Effect of Extended Conjugation of N-Heterocyclic Carbene-Based

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Cite This: Inorg. Chem. 2017, 56, 12987-12995

Effect of Extended Conjugation of N‑Heterocyclic Carbene-Based Sensitizers on the Performance of Dye-Sensitized Solar Cells Suri Babu Akula,†,§ Huei-Siou Chen,‡,§ Chaochin Su,*,‡ Bo-Ren Chen,† Jiunn-Jie Chiou,† Chia-Hsuan Shieh,† Ya-Fen Lin,† and Wen-Ren Li*,† †

Department of Chemistry, National Central University, Chungli, Taiwan 32001, ROC Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei, Taiwan 10608, ROC

‡

S Supporting Information *

ABSTRACT: We report the synthesis, characterization, and photovoltaic properties of four ruthenium complexes (CI101, CBTR, CB111, and CB108) having various N-heterocyclic carbene ancillary ligands, pyridine-imidazole, -benzimidazole, -dithienobenzimidazole, and -phenanthroimidazole, respectively. These complexes were designed to investigate the effect of extended conjugation ordained from ring fusion on the power conversion efficiencies of the solar cells. The device sensitized by CB108, the pyridine-phenanthroimidazole conjugated complex, showed an improved efficiency (9.89%) compared to those of pyridine-benzimidazole conjugated system (CBTR, 9.72%) and the parent unfused ring system (CI101, 6.24%). Surprisingly, the sulfur-incorporated pyridine-dithienobenzimidazole system (CB111, 9.24%) exhibited a little lower efficiency than that of N719 (9.41%). The enhanced photovoltaic performance of CB108 was mainly attributed to the increase in electron lifetime and diffusion length confirmed by the electrochemical impedance spectroscopy.

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INTRODUCTION Dye-sensitized solar cells (DSSCs) have emerged as costeffective photovoltaic devices by virtue of the benefits of inexpensive fabrication methods and tunable electrochemical and photophysical properties.1−8 A typical DSSC consists of a dye-sensitized semiconductor TiO2 electrode, an electrolyte containing I−/I3− redox couple, and a Pt-coated counter electrode.9−11 Among these device elements, the sensitizing dye plays a crucial role in light harvesting and energy conversion efficiency.12 Intensive efforts have been made toward improving the performance of the DSSCs with numerous ruthenium metal complexes, nonruthenium organometallic dyes, and metal-free organic photosensitizers.12−16 Recently, another promising energy conversion technology, perovskite solar cells, came into limelight with efficiencies above 20%.17−19 However, the instability and environmental hazards of the methylammonium lead iodide perovskite-based devices remain the significant challenges.20 Ruthenium(II)-based complexes remain dominant for the DSSC application with their intense absorption of visible light and favorable photovoltaic properties.21,22 Most of the structural modifications were performed with the anchoring and/or the ancillary ligands of the ruthenium sensitizers.12 The anchoring ligands were employed to adsorb the dye onto the photoanode and facilitate the injection of the excited electrons to the conduction band of the semiconductor. Different anchoring groups such as carboxylic acid, phosphate, or boronic acid were studied and developed to increase the binding ability of the dye molecule.23−27 Parallel to this research, ancillary ligands were also modified to broaden the © 2017 American Chemical Society

absorption spectrum, adjust the orbital energy levels, extend the excited electron lifetime, and thus enhance the overall performance of the device.12 Higher efficiencies were achieved by incorporating the appropriate conjugation length on the ancillary ligand using thiophene, triphenylamine, carbazole, or other substituents.28−34 In spite of the remarkable advances that have been made, the efficiency of the DSSCs can be further enhanced by tuning the sensitizers to become competitive with the conventional solar cells. Earlier, we demonstrated that the replacement of traditional bipyridine framework with the N-heterocyclic carbene (NHC)pyridine as the ancillary ligand in the ruthenium complexes enhanced the photoconversion efficiency.35 However, less efficient cell performance was observed for the ancillary ligands containing the imidazole derivatized carbene compared to the benzimidazole moiety.36 On the basis of several reports, suitable lengthening of conjugation of an ancillary ligand can enhance the device efficiency, but further elongation dropped the efficiency.37 An alternative method for expansion in the conjugation by incorporating the fused aromatic rings in ruthenium polypyridyl complexes resulted in lower photoconversion efficiency.38−41 NHC-pyridine-based ligands, with their unique set of electronic properties such as strong σdonating and π-accepting characteristics,35,42 show the promises for structural modification in the sensitizers. This exceptional class of donors prompted us to study the effect of Received: July 11, 2017 Published: October 11, 2017 12987

DOI: 10.1021/acs.inorgchem.7b01714 Inorg. Chem. 2017, 56, 12987−12995

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molecular geometry of the ester 3 was confirmed unequivocally through single-crystal structural analysis (see Figure S2 and Table S1 in the Supporting Information). The ester functional group in ruthenium complex 3 was hydrolyzed by basic KOH(aq) solution to yield the desired sensitizer CI101. Scheme 2 outlines the synthesis of ruthenium sensitizers CB111 and CB108 incorporated with fused-conjugated

fused aromatic conjugation in the ancillary ligands of carbenebased ruthenium photosensitizers on the cell performance. Furthermore, we wish to investigate the electron behavior of the metal complexes bearing different fused π-extended carbene-based ligands. Therefore, we designed three new imidazole derivatized carbene-based ruthenium complexes, namely, CI101, CB111, and CB108, and compared their photovoltaic performance with that of CBTR35 sensitizer, which was reported previously (Figure 1). A DSSC sensitized

Scheme 2. Synthesis of the Sensitizers CB111 and CB108

Figure 1. Molecular structures of the ruthenium complex sensitizers.

with CB108 showed the best light-harvesting capacity among the devices, with higher power conversion efficiency than those of N719 (Figure S1) and CBTR dye under the same cell fabrication and measuring procedures.

imidazole systems. The fused imidazole precursors 5a and 5b were prepared by a multicomponent condensation of respective diketo compounds 4a and 4b with formaldehyde and ammonium acetate in acetic acid and subsequent reaction with 2-fluoro-4-methylpyridine. The reaction of substituted imidazoles 5a and 5b with 1-bromooctane afforded the corresponding imidazolium salts 6a and 6b. Further, the ruthenium complexes CB111 and CB108 were prepared using a method similar to that described for the synthesis of CBTR.35 The ruthenium complexes 7a and 7b were synthesized by the chelation of the respective imidazolium ligands 6a and 6b with [RuCl 2 (p-cymene)] 2 in the presence of lithium bis(trimethylsilyl)amide (LHMDS), followed by coordination with 4,4′-bis(methoxy-carbonyl)-2,2′-bipyridine and replacement of chlorides with NCS ligands. Single-crystal structural analysis verified the molecular geometry of the esters 7a and 7b (see Figures S3 & S4 and Tables S2 & S3 in the Supporting Information). Finally, basic hydrolysis of the complexes 7a and 7b yielded the sensitizers CB111 and CB108, respectively. Photophysical Properties. The optical properties of the ruthenium complexes CI101, CBTR, CB111, and CB108 were investigated using UV−vis absorption spectroscopy and displayed in Figures 2 and S5 (Supporting Information). The UV−vis spectra of these dyes in CH3CN/t-BuOH solution showed similar absorption profiles consisting of three bands. The absorption band featured around 350−420 nm is characteristic intraligand π−π* transitions of the 4,4′dicarboxylic acid-2,2′-bipyridine and conjugated carbene− pyridine ancillary ligands. The broad band in the range of 420−490 nm can be attributed to the intraligand π−π* as well as to the higher energy metal-to-ligand charge transfer (MLCT) transitions.43,44 Similar to other ruthenium polypyridyl complexes, the band in the visible region of 500−570 nm

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RESULTS AND DISCUSSION Synthesis of Materials. The NHC-pyridine ruthenium sensitizer CI101 was prepared via efficient synthetic protocol illustrated in Scheme 1. The ligand 2 was synthesized by the Scheme 1. Synthesis of the NHC-Pyridine-Based Ruthenium Sensitizer CI101

reaction of imidazole with 2-fluoro-4-methylpyridine and subsequent alkylation with 1-bromooctane. The ruthenium complex 3 was obtained from the sequential reaction of imidazole salt 2 with [RuCl2(p-cymene)]2 in the presence of Ag2O and then with 4,4′-bis(methoxycarbonyl)-2,2′-bipyridine, followed by replacement of chlorides with NCS ligands. The 12988

DOI: 10.1021/acs.inorgchem.7b01714 Inorg. Chem. 2017, 56, 12987−12995

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and well-matched to the conduction band edge of TiO2. The energy levels of the LUMOs for four sensitizers were gradually increased with the increase in fused conjugation (CB108 > CB111 > CBTR > CI101). The difference between the LUMO energy levels might be the influence of the delocalized πelectron conjugation on the ancillary ligand of these dyes. The LUMO and HOMO energy levels of these sensitizers, compared to the levels of the conduction band of TiO2 film and the I−/I3− redox electrolyte, are suitable for efficient electron injection and dye regeneration. Photovoltaic Devices. To further evaluate the photovoltaic properties, the DSSCs were fabricated using the dyes N719, CI101, CBTR, CB111, and CB108 under the same fabrication conditions. This process employed the nonaqueous TiO2 paste, which is suitable for screen printing and massive production of DSSCs instead of the TiO2 water paste as reported in our previous publications.35,36 The photovoltaic performance data of these complexes, the short-circuit photocurrent densities (Jsc), open-circuit voltages (Voc), fill factors (FF), and overall cell efficiencies, are summarized in Table 1. Figure 3A,B depicts the photocurrent−voltage (J−V) curves and incident photon-to-current conversion efficiency (IPCE) spectra for these dyes. The short-circuit photocurrent densities (Jsc) of solar cells fabricated with CI101, CBTR, CB111, and CB108 sensitizers are 13.8, 19.7, 18.4, and 19.9 mA cm−2, respectively. The Jsc value of the device using fused conjugated CB108 sensitizer was the highest but dropped greatly for the cell based on the parent imidazole dye CI101. DSSC based on CI101 showed the lowest open-circuit photovoltage (660 mV) compared to those of the DSSCs fabricated with the conjugated dyes CBTR, CB111, and CB108 (710−720 mV). Since the open-circuit photovoltage (Voc) of DSSCs depends on the Fermi level of TiO2,44,47 the above results clearly indicate that the fused π-conjugated dyes have a better effect on the Fermi level of TiO2 than that of parent CI101 sensitizer when they bind to the conduction band of the TiO2 electrode. Subsequently, the η value also increased with increasing the extended π-conjugated system except the dye CB111 (CI101-6.24%, CBTR-9.72%, CB111-9.24%, and CB108-9.89%). Note that the CB108-sensitized DSSC gave the best conversion efficiency (9.89%) of the solar cells investigated, which was also higher than that of the standard N719-sensitized cell (9.41%) under the same fabrication conditions. The higher η value of CB108-sensitized cell is mainly attributed to the higher Jsc (19.9 mA cm−2) value. On the basis of the literature, extending the conjugation length of ancillary ligands using thiophene moieties increased the IPCE and efficiency,48−52 but in NHC-pyridine ruthenium sensitizers,

Figure 2. Absorption spectra of CI101, CBTR, CB111, and CB108 in CH3CN/t-BuOH (1/1, v/v). Concentration = 8.0 × 10−5 M.

with a shoulder peak around 650 nm extending to 750 nm is ascribed to the lower-energy MLCT transitions that originated from central ruthenium metal to anchoring, ancillary, and NCS ligands.41,45 It is noteworthy that the MLCT transition is one of the dominant factors responsible for the higher efficiency of a cell. Compared to those of CB111 and CB108 bearing fused ancillary ligands, the lower-energy MLCT band of the complex CI101 was slightly red-shifted and showed a similar molar absorption coefficient. These results revealed that the carbenebased complexes incorporated with extended fused conjugation could not significantly alter the optical properties of the sensitizers CB111 and CB108, which was presumably due to the unique electronic properties of carbene. However, this type of fused-conjugated insertion in the ancillary ligand of the carbene metal complexes perhaps influences the other photophysical or electrochemical properties to enhance the solar cell performance. Electrochemical Properties. The energy levels of the sensitizer at the semiconductor oxide and the electrolyte interface are fundamentally important for electron transfer process and thus the function of the DSSC. Before making the DSSC devices, the highest occupied molecular orbitals (HOMOs) and lowest-unoccupied molecular orbitals (LUMOs) energy levels of all sensitizers were calculated from their cyclic voltammograms and the absorption edge obtained from the UV−vis absorption spectra (Table 1).28−33,46 The data showed that the dye molecules CI101, CB111, and CB108 have similar oxidation potentials compared to that of our previously reported CBTR.35 CI101 sensitizer showed the lowest LUMO energy level; however, it is sufficiently higher

Table 1. Physical Data and Performance of Studied Fused Ring Complexes and N719-Sensitized Cells cell performancea sensitizer

EOXb [V vs Fc/Fc+]

EHOMOc [eV]

ELUMOd [eV]

E0−0d [eV]

JSC [mA/cm2]

VOC [mV]

FF

η [%]

CI101 CBTR CB111 CB108 N719

0.32 0.34 0.33 0.31 0.31

5.11 5.14 5.13 5.12 5.11

3.41 3.39 3.33 3.32 3.46

1.70 1.75 1.80 1.80 1.65

13.8 19.7 18.4 19.9 18.6

660 710 710 720 710

0.683 0.694 0.705 0.688 0.710

6.24 9.72 9.24 9.89 9.41

a

The average efficiency of each sensitizer was selected from six different cells and deviation in the cell performance of each cell from the chosen cell was within ±0.4% (±0.3). bThe Ag/AgNO3 reference electrode was calibrated with a ferrocene/ferrocinium (Fc/Fc+) redox couple. Same value of Eox of RuIII/II was obtained when the electrochemical experiments were performed using 0.1 M [nBu4N]PF6 in DMF. cEHOMO = Eox − EFc/Fc++ 4.8 eV. dELUMO = EHOMO − E0−0. The band gap E0−0 was estimated from the onset of absorption spectrum measured in DMF. 12989

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Figure 3. (A) Current density−voltage characteristics of photovoltaic devices incorporating CI101, CBTR, CB111, and CB108 as sensitizers under illumination with AM 1.5 simulated sunlight (100 mW cm−2) and (B) monochromatic IPCE spectra of the photovoltaic devices (TiO2 thickness: 16 μm; cell active area tested with a mask: 0.16 cm2).

we observed diminishing of the efficiency with the introduction of fused thiophene system. The decrease in the η value for CB111 dye, compared to CB108, might be due to the susceptibility of the sulfur atoms in the delocalized conjugate system to induce dye−iodine interactions, which result in increased charge recombination.53,54 The higher Jsc value of CB108-containing cell is consistent with the IPCE data (Figure 3B). The IPCE spectra of these dyes covered the entire visible region from 350 to 800 nm. The maxima of IPCE for DSSCs anchored with CI101 and CB111 showed 40% and 70%, respectively, lower value than that of CBTR, and their data were coherent with the conversion efficiencies (η). The IPCE spectra of CB108 and CBTR sensitized DSSCs exhibited almost similar characteristic curves, which exceeded 70% in the range from 410 to 590 nm, reaching their maxima of 80%. The CB108-sensitized solar cell exhibited excellent IPCE characteristics and showed the highest light-harvesting capacity among the DSSCs anchored with carbene-based ruthenium sensitizers. Until now, the use of ruthenium polypyridyl complexes bearing ligands functionalized with fused conjugation resulted in lower photoconversion efficiency.38−41 But, the carbene-based sensitizers incorporating with fused aromatic rings in the ancillary ligand showed an enhancement in the efficiency and offers a promising alternative for future studies involving the structural tuning of the dyes. To gain more insight into the variation in the light-harvesting performance of CI101, CBTR, CB111, and CB108, electrochemical impedance spectroscopy (EIS) was performed to analyze the effect of fused conjugation of ancillary groups in the sensitizers on the interfacial charge transfer in DSSCs. Figure 4

shows the electrochemical impedance spectra measured under illumination for the DSSCs anchoring different dyes. The semicircle in the medium-frequency regime of EIS spectra represents the resistances at the TiO2/dye/electrolyte interface and TiO2 network. A smaller radius of the middle semicircle in the Nyquist diagram implies a higher rate of electron generation and transport.37,49,55−57 The radius of this semicircle decreased in the order of CI101 > N719 > CBTR > CB111 > CB108, indicating the overall resistance of the CB108-sensitized DSSC was smaller than those of less conjugated dyes and N719. These results also support that photovoltaic performance of CB108 is superior and CI101 anchored cell exhibits the lowest rate of electron transport and energy conversion efficiency. The fitted EIS data, electron lifetime in photoanode (τ); the effective rate constant for recombination (keff); the electron transport resistance in the photoanode (Rw); the charge transfer resistance related to the recombination of electron at the interface (Rk); the effective electron diffusion coefficient (Deff); and electron diffusion length (Ln) are summarized in Table 2. The Rw of each dye sensitized device, CI101 (6.42 Ω), Table 2. Parameters Determined by Fitting the EIS Experimental Data of the Devices to the Equivalent Circuit sensitizer

Rk (Ω)

Rw (Ω)

CI101 CBTR CB111 CB108 N719

15.52 9.94 7.85 8.89 8.75

6.42 1.55 2.30 1.42 2.55

L (μm) 16 16 16 16 15

keff

τ (ms−1)

Ln (μm)

Deff (1 × 10−5 cm2/s)

12.80 14.80 14.13 12.80 14.13

78 67 70 78 70

24 40 29 40 27

7.92 24.30 12.31 20.52 10.90

CB111 (2.30 Ω), CBTR (1.55 Ω), and CB108 (1.42 Ω), was in good agreement with the experimental trend observed in the efficiency. The Ln of the CB111 sensitizer incorporated with dithienobenzene-imidazole carbene ligand (29 μm) was smaller than that of phenanthrene-imidazole derivatized CB108 (40 μm). The photogenerated electrons from the excited state of CB111 were less efficiently collected into the TiO2 conduction band, thus resulting in lower efficiency. The CB108 showed both the higher electron lifetime and the longer diffusion length, indicating that the CB108 sensitizer efficiently suppressed the back reaction with the electrolyte and also exhibited excellent electron collection.58−60 The EIS analysis supports the conclusion that the device performance of CB108 dye is superior, resulting in improved photocurrent and enhanced

Figure 4. Electrochemical impedance spectra of studied fused ring complexes and N719-sensitized solar cells in the form of a Nyquist plot. 12990

DOI: 10.1021/acs.inorgchem.7b01714 Inorg. Chem. 2017, 56, 12987−12995

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Synthesis of Ru(1-(4-methylpyridin-2-yl)-3-octyl-1H-imidazole)(dimethyl 2,2′-bipyridine-4,4′-dicarboxylate)(NCS)2 (3). To a solution of compound 2 (295 mg, 0.84 mmol) in CH2Cl2 (20 mL) was added Ag2O (102 mg, 0.44 mmol), and the reaction mixture was stirred at room temperature in the dark for 6 h. After filtration of the reaction mixture through diatomaceous earth, [RuCl2(p-cymene)]2 (250 mg, 0.41 mmol) was added, and the mixture was stirred at room temperature for 1 h. Afterward, the solvent was evaporated, and the resulting yellow solid was dissolved in MeOH (30 mL) and then filtered through diatomaceous earth. The filtrate was concentrated under reduced pressure, and the resulting residue was dissolved with 4,4′-dimethoxy-carbonyl-2,2′-bipyridine (223 mg, 0.82 mmol) in 1,2dichloroethane (40 mL). The reaction mixture was stirred at 80 °C for 16 h. Subsequently, the solvent was removed under reduced pressure to yield a black powder, which was dried and added to a suspension of potassium thiocyanate (797 mg, 8.2 mmol) in H2O/DMF (1:9, 30 mL). The reaction mixture was heated at 80 °C for 6 h. After it was cooled to room temperature, the solvent was removed under reduced pressure. The residue was dissolved in CH2Cl2 (80 mL), washed with H2O (3 × 40 mL), dried (Na2SO4), and concentrated to yield the crude product, which was purified by column chromatography (EtOAc/CH2Cl2, 1:9) and then recrystallized (ether/CH2Cl2) to afford compound 3 as deep red crystals. Yield: 33%. 1H NMR (500 MHz, DMSO-d6): δ 9.45 (d, J = 5.5 Hz, 1H), 9.20 (s, 1H), 8.95 (s, 1H), 8.49 (d, J = 2.0 Hz, 1H), 8.47 (d, J = 6.0 Hz, 1H), 8.01 (d, J = 6.0 Hz, 1H), 7.97 (s, 1H), 7.83 (d, J = 2.0 Hz, 1H), 7.57 (d, J = 6.0 Hz, 1H), 7.17 (d, J = 6.0 Hz, 1H), 6.84 (d, J = 6.0 Hz, 1H), 4.44−4.55 (m, 2H), 4.06 (s, 3H), 3.89 (s, 3H), 2.38 (s, 3H), 1.89−1.93 (m, 2H), 1.45−1.46 (m, 2H), 1.22−1.32 (m, 8H), 0.82 (t, J = 5.5 Hz, 3H); 13C NMR (125 MHz, DMSO-d6): δ 194.8, 164.5, 164.0, 159.1, 156.7, 155.8, 154.3, 151.2, 150.5, 150.2, 137.5, 135.9, 134.8, 133.2, 126.3, 124.8, 124.6, 122.7, 122.5, 116.6, 111.6, 53.2, 53.0, 49.5, 31.3, 31.1, 29.0, 28.6, 26.1, 22.1, 20.7, 13.9; high-resolution mass spectrometry (HRMS) (M+): calcd for C33H37N7O4RuS2, 761.1392, found: 761.1379. Synthesis of Ru(1-(4-methylpyridin-2-yl)-3-octyl-1H-imidazole)(2,2′-bipyridine-4,4′-dicarboxylic acid)(NCS)2 CI101. To a solution of compound 3 (200 mg, 0.26 mmol) in H2O/DMF (1:9, 10 mL) was added 1.0 M KOH(aq) (0.78 mL), and the mixture was stirred at room temperature for 1 h. After completion of the hydrolysis, the solvent was evaporated under reduced pressure. The resulting residue was dissolved in water (10 mL), and the product was precipitated using 1.0 M HCl(aq). The resulting precipitate was filtered, washed with water (3 × 6 mL), and dried under vacuum to yield CI101 as a dark red solid. Yield: 90%. 1H NMR (500 MHz, DMSO-d6): δ 9.43 (d, J = 5.5 Hz, 1H), 9.13 (s, 1H), 8.88 (s, 1H), 8.51 (s, 1H), 8.45 (d, J = 5.5 Hz, 1H), 7.97−7.99 (m, 2H), 7.85 (s, 1H), 7.56 (d, J = 6.0 Hz, 1H), 7.21 (d, J = 5.5 Hz, 1H), 6.86 (d, J = 6.0 Hz, 1H), 4.45−4.57 (m, 2H), 2.39 (s, 3H), 1.90−1.94 (m, 2H), 1.46−1.48 (m, 2H), 1.24−1.33 (m, 8H), 0.83 (t, J = 6.0 Hz, 3H). 13C NMR (125 MHz, DMSO-d6): δ 195.3, 165.4, 165.0, 158.9, 156.4, 155.7, 154.3, 150.9, 150.3, 150.2, 139.5, 136.9, 135.5, 133.0, 126.3, 124.8, 124.7, 122.7, 122.5, 116.6, 111.5, 49.5, 31.2, 31.1, 28.9, 28.6, 26.1, 22.1, 20.6, 13.9; HRMS (M+): calcd for C31H33N7O4S2Ru, 733.1079, found: 733.1086; Anal. Calcd (%) for C31H33N7O4RuS2: C 50.81, H 4.54, N 13.38; found: C 50.80, H 4.62, N 13.06. Synthesis of 1-(4-Methylpyridin-2-yl)-1H-dithieno[2,3-e:3′,2′-g]benzimidazole (5a). Thieno[3,2-e]benzothiophene-4,5-dione 4a (1 g, 4.54 mmol), ammonium acetate (3.5 mg, 45.4 mmol), and formaldehyde (5.90 mmol) were dissolved in acetic acid (60 mL), and the resulting mixture was refluxed for 24 h. After that, the reaction mixture was diluted with water (30 mL), cooled, and poured into aqueous 20% NaHCO3 solution (200 mL) to obtain a precipitate. The solid was dried and added to a mixture of potassium carbonate (486 mg, 3.39 mmol) and 2-fluoro-4-methylpyridine (652 mg, 5.87 mmol) in DMF (22 mL) in a round-bottom flask. The resulting suspension was stirred at 140 °C for 12 h. After the solvent was evaporated under reduced pressure, the crude product was dissolved in dichloromethane. After filtration, the solvent was removed under reduced pressure to afford compound 5a (1.021 g). Yield: 70%. 1H NMR (300 MHz,

light-harvesting efficiency, compared to those of CI101, CBTR, CB111, and N719 sensitized DSSCs.

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CONCLUSION In this study, we have successfully demonstrated that the NHCpyridine ruthenium complexes bearing an ancillary ligand with exceptional set of electronic properties are excellent sensitizers for DSSCs. The study of the fused ring effect on the carbenebased ruthenium sensitizers revealed that the pyridinebenzimidazole carbene ligand (CBTR) significantly enhanced the efficiency compared to that of the parent unfused ligand (CI101), whereas the further extension of CBTR to a pyridinephenanthroimidazole complex (CB108) achieved a slight improvement in the efficiency. The results indicated that the trend of fusion effect almost reached at maximum with the pyridine-phenanthroimidazole-fused ring system, and the introduction of sulfur atom (CB111) decreased the cell efficiency to a little extent. EIS data revealed that the electron lifetime of CB108-sensitized DSSC (78 ms−1) was higher than that of CBTR (67 ms−1); however, the electron diffusion lengths are the same and hence improved the charge injection efficiency (Jsc) marginally. These carbene-based ruthenium sensitizers showed good promise toward improvement in the conjugation through the incorporation of fused rings in the ancillary ligands. These findings not only permit the lightharvesting efficiency in carbene-based ruthenium complexes containing conjugated aromatics/heterocycles to be optimized but also provide a basis for the future design and synthesis of efficient sensitizers for solar cell applications.

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EXPERIMENTAL SECTION

Materials and Methods. All organic chemicals were purchased from Sigma−Aldrich, Fluka, Merck, Alfa Aesar, Acros, and Matrix. N719 was purchased from Solaronix. CH2Cl2 was dried using a patented solvent purifier (VAC 103991, Vacuum Atmospheres). CH3CN and dimethylformamide (DMF) were of high-performance liquid chromatography (HPLC) grade and dried using a VAC solvent purifier. All other solvents were of HPLC grade. NMR spectra were measured using a Bruker 300 or 500 MHz spectrometer with chloroform-d and/or dimethyl sulfoxide-d6 (DMSO-d6) as solvents. UV−Vis spectra were recorded using a Shimadzu UV3600 UV−Vis− NIR spectrophotometer, with CH3CN/t-BuOH as the solvent. Electrospray ionization mass spectrometry (ESI-MS) was performed using a JMS-700 HRMS spectrometer. Single-crystal X-ray structural determination was performed using a Bruker KAPPA APEX II apparatus. Cyclic voltammograms were collected using a CHI 627C electrochemical analyzer (CH Instruments). Synthesis of 1-(4-Methylpyridin-2-yl)-3-octyl-1H-imidazol-3-ium bromide (2). In a round-bottom flask, imidazole (500 mg, 7.34 mmol), and sodium hydride (60% dispersion in mineral oil, 269 mg, 6.73 mmol) were dissolved in DMF (25 mL) at 0 °C. To the above reaction mixture, 2-fluoro-4-methylpyridine (680 mg, 6.12 mmol) was added, and the mixture was stirred at room temperature for 10 min. The temperature of the reaction mixture was then raised to 80 °C and maintained overnight. The resulting solution was then cooled to room temperature, and the solvent was evaporated under reduced pressure. After CH2Cl2 (50 mL) was added, the organic phase was extracted with H2O (3 × 25 mL) and dried (Na2SO4) and concentrated. The residue was dissolved in DMF (25 mL), and 1-bromooctane (2364 mg, 12.24 mmol) was added to the solution, which was then stirred at 100 °C for 12 h. After removal of the solvent, the crude compound 2 was washed by ether (20 mL) and used in the next step without further purification. Yield: 86%. 1H NMR (300 MHz, CDCl3): δ 11.82 (s, 1H), 8.53 (s, 1H), 8.34 (d, J = 5.1 Hz, 1H), 8.28 (s, 1H), 7.36 (s, 1H), 7.25 (d, J = 5.1 Hz, 1H), 4.54 (t, J = 7.2 Hz, 2H), 2.56 (s, 3H), 1.97−2.07 (m, 2H), 1.19−1.37 (m, 10H), 0.86 (t, J = 6.6 Hz, 3H). 12991

DOI: 10.1021/acs.inorgchem.7b01714 Inorg. Chem. 2017, 56, 12987−12995

Article

Inorganic Chemistry CDCl3): δ 8.57 (d, J = 5.1 Hz, 1H), 8.35 (s, 1H), 7.77 (m, 2H), 7.52 (dd, J = 5.4, 0.6 Hz, 1H), 7.43 (dd, J = 5.4, 0.6 Hz, 1H), 7.40 (s, 1H), 7.23 (d, J = 5.1 Hz, 1H), 2.49 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 151.03, 149.16, 148.69, 139.48, 136.64, 133.66, 132.23, 128.96, 125.09, 124.51, 124.22, 122.51, 122.32, 122.22, 117.19, 21.24; HRMS (M+): calcd for C17H11N3S2, 321.0394, found 321.0399. Synthesis of 1-(4-Methylpyridin-2-yl)-1H-phenanthro[9,10-d]imidazole (5b). Prepared from phenanthrene-9,10-dione (4b) using a method similar to that described for compound 5a. Yield: 75%. 1H NMR (300 MHz, CDCl3): δ 8.68−8.77 (m, 3H), 8.59 (d, J = 5.1 Hz, 1H), 8.14 (s, 1H), 7.72 (t, J = 7.2 Hz, 1H), 7.64 (t, J = 7.5 Hz, 1H), 7.54 (dd, J = 8.1, 7.2 Hz, 1H), 7.48 (d, J = 8.1 Hz, 1H), 7.33−7.38 (m, 3H), 2.49 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 151.32, 151.10, 149.87, 141.96, 139.00, 129.75, 128.87, 127.71, 127.65, 126.56, 126.10, 126.02, 125.65, 124.49, 123.49, 123.03, 122.96, 122.31, 121.80, 21.48; HRMS (M+): calcd for C21H15N3, 309.1266, found 309.1263. Synthesis of 1-(4-Methylpyridin-2-yl)-3-octyl-1H-dithieno[2,3e:3′,2′-g] benzimidazol-3-ium bromide (6a). To a solution of 5a (1.0 g, 3.11 mmol) in dioxane (5 mL), 1-bromooctane (8.11 mL, 46.6 mmol) was added and stirred at 140 °C for 12 h. The resulting solution was cooled to room temperature, and the solvent was evaporated under reduced pressure. After CH2Cl2 (10 mL) was added, the crude product was triturated with hexane (20 mL) and filtered off. The crude solid was recrystallized (CH2Cl2/ether) to afford 6a (1.15 g). Yield: 72%. 1H NMR (500 MHz, CDCl3): δ 11.81 (s, 1H), 8.57 (d, J = 5.0 Hz,1H), 8.48 (s, 1H), 7.83 (d, J = 5.0 Hz, 1H), 7.77 (d, J = 5.5 Hz, 1H), 7.73 (d, J = 5.5 Hz, 1H), 7.63 (d, J = 5.5 Hz, 1H), 7.39 (d, J = 5.0 Hz, 1H), 4.93 (t, J = 7.5 Hz, 2H), 2.58 (s, 3H), 2.11 (t, J = 7.5 Hz, 2H), 1.53 (t, J = 7.5 Hz, 2H), 1.31−1.37 (m, 2H), 1.19−1.27 (m, 6H), 0.80 (t, J = 7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3): δ 153.51, 147.90, 146.75, 138.71, 135.76, 135.48, 129.79, 127.69, 126.73, 124.69, 123.77, 123.04, 122.90, 122.23, 119.83, 118.82, 49.65, 31.58, 30.61, 28.94, 26.25, 22.46, 21.36, 13.94; HRMS (M+): calcd for C25H28N3S2, 434.1725, found 434.1724. Synthesis of 1-(4-Methylpyridin-2-yl)-3-octyl-1H-phenanthro[9,10-d]imidazol-3-ium bromide (6b). Compound 5b (0.50 g, 1.62 mmol) and 1-bromooctane (7.03 mL, 40.39 mmol) were combined and dissolved in 5 mL of DMF in a microwave reaction vessel containing a magnetic stir bar. The reaction vessel was irradiated in the Synthos 3000 microwave oven at 500 W for 6 h (up to 120 °C). After removal of the solvent, the crude product was triturated with mixture of acetone and hexane. The crude solid was recrystallized (CH2Cl2/ ether) to afford 6b (633 mg). Yield: 78%. 1H NMR (300 MHz, CDCl3): δ 11.19 (s, 1H), 8.85 (dd, J = 6.0, 3.0 Hz,1H), 8.77 (d, J = 8.4 Hz, 1H), 8.56 (d, J = 5.1 Hz,1H), 8.32 (dd, J = 6.6, 3.0 Hz, 1H), 8.12 (d, J = 0.3 Hz, 1H), 7.84 (dd, J = 6.3, 3.3 Hz, 2H), 7.72 (t, J = 7.8 Hz, 1H), 7.52 (d, J = 5.1 Hz, 1H), 7.44 (t, J = 7.5 Hz, 1H), 7.26 (s, 1H), 5.25 (t, J = 7.5 Hz, 2H), 2.62 (s, 3H), 2.22 (t, J = 7.5 Hz,2H), 1.56− 1.66 (m, 2H), 1.37−1.39 (m, 2H), 1.24−1.31 (m, 6H), 0.83 (t, J = 6.6 Hz, 3H); 13C NMR (75 MHz, CDCl3): δ 153.26, 149.59, 147.52, 141.21, 130.09, 130.01, 128.39, 128.26, 128.12, 127.71, 126.43, 125.31, 124.48, 124.03, 122.78, 122.18, 121.37, 120.16, 119.85, 50.95, 31.47, 29.39, 28.82, 26.07, 22.37, 21.22, 13.87; HRMS (M+): calcd for C29H32N3, 422.2596, found 422.2598. Synthesis of Ru(1-(4-methylpyridin-2-yl)-3-octyl-1H-dithieno[2,3e:3′,2′-g] benzimidazole) (dimethyl 2,2′-bipyridine-4,4′dicarboxylate)(NCS)2 (7a). Compound 6a (0.5 mg, 0.97 mmol) and [RuCl2(p-cymene)]2 (298 mg, 0.49 mmol) were dissolved in anhydrous CH2Cl2 (16 mL) at 0 °C. LiN(SiMe3)2 (1.0 M in THF, 1.22 mL, 1.22 mmol) was added slowly, and the mixture was stirred at room temperature for 12 h. After completion, the reaction mixture was diluted with CH2Cl2 (100 mL) and washed with H2O (200 mL). The collected CH2Cl2 extracts were dried (Na2SO4), filtered, and concentrated to afford a crude black product, which, together with 4,4′-dimethoxycarbonyl-2,2′-bipyridine (292 mg, 1.07 mmol), was dissolved in 1,2-dichloroethane (24 mL) and maintained at 80 °C for 6 h. Subsequently, the solvent was evaporated under reduced pressure to give a black powder. The dried black solid and potassium thiocyanate (3.77 g, 38.8 mmol) were dissolved in H2O/DMF (1:9, 20 mL) and stirred at 80 °C for 10 h. The reaction mixture was cooled to room

temperature, and the solvent was evaporated. The residue was dissolved in CH2Cl2 (100 mL) and washed with H2O (100 mL), dried (Na2SO4), and concentrated. The crude compound was purified by column chromatography (EtOAc/CH2Cl2, 1:19) and then recrystallized (ether/CH2Cl2) to give 7a as deep red crystals (188 mg). Yield: 21%. 1H NMR (500 MHz, DMSO-d6): δ 9.53 (d, J = 5.5 Hz, 1H), 9.28(s, 1H), 9.01 (s, 1H), 8.64 (s, 1H), 8.56 (dd, J = 5.5, 1.5 Hz, 1H), 8.28 (d, J = 2.5, 3H), 8.15 (dd, J = 5.5, 2.0 Hz, 1H), 7.99 (dd, J = 5.0, 2.0 Hz, 1H), 7.43−7.46 (m, 2H), 7.01 (d, J = 6.0 Hz, 1H), 5.14 (br, 2H), 4.09 (s, 3H), 3.87 (s, 3H), 2.54 (s, 3H), 2.10 (d, 2H), 1.71 (br, 2H), 1.38 (br, 2H), 1.27 (br, 6H), 0.80 (br, 3H); 13C NMR (125 MHz, DMSO-d6): δ 206.67, 164.92, 164.44, 159.06, 157.77, 155.97, 155.24, 151.69, 151.61, 150.75, 138.69, 136.44, 136.12, 134.30, 133.88, 133.70, 130.08, 128.22, 127.01, 126.04, 125.14, 124.93, 124.05, 123.64, 123.48, 123.18, 123.09, 121.12, 120.28, 113.87, 53.78, 53.49, 49.54, 31.95, 31.65, 29.55, 29.00, 26.68, 22.54, 21.27, 14.40; HRMS (M+): calcd for C41H39N7O4RuS4, 923.0990, found 923.0992. Synthesis of Ru(1-(4-methylpyridin-2-yl)-3-octyl-1H-phenanthro[9,10-d] imidazole) (dimethyl 2,2′-bipyridine-4,4′-dicarboxylate) (NCS)2 (7b). Prepared from compound 6b using a method similar to that described for compound 7a. Yield: 18%. 1H NMR (500 MHz, DMSO-d6): δ 9.82 (d, J = 5.5 Hz, 1H), 8.88 (m, 3H), 8.64 (s,1H), 8.57 (d, J = 8.0 Hz, 1H), 8.36 (dd, J = 5.5, 1.0 Hz, 1H), 8.13 (d, J = 6.0 Hz,1H), 8.10 (d, J = 8.0 Hz, 1H), 7.87 (t, J = 7.5 Hz, 1H), 7.82 (t, J = 7.5 Hz, 1H), 7.73 (m, 2H), 7.60 (t, J = 7.5 Hz, 1H), 7.39 (dd, J = 6.0, 1.0 Hz, 1H), 7.28 (s, 1H), 6.70 (d, J = 6.0, 1H), 5.43 (br, 2H), 4.13 (s, 3H), 3.94 (s, 3H), 2.34 (s, 3H), 2.19 (br, 2H), 1.78 (br, 2H), 1.36 (br, 2H), 1.26 (br, 6H), 0.82 (br, 3H); 13C NMR (125 MHz, DMSO-d6): δ 209.26, 164.26, 163.83, 159.50, 156.47, 155.67, 152.52, 150.14, 149.54, 138.16, 136.64, 135.26, 134.16, 131.10, 129.87, 128.97, 128.39, 126.94, 126.78, 126.63, 126.41, 126.29, 124.93, 124.53, 124.31, 122.95, 122.47, 122.12, 121.74, 121.66, 120.99, 120.50, 114.28, 53.49, 53.22, 51.00, 31.82, 30.87, 29.48, 29.18, 26.80, 22.64, 21.50, 14.12; HRMS (M+): calcd for C45H43N7O4S2Ru, 911.1861, found 911.1851. Synthesis of Ru(1-(4-methylpyridin-2-yl)-3-octyl-1H-dithieno[2,3e:3′,2′-g] benzimidazole) (2,2′-bipyridine-4,4′-dicarboxylic acid)(NCS)2 CB111. To a solution of compound 7a (100 mg, 0.11 mmol) in DMF (5.49 mL) was added 0.5 M NaOH(aq) (0.65 mL), and the mixture was stirred at room temperature for 1 h. After completion of the hydrolysis, the solvent was evaporated under reduced pressure and purified by LH-20 column chromatography using methanol (2 mL) and two drops of water. After evaporation, the residue was dissolved in MeOH (10 mL), and the product was precipitated by adding 0.2 M HCl in MeOH. The resulting precipitate was filtered, washed with water (3 × 6 mL), and again purified by LH20 column chromatography using methanol/dichloromethane (1:1, 10 mL) and two drops of water to yield CB111 as a dark red solid. Yield: 80%. 1H NMR (500 MHz, MeOH-d4): δ 9.64 (d, J = 5.7 Hz, 1H), 9.09 (s, 1H), 8.83 (d, J = 1.2 Hz, 1H), 8.55 (s, 1H), 8.56 (dd, J = 5.7, 1.5 Hz, 1H), 7.84−7.78 (m, 4H), 7.54−7.51 (m, 2H), 7.35 (dd, J = 6, 1.5 Hz, 1H), 6.93 (d, J = 4.8 Hz, 1H), 5.31 (br, 2H) 2.55 (s, 3H), 2.26− 2.14 (br, 2H), 1.91−1.80 (br, 2H), 1.43−1.30 (br, 8H), 0.88−0.39 (br, 3H); HRMS (M+): calcd for C39H35N7O4RuS4, 895.0677, found 895.0686; Anal. Calcd (%) for C39H35N7O4RuS4·4H2O: C 48.43, H 4.48, N 10.14; found: C 48.19, H 4.10, N 9.98. Synthesis of Ru(1-(4-methylpyridin-2-yl)-3-octyl-1H-phenanthro[9,10-d]imidazole) (2,2′-bipyridine-4,4′-dicarboxylic acid)(NCS)2 CB108. This was prepared from compound 7b using a method similar to that described for CB111. Further, the compound CB108 was recrystallized from dichloromethane/ether to obtain crystals for elemental analysis. Yield: 80%. 1H NMR (500 MHz, DMSO-d6): δ 9.45 (d, J = 5.5 Hz, 1H), 9.14−9.13 (m, 2H), 9.08 (d, J = 8.4 Hz, 1H), 8.90 (s, 1H), 8.69 (d, J = 8.5 Hz, 1H), 8.45 (d, J = 5.4 Hz, 1H), 8.25 (d, J = 7.8 Hz, 2H), 7.97 (t, J = 7.7 Hz, 1H), 7.89−7.86 (m, 1H), 7.83 (s, 1H), 7.80−7.77 (m, 1H), 7.74 (t, J = 7.6 Hz, 1H), 7.44 (d, J = 5.0 Hz, 2H), 6.95 (d, J = 6.0 Hz, 1H), 5.35 (br, 2H), 2.35 (s, 3H), 2.15− 2.04 (br, 2H), 1.73−1.65 (br, 2H), 1.39−1.31 (br, 2H), 1.28−1.18 (br, 6H) 0.80−0.76 (br, 3H); HRMS (M+): calcd for C43H39N7O4RuS2, 883.1548, found 883.1544; Anal. Calcd (%) for C43H39N7O4RuS2· 2CH2Cl2: C 51.33, H 4.12, N 9.31; found: C 51.66, H 4.44, N 9.66. 12992

DOI: 10.1021/acs.inorgchem.7b01714 Inorg. Chem. 2017, 56, 12987−12995

Inorganic Chemistry

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DSSCs Fabrication. The dye-absorbed layer and the lightscattering TiO2 film of the photoelectrodes in this work were prepared using the following procedure from anatase-TiO2 (a-TiO2) sol and TiO2 powder, respectively. To prepare TiO2 paste of the dye-absorbed layer from sol solution, 14.6723 g of titanium(IV) n-butoxide (Ti(OBu)4, ACROS) was mixed with 2.0 M CH3COOH (72.3 mL) at room temperature under magnetic stirring to form a translucent solution, which was then transferred to a Teflon-lined autoclave to perform the hydrothermal treatment at 200 °C for 5 h. The obtained a-TiO2 sol was washed/centrifuged twice with ethanol and dispersed in ethanol/ acetic acid. The resulting solution was then dispersed in the αterpineol/ethanol with mixed ethyl celluloses (10 cps and 45 cP) using an ultrasonic homogenizer (BRANSON, S-250D). Finally, the welldispersed solution was concentrated by rotary evaporation followed by grinding to obtain an unagglomerated TiO2 paste using a three-rollermill grinder (CHII MAW, CM-075T). The above procedure was applied in the preparation of QF-1125 TiO2 paste for the scattering layer except that the a-TiO2 sol was replaced by commercial TiO2 powder (QF-Ti-1125, Yong-Zhen Technomaterial CO.,LTD). TiO2 photoelectrodes were prepared by coating a-TiO2 sol paste on the fluorine-doped tin oxide conducting (FTO) glass (TEC7 Hartford glass, transmission ≥80%, sheet resistivity: 8 ohm/□, USA) using the doctor-blade technique. Adhesive tapes were placed on the edges of FTO to form a mask for spreading the paste. The coated TiO2 film was dried at 110 °C for 30 min followed by programmed sintering in air at 125 °C for 15 min, at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C for 15 min. The final TiO2 film thickness was controlled by repeated doctor-blade coating, drying, and sintering steps. The thickness of the TiO2 film was measured by a Force EZstep surface profiler. The TiO2 scattering layers were coated with QF-1125 paste using a procedure similar to that described for the a-TiO2 dye-absorbed layer. Such prepared (a-TiO2 + QF-1125) electrodes were preheateded at 80 °C before being immersed in dye solutions [3 × 10−4 M in CH3CN/t-BuOH, 1:1 (v/v)] of N719, CI101, CBTR, CB111, and CB108 for 36 h.61,62 The Pt counter electrodes were prepared by sputtering-deposition (Hitachi E-1045 ion sputter) of 20 nm thick layers of Pt onto FTO substrates and placed over the dye-adsorbed TiO2 electrodes. The edges of the cell were sealed with Dupont Surlyn 1706 (thickness: 60 μ) spacer at 175 °C using a hot plate.63−66 To complete the cell assembly, the electrolyte comprised of 0.1 M lithium iodide, 0.05 M iodide, 0.5 M 1,2-dimethyl-3-propylimidazolium iodide, and 0.5 M 4-tert-butylpyridine in acetonitrile was injected into the intervening space between the TiO2 and Pt electrodes through the two predrilled holes, which were then covered with a microscope slide and sealed with Dupont Surlyn 1706 (thickness: 30 μ). The size of DSSC is 1.5 × 2.5 cm2, and the active area is 0.4 × 0.4 cm2. The photovoltaic performance data were measured with a shadow mask (0.16 cm2) made of a black polyethylene terephthalate (PET) sheet.67 DSSCs Performance Measurements. The current−voltage characteristics of the DSSCs were measured using a Keithley model 2400 source measuring unit. An A-class solar simulator equipped with a 300 W xenon lamp (Oriel, No. 91160) and an AM 1.5 filter (Oriel, #No. 81094) served as a light source. The light intensity at the cell measuring position was adjusted to 100 mW cm−2 using an NRELcalibrated monocrystalline silicon solar cell (PVM134 reference cell, PV Measurement, Inc.). The IPCE spectra of the cells were obtained by an equipment system comprised of a 150 W xenon lamp (Oriel, No. 66902), a monochromator (Oriel CornerstoneTM 130), and a Keithley model 2400 digital source meter. EIS Measurements. The electron transport properties were studied by EIS with an impedance analyzer (IM6ex, Zahner, Germany) under illumination condition of 100 mW/cm−2 with an alternating current (ac) amplitude of 10 mV and frequency range from 1 × 10−1 to 1 × 105 Hz. Modeling and fitting the EIS data of DSSCs was performed using the ZView software to obtain the electron transport parameters of cells. The equivalent circuit (Figure S6) and calculations of EIS parameters are provided in the Supporting Information.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01714. Molecular structure of N719, crystallographic data of compounds 3, 7a, and 7b, absorption spectra of carbene dyes on TiO2 films, and details of EIS measurements (PDF) Accession Codes

CCDC 878365−878367 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wen-Ren Li: 0000-0001-9230-4957 Author Contributions §

These two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology, ROC, for financial support (Grant Nos. MOST 103-2113-M-008-008MY3 and MOST 104-2113-M-027-005-MY3).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01714 Inorg. Chem. 2017, 56, 12987−12995