Direct Evidence of FÃ¶rster Resonance Energy Transfer for the

Article pubs.acs.org/JPCC

Direct Evidence of Förster Resonance Energy Transfer for the Enhanced Photocurrent Generation in Dye-Sensitized Solar Cell Hyunbong Choi,† Nara Cho,‡ Sanghyun Paek,‡ and Jaejung Ko*,‡ †

Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Department of Materials Chemistry, Korea University, Sejong City, 339-700, Republic of Korea S Supporting Information *

ABSTRACT: We meticulously designed organic antenna for the ruthenium phthalocyanine. A significant enhance-ment of photocurrent in JK-107 is attributed to a Förster energy transfer and intramolecular electron transfer from the antenna to the acceptor RuPc3. We demonstrate the direct evidence of Förster energy transfer and electron transfer for the enhanced photocurrent generation using photoluminescence spectroscopy and transient absorption spectroscopy, respectively.

coverage.9−13 As an extension of this concept, Odobel et al.14 described the facile assembly of panchromatic sensitizer formed through a coordinative interaction of chromophoric unit and a zinc porphyrin sensitizer. The incorporation of antenna unit to the porphyrin covers a wide window of the solar spectrum and shows a remarkable improvement to antenna-free systems. Herein, we extend the intramolecular energy transfer to supramolecular system using the six-coordination preference of ruthenium phthalocyanine RuPc3.15,16 We chose ruthenium phthalocyanine for its strong absorption in the red region, the versality provided by its axial ligands, as well as the reduction of its aggregation when coordinating axial ligands. We meticulously designed the cyano-vinylene D1 and dithiophene D2 derivatives as light harvester’s antennas because they exhibited absorbance across the 450−550 nm window where the RuPc3 absorption is absent. The antenna D1 was designed to have a significant emission overlap with the absorption spectrum of RuPc3, resulting in an efficient Förster energy transfer. In this study, we demonstrate the direct evidence of Fö rster Resonance Energy Transfer and electron transfer through the supramolecular system formed by the coordination bonding of antennas D to the ruthenium phthalocyanine (Figure 1).

1. INTRODUCTION Sensitization of wide band gap semiconductor surface by covalently grafting sensitizers such as polypyridyl ruthenium and organic dyes is central in applications to solar energy conversion in dye-sensitized solar cells (DSSCs).1−3 Essential for efficient conversion of solar energy by DSSCs is the development of panchromatic sensitizer which covers the whole solar radiation.4 Therefore, the development of optimal sensitizer, which combines the broad spectral coverage from 400 to 800 nm with a proper excited-state energy for favorable electron-transfer dynamics is a primary target. In the second approach, the simultaneous adsorption of the multiple dyes with complementary absorptions in the visible region was utilized to achieve the panchromatic sensitization of DSSCs.5 However, the drawback of such a technology is the limited number of sites on the TiO2 films for anchoring the dyes. To supplement this point, Durrant et al.6 and Choi et al.7 introduced the stepwise cosensitization of TiO2 films utilizing Al2O3 layers. A third approach is to use the concept of inter- or intramolecular energy transfer based on Förster Resonance Energy Transfer (FRET). The former developed by McGehee et al.8 proposes to use an unanchored energy relay dyes (ERD) dissolved in the electrolyte which acts as an energy donor molecule to the sensitized dyes (SD) anchored on the TiO2 films. However, as the FRET rate is a function of the distance between the ERD and the SD acceptor molecule, an efficiency enhancement could be modest because the separation distance between ERD/SD array is large enough. The latter is made of covalently linked donor dye, so-called “antenna unit” and the chemisorbed acceptor sensitizer, showing an enhanced photovoltaic performance in DSSCs due to the broad spectral © XXXX American Chemical Society

Special Issue: Michael Grätzel Festschrift Received: July 26, 2013 Revised: October 17, 2013

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dx.doi.org/10.1021/jp407475b | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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heated under an air flow 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 thickness of the transparent layer was measured by using an αstep 250 surface profilometer (Tencor Instruments, San Jose, CA), a paste for the scattering layer containing 400 nm sized anatase particles (CCIC, PST-400C) was deposited by doctor blade printing and then dried for 2 h at 25 °C. The TiO2 electrodes were gradually heated under an air flow 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 resulting layer was composed of 8 μm thickness of transparent layer and 4 μm thickness of scattering layer. The TiO2 electrodes were treated again by TiCl4 at 70 °C for 30 min and sintered at 500 °C for 30 min. The TiO2 electrodes were immersed into the JK-107 and JK-109 (0.1 mM in THF containing 0.1 mM 3a,7a-dihydroxy-5b-cholic acid (Cheno)) and kept at room temperature for 24 h. The FTO plate for counter electrodes cleaned with ultrasonic bath in H2O, acetone and 0.1 M HCl aq., subsequently. Counter electrodes were prepared by coating with a drop of H2PtCl6 solution (2 mg of Pt in 1 mL of ethanol) on a FTO plate and heating at 400 °C for 15 min. The dye adsorbed TiO2 electrode and Pt-counter electrode were assembled into a sealed sandwich-type cell by heating at 80 °C with a hot-melt ionomer film (Surlyn) as a spacer between the electrodes. A drop of electrolyte solution (electrolyte of 0.5 M LiI, 20 mM iodine in propylene carbonate) was placed on the drilled hole in the counter electrode of the assembled cell and was driven into the cell via vacuum backfilling. Finally, the hole was sealed using additional Surlyn and a cover glass (0.1 mm thickness). 2.3. Photoelectrochemical Measurement. Photoelectrochemical data were measured using a 1000 W xenon light source (Oriel, 91193) that was focused to give 1000 W/m2, the equivalent of one sun at Air Mass (AM) 1.5, at the surface of the test cell. The light intensity was adjusted with a Si solar cell that was doubled-checked with an NREL-calibrated Si solar cell (PV Measurement Inc.). The applied potential and measured cell current were measured using a Keithley model 2400 digital source meter. The current−voltage characteristics of the cell under these conditions were determined by biasing the cell externally and measuring the generated photocurrent. This process was fully automated using Wavemetrics software. 2.4. Electron Transport Measurements. The electron diffusion coefficients and lifetimes were measured by the stepped light-induced transient measurements of photocurrent and voltages (SLIM-PCV).12 The transients were induced by a stepwise change in the laser intensity. A diode laser (λ = 635 nm) as a light source was modulated using a function generator. The initial laser intensity was a constant 90 mW cm−2 and was attenuated up to approximately 10 mW cm−2 using a ND filter which was positioned at the front side of the fabricated samples (0.04 cm2). For the measurement of SLIM-PCV, the TiO2 thickness of the photoelectrode was controlled as approximately 3.3 μm. The photocurrent and photovoltage transients were monitored using a digital oscilloscope through an amplifier. A total of five points were measured to determine the electron diffusion coefficients and lifetimes. 2.5. The ac Impedance Measurements. The ac impedance measurements were carried out under illumination (1 sun) using an impedance analyzer (1260A, Solartron, U.K.). 2.6. Femtosecond Laser Flash Photolysis. Femtosecond transient absorption experiments were conducted using a CPA2010 1 kHz amplified Ti:sapphire laser system from Clark MXR, combined with Helios optical detection system provided

Figure 1. (A) Chemical structures of JK-107, JK-109, antennas D1 and D2, and RuPc3. (B) Energy and electron transfer processes in TiO2/ JK-107 (bottom arrow: energy transfer from excited D1 to RuPc and upper arrow: electron transfer from excited RuPc into TiO2).

2. EXPERIMENTAL SECTION 2.1. General Experimental. All reactions were carried out under an argon atmosphere. Solvents were distilled from appropriate reagents. All reagents were purchased from SigmaAldrich. 6-(Bis(9,9-dimethylfluoren-2-yl)amino)-2formylbenzo[b]thiophene17 and 2,2′-bithiophen-5-yltrimethylstannane18 and [{(t-Bu)4Pc}Ru(PhCN)2]19 were synthesized using a modified procedure of previous references. 1H and 13C NMR spectra were recorded on a Varian Mercury 300 spectrometer. Elemental analyses were performed with a Carlo Elba Instruments CHNS-O EA 1108 analyzer. Mass spectra were recorded on a JEOL JMS-SX102A instrument. The absorption and photoluminescence spectra were recorded on a Perkin-Elmer Lambda 2S UV−visible spectrometer and a Perkin LS fluorescence spectrometer, respectively. 2.2. DSSC Cell Fabrication Method. FTO glass plates (Pilkington TEC Glass-TEC 8, solar 2.3 mm thickness) were cleaned in a detergent solution using an ultrasonic bath for 30 min, rinsed with water and ethanol. The FTO glass plates were immersed in 40 mM TiCl4 (aqueous) at 70 °C for 30 min and washed with water and ethanol. A transparent nanocrystalline layer on the FTO glass plate was prepared by doctor blade printing TiO2 paste (Solaronix, Ti-Nanoxide T/SP) and then dried for 2 h at 25 °C. The TiO2 electrodes were gradually B



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144.1, 140.8, 138.8, 138.7, 136.6, 135.4, 134.5, 132.7, 131.9, 126.9, 126.5, 125.2, 122.6, 122.1, 122.0, 120.9, 120.8, 119.9, 119.6, 119.4, 116.7, 115.1, 114.9, 46.9, 27.1. MS: m/z 743 [M+]. Anal. Calcd for C50H37BN3S2: C, 80.72; H, 5.01. Found: C, 79.94; H, 4.91. 2.7.4. 2-(2,2′-Bithiophen-5-yl)-N,N-bis(9,9-dimethyl-9Hfluoren-2-yl)benzo[b]thiophen-6-amine (3). A stirred mixture of 1 (0.4 g, 0.65 mmol), 2,2′-bithiophen-5-yltrimethylstannane (0.32 g, 0.97 mmol), and Pd(PPh3)4 (0.04 g, 0.032 mmol) in toluene (50 mL) was refluxed for 12 h. After cooling the solution, H2O (10 mL) and brine were added to the solution. The organic layer was separated and dried in MgSO4. The solvent was removed in vacuo. The pure product 3 was obtained by chromatographic workup (eluent MC/Hx = 1:3, Rf = 0.3) as a yellow solid in 70% yield. Mp: 183 °C. 1H NMR (CDCl3): δ 7.64 (t, J = 7.2 Hz, 2H), 7.60 (d, J = 7.8 Hz, 2H), 7.50 (s, 1H), 7.38 (t, J = 7.2 Hz, 2H), 7.32 (d, J = 7.8 Hz, 2H), 7.30−7.26 (m, 7H), 7.25 (s, 2H), 7.20 (d, J = 3.9 Hz, 1H), 7.12 (d, J = 7.6 Hz, 2H), 7.10 (d, J = 3.9 Hz, 1H), 7.04 (d, J = 3.9 Hz, 1H), 1.42 (s, 12H). 13C{1H} NMR (CDCl3): δ 154.5, 152.7, 147.5, 146.0, 143.8, 142.1, 139.3, 138.9, 135.7, 135.0, 133.9, 132.5, 130.5, 130.1, 129.5, 128.8, 127.2, 126.8, 126.2, 125.4, 123.8, 122.9, 122.3, 121.9, 119.7, 119.5, 119.2, 117.7, 47.0, 27.1. MS: m/z 697 [M+]. Anal. Calcd for C46H35NS3: C, 79.16; H, 5.05. Found: C, 78.95; H, 5.97. 2.7.5. 2-(5′-Bromo-2,2′-bithiophen-5-yl)-N,N-bis(9,9-dimethyl-9H-fluoren-2-yl)benzo[b]thiophen-6-amine (4). Compound 4 was synthesized by the same procedure of 2 except that compound 3 (0.38 g, 0.54 mmol) was used instead of compound 1. Yield: 83%. Mp: 187 °C. 1H NMR (CDCl3): δ 7.64 (t, J = 7.8 Hz, 2H), 7.61 (d, J = 8.1 Hz, 2H), 7.56 (s, 1H), 7.39 (t, J = 7.8 Hz, 2H), 7.32 (d, J = 8.1 Hz, 2H), 7.30−7.26 (m, 5H), 7.25 (s, 2H), 7.12 (d, J = 8.1 Hz, 2H), 7.10 (d, J = 4.8 Hz, 1H), 7.03 (d, J = 3.9 Hz, 1H), 6.99 (d, J = 4.8 Hz, 1H), 6.93 (d, J = 3.9 Hz, 1H), 1.42 (s, 12H). 13C{1H} NMR (CDCl3): δ 155.4, 153.7, 147.4, 147.0, 143.0, 142.1, 139.1, 138.9, 135.7, 135.0, 133.9, 132.5, 130.5, 130.1, 129.5, 128.4, 127.1, 126.8, 126.2, 125.0, 123.8, 122.6, 122.2, 120.9, 119.7, 119.2, 117.1, 115.4, 47.0, 27.1. MS: m/z 777 [M+]. Anal. Calcd for C46H34BrNS3: C, 71.12; H, 4.41. Found: C, 70.56; H, 4.32. 2.7.6. N,N-Bis(9,9-dimethyl-9H-fluoren-2-yl)-2-(5′-(pyridin4-yl)-2,2′-bithiophen-5-yl)benzo[b]thiophen-6-amine (D2). Compound D2 was synthesized by the same procedure as D1, except that compound 4 (0.38 g, 0.54 mmol) was used instead of compound 2. Yield: 74%. Mp: 193 °C. 1H NMR (CDCl3): δ 8.54 (d, J = 4.5 Hz, 2H), 7.66 (t, J = 7.6 Hz, 2H), 7.60 (d, J = 7.8 Hz, 2H), 7.53 (d, J = 4.5 Hz, 2H), 7.48 (s, 1H), 7.40 (d, J = 7.8 Hz, 2H), 7.32 (t, J = 7.6 Hz, 2H), 7.30−7.20 (m, 11H), 7.09 (d, J = 8.1 Hz, 2H), 1.40 (s, 12H). 13C{1H} NMR (CDCl3): δ 156.0, 154.4, 151.5, 149.0, 147.0, 146.4, 143.0, 142.1, 139.9, 138.5, 135.3, 135.2, 134.9, 132.5, 131.7, 130.5, 130.1, 129.7, 128.4, 127.3, 126.8, 126.1, 125.0, 122.8, 122.6, 122.1, 120.9, 119.7, 119.2, 118.1, 117.4, 47.0, 27.1. MS: m/z 774 [M+]. Anal. Calcd for C51H38N2S2: C, 79.03; H, 4.94. Found: C, 78.86; H, 4.82. 2.7.7. Synthesis of the JK-107 Dye. A solution of D1 (0.07 g, 0.094 mmol), [{(t-Bu)4Pc}Ru(PhCN)2] (0.098 g, 0.094 mmol), and 4-carboxypyridine (0.0347 g, 0.2822 mmol) in THF (30 mL) was refluxed for 6 h. The solvent was evaporated. The pure product JK-107 was obtained by silica gel chromatography (eluent MC: MeOH = 10: 1, Rf = 0.3) to afford JK-107 in 40% yield. Mp: 231 °C. 1H NMR (DMSO-d6): δ 9.10 (m, 4H), 9.01 (m, 4H), 8.02 (m, 4H), 7.76 (t, J = 7.8

by Ultrafast Systems. The fundamental output of the CPA-2010 laser system (775 nm, 1 mJ per pulse, pulse width 150 fs) was split into two beams: a pump (95%) and a probe (5%). The pump beam was directed through a second harmonic generator to provide 387 nm excitation wavelength. The probe beam passed through an optical delay rail, allowing regulation of an appropriate delay time between the pump and the probe. 2.7. Synthetic Method of Sensitizers. 2.7.1. Synthesis of (E)-3-(6-(Bis(9,9-dimethyl-9H-fluoren-2-yl)amino)benzo[b]thiophen-2-yl)-2-(thiophen-2-yl)acrylonitrile (1). 6-(Bis(9,9dimethylfluoren-2-yl)amino)-2-formylbenzo[b]thiophene (0.49 g, 0.872 mmol) was dissolved in 30 mL of N,Ndimethylformamide (DMF), and acetic acid (0.15 mL) and piperidine (0.288 mL) were added to the solution, which was kept at 90 °C for 1 h. Thiophene-2-yl-acetonitrile (0.5 mL) was added to the solution, which was stirred at 90 °C for another 30 min. Addition of methanol (90 mL) to this solution system followed by cooling afforded crystals of the pure product 1 as a red solid. Yield: 82%. Mp: 197 °C. 1H NMR (CDCl3): δ 7.68 (t, J = 7.8 Hz, 2H), 7.63 (d, J = 8.1 Hz, 2H), 7.60 (s, 1H), 7.51 (s, 1H), 7.39 (t, J = 7.8 Hz, 2H), 7.40 (d, J = 8.1 Hz, 2H), 7.30−7.25 (m, 7H), 7.24 (s, 2H), 7.15 (d, J = 8.4 Hz, 2H), 7.07 (t, J = 5.1 Hz, 1H), 1.42 (s, 12H). 13C{1H} NMR (CDCl3): δ 155.4, 153.7, 147.4, 147.0, 143.0, 142.1, 139.1, 138.9, 135.7, 135.0, 133.9, 132.5, 129.5, 128.4, 127.1, 126.8, 126.2, 125.0, 123.8, 122.6, 122.2, 121.8, 120.9, 119.7, 119.2, 117.1, 115.4, 47.0, 27.1. MS: m/z 662 [M+]. Anal. Calcd for C45H34N2S2: C, 81.04; H, 5.14. Found: C, 80.88; H, 5.08. 2.7.2. (E)-3-(6-(Bis(9,9-dimethyl-9H-fluoren-2-yl)amino)benzo[b]thiophen-2-yl)-2-(5-bromothiophen-2-yl)acrylonitrile (2). Compound 1 (0.57 g) was dissolved in 30 mL of DMF at 40 °C and then 5 mL of DMF solution containing 0.89 g N-bromosuccinimide was added to this solution, which was stirred for 30 min. Addition of methanol (90 mL) to this solution system followed by cooling afforded crystals of the pure product 2 as a red solid. Yield: 82%. Mp: 191 °C. 1H NMR (CDCl3): δ 7.67 (d, J = 7.6 Hz, 2H), 7.65 (t, J = 8.7 Hz, 2H), 7.60 (s, 1H), 7.55 (s, 1H), 7.39 (t, J = 8.7 Hz, 2H), 7.35 (d, J = 7.6 Hz, 2H), 7.30−7.25 (m, 5H), 7.24 (s, 2H), 7.14 (d, J = 7.8 Hz, 2H), 7.09 (d, J = 4.2 Hz, 1H), 7.03 (d, J = 4.2 Hz, 1H), 1.42 (s, 12H). 13C{1H} NMR (CDCl3): δ 158.6, 156.4, 155.4, 153.9, 153.7, 147.6, 146.8, 143.1, 140.3, 138.9, 135.3, 135.1, 133.7, 132.7, 131.2, 130.1, 127.1, 126.8, 125.1, 123.8, 122.9, 122.2, 120.9, 119.7, 119.3, 116.5, 115.2, 47.0, 27.1. MS: m/z 746 [M+]. Anal. Calcd for C45H33BrN2S2: C, 72.47; H, 4.46. Found: C, 72.26; H, 4.38. 2.7.3. (E)-3-(6-(Bis(9,9-dimethyl-9H-fluoren-2-yl)amino)benzo[b]thiophen-2-yl)-2-(5-(pyridin-4-yl)thiophen-2-yl)acrylonitrile (D1). A mixture of 2 (0.40 g, 0.536 mmol), 4pyridineboronic acid pinacol ester (0.14 g, 0.697 mmol), Pd(PPh3)4 (0.03g, 0.027 mmol), and 2 M K2CO3 aqueous solution (2 mL) in THF (30 mL) was refluxed for 12 h. After cooling the solution, H2O (20 mL) and brine were added to the solution. The organic layer was separated and dried in MgSO4. The solvent was removed in vacuo. The pure product D1 was obtained by chromatographic workup (eluent EA: Hx = 1: 1, Rf = 0.3) as a red solid in 70% yield. Mp: 187 °C. 1H NMR (CDCl3): δ 8.63 (d, J = 4.5 Hz, 2H), 7.72 (d, J = 4.5 Hz, 2H),7.68 (t, J = 7.8 Hz, 2H), 7.63 (d, J = 8.1 Hz, 2H), 7.57 (s, 1H), 7.56 (s, 1H), 7.47 (d, J = 3.9 Hz, 2H), 7.39 (t, J = 7.8 Hz, 2H), 7.35 (d, J = 8.1 Hz, 2H), 7.32−7.25 (m, 5H), 7.24 (s, 2H), 7.15 (d, J = 7.8 Hz, 2H, 1.42 (s, 12H). 13C{1H} NMR (CDCl3): δ 156.3, 155.5, 154.6, 153.7, 150.6, 147.9, 146.7, C



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Scheme 1. Schematic Diagram for the Synthesis of JK-107 and JK-109

Hz, 2H), 7.68 (d, J = 8.1 Hz, 2H), 7.60 (s, 1H), 7.58 (s, 1H), 7.53 (m, 4H), 7.26 (m, 5H), 7.01 (m, 4H), 6.86 (d, J = 7.8 Hz, 2H), 5.88 (d, J = 6.1 Hz, 2H), 5.76 (d, J = 7.1 Hz, 2H), 2.41 (d, J = 6.1 Hz, 2H), 2.34 (d, J = 7.1 Hz, 2H), 1.68 (m, 36H), 1.32 (s, 12H). MS: m/z 1704 [M + ]. Anal. Calcd for C104H90N12O2RuS2: C, 73.26; H, 5.32. Found: C, 73.08; H, 5.27. 2.7.8. Synthesis of the JK-109 Dye. JK-109 was synthesized by the same procedure as JK-107, except that compound D2 (0.07 g, 0.09 mmol) was used instead of compound D1. Yield: 33%. Mp: 222 °C. 1H NMR (THF-d8): δ 9.12 (m, 4H), 8.95 (m, 4H), 7.96 (m, 4H), 7.63 (t, J = 8.1 Hz, 2H), 7.53 (d, J = 7.8 Hz, 2H), 7.51 (s, 1H), 7.39 (t, J = 8.1 Hz, 2H), 7.32 (d, J = 7.8 Hz, 2H), 7.20−7.05 (m, 8H), 7.02 (d, J = 7.5 Hz, 2H), 6.83 (s, 2H), 6.78 (d, J = 4.8 Hz, 1H), 5.55 (d, J = 6.2 Hz, 2H), 5.46 (d,

J = 7.1 Hz, 2H), 2.38 (d, J = 6.2 Hz, 2H), 2.31 (d, J = 7.1 Hz, 2H), 1.73 (m, 36H), 1.36 (s, 12H). MS: m/z 1735 [M+]. Anal. Calcd for C105H91N11O2RuS3: C, 72.64; H, 5.28. Found: C, 72.45; H, 5.19

3. RESULT AND DISCUSSION Scheme 1 illustrates the synthetic procedures of ruthenium phthalocyanine dyes (JK-107 and JK-109) starting from a 6bis(9,9-dimethylfluoren-2-yl)amino-2-benzo[b]thiophene. The key step in the synthesis of JK-107 and JK-109 is the synthesis of two organic ligands, D1 and D2. The pyridyl substituted ligand D1 was synthesized by condensation reaction of 2formylbenzo[b]thiophene and thiophene-2-yl-acetonitrile, bromination of 1 with NBS, followed by Suzuki coupling reaction of 2 with 4-pyridineboronic acid pinacol ester. The ligand D2 D



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organic ligand D2 gives a relatively poor spectral overlap. The emission intensity at 637 nm in JK-107 is decreased to 96% by a coordination interaction between the ligand D1 and RuPc3. On the other hand, the emission intensity at 562 nm in JK-109 is decreased to 58% (see Supporting Information, Figure S1). The strong diminishment in donor emission intensity in JK-107 is an indication of high energy transfer efficiency to the RuPc3 acceptor. To determine the photophysical properties, molecular orbital calculations of JK-107 and JK-109 sensitizers were performed with the B3LYP/3-31G*. The calculation illustrates that the HOMO of two sensitizers is delocalized over the π-conjugated system centered on the triphenylamino unit, and LUMO is delocalized over the carboxypyridine unit through the ruthenium phthalocyanine group. From these results, we could induce that the photoinduced electron transfer from JK-107 and JK-109 dyes to TiO2 electrode can efficiently occur by the HOMO → LUMO transition as shown in Figure 3.

was also synthesized by Stille reaction of 2-bromobenzo[b]thiophene and 2,2′-bithiophene-5-yltri-methylstannane, bromination of 3 with NBS in CH2Cl2, followed by Suzuki coupling reaction of 4 with 4-pyridineboronic acid pinacol ester. Two new sensitizers JK-107 and JK-109, were synthesized from the reaction of [{(t-Bu)4Pc}Ru(PhCN)2]19 with 1 equiv of the ligands D and three equivalent of the isonicotinic acid by refluxing THF with 40% yield. Figure 2 shows the electronic spectra of dyads together with those of the parent chromophores. The dye JK-107 presents

Figure 3. HOMO and LUMO surface plots for JK-107 and JK-109.

The photocurrent action spectra of the two supramolecular dyes are presented in the Figure 4B. The incident-photon-tocurrent conversion (IPCE) of JK-107 exceeds 50% in a broad spectral range from 480 to 640 nm, reaching its maximum of 58% at 508 nm. The spectrum tails off toward 850 nm, contributing to the broad spectral light harvesting. The photoaction spectrum clearly shows that the IPCE in the 440−580 nm region is remarkably enhanced when the antenna D1 is coordinated to the RuPc3. The J−V curves for the devices based on two sensitizers are depicted in Figure 4A. The JK-107 sensitized cell gave a short circuit photocurrent density (Jsc) of 10.43 mAcm−2, an open circuit voltage (Voc) of 0.45 V, and a fill factor (FF) of 0.44, affording an overall conversion efficiency (η) of 2.06%. Under the same conditions, the JK-109 sensitized cell gave Jsc of 3.39 mAcm−2, Voc of 0.32 V, and FF of 0.51, corresponding to η of 0.56%. For reference, the RuPc3 sensitized cell gave η of 0.24%. The 370% increase in photovoltaic performance is mainly attributed to the increase in short-circuit photocurrent (Jsc) caused by an increase in the EQE from 380 to 830 nm, while the open-circuit voltage (Voc) also increased. The rationale for a significant increase of Jsc and Voc in JK-107 relative to JK-109 is attributed to more efficient FRET and electron transfer from the antenna D1 to the RuPc3. To further probe the intramolecular electron transfer between the antenna D1 and RuPc3, we investigated the dynamics of excited the antenna D1 and JK-107 using transient absorption spectroscopy. Figure 5A and B show time-resolved transient absorption spectra recorded following 387 nm laser

Figure 2. Absorption spectra of (A) (a) D1 (red line) in THF (b) JK107 (purple line) in THF and (B) (a) D2 (blue line) in THF (b) JK109 (lime line) in THF. Emission spectra of (A) (c) D1 (red-dashed line) in THF and (B) (c) D2 (blue-dashed line) in THF. The emission spectra were obtained using the same solution by exciting at 493 nm for D1 and 440 nm for D2 at 298 K.

two strong bands with maxima at 500 and 626 nm. The band at 500 nm is assigned as D1 and one band at 626 nm as RuPc3. There is one new band at 570 nm. As the position of this band is sensitive to the nature of axial ligand D, this band can be assigned as charge transfer transition between the ruthenium metal and the ligand. Under similar conditions, the JK-109 sensitizer exhibits its maximum at 474 nm that is 35 nm blueshifted with respect to its cyano-vinylene counterpart. A significant red shift of D1 relative to D2 was derived from the delocalization over an entire conjugated system with an electron deficient character. The emission spectra of antennas D1 and D2 exhibit an emission band at 637 and 562 nm when excited at 500 and 440 nm, respectively. Organic antenna D1 has a significant emission overlap with the absorption spectrum of RuPc3, resulting in an efficient FRET. On the other hand, E



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pulse excitation of D1 in CH2Cl2 and JK-107 adsorbed onto a mesoscopic SiO2 film, repectively. In case of antenna D1, the difference spectrum of the singlet excited dye exhibits two broad absorption maximum around 600−730 nm and a strong bleaching in the region of the ground-state absorption (500 nm region). This transient species is attributed to the excited singlet state of the compound D1. From the exponential decay of the transient, the singlet excited state lifetime is calculated to be about 193 ps. On the other hand, the difference absorption spectra recorded following laser pulse excitation of the JK-107 adsorbed on the SiO2 film exhibits two bleaching at 500 and 650 nm arising from ground-state absorption of ligand (500 nm) and RuPc3 (650 nm) and a broad absorption maximum around ∼730 nm within short delay time region. Because SiO2 is an insulator, it serves as a neutral substrate, thus, not directly participating in any electron transfer process. The observed spectral characteristics correspond to the singlet excited dye. Interestingly, the two bleaching recoveries show different aspects. The bleaching recovery arising from the ligand (at 500 nm) is so fast compared to the bleaching recovery at 650 nm. The calculated bleaching recovery lifetime from exponential decay of the transient is about 1 ps, which is in good agreement with the lifetimes obtained from the lifetime of the transient at 730 nm (1 ps). On the basis of these results, it is evident that this bleaching (∼500 nm) and absorption (∼730 nm) arise from the antenna part in the JK-107. This lifetime of the transient is much faster than that of the D1. So we can conclude that this fast disappearance of ground state bleaching and excited state absorption is attributed to an efficient electron transfer from the ligand D1 to RuPc3. We also want to extend our studies of the time scale of electron injection to achieve an efficient DSSC function to

Figure 4. (A) J−V curve of (a) JK-107 (solid line) and (b) JK-109 (dashed line). The dark-current-bias potential is shown as dashed and dotted curves and (B) IPCE spectra of (a) JK-107 (circle symbol line) and (b) JK-109 (inverted triangle symbol line).

Figure 5. Differential absorption spectra obtained upon femtosecond flash photolysis (387 nm) of (A) ligand D1 in CH2Cl2 and (C) JK-107 on SiO2. Time-absorption profiles of (B) ligand D1 and (D) JK-107 on SiO2 at 500 and 730 nm. F



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case of the TiO2, the bleaching recovery at 635 nm (Figure 6b) exhibits a short- and long-lived component. The fast component that corresponds to a singlet excited state of JK107 decays with a lifetime of 9 ps. This faster decay represents deactivation of the excited state via a charge injection process (reaction 2). The long-lived component confirms the formation of cation radical of the dye following the charge injection into TiO2. To further understand the electron-injection property and the change in Voc of JK-107 and JK-109 more clearly, we measured the electron-diffusion coefficients and lifetimes of the photoelectrode. Figure 7 shows the electron diffusion coefficients and lifetimes of the DSSCs (employing JK-107 and JK-109, respectively) displayed as a function of the Jsc and Voc, respectively. There is no significant difference among the De values19 at the identical short-circuit current conditions. On the other hand, the τe values show a significant gap between the dyes. The τe values20 of JK-107 were much larger than those of JK-109 at the identical open-circuit voltage conditions. The results of the electron lifetime are also well consistent with those of the Voc shown in Figure 4 and Table S1. The ac impedance spectra of the cells were measured at the dark conditions. Figure 8 shows the ac impedance spectra of

axially coordinated ruthenium phthalocyanine JK-107. For this purpose, the sensitizer JK-107 was anchored on mesoscopic TiO2 and SiO2 films. These two films were subjected to 387 nm laser pulse excitation in a pump−probe spectrophotometer, and the excited state deactivation of the dye on two oxide surfaces were compared (reactions 1 and 2). SiO2 − JK107 + hν → SiO2 − 1JK107* → SiO2 − JK107 + h′

(1)

TiO2 − JK107 + hν → TiO2 − 1JK107* → TiO2 (e) − JK107+•

(2)

The recovery of the bleach following the laser pulse excitation of JK-107 linked to SiO2 and TiO2 films are shown in Figures 6. On SiO2, the odbersrved spectral fingerprints

Figure 6. Time-absorption profiles of (a) JK-107 on SiO2 (black circle) and (b) JK-107 (red circle) on TiO2 at 635 nm. Figure 8. Electrochemical impedance spectra measured under the dark for the cells with different dye adsorption conditions.

correspond to silget excited state with a bleaching at 500 and 635 nm and broad absorption around 730 nm (Figure 5c). The lifetime of JK-107* as monitored from the bleaching recovery at 635 nm (Figure 6a) was ∼180 ps (reaction 1). Because SiO2 is an insulator, it does not directly participate in the electron transfer process. This is also evident from the single exponential decay kinetics of the excited state. On the other hand, in the

the DSSCs measured in dark conditions. In the dark under forward bias (−0.68 V), the semicircle in intermediate frequency regime demonstrates the dark reaction impedance caused by the electron transport from the conduction band of TiO2 to I3− ions in electrolyte.21 A larger radius of the

Figure 7. Electron diffusion coefficients (A) and lifetimes (B) of the photovoltaic cells employing JK-107 and JK-109, respectively. G



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(4) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt(II/III)-Based Redox Electrolyte Exceed 12% Efficiency. Science 2011, 334, 629−634. (5) Yum, J. H.; Jang, S. R.; Walter, P.; Geiger, T.; Nüesch, F.; Kim, S.; Ko, J.; Grätzel, M.; Nazeeruddin, M. K. Efficient Co-Sensitization of Nanocrystalline TiO2 Films by Organic Sensitizers. Chem. Commun. 2007, 4680−4682. (6) Clifford, J. N.; Palomares, E.; Nazeeruddin, M. K.; Thampi, R.; Grätzel, M.; Durrant, J. R. Multistep Electron Transfer Processes on Dye Co-sensitized Nanocrystalline TiO2 Films. J. Am. Chem. Soc. 2004, 126, 5670−5671. (7) Choi, H.; Kim, S.; Kang, S. O.; Ko, J.; Kang, M.-S.; Clifford, J. N.; Forneli, A.; Palomares, E.; Nazeeruddin, M. K.; Grätzel, M. Stepwise Cosensitization of Nanocrystalline TiO2 Films Utilizing Al2O3 Layers in Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2008, 47, 8259− 8263. (8) Hardin, B. E.; Hoke, E. T.; Armstrong, P. B.; Yum, J.-H.; Comte, P.; Torres, T.; Frétchet, J. M.; Nazeeruddin, M. K.; Grätzel, M.; McGehee, M. D. Increased Light Harvesting in Dye-Sensitized Solar Cells with Energy Relay Dyes. Nat. Photonics 2009, 3, 406−411. (9) Siegers, C.; Hohl-Ebinger, J.; Zimmermann, B.; Würfel, U.; Mülhaupt, R.; Hinsch, A.; Haag, R. A Dyadic Sensitizer for Dye Solar Cells with High Energy-Transfer Efficiency in the Device. ChemPhysChem 2007, 8, 1548−1556. (10) Odobel, F.; Zabri, H. Preparations and Characterizations of Bichromophoric Systems Composed of a Ruthenium Polypyridine Complex Connected to a Difluoroborazaindacene or a Zinc Phthalocyanine Chromophore. Inorg. Chem. 2005, 44, 5600−5611. (11) Warnan, J.; Buchet, F.; Pellegrin, Y.; Blart, E.; Odobel, F. Panchromatic Trichromophoric Sensitizer for Dye-Sensitized Solar Cells Using Antenna Effect. Org. Lett. 2011, 13, 3944−3947. (12) Amadelli, R.; Argazzi, R.; Bignazzi, C. A.; Scandola, F. Design of Antenna-Sensitizer Polynuclear Complexes. Sensitization of Titanium Dioxide with [Ru(bpy)2(CN)2]2Ru(bpy(COO)2)22−. J. Am. Chem. Soc. 1990, 112, 7099−7103. (13) Tian, H.; Yang, X.; Pan, J.; Chen, R.; Liu, M.; Zhang, Q.; Hagfeldt, A.; Sun, L. A Triphenylamine Dye Model for the Study of Intramolecular Energy Transfer and Charge Transfer in DyeSensitized Solar Cells. Adv. Funct. Mater. 2008, 18, 3461−3468. (14) Warnan, J.; Pellegrin, Y.; Blart, E.; Odobel, F. Supramolecular Light Harvesting Antennas to Enhance Absorption Cross-Section in Dye-Sensitized Solar Cells. Chem. Commun. 2012, 48, 675−677. (15) O’Regan, B. C.; López-Duarte, I.; Martinez-Diaz, M. V.; Forneli, A.; Albero, J.; Morandeira, A.; Palomares, E.; Torres, T.; Durrant, J. R. Catalysis of Recombination and Its Limitation on Open Circuit Voltage for Dye Sensitized Photovoltaic Cells Using Phthalocyanine Dyes. J. Am. Chem. Soc. 2008, 130, 2906−2907. (16) Morandeira, A.; López-Duarte, I.; O’Regan, B.; Martinez-Diaz, M. V.; Forneli, A.; Palomares, E.; Torres, T.; Durrant, J. R. Ru(II)phthalocyanine Sensitized Solar Cells: The Influence of CoAdsorbents Upon Interfacial Electron Transfer Kinetics. J. Mater. Chem. 2009, 19, 5016−5026. (17) Choi, H.; Lee, J. K.; Song, K.; Kang, S. O.; Ko, J. Novel Organic Dyes Containing Bis-Dimethylfluorenyl Amino Benzo[b]thiophene for Highly Efficient Dye-Sensitized Solar Cell. Tetrahedron 2007, 63, 3115−3121. (18) Bilge, A.; Zen, A.; Forster, M.; Li, H.; Galbrecht, F.; Nehls, B. S.; Farrell, T.; Neher, D.; Scherf, U. Swivel-Cruciform Oligothiophene Dimers. J. Mater. Chem. 2006, 16, 3177−3182. (19) Rawling, T.; Xiao, H.; Lee, S.-T.; Colbran, S. B.; McDonagh, A. M. Optical and Redox Properties of Ruthenium Phthalocyanine Complexes Tuned with Axial Ligand Substituents. Inorg. Chem. 2007, 48, 2805−2813. (20) Nakade, S.; Kanzaki, T.; Wada, Y.; Yanagida, S. Stepped LightInduced Transient Measurements of Photocurrent and Voltage in Dye-Sensitized Solar Cells: Application for Highly Viscous Electrolyte Systems. Langmuir 2005, 21, 10803−10807.

semicircle in this intermediate frequency regime implies a lower rate of electron recombination at the TiO2/dye/electrolyte interface. In dark, the radius of the intermediate-frequency semicircle showed the increasing order of JK-109 (177.5 Ω) < JK-107 (216.3 Ω), in accord with the trends of the Voc.

4. CONCLUSION In conclusion, we meticulously designed two organic antennas D1 and D2 for RuPc3 in which one antenna has a significant emission overlap and another one has a poor emission overlap with the absorption spectrum of RuPc3 to check to what extent an efficient FRET occurs between the antennas and RuPc3. The incorporation of organic antennas D bound to the RuPc3 via a coordinative interaction leads to panchromatic response and shows a remarkable photovoltaic performance to antennafree system. A significant enhancement of photocurrent in JK107 is attributed to the efficient FRET and intramolecular electron transfer from the antenna D1 to the acceptor RuPc3. We demonstrate the direct evidence of the electron transfer process using transient absorption spectroscopy. This concept could be applicable to a variety of fields such as organic photovoltaic, solid-state dye sensitized cells, and hybrid quantum dot solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

Optical and DSSC performance parameters, orbital energy level data, emission spectra of dyes are shown in the Tables S1 and S2 and Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +82-44-860-1331. Tel.: +84-44-860-1337. E-mail: jko@ korea.ac.kr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support for this work by the WCU (the Ministry of Education and Science) program (No. R312012-000-10035-0), the International Science and Business Belt Program through the Ministry of Education, Science and Technology (No. 2012K001573), ERC (the Korean government (MEST)) program (No. 2012-0000591), and the Converging Research Center Program through the Ministry of Education, Science and Technology (2012K001287).

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REFERENCES

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(21) Wang, Q.; Moser, J.-E.; Grätzel, M. Electrochemical Impedance Spectroscopic Analysis of Dye-Sensitized Solar Cells. J. Phys. Chem. B 2005, 109, 14945−14953.

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