexes, it is important to focus on the fact that the LE values of Zn complexes (1Zn: 1.7 × 102, 1Zn-PPh3: 5.0 × 102, 2Zn: 2.6 × 102, 2Zn-PPh3: 2.8 × 102) are 1.3-1.6 times those of the corresponding Mg complexes (1Mg: 1.3 × 102, 1Mg-PPh3: 3.3 × 102, 2Mg: 1.7 × 102, 2MgPPh3: 1.8 × 102). In particular, it is worth to note that the LE value at -0.12 V is obviously larger in 1Zn-PPh3 (LE = 5.0 × 102) than that in 1Mg-PPh3 (3.3× 102), while those at +0.2 V are almost similar between 1Zn-PPh3 (1.1 × 102) and 1Mg-PPh3 (1.1 × 102). Thus, the plots of photocurrent versus electrode potential have the form of an arc for 1Mg-PPh3 and a valley for


20

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1Zn-PPh3, and the minimum of the valley for 1Zn-PPh3 is seen at an electrode potential of +0.2 V (Figure 3). This can be explained by the difference in the excited-state dynamics. For 1Zn and 1Mg, the quantum yields and lifetimes for the S1 and T1 states are summarized in Table 3.73-75 Because of the heavy atom effect based on the strong spin-orbit coupling on Zn, the triplet yield (ΦT) of 1Zn is greater (= 0.50) than that of 1Mg (= 0.18). The differences between the S1 and T1 states can be described by two factors, i.e., the lifetime and the excitation energy. For these complexes, the lifetimes (several 100 µs) of the T1 state are much longer than that of the S1 state (several ns), and therefore, the T1 state is more favorable for the electrochemical reduction reaction that forms Pc-•. Accordingly, the photocurrent of the Zn complex is larger at -0.12 V than that of the corresponding Mg complex, which strikingly demonstrates the contribution of the electrochemical reduction of the T1 Pc. On the other hand, because the S1 energy (1.85 eV) is higher than the T1 energy (1.13 eV), the electrochemical reduction of the S1 Pc can occur at the more positive electrode potentials than that of the T1 Pc. The photocurrent generation mechanism for 1Zn-PPh3, which has a high ΦT value, can be separated into the contribution from the T1 Pc and that from the S1 Pc. That is, the electrochemical reduction can occur at +0.6 ― +0.2 V only via the S1 1Zn-PPh3, while it can occur at < +0.2 V even via the T1 1Zn-PPh3 in addition to the S1 1Zn-PPh3. This is consistent with the following theoretical points: (1) The electrochemical reduction of the T1 state of 1Zn-PPh3 can occur at electrode potentials below the valley minimum (+0.2 V) as determined from the relationship between the photocurrent and the electrode potential. This most positive electrode potential (+0.2 V) is consistent with the reduction potential (0.19 V) of the T1 state of 1Zn, as calculated from the reduction potential (0.94 V) and the T1 energy (1.13 eV). (2) The reduction potential of the S1 1Zn was calculated to be +0.91 V using the reduction potential (-0.94 V) and the S1 energy (1.85 eV). The most


21


Page 22 of 34

positive electrode potential at which the photocurrent is produced is at approximately +0.6 V, which is more negative than the reduction potential of the S1 1Zn. Using this deviation (0.31 V) between the reduction potential of the S1 1Zn and the most positive electrode potential, the ∆G value required to generate the observable photocurrent was evaluated to be > 30 kJ/mol. That is, the S1 1Zn needs the larger ∆G value compared with the T1 1Zn in order to gain the observable photocurrent. This is because the excited-state decay rate is much faster in the S1 state than in the T1 state: the photocurrent results from the competition between the electrochemical reduction reaction rate and the excited-state decay rate.

Table 3. ΦF, fluorescence lifetimes (τF), triplet quantum yields (ΦT), and triplet lifetimes (τT) for 1Zn and 1Mg.73-75

ΦF

τF

ΦT

τT

Ref

1Zn

0.32

4.73 ns

0.50

250 µs

74

1Mg

0.60

7.49 ns

0.18

214 µs

75

ORR-induced photocurrent generation mechanism in Pcs

The following reaction mechanisms

are proposed in order to explain the ORR-induced photocurrent. In the case of the Mg complexes whose ΦT values are low, the reaction mechanism (4-1) that occurs only via the S1 state is the main contributor to the ORR-induced photocurrent (Scheme 2). On the other hand, in the case of the Zn complexes whose ΦT values are high, the reaction mechanism (4-2) that occurs via the T1 state can contribute to the ORR-induced photocurrent only at voltages less than +0.2 V in addition to the reaction mechanism (4-1) via the S1 state (Scheme 2). Thus, the S1 state is useful


22

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for shifting the photoreduction potential to more positive values in terms of the high excitation energy, whereas the T1 state can contribute to increase the photocurrent at < 0.2 V because of the long excited-state lifetime (Figure 6).

Scheme 2. Proposed reaction mechanisms after the visible light irradiation of Pcs on the electrode.

Figure 6. Photoelectrochemical dynamics of 1Zn when the electrode potentials are 0.5 (blue part) and 0.1 V (red part), respectively.


23


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Conclusions In this study, we have investigated the photoelectrochemical ORR using various Pcs. By reconsidering the experimental results on the basis of the photochemical properties of monomeric Pcs, we have clarified the ORR-induced photocurrent generation mechanism for the first time: the electrochemical reduction of the photoexcited Pc is the initial process, and the subsequent electron transfer from Pc-• to O2 occurs, i.e. Pc-• + O2 → Pc + O2-•. Based on the comparison between ZnPc and MgPc derivatives, the reaction via the S1 state has been clearly distinguished from that via the T1 state: the high photocurrent mainly originates from the electrochemical reduction of Pcs in the S1 state, although, the reaction via the T1 state can contribute to an increase in photocurrent when the electrode potential is below +0.2 V. This study will be useful for the development and design of novel photoelectrochemical catalysts.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI. Cyclic voltammograms of Pc complexes and NIR luminescence spectra for singlet oxygen (1∆g).

Acknowledgments This work was supported by JSPS KAKENHI Grant Numbers 17H06375 and JP16H04128, and The Sumitomo Foundation.


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−•

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The cathodic current corresponding to the ORR increased when the electrode potential

was scanned to the more negative side even in the dark, although, the cathodic current was measured at -0.12 V in order to extract the light irradiation effects. (68)

The number of photons absorbed by thin films is represented by

~

~ Abs (ν~ ) dν~ ,

∫ I (ν ) /ν

where I (ν~ ) and Abs (ν~ ) denote the intensity of light and the absorbance of the thin films, respectively. Since the wavelength dependence of the light intensity is relatively small between 500 nm and 900 nm for the used Xe lamp, the integral of the Q band absorbance, i.e.,

∫

20000

11110

1 /ν~ Abs(ν~ ) dν~ , was evaluated between 500 nm and 900 nm, which was employed in

order to compare the light irradiation effects of various thin films. (69)

Without the filters, we confirmed that the photocurrent of the thin film of 1Zn was larger

than that of the thin film of ZnTPP. This relationship is consistent with the previous report.29 Although the photocurrent (-20 µA/cm2) at -0.12 V was smaller than that (-130 µA/cm2) observed in the previous report, this deviation may originate from the difference in the power of light source (100 W and 500 W for the present and previous systems, respectively) and the existence of filter in the present system. (70)

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Nyokong, T. Effects of Substituents on the Photochemical and Photophysical Properties

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Zhang,

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Guo,

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Imidazole

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Phthalocyanine

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