Importance of the Molecular Orientation of an Iridium(III)-Heteroleptic

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Article

Importance of the molecular orientation of an Ir(III)heteroleptic photosensitizer immobilized on TiO nanoparticles 2

Atsushi Kobayashi, Shuhei Watanabe, Masaki Yoshida, and Masako Kato ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00538 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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ACS Applied Energy Materials

Importance of the Molecular Orientation of an Ir(III)-heteroleptic Photosensitizer Immobilized on TiO2 Nanoparticles Atsushi Kobayashi,* Shuhei Watanabe, Masaki Yoshida, Masako Kato* Department of Chemistry, Faculty of Science, Hokkaido University, North-10 West-8, Kita-ku, Sapporo 060-0810, Japan

KEYWORDS: Photosensitizer, Artificial photosynthesis, Ir(III) complexes, Immobilization, Molecular orientation

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ABSTRACT: To elucidate the effect of the molecular orientation of a photosensitizing (PS) dye molecule on photo-induced interfacial electron transfer to a semiconductor substrate, we have synthesized two new Ir(III) heteroleptic complexes each comprising two phosphonic acid groups: [Ir(ppy)2(CPbpy)]+ and [Ir(CPppy)2(bpy)]+ (1B and 1P, respectively; Hppy = 2phenylpyridine, bpy = 2,2′-bipyridine, CPbpy = 4,4′-bis(methylphosphonic acid)-2,2′-bipyridine, CPppy = 4-(methylphosphonic acid)-2-phenylpyridine). Both Ir(III) complexes exhibit similar UV-vis absorption spectra and quasi-reversible Ir(IV)/Ir(III) redox behavior at a potential of 1.67 V vs NHE. On the other hand, the triplet metal-to-ligand charge-transfer (3MLCT) phosphorescence energy of 1B was ~0.12 eV higher than that of 1P. This difference was attributed to the electron-donating methyl phosphonate groups attached to the bpy ligand that destabilize the 3MLCT excited state in which the photo-excited electron is localized in the bpy moiety. Both Ir(III) PS dyes were immobilized onto the surface of the Pt-cocatalyst-loaded TiO2 nanoparticles (1B@Pt-TiO2 and 1P@Pt-TiO2). Immobilization was comparable, suggesting that the effect of the positions of the methyl phosphonate groups on the immobilization behavior was negligible. On the other hand, the photocatalytic H2 evolution activity of 1B@Pt-TiO2 was about six-fold higher than that of 1P@Pt-TiO2, indicating the importance of the methyl phosphonate anchoring group position in regulating not only the redox potentials but also the orientation of the molecular photosensitizer on the semiconductor substrate.


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Introduction Over the past decades, the solar water splitting reaction has drawn considerable attention as one of the most promising methods to provide a renewable and clean energy resource, H2, without the formation of environmental pollutants.1-4 Following the discovery of the “Honda-Fujishima effect,”5 many semiconductor materials have been developed as photocatalysts for the water splitting reaction.6-10 In addition, the combination of semiconductor substrates and molecular photosensitizing (PS) dyes has provided many promising photoelectrochemical (PEC) cells for solar hydrogen production.11-17 Since PS molecules immobilized on the surface of semiconductor electrodes play key roles on the photoinduced charge separation process, many PS molecules have been developed to date.18-21 Among them, heteroleptic Ir(III) complexes have drawn much attention because of their unique electronic structure in the photoexcited state.22-40 Following the pioneering work on the heteroleptic Ir(III) complex [Ir(ppy)2(bpy)]+ (1, Scheme 1, Hppy = 2phenylpyridine, bpy = 2,2′-bipyridine) by Bernhard et al.,23 extensive studies have been devoted towards clarifying the role of the two different ligands as well as the effect of the various pendant functional groups attached to both ligands. These studies clearly indicate that the electronic structure of these heteroleptic [Ir(ppy)2(bpy)]+-type complexes favors photo-induced charge separation. Thus, the photo-excited electron and the remaining hole are spatially separated on the π* orbital of the bpy ligand and the Ir 5d orbital conjugated with the π orbital of the ppy ligand, respectively.22 This feature of the cyclometalated Ir(III) heteroleptic PS dyes is not observed in widely employed homoleptic PS dyes such as [Ru(bpy)3]2+. Thus, taking advantage of this anisotropic feature, several photocatalysts for CO2 reduction,41-44 H2 production,45-49 and dye-sensitized solar cells (DSSC)50-55 have been developed via the immobilization of Ir(III) PS molecules by attachment to the bpy ligand. Some of these


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photocatalysts have exhibited superior performance in photo-induced charge separation processes. However, the effect of the molecular orientation of the Ir(III) PS on the photocatalytic activity has been scarcely reported to date. This is because most of the Ir(III) PS molecules are commonly immobilized onto the semiconductor substrate via the introduction of several anchoring groups in the bpy ligand.45-55 One of the exceptions is a study of the immobilization of an [Ir(ppy)2(bpy)]-type photosensitizing dye onto a NiO cathode by Collomb et al.56 We have recently focused on the importance of the molecular orientation of the PS molecule on the semiconductor substrate. In this work, we report on the syntheses, PEC properties, and immobilization behavior of two phosphonate-functionalized Ir(III) heteroleptic photosensitizers, [Ir(ppy)2(CPbpy)]+ and [Ir(CPppy)2(bpy)]+ (1B and 1P, respectively; Scheme 1; CPbpy = 4,4′bis(methylphosphonic acid)-2,2′-bipyridine, CPppy = 4-(methylphosphonic acid)-2phenylpyridine) towards Pt-cocatalyst-loaded TiO2 nanoparticles (Pt-TiO2). We demonstrate that the introduction of methyl phosphonic acid groups to the [Ir(ppy)2(bpy)]+ moiety affects the redox potential in the photo-excited state so that 1B is more favorable for application to the photocatalytic H2 evolution reaction than 1P. Further, the nanoparticle photocatalyst 1B@PtTiO2 exhibited remarkably higher H2 evolution activity than 1P@Pt-TiO2.


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Scheme 1. Molecular structures of complexes [Ir(ppy)2(bpy)]+ (1), [Ir(CPppy)2(bpy)]+ (1P), and [Ir(ppy)2(CPbpy)]+ (1B).

Experimental Procedures Materials and Syntheses Caution! Although we did not come across any difficulties, most of the chemicals used in this study are potentially harmful and should be used in small quantities and handled with care in a fume hood. All commercially available starting materials were used as received without further purification. The TiO2 nanoparticles (SSP-25, ~9 nm in diameter) were purchased from Sakai Chemical Industry Co. Ltd. Pt-TiO2 (2 wt%) was prepared using a previously reported photodeposition method.57 The starting Ir(III) complexes bearing two diethyl phosphonate groups {[Ir(ppy-PE)2(bpy)]PF6 and [Ir(ppy)2(bpy-dPE)]PF6; Hppy-PE = 4-(diethylphosphonomethyl)-2-

phenylpyridine, bpy-dPE = 4,4′-bis(diethylphosphonomethyl)-2,2′-bipyridine)}58 and the nonsubstituted Ir(III) complex [Ir(ppy)2(bpy)]PF659 were prepared according to methods reported in the literature.


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Synthesis of 1B [Ir(ppy)2(bpy-dPE)]PF6 (184.5 mg, 0.17 mmol) was dissolved in 6 mL CH3CN/CH2Cl2 mixture (1:1 v/v) to which bromotrimethylsilane (0.3 mL, 2.27 mmol) was subsequently added. The mixture was refluxed for 2 d under N2 atmosphere. After cooling to room temperature, 75 mL cooled deionized water was slowly added to the reaction mixture to form a yellow-orange precipitate. The afforded precipitate was collected by filtration, washed with a small amount of deionized water and CHCl3, and subsequently dried in vacuo for one night to afford the crude product as a yellow powder. Recrystallization from an aqueous ammonia/acetone mixture afforded pure 1B as a yellow crystalline powder. Yield: 130 mg (0.15 mmol, 88 %). 1H NMR (270 MHz, DMSO-d6, δ ppm): 8.80 (s, 1H), 8.23 (d, 1H), 7.87 (m, 2H), 7.65 (d, 1H), 7.56 (d, 1H), 7.45 (d, 1H), 7.15 (t, 1H), 7.00 (t, 1H), 6.88 (t, 1H), 6.17 (d, 1H), 3.10 (d, 2H). Anal. Calcd. (%) for C34H34IrN5O6P2⋅3H2O: C, 44.64; H, 4.19; N, 7.66. Found: C, 44.70; H, 3.70; N, 7.05. Synthesis of 1P [Ir(ppy-PE)2(bpy)]PF6 (184.5 mg, 0.17 mmol) was dissolved in 6 mL CH3CN/CH2Cl2 mixture (1:1 v/v) to which bromotrimethylsilane (0.3 mL, 2.27 mmol) was subsequently added. The mixture was refluxed for 2 d under N2 atmosphere. After cooling to room temperature, 75 mL cooled water was slowly added to the reaction mixture to form a yellow-orange precipitate. The afforded precipitate was collected by filtration, washed with a small amount of deionized water and CHCl3, and subsequently dried in vacuo for one night to afford 1P as a yellow powder. Yield: 143 mg (0.17 mmol, 99%). 1H NMR (D2O, NaOD, 298 K): 8.48 (d, 1H), 8.10 (d, 1H), 8.05 (t, 2H), 7.79 (t, 2H), 7.65 (d, 1H), 7.42 (t, 1H), 7.15 (d, 1H), 6.93 (t, 1H), 6.30 (s, 1H), 2.61


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(d, 2H). Anal. Calcd. (%) for C34H31IrN4O7P2⋅3H2O: C, 44.59; H, 4.07; N, 6.12. Found: C, 44.41; H, 3.63; N, 6.48. Preparation of the Nanoparticle Photocatalysts (1B@Pt-TiO2 and 1P@Pt-TiO2) Pt-TiO2 nanoparticle powder (120 mg) was dispersed in basic water (20 mL) by adding a 1 M NaOH aqueous solution (0.1 mL). Next, 1B or 1P (27 mg, 32 µmol) was added to the Pt-TiO2dispersed solution and stirred continuously for 3 d in the dark. The afforded Ir(III)photosensitizer-immobilized Pt-TiO2 nanoparticles (1B@Pt-TiO2 or 1P@Pt-TiO2, respectively) were collected by ultracentrifugation (100,000 rpm; 30 min) and the supernatant solution was removed. After washing twice with ~6 mL H2O/CH3CN solvent (4:1 v/v), the product was dried in vacuo at 298 K for 1 d. The amount of Ir(III) photosensitizer immobilized on the Pt-TiO2 nanoparticle surface was estimated by X-ray fluorescence (XRF) spectroscopy and UV-vis absorption analyses of the supernatant solution (see “Calculation of the amount of immobilized Ir(III) complexes on the Pt-TiO2 nanoparticles” section in ESI). Measurements Elemental analysis was performed using a MICRO CORDER JM 10 analyzer at the Analysis Center, Hokkaido University. 1H NMR spectra were recorded on a JEOL EX-270 NMR spectrometer at room temperature. Energy dispersive XRF spectra were acquired on a JEOL JSX-3100RII spectrometer using a Rh target. Dynamic light scattering (DLS) analysis was conducted using an OTSUKA ELSZ-1000SCI analyzer. IR spectra were recorded on a JASCO FT-IR 4100 spectrophotometer using KBr pellets; alternatively, the spectrometer was equipped with a Smart-Orbit (ZnSe) attenuated total reflection accessory. UV-vis absorption and luminescence spectra were recorded on a Shimadzu UV-2400PC spectrophotometer and JASCO


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FP-8600 spectrofluorometer, respectively. Quartz cells with a 1-cm optical path length were used for both spectroscopic analyses. Prior to the luminescence measurements, the sample solutions were degassed by N2 bubbling for 20 min. Emission quantum yields (Φem) were measured using a Hamamatsu C9920-02 absolute photoluminescence quantum yield measurement system equipped with an integrating sphere apparatus and a 150-W continuous-wave xenon light source. Emission lifetime measurements were conducted using a Hamamatsu Photonics C4334 system equipped with a streak camera as the photodetector and nitrogen laser as the excitation light source (λex = 337 nm). Cyclic voltammetry (CV) was recorded using a HOKUTO DENKO HZ3000 electrochemical measurement system equipped with Pt wire and Ag/AgCl electrodes as the counter and reference electrodes, respectively. A glassy carbon electrode or an Ir(III) complexmodified indium tin oxide (ITO) electrode was used as the working electrode. The ITO electrode was prepared according to a slightly modified literature method60 as follows: a bare ITO electrode was cleaned by heating in basic hydrogen peroxide solution for 30 min at 80 °C and then washed with water. Next, the pre-cleaned ITO electrode was immersed in 0.1 mM 1B or 1P aqueous solution for 1 d at room temperature. Subsequently, the electrode was washed with MeOH and dried under reduced pressure. A CH3CN solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte was deaerated by N2 bubbling for 30 min and subsequently used in the CV experiments. Photocatalytic H2 Evolution Reactions For the photochemical H2 evolution reactions, each sample was prepared using a hand-made Schlenk flask equipped quartz cell (volume = 257 mL). Typical sample preparation was as follows: Ir(II)-photosensitizer-immobilized Pt-TiO2 nanoparticles and L-ascorbic acid (H2A) were added to the quartz cell as the photocatalyst and sacrificial electron donor (SED),


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respectively. The total sample volume was adjusted to 10 mL by adding a CH3CN/water mixture (1:1 v/v). The standard concentrations of the Ir(III) complex and SED used in this study were 40 µM and 0.2 M H2A, respectively. Each sample flask was doubly sealed using rubber septa and degassed by N2 bubbling for 3 h. Prior to irradiation, the gas (0.3 mL) was collected from the headspace using a gas-tight syringe (Hamilton 1001LTN) and analyzed by gas chromatography (GC) to confirm N2 purging. Subsequently, the samples were irradiated using a 300-W xenon lamp (MAX-303, ASAHI Spectra) in a water bath (293 K) combined with a visible-light-passed mirror module (385 nm 2 h) was observed for 1P@Pt-TiO2. This was attributed to the unfavorable orientation of 1P for interfacial electron injection to the TiO2; that is, the electron should be localized on the bpy ligand that faces the outer-edge of the 1P@Pt-TiO2 nanoparticle.


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Scheme 2. Schematic of the plausible electron transfer mechanism of Pt-cocatalyst-loaded TiO2 nanoparticles 1B@Pt-TiO2 and 1P@Pt-TiO2 in the presence of L-ascorbic acid (H2A) as sacrificial electron donor. The Ir(III)/Ir(II) reduction potentials of 1B and 1P are presumed to be the near-identical to those of their diethyl phosphonate analogues, [Ir(ppy)2(bpy-dPE)]+ and [Ir(ppy-PE)2(bpy)]+, respectively.58 The redox potential of H2A and the position of the CB minimum of TiO2 were inferred from the literature.69,70


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Conclusion We have newly synthesized two phosphonic-acid-functionalized Ir(III) complexes, [Ir(CPppy)2(bpy)]+ and [Ir(ppy)2(CPbpy)]+ (1P and 1B, respectively). Characterization by UVvis absorption and emission spectroscopy as well as electrochemical measurements allowed us to elucidate the importance of the molecular orientation of the heteroleptic Ir(III) PS in the interfacial electron transfer process from the immobilized PS to the semiconductor substrate. The introduction of two methyl phosphonic acid groups to the ppy or bpy ligand had negligible effect on the Ir(IV)/Ir(III) redox potential in the ground state. Conversely, the 3MLCT emission wavelength and reactivity with electron-donating emission quenchers strongly depended on the positions of these functional groups. Thus, 3MLCT emission of 1B was observed at a shorter wavelength (by ~34 nm) than that of 1P, while the electron-transfer quenching rate of 1B was higher than the reactivity of 1P in the presence of sacrificial electron-donating H2A and TEA. Two H2 evolution photocatalysts, 1P@Pt-TiO2 and 1B@Pt-TiO2 were successfully prepared by immobilizing these Ir(III) photosensitizers to the surface of Pt-cocatalyst-loaded TiO2 nanoparticles. The H2 evolution photocatalytic activity of 1B@Pt-TiO2 was remarkably higher than that of 1P@Pt-TiO2. This was attributed to the effective electron transfer quenching from both the photoexcited 1B* and one-electron-reduced 1B− to the CB of the TiO2 nanoparticle. These results clearly indicate the importance of the positions of the anchoring groups to regulate the molecular orientation of the molecular photosensitizer on the surface of the semiconductor substrate.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. X-Ray crystallographic data of 1P (CIF) UV-Vis absorption spectra of the supernatant solutions to estimate the adsorbed amount of Ir(III) photosensitizers on Pt-TiO2, IR spectra, particle size distribution, emission decay curves, and emission quenching by TiO2. (PDF) This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (A.K.); [email protected] (M.K.) Author Contributions The manuscript was written through contributions of all the authors. All authors have given their approval to the final version of the manuscript. Funding Sources This study was supported by the Shimadzu Science Foundation, the Shorai Science and Technology Foundation, the Murata Science Foundation, Grant-in-Aid for Scientific Research (C)(26410063), Artificial Photosynthesis (area No. 2406, No.15H00858), and Soft Crystals (area No. 2903, No. JP17H06367) from MEXT, Japan. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This study was supported by the Shimadzu Science Foundation, the Shorai Science and Technology Foundation, the Murata Science Foundation, Grant-in-Aid for Scientific Research (C)(26410063), Artificial Photosynthesis (area No. 2406, No.15H00858), and Soft Crystals (area No. 2903, No. JP17H06367) from MEXT, Japan. ABBREVIATIONS PS, Photosensitizer; PEC cell, photoelectrochemical cell; Hppy, 2-phenylpyridine; bpy, 2,2′bipyridine;

CPbpy,

4,4′-bis(methylphosphonic

acid)-2,2′-bipyridine;

CPppy,

4-

(methylphosphonic acid)-2-phenylpyridine; Pt-TiO2, Pt-cocatalyst-loaded TiO2 nanoparticles; Hppy-PE,

4-(diethylphosphonomethyl)-2-phenylpyridine;

bpy-dPE,

4,4′-

bis(diethylphosphonomethyl)-2,2′-bipyridine); XRF, X-ray fluorescence spectra; CV, Cyclic voltammetry; ITO, indium tin oxide; DLS, dynamic light scattering; DFT, density functional theory; LLCT, ligand-to-ligand charge-transfer ; MLCT, metal-to-ligand charge-transfer; SED, sacrificial electron donor, TEA, triethylamine, H2A, L-ascorbic acid; CB, conduction band.


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Table of Contents Synopsis.


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Figure 1. UV-visible absorption (broken lines) and luminescence spectra (solid lines; λex = 400 nm) of 1 (black), 1P (red), and 1B (blue) in 20 µM CH3CN/H2O (4:1 v/v) solution at 298 K. Inset: magnified view of the absorption spectra (450 to 540 nm) 79x74mm (300 x 300 DPI)



Figure 2. Stern–Volmer plots of 40-µM solutions of (black) 1, (blue) 1B, (red) 1P in the presence of various concentrations of (a) triethylamine (TEA; pH = 10) and (b) L-ascorbic acid (H2A; pH = 4.5) at room temperature in a CH3CN/H2O mixture (4:1 v/v) as solvent. 88x49mm (300 x 300 DPI)


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Figure 3. X-ray fluorescence (XRF) spectra of Pt-cocatalyst-loaded TiO2 nanoparticles: Pt-TiO2 (black), 1B@Pt-TiO2 (blue), and 1P@Pt-TiO2 (red). The peak marked by an asterisk originates from the Rh Lα radiation of the X-ray source. 88x93mm (300 x 300 DPI)



Figure 4. Photocatalytic H2 evolution reaction driven by the Pt-cocatalyst-loaded TiO2 nanoparticles 1P@PtTiO2 (red closed circles) and 1B@Pt-TiO2 (blue closed circles; 40 µM of the Ir(III) complex) in 0.2 M H2A solution (pH = 4.5, CH3CN:H2O = 1:1 v/v) under Ar atmosphere. A 300-W Xenon lamp with a longpass filter (λ >420 nm) was used as the irradiation source. Open symbols (black, red, and blue circles) indicate the results afforded by the 40-µM Ir(III) complex solution as homogeneous photosensitizer (1, 1P, and 1B, respectively) and Pt-cocatalyst-loaded TiO2 nanoparticles as the H2 evolution catalyst. 84x84mm (300 x 300 DPI)


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Scheme 1. Molecular structures of complexes [Ir(ppy)2(bpy)]+ (1), [Ir(CPppy)2(bpy)]+ (1P), and [Ir(ppy)2(CPbpy)]+ (1B). 49x15mm (600 x 600 DPI)



Scheme 2. Schematic of the plausible energy and electron transfer mechanism of Pt-cocatalyst-loaded TiO2 nanoparticles 1B@Pt-TiO2 and 1P@Pt-TiO2 in the presence of L-ascorbic acid (H2A) as sacrificial electron donor. The Ir(III)/Ir(II) reduction potentials of 1B and 1P are presumed to be the near-identical to those of their diethyl phosphonate analogues, [Ir(ppy)2(bpy-dPE)]+ and [Ir(ppy-PE)2(bpy)]+, respectively.58 The redox potential of H2A and the position of the CB minimum of TiO2 were inferred from the literature.69,70 62x24mm (300 x 300 DPI)


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Importance of the Molecular Orientation of an Iridium(III)-Heteroleptic

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