Molecular Design of Anthracene-Bridged Metal-Free Organic Dyes for

J. Phys. Chem. C 2010, 114, 9101–9110

9101

Molecular Design of Anthracene-Bridged Metal-Free Organic Dyes for Efficient Dye-Sensitized Solar Cells Chao Teng,† Xichuan Yang,*,† Chao Yang,† Shifeng Li,† Ming Cheng,† Anders Hagfeldt,†,‡ and Licheng Sun*,†,§ State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular DeVices, Dalian UniVersity of Technology (DUT), Dalian, China, School of Chemical Science and Engineering, Center of Molecular DeVices, Physical Chemistry, Royal Institute of Technology (KTH), Stockholm, Sweden, Department of Chemistry, Organic Chemistry, Royal Institute of Technology (KTH), Stockholm, Sweden ReceiVed: February 8, 2010; ReVised Manuscript ReceiVed: April 11, 2010

A series of metal-free organic dyes bridged by anthracene-containing π-conjugations were designed and synthesized as new chromophores for the application of dye-sensitized solar cells (DSCs). Detailed investigations on the relationship between the dye structures, photophysical properties, electrochemical properties, and performances of DSCs are described. With the introduction of the anthracene moiety, together with a triple bond for the fine-tuning of molecular planar configurations and to broaden absorption spectra, the short-circuit photocurrent densities (Jsc) and open-circuit photovoltages (Voc) of DSCs were improved to a large extent. The improvement of Jsc is attributed to much broader absorption spectra of the dyes with the anthracene moiety. Electrochemical impedance spectroscopy (EIS) analysis reveals that the introduction of the anthracene moiety suppresses the charge recombination arising from electrons in TiO2 films with I3- ions in the electrolyte, thus improving Voc considerably. On the basis of optimized molecular structures and DSC test conditions, the dye TC501 shows a prominent solar energy conversion efficiency (η) up to 7.03% (Jsc ) 12.96 mA · cm-2, VOC ) 720 mV, ff ) 0.753) under simulated AM 1.5 irradiation (100 mW · cm-2). Introduction The increasing energy demand and the concerns about climate changes have led to a great focus on renewable energy sources during the last years. Dye-sensitized solar cells (DSCs) have attracted intense interest for their high performance in converting solar energy to electricity at low cost.1 At present, DSCs based on Ru(II)-polypyridyl complexes have an overall solar energy conversion efficiency (η) approaching 12% under simulated AM 1.5 irradiation (100 mW · cm-2).2 The use of environmentally friendly metal-free organic dyes to replace the expensive and source-limited ruthenium dyes is highly demanded and certainly is a state of the art research topic. Compared with the traditional ruthenium dyes, metal-free organic dyes have many advantages such as lower cost, easier structural modification, and higher molar extinction coefficient. Recently, novel organic dyes based on coumarin,3 merocyanine,4 cyanine,5 indoline,6 hemicyanine,7 perylene,8 oligoene,9 xanthene,10 triphenylamine,11 dialkylaniline,11e,12 [bis(dimethylfluorenyl)amino]phenyl,13 phenothiazine,14 tetrahydroquinoline,15 and carbazole16 have been used in DSCs with the record highest solar energy conversion efficiency exceeding 9%.11b,17 The donor-(π-conjugation)-acceptor (D-π-A) system is the basic feature for most metal-free organic dyes due to the effective photoinduced intramolecular charge transfer property. Appropriate use of electron-excessive chains as π-conjugation between an electron donor and an electron acceptor was reported to be beneficial to red shift the charge-transfer transition. Fused aromatic compounds such as anthracene have big π-conjugations * Corresponding author. E-mail: [email protected]. † Dalian University of Technology. ‡ Physical Chemistry, Royal Institute of Technology. § Organic Chemistry, Royal Institute of Technology.

and are available as fine chemicals. Here we report the design, synthesis, and application of 10 novel organic dyes (TC201-TC602; the structures are shown in Chart 1), in which triphenylamine (TPA) derivatives are used as electron-donating moieties, cyanoacrylic acid as electron acceptor, and anthracene moiety as π-conjugations to bridge the donor-acceptor (D-A) systems. A double bond and triple bond are also introduced into different positions of the π-conjugation systems to tune the molecular structure and their configurations. Experimental Section Analytical Instruments and Measurements. Absorption and emission spectra were recorded in a quartz cell with 1 cm path length on HP8453 (USA) and PTI700 (USA), respectively. 1H NMR spectra were measured with VARIAN INOVA 400 MHz (USA) with the chemical shifts by using TMS as standard. MS data were obtained with GCT CA156 (UK), HP1100 LC/MSD (USA), and LC/Q-TOF MS (UK). Electrochemical redox potentials were obtained by cyclic voltammetry (CV) using a three-electrode cell and an electrochemistry workstation (BAS100B, USA). The working electrode was a glass carbon disk electrode; the auxiliary electrode was a Pt wire; and Ag/ Ag+ was used as the reference electrode. Tetrabutylammonium hexaflourophosphate (TBAPF6, 0.1 M) was used as supporting electrolyte in DMF. Ferrocene was added to each sample solution at the end of the experiments, and the ferrocenium/ ferrocene (Fc/Fc+) redox couple was used as an internal potential reference. The potentials vs NHE were calibrated by addition of 630 mV to the potentials vs Fc/Fc+.11a Electrochemical impedance spectroscopy (EIS) for DSC with forward bias -0.7 V under dark was measured with an impedance/gain-phase analyzer (PARSTAT 2273, USA). The spectra were scanned

10.1021/jp101238k  2010 American Chemical Society Published on Web 04/23/2010

9102

J. Phys. Chem. C, Vol. 114, No. 19, 2010

Teng et al.

CHART 1: Molecular Structures of 10 Novel Metal-Free Organic Dyes (TC201-TC602) Bridged by Anthracene-Containing π-Conjugations

in a frequency range of 10-1-105 Hz at room temperature. The alternate current (AC) amplitude was set at 10 mV. DSC Fabrication. A layer of ca. 2 µm TiO2 (13 nm paste, T/SP, Solaronix, Switzerland) was coated on the F-doped tin oxide conducting glass (TEC15, 15Ω/square, Pilkington, USA) by screen printing and then dried for 6 min at 125 °C. This procedure was repeated 6 times (ca. 12 µm) and finally coated by a layer (ca. 4 µm) of TiO2 paste (DHS-SLP1, Heptachroma, China) as the scattering layer. The double-layer TiO2 electrodes (area: 6 × 6 mm) 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 sintered film was further treated with 40 mM TiCl4 aqueous solution at 70 °C for 30 min, then washed with ethanol and water, and annealed at 500 °C for 30 min. After the film was cooled to 40 °C, it was immersed into a 2 × 10-4 M dye bath in CH2Cl2 solutions and maintained under dark for 8 h. The electrode was then rinsed with CH2Cl2 and dried. The hermetically sealed cells were fabricated by assembling the dye-loaded film as the working electrode and Pt-coated conducting glass as the counter electrode separated with a hot-melt Surlyn 1702 film (25 µm, Dupont). The electrolyte consisting of 0.6 M 1,2-dimethyl-3-propylimidazo-

lium iodide (DMPII), 0.06 M LiI, 0.04 M I2, and 0.4 M 4-tertbutylpyridine (TBP) in dried CH3CN solutions was introduced into the cell via vacuum backfilling from a hole in the back of the counter electrode. Finally, the hole was also sealed using Surlyn 1702 film and a cover glass. Photovoltaic Properties Measurements. The irradiation source for the photocurrent-voltage (J-V) measurement is an AM 1.5 solar simulator (16S-002, Solar Light Co. Ltd., USA). The incident light intensity was 100 mW · cm-2 calibrated with a standard Si solar cell. The tested solar cells were masked to a working area of 0.159 cm2. The photocurrent-voltage curves were obtained by the linear sweep voltammetry (LSV) method using an electrochemical workstation (LK9805, Lanlike Co. Ltd., China). The measurement of the incident photon-to-current conversion efficiency (IPCE) was performed by a Hypermonolight (SM-25, Jasco Co. Ltd., Japan). The Jsc was calibrated by integrating the IPCE value tuned light density of AM 1.5 against wavelength.18 Results and Discussions Synthesis. TPA derivatives D1-4 as the electron donor units were synthesized (Scheme 1). Different anthracene-containing

Anthracene-Bridged Metal-Free Organic Dyes

J. Phys. Chem. C, Vol. 114, No. 19, 2010 9103

SCHEME 1: Synthetic Routes of D1-4a

(a) KI, H2SO4, H2O2, CH3OH, 60 °C, overnight; (b) aniline, 1,10-phenanthroline, CuCl, KOH, toluene, reflux, 12 h; (c) NBS, CCl4, rt, 4 h; (d) (1) nBuLi, THF, -78 °C, 1 h; (2) B(OMe)3, -78 °C, 1 h; (3) H3+O; (e) trimethylsilyl acetylene (TMSA), Pd(PPh3)2Cl2, CuI, THF, NEt3, rt, 1 day; (f) K2CO3, CH3OH, rt, 1 day. a

SCHEME 2: Synthetic Routes of M1-5a

a (a) (1) nBuLi, THF, -78 °C, 1 h; (2) B(OMe)3, -78 °C, 1 h; (3) H3+O; (b) 4-bromobenzaldehyde or 5-bromothiophene-2-carbaldehyde, Pd(PPh3)4, K2CO3 (2 M), THF, reflux, 3 days; (c) Br2, CCl4, reflux, 1 h; (d) Br2, CCl4, reflux, 1 h; (e) (1) PPh3 · HBr, CHCl3, reflux, 2 h; (2) terephthalaldehyde or thiophene-2,5-dicarbaldehyde, 18-crown-6, K2CO3, DMF, rt, 2 h; (f) (1) TMSA, Pd(PPh3)2Cl2, CuI, THF, NEt3, rt, 1 day; (2) K2CO3, CH3OH, rt, 1 day; (g) 9-bromo-10-iodoanthracene, Pd(PPh3)2Cl2, CuI, THF, NEt3, rt, 1 day.

π-conjugations were synthesized as shown in Scheme 2. The palladium-catalyzed Suzuki coupling reaction and Sonogashira coupling reaction were used to synthesize aldehydes DM1-10. Finally, condensation of the aldehydes DM1-10 with cyanoacetic acid by the Knoevenagel reaction in the presence of piperidine gave the dyes TC201-TC602 (Scheme 3). The detailed synthetic procedures are described in the Supporting Information. 1-Iodo-4-methoxybenzene (2). Methoxybenzene (25 mL, 0.23 mol) and KI (40 g, 0.24 mol) were added slowly to a stirred mixture of CH3OH (400 mL) and H2SO4 (19 mL) at 0 °C. After

the mixture was warmed to room temperature, a H2O2 aqueous solution (30%, 50 mL) was added dropwise. Then, the reaction was performed at 60 °C under stirring overnight. After cooling to room temperature, the mixture was poured into H2O (1 L) and extracted with CH2Cl2 (3 × 50 mL). The combined organic layers were dried over Na2SO4. After removal of the solvent, the crude product was recrystallized from ethyl acetate:petroleum ether (v:v, 1:10) to afford compound 2 as a white solid (35 g, yield 65%). 1H NMR (400 MHz, CDCl3, ppm): δ 7.55 (d, 2H), 6.68 (d, 2H), 3.78 (s, 3H). GC/TOF HRMS-EI (m/z): [M]+ calcd for C7H7IO, 233.9542; found, 233.9547.

9104


Teng et al.

SCHEME 3: Synthetic Routes of the Dyes TC201-TC602a

a (a) Pd(PPh3)4, Na2CO3 (2 M), toluene, C2H5OH, reflux, 1 day; (b) cyanoacetic acid, piperidine, CH3CN, reflux, 3 h; (c) Pd(PPh3)4, Na2CO3 (2 M), toluene, C2H5OH, reflux, 1 day; (d) cyanoacetic acid, piperidine, CH3CN, reflux, 3 h.

Figure 1. Absorption spectra of the dyes TC201-TC602 in CH2Cl2 solutions (2 × 10-5 M).

Figure 2. Absorption spectra of the dyes TC201-TC602 on TiO2 films.

4-Methoxy-N-(4-methoxyphenyl)-N-phenylaniline(3b).Compound 2 (14.4 g, 62.5 mmol), aniline (2.28 mL, 25 mmol), and 1,10-phenanthroline (0.90 g, 5 mmol) were dissolved in toluene (50 mL). After the solution was heated to 100 °C, CuCl (0.495 g, 5 mmol) and KOH (11.22 g, 0.2 mol) were added under N2 purge. The mixture was refluxed for 12 h. After cooling to room temperature, acetic acid (6.8 mL, 0.12 mol) and toluene (30 mL) were added. The mixture was washed with H2O (90 mL) three times, and the organic phase was dried over Na2SO4. After removal of the solvent, the residual was purified on a silica gel column with ethyl acetate:petroleum (v:v, 1:10) to afford compound 3b as a white solid (4.36 g, yield 57%). 1H NMR

(400 MHz, CDCl3, ppm): δ 7.16-6.81 (m, 13H), 3.40 (s, 6H). GC/TOF HRMS-EI (m/z): [M]+ calcd. for C20H19NO2, 305.1416; found, 305.1424. 4-Bromo-N,N-bis(4-methoxyphenyl)aniline (4b). Compound 3b (5.28 g) and NBS (2.575 g) were added to CCl4 (80 mL), and the reaction was performed at room temperature for 4 h under N2 purge. After that the mixture was poured into water and extracted with CH2Cl2 (3 × 50 mL). The organic layer was washed with H2O and subsequently dried over anhydrous Na2SO4. After removing the solvent under reduced pressure, the residue was purified on a silica gel column with CH2Cl2: hexane (v:v, 1:1) as eluent to obtain compound 4b as a white-



TABLE 1: Absorption, Emission, and Electrochemical Properties of the Dyes TC201-TC602 absorption dye

λmax in CH2Cl2(nm)

TC201 TC202 TC203 TC401 TC402 TC403 TC501 TC502 TC601 TC602

401 418 464 402 464 494 452 490 449 511

a

-1

emission

-1

ε (M · cm ) λmax on TiO2 (nm) 5600 18200 24100 12100 13000 14100 20100 18300 17100 14000

b

397 416 440 400 458 470 455 461 454 512

λmax (nm) 583 629 568 540 648 619 647 630 540 632

oxidation potential c

E0-0 (V Abs/Em) 2.55 2.37 2.34 2.65 2.10 2.25 2.38 2.26 2.54 2.17

d

Eox (V vs NHE)e Eox - E0-0 (V vs NHE) 1.22 1.23 1.21 1.17 1.18 1.19 0.91 0.90 1.14 1.07

-1.33 -1.14 -1.13 -1.48 -0.92 -1.06 -1.47 -1.36 -1.40 -1.10

Absorptions in CH2Cl2 solutions (2 × 10-5 M) at rt. b Absorptions of the dye-loaded TiO2 films immerged in CH2Cl2 solutions. c Emission spectra in CH2Cl2 solutions (2 × 10-5 M) at rt. d The zeroth-zeroth transition E0-0 values were estimated from the intersection of the absorption and emission spectra. e The oxidation potentials of the dyes were measured in DMF solutions with tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) as electrolyte (working electrode: glassy carbon; reference electrode: Ag/Ag+; calibrated with ferrocene/ ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by addition of 630 mV;11a counter electrode: Pt). a

TABLE 2: Optimized Conformations Calculated with TD-DFT on B3LYP/6-31+G (d) of the Selected Dyes

yellow oil (5.78 g, yield 90%). 1H NMR (400 MHz, CDCl3, ppm): δ 7.18-6.83 (m, 12H), 3.40 (s, 6H). GC/TOF HRMSEI (m/z): [M]+ calcd. for C20H18BrNO2, 383.0521; found, 383.0528. 4-(Bis(4-methoxyphenyl)amino)phenylboronic Acid (D2). To a 100 mL two-necked flask containing the solution of compound 4b (8.43 g, 22 mmol) in dried THF (20 mL) equipped with a magnetic stirrer, a N2 purge and a -78 °C acetone-dry ice bath were dropwise added to nBuLi (17.6 mL, 26.4 mmol, 1.5 M) while maintaining a good stirring. After stirring for 1 h, trimethyl borate (3.00 mL, 26.4 mmol) was carefully added. After stirring at room temperature for another 2 h, water was first added to the reaction mixture, and then HCl (6 M) was added in a dropwise fashion until an acidic mixture was obtained. The reaction mixture was poured into water and extracted with CH2Cl2 (3 × 50 mL). The combined organic layer was dried with anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by column chromatography using CH2Cl2:ethyl acetate (v: v, 1:1) as eluent. The boronic acid compound D2 was obtained as a light yellow oil with an isolated yield of 77%. 1H NMR (400 MHz, DMSO-d6, ppm): δ

Figure 3. Cyclic voltammogram of TC501 measured in DMF solutions with TBAPF6 (0.1 M) as the electrolyte (working electrode: glassy carbon; reference electrode: Ag/Ag+; counter electrode: Pt; calibrated with Fc/Fc+ as an internal reference; scan rate: 100 mV s-1).

7.88 (s, 2H), 7.68 (d, J ) 8.1 Hz, 2H), 7.30 (t, J ) 8.1 Hz, 4H), 7.00-7.08 (m, 6H), 6.88 (d, J ) 8.1 Hz, 2H), 3.40 (s, 6H). 4-Ethynylbenzaldehyde (12a). To a mixture of PdCl2(PPh3)2 (175 mg, 0.25 mmol) and CuI (95 mg, 0.5 mmol) in THF (40

9106


Teng et al.

TABLE 3: Frontier Molecular Orbitals of the HOMO and LUMO Calculated with DFT on a B3LYP/6-31+G(d) Level of the Selected Dyes

mL) were added successively 4-bromobenzaldehyde (1.85 g, 10 mmol), trimethylsilyl acetylene (1.56 mL, 11 mmol), and NEt3 (40 mL). The resulting mixture was stirred for 24 h at room temperature. The dark solution was pumped dry, and the resulting black solid was extracted with CH2Cl2 and further purified by chromatography on silica gel with CH2Cl2:hexane (v:v, 1:2) to give the intermediate trimethylsilyl-protected acetylene derivatives as white solid (yield 85%). To a stirred solution of the trimethylsilyl-protected acetylene derivatives (5 mmol) in CH3OH (30 mL) was added K2CO3 (0.5 mmol). The mixture was stirred for 24 h at room temperature and concentrated, and the residue was diluted with Et2O and washed with water (3 × 30 mL). The organic phase was dried over MgSO4, filtered, and pumped dry. The resulting organic compound was dissolved in Et2O (5 mL) and filtered through a silica gel column with Et2O to afford compound 12a as white solid (546 mg, yield 84%). 1H NMR (400 MHz, CDCl3, ppm): δ 10.23 (s, 1H), 7.82 (m, 2H), 7.73 (m, 2H), 3.53 (s, 1H). GC/TOF HRMS-EI (m/z): [M]+ calcd for C9H6O, 130.0419; found, 130.0416. (4-(10-Bromoanthracen-9-yl)ethynyl)benzaldehyde (M5). To a mixture of PdCl2(PPh3)2 (175 mg, 0.25 mmol) and CuI (95 mg, 0.5 mmol) in THF (40 mL) were added successively 9-bromo-10-iodoanthracene (3.83 g, 10 mmol), compound 12a (1.43 g, 11 mmol), and NEt3 (40 mL). The resulting mixture was stirred for 24 h at room temperature. The dark solution was pumped dry, and the resulting black solid was extracted with CH2Cl2 (3 × 50 mL) and further purified by chromatography on silica gel with CH2Cl2:hexane (v:v, 1:1) to give compound M5 as yellow solid (3.46 g, yield 90%). 1H NMR (400 MHz, CDCl3, ppm): δ 10.08 (s, 1H), 8.66-8.64 (m, 2H), 8.60-8.58 (m, 2H), 7.96 (d, J ) 8.0 Hz, 2H), 7.90 (d, J ) 8.0 Hz, 2H), 7.67-7.66 (m, 4H). GC/TOF HRMS-EI (m/z): [M]+ calcd for C23H13BrO, 384.0150; found, 384.0162.

(4-(10-(4-(Bis(4-methoxyphenyl)amino)phenyl)anthracen9-yl)ethynyl)benzaldehyde (DM7). Compound M5 (960 mg, 2.5 mmol), compound D2 (872 mg, 2.5 mmol), Pd(PPh3)4 (347 mg, 0.3 mmol), aqueous Na2CO3 (2.0 M, 10 mL), C2H5OH (5 mL), and toluene (15 mL) were mixed in a flask. The mixture was degassed and refluxed for 24 h under N2 purge. After being cooled, the solvent was evaporated under vacuum, and the product was extracted with CH2Cl2 (3 × 50 mL). Evaporation of the solvent, followed by column chromatography with CH2Cl2:hexane (v:v, 1:2) on silica gel, gave compound DM7 as red-yellow solid (1.11 g, yield 73%). 1H NMR (400 Hz, CDCl3, ppm): δ 10.08 (s, 1H), 8.68 (d, J ) 8.8 Hz, 2H), 7.99-7.92 (m, 4H), 7.87 (d, J ) 8.8 Hz, 2H), 7.62 (t, J ) 8.0 Hz, 3H), 7.45 (t, J ) 7.6 Hz, 3H), 7.22 (t, J ) 8.4 Hz, 6H), 6.90 (d, J ) 8.8 Hz, 4H), 3.83 (s, 6H). GC/TOF HRMS-EI (m/z): [M]+ calcd for C43H31NO3, 609.2304; found, 609.2308. (3-(4-(10-(4-(Bis(4-methoxyphenyl)amino)phenyl)anthracen9-yl)ethynyl)phenyl)-2-cyanoacrylic Acid (TC501). A CH3CN (20 mL) solution of compound DM1 (264 mg, 0.5 mmol) and cynaoacetic acid (85 mg, 1 mmol) was refluxed in the presence of piperidine (0.2 mL) for 3 h. After removing the solvent, the residue was purified by column chromatography using silica gel and CH2Cl2:CH3OH (v:v, 10:1) mixed as the eluent to give the dye TC501 as purple solid (270 mg, yield 80%). 1H NMR (400 Hz, DMSO, ppm): δ 8.68 (d, J ) 8.8 Hz, 2H), 8.10 (s, 1H), 8.04 (d, J ) 7.6 Hz, 2H), 7.93 (d, J ) 8.0 Hz, 2H), 7.82 (d, J ) 8.8 Hz, 2H), 7.69 (t, J ) 7.6 Hz, 2H), 7.51 (t, J ) 7.2 Hz, 2H), 7.20 (d, J ) 8.8 Hz, 6H), 7.02 (d, J ) 8.0 Hz, 2H), 6.94 (d, J ) 8.8 Hz, 3H), 3.79 (s, 6H). GC/TOF HRMS-EI (m/z): [M - CO2]+ calcd for C45H32N2O2, 632.2464; found, 632.2470. Photophysical Properties. In DSCs, sensitizer is a pivotal and unique component with a function of light harvesting. Its spectral response overlapped with the solar emission will affect


J. Phys. Chem. C, Vol. 114, No. 19, 2010 9107 TABLE 4: Detailed Photovoltaic Parametersa of DSCs Sensitized by the Dyes TC201-TC602 Measured under Simulated AM 1.5 Irradiation (100 mW · cm-2)

Figure 4. Photocurrent action spectra of DSCs sensitized by the dyes TC201-TC602 with electrolyte containing 0.6 M DMPII, 0.06 M LiI, 0.04 M I2, and 0.4 M TBP in dried CH3CN solutions.

Figure 5. Photocurrent density vs voltage curves of DSCs sensitized by the dyes TC201-TC602 with electrolyte containing 0.6 M DMPII, 0.06 M LiI, 0.04 M I2, and 0.4 M TBP in dried CH3CN solutions.

the device photocurrent to a large extent; thus, we measured the UV-vis absorptions of the dyes TC201-TC602 both in CH2Cl2 solutions and on TiO2 films (Figure 1 and Figure 2, respectively). The characteristic data are collected in Table 1. All of the dyes with a strong absorption maximum in the visible region corresponding to the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) transitions are observed (Figure 1 and Table 1).19 The absorption spectrum of TC201 shows an absorption maximum (λmax) at 401 nm (ε ) 5600 M-1 · cm-1) (Figure 1a). Compared to TC201, the absorption spectrum of TC202 is red-shifted 17 nm corresponding to a 0.12 eV decrease in energy, with λmax appearing at 418 nm (ε ) 18 200 M-1 · cm-1) because of the introduction of a double bond. The introduction of a triple bond to TC201, giving TC203, causes a further red shift to 464 nm (ε ) 24 100 M-1 · cm-1). These can be understood from molecular modeling studies of the dyes TC201, TC202, and TC203 (Table 2). Theoretical computation shows that the ground state structure of TC201 possesses a 71.6° twist between the anthrathene moiety and electron acceptor, and the dihedral angle in TC202 is 48.4° because of the introduction of a double

dye

Jsc (mA · cm-2)

Voc (V)

ff (%)

η (%)

TC201 TC202 TC203 TC401 TC402 TC403 TC501 TC502 TC601 TC602

5.54 8.78 11.70 5.39 10.60 10.05 12.96 13.37 8.15 13.07

708 726 766 678 642 608 720 677 648 599

78.8 80.7 75.6 79.7 71.4 73.6 75.3 75.3 74.4 72.3

3.09 5.14 6.78 2.91 4.86 4.50 7.03 6.82 3.93 5.66

a The spectral distribution of our measurement system simulates AM 1.5 irradiation (100 mW · cm-2). Incident power intensity: Pin. Short-circuit photocurrent density: Jsc. Open-circuit photovoltage: Voc. Maximum electricity output power density: Pmax. Fill factor: ff ) Pmax/Pin. Solar energy conversion efficiency: η. Cell area tested with a metal mask: 0.159 cm2. The electrolyte composition is 0.6 M DMPII, 0.06 M LiI, 0.04 M I2, and 0.4 M TBP in dried CH3CN solutions.

bond, giving less twist than that of TC201. Furthermore, the dihedral angle in TC203 is 0° with the appearance of triple bond, giving almost a planar configuration. A significant red shift of TC203 relative to TC201 and TC202 derives from the full delocalization over an entire conjugated system in TC203.20 The conclusion can be confirmed by comparing the absorption spectra and molecular modeling studies of the dyes TC401, TC402, and TC403 (Figure 1b). Changing the position of the triple bond in TC203 close to the TPA donor, giving TC601, causes a blue shift to 449 nm (ε ) 17 100 M-1 · cm-1) (Figure 1d). In TC203, the electronegative property of the triple bond will strengthen the electron-withdrawing ability of the electron acceptor because of the identical dipole moment of the triple bond and electron acceptor. In TC601, however, the opposite dipole moment of the triple bond and electron acceptor exists, leading to weaker electron-withdrawing ability of the electron acceptor. Thus, the absorption spectrum of TC601 is blue shifted in comparison with that of TC203. Introducing two methoxyl groups and two triple bonds, giving TC602 (Chart 1), causes the largest red shift to 511 nm (ε ) 14 100 M-1 · cm-1) (Figure 1d) because of not only enhancement of the electron-donating ability of the TPA donor but also the most excellent planar configuration of TC602. When the dyes were attached on TiO2 films, the absorption spectra may shift more or less as compared to that in solutions because of strong interactions between the dyes and the semiconductor surface, which can lead to aggregated state of the dyes on semiconductor surface, such as H-aggregation for the blue shift or J-aggregation for the red shift. We can see from Figures 1 and 2 and Table 1 that absorption spectra of the dyes on TiO2 films only blue shifted slightly maybe due to H-aggregation or almost did not shift compared to that in CH2Cl2 solutions. In general, the smaller shift of the dye’s absorption spectrum on TiO2 films compared with that in solutions could have a smaller tendency to form an aggregation state on TiO2 film,21 thus anchoring monolayer dyes onto the TiO2 surface and accomplishing higher solar energy conversion efficiency of DSC. Electrochemical Properties. As shown in Figure 3, the cyclic voltammogram of TC501 was typical for so-called quasireversible behavior with a peak separation of 136 mV, rather than the 59 mV expected for the reversible case.22 The first oxidation potential (Eox) corresponding to the HOMO level of

9108


Teng et al.

CHART 2: Molecular Structures of the Anthracene-Containing Dyes TC202 and TC203 and Reference Dyes TPC1 and TC102

the dye was measured in DMF solution by cyclic voltammetry (CV). The values of Eox of all the dyes (Table 1) are sufficiently more positive than the potential of the I-/I3- redox mediator (about 0.4 V vs NHE),12a,23 indicating that the oxidized dyes formed after electron injection into the conduction band of TiO2 could accept electrons from I- ions in the electrolyte thermodynamically. The LUMO levels of the dyes were calculated by Eox - E0-0, where E0-0 is the zero-zero energy of the dyes estimated from the intersection between the absorption and emission spectra (Table 1). It is evident that the values of Eox - E0-0 are more negative than the conduction band of TiO2 (about -0.5 V vs NHE),24 thus all the photoexcited dyes can complete the process of electron injection into the conduction band of TiO2 thermodynamically. Noticeably, the relatively large energy gaps between the LUMO levels of the dyes and the conduction band of the TiO2 semiconductor allow for the addition of the 4-tertbutylpyridine (TBP) into the electrolyte, which can shift the conduction band of TiO2 negatively about 0.3 V and consequently improve the open-circuit voltage and total solar energy conversion efficiency.12b We can also see from Table 1 that the introduction of the triple bond or double bond in π-conjugations could change HUMO-LUMO energy gaps of the dyes more narrowly resulting from the enhancement of molecular planar configurations and red shift of the absorption spectra. Molecular Orbital Calculations. To gain insight into the geometrical electronic structures of the dyes, we performed DFT calculations on the dyes using the Gaussian 03 program package. In particular, we used B3LYP as an exchange-correlation functional and 6-31+G(d) as the basis set (Table 3).18,25 At the ground state (HOMO) of the dyes, electrons are homogeneously distributed on the electron donor (TPA derivatives). At the excited state (LUMO), the intramolecular charge transfer occurs, resulting in the electron movement to the electron acceptor (cyanoacrylic acid). The frontier molecular orbital of the dyes reveals that HOMO-LUMO excitation moves the electron density distribution from electron donor (TPA derivatives) to acceptor (cyanoacrylic acid) via different anthracene-containing π-conjugations. Photovoltaic Performance. The plots of incident photonto-current conversion efficiencies (IPCEs) versus wavelengths of DSCs with the dyes TC201-TC602 are shown in Figure 3. The photocurrent action spectra of the dyes TC202, TC203, and TC501 exhibit very high plateaus where IPCE values reach 85%. Considering the light absorption and scattering loss by the conducting glass, the maximum efficiency for absorbed

photon-to-collected electron conversion efficiency (APCE) is almost unity over a broad spectral range, suggesting a very high charge collection yield. The increased IPCE values of TC202 and TC203 relative to that of TC201 are attributed to the enhancement of molecular planar configurations (Figure 4a), and the photocurrent action spectra of TC501 and TC502 are red shifted compared to those of TC203 and TC403, which is in agreement with the absorption spectra of the dyes on TiO2 films. As presented in Figure 5c and Table 4, the short-circuit photocurrent densities (Jsc), open-circuit photovoltages (Voc), and fill factors (ff) of DSCs sensitized by the dyes TC501 and TC502 under simulated AM 1.5 irradiation (100 mW · cm-2) are 12.96 mA · cm-2, 720 mV, 0.753 and 13.37 mA · cm-2, 677 mV, 0.753, respectively, yielding overall conversion efficiencies (η) up to 7.03% and 6.82%, respectively. We attribute the excellent efficiencies to both much broader spectra matched with the solar irradiance and much better molecular planar configurations of the dyes TC501 and TC502. Influence of Anthracene Moiety on DSC Parameters. To gain insight into the influence of the anthracene moiety in π-conjugations on DSC parameters, DSCs sensitized by the anthracene-containing dyes TC202 and TC203 and reference dyes TPC118 and TC102 (the structures are shown in Chart 2) were tested, and the distinguishing DSC parameters Jsc, Voc, and η were obtained. The detailed photovoltaic parameters are summarized in Table 5. We can find that higher Jsc was obtained by DSC sensitized by TC203 than that by TC102, but lower Jsc was obtained by DSC sensitized by TC202 than that by TPC1. With the appearances of a triple bond and double bond, the dihedral angles in TC202 and TC203 are 48.4° and 0°, respectively. Better molecular planar configuration of TC203 than that of TC202 leads to broader absorption spectrum and thus higher Jsc of DSC sensitized by TC203. This result reveals that with suitable planar configuration introducing the anthracene moiety into π-conjugations could improve Jsc to a large extent. Electrochemical impedance spectroscopy (EIS) analysis26 was performed to study the interfacial charge transfer processes in DSCs sensitized by the dyes TC202 and TC203 and reference dyes TPC1 and TC102. The Nyquist plots and Bode phase plots are shown in Figure 6. The small semicircle in the Nyquist plot, which corresponds to the high-frequency peaks in the Bode phase plots, represents the electron transfer from the Pt counterelectrode to the oxidized species in the electrolyte, that is, the reduction of the oxidized species to the reduced species. The large semicircle, which corresponds to the midfrequency peaks



TABLE 5: Detailed Photovoltaic Parametersa and the Electron Recombination Lifetimes (τ)b of DSCs Sensitized by the Anthracene-Containing Dyes TC202 and TC203 and Reference Dyes TPC1 and TC102 dye

Jsc (mA · cm-2)

Voc (mV)

ff

η (%)

τ (ms)

TPC1 TC102 TC202 TC203

10.39 7.72 8.78 11.70

702 694 726 766

0.785 0.803 0.807 0.756

5.73 4.30 5.14 6.78

20 24 35 38

a

The spectral distribution of our measurement system simulates AM 1.5 solar emission. Incident power intensity: Pin. Short-circuit photocurrent density: Jsc. Open-circuit photovoltage: Voc. Maximum electricity output power density: Pmax. Fill factor: ff ) Pmax/Pin. Total power conversion efficiency: η. Cell area tested with a metal mask: 0.159 cm2. The electrolyte composition is 0.6 M DMPII, 0.06 M LiI, 0.04 M I2, and 0.4 M TBP in dried CH3CN solution. b The electron recombination lifetime (τ) can be extracted from the angular frequency (ωmin) at the midfrequency peak in the Bode phase plot using τ ) 1/ωmin.

library of new synthesized dyes and under the optimized cell test conditions, the dye TC501 shows a prominent solar energy conversion efficiency (η) up to 7.03% (Jsc ) 12.96 mA · cm-2, Voc ) 720 mV, ff ) 0.753) under simulated AM 1.5 irradiation (100 mW · cm-2). Further improvement of the dye structures based on TC501 by molecular engineering aiming for even better performance of DSCs is in progress. Acknowledgment. We gratefully acknowledge the financial support of this work from the following sources: China Natural Science Foundation (Grant 20633020), the National Basic Research Program of China (Grant No. 2009CB220009), the MinistryofScienceandTechnology(MOST)(Grant2001CCA02500), the Ministry of Education (MOE), the Program for Changjiang Scholars and Innovation Research Team in university (PCSIRT), the Swedish Energy Agency, the Swedish Research Council, and K&A Wallenberg Foundation. The authors are grateful to Prof. Can Li and Min Zhong at Dalian Institute of Chemical Physics, CAS, China, for EIS measurement and helpful discussions. Supporting Information Available: Synthetic routes and frontier molecular orbitals of the HOMO and LUMO of the dyes TC201-TC602. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 6. Electrochemical impedance spectra, scanned from 10-1 to 105 Hz at room temperature, for DSCs sensitized by TPC1, TC102, TC202, and TC203, respectively: (a) Nyquist plots and (b) Bode phase plots. The cells were measured at -0.7 V bias in the dark. The alternate current (AC) amplitude was set at 10 mV.

in the Bode phase plots, represents the charge recombination arising from electrons in the TiO2 film recombinating with the I3- ion in the electrolyte. The electron recombination lifetimes (τ), extracted from the angular frequency (ωmin) at the midfrequency peak in the Bode phase plot using τ ) 1/ωmin, are 35 and 38 ms for DSCs sensitized by the anthracene-containing dyes TC203 and TC205, which are much larger than 20 and 24 ms for DSCs sensitized by reference dyes TPC1 and TC102, respectively. The significant increase in electron recombination lifetime means that the charge recombination arising from electrons in TiO2 film with the I3- ion in electrolyte is suppressed by the introduction of the anthracene moiety. Thus, we can conclude that a significant increase in Voc by introducing the anthracene moiety is attributed to the suppressed charge recombination. Conclusion In summary, a series of novel metal-free organic dyes bridged by anthracene-containing π-conjugations were synthesized and applied in DSCs. With the introduction of the anthracene moiety, together with a triple bond to keep suitable molecular planar configuration and broaden absorption spectra, short-circuit photocurrent densities (Jsc) and open-circuit photovoltages (Voc) were improved to a large extent. The improvement of Jsc is attributed to the much broader absorption spectra of the dyes with the anthracene moiety. Electrochemical impedance spectroscopy (EIS) analysis reveals that the introduction of the anthracene moiety could suppress the charge recombination arising from electrons in TiO2 film and the I3- ion in electrolyte, thus improving the Voc of DSCs considerably. Among this

(1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) (a) Gra¨tzel, M. Nature 2001, 414, 338. (b) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Grta¨zel, M. J. Am. Chem. Soc. 2001, 123, 1613. (c) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gra¨tzel, M. J. Am. Chem. Soc. 2005, 127, 16835. (d) Chiba, Y.; Islam, A.; Watanabe, Y.; Komiya, R.; Koide, N.; Han, L. Jpn. J. Appl. Phys. 2006, 45, 638. (e) Nazeeruddin, M. K.; Bessho, T.; Cevey, L.; Ito, S.; Klein, C.; De Angelis, F.; Fantacci, S.; Comte, P.; Liska, P.; Imai, H.; Gra¨tzel, M. J. Photochem. Photobiol. A 2007, 185, 331. (f) Gao, F.; Wang, Y.; Zhang, J.; Shi, D.; Wang, M.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. Chem. Commun. 2008, 23, 2635. (3) (a) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597. (b) Wang, Z.; Cui, Y.; Hara, K.; Dan-oh, Y.; Kasada, C.; Shinpo, A. AdV. Mater. 2007, 19, 1138. (c) Wang, Z.; Cui, Y.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Hara, K. J. Phys. Chem. C 2007, 111, 7224. (4) (a) Sayama, K.; Hara, K.; Mori, N.; Satsuki, M.; Suga, S.; Tsukagoshi, S.; Abe, Y.; Sugihara, H.; Arakawa, H. Chem. Commun. 2000, 13, 1173. (b) Sayama, K.; Tsukagoshi, S.; Hara, K.; Ohga, Y.; Shinpou, A.; Abe, Y.; Suga, S.; Arakawa, H. J. Phys. Chem. B 2002, 106, 1363. (5) (a) Ma, X.; Hua, J.; Jin, Y.; Meng, F.; Zhan, W.; Tian, H. Tetrahedron 2008, 64, 345. (b) Zhan, W.; Wu, W.; Hua, J.; Jing, Y.; Meng, F.; Tian, H. Tetrahedron Lett. 2007, 48, 2461. (6) (a) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126, 12218. (b) Schmidt-Mende, L.; Bach, U.; Humphry-Baker, R.; Horiuchi, T.; Miura, H.; Ito, S.; Uchida, S.; Gra¨tzel, M. AdV. Mater. 2005, 17, 813. (7) (a) Wang, Z.; Li, F.; Huang, C. J. Phys. Chem. B 2001, 105, 9210. (b) Wang, Z.; Li, F.; Huang, C. Chem. Commun. 2000, 20, 2063. (c) Chen, Y.; Li, C.; Zeng, Z.; Wang, W.; Wang, X.; Zhang, B. J. Mater. Chem. 2005, 15, 1654. (8) (a) Ferrere, S.; Zaban, A.; Gregg, B. A. J. Phys. Chem. B 1997, 101, 4490. (b) Ferrere, S.; Gregg, B. A. New J. Chem. 2002, 26, 1155. (9) Kitamura, T.; Ikeda, M.; Shigaki, K.; Inoue, T.; Anderson, N. A.; Ai, X.; Lian, T.; Yanagida, S. Chem. Mater. 2004, 16, 1806. (10) Hara, K.; Horiguchi, T.; Kinoshita, T.; Sayama, K.; Sugihara, H.; Arakawa, H. Chem. Lett. 2000, 29, 316. (11) (a) Hagberg, D. P.; Edvinsson, T.; Marinado, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Chem. Commun. 2006, 21, 2245. (b) Hwang, S.; Lee, J. H.; Park, C.; Lee, H.; Kim, C.; Park, C.; Lee, M. H.; Lee, W.; Park, J.; Kim, K.; Park, N. G.; Kim, C. Chem. Commun. 2007, 46, 4887. (c) Hagberg, D. P.; Marinado, T.; Karlsson, K. M.; Nonomura, K.; Qin, P.; Boschloo, G.; Brinck, T.; Hagfeldt, A.; Sun, L. J. Org. Chem. 2007, 72, 9550. (d) Liang, M.; Xu, W.; Cai, F.; Chen, P.; Peng, B.; Chen, J.; Li, Z. J. Phys. Chem. C 2007, 111, 4465. (e) Xu, W.; Peng, B.; Chen, J.; Liang, M.; Cai,

9110


F. J. Phys. Chem. C 2008, 112, 874. (f) Tsai, M. S.; Hsu, Y. C.; Lin, J. T.; Chen, H. C.; Hsu, C. P. J. Phys. Chem. C 2007, 111, 18785. (g) Kitamura, T.; Ikeda, M.; Shigaki, K.; Inoue, T.; Anderson, N. A.; Ai, X.; Lian, T.; Yanagida, S. Chem. Mater. 2004, 16, 1806. (h) Ning, Z.; Zhang, Q.; Wu, W.; Pei, H.; Liu, B.; Tian, H. J. Org. Chem. 2008, 73, 3791. (i) Velusamy, M.; JustinThomas, K. R.; Lin, J. T.; Hsu, Y. C.; Ho, K. C. Org. Lett. 2005, 7, 1899. (12) (a) Li, S. L.; Jiang, K. J.; Shao, K. F.; Yang, L. M. Chem. Commun. 2006, 26, 2792. (b) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Yoshihara, T.; Murai, M.; Kurashige, M.; Ito, S.; Shinpo, A.; Suga, S.; Arakawa, H. AdV. Funct. Mater. 2005, 15, 246. (13) (a) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J. H.; Fantacci, S.; DeAngelis, F.; DiCenso, D.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 16701. (b) Jung, I.; Lee, J. K.; Song, K. H.; Song, K.; Kang, S. O.; Ko, J. J. Org. Chem. 2007, 72, 3652. (c) Choi, H.; Lee, J. K.; Song, K.; Kang, S. O.; Ko, J. Tetrahedron 2007, 63, 3115. (d) Choi, H.; Baik, C.; Kang, S. O.; Ko, J.; Kang, M. S.; Nazeeruddin, M. K.; Gra¨tzel, M. Angew. Chem., Int. Ed. 2008, 47, 327. (14) Tian, H. N.; Yang, X. C.; Chen, R. K.; Pan, Y. Z.; Li, L.; Hagfeldt, A.; Sun, L. Chem. Commun. 2007, 36, 3741. (15) (a) Chen, R.; Yang, X.; Tian, H.; Wang, X.; Hagfeldt, A.; Sun, L. Chem. Mater. 2007, 19, 4007. (b) Chen, R.; Yang, X.; Tian, H.; Sun, L. J. Photochem. Photobiol. A 2007, 189, 295. (16) (a) Koumura, N.; Wang, Z. S.; Mori, S.; Miyashita, M.; Suzuki, E.; Hara, K. J. Am. Chem. Soc. 2006, 128, 14256. (b) Kim, D.; Lee, J. K.; Kang, S. O.; Ko, J. Tetrahedron 2007, 63, 1913. (17) (a) Ito, S.; Zakeeruddin, S. M.; Humphry-Baker, R.; Liska, P.; Charvet, R.; Comte, P.; Nazeeruddin, M. K.; Pechy, P; Takata, M.; Miura, H.; Uchida, S.; Gra¨tzel, M. AdV. Mater. 2006, 18, 1202. (b) Ito, S.; Miura, H.; Uchida, S.; Takata, M.; Sumioka, K.; Liska, P.; Comte, P.; Pechy, P.; Gra¨tzel, M. Chem. Commun. 2008, 41, 5194. (18) Tian, H.; Yang, X.; Chen, R.; Zhang, R.; Hagfeldt, A.; Sun, L. J. Phys. Chem. C 2008, 112, 11023. (19) Lin, J.; Elangovan, A.; Ho, T. J. Org. Chem. 2005, 70, 7397. (20) Choi, H.; Lee, J. K.; Song, K. H.; Song, K.; Kang, S. O.; Koa, J. Tetrahedron 2007, 63, 1553.

Teng et al. (21) Roquet, S.; Cravino, A.; Leriche, P.; Ale´veˆque, O.; Fre`re, P.; Roncali, J. J. Am. Chem. Soc. 2006, 128, 3459. (22) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley and Sons: New York, 2001. (b) Cameron, P. J.; Peter, L. M.; Zakeeruddin, S. M.; Gra¨tzel, M. Coord. Chem. ReV. 2004, 248, 1447. (23) Kim, S.; Choi, H.; Kim, D.; Song, K.; Kang, S. O.; Ko, J. Tetrahedron 2007, 63, 9206. (24) Hagfeldt, A.; Gra¨zel, M. Chem. ReV. 1995, 95, 49. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (a) Beche, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Ditchfield, R.; Herhe, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (26) (a) Adachi, M; Sakamoto, M; Jiu, J; Ogata, Y; Isoda, S. J. Chem. Phys. B 2006, 110, 13872. (b) Wang, Q; Moser, J; Gra¨tzel, M. J. Chem. Phys. B 2005, 109, 14945. (c) Wang, Z; Koumura, N; Cui, Y; Takahashi, M; Sekiguchi, H; Mori, A; Kubo, T; Furube, A; Hara, K. Chem. Mater. 2008, 20, 3993. (d) Kuang, D; Uchida, S; Humphry-Baker, R; Zakeeruddin, S; Gra¨tzel, M. Angew. Chem., Int. Ed. 2008, 47, 1923.

JP101238K

Molecular Design of Anthracene-Bridged Metal-Free Organic Dyes for

Recommend Documents