Pyrrole-Based Organic Dyes for Dye-Sensitized Solar Cells

J. Phys. Chem. C 2008, 112, 12557–12567

12557

Pyrrole-Based Organic Dyes for Dye-Sensitized Solar Cells Yung-Sheng Yen,† Ying-Chan Hsu,† Jiann T. Lin,*,† Che-Wei Chang,† Chao-Ping Hsu,*,† and Da-Jong Yin‡ Institute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan, and EVerlight Chemical Industrial Corporation, 328 Taoyuan, Taiwan ReceiVed: February 4, 2008; ReVised Manuscript ReceiVed: May 14, 2008

Series of dipolar dyes containing pyrrole-based conjugate spacers between arylamine donor and the 2-cyanoacrylic acid acceptor have been synthesized. These compounds exhibit both π-π*and charge-transfer transition in the absorption spectra. Quantum computation results indicate that the pyrrole moieties have larger dihedral angles with its neighboring units, leading to less interaction between the units and, therefore, better charge separation. It also acts as a secondary electron donor upon photoexcitation. The oxidation potentials of arylamine in these compounds are affected by the heteroaromatic conjugated spacers. Dye-sensitized solar cells (DSSCs) using these materials as the sensitizers exhibit good performance, with conversion efficiencies ranging from 4.77% to 6.18%. These values are 66-86% of the standard cell from N719 (7.19%). Introduction Because of energy deficit as well as environmental concern, renewable energy resources such as hydrogen fuel, biomass fuel, wind, hydro, geothermal, and solar energies receive worldwide attention.1 Among these, solar energy appears to be more attractive because of its unlimited supply.2 While crystalline and amorphous silicon, and inorganic semiconductors dominate current photovoltaics, they suffer from two drawbacks: higher cost and incompatibility with flexible substrates. Consequently, both organic semiconductor-based solar cells3 and dye-sensitized solar cells (DSSCs)4 were also extensively studied in the past decade. Significant progress has been made and up to 5% and 11% efficiencies have been achieved for organic semiconductorbased solar cells5 and DSSCs, respectively.6 Though dye sensitization for photoelectric power generation was realized around 1970,7 no breakthrough was made until Gra¨tzel’s seminal report on DSSC using cis-di(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) (N3) as a sensitizer.8 Since then quite a few DSSCs using similar Ru(II) dyes containing bipyridine9 or terpyridine ligands10 (such as black dye) have been reported with efficiencies as high as ∼10%, a value close to amorphous silicon-based photovoltaic cell. These Ru(II) complexes possess strong metal-to-ligand charge transfer (MLCT) with absorption extending to >700 nm when adsorbed on TiO2. Photoexcitation of MLCT leads to transfer of an electron from the metal to the π* orbital of the bipyridine ligand anchored on the TiO2 surface, with subsequent electron injection into the conduction band of TiO2. Metal-free dipolar organic compounds possessing charge-transfer character, when anchored on the TiO2 surface, are expected to behave similarly upon photoexcitation of their charge-transfer band. Impressively high efficiency of 9% has also been achieved for DSSC based on a metal-free sensitizer, indoline dye.11 The high performance of this device can be attributed partly to the high molar extinction coefficient of the sensitizer, 68 700 M-1 cm-1 (at λ 526 nm). In contrast to Ru(II) dyes which normally * Corresponding authors. E-mail: [email protected]; cherri@ chem.sinica.edu.tw. † Academia Sinica. ‡ Everlight Chemical Industrial Corporation.

have molar extinction coefficients lower than 20 000 M-1 cm-1 at the absorption maximum, it is relatively easy for sheer organic dyes to have high molar extinction coefficients. Besides, they can generally be prepared at a lower cost. The number of metalfree organic sensitizers has rapidly grown in recent years, and representative dyes include coumarin-,12 indoline-,13 cyanine-,14 hemicyanine-,15 merocyanine-,16 perylene-,17 xanthene-,18 triarylamine-,19 oligoene-,20 and thiophene-based compounds.21 Besides high extinction coefficients, the broader spectral coverage (especially in the red to near-infrared region) is also considered important for sensitizers to have better harvest of light.22 Appropriate use of electron-excessive heteroaromatic rings such as thiophene, furan, and pyrrole as the spacer between an electron donor and an electron acceptor was reported to be beneficial to red shifting the wavelength of charge-transfer transition, nevertheless with negligible tradeoff for stability.23 A few reports on using thiophene-based spacer exhibiting good device efficiencies have appeared.12b,21 So far there is no report on sensitizers with pyrrole in the spacer. Thereby we set out to develop and evaluate the potential of pyrrole-based sensitizers. A series of new pyrrole-based dyes have been synthesized, and DSSCs using these pyrrole-based sensitizers will be discussed. Experimental Section General Information. Unless otherwise specified, all the reactions were performed under nitrogen atmosphere with standard Schlenk techniques. THF was distilled from sodium and benzophenone under nitrogen atmosphere. DMF was distilled from CaH2 under nitrogen atmosphere. Dichloromethane for spectroscopic measurements was distilled from calcium hydride under nitrogen atmosphere. 1H and 13C NMR spectra were recorded on a Bruker 400 MHz spectrometer operating at 400.13 and 100.03 MHz, respectively. Absorption spectra were recorded on a Perkin-Elmer spectrofluorometer. All chromatographic separations were carried out on silica gel (60M, 230-400 mesh). Mass spectra (FAB) were recorded on a VG70-250S mass spectrometer. Elementary analyses were performed on a Perkin-Elmer 2400 CHN analyzer. 4-(1-Methyl-1H-pyrrole-2-yl)-N,N-diphenylaniline (1a). A mixture of 4-bromo-N,N-diphenylaniline (3.24 g, 10 mmol),

10.1021/jp801036s CCC: $40.75  2008 American Chemical Society Published on Web 07/17/2008

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Yen et al.

Figure 1. The structure of the dyes.

1-methyl-2-(tributylstannyl)-1H-pyrrole (4.0 g, 10.8 mmol), PdCl2(PPh3)2 (0.070 g, 0.1 mmol), and dry DMF was placed into a two-necked flask under nitrogen atmosphere and stirred overnight at 100 °C. After cooling, it was quenched with aqueous KF and extracted with diethyl ether. The organic extracts were washed with brine solution and dried over anhydrous MgSO4. The crude product was purified by column chromatography, using a dichloromethane/hexane mixture as eluent to give 1a as a viscous oil. 1H NMR (CDCl3) δ 7.36-7.30 (m, 6H), 7.11-7.05 (m, 8H), 6.74 (t, J ) 2.0 Hz, 1H), 6.13 (dd, J ) 3.6, 2.0 Hz, 1H), 6.07(dd, J ) 3.6, 2.0 Hz, 1H), 3.69 (s, 3H). MS (FAB) m/z 324.2. 4-(5-(4-(N,N-Diphenylamino)phenyl)-1-methyl-1H-pyrrol2-yl)benzaldehyde (1b). 1b was prepared from 4-(1-methyl5-(tributylstannyl)-1H-pyrrole-2-yl)-N,N-diphenylaniline and 4-bromobenzaldehyde as described above for 1a. Orange solid. Spectroscopic data for 1b: 1H NMR (CDCl3) δ 9.99 (s, 1H), 7.89 (d, 8.4 Hz, 2H), 7.60 (d, J ) 8.4 Hz, 2H), 7.31-7.24 (m, 6H), 7.14-7.03 (m 6H), 6.45 (d, J ) 4.0 Hz, 1H), 6.30 (d, J ) 4.0 Hz, 1H), 3.66 (s, 3H). MS (FAB) m/z 428.2. (E)-2-Cyano-3-[4-(5-(4-(N,N-diphenylamino)phenyl)-1-methyl-1H-pyrrol-2-yl)phenyl]acrylic Acid (1). To a flask containing a mixture of 1b (0.80 g, 1.87 mmol), cyanoacetic acid (0.20 g, 2.43 mmol), and ammonium acetate (0.040 g, 0.60 mmol) was added acetic acid (10 mL). The mixture was heated at 120 °C for 8 h and allowed to cool to room temperature. The resulting solid was filtered and washed with distilled water, diethyl ether, and methanol to give a dark brown solid. The pure product was obtained by silica gel chromatography to afford 1. Spectroscopic data for 1: 1H NMR (CDCl3) δ 8.32 (s, 1H), 8.11 (d, J ) 8.4 Hz, 2H), 7.64 (d, J ) 8.4 Hz, 2H), 7.36-7.06 (m, 8H), 6.55 (d, J ) 4 Hz, 1H), 6.35 (d, J ) 4 Hz, 1H), 3.73 (s, 3H). 13C NMR (CDCl3) δ 166.5, 155.7, 147.4, 147.2, 139.8, 138.8, 135.2, 132.0, 129.5, 129.3, 128.7, 127.9, 126.3, 124.7, 123.2, 122.9, 115.6, 111.6, 109.3, 100.0, 35.0. MS (FAB) m/z 495.2. Anal. Calcd for C33H25N3O2: C, 79.98; H, 5.08; N, 8.48. Found: C, 79.48; H, 5.15; N, 8.22. 5-[5-(4-(Diphenylamino)phenyl)1-methyl-1H-pyrrol-2-yl]thiophene-2-carbaldehyde (2a). 2a was prepared from 4-(1methyl-5-(tributylstannyl)-1H-pyrrol-2-yl)-N,N-diphenyla-

niline and 5-bromo-2-thiophenecarboxaldehyde as described above for 1b. Spectroscopic data for 2a: 1H NMR (CDCl3) δ 9.84 (s, 1H), 7.69 (d, J ) 4.0 Hz, 1H), 7.29-7.24 (m, 5H), 7.16-7.04 (m, 10H), 6.60 (d, J ) 4.0 Hz), 6.25 (d, J ) 4 Hz, 1.0H), 3.76 (s, 3H). MS (FAB) m/z 434.2. (E)-2-Cyano-3-(5-(5-(4-(diphenylamino)phenyl)-1-methyl1H-pyrrol-2-yl)thiophen-2-yl)acrylic Acid (2). 2 was synthesized by the same procedure as described above for 1. Spectroscopic data for 2: 1H NMR (acetone-d6) δ 8.41 (s, 1H), 7.96 (d, J ) 4.0 Hz, 1H), 7.44-7.41 (m, 3H), 7.36-7.32 (m, 4H), 7.13-7.08 (m, 8H), 6.71 (d, J ) 4.0 Hz, 1H), 6.33 (d, J ) 4.0 Hz, 1H), 3.87 (s, 3H). 13C NMR (CDCl3) δ 167.3, 147.6, 147.4, 147.2, 140.7, 139.8, 132.9, 129.6, 129.4, 128.3, 125.5, 124.9, 124.1, 123.4, 122.7, 116.2, 113.3, 110.1, 95.0, 34.8. MS (FAB) m/z 501.2. Anal. Calcd for C31H23N3O2S: C, 74.23; H, 4.62; N, 8.38. Found: C, 73.86; H, 4.87; N, 8.76. 2-(7-Bromo-9,9-diethyl-9H-fluoren-2-yl)-1-methyl-1H-pyrrole (3a). 3a was obtained from 1-methyl-2-(tributylstannyl)1H-pyrrole and 2,7-dibromo-9,9-diethyl-9H-fluorene as described above for 1a. Spectroscopic data for 3a: 1H NMR (CDCl3) δ 7.66 (d, J ) 8.4 Hz, 1H), 7.54 (d, J ) 8.4 Hz, 1H), 7.46- 7.43 (m, 2H), 7.36 (dd, J ) 8.0, 1.6 Hz, 2H), 7.31 (dd, J ) 1.6, 0.4 Hz, 1H), 6.73 (t, J ) 2.4 Hz, 1H), 6.27 (dd, J ) 3.2, 2.4 Hz, 1H), 6.21 (dd, J ) 3.2, 2.4 Hz, 1H), 3.68 (s, 3H), 2.03-1.97 (m, 4H), 0.35 (t, J ) 7.6 Hz, 6H). MS (FAB) m/z 380.2. 9,9-Diethyl-7-(1-methyl-1H-pyrrol-2-yl)-N,N-diphenyl-9Hfluoren-2-amine (3b). A stirred mixture of 2-(7-bromo-9,9diethyl-9H-fluoren-2-yl)-1-methyl-1H-pyrrole (3a, 3.0 g, 7.90 mmol), diphenylamine (1.54 g, 9.10 mmol), sodium tertbutoxide (1.14 g 11.85 mmol), Pd(dba)2 (0.090 g 0.16 mmol), and tri-tert-butylphosphine (65 mg, 0.32 mmol) in toluene (50 mL) was heated to 80 °C for 8 h. The reaction was quenched by addition of water and extracted with diethyl ether. The organic extract was dried over anhydrous MgSO4 and evaporated under vacuum. The residue was purified by column chromatography on silica gel by eluting with hexane/dichloromethane (4:1). Spectroscopic data for 3b: 1H NMR (CDCl3) δ 7.75 (d, J ) 8.0 Hz, 1H), 7.72 (d, J ) 8.0 Hz, 1H), 7.43-7.39 (m, 2H), 7.32-7.28 (m, 4H), 7.16 (d, J ) 2.0 Hz, 1H), 7.09- 7.01 (m,

Dye-Sensitized Solar Cells

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12559

SCHEME 1: Outline of the Synthetic Scheme for Dyesa

a Reagents and conditions: (i) PdCl2(PPh3)2. (ii) (a) n-BuLi, (b) Bu3SnCl. (iii) PdCl2(PPh3)2, 2-bromothiophene-5-carbaldehyde or 1-bromo-4benzaldehyde. (iv) Cyanoacetic acid, NH4OAc, acetic acid. (v) Pd(dba)2, Na OtBu, P(tBu)3. (vi) NaCN, DMF. (vii) MeNH2, reflux. (viii) (a) n-BuLi, (b) DMF, (c) 2 N HCl.

7H), 6.78 (dd, J ) 4.0, 2.0 Hz, 1H), 6.20 (t. J ) 2.0 Hz, 1H), 6.10 (dd, J ) 4.0, 2.0 Hz, 1H), 3.72 (s, 3H), 2.01-1.92 (m, 4H), 0.37 (t, J ) 7.6 Hz, 6H). MS (FAB) m/z 468.2. 5-[N-Methyl-2-(7-diphenylamino-9,9-diethyl-9H-fluoren2-yl)-pyrrol-5-yl]thiophene-2-carbaldehyde (3c). 3c was obtained from N-methyl-2-(7-diphenylamino-9,9-diethyl-9Hfluoren-2-yl)-5-tributylstannylpyrrole and 5-bromo-2-thiophenecarboxaldehyde as described above for 1b. Spectroscopic data for 3c: 1H NMR (CDCl3) δ 9.91 (s, 1H), 7.95 (d, J ) 4.0 Hz, 1H), 7.82 (d, J ) 8.0 Hz, 1H), 7.76 (d, J ) 8.0 Hz, 1H), 7.53-7.48 (m, 2H), 7.40 (d, J ) 4.0 Hz, 1H), 7.33-7.29 (m, 4H), 7.17-7.03 (m, 8H), 6.67 (d, J ) 4.0 Hz, 1H), 6.38 (d, J ) 4.0 Hz, 1H), 3.87 (s, 3H), 2.12-2.04 (m, 2H), 1.99-1.94 (m, 2H), 0.39 (t, J ) 7.6 Hz, 6H). MS (FAB) m/z 578.2. (E)-2-Cyano-3-[5-(N-methyl-2-(7-diphenylamino-9,9-diethyl-9H-fluoren-2-yl)-pyrrol-5-yl)thiophen-2-yl]acrylic Acid (3). 3 was synthesized by the same procedure as described above for 1. Spectroscopic data for 3: 1H NMR (THF-d8) δ 8.33 (s, 1H), 7.83 (d, J ) 4.0 Hz, 1H), 7.73 (d, J ) 8.0 Hz, 1H), 7.64 (d, J ) 8.0 Hz, 1H), 7.46 (s, 1H), 7.44 (d, J ) 8.0 Hz, 1H), 7.33 (d, J ) 4.0 Hz, 1H), 7.25-7.21 (m, 4H), 7.15 (d, J ) 2.0 Hz, 1H), 7.11-6.96 (m, 7H), 6.69 (d, J ) 4.0 Hz, 1H), 6.35 (d, J ) 4.0 Hz, 1H), 3.85 (s, 3H), 2.06-1.91 (m, 4H), 0.39 (t, J ) 7.2 Hz, 6H). 13C NMR (THF-d8) δ 164.6, 152.5, 151.2, 149.2, 148.7, 146.5, 142.0, 141.8, 140.1, 137.4, 134.7, 131.8, 130.2, 129.8, 128.9, 125.1, 125.0, 124.6, 124.1, 123.6, 121.6, 120.2, 117.1, 113.4, 111.0, 98.7, 57.2, 35.4, 33.5, 9.1. MS (FAB) m/z 645.2 Anal. Calcd for C42H35N3O2S: C, 78.11; H, 5.46; N, 6.51. Found: C, 78.37; H, 5.48; N, 6.25.

N-Methyl-2-(7-(naphthalen-1-yl(phenyl)amino)-9,9-diethyl-9H-fluoren-2-yl)pyrrole (4a). 4a was obtained from N-methyl-2-(7-bromo-9,9-diethyl-9H-fluoren-2-yl)pyrrole (3a) and N-phenylnaphthalen-1-amine as described above for 3b. Spectroscopic data for 4a: 1H NMR (acetone-d6) δ 7.98 (d, J ) 8.0 Hz, 2H), 7.87 (d, J ) 8.0 Hz, 1H), 7.70 (d, J ) 8.0 Hz, 1H), 7.64 (d, J ) 8.0 Hz, 1H), 7.56 (t, J ) 8.0 Hz, 1H), 7.49 (t, J ) 8.0 Hz, 1H), 7.40-7.36 (m, 4H), 7.27-7.23(m, 2H), 7.13 (d, J ) 2.0 Hz, 1H), 7.04-6.93 (m, 4H), 6.77 (t, J ) 4.0 Hz, 1H), 6.18 (dd, J ) 4.0, 2.0 Hz, 1H), 6.08 (dd, J ) 4.0, 2.0 Hz, 1H), 3.70 (s, 3H), 2.04-1.83 (m, 4H), 0.33 (t, J ) 7.6 Hz). MS (FAB) m/z 518.2. 5-[N-Methyl-2-(7-(naphthalen-1-yl(phenyl)amino)-9,9-diethyl-9H-fluoren-2-yl)-pyrrol-5-yl]thiophene-2-carbaldehyde (4b). 4b was obtained from N-methyl-2-(7-(naphthalen1-yl(phenyl)amino)-9,9-diethyl-9H-fluoren-2-yl)-5tributylstannylpyrrole and 5-bromo-2-thiophenecarboxaldehyde as described above for 1b. Spectroscopic data for 4b: 1H NMR (acetone-d6) δ 9.91 (s, 1H), 7.99 (d, J ) 8.0 Hz, 1H), 7.97 (d, J ) 8.0 Hz, 1H), 7.94 (d, J ) 4.0 Hz, 1H), 7.88 (d, J ) 8.0 Hz, 1H), 7.77 (d, J ) 8.0 Hz, 1H), 7.68 (d, J ) 8.0 Hz, 1H), 7.57 (t, J ) 8.0 Hz, 1H), 7.52-7.45 (m, 3H), 7.40-7.36 (m, 3H), 7.28-7.23 (m, 2H), 7.13 (d, J ) 2.0 Hz, 1H), 7.06-6.94 (m, 4H), 6.66 (d, J ) 4.0 Hz, 1H), 6.36 (d, J ) 4.0 Hz, 1H), 3.86 (s, 3H), 2.06-2.01 (m, 2H), 1.92-1.84 (m, 2H), 0.35 (t, J ) 7.6 Hz, 6H). MS (FAB) m/z 628.2. (E)-2-Cyano-3-[5-(N-methyl-2-(7-(naphthalen-1-yl(phenyl)amino)-9,9-diethyl-9H-fluoren-2-yl)-pyrrol-5-yl)thiophen-

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Figure 2. (a) Absorption and (b) emission spectra of the dyes recorded in THF.

2-yl]acrylic Acid (4). 4 was synthesized by the same procedure as described above for 1. Spectroscopic data for 4: 1H NMR (THF-d8) δ 8.32 (s, 1H), 7.95 (d, J ) 8.0 Hz, 1H), 7.90 (d, J ) 8.0 Hz, 1H), 7.83 (d, J ) 4.0 Hz, 1H), 7.78 (d, J ) 8.0 Hz, 1H), 7.69 (d, J ) 8.0 Hz, 1H), 7.57 (d, J ) 8.0 Hz, 1H), 7.47 (t, J ) 8.0 Hz, 1H), 7.43-7.28 (m, 6H), 7.18 (t, d ) 8.0 Hz, 2H), 7.11 (d, J ) 2.0 Hz, 1H), 7.05 (d, J ) 8.0 Hz, 2H), 6.95-6.91 (m, 2H), 6.69 (d, J ) 4.0 Hz, 1H), 6.34 (d, J ) 4.0 Hz, 1H), 3.84 (s, 3H), 2.01-1.83 (m, 4H), 0.32 (t, J ) 7.2 Hz). 13C NMR (THF-d8) δ 164.4, 152.4, 151.0, 150.0, 149.6, 146.7, 146.6, 145.1, 142.2, 141.9, 140.1, 136.7, 136.4, 134.7, 132.1, 131.5, 130.1, 129.7, 129.4, 128.8, 127.7, 127.2, 127.0, 126.9, 125.4, 125.1, 124.1, 123.1, 122.8, 122.5, 121.5, 120.0, 117.9, 117.0, 113.5, 110.9, 98.4, 57.1, 35.3, 33.5, 9.1. MS (FAB) m/z 695.2 Anal. Calcd for C46H37N3O2S: C, 79.40; H, 5.36; N, 6.04. Found: C, 78.88; H, 5.40; N, 5.88. N-Methyl-2-(3-bromo-9-hexyl-9H-carbazol-6-yl)pyrrole (5a). 5a was obtained from N-methyl-2-tributylstannylpyrrole and 3,6dibromo-9-hexyl-9H-carbazole as described above for 1a. Spectroscopic data for 5a: 1H NMR (acetone-d6) δ 8.37 (d, J ) 2.0 Hz, 1H), 8.22 (d, J ) 2.0 Hz, 1H), 7.56-7.48 (m, 4H), 6.76 (t, J ) 2.4 Hz), 6.19 (dd, J ) 3.2, 2.4 Hz, 1H), 6.12 (dd, J ) 3.2, 2.4 Hz, 1H), 4.37 (t, J ) 7.2 Hz, 2H), 3.71 (s, 3H), 1.85-1.81 (m, 2H), 1.56-1.25 (m, 6H), 0.84 (t, J ) 7.2 Hz, 3H). MS (FAB) m/z 409.2. N-Methyl-2-(3-diphenylamino-9-hexyl-9H-carbazol-6-yl)pyrrole (5b). 5b was obtained from N-methyl-2-(3-bromo-9-hexyl9H-carbazol-6-yl)pyrrole (5a) and diphenylamine as described above for 3b. Spectroscopic data for 5b: 1H NMR (acetone-d6)

Yen et al. δ 8.10 (d, J ) 2.0 Hz, 1H), 8.02 (d, J ) 2.0 Hz, 1H), 7.62-7.50 (m, 3H), 7.28-7.21 (m, 5H), 7.06-6.92 (m, 6H), 6.72 (t, J ) 3.6, 2.4 Hz, 1H), 6.12 (dd, J ) 3.6, 2.4 Hz, 1H), 6.06 (dd, J ) 3.6, 2.4 Hz, 1H), 4.45 (t, J ) 7.2 Hz, 3H), 1.97-1.87 (m, 2H), 1.57-1.30 (m, 6H), 0.87 (t, J ) 7.2 Hz, 3H). MS (FAB) m/z 497.2. 5-[N-Methyl-2-(3-diphenylamino-9-hexyl-9H-carbazol-6yl)pyrrol-5-yl]thiophene-2-carbaldehyde (5c). 5c was obtained from N-methyl-2-(3-diphenylamino-9-hexyl-9H-carbazol-6-yl)5-tributylstannylpyrrole and 5-bromo-2-thiophenecarboxaldehyde as described above for 1b. Spectroscopic data for 5c: 1H NMR (acetone-d6) δ 9.89 (s, 1H), 8.21 (s, 1H), 8.03 (d, J ) 2.0 Hz, 1H), 7.93 (d, J ) 4.0 Hz, 1H), 7.70-7.59 (m, 3H), 7.37 (d, J ) 4.0 Hz, 1H), 7.31-7.23 (m, 5H), 7.05 (d, J ) 8.0 Hz, 4H), 6.95 (t, J ) 7.2 Hz, 2H), 6. 55 (d, J ) 4.0 Hz, 1H), 6.31 (d, J ) 4.0 Hz, 1H), 4.48 (t, J ) 7.2 Hz, 2H), 3.85 (s, 3H), 1.96-1.89 (m, 2H), 1.58-1.31 (m, 6H), 0.87 (t, J ) 7.2 Hz, 3H). MS (FAB) m/z 607.2. (E)-2-Cyano-3-[5-(N-methyl-2-(3-diphenylamino-9-hexyl9H-carbazol-6-yl)pyrrol-5-yl)thiophen-2-yl]acrylic Acid (5). 5 was synthesized by the same procedure as described above for 1. Spectroscopic data for 5: 1H NMR (acetone-d6) δ 8.40 (s, 1H), 8.25 (d, J ) 1.2 Hz, 1H), 8.05 (d, J ) 2.0 Hz, 1H), 7.95 (d, J ) 4.0 Hz, 1H), 7.71-7.61 (m, 3H), 7.43 (d, J ) 4.0 Hz, 1H), 7.31-7.22 (m, 5H), 7.07-6.94 (m, 6H), 6.73 (d, J ) 4.0 Hz, 1H), 6.36 (d, J ) 4.0 Hz, 1H), 4.49 (t, J ) 7.2 Hz, 3H), 1.97-1.92 (m, 2H), 1.50-1.29 (m, 6H), 0.87 (t, J ) 7.2 Hz, 3H). 13C NMR (CDCl3) δ 167.8, 148.6, 147.9, 147.3, 141.9, 140.5, 140.0, 138.0, 135.2, 132.6, 129.1, 127.9, 127.0, 125.9, 125.0, 123.9, 123.5, 122.7, 122.6, 121.6, 121.5, 121.2, 118.7, 116.2, 113.4, 110.3, 109.8, 108.9, 94.5, 43.4, 31.5, 29.0, 27.0, 22.5, 14.0. MS (FAB) m/z 674.2 Anal. Calcd for C46H37N3O2S: C, 76.53; H, 5.68; N, 8.30. Found: C, 76.14; H, 5.51; N, 8.17. 1-(5-(7-Diphenylamino-9,9-diethyl-9H-fluoren-2-yl)thiophenyl-2-yl)-4-(thiophen-2-yl)butane-1,4-dione (6a). Sodium cyanide (0.12 g, 1.20 mmol) was finely ground and stirred with DMF (1.0 mL) under nitrogen. A solution of 5-(7-diphenylamino-9,9-diethyl-9H-fluoren-2-yl)thiophene-2-carbaldehyde (1.2 g, 2.40 mmol) in DMF (5.0 mL) was added. After being stirred for 10 min, a solution of 3-dimethylamino-1-(2-thienyl)-1prapanone (0.44 g, 2.40 mmol) in DMF (5.0 mL) was added slowly over a period of 10 min. The dark red mixture was stirred overnight under nitrogen, and then poured into water (100 mL) and extracted with CH2Cl2. The combined organic extract was washed with 10% HCl (40 mL), saturated aqueous NaHCO3 (40 mL), and water (50 mL), dried (MgSO4), concentrated in vacuo, and purified by column chromatography on silica (1:1 hexanes/CH2Cl2) to afford an orange solid. Crystallization from ethanol and diethyl ether gave the diketone 6a. Spectroscopic data for 6a: 1H NMR (300 MHz, CDCl3) δ 7.82 (d, J ) 3.3 Hz, 1H), 7.78 (d, J ) 3.3 Hz, 1H), 7.64-7.54 (m, 5H), 7.36 (d, J ) 6.0 Hz, 1H), 7.27-7.22 (m, 3H), 7.15-6.98 (m, 9H), 3.41-3.38 (m, 4H), 2.00-1.85 (m, 4H), 0.36 (t, J ) 7.2 Hz). 13C NMR (CDCl ) δ 191.5, 191.1, 153.7, 151.7, 150.8, 147.9, 3 147,8, 143.8, 142.6, 141.5, 135.5, 133.6, 133.2, 132.1, 131.2, 129.2, 128.1, 125.4, 124.0, 123.5, 122.7, 120.7, 120.4, 119.5, 118.9, 56.2, 33.3, 32.8, 32.6, 8.6. MS (FAB) m/z 637.1. 9,9-Diethyl-7-(5-(N-methyl-5-(thiophen-2-yl)pyrrol-2-yl)thiophen-2-yl)-N,N-diphenyl-9H-fluoren-2-amine (6b). A solution of diketone 6a (0.94 g, 1.47 mmol), methylamine (0.27 g, 3.48 mmol, 40% in water), and glacial acetic acid (0.2 mL) in toluene (30 mL) was refluxed under nitrogen in a flask equipped with a Dean-Stark trap for 48 h. The mixture was washed with water and the organic layer was concentrated in vacuo. The

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TABLE 1: Electrooptical Parameters of the Dyes dye 1 2 3 4 5 6 S1 S2 S3

λabs (
× 10-4 M-1 cm-1),a nm 437 472 475 475 497 462 420 470 473

(3.56), 309 (4.46) (3.84), 311 (3.62) (5.27), 366 (5.35), 310 (3.63) (5.10), 355 (5.52) (0.705), 427 (1.41), 304 (3.43) (5.38), 388 (6.62), 302 (3.47) (5.29), 333 (0.139) (5.00), 363 (3.00) (4.35), 351 (1.79), 302 (2.15)

E1/2 (ox), mVb 283 308 342 350 297 358 509 464 490

(195), 708 (61) (90), 928 (83) (94), 835 (97) (102), 836 (101) (74), 837 (112) (116), NA (82) (80), 747 (119) (108)

HOMO/LUMO, eV

E0-0,c eV

E0-0*,d V

5.08/2.71 5.11/3.08 5.14/2.92 5.15/2.93 5.10/3.02 5.16/2.99

2.37 2.03 2.22 2.22 2.08 2.17

-1.29 -0.93 -1.09 -1.08 -0.99 -1.02

a Recorded in THF solutions at 298 K. b Recorded in CH2Cl2 solutions. Oxidation potential reported is adjusted to the potential of ferrocene (E1/2(ox) ) 293 mV vs Ag/AgNO3), which was used as an internal reference. Scan rate: 100 mV/s. c The bandgap, E0-0, was derived from the observed optical edge. d E0-0* is the excited state oxidation potential vs NHE.

Figure 3. UV-vis curves of the dyes on a 4 µm TiO2 electrode.

Figure 4. Cyclic voltammograms of 3, 4, and 6 in deoxygenated CH2Cl2 containing 0.1 M TBAPF6 at 25 °C. Ferrocene (Fc) was added as an internal standard. All potentials are in volts vs Ag/AgNO3 (0.01 M in MeCN; the scan rate is 100 mV s-1).

residue was redissolved in CH2Cl2, washed with saturated aqueous NaHCO3, water, and brine, and dried (MgSO4). Removal of the solvent under reduced pressure produced an orange solid that was purified by column chromatography on silica (1:1 hexanes/CH2Cl2) to afford 6b. Spectroscopic data for 6b: 1H NMR (300 MHz, CDCl3) δ 7.60-7.50 (m, 4H), 7.32-7.21 (m, 5H), 7.11-6.97 (m, 12H), 6.40 (d, J ) 3.6 Hz, 1H), 6.36 (d, J ) 3.6 Hz, 1H), 2.02-1.86 (m, 4H), 0.38 (t, J ) 7.2 Hz). 13C NMR (CDCl3) δ 151.5, 150.7, 148.0, 147.3, 144.3, 141.0, 136.1, 134.9, 133.8, 132.2, 129.4, 129.2, 127.4, 126.2,

125.5, 125.0, 124.6, 123.9, 123.6, 123.0, 122.5, 120.4, 119.7, 119.5, 119.3, 110.3, 110.0, 56.2, 33.8, 32.7, 8.59. MS (FAB) m/z 632.2. 5-(5-(5-(7-Diphenylamino-9,9-diethyl-9H-fluoren-2-yl)thiophenyl-2-yl)-N-methylpyrrol-2-yl)thiophene-2-carbaldehyde (6c). A mixture of 6b (0.40 g, 0.63 mmol) and THF was cooled to -78 °C with an acetone-liquid N2 bath. n-Butyllithium (0.40 mL, 1.6 M solution in hexanes, 0.64 mmol) was added dropwise over 10 min with vigorous stirring. The solution was then brought to 0 °C over 1 h and kept at this temperature for an additional 1 h. Again the mixture was cooled to -78 °C and DMF was added at once. The solution was brought to room temperature and stirred overnight. The reaction was quenched by 1 N HCl and extracted with diethyl ether. The combined organic extract was dried over anhydrous MgSO4 and filtered. The filtrate was evaporated to yield the crude product as an orange solid. It was purified by column chromatography on silica gel with a hexane/CH2Cl2 mixture (1:1) as eluent. Orange solid. Spectroscopic data for 6c: 1H NMR (300 MHz, CDCl3) δ 9.86 (s, 1H), 7.72 (d, J ) 3.3 Hz, 1H), 7.62-7.50 (m, 3H), 7.33 (d, J ) 4.0 Hz, 2H), 7.27-7.21 (m, 3H), 7.18 (d, J ) 4.0 Hz, 2H), 7.12-6.97 (m, 9H), 6.58 (d, J ) 3.6 Hz, 1H), 6.43 (d, J ) 3.6 Hz, 1H), 3.89 (s, 3H), 1.99-1.89 (m, 4H), 0.37 (t, J ) 7.2 Hz). MS (FAB) m/z 660.2. (E)-2-Cyano-3-(5-(5-(5-(7-diphenylamino-9,9-diethyl-9Hfluoren-2-yl)thiophenyl-2-yl)-N-methylpyrrol-2-yl)thiophene2-yl)acrylic Acid (6). 6 was synthesized by the same procedure as described above for 1. Spectroscopic data for 6: 1H NMR (acetone-d6) δ 8.43 (s, 1H), 7.98 (d, J ) 4.0 Hz, 1H), 7.80-7.69 (m, 4H), 7.59 (d, J ) 4.0 Hz, 1H), 7.47 (d, J ) 4.0 Hz, 1H), 7.33-7.28 (m, 5H), 7.16-7.02 (m, 8H), 6.71 (d, J ) 4.0 Hz, 1H), 6.50 (d, J ) 4.0 Hz, 1H), 4.00 (s, 3H), 2.11-1.96 (m, 4H), 0.38 (t, J ) 7.2 Hz, 6H). 13C NMR (CDCl3) δ 152.5, 151.7, 149.3, 148.8, 146.4, 145.9, 145.7, 142.4, 139.7, 137.4, 139.7, 136.1, 135.3, 134.1, 133.5, 130.2, 127.9, 126.1, 125.8, 125.0, 124.6, 124.4, 123.6, 121.5, 120.7, 120.6, 117.0, 113.5, 111.9, 57.2, 35.0, 33.5, 9.1. MS (FAB) m/z 727.1 Anal. Calcd for C46H37N3O2S: C, 75.90; H, 5.12; N, 5.77. Found: C, 75.25; H, 5.01; N, 5.64. Assembly and Characterization of DSSCs. A thin film of TiO2 (∼18 µm in thickness) was used as a photoanode. It was coated on a 0.5 × 0.5 cm2 FTO glass substrate.24 The TiO2 film thickness was measured by a profilometer: Dektak3, Veeco/Sloan Instruments Inc., USA. An FTO with sputtering 100 nm thick Pt was used as the counter electrode. The active area was controlled at a dimension of 0.5 × 0.5 cm2 by adhering 60 µm thick polyester tape (3M) on the Pt electrode. After heating the TiO2 thin film to 80 °C, the film was taken out from the oven and soaked in THF solution containing 3

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TABLE 2: Calculated Low-Lying Transition for the Compounds

excitationa

λcal, eV

S1

H f L (99%)

2.29

0.524

S2

H1 f L (97%)

2.95

0.556

dye

state

1

S3

2

4

5

3.68

b

0.571

S1

HfL (98%)

2.30

0.599

S2

H1 f L (96%)

2.94

0.547

S3

3

H f L1 (97%)

f

H2 f L (12%) H f L1 (85%)

3.65

0.436

S1

H f L (99%)

2.21

0.501

S2

H1 f L (96%)

2.72

0.847

S3

H f L1 (91%) H2 f L (5%)

3.29

0.611

S1

H f L (99%)

2.23

0.521

S2

H1 f L (96%)

2.72

0.832

S3

H f L1 (85%) H2 f L (7%)

3.17

0.365

S1

H f L (99%)

2.17

0.114

S2

H1 f L (98%)

2.60

0.916

S3

H f L1 (94%)

3.22

0.027

∆(Mulliken charge),c |e| (Am, B1, B2, B3, and Ac)c Am: +0.675 B1: +0.0956 B2: -0.262 Ac: -0.509 Am: +0.349 B1: +0.315 B2: -0.183 Ac: -0.481 Am: +0.0166 B1: +0.0672 B2: -0.00810 Ac: -0.0757 Am: +0.632 B1: +0.0471 B2: -0.291 Ac: -0.388 Am: +0.329 B1: +0.180 B2: -0.184 Ac: -0.325 Am: +0.0234 B1: +0.0816 B2: -0.0334 Ac: -0.0716 Am: +0.770 B1: -0.033 B2: -0.323 Ac: -0.414 Am: +0.277 B1: +0.220 B2: -0.181 Ac: -0.316 Am: +0.053 B1: +0.027 B2: -0.021 Ac: -0.059 Am: +0.760 B1: -0.028 B2: -0.321 Ac: -0.412 Am: +0.296 B1: +0.213 B2: -0.188 Ac: -0.321 Am: -0.023 B1: +0.042 B2: +0.001 Ac: -0.020 Am: +0.922 B1: -0.106 B2: -0.365 Ac: -0.451 Am: +0.312 B1: +0.216 B2: -0.200 Ac: -0.328 Am: -0.007 B1: +0.016 B2: -0.002 Ac: -0.008

excitationa

λcal, eV

f

b

dye

state

6

S1

H f L (99%) 2.09

0.619

S2

H1 f L (96%) 2.51

0.908

S3

H f L1 (88%) 2.98 H2 f L (7%)

0.765

S1

H f L (99%) 2.22

0.574

S2

H1 f L (94%) 3.01

0.590

S3

H f L1 (90%) 3.19

0.065

S1

H f L (99%) 2.07

0.633

S2

H1 f L (94%) 2.71 H f L1 (5%)

1.11

S3

H f L1 (74%) 3.10 H f L2 (13%)

0.32

S1

H f L (99%) 2.18

0.810

S2

H1 f L (90%) 2.97 H f L1 (8%)

0.787

S3

H f L1 (85%) 3.36 H1 f L (7%) H2 f L (6%)

0.143

S1

S2

S3

∆(Mulliken charge),c |e| (Am, B1, B2, B3, and Ac)c Am: +0.701 B1: +0.055 B2: -0.042 B3: -0.317 Ac: -0.398 Am: +0.336 B1: +0.089 B2: +0.138 B3: -0.227 Ac: -0.336 Am: +0.233 B1: -0.143 B2: +0.019 B3: -0.032 Ac: -0.078 Am: +0.716 B1: -0.310 Ac: -0.406 Am: +0.525 B1: -0.196 Ac: -0.329 Am: +0.002 B1: +0.009 Ac: -0.012 Am: +0.782 B1: -0.123 B2: -0.303 Ac: -0.356 Am: +0.322 B1: +0.084 B2: -0.149 Ac: -0.258 Am: +0.237 B1: -0.062 B2: -0.065 Ac: -0.110 Am: +0.654 B1: -0.037 B2: -0.274 Ac: -0.343 Am: +0.274 B1: +0.059 B2: -0.101 Ac: -0.232 Am: +0.372 B1: -0.120 B2: -0.091 Ac: -0.161

a H ) HOMO, L ) LUMO, H1 ) The next highest occupied molecular orbital, or HOMO-1, H2 ) HOMO-2, L1 ) LUMO+1, L2 ) LUMO+2. In parentheses is the population of a pair of MO excitations. b Oscillator strength. c The difference of the Mulliken charge between the ground state and excited state.

× 10-4 M dye sensitizers for at least 12 h. After being rinsed with THF and dried in the air, the photoanode was placed on top of the counter electrode and they were tightly clipped together to form a cell. Electrolyte was then injected into

the seam and then the cell was sealed with Torr Seal cement (Varian, MA, USA). The electrolyte was composed of 0.5 M lithium iodide (LiI), 0.05 M iodine (I2), and 0.5 M 4-tertbutylpyridine dissolved in acetonitrile. A 0.6 × 0.6 cm2

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Figure 5. The plot of the difference in the Mulliken charge between the ground state and excited state.

TABLE 3: Performance Parameters of DSSCs Constructed by Using the Dyesa dye

JSC (mA/cm2)

VOC (V)

FF

η (%)

1 2 3 4 5 6 S1 S2 S3 N719

13.47 14.20 18.14 16.79 12.93 13.54 16.48 13.40 16.00 16.08

0.60 0.57 0.61 0.64 0.58 0.60 0.67 0.62 0.63 0.72

0.59 0.60 0.56 0.58 0.64 0.64 0.64 0.62 0.63 0.63

4.77 4.79 6.16 6.18 4.80 5.25 6.90 5.70 6.15 7.19

a Experiments were conducted with TiO2 photoelectrodes with approximately 18 µm thickness and 0.25 cm2 working area on the FTO (15 Ω/sq) substrates.

Figure 6. J-V curves of DSSCs based on the dyes.

cardboard mask was clipped onto the device to constrain the illumination area. The photoelectrochemical characterizations on the solar cells were carried out by using a modified light source, a 300 W Xe lamp (Oriel, No. 6258) equipped with a waterbased IR filter and AM 1.5 filter (Oriel, No. 81080 kit). Light intensity, attenuated by a neutral density filter (Optosigma, No. 078-0360) at the measuring (cell) position, was estimated to be ca. 100 mW cm-2 from a radiant power meter (Oriel, No. 70310) connected to a thermopile probe (Oriel, No. 71964). Photoelectrochemical characteristics of the DSSCs, including photocurrent-voltage curves and electrochemical impedance spectra, were recorded through a potentiostat/ galvanostat (CHI650B, CH Instruments, Inc., USA).

Figure 7. IPCE plots for the DSSCs.

Measurement of Recombination Time Constant by Transient Photovoltage. The photovoltage transients of assembled devices were recorded with a digital oscilloscope (LeCroy, WaveSurfer 24Xs). Pulsed laser excitation was applied by a Q-switched Nd:YAG laser (Continuum, model Minilite II) with a 1 Hz repetition rate at 532 nm and a 5 ns pulse width at halfheight. The beam size was slightly larger than 0.5 × 0.5 cm2 to cover the area of the device with an incident energy of 1 mJ/ cm2. The recombination lifetime of photoinjected electrons with oxidized dyes was measured by transient photovoltages at open circuit with the presence of LiClO4 or LiI electrolyte (0.5 M). The average electron lifetime can be estimated approximately by fitting a decay of the open circuit voltage transient with exp(-t/τR), where t is time and τR is an average time constant before recombination. Quantum Chemistry Computation. The predicted structures of the molecules were optimized by using B3LYP hybrid functional and 6-31G* basis sets. For each of the molecules, a number of conformational isomers were examined and the one with the lowest energy was used. For the excited states, we have employed the time-dependent density functional theory (TDDFT) with the B3LYP functional. All of the analyses were performed under Q-Chem 3.0 software.25 There exist a number of previous works that employed TDDFT to characterize excited states with charge-transfer character.26 In some cases, underestimation of the excitation energies was seen.26,27 Therefore, in the present work, we use TDDFT to characterize the extent of charge-shift, and avoid drawing conclusions from the excitation energy.

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Figure 8. The dihedral angles between two neighboring aromatic rings or between the aromatic ring and 2-cyanoacrylic acid.

Results and Discussion Synthesis of the Materials. The new dyes synthesized are illustrated in Figure 1. Figure 1 also includes dyes S1,21b S2 (Supporting Information), and S328 for relevant comparison (vide infra). In these new dyes diarylamine moiety (electron donor) and 2-cyanoacrylic acid (electron acceptor and anchoring group for attachment on the TiO2 surface) are linked by different conjugated spacers containing a pyrrole moiety. These compounds were obtained in good yields by the sequential steps illustrated in Scheme 1. The intermediate of 6, 6a, was synthesized from 5-(7-diphenylamino-9,9-diethyl-9H-fluoren2-yl)thiophene-2-carbaldehyde21b adopting the reported procedures.29 Other key steps involved in these reactions include the following: (1) Stille’s coupling reaction30 of 1-methyl-2tributylstannyl-1H-pyrrole with aryl bromide (steps i and iii); (2) palladium-catalyzed aromatic C-N coupling reactions (step v) developed by Koie31 and Hartwig;32 and (3) condensation of aromatic aldehyde with cyanoacetic acid with the formation of 2-cyanoacrylic acid (step iv). The dyes are freely dissolved in THF and CH2Cl2. Optical Properties. The optical absorption spectra of the dyes in THF are displayed in Figure 2 and the data are collected in Table 1. All the compounds show two prominent bands: (A) a solvent-independent band at λmax < 400 nm attributed to localized ππ* transition and (B) a solvent-dependent band at λmax ≈ 400-450 nm due to charge transfer (from the donor, diarylamine, to the acceptor, 2-cyanoacrylic acid) transition mixed with more delocalized ππ* transition. The charge-transfer character in band B is evident based on the comparison of λmax between 3c (410 nm in THF) and 3 (475 nm in THF) and theoretical computation (vide infra). While a ππ* transition band normally has a longer wavelength as the effective conjugation length increases, a charge-transfer band will be affected by the electron donor, electron acceptor, and the spacer between the two. The λmax value of B band is in the order of 5 > 4 ≈ 3 ≈ S3 ≈ 2 ≈ S2 > 6 > 1 > S1. Compared to 3, 4, and S2, an additional heteroaromatic ring in 6 does not lead to an increment in λmax value for both ππ* transition and charge-transfer transitions. Thienyl was known to be a better unit than phenyl for charge transfer to occur due to the reduced aromatic character

of the thiophene ring than the benzene ring.23a,c–e,g,33 This accounts for the much smaller λmax value of 1 compared to 2. Because of less efficient electronic communication between the biphenyl unit of the carbazolyl moiety with 3- and 6-substituents, the B band of 5 has weaker intensity compared to that of other compounds. The broad feature of the absorption of 5 may be due to the mixing of the B and with the charge-transfer band from the carbazole to the 2-cyanoacrylic acid. This observation is also supported by the computation results of 5, which show a lower calculated oscillator strength (vide infra). A comparison between 4 and S2 suggests that bithiophene is slightly more effective than thienylpyrrole in lowering the energy of the charge-transfer band. The charge-transfer bands in these compounds exhibit a negative solvatochromism, i.e., blue shift in more polar solvents. This phenomenon can be attributed to the deprotonation of the carboxylic acid, which decreases the strength of the electron acceptor. Blue shift of λmax upon NEt3 treatment of the THF solution of the dye provides additional support of this argument. Similar observation has been reported for the MLCT band of the black dye10a and the charge-transfer band of metal-free organic dyes.21e Both blue21e and red13a,34 shift of the absorption were reported for dipolar organic sensitizers upon adsorption on the TiO2 surface, due to different interaction between the sensitizers and TiO2, or between the sensitizers. The absorption spectra of the dyes in this study become broadened and redshifted to a different extent upon adsorption on the TiO2 surface (Figure 3). Compounds 3 and 4 have the smallest shift of λmax when going from the solution to being adsorbed as film on the TiO2 surface, whereas compounds 2, 5, and 6 have significant red shifts of λmax when adsorbed on TiO2 surface. These observations suggest that the dyes have different degrees of J-aggregation in the film state. All the compounds are weakly emissive, and the emission wavelength in THF is in the order of 6 ≈ 3 > 4 ≈ 2 > 5 ≈ 1 (Figure 2). Compounds 1 and 5 have significantly shorter emission wavelengths than others, which is consistent with the weaker charge-transfer character of the two (vide supra). Electrochemical Properties. The electrochemical characteristics of the dyes were investigated by using cyclic voltam-

Dye-Sensitized Solar Cells metry (CV) and differential pulse voltammetric (DPV) methods. Representative cyclic voltammograms are shown in Figure 4. A quasi-reversible one-electron oxidation wave detected at ∼300-360 mV more positive than ferrocene/ferrocenium for 2-4 and 5 is attributed to the removal of electron from the arylamine. The second quasi-reversible one-electron oxidation wave at a higher potential was attributed to the oxidation of oligo(heteroaromatic ring) in viewing that oligo(pyrrolyl/thienyl ring) is a potent electron donor.35 These two oxidation waves appear at nearly the same potential (i.e., two-electron process) for 6 and were resolvable only with DPV (differential pulse voltammetry). For 1, the first oxidation potential is somewhat irreversible. The oxidation potential of the compounds decreases in the order of 6 > 4 > 3 > 2 > 5 > 1. Apparently the electronic coupling between arylamine and electron-rich oligo(pyrrolyl/ thienyl ring) affects the oxidation potential of each, and it is anticipated that stronger interaction will result in more facile oxidation of arylamine and render the second oxidation less facile. Such an electronic effect decreases as the distance between the two becomes larger (e.g., 3 vs 2), or as the heteroaromatic ring becomes less electron rich (e.g., 2 vs 1). Consistent with our previous observations, the diarylamine unit attached to the 3-position of the N-ethylcarbazole unit has significantly lower oxidation potential than ordinary triarylamines due to the electronic interaction between the two amines.36 The excited state potential (E0-0*) of the sensitizer, estimated from the first oxidation potential at the ground state and the zero-zero excitation energy, E0-0, is more negative (-0.93 to -1.29 V vs NHE, see Table 1) than the conduction-band-edge energy level of the TiO2 electrode, -0.5 V vs NHE, where the zero-zero excitation energy was estimated from the absorption onset. Therefore, the electron injection from the excited dye to TiO2 should be energetically favorable. On the other hand, dye regeneration is also ensured because the first oxidation potentials of the dyes (1.07-1.15 V vs NHE) are more positive than the I-/I3- redox couple (∼0.4 V vs NHE). Theoretical Approach. To gain further insight in the correlation between structure and the physical properties as well as the device performance, quantum chemistry computation was conducted. The results for the theoretical approach are summarized in Table 2 and Figure S1 (Supporting Information). For comparison, computation was also conducted for compounds with closely related structure, i.e., S1-S3. The molecules were divided into four or five segments: the arylamine group groups (Am), the heteroaromatic ring next to Am (B1), the heteroaromatic ring next to B1 (B2), the heteroaromatic ring next to B2 (B3), and 2-cyanoacrylic acid (Ac). Except for Ac, the neighboring segment, deviation between the two successive segments from planarity was found in the optimized structure of each compound. The largest twist angle was found between Am and B1 (26.6-44.5°). The Mulliken charges for the S1, S2, and S3 states were also calculated as a projection from the TDDFT results. Differences in the Mulliken charges in the excited state and ground states were calculated and grouped into several segments in the molecules to estimate the extent of charge separation upon excitation (Figure 5). The two electronic transitions with lowest energies, S1 (mainly HOMO f LUMO transition) and S2 (mainly HOMO-1 f LUMO transition), have significant oscillator strength (f) and extent of charge separation (i.e., change in Mulliken charges in the transition) from the arylamine to the 2-cyanoacrylic acid. It is interesting to note that the pyrrole moiety also has significant contribution in charge transfer to the 2-cyanoacrylic acid. In

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12565 contrast, the effect diminishes if the pyrrole is replaced by the thiophene (compounds S1-S3). Internal conversion of S2 to S1, that is, long-range charge shift from the pyrrole to the arylamine group, may also take place after the S0 f S2 transition. The computation results appear to be consistent with the electronic spectra observed in these compounds. The B band is likely to be the combination of S0 f S1 and S0 f S2 transitions. Among the six compounds synthesized, 3, 4, and 6 have large oscillator strength values (f) for both S0 f S1 and S0 f S2 transitions. Indeed, the B bands of compounds 3, 4, and 6 were found to have higher intensities than those of compounds 1, 2, and 5. The very weak charge-transfer band calculated for compound 5 was consistent with that observed experimentally (see Table 1). Photovoltaic Devices. The dye-sensitized solar cells were constructed by using 1-6 as the sensitizer for nanocrystalline anatase TiO2. Typical solar cells, with an effective area of 0.25 cm2, were fabricated with an electrolyte composed of 0.05 M I2/0.5 M LiI/0.5 M tert-butylpyridine in acetonitrile solution. The device performance statistics under AM 1.5 illumination are collected in Table 3. Figure 6 shows the photocurrent-voltage (J-V) curves of the cells. The incident photon-to-current conversion efficiencies (IPCE) of the dyes on TiO2 are plotted in Figure 7. The devices exhibit good conversion efficiencies, ranging from 66% to 86% of the standard ruthenium dye N71937 fabricated and measured under similar conditions. Cells based on compounds 3, 4, and 6 have higher conversion efficiencies than others, and possibly their more efficient light-harvesting capabilities (i.e., better coverage of the absorption spectra in the visible range) play an important role. This argument is supported by theoretical calculation (vide supra), which shows that the two electronic transitions with the lowest energies, S1 and S2, have significant oscillator strength and extent of charge separation from the arylamine to the 2-cyanoacrylic acid. It is somewhat surprising that the device performances of 6, both the conversion efficiency and the short-circuit current, are inferior to that of 3 or 4, even though 6 has the widest absorption band in the visible region with the highest extinction coefficient (Figure 3). We speculate that the aggregation observed in 6 resulting in less efficient electron injection38 to TiO2 might be due to quenching of the excited state. Both 3- and 4-based DSSCs not only have high efficiencies (∼86% of N719-based DSSC), but also have higher short-circuit current (JSC) than N719-based DSSC. The high JSC values for the devices of 3 and 4 may be due to relatively lower HOMO levels among all (except for 6, which forms aggregation), which leads to more facile regeneration of the oxidized dyes and suppresses the recombination between the conduction band electrons and dye cations. DSSCs fabricated from 1, 2, and 5 have similar efficiencies (∼70% that of N719-based cell) even though they exhibit different light-harvesting behaviors both in THF and as a film on TiO2. Less efficient light harvesting capabilities of 1, 2, and 5 likely led to lower JSC values and conversion efficiencies of the cells thus fabricated. It is important to note that although 2 and 5 exhibit more intense absorption than 1 when adsorbed on the TiO2 surface, the aggregation-induced quenching of the excited state may counteract the better lightharvesting behavior in the former. The effect of the pyrrole ring was also evaluated by comparison of the compounds with some closely related compounds. The efficiency of the device from 4 is slightly better than that from S2 (η ) 5.7%; VOC ) 0.62 V; JSC ) 13.4 mA/ cm;2 FF ) 0.62). The λφabs of the ππ* transition and the charge-

12566 J. Phys. Chem. C, Vol. 112, No. 32, 2008 transfer transition bands for S2 appear at 363 (
) 3.00 × 104 M-1 cm-1) and 470 nm (
) 5.00 × 104 M-1 cm-1), respectively, which appear to be slightly inferior to those of 4. The recombination lifetime (τR) of the photoinjected electron with the oxidized dye was measured by transient photovoltage39 at open circuit. To avoid dark current, I2 was omitted and the electrolyte used was LiI in CH3CN. The S2-based cell was found to have a larger τR value than the 4-based cell (S2, 3.83 ms; 4, 2.38 ms). Therefore, the better performance of the 4-based cell may be rationalized by the slightly better light-harvesting efficiency (Table 1) as well as higher adsorbed dye density of 4 than S2 (4: 3.8 × 10-7 mol/cm2; S2: 3.6 × 10-7 mol/cm2). In comparison, though 2 has a higher adsorbed dye density than S3 (2: 5.1 × 10-7 mol/cm2; S3: 4.7 × 10-7 mol/cm2), the 2-based device has an inferior performance than the S3-based device (η ) 6.15%; VOC ) 0.63 V; JSC ) 16.0 mA/cm;2 FF ) 0.61).28 Such an outcome may be attributed to the significantly larger τR value (S3, 3.54; 2, 1.09 ms) as well as better light harvesting of S3 than 2 (Table 1). DSSC based on compound S1 also has much higher conversion efficiency than that based on the congener of S1, P1 (Figure S2 in Supporting Information) where the thiophene is replaced by N-methylpyrrole (6.90% vs 5.12%). Our computational results indicate that the dihedral angles for a pyrrole moiety and its neighboring functional groups are generally larger (Figure 8). A larger dihedral angle implies a smaller interaction between the two neighboring aromatic moieties, leading to a better charge separation but with lower optical transition probabilities. The presence of the pyrrole moiety is advantageous for the charge transfer to 2-cyanoacrylic acid as seen in Table 2 and Figure 5. When the pyrrole- and thiophene-spaced molecules are compared (such as 4 vs S2 and 2 vs S3), the additional negative charges gained on the 2-cyanoacrylic acid in their first excited states are larger in the pyrrole-spaced compounds. In addition, both the pyrrole-spaced compounds have a second excited state (S2) that uses pyrrole as an electron donor. However, the oscillator strengths are lower in the pyrrole-spaced molecules, limiting the light-harvesting efficiencies, and this is also a consequence of less interaction between aromatic units. Though incorporation of the pyrrole ring in the spacer of dipolar compounds in this study did not lead to DSSCs with efficiencies surpassing those based on thiophene dyes, interposing appropriate five-membered auxiliary fragments between strong donor or acceptor groups is expected to result in better charge transfer and deserves further studies.40 Conclusion In summary, we have synthesized dipolar compounds where the electron-donating arylamine and electron-accepting 2-cyanoacrylic acid were linked by pyrrole-containing conjugated spacers. DSSCs using these compounds exhibited efficiencies ranging from 4.77% to 6.18% under AM 1.5 illumination. The best performance of the device reaches ∼86% of N719-based DSSC (7.19%) fabricated and measured under similar conditions. However, the short-circuit current of the devices surpasses that of the standard device. Theoretical investigations (TDDFT) indicated that the compounds with efficient charge transfer from arylamine to 2-cyanoacrylic acid resulted in DSSCs of better performance. The pyrrole moieties interact less with its neighboring aromatic units, leading to better charge separation in excited states. It also acts as a secondary electron donor via a higher energy excited state. In contrast, the characteristic of the second donor is diminished for the thiophene moiety in thiophene-containing congeners. Further investigation on the

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