pubs.acs.org/cm Published on Web 07/12/2010 r2010 American Chemical Society DOI:10.1021/cm100770x
Toward Optimization of Oligothiophene Antennas: New Ruthenium
Sensitizers with Excellent Performance for Dye-Sensitized Solar Cells
Jen-Fu Yin,
‡Jian-Ging Chen,
§Zong-Zhan Lu,
†Kuo-Chuan Ho,
§Hong-Cheu Lin,*
,‡and
Kuang-Lieh Lu*
,††Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan,‡Department of Materials Science and
Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, and§Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
Received March 16, 2010. Revised Manuscript Received June 9, 2010
This paper reports on the design and synthesis of three new ruthenium sensitizers, as well as the optimization of their linear or dendritic light-harvesting oligothiophene antennas to achieve superior device performance. The three new ruthenium sensitizers, [Ru(dcbpy)(obtip)(NCS)2] (JF-5, dcbpy =4,40
-dicarboxylic acid-2,20-bipyridine, obtip =2-(5-octyl-(2,20-bithiophen)-50 -yl)-1H-imidazo[4,5-f][1,10]-phenanthroline), [Ru(dcbpy)(ottip)(NCS)2] (JF-6, ottip =2-(5-octyl-2,20,50,200-terthiophen)-500
-yl)-1H-imidazo[4,5-f][1,10]phenanthroline), and [Ru(dcbpy)(dottip)(NCS)2] (JF-7, dottip
=2-(2,3-di-(5-octylthiophen-2-yl)thiophen-5-yl)-1H-imidazo[4,5-f][1,10]phenanthroline), were synthesized in a typical one-pot reaction. The ruthenium sensitizer JF-5 incorporating a linear and planar 2,20 -bithiophene antenna showed the best DSCs performance (9.5%; compared to N3, 8.8%). The difference in the performance of these sensitizers demonstrates that elongating the linear and planar light-harvesting antenna result in an enhancement in MLCT intensity, but a reduction in the quantity of dye-loading. This finding not only permitted the power-conversion efficiency in ruthenium sensitizers containing oligothiophene antennas to be optimized, but also points to a promising direction for molecule engineering in DSCs.
Introduction
With the likely increase in oil prices on the horizon, the development of alternative energy sources has become a subject of major research interest. The solar energy incident on the earth’s surface, whose globally convertible power is estimated to be 120000 TW, constitutes an inexhaustible, plentiful, and clean energy source, which is sufficient to satisfy the world’s needs.1Therefore, the practical and efficient use of solar energy has been a consistently significant topic in recent years. The dye-sensitized solar cells (DSCs) have great potential for replacing traditional silicon solar cells because of their high photon-to-current conversion effici-ency, ease, and low cost of preparation.2,3The best power-conversion efficiency (up to 11%) was achieved by using ruthenium-based dyes in DSCs, such as N3- and N719-sensitized solar cells.4,5Attempts to design and synthsize new heteroleptic ruthenium sensitizers with higher device
performances is ongoing.6-15There are two crucial factors that affect the performance of DSCs, the intensity of the metal-to-ligand charge transfer (MLCT) band16,17and the quantity of dye that can be loaded.18,19Several groups have modified ancillary ligands in ruthenium complexes by elon-gating20-22or bifurcating23,24conjugated light-harvesting
*To whom correspondence should be addressed. Fax:þ886-2-27831237. E-mail: [email protected].
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chromophores to enhance and broaden the MLCT band. However, this route also involves a decrease in the quantity of dye that can be loaded on the TiO2surface, because of the
larger volume of dye molecules, a factor that lowers device performance.23Therefore, the issue arises as to what is the optimum length of oligothiophene antennas required to optimize these two inversely proportional factors and result in superior device performance. Herein, we report on an excellent resolution to this problem by a comparison of four ruthenium sensitizers, 5, 6, 7 and 2 (where JF-2 has been reported in our previous study15), with ancillary ligands of linear or dendritic conjugated oligothiophene antennas (Chart 1). The power-conversion efficiencies of these dyes were optimized by incorporating a linear 2,20 -bithiophene antenna and this optimization can be explai-ned by the experimental data collected and was further confirmed in theoretical studies. This finding not only optimizes the power-conversion efficiency in ruthenium sensitizers containing oligothiophene antennas but also represents an alternative strategy for improving DSCs.
Results and Discussion
The 5-octyl-(2,20-bithiophene) and 5-octyl-(2,20,50,200 -terthiophene) were obtained by forming the dianion of their oligothiophene precursors with 1.2 equiv of n-butylli-thium, followed by reaction with 1-bromooctane. 5-Octyl-(2,20-bithiophene)-50-carbaldehyde and 5-octyl-(2,20,50,200 -terthiophene)-500-carbaldehyde were prepared using the same synthetic procedure as described above, only N-for-mylpiperidine was used instead of 1-bromooctane. Coupling with 1,10-phenanthroline-5,6-dione and am-monium acetate as the source of ammonia in glacial acetic acid to form the ancillary ligands, obtip and ottip, according to protocols reported by Steck and Day.25 The synthetic procedure for preparing dottip is very similar to that for obtip and ottip. The synthesis of the dottip ligand started by coupling 2,3-dibromothiophene and trimethyl(5-octylthiophen-2-yl)stannane via a Stille coupling, followed by a reaction with N-formylpiperidine and coupling with 1,10-phenanthroline-5,6-dione. Scheme 1
depicts the synthetic routes of the ancillary ligands, obtip, ottip and dottip. The ruthenium complexes, JF-5, JF-6 and JF-7, were prepared by treating a ruthenium dimer with each of the ancillary ligands, i.e., obtip, ottip and dottip, respectively, in dimethylformamide (DMF) under an ar-gon atmosphere using a typical one-pot synthesis.26The reaction of [RuCl2(p-cymene)]2with an ancillary ligand in
a 1:2 stoichiometry in dry DMF at 80°C resulted in the formation of a mononuclear Ru(II) complex. Afterward, the anchoring ligand (dcbpy, 4,40-dicarboxylic acid-2,20 -bipyridine) was added, and the reaction mixture was refluxed in the dark. In this process, the cymene ligand from the Ru(II) coordination sphere is replaced by dcbpy. Finally, excess ammonium thiocyanate was added to af-ford the desired ruthenium sensitizers as reaction products. The three new ruthenium sensitizers, [Ru(dcbpy)(obtip)-(NCS)2] (JF-5, dcbpy =4,40-dicarboxylic acid-2,20
-bipyri-dine, obtip =2-(5-octyl-(2,20-bithiophen)-50 -yl)-1H-imidazo-[4,5-f][1,10]phenanthroline), [Ru(dcbpy)(ottip)(NCS)2]
(JF-6, ottip =2-(5-octyl-2,20,50,200-terthiophen)-500 -yl)-1H-imidazo[4,5-f][1,10]phenanthroline), and [Ru(dcbpy)-(dottip)(NCS)2] (JF-7, dottip
=2-(2,3-di-(5-octylthiophen-2-yl)thiophen-5-yl)-1H-imidazo[4,5-f][1,10]phenanthroline), were synthesized in a typical one-pot reaction. The UV-vis absorption spectra of the series of ruthenium sensiti-zers, JF-5, JF-6, and JF-7, in DMF solution consist of three main features, assigned as bands I, II and III with in-creasing energy order (Figure 1 and Table 1). Band III at 310 nm is assigned to the overlap of an intraligandπ-π* transition. Band II, at around 358-430 nm, includes both intraligandπ-π* and MLCT transitions. The molar ex-tinction coefficient (ε) of the lower energy MLCT band I at 520 nm in JF-6 is 1.49 104M-1cm-1, higher than that of JF-5 and JF-7. This enhancement is because JF-6 has a longer and planar oligothiophene main chain which not only increases the light-harvesting capability but also maintains the delocalizedπ-framework throughout en-tire molecule. Moreover, band II could be red-shifted and increased itsε value, presumably due to the elongation of the length of the oligothiophene main chain. Although JF-7 also contains three thiophene moieties, the bifurcated and twisted oligothiophene moiety destroys the connection of the delocalizedπ-framework in dottip (illustrated in Figure 3). For this reason, JF-7 shows a lower MLCT in-tensity compared with the corresponding values for JF-5 and JF-6. Therefore, it can be generally assumed that the MLCT intensity would increase by elongating rather than bifurcating the oligothiophene chain.
The electrochemical behaviors of the ruthenium dyes, JF-5, JF-6, and JF-7, were investigated by cyclic and square-wave voltammetry (Figure 2) and their oxidation potentials are listed in Table 1. The cyclic voltammograms showed that the oxidation and reduction potentials of all dyes are very close to each other. Moreover, the oxidation potentials of JF-5, JF-6, and JF-7 were unequivocally Chart 1. Molecular Structures of JF-2, JF-5, JF-6, and JF-7
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determined to be 0.30, 0.32, and 0.31 (V vs Fc/Fcþ) by square-wave voltammetry, respectively. The highest-occupied molecular orbitals (HOMOs) and lowest-unoccu-pied molecular orbitals (LUMOs) energy levels of all ruthe-nium dyes were calculated from their oxidation potentials and the absorption edge obtained from the UV-vis absorp-tion spectra (Table 1 and Figure 3). These HOMO and LUMO energy levels, compared to the levels of the conduc-tion band of TiO2and the redox electrolyte, are appropriate
for electron injection and dye regeneration.
The optimized molecular structures for JF-5, JF-6, and JF-7, as well as the frontier molecular orbitals of their calculated HOMOs and LUMOs energy levels are shown in Figure 3. On closer inspection, the frontier molecular orbitals of JF-5, JF-6, and JF-7 indicate that the HOMOs are localized on the mixed Ru-t2gand NCS-π orbital.
The LUMOs of these series ruthenium sensitizers are localized homogeneously on the anchoring ligand. The selected and calculated results of time dependent DFT (TDDFT) singlet excited-state transitions for MLCT band I of the series ruthenium sensitizers are listed in Table 2. An-alyses of the transitions and orbital contributions indicate
that band I of JF-6 included a higher oscillator strength (f = 0.1625) and a higher probability of MLCT transi-tions (72%) compared with that of JF-5 (f = 0.1171; 66%) and JF-7 (f = 0.1152; 57%). This result points out again that ruthenium sensitizers with longer and planar oligothiophene antenna have the ability to enhance MLCT intensity. The experimental and predicted electronic spectra are in overall good agreement.
Figure 4 shows the current density-voltage (J-V) cha-racteristics and monochromatic incident photon-to-cur-rent conversion efficiency (IPCE) curves for the JF-5-, JF-6-, and JF-7-sensitized solar cells. The JF-5-sensitized solar cell gave a short-circuit photocurrent density (Jsc) of
18.3 mA cm-2, an open-circuit voltage (Voc) of 0.73 V and
a fill factor (ff) of 0.71, which correspond to a surprisingly high overall power-conversion efficiency (η) of 9.5% com-pared with JF-6 (8.7%), JF-7 (6.4%), and N3 (8.8%) under standard global AM 1.5 solar conditions (device data listed in Table 1). The calculated Jscfrom the overlap integral of
the IPCE with the standard AM 1.5 solar emission spec-trum is 17.0 mA cm-2, and the mismatch factor27of the calculated and measured photocurrent density is less than 1.08. The values for Jsc calculated by integration of the
IPCE with the AM 1.5G solar spectrum for all dyes are listed in Table 1. Moreover, the IPCE curve for JF-5 (as shown Figure 4) is over 80% in the 400-600 nm range, with the highest value of 92% at 523 nm compared with that of JF-6 (84%) and JF-7 (76%). The dye-loading measure-ment (listed in Table 1) indicated that JF-5 contains a higher quantity of loaded dye (1.8 10-7 mol cm-2), compared with that of JF-6 (1.1 10-7 mol cm-2) and JF-7 (0.5 10-7mol cm-2). The decreasing dye-loading quantity in JF-5 to JF-7 is because of the increased vol-ume of dye molecules, produced by further elongating or bifurcating the oligothiophene antenna.
Figure 5 shows the relationship betweenε for MLCT band I,Γ, and η of JF-2,15JF-5, JF-6, and JF-7. Although Scheme 1. Synthetic Procedure of the Ancillary Ligands, obtip, ottip and dottipa
a(a) n-Butyllithium, 1-bromooctane, THF,-78 °C; (b) n-butyllithium, N-formylpiperidine, THF, -78 °C; (c) NH
4OAc, 1,10-phenanthroline-5,6-dione, glacial acetic acid, reflux 2 h; (d) Pd(PPh3)2Cl2, DMF, reflux 22 h.
Figure 1. UV-vis absorption spectra of the series ruthenium sensitizers, JF-5, JF-6, and JF-7 in DMF solution.
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JF-5 displayed the second highest MLCT intensity and dye-loading quantity, their curves intersect around JF-5, indicating that the oligothiophene antenna (2,20 -bithi-ophene) in JF-5 is able to simultaneously acquire the best overall benefit from these two factors, resulting in the highest power-conversion efficiency.
Electrochemical impedance spectra (EIS) indicated that the DSCs consist of mainly three resistances, corresponding to three semicircles, which from first to third are the resistance of the charge transfer at the counter electrode,
the resistance at the dye-adsorbed TiO2/electrolyte
inter-face together with electron transport in the TiO2network,
and the resistance of triiodide to diffusion in the electro-lyte, respectively.28-31It is clear from Figure 6 that the
Table 1. Optical, Electrochemical Data, and Cell Performance of JF-5, JF-6, and JF-7 ε ( 104 M-1cm-1) cell performanced dye π-π* π-π* or 4d-π* 4d-π* Eoxbof RuIII/II (V vs Fc/Fcþ) EHOMOc (eV) ELUMOc (eV) Jsc (mA cm-2) Voc(V) ff η (%)e Γf ( 10-7mol cm-2) JF-5 3.59 (302)a 3.95 (383) 1.26 (520) 0.30 5.40 3.66 18.3 (17.0)g 0.73 0.71 9.5 1.8 JF-6 3.83 (299) 4.79 (417) 1.49 (520) 0.32 5.42 3.70 17.0 (15.9) 0.72 0.71 8.7 1.1 JF-7 4.87 (300) 2.75 (375) 1.05 (520) 0.31 5.41 3.65 13.1 (12.5) 0.70 0.70 6.4 0.5 aAbsorption maxima,λ
max(nm).bThe Ag/AgNO3reference electrode was calibrated with a ferrocene/ferrocinium (Fc/Fcþ) redox couple. The electrochemical experiments were carried out in 0.1 M tetrabutylammonium tetrafluoroborate/DMF solution.cThe values of E
HOMOand ELUMOwere calculated with the following formula: HOMO (eV) = Eox- EFc/Fcþþ 5.1; LUMO (eV) = HOMO - Eg. Egis the absorption onset estimated from the electronic absorption spectra of the sensitizers.dThe cell performance data of JF-5, JF-6, and JF-7 were the average of four measurements.eThe power-conversion efficiency of N3-sensitized solar cell (where N3 is [Ru(dcbpy)2(NCS)2]) measured by the same device fabrication process is 8.8%.
f Γ is the surface concentration of the dye molecules on TiO2film.gThe values in parentheses were calculated by integration of the IPCE with the AM 1.5G solar spectrum.
Figure 2. (a) Cyclic and (b) square-wave voltammograms of ruthenium sensitizers, JF-5, JF-6, and JF-7, in DMF.
Figure 3. Schematic representation of the frontier orbitals of JF-5, JF-6, and JF-7, along with isodensity plots of the HOMO and LUMO orbitals. The experimental and calculated energy levels are displayed in solid and dashed line, respectively.
Table 2. Selected Results of Time Dependent DFT (TDDFT) Singlet Excited-State Transitions for MLCT Band I of JF-5, JF-6, and JF-7,
Respectively, in Dimethylformamide
dye wavelength (nm) f major contribution
JF-5 672.8 0.0239 HOMOf LUMO (79%) 595.9 0.0006 HOMO-3 f LUMO (67%) 548.6 0.1171 HOMO-2 f LUMO (66%) 523.8 0.0034 HOMO-1 f LUMO (73%) JF-6 672.3 0.0237 HOMO-1 f LUMO (86%) 597.2 0.0006 HOMO-2 f LUMO (71%) 564.2 0.0003 HOMOf LUMO (79%) 548.1 0.1625 HOMO-3 f LUMO (72%) JF-7 671.1 0.0233 HOMOf LUMO (85%) 594.4 0.0006 HOMO-3 f LUMO (58%) 547.4 0.1152 HOMO-2 f LUMO (57%) 514.2 0.0005 HOMO-1 f LUMO (80%)
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radii of the second semicircles increase in the order JF-5< JF-6 < JF-7. This indicates that the overall resistance at the TiO2/electrolyte interface and TiO2network of the
JF-5-sensitized solar cell is smaller than that for the JF-6-and JF-7-sensitized solar cells. An EIS analysis also supports the conclusion that device performance of JF-5 is superior. Therefore, the JF-JF-5-sensitized solar cell exhibits the best overall device performance, as compared to that of the JF-2-, JF-6-, and JF-7-sensitized solar cells. This optimization of power-conversion efficiency by var-ying the length of the light-harvesting oligothiophene
antenna can be explained by UV-vis absorption spectra, electrochemical data, dye-loading measurements, electro-chemical impedance spectroscopy (EIS), density functional theory (DFT), and time-dependent density functional theory (TDDFT) studies.
Conclusion
In conclusion, we report on the design and synthesis of three new ruthenium sensitizers, JF-5, JF-6, and JF-7, with excellent power-conversion efficiency and optimize the light-harvesting oligothiophene antennas that contain 2,20-bithiophene, which shows superior device perfor-mance. The difference in the performance of these sensi-tizers demonstrates that elongating the linear and planar light-harvesting antenna result in an enhancement in MLCT intensity, but a reduction in the quantity of dye-loading. Moreover, the sensitizer incorporating a longer ancillary ligand employed in DSCs would result in an increased cell resistance. This finding not only permitted the power-conversion efficiency in ruthenium sensitizers containing oligothiophene antennas to be optimized, but also points to a promising direction for molecule engine-ering in DSCs.
Experimental Section
Materials and Measurements. All reactants and solvents were purchased from commercial sources and used as received.1H NMR spectra were recorded on a Bruker AMX400 or AV400 spectrometer with tetramethylsilane as the internal standard. Elemental analysis was determined on a Perkin-Elmer 2400 CHN analyzer. Mass spectrometry was performed with a JMS-700 double focusing mass spectrometer (JEOL, Tokyo, Japan). Absorption spectra were recorded with a UV-vis spectrophot-ometer (Hewlett-Packard 8453). Cyclic and square-wave vol-tammetry were performed on a CH Instruments electrochemical analyzer in DMF solutions (10-3M) with a platinum plate as the working electrode, a platinum wire auxiliary electrode, and a nonaqueous Ag/AgNO3reference electrode. The supporting
elec-trolyte was tetrabutylammonium tetrafluoroborate (0.1 M), and ferrocene was selected as the internal standard. The solutions were bubbled with N2for 10 min before measurements. The scan
rate for CV was 100 mV s-1. The square-wave voltammograms swept with potential step increment of 10 mV and frequency of 25 Hz. The dye loading measurement on TiO2films was carried
out by desorbing the dye into 0.1 M NaOH solution in CH3OH
and then measuring the ultraviolet-visible absorption spectra of the resultant solution with the same dilution. The adsorbed quantity of each dye was calculated from the difference in con-centration for each solution before and after TiO2film
immer-sion. Electrochemical impedance spectra (EIS) of the DSCs were obtained using a potentiostat/galvanostat equipped with FRA2 modules under a constant light illumination of 100 mW/cm2and the frequency range used was from 10 mHz to 65 kHz. The app-lied bias voltage and ac amplitude were set at the open-circuit voltage (Voc) of the DSCs and 10 mV, respectively.
5-Octyl-(2,20-bithiophene)-50-carbaldehyde. The precursor, 5-octyl-(2,20-bithiophene), was synthesized by previously repor-ted procedures.85-Octyl-(2,20-bithiophene) (277.5 mg, 1.0 mol) was dissolved in 30 mL THF in a well-dried flask under the pro-tection of a N2flow. The solution was cooled in a liquid nitrogen/
acetone cooling bath, and n-butyllithium (0.8 mL, 1.28 mmol, 1.6 M
Figure 4. Current density-voltage characteristics of the photovoltaic devices with JF-5, JF-6, and JF-7 as photosensitizers under AM 1.5 simulated sunlight (100 mW cm-2) illumination. (thickness of TiO2= 12 μm; cell active area = 0.16 cm2
); inset: the incident photon-to-current conversion efficiency spectra of the photovoltaic devices with different photosensitizers.
Figure 5. Variation in the optical, dye-loading properties and power-conversion efficiencies for JF-2, JF-5, JF-6, and JF-7.
Figure 6. Electrochemical impedance spectra of JF-5-, JF-6-, and JF-7-sensitized solar cells in the form of a Nyquist plot.
in hexane) was then added dropwise. The cooling bath was re-moved, and the solution was allowed to warm to room tempera-ture and N-formylpiperidine (139.8 mg, 1.2 mmol) was then added in one portion. After 6 h, the solution was poured into 200 mL of cool water. The organic layer was separated, and the aqueous layer was extracted with ether. The organic layers were collected, dried over anhydrous MgSO4, and the removal of the solvent
gave the crude product. The crude product was purified by chro-matography using CH2Cl2/hexane (1:1) as the eluent to afford
239.1 mg (0.78 mmol, yield 78.0%) of 5-octyl-(2,20 -bithiophene)-50-carbaldehyde as light yellow liquid.1H NMR (CD2Cl2, 400 MHz)
δ ppm: 9.86 (s, 1H), 7.69 (d, J = 4.0 Hz, 1H), 7.24 (d, J = 3.6, 1H), 7.22 (d, J = 4.0, 1H), 6.82 (d, J = 3.6, 1H), 2.82 (t, 2H), 1.68 (m, 2H), 1.34 (m, 10H), 0.95 (t, 3H). EI-MS (m/z): 306.2 [M]þ.
2-(5-Octyl-(2,20-bithiophen)-50-yl)-1 H-imidazo[4,5-f][1,10]phe-nanthroline (obtip). A mixture of 1,10-pheH-imidazo[4,5-f][1,10]phe-nanthroline-5,6-dione (254.3 mg, 1.2 mmol), 5-octyl-(2,20-bithiophen)-20-carbaldehyde (306.7 mg, 1.0 mmol), ammonium acetate (1548.9 mg, 20.2 mmol), and glacial acetic acid (30 mL) was refluxed for 2 h. After the reaction, the mixture was poured into 200 mL of cool water, and the resulting precipitate was isolated by filtration. The crude products were washed with water and purified by chromatography using CH2Cl2/hexane/MeOH (5:5:2) as the eluent to afford
2-(5-octyl-(2,20-bithiophen)-50 -yl)-1H-imidazo[4,5-f][1,10]phenanthro-line (362.1 mg, 0.729 mmol, 73%) as brown solid, mp 198°C.1H
NMR ([D6] DMSO, 300 MHz)δ ppm: 9.03 (d, J = 1.2 Hz, 2H),
8.83 (d, J = 7.8 Hz, 2H), 7.82 (m, 3H), 7.33 (d, J = 3.9 Hz, 1H), 7.26 (d, J = 3.6 Hz, 1H), 6.85 (d, J = 3.6 Hz, 1H), 2.79 (t, J = 7.4 Hz, 2H), 1.61 (m, 2H), 1.24 (m, 10H), 0.83 (t, J = 6.6 Hz, 3H). EI-MS (m/z): 496.2 [M]þ.
[Ru(dcbpy)(obtip)(NCS)2] (JF-5). [RuCl2(p-cymene)]2(306.3
mg, 0.5 mmol) and obtip (500.3 mg, 1.0 mmol) were added to dry DMF (20 mL). The reaction mixture was heated at 80°C under N2for 4 h, and then dcbpy (4,40-dicarboxylic acid-2,20
-bipyr-idine; 244.0 mg, 1 mmol) was added. The reaction mixture was refluxed at 160°C for another 4 h in the dark. Excess NH4NCS
was added to the reaction mixture and heated at 130°C for 5 h. After the reaction, the solvent was removed by rotary evapora-tor. The product was collected and washed with water and di-ethyl ether. The crude product was dissolved in methanol then passed through a column using methanol as the eluent. The main band was collected and concentrated, 335.3 mg (0.349 mmol, 35%) of black solid was obtained, mp >400°C.1H NMR ([D6]
DMSO, 400 MHz)δ ppm: 9.53 (m, 2H), 9.11 (m, 2H), 8.93 (s, 1H), 8.71 (d, J = 8.1 Hz, 1H), 8.35 (m, 2H), 7.88 (d, J = 3.3 Hz, 1H), 7.83 (d, J = 4.8 Hz, 1H), 7.68 (d, J = 5.7 Hz, 1H), 7.59 (t, J= 6.5 Hz, 1H), 7.47 (d, J = 4.5 Hz, 1H), 7.34 (d, J = 3.6 Hz, 1H), 7.26 (d, J = 3.3 Hz, 1H), 6.85 (d, J = 3.3 Hz, 1H), 2.78 (t, J= 7.2 Hz, 2H), 1.53 (t, J = 7.2 Hz, 2H), 1.28 (m, 10H), 0.85 (t, J= 5.4 Hz, 3H). FAB-MS (m/z): 900.4 [M- NCS]þ. Anal. Calcd for C43H36N8O4RuS4: C 53.90, H 3.79, N 11.70, S 13.39.
Found: C 53.72, H 3.86, N 11.49, S 12.98.
5-Octyl-(2,20,50,200-terthiophene). The precursor, 2,20,50,200 -ter-thiophene, was synthesized by previously reported procedures.32 2,20,50,200-Terthiophene (247.6 mg, 1.0 mmol) was dissolved in 30 mL THF in a well-dried flask under the protection of a N2flow. The
solution was cooled in a liquid nitrogen/acetone cooling bath, and n-butyllithium (0.8 mL, 1.28 mmol, 1.6 M in hexane) was then added dropwise. The cooling bath was removed, and the solution was allowed to warm to room temperature and 1-bromooctane (219.8 mg, 1.1 mmol) was then added in one portion. After 6 h, the
solution was poured into 200 mL of cool water. The organic layer was separated, and the aqueous layer was extracted with ether. The organic layers were collected, dried over anhydrous MgSO4, and
the removal of the solvent gave the crude product. The crude pro-duct was purified by chromatography using hexane as the eluent to afford 183.8 mg (0.51 mmol, yield 51%) of 5-octyl-(2,20,50,200 -terthio-phene) as light yellow solid, mp 69°C.1H NMR (CDCl
3, 400 MHz)
δ ppm: 7.18 (d, J = 5.2 Hz, 1H), 7.13 (d, J = 3.2 Hz, 1H), 7.03 (d, J= 4.0 Hz, 1H), 6.97 (m, 3H), 6.66 (d, J = 3.2 Hz, 1H), 2.77 (t, J = 7.6 Hz, 2H), 1.66 (m, 2H), 1.27 (m, 10H), 0.86 (t, J = 6.8 Hz, 3H). FAB-MS (m/z): 360.1 [M]þ.
5-Octyl-(2,20,50,200-terthiophene)-500-carbaldehyde. 5-Octyl-(2,20 ,-50,200-terthiophene) (361.3 mg, 1.0 mmol) was dissolved in 30 mL THF in a well-dried flask under the protection of a N2flow. The
solution was cooled in a liquid nitrogen/acetone cooling bath, and n-butyllithium (0.8 mL, 1.28 mmol, 1.6 M in hexane) was then added dropwise. The cooling bath was removed, and the solution was allowed to warm to room temperature and N-formylpiperidine (132.3 mg, 1.2 mmol) was then added in one portion. After 6 h, the solution was poured into 200 mL of cool water. The organic layer was separated, and the aqueous layer was extracted with ether. The organic layers were collected and dried over anhydrous MgSO4,
and the removal of the solvent gave the crude product. The crude product was purified by chromatography using CH2Cl2/hexane
(1:1) as the eluent to afford 285.5 mg (0.73 mmol, yield 73%) of 5-octyl-(2,20,50,200-terthiophene)-500-carbaldehyde as yellow solid, mp 90°C.1H NMR (CD2Cl2, 400 MHz)δ ppm: 9.82 (s, 1H), 7.65
(d, J = 3.6 Hz, 1H), 7.26 (d, J = 4.0 Hz, 1H), 7.22 (d, J = 4.0 Hz, 1H), 7.05 (d, J = 1.2 Hz, 1H), 7.04 (d, J = 1.2 Hz, 1H), 6.71(d, J = 3.6 Hz, 1H), 2.79 (t, J = 8.0 Hz, 2H), 1.67 (m, 2H), 1.27 (m, 10H), 0.88 (t, J = 6.8 Hz, 3H). FAB-MS (m/z): 389.1 [Mþ H]þ.
2-(5-Octyl-(2,20,50,200-terthiophen)-500-yl)-1 H-imidazo[4,5-f]-[1,10]phenanthroline (ottip). A mixture of 1,10-phenanthroline-5,6-dione (221.0 mg, 1.1 mmol), 5-octyl-(2,20,50,200 -terthiophene)-50-carbaldehyde (388.3 mg, 1.0 mmol), ammonium acetate (1553.2 mg, 20.2 mmol), and glacial acetic acid (30 mL) was re-fluxed for 2 h. After the reaction, the mixture was poured into 200 mL of cool water, and the resulting precipitate was isolated by filtration. The crude products were washed with water and purified by chromatography using CH2Cl2/hexane/MeOH (5:5:2)
as the eluent to afford 2-(5-octyl-2,20,50,200-terthiophen)-500 -yl)-1H-imidazo[4,5-f][1,10]phenanthroline (437.9 mg, 0.76 mmol, 76%) as brown solid, mp 223°C.1H NMR ([D6] DMSO, 400 MHz)
δ ppm: 9.04 (d, J = 3.6 Hz, 2H), 8.86 (s, 2H), 7.84 (m, 3H), 7.45 (d, J= 3.6 Hz, 1H), 7.40 (d, J = 3.6 Hz, 1H), 7.22 (d, J = 3.6 Hz, 1H), 7.17 (d, J = 3.2 Hz, 1H), 6.82 (d, J = 3.2 Hz, 1H), 2.78 (t, J = 7.6 Hz, 2H), 1.61 (m, 2H), 1.24 (m, 10H), 0.85 (t, J = 6.4 Hz, 3H). FAB-MS (m/z): 579.2 [Mþ H]þ.
[Ru(dcbpy)(ottip)(NCS)2] (JF-6). [RuCl2(p-cymene)]2(306.5
mg, 0.5 mmol) and ottip (580.4 mg, 1.0 mmol) were added to dry DMF (20 mL). The reaction mixture was heated at 80°C under N2for 4 h and then dcbpy (4,40-dicarboxylic acid-2,20
-bipyr-idine; 244.4 mg, 1.0 mmol) was added. The reaction mixture was refluxed at 160°C for another 4 h in the dark. Excess NH4NCS
was added to the reaction mixture and heated at 130°C for 5 h. After the reaction, the solvent was removed by rotary evapora-tor. The product was collected and washed with water and diethyl ether. The crude product was dissolved in methanol then passed through a column using methanol as the eluent. The main band was collected and concentrated; 298.3 mg (0.29 mmol, 29%) of black solid was obtained, mp >400°C.1
H NMR ([D6] DMSO,
400 MHz)δ ppm: 9.53 (m, 2H), 9.09 (m, 2H), 8.90 (s, 1H), 8.72 (d, J= 5.2 Hz 1H), 8.35 (m, 2H), 7.89 (d, J = 8.4 Hz, 1H), 7.83 (d, J= 5.4 Hz, 1H), 7.68 (d, J = 5.2 Hz, 1H), 7.60 (d, J = 4.8 Hz,
(32) Leriche, P.; Aillerie, D.; Roquet, S.; Allain, M.; Cravino, A.; Frere, P.; Roncali, J. Org. Biomol. Chem. 2008, 6, 3202.
1H), 7.46 (m, 2H), 7.40 (d, J = 3.8 Hz, 1H), 7.20 (d, J = 3.8 Hz, 1H), 7.16 (d, J = 3.4 Hz, 1H), 6.81 (d, J = 3.4 Hz, 1H), 2.76 (t, J = 6.8 Hz, 2H), 1.58 (m, 2H), 1.25 (m, 10H), 0.84 (t, J = 6.8 Hz, 3H). FAB-MS (m/z): 1040.2 [M]þ. Anal. Calcd for C47H38N8O4
-RuS5: C 54.27, H 3.68, N 10.77, S 15.41. Found: C 54.67, H 4.06,
N 10.83, S 15.11.
2,3-Di-(5-octylthiophen-2-yl)thiophene. The precursor, trime-thyl(5-octylthiophen-2-yl)stannane, was synthesized by previ-ously reported procedures.92,3-Dibromothiophene (240.3 mg, 1.0 mmol) and trimethyl(5-octylthiophen-2-yl)stannane (810.8 mg, 2.3 mmol) were dissolved in 30 mL anhydrous DMF then Pd-(PPh3)2Cl2(35.5 mg, 0.051 mmol) was added as a catalyst. The
mixture was refluxed under N2for 22 h. After the mixture was
cooled to room temperature, 5 wt % NH4Cl(aq)was added to
ter-minate the reaction and the product was extracted with CH2Cl2.
The organic layer was washed with saturated NaHCO3(aq),
dis-tilled water and saturated NaCl(aq), respectively. The crude
pro-duct was purified by chromatography using hexane as the eluent to afford 353.4 mg (0.747 mmol, 75%) of 2,3-di-(5-octylthiophen-2-yl)thiophene as yellow liquid.1H NMR (CDCl
3, 400 MHz)δ ppm: 7.25 (d, J = 5.2 Hz, 1H), 7.19 (d, J = 5.2 Hz, 1H), 7.02 (d, J= 3.6 Hz, 1H), 6.95 (d, J = 3.6 Hz, 1H), 6.75 (d, J = 4.0 Hz, 1H), 6.73 (d, J = 4.0 Hz, 1H), 2.85 (m, 4H), 1.75 (m, 4H), 1.43 (m, 20H), 0.97 (m, 6H). EI-MS (m/z): 472.3 [M]þ. octylthiophen-2-yl)thiophene-5-carbaldehyde. 2,3-Di-(5-octylthiophen-2-yl)thiophene (470.8 mg, 1.0 mmol) was dissolved in 30 mL THF in a well-dried flask under the protection of a N2flow.
The solution was cooled in a liquid nitrogen/acetone cooling bath, and n-butyllithium (0.8 mL, 1.28 mmol, 1.6 M in hexane) was then added dropwise. The cooling bath was removed, and the solu-tion was allowed to warm to room temperature and N-formylpiper-idine (140.3 mg, 1.2 mmol) was then added in one portion. After 6 h, the solution was poured into 200 mL of cool water. The organic layer was separated, and the aqueous layer was extracted with ether. The organic layers were collected, washed with water, and dried over anhydrous MgSO4, and the removal of the solvent gave the crude
product. The crude product was purified by chromatography using CH2Cl2/hexane (1:1) as the eluent to afford 341.2 mg (0.681 mmol,
yield 68%) of 2,3-di-(5-octylthiophen-2-yl)thiophene-5-carbalde-hyde as yellow liquid.1H NMR (CDCl
3, 400 MHz)δ ppm: 9.86 (s, 1H), 7.70 (s, 1H), 7.10 (d, J = 3.6 Hz, 1H), 6.92 (d, J = 3.6 Hz, 1H), 6.74 (d, J = 3.6 Hz, 1H), 6.71(d, J = 3.6 Hz, 1H), 2.82 (m, 4H), 1.68 (m, 4H), 1.29 (m, 20H), 0.89 (m, 6H). EI-MS (m/z): 500.3 [M]þ. 2-(2,3-Di-(5-octylthiophen-2-yl)thiophen-5-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (dottip). A mixture of 1,10-phenanthro-line-5,6-dione (229.5 mg, 1.1 mmol), 2,3-di-(5-octylthiophen-2-yl)thiophene-5-carbaldehyde (510.3 mg, 1.0 mmol), ammonium acetate (1632.5 mg, 21.2 mmol) and glacial acetic acid (30 mL) was refluxed for 2 h. After the reaction, the mixture was poured into 200 mL of cool water and the resulting precipitate was iso-lated by filtration. The crude products were washed with water and purified by chromatography using CH2Cl2/hexane/MeOH
(5:5:2) as the eluent to afford 2-(2,3-di-(5-octylthiophen-2-yl)-thiophen-5-yl)-1H-imidazo[4,5-f][1,10]phenanthroline (456.3 mg, 0.66 mmol, 65%) as brown solid, mp 208°C.1H NMR ([D
6] DMSO, 400 MHz)δ ppm: 9.01 (d, J = 3.2 Hz, 2H), 8.85 (d, J = 8.4 Hz, 2H), 7.92 (s, 1H), 7.77 (dd, J = 3.2, 8.4 Hz, 2H), 7.09 (d, J= 3.6 Hz, 1H), 7.00 (d, J = 3.6 Hz, 1H), 6.79 (d, J = 3.2 Hz, 1H), 6.77 (d, J = 3.2 Hz, 1H), 2.77 (m, 4H), 1.62 (m, 4H), 1.27 (m, 20H), 0.86 (m, 6H). EI-MS (m/z): 690.4 [M]þ.
[Ru(dcbpy)(dottip)(NCS)2] (JF-7). [RuCl2(p-cymene)]2(306.0
mg, 0.5 mmol) and dottip (692.8 mg, 1.0 mmol) were added to dry DMF (20 mL). The reaction mixture was heated at 80°C
under N2for 4 h and then dcbpy (4,40-dicarboxylic acid-2,20
-bipyridine; 244.9 mg, 1.0 mmol) was added. The reaction mix-ture was refluxed at 160°C for another 4 h in the dark. Excess NH4NCS was added to the reaction mixture and heated at 130°C
for 5 h. After the reaction, the solvent was removed by rotary evaporator. The product was collected and washed with water and diethyl ether. The crude product was dissolved in methanol then passed through a column using methanol as the eluent. The main band was collected and concentrated, 380.2 mg (0.329 mmol, 33%) of black solid was obtained, mp >400°C.1H NMR ([D6]
DMSO, 400 MHz)δ ppm: 9.57 (d, J = 6.0 Hz, 1H), 9.53 (d, J = 4.8 Hz, 1H), 9.20 (s, 1H), 9.07 (s, J = 7.6 Hz, 1H), 8.98 (s, 1H), 8.72 (dd, J = 8.4, 5.2 Hz, 1H), 8.39 (m, 2H), 7.99 (d, J = 5.2 Hz, 1H), 7.86 (d, J = 4.4 Hz, 1H), 7.75 (d, J = 4.8 Hz, 1H), 7.62 (d, J= 5.2 Hz, 1H), 7.50 (d, J = 4.8 Hz, 1H), 7.17 (d, J = 6.0 Hz, 1H), 7.09 (d, J = 5.2 Hz, 1H), 6.86 (m, 2H), 2.78 (m, 2H), 1.60 (m, 2H), 1.24 (m, 10H), 0.85 (t, J = 3.2 Hz, 3H). FAB-MS (m/z): 1094.4 [M- NCS]þ. Anal. Calcd for C55H54N8O4RuS5: C 57.32,
H 4.72, N 9.72, S 13.91. Found: C 56.88, H 5.16, N 9.30, S 13.14. Molecular Modeling. The geometrical and electrochemical properties of the dye, JF-5, JF-6, and JF-7, were studied with density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations using Gaussian 03 (G03) program package,33employing the DFT method with Becke’s three-parameter hybrid function34 and Lee
-Yang-Parr’s gradient corrected correlation function35(B3LYP). The LanL2DZ effective core potential36was used for the ruthenium atom and the split-valence 6-31G** basis set37was applied for hydrogen, sulfur, carbon, oxygen and nitrogen atoms. The ground-state geometries of the dye molecules were optimized in the gas phase. Molecular orbitals were visualized using “Gauss View 3.09”. TDDFT calculations for JF-5, JF-6, and JF-7 were per-formed using the conductor-like polarizable continuum model method (C-PCM)38-40with dimethylformamide (DMF) as the solvent.41The empirical solvent data, molecular radius and die-lectric constant (ε), employed in C-PCM are 2.647 A˚ and 36.71, respectively. Ninety singlet excited states were determined start-ing from geometry-optimized structures of 5, 6, and JF-7. The GaussSum 1.0542 was used for analyzing the data of singlet excited-state transitions.
(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr., 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.; 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. G.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. GAUSSIAN 03, revision D.01, Gaussian Inc.: Wallingford, CT, 2004. (34) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(35) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (36) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (37) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (38) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem.
2003, 24, 669.
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Preparation of TiO2Electrode. The preparation of the TiO2
precursor and the electrode fabrication were carried out based on previous reports.8,43The TiO2film, serving as the photoanode
in this work, was prepared through the general sol-gel method. The precursor solution was prepared according to the following procedure: To 430 mL of 0.1 M nitric acid solution under vigorous stirring, 72 mL of Ti(C3H7O)4slowly added dropwise
to form a mixture. After hydrolysis, the mixture was heated at 85°C in a water bath and stirred vigorously for 8 h to achieve peptization. When the mixture was cooled to room temperature, the resultant colloid was filtered, and then heated in an auto-clave at a temperature of 240°C for 12 h to grow the TiO2
par-ticles. After the colloid was cooled to room temperature, it was ultrasonically vibrated for 10 min. The TiO2colloid was
con-centrated to 13 and 30 wt % (with respect to TiO2weight) of
poly(ethylene glycol) (PEG, MW = 20 000 and 200 000), which was added to prevent the film from cracking during drying. To fabricate the TiO2electrode, TTIP (titanium(IV) isopropoxide)
was vigorously mixed with ME (2-methoxylethanol) (in the wei-ght ratio of 1:3) to form a metallorganic solution. The metallor-ganic solution was then spin-coated onto clean conducting fluorine-doped tin oxide (FTO) glasses with a sheet resistivity of 13Ω/square, followed by annealing at 500 °C for 30 min to form a compact, thin TiO2layer. A TiO2paste was applied three
times to the top of this compact film using a glass rod to obtain the appropriate thickness. For the first coating (paste 1), the TiO2 colloid mixed with PEG having a molecular weight of
200 000 was used. The second coating, used a TiO2paste (paste 2)
containing a TiO2colloid and PEG with a molecular weight of
20000. Paste 2 mixed with the light scattering particles of TiO2
(300 nm, 30 wt % in total TiO2) was used for the third (final)
coating to reduce light loss by back scattering.
Device Fabrication of Dye-Sensitized Solar Cells. The TiO2
film electrode with a 0.4 0.4 cm2geometric area was immersed overnight in acetonitrile/tert-butanol mixtures (volume ratio 1:1) containing 2 10-4M dye sensitizers. A platinized FTO was used as a counter electrode and contained an active area of 0.16 cm2, produced by adhering a polyester tape with a thickness of 60 μm. The dye-sensitized photoanode was rinsed with acetonitrile and air-dried. After filling the electrolyte in the spacer, the photoanode was placed on top of the counter electrode, and they were tightly clipped to form a cell. The electrolyte was composed of 0.6 M butylmethylimidazolim (BMII), 0.1 M LiI, 0.5 M 4-tert-butylpyridine, 0.03 M I2, 0.5 M
guanidinium thiocyanate (GuSCN) dissolved in acetonitrile. The photovoltaic characterizations of the solar cells with a mask (0.5 0.5 cm2) were carried out using a 150 W Peccell solar sim-ulator (PEC-L11). Light intensity attenuated by neutral density filter (Optosigma, 078-0360) at the measuring (cell) position, was calibrated to be 100 mW/cm2according to the reading from a radiant power meter (Oriel, 70260) connected by means of a thermopile probe (Oriel, 70263). Photoelectrochemical charac-teristic photocurrent density-voltage curves of the DSCs were recorded using a potentiostat/galvanostat (PGSTAT 30, Auto-lab, Eco-Chemie, The Netherlands).
Acknowledgment. This work was financially supported by Academia Sinica, Taiwan. We thank Dr. H.-F. Lu, C.-H. Huang, Dr. Y.-C. Hsu, Dr. Y.-S. Yen, and D.-C. Huang for valuable discussion.
(43) Hsu, Y. C.; Zheng, H.; Lin, J. T.; Ho, K. C. Sol. Energy Mater. Sol. Cells 2005, 87, 357.
