FULL PAPER DOI: 10.1002/asia.201402232

Dye-Sensitized Solar Cells Based on (Donor-p-Acceptor)2 Dyes With Dithiafulvalene as the Donor Ting-Hui Lee,[a, b] Chih-Yu Hsu,[a] You-Ya Liao,[a, b] Hsien-Hsin Chou,[a] Heather Hughes,[c] and Jiann T. Lin*[a] Abstract: Dipolar metal-free sensitizers (D-p-A; D = donor, p = conjugated bridge, A = acceptor) consisting of a dithiafulvalene (DTF) unit as the electron donor, a benzene, thiophene, or fluorene moiety as the conjugated spacer, and 2-cyanoacrylic acid as the electron acceptor have been synthesized. Dimeric congeners of

these dyes, (D-p-A)2, were also synthesized through iodine-induced dimerization of an appropriate DTF-containing segment. Dye-sensitized solar Keywords: dimerization · dyes/ pigments · sensitizers · solar cells · sulfur heterocycles

Introduction

cells (DSSCs) with the new dyes as the sensitizers have cell efficiencies that range from 2.11 to 5.24 %. In addition to better light harvesting, more effective suppression of the dark current than the D-p-A dyes is possible with the (D-p-A)2 dyes.

bridge, A = acceptor), which induces charge transfer and subsequent electron injection upon photoexcitation. Recently there has also been increasing interest in organic sensitizers with di- or multi-anchors[6] because they can provide more electron extraction paths and/or lead to a redshift in the absorption spectra compared with those with only one anchor. Compounds with two pinched D-p-A motifs, LACHTUNGRE(D-pA)2 (L is the linker between the two D-p-A segments), represent a special type of sensitizer with multi-anchors.[7] Better cell performance (both photocurrent and voltage) can be achieved with the LACHTUNGRE(D-p-A)2 sensitizer with proper design[7a,b] because more compact packing of the dye results in higher D-p-A density and more effective dark current suppression. In a continuation of our interest in dyes that have two anchors,[6g, h] we also turned our attention to LACHTUNGRE(Dp-A)2 sensitizers. Arylamine is the most popularly used electron donor in metal-free sensitizers.[8] Though the electron-donating ability of the dithiafulvenyl entity (DTF) rivals that of arylamine, metal-free sensitizers that use the dithiafulvenyl entity as an electron donor are still very limited so far.[9] Of the available examples, the sensitizers reported by Yang et al. have the highest efficiency and reach 8.29 %.[9c] Because oxidants were reported to induce dimerization of the dithiafulvenyl entity,[10] we felt that the same strategy could be applied for construction of LACHTUNGRE(D-p-A)2 molecules by using the dithiafulvenyl entity as the donor. Herein we report new dithiafulvenyl-based sensitizers with a LACHTUNGRE(D-p-A)2 skeleton, and their physical properties and application in DSSCs.

The ability to harness the power of sunlight as a renewable energy is currently attracting great attention because of the rapid depletion of fossil fuel reserves and degradation of the environment. Two decades after the seminal work of ORegan and Grtzel in 1991,[1] dye-sensitized solar cells (DSSCs) have been deemed a very promising type of solar cell because of their easy fabrication and low cost. There are three classes of organic sensitizers:[2] ruthenium dyes, porphyrins, and metal-free dyes. Until now, record high cell efficiencies of 11.3, 13.0, and 11.5 % have been reached for ruthenium,[3] porphyrins[4] and metal-free dyes,[5] respectively. Compared with metal-containing sensitizers, metal-free organic dyes normally have higher molar extinction coefficients and are of lower cost, in addition to being easier to prepare and purify. Most metal-free dyes possess a prototype dipolar architecture, D-p-A (D = donor, p = conjugated

[a] T.-H. Lee, Dr. C.-Y. Hsu, Y.-Y. Liao, Dr. H.-H. Chou, Prof. Dr. J. T. Lin Institute of Chemistry, Academia Sinica 128 Academia Road Sec. 2 Nankang Taipei 115 Taiwan (ROC) Fax: (+ 886)-2-2783-1237 E-mail: [email protected] [b] T.-H. Lee, Y.-Y. Liao Department of Chemistry National Taiwan Normal University 117, Taipei (Taiwan) [c] H. Hughes Department of Mechanical Engineering University of California, Berkeley 2016, California, 94720-1500 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402232.

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Results and Discussion

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UV/Vis Absorption Properties

Synthesis of the Materials

Figure 2 shows the UV/Vis absorption spectra of organic dyes in THF, and the corresponding data are summarized in Table 1. All of the dyes except D5 have two prominent absorption bands in the range of l = 300 to 600 nm when measured in THF. The band of the shorter wavelength is due to a more localized aromatic p–p* transition, and that of the longer

The structures of new dyes are shown in Figure 1. The synthetic routes to these compounds are depicted in Scheme S1 in the Supporting Information. DTF-based organic sensitizers of the monomer type M1, M2, or M4 were synthesized through two key steps: 1) a Horner–Wittig condensation of 4,5-bis(butylthio)-1,3-di-

Figure 1. The structures of the dyes.

wavelength is attributed to a more delocalized p–p* transition with charge-transfer character (from the DTF unit to the 2-cyanoacrylic acid). The band at shorter wavelength appears as a shoulder in D5. The wavelength of the chargetransfer band in M2 is longer than that in M1 because the

thiole-2-thione (BDT) with an appropriate dialdehyde afforded DTF-containing intermediates 1, 2, or 7; and 2) a condensation reaction of the intermediate with cyanoacetic acid provided the desired product. While this work was ongoing, we noted that a compound with a structure similar to that of M4, but with butyl substituents replaced by hexyl substituents, was reported.[9c] Dimer-type sensitizers D2 and D4 were synthesized in two steps: 1) an iodine-induced dimerization[10] of 2 (or 7) to afford intermediate 3 (or 8); and 2) a condensation reaction of 3 (or 8) with 2-cyanoacetic acid. For the preparation of D3 and D5, the iodine-induced dimerization of the DTFcontaining entity (4 or 9) proceeded before incorporation of the formyl group. A Vilsmeier–Haack formylation of 4 and 9 provided 6 and 11, which then underwent a condensation reaction with cyanoacetic acid to afford the desired products. All new sensitizers were characterized by using 1H and 13 C NMR spectroscopy and HRMS.

Figure 2. Absorption spectra of the dyes in THF; &: M1, D3, ^: M4, ^: D4, I: D5.

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Table 1. Optical and electrochemical data for the dyes in THF.

M1 M2 D2 D3 M4 D4 D5

labs [nm] ACHTUNGRE(e104 ACHTUNGRE[m1 cm1])[a]

E00*[d] lem[a] Egapopt[b] E1/2 (ox)[c] [nm] [eV] [V vs. NHE] [V vs. NHE]

448 461 488 447 438 436 439

551 584 653 611 559 611 617

(2.52), (2.51), (5.31), (4.39), (4.24), (7.27), (7.78),

314 348 366 368 381 380 381

(0.750) (0.998) (2.74) (4.33) (2.74) (5.98) (5.74)

2.52 2.35 2.16 2.34 2.48 2.50 2.47

1.03 0.93 0.92 0.76 0.95 0.74 0.71

1.49 1.42 1.24 1.58 1.5 1.76 1.76

[a] Recorded in THF at 298 K. [b] Egapopt = 1240/l00. [c] Recorded in THF; Eox = 1/2ACHTUNGRE(Epa+Epc); the reported oxidation potential is adjusted to the potential of ferrocene (0.70 V vs. NHE). [d] E00*: The excited-state oxidation potential vs. NHE.

Figure 3. DPV plots of selected dyes in THF solution; &: M1, D2, ^: D4, I: D5.

extra thiophene entity increases the conjugation length of the former. The band of M2 is also redshifted compared with that of M4. This can be attributed to the lower resonance energy of the thiophene ring than the benzene ring,[11] which benefits formation of a quinoid structure. The longer wavelength absorption of D2 compared with D4 and D3 can be rationalized by the same reason. The spectra of the dyes adsorbed on TiO2 (Figure S1 in the Supporting Information) broadened significantly in the long wavelength region, especially for M1 and M2. This can be attributed to the J-aggregation of the dyes on TiO2. Compared with the monomer-type sensitizers, the dimertype dyes have doubled quantities of D-p-A units and, therefore, exhibit significantly higher absorption intensities for both bands. However, the wavelength shift in the charge-transfer band between M2 and D2 is different from that between M4 and D4. Dimer D2 has longer wavelength than M2, whereas D4 has a wavelength that is comparable to M4. Compared with M2, the longer wavelength of D2 can be attributed to two reasons based on theoretical computations (see below): 1) each D-p-A unit retains good planarity similar to M2; and 2) the two acceptors are conjugated through the spacer between them. In contrast, the donor in the D-p-A unit of D4 has a large dihedral angle (  378) with the spacer because of greater steric congestion (Figure S2 in the Supporting Information), which leads to less effective charge transfer when compared with M4.

The electrochemical properties of the organic dyes were measured in THF by using differential pulse voltammetry (DPV), and the results are listed in Table 1. DPV plots of representative compounds are shown in Figure 3. The first oxidation potential (Eox) was used to deduce the highest occupied molecular orbital (HOMO). This together with the HOMO/LUMO energy gap (E00) estimated from the intersection of the normalized absorption and emission spectra were used to obtain the excited-state potential (E00*) of the sensitizer. To assure electron injection from the excited sensitizer to the TiO2 electrode, E00* should be more negative than the conduction band edge energy level of TiO2 (0.5 V vs. NHE). However, the first oxidation potential of

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M2,

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the sensitizer should be more positive than the I/I3 redox couple (  0.4 V vs. NHE)[12] to assure regeneration of the oxidized sensitizer. The E00* values and the first oxidation potentials shown in Table 1 indicate that electron injection and dye regeneration are energetically favored for these dyes. The first redox wave is attributed to the oxidation of the DTF entity. The potential for the monomer-type dyes increases in the order of M2 (0.93 V)ffiM4 (0.95 V) < M1 (1.03 V). The greatest electronic communication between the cyanoacrylic acid and the DTF entity in M1 can be ascribed to the shortest conjugated spacer between the two. Though M2 and M4 have comparable oxidation potentials, D4 has significantly lower potential than D2 (D2: 0.92 V; D4: 0.74 V). This outcome can be attributed to the different dihedral angle between the two DTF entities in the dye molecule. In D2, the two DTF entities are nearly perpendicular with a dihedral angle of 92.48 (see below, theoretical computation section). In contrast, the two DTF entities in D4 have a dihedral angle of 74.08, which allows a stronger interaction between the two. The dihedral angles between the two DTF entities of D3 and D5 (D3: 74.38; D5: 73.18) are comparable to that of D4. Consequently, the two compounds also have significantly lower oxidation potentials (D3: 0.76 V; D5: 0.71 V) compared with D2.

Electrochemical Properties

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Theoretical Approach Density functional theory (DFT) and time-dependent DFT calculations by using the Q-Chem 4.0 software[15] were conducted to gain further insight in the correlation between structure and the physical properties and device performance. The ground-state geometries of the dyes and the corresponding dihedral angles between two neighboring conjugated segments are shown in Figure S2 in the Supporting Information. Sensitizer M2 has a nearly planar structure (largest dihedral angle = 1.98), whereas the other two monomertype dyes, M3 and M4, have larger dihedral angles between the phenyl entity and its neighboring segment (13.5–22.28).

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This observation is consistent with the longer wavelength absorption of M2 among the three dyes (see above). A fairly large dihedral angle (73.1–92.48) between the two DTF entities in the dimer-type dyes is attributable to the steric congestion of the molecules. Therefore, the two D-pA units can be regarded as electronically independent or only weakly coupled. As such, both charge-transfer and p– p* transition bands have nearly double absorption intensities compared with the monomer-type dyes at the same molecular concentration (see above). The steric congestion also results in a larger dihedral angle (D2: 39.3, 35.58; D4: 37.2, 36.88; D5: 40.2, 37.98) between the DTF entity and its neighboring phenyl entity compared with the monomer-type congeners (M2: 15.78; M4: 13.58). Figures S3 and S4 in the Supporting Information show the frontier orbitals of the dyes and their corresponding energy states, respectively. For the monomer-type molecules, the HOMO has a significant contribution from DTF that extends to the conjugated bridge, whereas the LUMO has a significant contribution from the acceptor that extends to the conjugated bridge. Electronic communication between the two DTF entities can be clearly seen in the HOMO of the dimer-type, except for D2. This is consistent with the electrochemical data. A deficit of electronic communication between the two DTF entities in D2 can be attributed to the nearly perpendicular D-p-A units in the molecule. For the monomer-type dyes, the lowest energy band in the absorption spectra (Figure S5 in the Supporting Information) can be attributed to the S0 !S1 transition, which has prominent charge-transfer character. For the dimer-type dyes, the lowest energy band in the absorption spectra likely consists of several transitions, S0 !Sn (n = 1–4) and the charge-transfer character is also evident.

Jiann T. Lin et al.

(IPCE) spectra and photocurrent–voltage (J–V) curves of the cells based on the dyes are shown in Figures 4 and 5, respectively. The performance of a DSSC fabricated from bis(tetrabutylammonium)-cis-di(thiocyanato)-N,N’-bis(4-carboxylato-4’-carboxylic acid-2,2’-bipyridine)ruthenium (N719) under the same conditions is also shown for comparison.

Figure 4. IPCE plots of the DSSCs using the dyes and N719; M2, ~: D2, *: D3, ^: M4, ^: D4, I: D5, c: N719.

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Photovoltaic Performance DSSCs with an effective area of 0.25 cm2 were fabricated by using these dyes as the sensitizers and nanocrystalline anatase TiO2 particles as the photoanode. The electrolyte was composed of 0.05 m I2/0.5 m LiI/0.5 m tert-butylpyridine in acetonitrile. The performance parameters of the DSSCs measured under AM 1.5 illumination are listed in Table 2. The incident photon-to-current conversion efficiency

Figure 5. J–V curves of DSSCs based on the dyes; &: M1, *: D3, ^: M4, ^: D4, I: D5, c: N719.

M1 M2 D2 D3 M4 D4 D5 N719

VOC [V]

FF

h [%]

Dye loading[a] [mol cm2]

6.43 6.94 6.76 6.66 11.33 6.95 5.02 16.08

0.54 0.54 0.55 0.59 0.65 0.67 0.63 0.75

0.68 0.62 0.57 0.70 0.71 0.75 0.72 0.67

2.36 2.34 2.11 2.75 5.24 3.48 2.28 8.31

5.85  107 6.62  107 5.56  107 4.86  107 3.01  107 4.94  107 4.64  107

[a] The amount of dimer-type dyes has been doubled due to the presence of two D-p-A units.

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For monomer-type dyes, M1 and M2 exhibited comparable cell conversion efficiencies (M1: 2.36 %; M2: 2.34 %) and open-circuit voltages (M1: 0.54 V; M2: 0.54 V). The slightly higher JSC value for M2 (6.94 mA cm2) compared with M1 (6.43 mA cm2) can be attributed to the better light harvesting ability of the former (see above). However, the IPCE values for both are far from satisfactory. Compared with M1 and M2, M4 exhibited much better cell performance parameters (h = 5.24 %; JSC = 11.33 mA cm2, VOC = 0.65 V, FF = 0.71). This can be attributed to several factors: 1) M4 has significantly higher absorption intensity than the other two dyes; 2) M4 has more effective dark current suppression (see below); and 3) there is a significant upward shift in the conduction band edge of M4 compared with the other two (see below, see charge extraction).

Table 2. Performance parameters of DSSCs constructed by using the dyes. JSC [mA cm2]

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The cell performance of D2 (h = 2.11 %, JSC = 6.76 mA cm2, VOC = 0.55 V, FF = 0.57) is slightly inferior to that of M2 (h = 2.34 %, JSC = 6.94 mA cm2, VOC = 0.54 V, FF = 0.62). Given that the loading (Table 1) of the D-p-A units in D2 is lower than that in M2 by approximately 16 %, the lower efficiency of M2 can be attributed to more serious dye aggregation (see above) that quenches the excited state and hampers electron injection. More serious dye aggregation of M2 than D2 is observed from comparison of the absorption spectra of the dyes on TiO2 with and without the co-adsorbent (chenodeoxycholic acid (CDCA)). A blueshift in the absorption spectra is more prominent for M2 when more CDCA is added (Figure S6 in the Supporting Information). The lower IPCE value of M2 than D2 in the range of l = 350 to 600 nm is also consistent with dye aggregation of M2, which jeopardizes electron injection. In contrast, M4 (h = 5.24 %, JSC = 11.33 mA cm2, VOC = 0.65 V, FF = 0.71) has much better cell performance than D4 (h = 3.48 %, JSC = 6.95 mA cm2, VOC = 0.67 V, FF = 0.75). In contradiction to the loading of D-p-A units (M4 = 3.01  107 mol cm2 ; D4 = 4.94  107 mol cm2), there is a significant drop in the photocurrent output for D4. This may be attributed to the smaller regeneration driving force for D4, that is, the smaller gap between the HOMO of the dye and the redox potential of I/I3 (0.40 V vs. NHE). The HOMO energy level of D4 (0.74 V vs. NHE) is at a much higher potential than that of M4 (0.95 V vs. NHE), which reduces the regeneration driving force. The open-circuit voltage is affected by both the conduction band edge shift for TiO2 and the concentration of electrons in TiO2,[13] and the latter is affected by both electron injection and electron recombination. Though D4 can more effectively suppress the current (see below, see the electrochemical impedance section), M4 has an upward shift in the conduction band edge of TiO2 (see below, see the charge extraction section). As a result, the two cells have comparable VOC values. Slower dye regeneration likely also occurs in the cell of D5 (h = 2.28 %, JSC = 5.02 mA cm2, VOC = 0.63 V, FF = 0.72), which has an even lower photocurrent than D4 due to the higher HOMO energy level (0.71 V vs. NHE) of D5. To improve the dye regeneration efficiency, we adjusted the electrolyte composition in the cell of D5 by altering the I2 concentration, as shown in Figure S7 and Table S2 in the Supporting Information. The concentrations of LiI and 4-tert-butylpyridine (TBP) were kept constant so as not to perturb the conduction band edge of TiO2. As the concentration of I2 was decreased from 0.05 to 0.03 m, the JSC value increased due to the upward shift in redox potential for the I/I3 couple. Accordingly, the VOC value decreased with decreased I2 concentration. The overall cell efficiency was found to increase by approximately 1 % when the concentration of was I2 decreased from 0.05 to 0.03 m. Faster recombination of the electron in TiO2 with the oxidized electrolyte will result in a larger dark current, which leads to a lower VOC value. Therefore, electrochemical impedance analysis was measured in the dark to elucidate the influence of the dark current on the cell performance. Figure 6 shows the Nyquist plots under a forward bias of

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Figure 6. Nyquist plots of DSSCs in the dark; D3, ^: M4, ^: D4, I: D5, c: N719.

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M2,

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D2,

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0.55 V in the dark. The middle-frequency semicircle in the Nyquist plots is attributed to the dark reaction impedance caused by charge transportation at the TiO2/dye/electrolyte interface. The radius of the semicircle decreases in the order of D4 > D5ffiM4 > D3 > M2 > M1 > D2, which should be the order of decreased electron recombination resistance. The result is in agreement with the trend of the observed dark current (Figure 5). Dimer D4 can more effectively suppress the dark current than M4. The higher D-p-A loading density of D4 (4.04  107 mol cm2) than M4 (3.01  107 mol cm2) certainly leads to more compact packing of D-p-A units (4.04  107 mol cm2 for D4), which more efficiently blocks the electrolytes from approaching the TiO2 surface. Better dark current suppression of M2 than D2 can also be attributed to more compact packing of D-p-A units in the former (M2: 6.62  107 mol cm2 ; D2: 5.56  107 mol cm2). Although the dye loading density of M2 (6.62  107 mol cm2) is much higher than that of M4 (3.01  107 mol cm2), the dark current suppression is more efficient in M4. The less planar molecular structure and longer conjugated segment of M4 may be advantageous for blocking the electrolyte from approaching the TiO2 surface. The efficiency of the M4-based DSSC is significantly lower than the congener that has hexyl substituents[9c] instead of butyl substituents at the DTF unit. Possibly the shorter alkyl chain is less effective in blocking the electrolytes from approaching the TiO2 surface. The relative conduction band shift for TiO2 was estimated by means of the charge extraction method (CEM).[14] The cell of M4 exhibited an obvious upward shift in conduction band edge of TiO2 compared with M1 and M2, as shown by the higher VOC values when at the same dn (electron density) value (Figure 7). Together with more effective dark current suppression (see above), this renders M4 with a much better VOC value (0.65 V) than M1 (0.54 V) and M2 (0.54 V). Though D4 has the most effective dark current suppression of all the dyes, its VOC value (0.67 V) is only slightly higher than M4 (0.65 V) due to the downward shift in the conduction band edge of TiO2. 5

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solar cells were carried out by using an Oriel Class AAA solar simulator (Oriel94043A, Newport Corp.). Photocurrent–voltage characteristics of the DSSCs were recorded by using a potentiostat/galvanostat (CHI650B, CH Instruments) at a light intensity of 100 mW cm2 calibrated by using an Oriel reference solar cell (Oriel 91150, Newport Corp.). The monochromatic quantum efficiency was recorded through a monochromator (Oriel 74100, Newport Corp.) under short-circuit conditions. The intensity of each wavelength was in the range of 1 to 3 mW cm2. Electrochemical impedance spectra (EIS) were recorded for DSSC under illumination at open-circuit voltage (VOC) or in the dark at 0.55 V at RT. The frequencies explored ranged from mHz to 100 kHz. The charge-extraction method (CEM) was carried out by using the electrochemical workstation (Zahner, Zennium) under an intensity-modulated white-light-emitting diode driven by a Zahner (0982wlr02) source supply. Assembly and Characterization of DSSCs Figure 7. VOC as a function of electron density for DSSCs; &: M1, ~: D2, *: D3, ^: M4, ^: D4, I: D5.

~:

The photoanode used was a TiO2 thin film (12 mm of 20 nm particles as the absorbing layer and 6 mm of 400 nm particles as the scattering layer) coated on a FTO glass substrate with a dimension of 0.5  0.5 cm2, and the film thickness was measured by using a profilometer (Dektak3, Veeco/Sloan Instruments Inc., USA). The TiO2 thin film was dipped into a solution that contained 3  104 m dye sensitizers in THF for at least 12 h. After rinsing with THF, the photoanode, adhered with a polyimide tape of 30 mm in thickness and with a square aperture of 0.36 cm2, was placed on top of the counter electrode and the two were tightly clipped together to form a cell. Electrolyte was then injected into the seam between the two electrodes. The electrolyte was composed of 0.5 m lithium iodide (LiI), 0.05 m iodine (I2), and 0.5 m 4-tert-butylpyridine (TBP) dissolved in acetonitrile.

M2,

Conclusion Dipolar-type (D-p-A) metal-free dyes with dithiafulvalene (DTF) units as the electron donor have been synthesized. Their dimeric congeners with two pinched D-p-A motifs, (D-p-A)2, were also synthesized through an iodine-induced dimerization of the DTF-containing segments. Though steric congestion in the (D-p-A)2 dyes results in large twist angle between the two D-p-A entities, the two DTF units or the two acceptors may have electronic communication through the linkage between the two D-p-A entities. These dyes can be used as sensitizers for dye-sensitized solar cells. Though the (D-p-A)2 dyes have more intense and/or redshifted absorption bands compared with the D-p-A dyes, their better light-harvesting characteristic do not result in better cell performance because of their less efficient dye regeneration, which stems from the higher-lying HOMO energy level of the dye. However, appropriately designed (D-p-A)2 dyes can more effectively suppress the dark current compared with the D-p-A dyes.

Quantum Chemistry Computation Computations were performed with Q-Chem 4.0 software.[15] Geometry optimization of the molecules was performed by using a hybrid B3LYP functional and the 6-31G* basis set. For each molecule, a number of possible conformations were examined and the one with the lowest energy was used. The same functional was also applied for the calculation of excited states by using TD-DFT. A number of previous works exist that employ TD-DFT to characterize excited states with charge-transfer character.[16] In some cases, underestimation of the excitation energies was seen.[16, 17] Therefore, herein we used TD-DFT to visualize the extent of transition moments and their charge-transfer characters, and avoided drawing conclusions from the excitation energy. General Synthetic Procedure of Dyes Bis(tetraethylammonium)bis(1,3-dithiole-2-thione-4,5-dithiol)zincate and 4,5-bis(butylthio)-1,3-dithiole-2-thione (BDT) were synthesized according to the published procedures.[9c, 18]

Experimental Section

5-((4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)methyl)thiophene-2carbaldehyde (1)

Materials and Reagents

Dry toluene (35 mL) was added to a 100 mL flask that contained thiophene-2,5-dicarbaldehyde (0.90 g, 6.42 mmol) and BDT (1.99 g, 6.42 mmol) and the mixture was heated to 120 8C. PACHTUNGRE(OEt)3 (16 mL) was slowly added and the solution was heated for 4 h, and the yellow slurry turned into clear red solution. After removal of the solvent, the residue was extracted with a mixture of CH2Cl2 and H2O. The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The residue was purified by using column chromatography on silica gel with CH2Cl2/hexanes (1:1 v/v) as the eluent to give 1 as an orange-red liquid (yield: 0.40 g, 15 %). 1H NMR (400 MHz, CDCl3): d = 9.82 (s, 1 H; -CHO), 7.64 (d, J = 4.0 Hz, 1 H), 6.88 (d, J = 4.0 Hz, 1 H), 6.75 (s, 1 H; -C=CH-), 2.89–2.81 (m, 4 H; -CH2-), 1.67–1.59 (m, 4 H), 1.49–1.38 (m, 4 H), 0.94–0.90 ppm (m, 6 H; -CH3); 13C NMR (125 MHz, CDCl3): d = 181.86, 149.94, 140.49, 139.63, 136.97, 128.89, 127.00, 106.74, 35.95, 35.84, 31.68, 31.58, 21.58, 13.56 ppm; MS-LR-FAB: m/z calcd for C17H22OS5 : 402.03 [M] + ; found: 402.0.

Unless otherwise specified, all reactions and manipulations were carried out under a nitrogen atmosphere. Solvents were dried by using standard procedures. Bis(tetrabutylammonium)-cis-di(thiocyanato)-N,N’-bis(4-carboxylato-4’-carboxylic acid-2,2’-bipyridine)ruthenium (N719) and TiO2 paste were purchased from Solaronix S. A., Switzerland. Characterization 1 H and 13C NMR spectra were recorded by using a Bruker 400 MHz spectrometer. Mass spectra (FAB) were recorded by using a VG70-250S mass spectrometer. Elementary analyses were performed by using a Perkin–Elmer 2400 CHN analyzer. Absorption spectra were recorded by using a Dynamica DB-20 UV/Vis spectrophotometer. Fluorescence spectra were recorded by using a Hitachi F-4500 spectrophotometer. Voltammetry experiments were performed by using a CHI-621B electrochemical analyzer. All measurements were carried out at RT with a conventional three-electrode configuration that consisted of a platinum working electrode, an auxiliary electrode, and a nonaqueous Ag/AgNO3 reference electrode. The photoelectrochemical characterizations on the

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ACHTUNGRE(2E,2’E)-3,3’-(5’,5’’’-(1,2-BisACHTUNGRE(4,5-bis(butylthio)-1,3-dithiol-2ylidene)ethane-1,2-diyl)bis([2,2’-bithiophene]-5’,5-diyl))bis(2-cyanoacrylic acid) (D2)

(E)-3-(5-((4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)methyl)thiophen-2-yl)-2cyanoacrylic acid (M1) CH3CN (100 mL), dry THF (5 mL), and piperidine (0.10 mL) were added to a 100 mL flask that contained 1 (290 mg, 0.72 mmol) and cyanoacetic acid (306 mg, 3.60 mmol). The solution was then heated at 80 8C for 13 h. After removal of the solvent, the residue was extracted with a mixture of CH2Cl2 and H2O (200:200 v/v). The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The residue was purified by using column chromatography on silica gel with EA/CH3COOH (10:1 v/v) as the eluent to give M1 as a dark purple solid (yield: 150 mg, 44 %). 1H NMR (400 MHz, CDCl3): d = 8.25 (s, 1 H), 7.77 (d, J = 4.0 Hz, 1 H), 6.92 (d, J = 4.4 Hz, 1 H), 6.79 (s, 1 H), 2.89 (t, J = 7.2 Hz, 2 H), 2.84 (t, J = 7.2 Hz, 2 H), 1.68–1.59 (m, 4 H), 1.50– 1.39 (m, 4 H), 0.94–0.90 ppm (m, 6 H); 13C NMR (125 MHz, CDCl3): d = 168.60, 152.29, 147.14, 142.63, 139.52, 133.55, 129.68, 128.38, 125.36, 116.50, 106.78, 93.92, 36.50, 36.31, 32.01, 31.96, 21.89, 13.79, 13.77 ppm; MS-HR-MALDI: m/z calcd for C20H23NO2S5 : 469.0332 [M] + ; found: 469.0345; elemental analysis calcd (%) for C20H23NO2S5 : C 51.14, H 4.94, N 2.98; found: C 51.05, H 4.74, N 2.78.

CH3CN (6.0 mL), dry THF (3.0 mL), and piperidine (0.10 mL) was added to a 100 mL flask that contained 3 (100 mg, 0.19 mmol) and cyanoacetic acid (80 mg, 0.93 mmol). After being heated at 80 8C for 13 h, the solution turned from orange-red into dark red. The solution was pumped dry and the residue was purified by using column chromatography on silica gel with CH2Cl2, EA, and EA/CH3COOH (200:3 v/v) as the eluents to give the crude product, which was extracted by using a mixture of EA and H2O (200:200 v/v). The organic extract was pumped dry and the residue was recrystallized from CH2Cl2/hexane to afford D2 as a dark purple powder (yield: 55 mg, 49 %). 1H NMR (400 MHZ, [D6]acetone): d = 8.25 (s, 1 H; -CHO), 7.65 (d, JHH = 4.0 Hz, 1 H), 7.35 (d, JHH = 4.0 Hz, 1 H), 7.23 (d, JHH = 4.0 Hz, 1 H), 6.87 (d, JHH = 4.0 Hz, 1 H), 2.93 (t, JHH = 7.2 Hz, 2 H), 2.79 (t„ JHH = 7.4 Hz 2 H), 1.68 (quint, JHH = 7.3 Hz, 2 H), 1.56 (quint, JHH = 7.3 Hz, 2 H), 1.48 (sextet, JHH = 7.2 Hz, 2 H), 1.36 (sextet, JHH = 7.2 Hz, 2 H), 0.95 (t, JHH = 7.4 Hz, 3 H; -CH3), 0.84 ppm (t, JHH = 7.4 Hz, 3 H; -CH3); 13C NMR (100 MHz, CDCl3): d = 167.44, 149.43, 147.71, 143.07, 140.67, 140.38, 133.90, 131.38, 128.17, 127.54, 125.23, 124.34, 117.29, 116.06, 36.51, 36.07, 31.96, 31.87, 21.94, 21.83, 13.88, 13.77 ppm; MS-HR-MALDI: m/z calcd for C48H48N2O4S12 : 1100.0262 [M] + ; found: 1100.0300; elemental analysis calcd (%) for C48H48N2O4S12 : C 52.33, H 4.39, N 2.54; found: C 52.20, H 4.23, N 2.54.

5-(5-((4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)methyl)thiophen-2yl)thiophene-2-carbaldehyde (2) Dry toluene (20 mL), THF (5 mL), and piperidine (0.10 mL) were added to a 250 mL flask that contained 5-(5-formylthiophen-2-yl)thiophene-2carbaldehyde (1.0 g, 4.50 mmol) and BDT (1.39 g, 4.50 mmol), and the mixture was heated at 120 8C for 4 h. After slow addition of PACHTUNGRE(OEt)3 (10 mL), the solution was heated for 4 h and the color turned from yellow slurry into clear orange-red. After removal of the solvent, the residue was extracted with a mixture of CH2Cl2 and H2O (200 mL, 1:1 v/v). The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The residue was purified by using column chromatography on silica gel with CH2Cl2/hexane (1:8 v/v) and then CH2Cl2/hexane (1:4 v/v) as the eluent. The crude product was further recrystallized from MeOH to provide 2 as red crystals (yield: 1.0 g, 29 %). 1 H NMR (400 MHz, CDCl3): d = 9.82 (s, 1 H; -CHO), 7.63 (d, J = 4.0 Hz, 1 H), 7.28 (d, J = 4.0 Hz, 1 H), 7.21 (d, J = 4.0 Hz, 1 H), 6.78 (d, J = 4.0 Hz, 1 H), 6.64 (s, 1 H; -C=CH-), 2.87 (t, J = 7.4 Hz, 3 H; -CH2-), 2.82 (t, J = 7.4 Hz, 3 H; -CH2-), 1.68–1.59 (m, 4 H), 1.50–1.39 (m, 4 H), 0.93 (t, J = 7.2 Hz, 3 H; -CH3), 0.92 ppm (t, J = 7.2 Hz, 3 H; -CH3); 13C NMR (125 MHz, CDCl3): d = 182.42, 147.65, 142.72, 141.38, 137.61, 134.24, 134.10, 129.08, 126.87, 126.33, 125.07, 123.67, 107.36, 36.27, 36.12, 32.06, 31.96, 21.90, 13.82, 13.78 ppm; MS-LR-EI: m/z calcd for C21H24OS6 : 484.02 [M] + ; found: 484.0.

(E)-3-(5-(5-((4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)methyl)thiophen-2yl)thiophen-2-yl)-2-cyanoacrylic acid (M2) CH3CN (10 mL), dry THF (8.0 mL), and piperidine (0.10 mL) was added to a 100 mL flask that contained 2 (300 mg, 0.92 mmol) and cyanoacetic acid (263 mg, 3.10 mmol). The solution was then heated at 80 8C for 13 h. After removal of the solvent, the residue was extracted with CH2Cl2, EA, and a mixture of CH2Cl2 and EA (200:200 v/v). The organic extract was dried over anhydrous MgSO4 and pumped dry. The residue was purified by using column chromatography on silica gel with CH2Cl2, EA, and EA/ CH3COOH (9:1 v/v) as the eluents to give M2 as a dark purple powder (yield: 95 mg, 28 %). 1H NMR (400 MHz, CDCl3): d = 8.26 (s, 1 H; -CHO), 7.66 (d, J = 4.0 Hz, 1 H), 7.35 (d, J = 4.0 Hz, 1 H), 7.24 (d, J = 4.0 Hz, 1 H), 6.80 (d, J = 4.0 Hz, 1 H), 6.66 (s, 1 H; -C=CH-), 2.88 (t, J = 7.2 Hz, 2 H), 2.83 (t, J = 7.2 Hz, 2 H), 1.69–1.60 (m, 4 H), 1.49–1.43 (m, 4 H), 0.93 (t, JHH = 7.2 Hz, 3 H), 0.92 ppm (t, JHH = 7.2 Hz, 3 H); 13C NMR (100 MHz, [D8]THF): d = 164.11, 147.62, 146.28, 143.57, 140.33, 135.34, 134.99, 134.28, 130.26, 127.90, 126.97, 126.41, 124.73, 116.72, 108.55, 99.23, 36.69, 36.49, 32.98, 32.76, 22.51, 13.97 ppm; MS-HR-MALDI: m/z calcd for C24H25NO2S6 : 551.0209 [M] + ; found: 551.0224; elemental analysis calcd (%) for C24H25NO2S6 : C 52.23, H 4.57, N 2.54; found: C 52.65, H 4.41, N 2.57.

5-(5-(1,2-BisACHTUNGRE(4,5-bis(butylthio)-1,3-dithiol-2-ylidene)-2-(5-(5formylthiophen-2-yl)thiophen-2-yl)ethyl)thiophen-2-yl)thiophene-2carbaldehyde (3)

2-(4-(Thiophen-2-yl)benzylidene)-4,5-bis(butylthio)-1,3-dithiole (4)

Dry CH2Cl2 (60 mL) was added to a 100 mL flask that contained 1 (300 mg, 0.62 mmol), followed by I2 (565 mg, 2.23 mmol). The color of the solution turned from red to olive green. The solution was stirred at RT for 2 d, then aqueous Na2S2O3 (5.0 g) in H2O (100 mL) was added. After stirring for 6 h, the solution turned from olive green to red. A mixture of CH2Cl2 and H2O (200 mL, 1:1 v/v) was added for extraction. The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The residue was purified by using column chromatography on silica gel with CH2Cl2/hexane (1:1 v/v) and then recrystallized from MeOH to provide 3 as a red powder (yield: 100 mg, 33 %). 1 H NMR (500 MHz, CDCl3): d = 9.80 (s, 1 H; -CHO), 7.61 (d, JHH = 3.5 Hz, 1 H), 7.26 (d, J = 4.0 Hz, 1 H), 7.18 (d, J = 3.5 Hz, 1 H), 6.84 (d, J = 3.5 Hz, 1 H), 2.91 (t, J = 7.3 Hz, 2 H), 2.78 (t, J = 7.3 Hz, 2 H), 1.70–1.64 (m, 2 H), 1.58–1.53 (m, 2 H), 1.52–1.47 (m, 2 H), 1.39–1.32 (m, 2 H), 0.95 (t, J = 7.3 Hz, 3 H; -CH3), 0.83 ppm (t, J = 7.3 Hz, 3 H; -CH3); 13C NMR (125 MHz, CDCl3): d = 182.40, 147.42, 142.00, 141.45, 139.12, 137.51, 134.31, 131.03, 127.14, 127.11, 124.93, 123.86, 117.40, 36.35, 15.95, 31.91, 31.81, 21.88, 21.76, 13.82, 13.70 ppm; MS-LR-MALDI: m/z calcd for C42H46O2S12 : 966.0 [M] + ; found: 965.8.

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Dry toluene (30 mL) was added to a flask that contained 4-(thiophen-2yl)benzaldehyde (1.3 g, 6.90 mmol) and BDT (2.2 g, 7.08 mmol) and the mixture was heated to 120 8C. Then PACHTUNGRE(OEt)3 (15 mL) was slowly added and the solution was heated for 4 h, and the color turned from light yellow to orange-red. After removal of the solvent, the residue was extracted with a mixture of CH2Cl2 and H2O. The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The residue was purified by using column chromatography on silica gel with hexanes and CH2Cl2/hexanes (1:8 v/v) as the eluents to give the crude product. The crude product was further recrystallized from CH2Cl2/ MeOH to provide 4 as a yellow-orange syrup (yield: 1.1 g, 35 %). 1 H NMR (300 MHz, CDCl3): d = 7.59 (d, J = 8.4 Hz, 2 H), 7.29 (d, J = 3.6 Hz, 2 H), 7.25 (d, J = 4.2 Hz, 1 H), 7.20 (d, JHH = 8.4 Hz, 2 H) 7.06 (dd, JHH = 4.8, 3.9 Hz, 1 H), 6.45 (s, 1 H; -C=CH-), 2.82 (t, J = 7.4 Hz, 4 H), 1.68–1.57 (m, 4 H), 1.47–1.37 (m, 4 H), 0.94–0.88 ppm (m, 6 H; -CH3); 13 C NMR (75 MHz, CDCl3): d = 144.19, 135.50, 132.68, 131.69, 128.08, 127.60, 127.18, 125.93, 124.87, 124.66, 122.86, 113.78, 35.86, 35.75, 31.87, 31.75, 21.69, 13.60 ppm; MS-LR-FAB: m/z calcd for C22H26S5 : 450.06 [M] + ; found: 450.0.

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C52H52N2O4S10 : 1088.11 [M] + ; found: 1187.1106; elemental analysis calcd (%) for C52H52N2O4S10 : C 57.32, H 4.81, N 2.57; found: C 57.11, H 4.90, N 2.62.

2-(2-(4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)-1,2-bis(4-(thiophen-2yl)phenyl)ethylidene)-4,5-bis(butylthio)-1,3-dithiole (5) Dry CH2Cl2ACHTUNGRE(100 mL) was added to a 500 mL flask that contained 4 (1.1 g, 2.44 mmol), followed by I2 (2.23 g, 8.80 mmol), and the color of the solution turned from yellow into dark green. The solution was stirred at RT for 2 d, then Na2S2O3 (5.0 g) in H2O (100 mL) was added. After stirring for 6 h, the solution turned from olive green into red. A mixture of CH2Cl2 and H2O (200:200 v/v) was added for extraction. The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The residue was purified by using column chromatography on silica gel with CH2Cl2/hexanes (8:1 v/v) and CH2Cl2/hexanes (4:1 v/v) as the eluents. The light yellow-green band was collected and pumped dry, and the residue was recrystallized from CH2Cl2/MeOH to provide 5 as a yellow-orange syrup (yield: 0.60 g, 73 %). 1H NMR (400 MHz, CDCl3): d = 7.63 (d, J = 8.4 Hz, 2 H), 7.53 (d, J = 8.4 Hz, 2 H), 7.34 (d, J = 3.2 Hz, 1 H), 7.28 (d, J = 4.8 Hz, 1 H), 7.09 (dd, J = 4.8, 3.6 Hz, 1 H), 2.90 (t, J = 7.2 Hz, 2 H), 2.84 (t, J = 7.4 Hz, 2 H), 1.74–1.61 (m, 4 H), 1.57–1.40 (m, 4 H), 1.00 (t, J = 7.4 Hz, 3 H; -CH3), 0.94 ppm (t, J = 7.4 Hz, 3 H; -CH3); 13C NMR (100 MHz, CDCl3): d = 144.09, 136.97, 136.17, 132.68, 129.03, 128.15, 127.08, 126.06, 125.19, 124.85, 124.05, 123.03, 35.91, 35.67, 31.83, 21.77, 21.72, 13.77, 13.73 ppm; MS-LR-MALDI: m/z calcd for C44H50S10 : 898.11 [M] + ; found: 898.1.

4-(5-(4-((4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)methyl)phenyl)thiophen2-yl)benzaldehyde (7) Dry toluene (16 mL) was added to a 250 mL flask that contained 4,4’(thiophene-2,5-diyl)dibenzaldehyde (0.92 g, 3.15 mmol) and BDT (0.93 g, 3.00 mmol), and the mixture was heated to 130 8C. PACHTUNGRE(OEt)3 (8.0 mL) was slowly added and the solution was heated for 4 h, and the yellow slurry turned into clear orange solution. After removal of the solvent, the residue was extracted with CH2Cl2 and H2O. The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The residue was purified by using column chromatography on silica gel with CH2Cl2/hexanes (1:4 v/v) and CH2Cl2/hexanes (1:1 v/v) as the eluents. Further recrystallization of the crude product from CH2Cl2/MeOH provided 7 as a yellow-orange powder (yield: 54 mg, 30 %). 1H NMR (400 MHz, CDCl3): d = 9.98 (s, 1 H; -CHO), 7.88 (d, J = 8.0 Hz, 2 H), 7.76 (d, J = 8.4 Hz, 2 H), 7.61 (d, J = 8.0 Hz, 2 H), 7.43 (d, J = 3.6 Hz, 1 H), 7.32 (d, J = 3.6 Hz, 1 H), 7.23 (d, J = 8.4 Hz, 2 H), 2.83 (t, J = 7.2 Hz, 4 H), 1.63 (m, 4 H), 1.43 (m, 4 H), 0.91 ppm (m, 6 H; -CH3); 13C NMR (75 MHz, CDCl3): d = 191.58, 145.94, 141.68, 140.22, 136.32, 135.24, 133.66, 131.17, 130.73, 128.09, 127.46, 126.40, 126.00, 125.85, 125.07, 124.38, 113.74, 36.11, 35.99, 32.10. 31.97, 21.92, 13.84 ppm; MS-LR-FAB: m/z calcd for C29H30OS5 : 554.87 [M] + ; found: 554.0.

5-(4-(1,2-BisACHTUNGRE(4,5-bis(butylthio)-1,3-dithiol-2-ylidene)-2-(4-(5formylthiophen-2-yl)phenyl)ethyl)phenyl)thiophene-2-carbaldehyde (6) Dry dimethylformamide (DMF; 5.0 mL) was added to a 300 mL flask that contained 5 and the solution was cooled to 0 8C. POCl3 (0.15 mL, 1.50 mmol) was added and the solution was stirred for 30 min, then warmed to RT and stirred for 30 min, and then heated at 70 8C for 3 h. The solution turned from light yellow to dark brown. After addition of CH3COONa (5.0 g) in H2O (50 mL), the solution was stirred for 30 min. and then extracted with CH2Cl2 and H2O. The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The residue was purified by using column chromatography on alumina with CH2Cl2/hexanes (4:1 v/v) and CH2Cl2/hexanes (1:1 v/v) as the eluents. The light yellow-green band was collected and pumped dry and the residue was recrystallized from CH2Cl2/hexanes to provide 6 as a yelloworange syrup (yield: 195 mg, 41 %). 1H NMR (400 MHz, CDCl3): d = 9.85 (s, 1 H; -CHO), 7.69 (d, JHH = 3.6 Hz, 1 H), 7.62 (d, J = 8.4 Hz, 2 H), 7.46 (d, J = 8.4 Hz, 2 H), 7.35 (d, J = 4.0 Hz, 1 H), 2.83 (t, J = 7.2 Hz, 2 H), 2.75 (t, J = 7.2 Hz, 2 H), 1.66–1.51 (m, 4 H), 1.49–1.31 (m, 4 H). 0.92 (t, JHH = 7.4 Hz, 3 H), 0.84 ppm (t, J = 7.4 Hz, 3 H); 13C NMR (125 MHz, CDCl3): d = 182.82, 153.94, 142.34, 139.33, 138.06, 137.66, 131.10, 129.42, 127.15, 126.81, 125.65, 124.00, 123.11, 36.05, 35.78, 31.87, 21.83, 21.76, 13.80, 13.75 ppm; MS-LR-EI: m/z calcd for C46H50O2S10 : 954.10 [M] + ; found: 954.1.

(E)-3-(4-(5-(4-((4,5-Bis(butylthio)-1,3-dithiol-2ylidene)methyl)phenyl)thiophen-2-yl)phenyl)-2-cyanoacrylic acid (M4) Dry THF (3.0 mL) and piperidine (0.10 mL) was added to a 100 mL flask that contained 7 (170 mg, 1.53 mmol) and cyanoacetic acid (130 mg, 1.53 mmol). The solution was then heated at 80 8C for 13 h, then pumped dry and the residue was purified by using column chromatography on silica gel with CH2Cl2, EA, and EA/CH3COOH (9:1 v/v) as the eluents. The residue was extracted with EA and H2O and the organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The residue was recrystallized from CH2Cl2/hexanes to afford M4 as a deep red powder (yield: 120 mg, 63 %). 1H NMR (400 MHZ, [D6]DMSO): d = 8.30 (s, 1 H; -CHO), 8.09 (d, J = 8.0 Hz, 2 H), 7.90 (d, J = 8.4 Hz, 2 H), 7.78 (d, J = 3.6 Hz, 1 H), 7.75 (d, J = 8.8 Hz, 2 H), 7.63 (d, J = 4.0 Hz, 1 H), 7.29 (d, J = 8.4 Hz, 2 H), 6.76 (s, 1 H), 2.88 (t, J = 7.2 Hz, 4 H), 1.56 (m, 4 H), 1.39 (m, 4 H), 0.88 ppm (m, 6 H; -CH3); 13C NMR (100 MHz, [D8]THF): d = 163.89, 153.70, 146.34, 142.54, 139.44, 137.16, 133.66, 132.65, 132.11, 131.84, 128.97, 128.19, 127.41, 126.44, 126.39, 125.85, 125.42, 116.48, 114.69, 103.78, 36.49, 36.35, 32.93, 32.76, 22.52, 13.96 ppm; MS-HR-FAB: m/z calcd for C32H31NO2S5 : 621.0958 [M] + ; found: 621.097; elemental analysis calcd (%) for C32H31NO2S5 : C 61.80, H 5.02, N 2.25; found: C 61.90, H 5.21, N 2.38.

3,3’-(5,5’-((1,2-BisACHTUNGRE(4,5-bis(butylthio)-1,3-dithiol-2-ylidene)ethane-1,2diyl)bis(4,1-phenylene))bis(thiophene-5,2-diyl))bis(2-cyanoacrylic acid) (D3)

4,4’-(5,5’-((1,2-BisACHTUNGRE(4,5-bis(butylthio)-1,3-dithiol-2-ylidene)ethane-1,2diyl)bis(4,1-phenylene))bis(thiophene-5,2-diyl))dibenzaldehyde (8)

CH3CN (3.0 mL), dry THF (3.0 mL), and piperidine (0.10 mL) were added to a 100 mL flask that contained 6 (165 mg, 0.17 mmol) and acyanoacetic acid (75 mg, 0.85 mmol). The solution was then heated at 80 8C for 13 h. The solution turned from orange-red to dark red. After removal of the solvent, the residue was purified by using column chromatography on silica gel with CH2Cl2, EA, EA/CH3COOH (100:1 v/v), and EA/ CH3COOH (9:1 v/v) as the eluents to give the crude product. The crude product was extracted with EA and H2O. The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. Further recrystallization of the crude product from CH2Cl2/hexanes provided D3 as a purple powder (yield: 54 mg, 30 %). 1H NMR (400 MHz, [D8]THF): d = 8.34 (s, 1 H), 7.86 (d, J = 4.0 Hz, 1 H), 7.74 (d, J = 8.4 Hz, 2 H), 7.55 (d, J = 4.0 Hz, 2 H), 7.52 (d, J = 8.4 Hz, 2 H), 2.89 (t, J = 7.2 Hz, 2 H), 2.81 (t, J = 7.2 Hz, 2 H), 1.65–1.55 (m, 4 H), 1.52–1.35 (m, 4 H), 0.95 (t, J = 7.4 Hz, 3 H; -CH3), 0.88 ppm (t, J = 7.4 Hz, 3 H; -CH3); 13C NMR (125 MHz, [D8]THF): d = 164.04, 153.84, 146.47, 139.78, 138.94, 136.32, 132.12, 130.00, 128.12, 127.47, 126.80, 125.25, 124.40, 116.60, 100.12, 36.52, 36.31, 32.78, 32.73, 22.47, 13.98 ppm; MS-HR-MALDI: m/z calcd for

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Dry CH2Cl2 (120 mL) was added to a 300 mL flask that contained 7 (640 mg, 1.15 mmol), followed by I2 (1.05 g, 4.15 mmol), and the color of the solution turned from red to olive green. The solution was stirred at RT for 2 d, then Na2S2O3 (5.0 g) in H2O (100 mL) was added. After stirring for 1 d, the solution turned from olive green to yellow-orange. A mixture of CH2Cl2 and H2O (1:2 v/v) was added for extraction. The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The residue was purified by using column chromatography on silica gel with CH2Cl2/hexanes (1:2 v/v) and CH2Cl2/hexanes (1:1 v/v) as the eluents. Further recrystallization of the crude product from CH2Cl2/MeOH provided 8 as a yellow-orange powder (yield: 300 mg, 47 %). 1H NMR (400 MHz, [D6]acetone): d = 10.01 (s, 1 H; -CHO), 7.93 (d, J = 8.4 Hz, 2 H), 7.87 (d, J = 8.4 Hz, 2 H), 7.69 (d, J = 8.4 Hz, 2 H), 7.63 (d, J = 4.0 Hz, 1 H), 7.52 (d, J = 6.8 Hz, 2 H), 7.48 (d, J = 4.0 Hz, 1 H), 2.89 (t, J = 7.2 Hz, 2 H), 2.82 (t, J = 7.2 Hz, 2 H), 1.67–1.55 (m, 4 H), 1.50–1.35 (m, 4 H), 0.98–0.88 ppm (m, 6 H; -CH3); 13C NMR (125 MHz, CDCl3): d = 191.43, 145.72, 141.76, 140.10, 137.91, 136.85, 135.22, 132.11, 130.63, 129.33, 127.22, 126.33, 126.06, 125.79, 125.46,

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124.46, 123.80, 36.07, 35.82, 31.93, 21.86, 21.81, 13.81, 13.78 ppm; MS-LRMALDI: m/z calcd for C58H58O2S10 : 1106.16 [M] + ; found: 1105.9.

(m, 4 H), 1.55–1.35 (m, 4 H), 0.93 (t, J = 7.4 Hz, 3 H; -CH3), 0.88 (t, J = 7.4 Hz, 3 H; -CH3), 0.37 ppm (t, J = 7.4 Hz, 6 H; -CH3); 13C NMR (100 MHz, [D6]acetone): d = 152.10, 151.23, 145.58, 141.53, 141.13, 137.77, 136.20, 134.43, 129.15, 128.70, 126.98, 126.96, 126.28, 125.87, 125.71, 124.19, 122.84, 121.22, 121.03, 120.93, 57.03, 36.26, 36.19, 33.30, 32.70, 22.29, 14.05, 9.20 ppm; MS-LR-MALDI: m/z calcd for C66H74S10 : 1186.3 [M] + ; found: 1186.1.

ACHTUNGRE(2E,2’E)-3,3’-((5,5’-((1,2-BisACHTUNGRE(4,5-bis(butylthio)-1,3-dithiol-2ylidene)ethane-1,2-diyl)bis(4,1-phenylene))bis(thiophene-5,2-diyl))bis(4,1phenylene))bis(2-cyanoacrylic acid) (D4) CH3CN (3.0 mL), THF (3.0 mL), and piperidineACHTUNGRE(0.10 mL) was added to a 100 mL flask that contained 8 (300 mg, 0.27 mmol) and cyanoacetic acid (115 mg, 1.35 mmol). The solution was then heated at 80 8C for 13 h, then pumped dry and the residue was purified by using column chromatography on silica gel with CH2Cl2, EA, and EA/CH3COOH (9:1 v/v) as the eluents. The crude product was extracted with EA and H2O and the organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. Recrystallization of the crude product from CH2Cl2 afforded D4 as a black solid (yield: 80 mg, 34 %). 1H NMR (400 MHZ, [D8]THF): d = 8.24 (s, 1 H), 8.10 (d, J = 8.4 Hz, 2 H), 7.84 (d, J = 8.4 Hz, 2 H), 7.66 (d, J = 8.0 Hz, 2 H), 7.59 (d, J = 4.0 Hz, 1 H), 7.49 (d, J = 8.0 Hz, 2 H), 7.44 (d, J = 4.0 Hz, 1 H), 2.88 (t, J = 7.2 Hz, 2 H), 2.82 (t, J = 7.2 Hz, 2 H) 1.69–1.51 (m, 4 H), 1.49–1.36 (m, 4 H), 0.93 (t, J = 7.4 Hz, 3 H; -CH3), 0.88 ppm (t, J = 7.4 Hz, 3 H; -CH3); 13C NMR (75 MHz, CDCl3): d = 163.75, 154.56, 146.01, 142.64, 139.45, 133.22, 132.87, 131.72, 127.90, 126.83, 126.59,126.11, 116.71, 103.38, 36.22, 32.67, 22.32, 14.04 ppm; MS-HR-MALDI: m/z calcd for C64H60N2O4S10 : 1240.1760 [M] + ; found: 1240.1807; elemental analysis calcd (%) for C64H60N2O4S10 : C 61.90, H 4.87, N 2.26; found: C 61.79, H 4.78, N 2.25.

5-(7-(1,2-BisACHTUNGRE(4,5-bis(butylthio)-1,3-dithiol-2-ylidene)-2-(9,9-diethyl-7-(5formylthiophen-2-yl)-9 H-fluoren-2-yl)ethyl)-9,9-diethyl-9 H-fluoren-2yl)thiophene-2-carbaldehyde (11) Dry DMF (5.0 mL) was added to a 300 mL flask that contained 10 (0.80 g, 0.67 mmol) and the solution was cooled to 0 8C. POCl3 (0.18 mL, 2.01 mmol) was added and the solution was stirred for 30 min, then warmed to RT and stirred for 30 min, and finally heated at 70 8C for 3 h. The solution turned from light yellow to dark brown. After addition of CH3COONa (5.0 g) in H2O (50 mL), the solution was stirred for 30 min and then extracted with CH2Cl2 and H2O. The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry and the residue was purified by using column chromatography on alumina with CH2Cl2/hexanes (4:1 v/v) and CH2Cl2/hexanes (1:2 v/v) as the eluents. The crude product was recrystallized from CH2Cl2/hexanes to afford 11 as an orange powder (yield: 0.42 g, 50 %). 1H NMR (400 MHz, [D6]acetone): d = 9.94 (s, 1 H; -CHO), 7.97 (d, J = 4.0 Hz, 1 H), 7.90 (d, J = 1.6 Hz, 1 H), 7.89 (d, J = 6.8 Hz, 1 H), 7.87 (d, J = 6.8 Hz, 1 H), 7.81 (dd, J = 8.0, 2.0 Hz, 1 H), 7.74 (d, J = 4.0 Hz, 1 H), 7.59 (d, J = 1.6 Hz, 1 H), 7.49 (dd, J = 8.4, 2.0 Hz, 1 H), 2.88 (t, J = 7.2 Hz, 2 H), 2.80 (t, J = 7.2 Hz, 2 H), 2.22–2.12 (m, 4 H), 1.69–1.55 (m, 4 H), 1.51–1.35 (m, 4 H), 0.93 (t, J = 7.4 Hz, 3 H; -CH3), 0.88 (t, J = 7.4 Hz, 3 H; -CH3), 0.37 ppm (t, J = 7.4 Hz, 6 H; -CH3); 13C NMR (75 MHz, [D6]acetone): d = 183.68, 154.77, 152.35, 151.57, 143.40, 143.29, 140.65, 139.09, 138.28, 136.71, 132.91, 128.71, 127.05, 126.67, 126.62, 126.30, 125.45, 122.86, 121.64, 121.48, 57.17, 36.24, 36.17, 33.17, 32.67, 22.27, 14.04, 9.18 ppm; MS-LR-MALDI: m/z calcd for C68H74O2S10 : 1242.29 [M] + ; found: 1242.1.

4,5-Bis(butylthio)-2-((9,9-diethyl-7-(thiophen-2-yl)-9 H-fluoren-2yl)methylene)-1,3-dithiole (9) Dry toluene (16 mL) was added to a 250 mL flask that contained 9,9-diethyl-7-(thiophen-2-yl)-9H-fluorene-2-carbaldehyde (1.01 g, 3.06 mmol) and BDT (0.95 g, 3.06 mmol) and the mixture was heated to 130 8C. PACHTUNGRE(OEt)3 (8.0 mL) was slowly added and the solution was heated for 4 h, and the yellow slurry turned into a clear yellow-orange solution. After removal of the solvent, the residue was extracted with CH2Cl2 and H2O. The organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry and the residue was purified by using column chromatography on silica gel with CH2Cl2/hexanes (1:1 v/v) as the eluent. The residue was recrystallized from CH2Cl2/hexanes to afford 9 as a yellow-orange syrup (yield: 1.2 g, 66 %). 1H NMR (400 MHz, CDCl3): d = 7.65 (d, J = 8.0 Hz, 1 H), 7.64 (d, J = 8.0 Hz, 1 H), 7.58 (dd, J = 8.0, 2.0 Hz, 1 H), 7.52 (d, J = 1.2 Hz, 1 H), 7.35 (dd, J = 4.0, 1.2 Hz, 1 H), 7.26 (dd, J = 6.8, 1.2 Hz, 1 H), 7.18 (dd, J = 8.4, 1.6 Hz, 1 H), 7.15 (d, J = 1.6 Hz, 1 H), 7.08 (dd, J = 4.8, 1.6 Hz, 1 H), 6.55 (s, 1 H), 2.82 (t, JHH = 7.2 Hz, 4 H; -SCH2-), 2.03 (m, 4 H), 1.67–1.60 (m, 4 H), 1.49–1.39 (m, 4 H), 0.96–0.88 (m, 6 H), 0.36 ppm (t, J = 7.4 Hz, 6 H); 13C NMR (75 MHz, CDCl3): d = 150.96, 150.48, 145,11, 140.75, 138.94, 135.42, 133.12, 131.57, 128.09, 127.67, 126.05, 125.04, 124.74, 124.48, 122.85, 121.16, 120.15, 120.03, 119.84, 115.23, 56.13, 35.92, 35.76, 32.86, 31.90, 31.84, 21.74, 13.71, 8.66 ppm; MS-LR-FAB: m/z calcd for C33H38S5 : 594.16 [M] + ; found: 594.0.

ACHTUNGRE(2E,2’E)-3,3’-(5,5’-((1,2-BisACHTUNGRE(4,5-bis(butylthio)-1,3-dithiol-2-ylidene)ethane1,2-diyl)bis(9,9-diethyl-9 H-fluorene-7,2-diyl))bis(thiophene-5,2-diyl))bis(2cyanoacrylic acid) (D5) CH3CN (3.0 mL), THF (6.0 mL), and piperidine (0.10 mL) was added to a 100 mL flask that contained 11 (200 mg, 0.16 mmol) and cyanoacetic acid (68 mg, 0.80 mmol). The solution was then heated at 80 8C for 13 h. The solution was pumped dry, and the residue was purified by using column chromatography on silica gel with CH2Cl2, EA, and EA/ CH3COOH (20:1 v/v) as the eluents. After removal of the solvent, the residue was extracted with EA and H2O and the organic extract was dried over anhydrous MgSO4. After filtration, the filtrate was pumped dry. The crude product was recrystallized from CH2Cl2/hexanes to afford D5 as a dark purple powder (yield: 160 mg, 49 %). 1H NMR (400 MHz, [D8]THF): d = 8.36 (s, 1 H), 7.86 (d, J = 4.0 Hz, 1 H), 7.81–7.74 (m, 4 H), 7.64 (d, J = 4.0 Hz, 1 H), 7.56 (s, 1 H), 7.44 (d, J = 8.0 Hz, 1 H), 2.84 (t, J = 7.0 Hz, 2 H), 2.76 (t, J = 7.2 Hz, 2 H), 1.63 (quint, J = 7.3 Hz, 2 H), 1.56 (quint, J = 7.3 Hz, 2 H), 1.45 (sextet, J = 7.6 Hz, 2 H), 1.38 (sextet, J = 7.6 Hz, 2 H), 0.93 (t, J = 7.4 Hz, 3 H), 0.88 (t, J = 7.4 Hz, 3 H), 0.38 ppm (t, J = 7.4 Hz, 6 H); 13C NMR (100 MHz, [D8]THF): d = 164.10, 155.22, 152.38, 151.52, 146.71, 143.79, 140.66, 140.27, 138.61, 137.04, 136.03, 132.90, 128.91, 127.09, 126.71, 126.52, 125.24, 122.90, 121.44, 121.32, 121.27, 116.65, 99.67, 57.23, 36.38, 36.26, 33.34, 32.83, 32.76, 22.49, 22.46, 14.02, 14.00, 9.09 ppm; MS-HR-MALDI: m/z calcd for C74H76N2O4S10 : 1376.3013 [M] + ; found: 1376.3065; elemental analysis calcd (%) for C74H76N2O4S10 : C 64.50, H 5.56, N 2.03; found: C 64.42, H 5.53, N 2.29.

2-(2-(4,5-Bis(butylthio)-1,3-dithiol-2-ylidene)-1,2-bis(9,9-diethyl-7(thiophen-2-yl)-9 H-fluoren-2-yl)ethylidene)-4,5-bis(butylthio)-1,3-dithiole (10) Dry CH2Cl2 (40 mL) was added to a 300 mL flask that contained 9 (1.2 g, 2.02 mmol), followed by I2 (1.84 g, 7.26 mmol), and the color of the solution turned from red to olive green. The solution was stirred at RT for 2 d, then Na2S2O3 (5.0 g) in H2O (100 mL) was added. After stirring for 1 d, the solution turned from olive green to yellow. A mixture of CH2Cl2 and H2O (1:2 v/v) was added for extraction. The organic extract was dried over anhydrous MgSO4 and pumped dry. The residue was purified by using column chromatography on silica gel with CH2Cl2/hexanes (1:1 v/v) and CH2Cl2 as the eluents. Recrystallization of the crude product from hexanes provided 10 as a yellow powder (yield: 670 mg, 56 %). 1 H NMR (400 MHz, [D6]acetone): d = 7.81 (d, J = 8.0 Hz, 1 H), 7.80 (d, J = 7.6 Hz, 1 H), 7.75 (d, J = 1.6 Hz, 1 H), 7.66 (dd, J = 8.0, 1.6 Hz, 1 H), 7.59 (d, J = 1.6 Hz, 1 H), 7.53 (dd, J = 3.6, 1.2 Hz, 1 H), 7.45 (dd, J = 8.0, 1.6 Hz, 1 H), 7.43 (dd, J = 4.0, 1.2 Hz, 1 H), 7.12 (dd, J = 5.2, 3.6 Hz, 1 H), 2.86 (t, J = 7.2 Hz, 2 H), 2.78 (t, J = 7.2 Hz, 2 H), 2.12 (m, 4 H), 1.68–1.57

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Acknowledgements We acknowledge the support of the Academia Sinica (AS), the Ministry of Science and Technology (Taiwan), and the Instrumental Center of Institute of Chemistry (AS).

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Received: March 14, 2014 Published online: && &&, 0000

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FULL PAPER Pair up to catch the sun: Dipolar metal-free sensitizers (D-p-A) and their dimeric congeners ((D-p-A)2) consisting of a dithiafulvalene (DTF) unit as the electron donor, a benzene, thiophene, or fluorene moiety as the conjugated spacer, and 2-cyanoacrylic acid as the electron acceptor have been synthesized. In dye-sensitized solar cells, the (D-p-A)2 dyes show better light harvesting and more effective suppression of the dark current than the (D-p-A) dyes (see figure).

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Dye-Sensitized Solar Cells Ting-Hui Lee, Chih-Yu Hsu, You-Ya Liao, Hsien-Hsin Chou, Heather Hughes, &&&&—&&&& Jiann T. Lin* Dye-Sensitized Solar Cells Based on (Donor-p-Acceptor)2 Dyes With Dithiafulvalene as the Donor

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Dye-sensitized solar cells based on (donor-π-acceptor)2 dyes with dithiafulvalene as the donor.

Dipolar metal-free sensitizers (D-π-A; D=donor, π=conjugated bridge, A=acceptor) consisting of a dithiafulvalene (DTF) unit as the electron donor, a b...
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