Novel Ruthenium Sensitizers with a Phenothiazine Conjugated

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Novel Ruthenium Sensitizers with a Phenothiazine Conjugated Bipyridyl Ligand for High-Efficiency Dye-Sensitized Solar Cells Zhijie She, Yangyang Cheng, Luoqiang Zhang, Xiaoyu Li, Di Wu, Qiang Guo, Jingbo Lan, Ruilin Wang, and Jingsong You ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09160 • Publication Date (Web): 01 Dec 2015 Downloaded from http://pubs.acs.org on December 2, 2015

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ACS Applied Materials & Interfaces

Novel Ruthenium Sensitizers with a Phenothiazine Conjugated Bipyridyl Ligand for High-Efficiency Dye-Sensitized Solar Cells Zhijie She,† Yangyang Cheng,† Luoqiang Zhang,† Xiaoyu Li,† Di Wu,† Qiang Guo,† Jingbo Lan,*,† Ruilin Wang,*,‡ and Jingsong You*,† †

Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of

Chemistry, and State Key Laboratory of Biotherapy, West China Medical School, Sichuan University, 29 Wangjiang Road, Chengdu 610064, China. ‡

College of Materials Science and Engineering, Sichuan University, 29 Wangjiang Road,

Chengdu 610064, China. KEYWORDS: dye-sensitized solar cell, ruthenium sensitizer, phenothiazine, high-efficiency, light-harvesting capacity ABSTRACT: Two efficient ruthenium sensitizers with a phenothiazine-modified bipyridine as an ancillary ligand, coded SCZ-1 and SCZ-2, have been developed as dyes in dye-sensitized solar cells (DSSCs). Both sensitizers exhibit low-energy metal-to-ligand charge transfer (MLCT) bands centred at 539 nm with high molar extinction coefficients of 1.77 × 104 M-1 cm-1 for SCZ1 and 1.66 × 104 M-1 cm-1 for SCZ-2, which are significantly higher than the corresponding value for the reference N719 (1.27 × 104 M-1 cm-1), indicating that the light-harvesting capacity

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of ruthenium sensitizers can be reinforced by introducing phenothiazine moieties into the bipyridine ligand. Under AM 1.5G irradiation (100 mW cm-2), SCZ-1 and SCZ-2 sensitized DSSC devices show impressive power conversion efficiencies (PCE) up to 10.4% by using of iodide-based electrolytes, which exceeds that of N719 (9.9%) under the same conditions. Both of the open circuit voltage (VOC) and fill factor (FF) of SCZ-sensitized solar cells approximate to those of N719-sensitized cell. The relatively higher efficiencies of the SCZ-sensitized cells than that of N719-sensitized cell come from their higher short-circuit photocurrent density (JSC), which may be mainly attributed to the high absorption coefficient. The absorption spectrum and device efficiency of SCZ-1 are both quite close to those of SCZ-2, suggesting that the difference in alkyl chains on the N atom of phenothiazine is not a decisive factor in affecting the photovoltaic performance of dyes.

■ INTRODUCTION Ruthenium

bipyridyl

complex,

i.e.,

bis(2,2'-bipyridine)(2,2'-bipyridyl-4,4'-

dicarboxylate)ruthenium(II) [Ru(BIPY)2(BPCA)], was used for the first time to sensitize TiO2 electrode in 1979.1 In 1985, tris(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) dichloride [Ru(BPCA)3] was employed as a sensitizer, affording the first efficient dye-sensitized solar cell (DSSC) with incident photon-to-current efficiency (IPCE) over 40% in the wavelength range of 450–500 nm.2 The breakthrough of DSSC is that Grätzel and coworkers achieved a power conversion efficiency (PCE) of 7.9% in 1991 by using a CN-bridged trinuclear Ru-complex [Ru(BPCA)2(μ-(CN)Ru(CN)(BIPY)2)2] and a new mesoporous TiO2 film as photoanode.3,4 Soon after

that,

mononuclear

Ru-complexes

cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-

dicarboxylate)ruthenium(II), i.e. N3 or N719, depending on whether it contains four or two


2

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protons, were used as sensitizers, which display outstanding photovoltaic properties.5-7 In the last two decades, a great deal of attention has focused on the structural optimization of ruthenium complexes by extending the π-conjugated skeleton of the anchoring or ancillary ligand to tune the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, red-shift the metal-to-ligand charge transfer (MLCT) transition absorption band, enhance light-harvesting capacity, suppress dye aggregation, and decrease recombination rate of electrons and holes.8,9 It has been well-accepted that the introduction of electron-donating antenna substituents into the ancillary bipyridine ligand can considerably reinforce the spectral response of sensitizers.10-22 Recently, many bipyridyl ligand-based ruthenium sensitizers featuring electron-donating antennas, such as C10111 and CYC-B11,12 are emerging with efficient light-to-electric conversion over 10%. However, the relatively low MLCT transition absorption coefficient hinders the further improvement in the PCE of DSSCs. It remains a great challenge to develop well-performed ruthenium sensitizers for high-efficiency DSSCs. Phenothiazines (PTZ) are a class of important diheteroanthracene scaffolds that contain electron-rich S and N atoms. Their non-planar butterfly conformation can sufficiently inhibit the aggregation of dyes.23-27 Meanwhile, phenothiazines are also potential hole-transport materials, which can vectorially move positive charge away from the nanocrystalline TiO2 surface by intramolecular electron transfer, and thus decrease charge recombination rate.28,29 Furthermore, the alkyl substituent on N atom can create a hydrophobic environment to prevent the dye desorption by water from TiO2 surface.10,30 Therefor, we envisaged that the incorporation of electron-donating phenothiazines into the bipyridine ancillary ligand would endow ruthenium sensitizers with red-shifted MLCT band and enhanced light-harvesting capacity.


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H N

R N

R Br KOH, DMSO, rt

S

NBS THF, 0 oC

S

1a, R = hexyl 1b, R = 2-ethylhexyl

R N

R N

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R N

1) n-BuLi 2) (Bu)3SnCl o Br THF, -78 C~rt

S

S


Sn(Bu)3


R N

R N

R N

S

S S

S o

1) [Ru(p-cymene)Cl2]2, DMF, 80 C

4,4'-dibromo-2,2'-bipyridine o

Pd(PPh3)4, DMF,140 C

N


2) 2,2'-bipyridyl-4,4'-dicarboxylic acid, 140 C o HOOC 3) NH4NCS, 120 C

N

N

o

N

Ru

N N

N C S N C S

HOOC SCZ-1, R = hexyl SCZ-2, R = 2-ethylhexyl

Scheme 1 Synthetic routes for ruthenium sensitizers SCZ-1 and SCZ-2 with a phenothiazine conjugated bipyridyl ligand.

■ RESULTS AND DISCUSSION Synthesis. The synthetic routes for ruthenium sensitizers SCZ-1 and SCZ-2 were shown in Scheme 1. With the commercially available phenothiazine as starting material, the N-alkylation reactions were implemented by using of 1-hexyl bromide or 2-ethylhexyl bromide as alkylation regent, affording 10-alkylphenothiazine in excellent yields (1).23 Next, an electrophilic aromatic bromination using N-bromosuccinimide (NBS) in THF gave mono-brominated 2 as major products.31 Then, organostannane compounds (3) were prepared by the treatment of 2 with nBuLi and subsequent addition of tributylchlorostannane. The palladium-catalyzed Stille crosscoupling of 3 with 4,4'-dibromo-2,2'-bipyridine produced phenothiazine functionalized bipyridyl ligand 4 in good yield. Ruthenium sensitizers SCZ-1 and SCZ-2 were finally obtained through the reaction of ligand 4 with [Ru(p-cymene)Cl2]2, then addition of 2,2'-bipyridine-4,4'-


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dicarboxylic acid, and substitution of the Cl‾ with the NCS‾ monodentate ligand.11 SCZ-1 and SCZ-2 were purified by Sephadex LH-20 column chromatography. Their structures were confirmed by NMR, HRMS and elemental analysis. The spectroscopic and characterization data for both complexes are in complete agreement with the structures shown in Scheme 1. UV-vis Absorption Properties. The absorption spectra of SCZ-1, SCZ-2, and the reference N719 in DMF (1.0 × 10-5 M) are displayed in Figure 1a, and the detailed spectroscopic data are collected in Table 1. The UV-vis spectra of both SCZ-1 and SCZ-2 show three absorption bands centered at 539 nm, 407 nm, and 306 nm, respectively. The absorption band at 306 nm is assignable to the intramolecular π–π* transitions of 2,2'-bipyridine-4,4'-dicarboxylic acid. The band centered at 407 nm consists of the π–π* charge transition of the phenothiazine-conjugated bipyridine 4 and one of the MLCT bands for SCZ sensitizers.10,30 This band exhibits an approximately 2.5-fold-increased absorption coefficient compared with the corresponding band observed in N719, which may be ascribed to the π–π* transition of extending π–conjugated skeleton of ancillary ligand 4. The absorption band at 539 nm is the characteristic MLCT band, which is one of the major factors of affecting the PCE of devices. This lower-energy MLCT band may be attributed to the charge transfer from the ruthenium center to the bipyridine moiety as anchoring ligand.30 Both MLCT absorption bands of SCZ sensitizers are more red-shifted than those of N719, suggesting a shrinked band gap of MLCT transition by incorporating electron-donating phenothiazines into the bipyridine ligand. The molar extinction coefficients (ε) of the characteristic MLCT band at 539 nm are 1.77 × 104 M-1 cm-1 for SCZ-1 and 1.66 × 104 M1

cm-1 for SCZ-2, which are considerably higher than that of N719 (1.27 × 104 M-1 cm-1),

indicating that the light-harvesting capacity of Ru-bipyridine dyes can be reinforced by introducing phenothiazine moieties into the bipyridine ligand. The absorption profile and


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absorption maximum of SCZ-1 are quite close to those of SCZ-2, suggesting that the difference in alkyl chains on the N atom of phenothiazine is not a decisive factor in affecting lightabsorption ability of dyes. When SCZ sensitizers are anchored onto 2 μm thick transparent nanocrystalline TiO2 film, the lower-energy MLCT band blue-shifts slightly about 10 nm, which may come from the deprotonation of carboxylic groups in the dye self-assembly process.32

Figure 1. Absorption spectra of SCZ-1, SCZ-2 and N719 (a) in DMF (1.0 × 10-5 M) and (b) anchored on transparent TiO2 films (thickness of 2 μm). Table 1. Photophysical and Electrochemical Properties of Dyes

Dye SCZ-1 SCZ-2 N719 a

306 (6.49) 307 (5.86) 306 (4.58)

λmaxa (nm) (ε [× 104 M-1 cm-1]) 407 (3.06) 406 (2.92) 374 (1.29)

539 (1.77) 539 (1.66) 515 (1.27)

Eoxb (V)

E0-0c (eV)

Eox*d (V)

0.99

1.75

−0.76

1.04

1.75

−0.71

—

—

—

Absorption maxima (λmax) and molar extinction coefficients (ε) were measured in DMF (1.0

× 10-5 M). b Oxidation potential of dyes was measured in DMF. c E0–0 was determined from the onset of absorption spectrum. d Eox* = Eox – E0–0.


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Figure 2. Calculated isosurfaces of HOMO, HOMO−1, LUMO, and LUMO+1 of SCZ and N719 sensitizers. Theory Calculations. To gain insights into electronic structures, the density functional theory (DFT) calculation of SCZ and N719 sensitizers were performed based on B3LYP/LAN12DZ basis set (Figure 2). The HOMO-1 and HOMO of SCZ can very well be compared with those of N719, primarily located on the ruthenium and the thiocyanate ligands. The LUMO and LUMO+1 of N719 are nearly degenerate, which are homogeneous distribution on the two anchoring bipyridine ligands. Owing to the desymmetrization by incorporating electron-donating phenothiazine groups, the LUMO and LUMO+1 of SCZ sensitizer are non-degenerate.7 The LUMO of SCZ is located only on the bipyridine as anchoring ligand, which facilitates the direct charge transfer from the excited ruthenium sensitizers to TiO2 photoanodes. Considering that both anchoring ligands on N719 may not bind to TiO2 film simultaneously, the photo-excited electron on the anchoring ligand that is not anchored to TiO2 photoanodes show a weak effect to the PCE of devices, indicating that SCZ sensitizers may be better than N719 for DSSCs.30


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Electrochemical Properties. To evaluate the driving force for electron injection from photoexcited ruthenium sensitizers to conduction band in TiO2 and the regeneration of oxidized dye, the electrochemical properties of SCZ-1 and SCZ-2 in DMF were studied by cyclic voltammetry (Table 1 and Figure S1, SI†).33,34 The ground oxidation potentials (Eox) corresponding to the HOMO energy levels are 0.99 and 1.04 V versus normal hydrogen electrode (NHE), respectively. The band gaps (E0−0) for both sensitizers determined from their absorption onset are 1.75 eV. The excited-state oxidation potentials (Eox*) corresponding to the LUMO energy levels are −0.76 and −0.71 V, respectively. Their HOMO levels are more positive than the redox potential of I−/I3− couple (0.42 V vs NHE), ensuring that the oxidized dyes can be regenerated by the electrolyte. Similarly, their LUMO levels are more negative than the conduction-band potential of TiO2 (−0.5 V vs NHE), indicating a sufficient driving force for electron injection from photoexcited ruthenium sensitizers to TiO2 photoanodes. Solar Cell Performances of the Dye-Sensitized Solar Cells. The performance of SCZ sensitizers was then examined in DSSC devices. The devices were fabricated by using a double layer TiO2 film (13 μm thickness × 20 nm particle-sized TiO2 transparent film with 5 μm thickness × 400 nm particle-sized TiO2 scattering layer) and the I−/I3− electrolyte. As shown in Figure 3a, the IPCE of DSSC device with SCZ-1 as a sensitizer is more than 80% in the spectral region from 465 to 565 nm, up to 88% at 535 nm. And the IPCE of SCZ-2 sensitized device is more than 80% in the spectral region from 485 to 555 nm, up to 86% at 530 nm. In contrast, the solar cell device sensitized with N719 has a relatively lower IPCE values.


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Figure 3. (a) IPCE curves and (b) J–V plots under AM 1.5G irradiation. The characteristic photocurrent-voltage (J−V) curves of DSSC devices sensitized with SCZ-1, SCZ-2 and N719 measured under AM 1.5G standard testing condition (100 mW cm−2) are displayed in Figure 3b, and the detailed photovoltaic parameters are summarized in Table 2. As can be seen, SCZ-sensitized solar cells gave the short-circuit photocurrent density (JSC) of 19.85 and 19.88 mA cm-2, open circuit voltage (VOC) of 761 and 761 mV, and fill factor (FF) of 0.688 and 0.677, corresponding to an overall efficiency (η) of 10.4 and 10.2% for SCZ-1 and SCZ-2, respectively. The reference N719 sensitizer gave JSC, VOC, and FF of 18.77 mA cm-2, 760 mV, and 0.692, respectively, corresponding to an efficiency of 9.9%. Both the VOC and FF values of SCZ-sensitized cells approximate to those of the N719-sensitized cell. The relatively higher efficiencies of the SCZ-sensitized solar cells than that of N719-sensitized cell come from their higher JSC, which may mainly come from the high absorption coefficient. Table 2. Photovoltaic Parameters of DSSCs with SCZ-1, SCZ-2, and N719 as Sensitizersa

Dye SCZ-1 SCZ-2 N719

JSC (mA cm-2) 19.85 19.88 18.77

VOC (mV) 761 761 760

FF 0.688 0.677 0.692

η (%) 10.4 10.2 9.9

Rrecb (Ω cm-2) 107.8 105.2 101.6

Dye loadingc (10-7 mol cm-2) 1.8 1.7 2.2


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a

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Measured under standard AM 1.5G simulated sunlight (100 mW cm−2) with effective area of

0.16 cm2. SCZ dyes were dissolved in DMF (0.5 mM) and N719 in ethanol (0.5 mM). Electrolyte: 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in acetonitrile solution. b EIS was carried out in the dark conditions. c Dye loading amounts were determined by using 0.1 M aqueous solution of NaOH and THF (1:1) to desorb the dye on photoanode for 10 min and measuring the absorption spectra. Subsequently, dye loading amounts were obtained by desorbing the dye on the TiO2 photoanode and measuring the UV-vis spectra.35 The loading amounts of SCZ-1, SCZ-2 and N719 on TiO2 are 1.8×10-7, 1.7×10-7 and 2.2×10-7 mol cm-2, respectively (Table 2 and Figure S2, SI†). Notably, although both of SCZ sensitizers exhibited lower adsorption amount on TiO2 than N719, they has higher extinction coefficient (Figure 1). The light harvesting efficiency (LHE) of dyes also show that SCZ sensitizers possesses higher LHE than N719 (Figure S3, SI†),36 which further demonstrates that the much larger light-harvesting capacity of SCZ sensitizers compared to that of N719 is one of the major contribution to their relatively higher JSC values. Electronic impedance measurements (EIS) were performed and the calculated Rrec values of SCZ-1 and SCZ-2 are approximately equal to that of N719, which is coincidence with the trend of their same VOC values in DSSCs (Table 2 and Figure 4).37,38 The corresponding electron lifetimes for SCZ-1, SCZ-2 and N719 are 103, 99 and 91 ms, respectively.26 The techniques of open-circuit photovoltage decay (OCVD) is utilized to further investigate electron recombination in DSSCs.24,39 The plots of OCVD of SCZ-1, SCZ-2 and N719 as a function of elapsed time were recorded (Figure 5). The effective electron lifetimes (τn) derives from the slope of VOC decay curves. SCZ-1 and SCZ-2 exhibit nearly identical slope with N719, which are consistent with the results obtained from EIS spectra.


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Figure 4. EIS Nyquist plots for the DSSCs in the dark by using the equivalent circuit (inset).

Figure 5. Open-circuit photovoltage decay.

■ CONCLUSIONS We have developed two new ruthenium sensitizers, coded SCZ-1 and SCZ-2, by incorporating phenothiazines into the bipyridine ligand. Compared with the reference N719 dye, both of the new ruthenium sensitizers exhibit more red-shifted absorption bands and enhanced light-harvesting capacity. Under AM 1.5G simulated sunlight, the corresponding DSSC cells sensitized with SCZ-1 and SCZ-2 achieve impressive power conversion efficiencies over 10%, which exceeds N719 (9.9%) under the same conditions. The absorption spectrum and device


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efficiency of SCZ-1 are both quite close to those of SCZ-2, suggesting that the difference in alkyl chains on the N atom of phenothiazine is not a decisive factor in affecting the photovoltaic performance of dyes.

■ EXPERIMENTAL SECTION Materials and General Measurements. All solvents and reagents were purchased from commercial suppliers and used without further purification except those specified. Pt-counter electrode and TiO2 films were purchased from OPV Tech (China) Co., Ltd. 10Hexylphenothiazine (1a), 3-bromo-10-hexyl-10H-phenothiazine (2a), 10-(2-ethylhexyl)-10Hphenothiazine (1b), and 3-bromo-10-(2-ethylhexane)-10H-phenoxazine (2b) were prepared according to the previously reported methods.17,22 NMR spectra were recorded on a Bruker AV II-400. 1H NMR (400 MHz) chemical shifts were recorded with CDCl3 (δ = 7.26 ppm) or DMSO-d6 (δ = 2.50 ppm) as the internal standard.

13

C

NMR (100 MHz) chemical shifts were measured with CDCl3 (δ = 77.16 ppm) or DMSO-d6 (δ = 39.52 ppm) as the internal reference. High resolution mass spectra (HRMS) were obtained with a Waters-Q-TOF-Premier (ESI+). Elemental analysis data was obtained on an EA Flash 1112 analyzer. Absorption spectra were recorded on a HITACHI U-2910 UV-vis absorption spectrophotometer. The cyclic voltammetry (CV) of various dyes were measured on LK2005A electrochemical workstation. Synthesis of 10-Hexyl-3-(tributylstannyl)-10H-phenothiazine (3a). To a solution of anhydrous THF (100 mL) containing compound 2a (5.43 g, 15.0 mmol) under surrounding temperature of -78 ºC, n-BuLi (2.5 M in hexane, 7.2 mL, 18.0 mmol) was added dropwise under nitrogen atmosphere. The mixture was stirred at the same temperature for another 2 h. Then,


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tributylchlorostannane (5.3 mL, 19.5 mmol) was added dropwise, and the reaction mixture was stirred continuously at -78 ºC for an additional 1 h before elevating the temperature to ambient temperature for another 3 h of stirring. The product was washed with saturated aqueous NaCl (50 mL) and extracted with CH2Cl2 (50 mL) three times. The organic phase was dried over anhydrous MgSO4. The volatiles were evaporated under reduced pressure to leave the crude product 3a (5.23 g) as a mobile liquid, which was used directly in the next step. Synthesis of 4,4'-Bis(10-hexyl-10H-phenothiazin-3-yl)-2,2'-bipyridine (4a). The crude product 3a (5.23 g) and 4,4'-dibromo-2,2'-bipyridine (1.57 g, 5.0 mmol) were dissolved in 150 mL anhydrous DMF. Then Pd(PPh3)4 (0.60 g, 0.5 mmol) was added as a catalyst. The mixture was refluxed under nitrogen atmosphere for 24 h. After cooling to room temperature, the mixture was quenched with deionized water (100 mL) and extracted with EtOAc (100 mL) two times. The organic phase was dried over anhydrous MgSO4. After removal of the volatiles under reduced pressure, the residue was purified by silica gel column chromatography (CH2Cl2 as the eluent) to give the product 4a as a yellow solid (2.52 g, 70% yield). 1H NMR (400 MHz, CDCl3): δ = 0.89 (t, J = 6.8 Hz, 6H), 1.30-1.35 (m, 8H), 1.43-1.48 (m, 4H), 1.80-1.87 (m, 4H), 3.89 (t, J = 7.2 Hz, 4H), 6.88 (d, J = 8.0 Hz, 2H), 6.92-6.96 (m, 4H), 7.14-7.19 (m, 4H), 7.49 (td, J = 5.2 Hz, 1.6 Hz, 2H), 7.57-7.61 (m, 4H), 8.65 (d, J = 1.2 Hz, 2H), 8.70 (d, J = 5.2 Hz, 2H) ppm. 13C NMR (100 MHz, CDCl3): δ = 14.1, 22.8, 26.8, 27.0, 31.6, 47.8, 115.6, 115.7, 118.4, 120.9, 122.9, 124.3, 125.7, 125.9, 126.2, 127.5, 127.7, 132.2, 144.9, 146.3, 148.1, 149.8, 156.8 ppm. HRMS (ESI+): calcd for C46H46N4S2 [M+H]+ 719.3242, found 719.3237. Synthesis of SCZ-1. A mixture of [RuCl2(p-cymene)]2 (367 mg, 0.6 mmol) and compound 4a (863 mg, 1.2 mmol) were dissolved in 90 mL anhydrous DMF. The solution was heated at 80 ºC under nitrogen atmosphere for 4 h in the dark. After this period, 2,2′-bipyridyl-4,4′-dicarboxylic


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acid (293 mg, 1.2 mmol) was added. The reaction mixture was refluxed at 140 ºC for another 4 h. Then an excess of NH4NCS (1.83 g, 24.0 mmol) was added to the reaction mixture and heated at 120 ºC overnight. After reaction, the volatiles were evaporated under reduced pressure and deionized water was added to dissolve NH4NCS. The precipitate was filtered out, washed with deionized water and Et2O, and dried under reduced pressure. The crude ruthenium bipyridyl complex was dissolved in MeOH solution with 0.05 M tetrabutylammonium hydroxide and purified on a Sephadex LH-20 column with MeOH as eluent. The product was collected, concentrated, and precipitated with acidic MeOH solution with 0.01 M HNO3 to obtain SCZ-1 as a black solid. And then it was further purified by the same procedure for three times. The final precipitate was dried under a nitrogen atmosphere (368 mg, 52% yield). 1H NMR (400 MHz, DMSO-d6): δ = 0.82-0.87 (m, 6H), 1.25-1.45 (m, 12H), 1.69-1.76 (m, 4H), 3.92-4.00 (m, 4H), 6.95-7.29 (m, 10H), 7.43-7.56 (m, 2H), 7.65-8.35 (m, 9H), 8.97-9.20 (m, 4H), 9.41-9.55 (m, 1H), 14.09 (s, 2H) ppm. HRMS (ESI+): calcd for C60H54N8O4RuS4 [M+H]+ 1181.2273, found 1181.2697. Anal. calcd for C60H54N8O4RuS4: C, 61.05; H, 4.61; N, 9.49; S, 10.86. Found: C, 61.00; H, 4.56; N, 9.50; S, 10.81. Synthesis of 3-Bromo-10-(2-ethylhexyl)-10H-phenothiazine (3b). To a solution of anhydrous THF (100 mL) containing compound 2b (5.86 g, 15.0 mmol) under surrounding temperature of -78 ºC, n-BuLi (2.5 M in hexane, 7.2 mL, 18.0 mmol) was added dropwise under nitrogen atmosphere. The mixture was stirred at the same temperature for another 2 h. Then, tributylchlorostannane (5.3 mL, 19.5 mmol) was added to the reaction mixture, and the mixture was stirred continuously at -78 ºC for an additional 1 h before elevating the reaction temperature to ambient temperature for another 3 h of stirring. The product was washed with saturated aqueous NaCl (50 mL) and extracted with CH2Cl2 (50 mL) three times. The organic phase was


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dried over anhydrous MgSO4. The volatiles were evaporated under reduced pressure to leave the crude product 3b (5.98 g), which was used directly in the next step. Synthesis of 4,4'-Bis(10-(2-ethylhexyl)-10H-phenothiazin-3-yl)-2,2'-bipyridine (4b). The crude product 3b (5.98 g) and 4,4'-dibromo-2,2'-bipyridine (1.57 g, 5.0 mmol) were dissolved in 150 mL anhydrous DMF. Then Pd(PPh3)4 (0.60 g, 0.5 mmol) was added as a catalyst. The mixture was refluxed under nitrogen atmosphere for 24 h. After cooling to 25 ºC, the reaction was quenched with deionized water (100 mL) and extracted with EtOAc (100 mL) two times. The organic phase was dried over anhydrous MgSO4. After removal of the volatiles under reduced pressure, the residue was purified by silica gel column chromatography (CH2Cl2 as the eluent) to give the product 4b as a yellow solid (2.64 g, 68% yield). 1H NMR (400 MHz, CDCl3): δ = 0.85-0.91 (m, 12H), 1.27-1.29 (m, 8H), 1.37-1.50 (m, 8H), 1.94-2.00 (m, 2H), 3.78 (d, J = 7.2 Hz, 4H), 6.90-6.98 (m, 6H), 7.15-7.19 (m, 4H), 7.49 (dd, J = 5.2 Hz, 2.0 Hz, 2H), 7.59-7.62 (m, 4H), 8.65 (d, J = 1.2 Hz, 2H), 8.70 (d, J = 5.2 Hz, 2H) ppm. 13C NMR (100 MHz, CDCl3): δ = 10.7, 14.2, 23.2, 24.2, 28.7, 30.9, 36.0, 51.3, 116.18, 116.23, 118.5, 120.9, 122.9, 125.3, 126.1, 126.2, 126.7, 127.4, 127.8, 132.3, 145.4, 146.8, 148.2, 149.7, 156.8 ppm. HRMS (ESI+): calcd for C50H55N4S2 [M+H]+ 775.3868, found 775.3873. Synthesis of SCZ-2. A mixture of [RuCl2(p-cymene)]2 (367 mg, 0.60 mmol) and compound 4b (930 mg, 1.2 mmol) were dissolved in 90 mL anhydrous DMF. The solution was heated at 80 ºC under nitrogen atmosphere for 4 h in the dark. After this period, 2,2′-bipyridyl-4,4′dicarboxylic acid (293 mg, 1.2 mmol) was added. The reaction mixture was refluxed at 140 ºC for another 4 h. Then an excess of NH4NCS (1.83 g, 24.0 mmol) was added to the reaction mixture and heated at 120 ºC overnight. After reaction, the volatiles were evaporated under reduced pressure and deionized water was added to dissolve NH4NCS. The precipitate was


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filtered out, washed with deionized water and Et2O, and dried under reduced pressure. The crude ruthenium bipyridyl complex was dissolved in MeOH solution with 0.05 M tetrabutylammonium hydroxide and purified on a Sephadex LH-20 column with MeOH as eluent. The product was collected, concentrated, and precipitated with acidic MeOH solution with 0.01 M HNO3 to obtain SCZ-2 as a black solid. And then it was further purified by the same procedure for three times. The final precipitate was dried under a nitrogen atmosphere (378 mg, 51% yield). 1H NMR (400 MHz, DMSO-d6): δ = 0.76-0.89 (m, 12H), 1.19-1.46 (m, 16H), 1.80-1.91 (m, 2H), 3.83-3.92 (m, 4H), 6.96-7.33 (m, 10H), 7.44-8.32 (m, 11H), 8.98-9.21 (m, 4H), 9.48 (d, J = 5.6 Hz, 1H), 14.34 (s, 2H) ppm. HRMS (ESI+): calcd for C64H62N8O4RuS4 [M-H]– 1235.2742, found 1235.2745. Anal. calcd for C64H62N8O4RuS4: C, 62.16; H, 5.05; N, 9.06; S, 10.37. Found: C, 62.13; H, 5.15; N, 9.04; S, 10.21. Fabrication of Cells and Photovoltaic Characterization. The TiO2 photoanode was stained by immersing it into the DMF solution of dyes (0.5 mM) for 24 h. After washing with THF, the sensitized TiO2 photoanode and the counter electrode, which was stuck with parafilm (30 μm thickness, Sigma-Aldrich) to separate out a square aperture of 0.36 cm2, were clipped together. The internal space between the two electrodes was filled with a liquid electrolyte. The electrolyte consisted of 0.6 M DMPII, 0.1 M LiI, 0.05 M I2, and 0.5 M tert-butylpyridine in acetonitrile. An Oriel Class AAA solar simulator (Oriel 94023A, Newport Corp.) was employed to give an irradiance of 100 mW cm-2. The light intensity was calibrated by an Oriel reference solar cell (Oriel 91150V, Newport Corp.). Photocurrent-voltage (J−V) characteristics of the solar cells were performed with a potentiostat/galvanostat (Keithley Series 2000 SourceMeter). The IPCE for solar cell were performed with a monochromator (Oriel 77890, Newport Corp.). EIS was determined under dark conditions with an Autolab PGSTAT 30/302 electrochemical workstation


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(Eco Chemie B.V.) over a frequency range of 0.1–105 Hz with an alternate current amplitude of 10 mV. Z-View software (v3.1, Scribner Associates, Inc.) was employed to fit out the corresponding parameters.

■ SUPPORTING INFORMATION Cyclic voltammetry, dye-loaded amounts, light harvesting efficiency, and copies of NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J.L.); [email protected] (J.Y.); [email protected] (R.W.). Fax: (+86) 28-85412203 (J.L., J.Y.); (+86) 28-85418018 (R.W.). Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was supported by grants from 863 Program (2013AA031901), the National NSF of China (Nos 21432005, 21372164, 21321061, 21272160, 21172155, and J1103315), Sichuan Provincial Foundation (2012JQ0002), and Comprehensive Training Platform of Specialized Laboratory, College of chemistry, Sichuan University.

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Novel Ruthenium Sensitizers with a Phenothiazine Conjugated

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