Structurally Defined High-LUMO-Level 66π-[70]Fullerene

(12) These two fullerenes are not necessarily the best electron acceptor to be ... of new classes of structurally defined 66π-electron tetra-organo[7...
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Structurally Defined High-LUMO-Level 66π-[70]Fullerene Derivatives: Synthesis and Application in Organic Photovoltaic Cells Zuo Xiao,† Yutaka Matsuo,*,† Iwao Soga,†,‡ and Eiichi Nakamura*,† †

Department of Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Mitsubishi Chemical Group Science and Technology Research Center, Inc. 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan

‡

S Supporting Information *

ABSTRACT: Two new reactions for the synthesis of structurally defined 66π-electron [70]fullerene derivatives are reported. The first provides synthetic access to tetra-phenyl or [3 + 1] hybrid tetra-aryl C70 adducts via oxidation of a fullerene copper complex [Ar3C70−Cu−Ar′]− (Ar = Ph, 4-nBuC6H4; Ar′ = Ph, 4-MeOC6H4). The second provides access to alkyl fullerene ethers, C70Ar3(2-EH) via AgClO4-mediated coupling of a [70]fullerene bromide C70Ar3Br with 2-ethylhexanol (2-EH). The first reaction afforded two types of regioisomers including a 3,10,22,25-adduct (denoted type I) and a 7,10,22,25-adduct (type II). The haptotropic migration of the copper on a cuprio fullerene intermediate was suggested to be responsible for the generation of the two regioisomers. The second reaction gave only one regioisomer (type II). The eight new 66π-electron [70]fullerene derivatives synthesized are electrochemically and thermally stable, and their photoabsorption and electrochemical properties are closely related to the addition pattern. For example, the type II regioisomers have higher LUMO levels than the type I isomers. Through modification of the addends, the LUMO levels of these [70]fullerene derivatives can be raised by as much as 220 meV, that is, from −3.80 to −3.58 eV. Solution-processed p-n junction organic photovoltaic devices using five soluble compounds 5, 9, 10, 13, and 15 as the n-type semiconducting materials were fabricated. The device using compound 15 (LUMO = −3.58 eV) showed the highest open circuit voltage (Voc = 0.90 V) and a respectable PCE value of 3.33%. For Jsc and FF, type II compounds 10, 13, and 15 showed much higher values than did type I compounds 5 and 9, suggesting that the type II addition pattern on C70 may be superior to the type I pattern for efficient electron transport, likely because of better molecular packing in crystals as suggested by crystallographic data. KEYWORDS: copper, migration, fullerene, porphyrin, organic solar cells

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INTRODUCTION With the prospect of highly efficient organic photovoltaic devices (OPVs) being commercialized in the near future,1 there is a pressing need for the improvement of power conversion efficiency (PCE)2 and for the development of synthetically viable high-performance organic semiconductor materials.3,4 There is a rich repertoire of p-type materials such as π-conjugated polymers5 and oligomers,6 organic dyes,7 porphyrins,8 and phthalocyanines,9 but fullerene derivatives are the main components utilized as n-type materials,10 because of their high electron affinity, small reorganization energy, and unique spherical or ellipsoidal molecular shape that renders them able to form a dense packing structure for efficient electron transport in the active layer.11 Among the fullerene materials examined so far, two methanofullerene derivatives, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and [6,6]-phenyl-C71-butyric acid © 2012 American Chemical Society

methyl ester (PC71BM) derivatives, have been widely employed as the acceptor materials for blending with an electron-donating π-conjugated polymers in bulk-heterojunction (BHJ) solar cells.12 These two fullerenes are not necessarily the best electron acceptor to be combined with other donor materials. For example, when used together with tetrabenzoporphyrin (BP) in a p-i-n junction device, a 58π-electron 1,4-bis(dimethylphenylsilylmethyl) [60]fullerene (SIMEF) that we developed some time ago13 gave 5.4% PCE, which is better than the value for PCBM in the same device.8c One probable reason for this difference in performance is the LUMO levels of the fullerenes, which need to be matched precisely to the HOMO levels of the donors; the open circuit Received: April 23, 2012 Revised: June 5, 2012 Published: June 5, 2012 2572

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voltage (Voc) of the OPVs is proportional to the energy difference between the HOMO of the donor and the LUMO of the acceptor.14 Thus, we would be able to achieve higher Voc and hence higher PCE through the development of fullerenes with a higher LUMO level. One way to achieve this goal is to shrink the conjugation of the fullerene derivatives from the original 60πsystem to a less conjugated system, for example, 56π and 54π. Toward this goal, 56π-electron conjugated systems such as indene fullerene bis-adducts (IC60BA)15 and dihydromethanofullerene derivatives,16 and 54π-systems of hexa-organo[60]fullerenes have been developed.17,18 Such operations can raise the LUMO level by >0.2 eV and Voc by >0.2 V. Reduction of the conjugation system of [70]fullerene however has been less fruitful, largely because the reactions of [70]fullerene are more difficult to control than those of the more symmetric [60]fullerene.15c Here, we report the regiocontrolled synthesis and OPV application of new classes of structurally defined 66π-electron tetra-organo[70]fullerenes. An attractive feature of the present synthesis is that each compound is available in an isomerically pure form, in contrast to conventional preparations of organo[70]fullerene where products are obtained and tested as a mixture of isomers.19 This feature allows us to discuss the correlation between the structure of the [70]fullerene derivatives and their OPV performance. We thus found that certain isomers show much higher high Voc and PCE values.

Scheme 1. Synthesis and Purification of C70Ph4 Regioisomers 2 and 3

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RESULTS AND DISCUSSION Synthesis and Characterization of 66π-Electron C70 Tetra-aryl Derivatives. The synthesis of C70 tetra-phenyl (C70Ph4) compounds is first discussed. As described in our previous reports, the reaction of C70 and an organocopper reagent (Ph2Cu·MgBr) produces an indenyl-type C70 triphenyl copper complex intermediate [Ph3C70−Cu−Ph]− (A) in quantitative yield. Treating it with HCl or NH4Cl quantitatively affords a triadduct C70Ph3H (1).20 Treatment of the copper intermediate with N-bromosuccinimide (NBS) was found to afford a mixture of isomers of tetra-phenyl C70 derivatives (C70Ph4) in good total yield of 89%, instead of the expected bromide, C70Ph3Br.21 This is experimental proof that copper intermediate A bears a phenyl group, as indicated in Scheme 1.22 One of the isomers, 2, was separated from other isomers (3 and unidentified isomers) by gel permeation chromatography (GPC). Although 3 could not be purified by chromatography, pure 3 were obtained by thermal isomerization of the remaining isomers. Thus, heating a mixture of 3 and the other unidentified isomers very slowly (even at room temperature) or rapidly at 100 °C (completion in 2 h) quantitatively converted the mixture to pure 3. Therefore, we could ultimately obtain the isomerically pure tetra-phenyl derivatives 2 and 3 in 42 and 35% yields based on [70]fullerene (Scheme 1). Given the low likelihood of C−C bond cleavage at 100 °C and the steric congestion among the four phenyl groups (Figure 2), we consider that the isomers of 3 are atropisomers due to C−C bond rotation of one or more phenyl groups.21 The APCI-MS spectra of 2 and 3 showed the same molecular ion peak at 1148 in accordance with the molecular formula C94H20. The 1H NMR spectra of 2 and 3 exhibited an asymmetric pattern in the aromatic region indicating the C1 symmetric structures of both molecules. Due to low solubility, the 13C NMR measurements of both 2 and 3 were unsuccessful. The molecular structure of 2 was unambiguously determined by X-ray singlecrystal analysis (Figure 1a). Interestingly, the fourth-added phenyl group in 2 is located on the bottom hemisphere.

Figure 1. Crystal structure of compound 2. (a) Ellipsoids are drawn at the 30% level. Key: gray, carbon; white, hydrogen. (b) One-dimensional stacking structure along the a-axis (capped sticks model). (c) A close-up view of the purple square region. Intermolecular interactions: green, CH−π interaction between Ph(B) and Ph(C); cyan, π−π interaction between Ph(A) and Ph(D); yellow, π−π interaction between two adjacent C70 molecules.

To the best of our knowledge, this kind of 3,10,22,25-tetrakisaddition23 mode (type I) is unprecedented in [70]fullerene 2573

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multiaddition chemistry.19,20 Upon comparison of UV−vis spectra, 3 was assigned the same addition pattern as 6, whose structure was determined by X-ray analysis (vide infra). Compound 3 presents a 7,10,22,25-addition mode (type II), which is the same as that of C70Ph3H that we reported previously (Scheme 2).20

Scheme 3. Synthesis of C70 Tetra-aryl Adducts by Sequential Addition Reactions

Scheme 2. Mechanistic Rationale of the Generation of the Different Regioisomers of C70 Tetra-phenyl Adducts

The packing structure of 2 in the crystal is noteworthy. As shown in Figure 1b, the molecules are one-dimensionally stacked24 along the a-axis. Three different kinds of intermolecular interactions contribute to the formation of the alignment in the crystal (Figure 1c): a CH-π interaction between Ph(B) and Ph(C), where these two adjacent phenyl groups are perpendicular to each other and the distance between the proton of Ph(C) and the plane of Ph(B) is as small as 2.63 Å; a π−π interaction between Ph(A) and Ph(D), where these two Ph groups are approximately parallel to each other and the distance between the centroid of Ph(D) and the plane of Ph(A) is 3.53 Å; and a π−π interaction between the two adjacent C70 molecules, where the closest distance between them is 3.30 Å. The 1,4-relationship of the phenyl groups marked in red for 2 and 3 (Scheme 1) provides strong support for our hypothesis that phenylcuprate(I) intermediate A that produces 2, and the haptotropic25 isomer B that produces 3 (Scheme 2). Thus, NBS oxidized the cuprates to induced C−C bond formation.26−28 On the basis of this unexpected C−C bond-forming reaction, we explored the synthesis of a mixed tetra-aryl adduct C70Ar3Ar′ via a triaddition/monoaddition sequence (Scheme 3). A similar strategy was previously achieved for the synthesis of mixed triarylated [60]fullerenes in a simpler manner.29 First, C70 was quantitatively converted to C70Ar3H through the 3-fold addition of an organocopper reagent Ar2Cu·MgBr followed by proton quenching of the intermediate A.20 The C70Ar3H product thus obtained was quantitatively converted to C70Ar3Br by reaction with NBS/pyridine. The bromide was converted to [Ar3C70− Cu−Ar′]− by reaction with a second organocopper reagent Ar′2Cu·MgBr,30 and the resulting mixed cuprate [Ar3C70−Cu− Ar′]− was oxidized with NBS to obtain the desired mixed tetraaryl adduct C70Ar3Ar′. As was found for C70Ph4, the initial product mixture contained several isomers and only two isomers corresponding to 2 (type I) and 3 (type II) were isolated in pure form via thermal treatment. Thus, compounds 5 and 6 for C70Ph3(C6H4−OMe-4) were obtained in 31 and 34% yields, and compounds 9 and 10 for C70(C6H4-nBu-4)3(C6H4−OMe-4) were obtained in 25 and 22% yields based on [70]fullerene.

In contrast to C70Ph4, the anisyl triphenyl [70]fullerene derivatives (5, 6, 9, and 10) showed much improved solubility and were well characterized by spectroscopic methods. For example, the 13C NMR spectrum of 5 exhibits five peaks in the sp3 region (55.07 ppm for the methoxy group; 57.57, 58.13, 58.48, and 59.54 ppm for the four sp3 carbons of C70) and 73 peaks in the sp2 region (110−170 ppm range, for the 90 sp2 carbons of the fullerene and aryl substituents),31 indicating the C1 symmetry of the molecule. The structure of compound 6 was unambiguously determined by the X-ray single-crystal analysis (Figure 2). The crystal structure of 6 indicates that the last 4-MeOC6H4 group was added at the C7 (or C9) position.

Figure 2. Crystal structure of compound 6. Ellipsoids are drawn at the 30% level. Key: gray, carbon; red, oxygen; white, hydrogen.

For fullerene regioisomers, each isomer has a particular conjugated π-system on the fullerene core, and hence has its own characteristic absorption spectrum. Therefore, the UV−vis spectrum of each isomer provides a fingerprint for characterizing 2574

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isomeric fullerene adducts.32 By comparing the UV−vis spectra of the C70 tetra-aryl derivatives, we found that 2, 5, and 9 afford the same type of absorbance with four characteristic shoulder peaks at 340, 375, 475, and 570 nm, whereas 3, 6, and 10 afford the same type of absorbance with one characteristic peak at 414 nm (Figure 3). The results suggest that 2, 5, and 9 belong to

We chose the branched 2-ethylhexyl group to enhance the solubility of the molecules in organic solvents. As illustrated in Scheme 4, compound 13 was synthesized in three steps. First, the reaction of [70]fullerene with an Scheme 4. Synthesis of Tri-aryl-mono(2-ethylhexyloxyl) [70]fullerene Derivatives 13 and 15

organocopper reagent gave C70(C6H4−Ph-4)3H in 93% yield, which was converted to C70(C6H4−Ph-4)3Br in 96% yield by reaction with NBS/pyridine. Finally, the bromide was converted to C70(C6H4−Ph-4)3(2-EH) in 80% yield through AgClO4mediated cross-coupling with 2-ethylhexanol. The total yield of 13 from C70 was 71%. The use of excess alcohol in the last step suppresses the formation of the side product C70Ar3(OH) which forms because of adventitious water. Similarly, C70(C6H4− OMe-2)3(2-EH) (15) was synthesized in 40% yield based on [70]fullerene. The relatively low yield of 15 was due to competitive decomposition of the bromide under the reaction conditions. Spectroscopic data are in agreement with the assigned structures of 13 and 15 (Scheme 4), which are an approximately 1:1 mixture of diastereomers as judged by their 1H and 13C NMR spectra. The position of the 2-EH group in 13 and 15 was determined to be at C7 as judged by the similarity of their UV−vis spectra to that of compound 6. Likewise, 13 and 15 exhibit a characteristic peak at 414 nm, which indicates the type II adduct (see the Supporting Information). Electronic and Thermal Properties of the New C70 Derivatives. To serve as useful semiconductor materials for OPV devices, the compounds need to exhibit favorable electrochemical and thermal stability, a suitable LUMO level, and temperature-dependent morphological changes. The electrochemical properties of compounds 2, 3, 5, 6, 9, 10, 13, and 15 were first examined by cyclic voltammetry. All compounds showed at least two reversible reduction waves, indicating that they are stable up to the dianion stage. The values of their reversible half-wave potentials (E1/2red1, E1/2red2, and E1/2red3) and estimated LUMO levels5i are summarized in Table 1. The marked difference between the two regioisomers of the C70 tetra-aryl derivatives at the first reduction potential is noteworthy. For example, the type II adducts (3, 6, and 10) showed a more negative shift (90−100 mV) at the first reduction potential than their type I isomers (2, 5, and 9); that is, the LUMO levels of the type II isomers are much higher than those of the type I isomers. Such an isomer effect on the energy levels

Figure 3. UV−vis spectra of C70 tetra-aryl derivatives. (a) Compounds 2, 5, and 9. (b) Compounds 3, 6, and 10.

the same type of regioisomer, which is type I as judged by the crystal structure of 2, and that 3, 6, and 10 belong to the type II addition mode as judged by the crystal structure of 6. These tetraaryl [70]fullerene derivatives show much stronger absorption in the 400−700 nm region, where [60]fullerene derivatives absorb very weakly. The enhanced long-wavelength absorption is due to the reduced symmetry of C70 and has been shown to significantly contribute to the enhancement of OPV performance.33 Synthesis and Characterization of 66π-Electron Triarylmono(2-ethylhexyloxyl)[70]fullerene Derivatives. The synthetic yield and the solubility of the tetra-aryl [70]fullerene derivatives being unsatisfactory for practical application of these materials to OPVs, we explored the possibility of introducing an alkoxy group instead of the fourth aryl group, as inspired by the silver-mediated cross-coupling reaction of [60]fullerene bromide and alcohol developed by Gan’s group.34 Thus, new triarylmono(2-ethylhexyloxyl)[70]fullerene derivatives, 13 and 15 (C70Ar3(2-EH)), were synthesized through the AgClO4mediated cross-coupling between C70Ar3Br and 2-ethylhexanol (2-EH). Unlike the tetra-arylation reaction, this reaction afforded predominantly one isomer of C70Ar3(2-EH) in good yield. 2575

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Table 1. Reduction Potentials for the C70 Derivativesa compd

E1/2red1 (V)

E1/2red2 (V)

2 3 5 6 9 10 13 15

−1.00 −1.09 −1.01 −1.10 −1.08 −1.18 −1.11 −1.22

−1.55 −1.65 −1.58 −1.67 −1.68 −1.75 −1.69 −1.80

E1/2red3 (V)

−2.14

−2.36

[70]fullerene derivative 5, 9, 10, 13, or 1537 or the reference fullerene SIMEF, and then annealed at 120 °C for 5 min. NBphen (2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline) was used as a hole-blocking material in the buffer layer. Figure 4

LUMO level (eV)b −3.80 −3.71 −3.79 −3.70 −3.72 −3.62 −3.69 −3.58

a

Potential in volts vs ferrocene/ferrocenium measured by cyclic voltammetry in THF containing Bu4N+ClO4− (0.1 M) as a supporting electrolyte. Glassy-carbon, platinum wire, and Ag/Ag+ electrodes were used as working, counter, and reference electrodes, respectively. bThe values from the vacuum level were estimated by using the following equation: LUMO level = −(E1/2red1 + 4.8).5i

has also been reported for regioisomeric [60]fullerene diadducts (e.g., 1,2-, 1,4-, and 1,16-adducts) whose first reduction potentials vastly differ.35 On the other hand, the substituent effect on the LUMO levels of the tetra-aryl [70]fullerene derivatives was found to be relatively small. For example, compound 5 (E1/2red1 = −1.01 V) bearing a fourth-added electron-donating 4-MeOC6H4 group shows only a 10 mV negative shift of the first reduction potentials as compared with its Ph analogue 2 (E1/2red1 = −1.00 V). Similarly, compound 9 (E1/2red1 = −1.08 V) bearing three electron-donating 4-nBuC6H4 groups shows a 70 mV negative shift (∼23 mV contribution for each 4-nBuC6H4 group) of the first reduction potentials as compared with its Ph analogue 5 (E1/2red1 = −1.01 V). For the C70Ar3(2-EH) derivatives (13 and 15), relatively high LUMO levels were observed as expected from their type II structure. Bearing three electron-donating 2-MeOC6H4 groups, compound 15 possesses the highest LUMO level (−3.58 eV), which is 160 meV higher than that of SIMEF (−3.74 eV).13 Although there have been some efforts to tune the LUMO level of fullerenes by modifying organic residues on the fullerene core,5j,k,14d,e there have been few studies on the effects of regioisomerism on the LUMO level. By exploiting those effects, we could change the LUMO level by as much as 220 meV through a change from compound 2 to compound 15. We expected that this change alters the Voc value of the OPV devices (vide infra). All the compounds shown in Table 1 are thermally stable as determined by thermogravimetric analysis (TGA). The decomposition temperatures of these compounds are generally over 300 °C (see the Supporting Information). Unlike 1,4-di(organo) [60]fullerenes (e.g., SIMEF) which show multiple phase transitions in a temperature range of 20−300 °C,13 DSC data did not show any clear exothermic or endothermic transitions for compounds 5, 9, 10, 13, and 15 (see the Supporting Information). Organic Thin Film Photovoltaic Devices Using 66πElectron [70]fullerenes. In light of the high LUMO levels of the present 66π-electron [70]fullerenes (Table 1), we expected that they would show higher Voc and PCE values than those for our prototype [60]fullerene, SIMEF. To probe this expectation, we fabricated p-n junction OPV devices having a configuration of ITO/PEDOT:PSS/BP(p-layer)/fullerene(n-layer)/NBphen/ Al.36 The p-layer film of BP on the PEDOT:PSS layer was prepared by spin-coating of a precursor, 1,4:8,11:15,18:22,25tetraethano-29H,31H-tetrabenzo[b,g,l,q]porphyrin (denoted as CP), followed by thermal conversion to BP.8c The n-layer film was formed by spin-coating a CS2/PhCl (1:1) solution (0.8 wt %) of

Figure 4. J−V curves of the p−n structure OPVs based on the BP as donor and [70]fullerene or SIMEF as acceptors.

shows the current density−voltage (J−V) curves of the six OPV devices under AM1.5G illumination (100 mW/cm2). The device performance data including Voc, Jsc, FF, and PCE are shown in Table 2. To our satisfaction, the devices based on 9, 10, 13, and Table 2. Photovoltaic Performance of [70]fullerenes or SIMEF-Based Devices under the Illumination of AM 1.5G, 100 mW/cm2 fullerene

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

5 9 10 13 15 SIMEF

0.52 0.74 0.87 0.80 0.90 0.69

2.98 3.66 5.52 6.17 5.83 6.27

0.36 0.44 0.60 0.68 0.64 0.67

0.55 1.18 2.87 3.37 3.33 2.87

15 all showed higher Voc than that of the device based on SIMEF. The device based on compound 15 showed a Voc value of 0.90 V, and a PCE value of 3.33%, which is higher by 16% than that of the SIMEF-based device (2.87%). The best PCE of 3.37% was given by the device using compound 13 that showed a Voc value of 0.80 V and FF of 0.68. The Voc values plotted against the LUMO levels of the fullerenes in Figure 5 indicate that the Voc values are linearly correlated to the LUMO levels of the fullerenes.14,18 Interestingly, the type I and type II regioisomers show very different Jsc and FF values. As is seen from the data in Table 2, the devices based on type II fullerenes (10, 13, and 15) give much higher Jsc and FF values (about 50% higher) than those of the type I fullerenes (5 and 9). Because Jsc and FF values are generally correlated to the carrier mobility of the materials,38 and, for fullerene acceptors, close packing of the fullerene cores is the key factor for effective electron hoping in the OPV devices,17b,39 we attribute the higher performance of the type II compounds to their higher electron mobility. One possible structural factor would be a closer fullerene-fullerene distance in the crystals of the type II compounds, as compared with the type I compounds, 2576

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by APCI using a time-of-flight mass analyzer on a JEOL JMS-T100LC (AccuTOF) spectrometer. UV/vis absorption spectra were recorded on a JASCO V-570 spectrophotometer. IR spectra were measured by JASCO FT-IR-6100 spectrometer. Cyclic voltammetry (CV) was performed using a Hokuto Denko HZ-5000 voltammetric analyzer. All measurements were carried out in a one-compartment cell under Ar gas, equipped with a glassy-carbon working electrode, a platinum wire counter electrode, and a Ag/Ag+ reference electrode. Measurements were performed in THF solution containing tetrabutylammonium perchlorate (0.1 M) as a supporting electrolyte at 25 °C with a scan rate of 0.1 V/s. All potentials were corrected against Fc/Fc+. Differential scanning calorimetry (DSC) was performed on a NETZSCH thermal analyzer (DSC 204/F1). Samples (∼5 mg) were placed in aluminum pans and heated at 10 °C/min under N2 gas. Reagents and chemicals were purchased from Tokyo Chemical Industry Co., Sigma-Aldrich Co., Kanto Chemical Co., Inc., Wako Pure Chemical Industries, or other commercial suppliers and used as received. Synthesis of Compound 1 (C70Ph3H). To a suspension of CuBr·SMe2 (428 mg, 2.08 mmol, 35 equiv.) in THF (15 mL) was added a THF solution of C6H5MgBr (0.81 M, 2.2 mL, 30 equiv.) at 28 °C and the stirring was continued for 20 min at this temperature. To the resulting dark green suspension was added a degassed solution of C70 (50 mg, 0.060 mmol) in 1,2-dichlorobenzene (ODCB) (15 mL) and the stirring was continued for 48 h at 40 °C. The reaction mixture was quenched with saturated NH4Cl solution. The resulted solution was then evaporated under reduced pressure at 50 °C to remove THF and passed through a short silica gel column with CS2 as the eluent. The black solution was collected and concentrated to a small volume and precipitated by MeOH. The precipitated dark brown solid was washed with hexane (5 mL × 3). Finally, compound 1 with 97% purity was obtained (66 mg, 0.057 mmol, 97%) and was directly used as the starting material for the next step reaction. The spectra data of compound 1 has been reported in our previous paper.20 Synthesis of Compound 2 and Compound 3 (C70Ph4). To a suspension of CuBr·SMe2 (214 mg, 1.04 mmol, 35 equiv.) in THF (7.5 mL) was added a THF solution of C6H5MgBr (0.81 M, 1.1 mL, 30 equiv.) at 28 °C and the stirring was continued for 20 min at this temperature. To the resulting dark green suspension was added a degassed solution of C70 (25 mg, 0.03 mmol) in ODCB (7.5 mL) and the stirring was continued for 48 h at 40 °C. The reaction mixture was quenched with NBS (108 mg, 0.61 mmol, 20 equiv.). The resulted solution was then directly injected into a silica gel column using CS2 as the eluent. The brown solution was collected and concentrated to a small volume and precipitated by MeOH. Drying under vacuum for 3 h afforded the mixture of isomers of C70Ph4 (30.5 mg, total yield 89%). This mixture was further purified by GPC with toluene as the eluent. Two bands was obtained, the first band was the pure compound 2 (14.3 mg, 0.012 mmol, 42%). The second band was the mixture of compound 3 and unidentified isomers. Heating this mixture at 100 °C for 2 h quantitatively converted the unidentified isomers into compound 3. After this process, the pure compound 3 was obtained (12 mg, 0.010 mmol, 35%). Compound 2. 1H NMR (500 MHz, CDCl3/CS2) δ 7.10−7.26 (m, 10H, Ar), 7.30−7.33 (m, 2H, Ar), 7.41−7.44 (m, 4H, Ar), 7.62−7.66 (m, 4H, Ar); 13C NMR (125 MHz, CDCl3/CS2) because of the low solubility of the sample, not all carbon signals were adequately acquired. δ 126.85, 127.00, 127.11, 127.16, 127.42, 127.46, 127.49, 127.91, 128.44, 128.52, 128.68, 129.14, 130.22, 131.03, 131.44, 132.94, 134.68, 134.97, 139.64, 140.72, 143.50, 143.73, 143.80, 144.37, 144.74, 145.15, 145.43, 145.66, 146.12, 146.23146.50, 146.70, 146.75, 146.90, 147.97, 148.08, 148.29, 148.73, 149.09, 149.39, 154.39, 156.37, 165.51. IR (powder, cm−1): 3025 (m), 2922 (m), 1595 (s), 1489 (s), 1444 (s), 690 (s). UV/ vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 340 (32 000), 375 (25 000), 475 (12 000), 570 (6000); HR-APCI-MS (−) for C94H20 [M−]: calcd 1148.1570; found 1148.1604. Compound 3. 1H NMR (500 MHz, CDCl3/CS2) δ 6.48 (brs, 2H, Ar), 6.62−6.65 (m, 2H, Ar), 6.80−6.99 (brs, 5H, Ar), 7.30−7.40 (m, 6H, Ar), 7.46 (brs, 1H, Ar), 7.70−7.77 (m, 4H, Ar); The 13C NMR spectra of compound 3 could not be obtained due to its extremely low solubility. IR (powder, cm−1): 3056 (m), 2922 (m), 1597 (m), 1491 (s),

Figure 5. Voc vs LUMO level of [70]fullerene 5, 9, 10, 13, 15, and SIMEF.

because the organic addends are more compactly located in the former.

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CONCLUSION In conclusion, we have developed two new reactions of [70]fullerene that readily produce structurally defined, stable, soluble, and high-LUMO-level 66π-electron [70]fullerene derivatives, and found that they serve as acceptor materials in OPV devices. The sequential 4-fold addition of organocopper reagents produces two types of regioisomers due to a 3, 10,22,25addition mode (type I) and a 7,10,22,25-addition mode (type II), and for the first time, their structures were unambiguously determined by single-crystal X-ray analysis. The second reaction that we reported features AgClO4-mediated coupling between the [70]fullerene bromide C70Ar3Br and 2-ethylhexanol and affords only one regioisomer (type II) of C70Ar3(2-EH) in good yield. By using these [70]fullerenes in a solution-processable p-n junction OPVs, we achieved high Voc (up to 0.9 V) and good PCE (up to 3.37%). One important finding is that the Jsc and FF values depend on the addition patterns on [70]fullerene. The type II regioisomers typically show higher LUMO levels (90− 100 meV higher) than the corresponding type I regioisomers; and the devices based on type II fullerenes show much higher Jsc and FF values than their type I counterparts. We ascribe the difference to the availability of an open surface on the fullerene core which can allow the closest packing of the fullerene molecules in the crystal, so as to maximize the efficiency of electron hopping in the n-layer. This finding further suggests the necessity of developing selective and effective synthetic methods for the rational design of organic semiconductor materials.

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EXPERIMENTAL SECTION

General Considerations for Synthesis and Characterization. All reactions dealing with air- or moisture-sensitive compounds were carried out using standard Schlenk techniques. HPLC analyses were performed on a Shimadzu LC-10A system equipped with an SPD-M10A diode array detector and a Bucky-prep column (Nacalai Tesque Inc., 4.6 mm ID x 250 mm). Preparative HPLC was performed on a Buckyprep column (20 mm ID × 250 mm) using toluene/2-propanol (8:2) as eluent (flow rate 8 mL/min) detected at 350 nm with a UV spectrophotometric detector, Shimadzu SPD-6A. Gel permeation column (GPC) chromatography was performed on a Japan Analytical Industry LC-9201 (eluent: toluene, flow rate 3.5 mL/min) with JAIGEL 2H and 3H polystyrene column. Column chromatography was performed on silica gel 60N (Kanto Chemical, spherical and neutral, 140−325 mesh). NMR spectra were measured with a JEOL ECA-500 (500 MHz) spectrometer. High-resolution mass spectra were measured 2577

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1446 (s), 1430 (s), 693 (s). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 414 (21 000); HR-APCI-MS (−) for C94H22O [M−]: calcd 1148.1570; found 1148.1578. Synthesis of Compound 4 (C70Ph3Br). To the solution of compound 1 (C70Ph3H, 100 mg, 0.093 mmol) in ODCB (5 mL) was added pyridine (7.6 μL, 0.094 mmol, 1 equiv) and NBS (83 mg, 0.47 mmol, 5 equiv). The stirring was continued at 40 °C for 1 h. The solution was then poured into MeOH (30 mL) to precipitate the product. After filtration, the product 4 with 94% purity was obtained as a brown solid (105 mg, 0.091 mmol, 98%) and was directly used as the starting material for the next step reaction (Note: C70Ph3Br is unstable on silica gel; therefore, further purification by silica gel column chromatography is not suggested). 1H NMR (500 MHz, CDCl3/CS2) δ 7.22−7.27 (m, 2H, Ar), 7.35−7.45 (m, 8H, Ar), 7.53−7.55 (m, 1H, Ar), 7.81−7.83 (m, 2H, Ar), 7.92−7.94 (m, 2H, Ar). 13C NMR (125 MHz, CDCl3/CS2) all signals represent 1C (sp2) except as being noted. δ 55.53 (1C, sp3), 59.94 (1C, sp3), 60.79 (1C, sp3), 66.03 (1C, sp3), 126.03, 127.26 (2C), 127.30, 127.57, 127.61, 127.92, 127.93, 128.24, 128.34 (2C), 128.72, 128.92 (2C), 129.24 (2C), 130.31, 130.76, 131.05, 131.22, 131.92, 132.07, 132.48, 133.47, 133.71, 137.28, 137.59, 138.43, 138.85, 139.47, 139.75, 140.72, 141.51, 142.39, 142.46, 143.08, 144.22, 144.25, 144.69, 144.73, 144.90, 145.08, 145.23, 145.27, 145.44, 145.49, 145.63, 145.68, 146.15, 146.22, 146.69, 146.74, 146.77, 146.87, 147.00, 147.33, 147.84, 147.90, 147.96, 148.11, 148.19, 148.21, 148.42, 148.48, 148.92, 149.02, 149.06, 149.16, 149.21, 149.42, 150.04, 150.09, 150.23, 150.52, 152.31, 152.45, 152.93, 154.52, 155.07, 156.72, 162.21. 13C NMR (125 MHz, ODCB-d4) a part of the compound signals is covered by the ODCB signals; all signals represent 1C (sp2) except as noted. δ 55.51 (1C, sp3), 59.97 (1C, sp3), 60.85 (1C, sp3), 66.32 (1C, sp3), 127.25 (2C), 127.28 (2C), 127.50, 127.59, 127.93, 127.97, 128.42 (2C), 128.66, 128.93 (2C), 129.22, 130.66, 131.13, 133.25, 133.51, 137.18, 137.50, 138.67, 139.14, 139.29, 139.69, 140.49, 141.52, 142.21, 142.27, 142.83, 143.37, 143.98, 144.04, 144.47, 144.53, 144.69, 144.87, 145.01, 145.03, 145.19, 145.27, 145.29, 145.49, 145.92, 145.99, 146.07, 146.46, 146.51, 146.55, 146.66, 146.90, 147.11, 147.62, 147.76, 147.84, 147.98, 148.00, 148.13, 148.27 (2C), 148.77, 148.81, 148.84, 149.00, 149.07, 149.26, 149.83 (2C), 149.97, 150.13, 150.42, 152.24, 152.51, 152.98, 154.68, 154.87, 156.65, 162.05. IR (powder, cm−1): 3056 (m), 2922 (m), 1596 (s), 1491 (s), 1429 (s), 692 (s). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 414 (21,000); HR-APCI-MS (−) for C88H15 [M− − Br]: calcd 1071.1179; found 1071.1185. Synthesis of Compound 5 and Compound 6 (C70Ph3(C6H4OMe-4)). To a suspension of CuBr·SMe2 (54 mg, 0.26 mmol, 10 equiv.) in THF (1.5 mL) was added a THF solution of 4-MeOC6H4MgBr (0.725 M, 0.36 mL, 10 equiv.) at 28 °C and the stirring was continued for 20 min at this temperature. To the resulting dark green suspension was added a degassed solution of C70Ph3Br (30 mg, 0.026 mmol) in ODCB (1.5 mL) and the stirring was continued for 10 min. The reaction mixture was quenched with NBS (46 mg, 0.26 mmol, 10 equiv.). After 10 min, the reaction was precipitated with MeOH. After filtration, the solid was purified by silica gel column chromatography with CS2 as the eluent. Two main product bands were observed. The first purple brown band is pure compound 5 (10 mg, 0.0085 mmol, 33%). The second purple brown band is the mixture of compound 6 and unidentified isomers. Heating this mixture at 100 °C for 2 h quantitatively converted the unidentified isomers into compound 6. After this process, the pure compound 6 was obtained (11.2 mg, 0.0095 mmol, 36%). Compound 5. 1H NMR (500 MHz, CDCl3/CS2) δ 3.78 (s, 3H, OCH3), 6.86−6.87 (m, 2H, Ar), 7.14−7.29 (m, 9H, Ar), 7.47−7.49 (m, 4H, Ar), 7.58−7.60 (m, 2H, Ar), 7.66−7.68 (m, 2H, Ar); 13C NMR (125 MHz, CDCl3/CS2) all signals represent 1C (sp2) except as noted. δ 55.07 (1C, OCH3), 57.57 (1C, sp3), 58.13 (1C, sp3), 58.48 (1C, sp3), 59.54 (1C, sp3), 114.72 (2C), 123.68, 127.10 (2C), 127.34 (2C), 127.37, 127.62 (2C), 127.68 (3C), 128.01, 128.12, 128.37 (2C), 128.64 (2C), 128.75 (2C), 128.88 (2C), 130.58, 131.28, 131.75, 132.22, 133.27, 134.81, 134.98, 135.30, 137.82, 138.41, 138.51, 139.31, 140.21, 142.62, 143.67, 143.87, 144.04, 144.08, 144.35, 144.65, 144.87, 144.99 (2C), 145.25, 145.39, 145.66, 145.87, 145.93, 146.14, 146.40, 146.45, 146.55, 146.71, 146.87, 146.96, 147.10, 147.31, 147.57, 147.70, 147.90, 148.01,

148.17, 148.26 (3C), 148.30, 148.38, 148.57, 148.70, 149.09, 149.16, 149.41, 149.57, 149.71 (2C), 150.17 (2C), 154.74, 155.37, 156.72, 159.09, 159.34, 160.05, 161.87, 165.51. IR (powder, cm−1): 3056 (m), 2926 (m), 1599 (s), 1505 (s), 1490 (s), 1251 (s), 1179 (s), 1032 (m), 693 (s). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 340 (33,000), 375 (26,000), 475 (12,000), 570 (6,000); HR-APCI-MS (−) for C95H22O [M−]: calcd 1178.1676; found 1178.1686. Compound 6. 1H NMR (500 MHz, ODCB-d4/CS2) δ 3.21 (s, 3H, OCH3), 5.89 (brs, 2H, Ar), 6.33 (brs, 1H, Ar), 6.70−7.00 (brs, 5H, Ar), 7.13−7.25 (m, 6H, Ar), 7.42 (brs, 1H, Ar), 7.58−7.61 (m, 2H, Ar), 7.67−7.69 (m, 2H, Ar); 13C NMR (125 MHz, ODCB-d4/CS2) because of the low solubility of the sample, not all carbon signals were adequately acquired; a part of the compound signals is covered by the ODCB signals. δ 54.38 (1C, OCH3), 56.04 (1C, sp3), 61.40 (1C, sp3), 61.72 (1C, sp3), 64.41 (1C, sp3), 112.25, 126.43, 127.67, 128.11, 128.26, 128.80, 129.32, 129.38, 130.45, 131.97, 132.25, 133.30, 133.37, 133.56, 136.51, 139.36, 139.73, 139.94, 140.11, 140.93, 141.00, 141.61, 141.88, 142.53, 142.72, 143.16, 143.80, 144.31, 144.58, 144.77, 145.17, 145.24, 145.28, 145.35, 145.54, 145.59, 146.02, 146.41, 146.78, 146.80, 146.96, 147.25, 147.26, 147.43, 147.95, 147.97, 147.99, 148.43, 148.45, 148.49, 148.52, 148.61, 149.12, 149.24, 149.32, 149.51, 149.55, 149.93, 150.68, 150.80, 150.84, 152.63, 153.29, 155.68, 156.34, 157.50, 158.16, 161.33, 164.36. IR (powder, cm−1): 2923 (m), 1505 (s), 1491 (s), 1427 (s), 1250 (s), 1180 (s), 1023 (m). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 414 (21 000); HR-APCI-MS (−) for C95H22O [M−]: calcd 1178.1676; found 1178.1738. Synthesis of Compound 7 (C70(C6H4-nBu-4)3H). To a suspension of CuBr·SMe2 (4.28 g, 20.8 mmol, 35 equiv.) in THF (150 mL) was added a THF solution of 4-nBuC6H4MgBr (0.62 M, 28.8 mL, 30 equiv.) at 28 °C and the stirring was continued for 20 min at this temperature. To the resulting dark green suspension was added a degassed solution of C70 (500 mg, 0.60 mmol) in ODCB (150 mL) and the stirring was continued for 40 h at 45 °C. The reaction mixture was quenched with saturated NH4Cl solution. The resulted solution was then evaporated under reduced pressure at 50 °C to remove THF and passed through a short silica gel column with CS2 as the eluent. The black solution was collected and concentrated to a small volume and precipitated by MeOH. The precipitated dark brown solid was washed with hexane (10 mL × 3). Finally, compound 7 with 97% purity was obtained (730 mg, 0.59 mmol, 99%) and was directly used as the starting material for the next step reaction. 1H NMR (500 MHz, CDCl3/CS2) δ 0.89− 1.00 (m, 9H, CH3), 1.29−1.44 (m, 6H, CH2), 1.50−1.69 (m, 6H, CH2), 2.54−2.69 (m, 6H, CH2), 4.43 (s, 1H, CH), 7.07−7.18 (m, 6H, Ar), 7.49−7.51 (m, 2H, Ar), 7.63−7.65 (m, 2H, Ar), 7.69−7.71 (m, 2H, Ar). 13 C NMR (125 MHz, CS2, 192.40 ppm of CS2 was used as the reference for determination of chemical shifts) all signals represent 1C (sp2) except as noted. δ 14.41 (2C, CH3), 14.49 (1C, CH3), 22.84 (1C, CH2), 22.94 (1C, CH2), 22.96 (1C, CH2), 33.80 (1C, CH2), 33.84 (1C, CH2), 33.86 (1C, CH2), 35.57 (1C, CH2), 35.63 (1C, CH2), 35.69 (1C, CH2), 55.71 (1C, sp3, CH), 56.08 (1C, sp3), 56.16 (1C, sp3), 59.90 (1C, sp3), 126.54, 126.78, 126.92 (2C), 127.37 (2C), 127.62, 127.76 (2C), 128.73, 128.98 (2C), 129.06 (2C), 129.11 (2C), 130.79, 131.74, 131.90, 132.08, 132.15, 133.24 (2C), 135.79, 136.97, 138.08, 139.09, 139.73, 140.68, 141.78, 142.03, 142.18, 142.27, 142.35, 142.91, 143.14, 143.23, 143.64, 144.10, 144.47, 144.63, 144.89, 145.07 (2C), 145.13, 145.25, 145.46 (2C), 145.69, 145.97, 146.33, 146.40, 146.58, 146.74, 146.79, 147.14, 147.20, 147.32, 147.82, 147.87, 147.91, 148.02, 148.10, 148.26, 148.36, 148.44, 148.73, 148.82, 148.97, 149.22, 149.23, 149.31, 149.59, 149.64, 149.71, 150.01, 150.29, 152.32, 152.92, 153.48, 154.03, 155.41, 155.94, 161.18. IR (powder, cm−1): 2921 (s), 2850 (s), 1506 (s), 1428 (s), 1260 (s). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 414 (22 000); HR-APCI-MS (−) for C100H39 [M − H+]: calcd 1239.3057; found 1239.3026. Synthesis of Compound 8 (C70(C6H4-nBu-4)3Br). To the solution of compound 7 (C70(C6H4-nBu-4)3H, 500 mg, 0.4 mmol) in ODCB (10 mL) was added pyridine (32.5 μL, 0.4 mmol, 1 equiv) and NBS (359 mg, 2.02 mmol, 5 equiv). The stirring was continued at 40 °C for 1 h. The solution was then poured into MeOH (50 mL) to precipitate the product. After filtration, the product 8 with 95% purity was obtained as a brown solid (528 mg, 0.4 mmol, 99%) and was directly used as the 2578

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starting material for the next step reaction (Note: C70(C6H4-nBu-4)3Br is unstable on silica gel; therefore, further purification by silica gel column chromatography is not suggested). 1H NMR (500 MHz, CDCl3/CS2) δ 0.88−1.00 (m, 9H, CH3), 1.29−1.45 (m, 6H, CH2), 1.50−1.69 (m, 6H, CH2), 2.55−2.70 (m, 6H, CH2), 7.07−7.08 (m, 2H, Ar), 7.21−7.24 (m, 3H, Ar), 7.35−7.50 (m, 3H, Ar), 7.74−7.75 (m, 2H, Ar), 7.74−7.75 (m, 2H, Ar). 13C NMR (125 MHz, CS2, 192.40 ppm of CS2 was used as the reference for determination of chemical shifts) all signals represent 1C (sp2) except as noted. δ 14.39 (2C, CH3), 14.48 (1C, CH3), 22.79 (1C, CH2), 22.93 (1C, CH2), 23.03 (1C, CH2), 33.66 (1C, CH2), 33.71 (1C, CH2), 33.78 (1C, CH2), 35.54 (1C, CH2), 35.65 (1C, CH2), 35.69 (1C, CH2), 55.13 (1C, sp3), 59.56 (1C, sp3), 60.44 (1C, sp3), 66.41 (1C, sp3), 125.89, 127.20, 127.28 (2C), 127.37, 127.65 (2C), 127.79 (2C), 128.34 (2C), 128.73, 128.86 (2C), 129.20 (2C), 130.09, 130.35, 130.57, 130.88, 131.09, 131.78, 131.94, 132.39, 133.32, 133.57, 135.38, 136.19, 137.14, 137.29, 139.42, 139.61, 140.64, 141.38, 141.89, 142.15, 142.23, 142.32, 142.52, 142.86, 142.97, 143.52, 144.08, 144.14, 144.61 (2C), 144.82, 144.94, 145.14, 145.16, 145.32 (2C), 145.39, 145.53, 146.05, 146.08, 146.56, 146.64, 146.74, 146.86, 147.20, 147.71, 147.79, 147.97, 148.10, 148.26, 148.36, 148.78, 148.89, 148.93, 149.07, 149.27, 149.88 (2C), 149.98, 150.10, 150.45, 152.19, 152.30, 152.87, 154.49, 155.03, 156.60, 162.32. IR (powder, cm−1): 2921 (s), 2851 (s), 1505 (s), 1428 (s). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 414 (21 000); HR-APCI-MS (−) for C100H39 [M− − Br]: calcd 1239.3057; found 1239.3013. Synthesis of Compound 9 and Compound 10 (C70(C6H4-nBu-4)3(C6H4−OMe-4)). To a suspension of CuBr·SMe2 (156 mg, 0.76 mmol, 10 equiv.) in THF (5 mL) was added a THF solution of 4MeOC6H4MgBr (0.725 M, 1.05 mL, 10 equiv.) at 28 °C and the stirring was continued for 20 min at this temperature. To the resulting dark green suspension was added a degassed solution of C70(C6H4n Bu-4)3Br (100 mg, 0.076 mmol) in ODCB (5 mL) and the stirring was continued for 10 min. The reaction mixture was quenched with NBS (135 mg, 0.76 mmol, 10 equiv.). After 10 min, the reaction was precipitated with MeOH. After filtration, the solid was purified by silica gel column chromatography with CS2/hexane (1: 1) as the eluent. The first band was the recovered C70(C6H4-nBu-4)3Br (35 mg, 35% recovery). The second band is the mixture of isomers of C70(4-nBuC6H4)3(4-MeOC6H4) (50 mg, total yield 49%). This mixture was further purified by GPC with toluene as the eluent. The first band was the pure compound 9 (25 mg, 0.019 mmol, 25%). The second band was the mixture of compound 10 and unidentified isomers. Heating this mixture at 100 °C for 2 h quantitatively converted the unidentified isomers into compound 10. After this process, the pure compound 10 was obtained (22 mg, 0.016 mmol, 22%). Compound 9. 1H NMR (500 MHz, CDCl3) δ 0.88−0.94 (m, 9H, CH3), 1.28−1.35 (m, 6H, CH2), 1.50−1.59 (m, 6H, CH2), 2.52−2.59 (m, 6H, CH2), 3.80 (s, 3H, OCH3), 6.88−6.90 (m, 2H, Ar), 6.98−7.03 (m, 4H, Ar), 7.08−7.10 (m, 2H, Ar), 7.39−7.42 (m, 4H, Ar), 7.57−7.63 (m, 4H, Ar). 13C NMR (125 MHz, CDCl3) all signals represent 1C (sp2) except as noted. δ 13.92 (2C, CH3), 13.95 (1C, CH3), 22.13 (1C, CH2), 22.18 (1C, CH2), 22.23 (1C, CH2), 33.46 (1C, CH2), 33.51 (2C, CH2), 35.15 (1C), 35.16 (1C, CH2), 35.20 (1C, CH2), 55.41 (1C, OCH3), 57.59 (1C, sp3), 58.18 (1C, sp3), 58.58 (1C, sp3), 59.51 (1C, sp3), 114.78 (2C), 123.44, 127.13 (2C), 127.39 (2C), 127.82 (2C), 128.34, 128.49 (2C), 128.70, 128.75 (2C), 128.86 (2C), 129.02 (2C), 130.85, 131.44, 131.94, 132.37, 133.46, 135.08, 135.16, 135.39, 135.47, 136.23, 138.01, 138.68, 139.43, 141.12, 141.28, 142.08, 142.42, 142.52, 142.68, 143.89, 144.07, 144.24, 144.53, 144.73, 144.82, 145.03, 145.08, 145.19, 145.39, 145.51, 145.76, 145.94 (2C), 146.04, 146.28, 146.52 (2C), 146.67, 146.95, 147.03 (2C), 147.33, 147.56, 147.76, 147.84, 148.03, 148.19, 148.28, 148.36 (2C), 148.39, 148.45, 148.55, 148.72, 148.92, 149.30, 149.33, 149.59, 149.69, 149.85, 149.93, 150.38, 150.42, 155.10, 155.46, 157.11, 159.22, 159.45, 159.85, 162.07, 165.86. IR (powder, cm−1): 2952 (m), 2923 (s), 2853 (m), 1505 (s), 1251 (s), 1178 (m). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 340 (38 000), 375 (30 000), 475 (13 000), 570 (7,000); HR-APCI-MS (−) for C107H46O [M−]: calcd 1346.3554; found 1346.3522. Compound 10. 1H NMR (500 MHz, CDCl3) δ 0.82−1.00 (m, 9H, CH3), 1.11−1.71 (m, 12H, CH2), 2.35−2.72 (m, 6H, CH2), 3.50 (s, 3H,

OCH3), 6.04 (broad, 1H, Ar), 6.51−6.93 (broad, 2H, Ar), 7.16−7.19 (m, 4H, Ar), 7.40 (broad, 1H, Ar), 7.65−7.72 (m, 4H, Ar). 13C NMR (125 MHz, CDCl3) all signals represent 1C (sp2) except as noted. δ 13.87 (1C, CH3), 13.99 (1C, CH3), 14.02 (1C, CH3), 21.93 (1C, CH2), 22.29 (1C, CH2), 22.37 (1C, CH2), 33.46 (1C, CH2), 33.50 (1C, CH2), 33.60 (1C), 34.87 (1C, CH2), 35.31 (2C, CH2), 54.78 (1C, OCH3), 55.75 (1C, sp3), 61.07 (1C, sp3), 61.47 (1C, sp3), 64.33 (1C, sp3), 111.85 (2C), 126.92 (2C), 127.10 (2C), 127.40 (2C), 128.62 (2C), 129.09 (2C), 129.30 (2C), 130.19, 130.90, 131.66, 131.71 (2C), 132.01, 132.27, 132.73, 133.39, 133.58, 133.75, 136.36, 137.33, 139.17, 139.29, 139.71, 139.90, 140.20, 140.83, 141.07 (2C), 141.51, 142.43, 142.61, 142.74 (2C), 143.02, 143.82 (2C), 144.24, 144.62, 144.75, 145.14, 145.25 (2C), 145.27, 145.40 (2C), 145.47, 145.51, 145.70, 146.04, 146.34, 146.76, 146.78, 146.94, 147.22, 147.24, 147.40, 147.84, 147.93, 147.97, 148.30, 148.39, 148.43, 148.49, 148.69 (2C), 149.12, 149.21, 149.33, 149.51, 149.54, 149.91 (2C), 150.69, 150.74, 150.86, 152.62,153.50, 155.78, 156.65, 157.23, 158.11, 161.34, 164.57. IR (powder, cm−1): 2952 (s), 2925 (s), 2853 (m), 1508 (s), 1429 (s), 1252 (s), 1183 (s), 792 (m). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 414 (20 000); HR-APCI-MS (−) for C107H46O [M−]: calcd 1346.3554; found 1346.3638. Synthesis of Compound 11 (C 70(C6H 4−Ph-4) 3H). To a suspension of CuBr·SMe2 (4.28 g, 20.8 mmol, 35 equiv.) in THF (150 mL) was added a THF solution of 4-PhC6H4MgBr (0.617 M, 28.9 mL, 30 equiv.) at 28 °C and the stirring was continued for 20 min at this temperature. To the resulting yellow green suspension was added a degassed solution of C70 (500 mg, 0.60 mmol in ODCB (150 mL)) and the stirring was continued at 45 °C for 30 h. The reaction mixture was quenched with saturated NH4Cl solution. The resulted solution was then evaporated under reduced pressure at 50 °C to remove THF and passed through a short silica gel column with CS2 as the eluent. The black solution was collected and concentrated to a small volume and precipitated by MeOH. The precipitated dark brown solid was further purified by a silica gel column chromatography with CS2 as the eluent to afford the pure compound 11 (720 mg, 0.55 mmol, 93%). The spectra data of compound 11 has been reported in our previous paper.20 Synthesis of Compound 12 (C70(C6H4−Ph-4)3Br). To the solution of compound 11 (C70(C6H4−Ph-4)3H, 600 mg, 0.46 mmol) in ODCB (40 mL) was added pyridine (37 μL, 0.46 mmol, 1 equiv) and NBS (411 mg, 2.3 mmol, 5 equiv). The stirring was continued at 40 °C for 1 h. The solution was then poured into MeOH (100 mL) to precipitate the product. After filtration, the product 12 with 95% purity was obtained as a brown solid (609 mg, 0.44 mmol, 96%) and was directly used as the starting material for the next step reaction (Note: C70(C6H4−Ph-4)3Br is unstable on silica gel; therefore, further purification by silica gel column chromatography is not suggested). 1 H NMR (500 MHz, CDCl3/CS2) δ 7.26−7.54 (m, 14H, Ar), 7.57− 7.58 (m, 2H, Ar), 7.63−7.67 (m, 7H, Ar), 7.91−7.92 (m, 2H, Ar), 8.06− 8.08 (m, 2H, Ar). 13C NMR (125 MHz, CDCl3/CS2) because of the low solubility of the sample, not all carbon signals were adequately acquired. δ 55.36 (1C, sp3), 59.73 (1C, sp3), 60.65 (1C, sp3), 59.51 (1C, sp3), 126.04, 126.28, 126.64, 126.98 (4C), 127.01 (2C), 127.10 (2C), 127.31, 127.47, 127.53, 127.58, 127.63 (2C), 127.77 (2C), 127.93 (2C), 128.69 (2C), 128.82 (4C), 128.84 (2C), 129.15, 130.36, 130.82, 131.29, 131.49, 131.95, 132.11, 132.53, 133.50, 133.74, 137.30, 137.37, 137.67, 138.01, 139.35, 139.55, 139.78, 139.91, 140.07, 140.25, 140.39, 140.80 (2C), 141.17, 141.52, 142.47, 142.51, 143.12, 143.61, 144.26, 144.28, 144.73, 144.75, 144.94, 145.11, 145.27, 145.30, 145.49 (2C), 145.54, 145.70, 146.19, 146.26, 146.73, 146.78, 146.91, 147.10, 147.36, 147.87, 147.98 (2C), 148.17, 148.25 (2C), 148.44, 148.52, 148.96, 149.06, 149.09, 149.23, 149.44, 150.08 (2C), 150.15, 150.29, 150.62, 152.31, 152.48, 152.98, 154.53, 155.01, 156.82, 162.20. 13C NMR (125 MHz, ODCBd4) a part of the compound signals is covered by the ODCB signals; all signals represent 1C (sp2) except as noted. δ 55.49 (1C, sp3), 59.95 (1C, sp3), 60.86 (1C, sp3), 66.67 (1C, sp3), 125.92, 126.28, 126.65, 127.60, 127.68 (2C), 127.88 (2C), 127.92 (2C), 128.79 (2C), 128.89 (2C), 128.92 (2C), 130.33, 130.87, 131.11, 131.98, 132.45, 133.39, 133.66, 137.41, 137.73, 137.81, 138.49, 139.50, 139.66, 139.79, 139.84, 139.87, 140.35, 140.70, 140.84, 141.19, 141.77, 142.39, 142.48, 142.99, 143.55, 144.12, 144.20, 144.65, 144.68, 144.85, 145.03, 145.15, 145.18, 145.34, 2579

dx.doi.org/10.1021/cm301238n | Chem. Mater. 2012, 24, 2572−2582

Chemistry of Materials

Article

145.43, 145.46, 145.72, 146.15, 146.20, 146.60, 146.66, 146.70, 146.81, 147.13, 147.25, 147.76, 147.93, 148.07, 148.14, 148.17, 148.33, 148.40, 148.42, 148.91, 148.97, 149.13, 149.29, 149.41, 149.98, 150.18, 150.43, 150.65, 152.38, 152.72, 153.09, 154.81, 155.14, 156.85, 162.25. IR (powder, cm−1): 3027 (m), 2924 (m), 1485 (s), 1429 (s), 761 (s), 693 (s). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 414 (19 000); HR-APCI-MS (−) for C106H27 [M− − Br]: calcd 1299.2118; found 1299.2073. Synthesis of Compound 13 (C70(C6H4−Ph-4)3(2-EH)). To the solution of compound 12 (C70(C6H4−Ph-4)3Br, 200 mg, 0.15 mmol) in ODCB (10 mL) was added 2-ethylhexanol (2-EH) (450 μL, 20 equiv.) and AgClO4 (60 mg, 0.29 mmol, 2 equiv.). The stirring was continued at room temperature for 1.5 h. The solution was then poured into MeOH (100 mL) to precipitate the product. After filtration, the solid was purified by silica gel column chromatography with CS2/hexane (1:2) as the eluent to afford the pure compound 13 (166 mg, 0.12 mmol, 80%). 1 H NMR (500 MHz, CDCl3) δ 0.61−0.82 (two diastereomers, 6H, CH3), 0.97−1.24 (m, 9H, CH2 and CH), 3.92−4.00 (two diastereomers, 2H, CH2O), 7.21−7.64 (m, 23H, Ar), 7.79−7.81 (m, 2H, Ar), 7.86− 7.88 (m, 2H, Ar). 13C NMR (125 MHz, CDCl3) all signals represent 1C (sp2) except as noted. δ 11.40 (1C, CH3), 14.09−14.11 (two diastereomers, 1C, CH3), 23.00−23.07 (two diastereomers, 1C, CH2), 23.66−23.84 (two diastereomers, 1C, CH2), 29.23−29.31 (two diastereomers, 1C, CH2), 30.67−30.83 (two diastereomers, 1C, CH2), 40.16−40.19 (two diastereomers, 1C, CH), 55.77 (1C, sp3), 60.93 (1C, sp3), 64.55 (1C, sp3), 68.80−68.98 (two diastereomers, 1C, CH2), 81.11 (1C, sp3), 126.90, 126.92, 127.09 (2C), 127.16 (2C), 127.66 (2C), 127.72 (2C), 127.79, 127.81, 127.97 (2C), 128.63 (2C), 128.89 (2C), 128.91 (2C), 128.93 (2C), 129.18, 129.40 (2C), 130.33−130.39 (two diastereomers, 1C), 130.93−130.96 (two diastereomers, 1C), 132.16, 132.28, 133.04, 133.51, 133.68, 136.60, 137.00, 138.25−138.26 (two diastereomers, 1C), 138.85, 138.91, 139.14, 139.18, 139.27, 140.22, 140.27, 140.31, 140.33, 140.43, 140.47, 140.55, 140.87, 141.07, 141.15, 141.33−141.35 (two diastereomers, 1C), 142.93, 143.31, 143.61, 143.92, 144.32, 144.78, 144.88, 144.92, 145.05, 145.32, 145.42, 145.50, 145.58, 145.66, 145.75, 145.79, 146.43, 146.52, 146.63, 146.78, 147.16 (2C), 147.35, 147.50, 147.83, 147.85, 147.95, 148.11, 148.24 (2C), 148.38, 148.71, 148.75, 149.18, 149.30, 149.39, 149.42, 149.58, 149.60, 149.62, 149.67, 149.80, 149.98, 150.15, 150.30, 152.58, 155.67, 155.87, 156.28−156.36 (two diastereomers, 1C), 156.56− 156.63 (two diastereomers, 1C), 164.18−164.23 (two diastereomers, 1C). IR (powder, cm−1): 2924 (s), 1486 (s), 1430 (s), 1072 (m), 761 (s), 694 (s). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 414 (24 000); HR-APCI-MS (−) for C114H44O [M−]: calcd 1428.3398; found 1428.3351. Synthesis of Compound 14 (C70(C6H4−OMe-2)3H). To a suspension of CuBr·SMe2 (4.28 g, 20.8 mmol, 35 equiv.) in THF (150 mL) was added a THF solution of 2-MeOC6H4MgBr (0.80 M, 22.3 mL, 30 equiv.) at 28 °C and the stirring was continued for 20 min at this temperature. To the resulting dark green suspension was added a degassed solution of C70 (500 mg, 0.60 mmol in ODCB (150 mL)) and the stirring was continued at 45 °C for 24 h. The reaction mixture was quenched with saturated NH4Cl solution. The resulted solution was then evaporated under reduced pressure at 50 °C to remove THF and passed through a short silica gel column with CS2 as the eluent. The black solution was collected and concentrated to a small volume and precipitated by MeOH. The precipitated dark brown solid was further purified by a silica gel column chromatography with CS2 as the eluent to afford the pure compound 14 (600 mg, 0.52 mmol, 87%). 1H NMR (500 MHz, CDCl3/CS2) δ 3.39 (s, 3H, OCH3), 3.64 (s, 3H, OCH3), 3.68 (s, 3H, OCH3), 4.50 (s, 1H, CH), 6.78−6.92 (m, 6H, Ar), 7.17−7.28 (m, 3H, Ar), 7.75−7.77 (m, 2H, Ar), 7.90−7.92 (m, 1H, Ar). 13C NMR (125 MHz, CDCl3/CS2) because of the low solubility of the sample, not all carbon signals were adequately acquired. δ 54.01, 54.50, 54.70, 54.84, 58.07, 111.04, 111.06, 111.39, 120.56, 120.67, 120.82, 127.72, 128.04, 128.27, 128.52, 129.17, 129.20, 129.23, 129.63, 130.23, 131.17, 131.80, 131.88, 131.96, 132.30, 133.27, 134.32, 138.25, 139.50, 140.30, 141.02, 142.02, 142.85, 143.52, 143.76, 144.04, 144.52, 145.06, 145.13, 145.21, 145.35, 145.56, 145.59, 145.70, 145.73, 146.10, 146.32, 146.49, 146.72, 146.80, 147.02, 147.11, 147.20, 147.83, 147.90, 147.99, 148.14, 148.27,

148.38, 148.53, 148.67, 149.02, 149.29, 149.49, 149.55, 152.18, 152.31, 154.02, 154.07, 154.70, 155.75, 157.37, 157.51, 157.99, 160.63. 13C NMR (125 MHz, ODCB-d4/CS2) a part of the compound signals is covered by the ODCB signals; all signals represent 1C (sp2) except as noted. δ 53.56 (1C, OCH3), 54.17 (1C, sp3), 54.39 (1C, OCH3), 54.55 (1C, OCH3), 55.06 (1C, sp3), 55.11 (1C, sp3), 58.31 (1C, sp3), 111.01, 111.18, 111.44, 120.68, 120.72, 120.91, 127.76, 128.19, 128.27, 129.15, 131.31, 131.80, 131.93, 132.06, 133.30, 133.71, 134.34, 136.34, 138.44, 139.69, 140.42, 141.13, 142.04, 142.92, 143.73, 143.87, 144.12, 144.58, 144.95, 145.11, 145.13, 145.21 (2C), 145.40, 145.53, 145.63, 145.80, 146.00, 146.18, 146.52, 146.60, 146.73, 146.95, 147.05, 147.20, 147.28, 147.34, 147.91, 147.95, 147.98, 148.23, 148.25, 148.34, 148.47, 148.71, 148.78, 149.11, 149.16, 149.39, 149.41, 149.56, 149.58, 149.62, 149.70, 149.95, 152.31, 152.69, 154.23, 154.27, 154.90, 155.81, 157.45, 157.49, 158.07, 160.92. IR (powder, cm−1): 2926 (m), 1488 (s), 1458 (s), 1430 (s), 1249 (s), 1028 (s), 749 (s). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 414 (18 000); HR-APCI-MS (−) for C91H22O3 [M − H+]: calcd 1161.1496; found 1161.1506. Synthesis of Compound 15 (C70(C6H4−OMe-2)3(2-EH)). To the solution of compound 14 (C70(2-MeOC6H4)3H, 435 mg, 0.37 mmol) in ODCB (20 mL) /THF (10 mL) was added tBuOK (1M, 0.36 mL, 1.2 equiv.) and the stirring was continued at room temperature for 15 min. Then NBS (100 mg, 0.56 mmol, 1.5 equiv.) was added. The solution was poured into MeOH to precipitate the unstable C70 bromide. After filtration, the solid was washed with hexane and quickly used for the next step reaction. To the solution of the resulted C70 bromide (430 mg) in ODCB (30 mL) was added 2-ethylhexanol (2-EH) (750 μL) and AgClO4 (100 mg, 0.48 mmol). The stirring was continued at room temperature for 1.5 h. The solution was then poured into MeOH to precipitate the product. After filtration, the solid was purified by silica gel column chromatography with CS2 as the eluent to afford compound 15 with 95% purity (220 mg, 0.17 mmol, 46%). Further HPLC purification afforded 200 mg of compound 15 with >99% purity. 1H NMR (500 MHz, CDCl3) δ 0.62−0.85 (two diastereomers, 6H, CH3), 0.95−1.21 (m, 9H, CH2 and CH), 3.06 (brs, 3H, OCH3), 3.54 (s, 3H, OCH3), 3.77−3.78 (two diastereomers, 3H, OCH3), 3.83− 4.01 (two diastereomers, 2H, CH2O), 6.31−6.35 (m, 1H, Ar), 6.60− 6.69 (m, 2H, Ar), 6.82−7.08 (m, 5H, Ar), 7.28−7.48 (m, 3H, Ar), 7.95− 7.96 (m, 1H, Ar). 13C NMR (125 MHz, CDCl3) all signals represent 1C (sp2) except as being noted. δ 11.34−11.38 (two diastereomers, 1C, CH3), 14.17 (1C, CH3), 22.99−23.03 (two diastereomers, 1C, CH2), 23.45−23.70 (two diastereomers, 1C, CH2), 29.33−29.49 (two diastereomers, 1C, CH2), 30.61−30.75 (two diastereomers, 1C, CH2), 40.45−40.47 (two diastereomers, 1C, CH), 53.95 (brs, 1C, OCH3), 54.17−54.24 (two diastereomers, 1C, OCH3), 55.02 (1C, sp3), 55.43 (1C, OCH3), 58.62 (1C, sp3), 63.53−63.56 (two diastereomers, 1C, sp3), 69.30−69.48 (two diastereomers, 1C, CH2), 80.94−80.99 (two diastereomers, 1C, sp3), 109.85−110.04 (two diastereomers, 1C), 111.53, 111.97−112.00 (two diastereomers, 1C), 119.62−119.66 (two diastereomers, 1C), 120.58−120.60 (two diastereomers, 1C), 121.20− 121.22 (two diastereomers, 1C), 126.35−126.40 (two diastereomers, 1C), 127.28−127.30 (two diastereomers, 1C), 127.86−127.91 (two diastereomers, 1C), 128.52−128.54 (two diastereomers, 1C), 128.59− 128.61 (two diastereomers, 1C), 128.71−128.76 (two diastereomers, 1C), 128.93, 129.09, 129.48, 129.66−129.68 (two diastereomers, 1C), 130.30−130.36 (two diastereomers, 1C), 130.92−130.97 (two diastereomers, 1C), 132.01, 132.15−132.17 (two diastereomers, 1C), 132.53, 132.74−132.76 (two diastereomers, 1C), 133.45, 133.57− 133.59, 133.73, 135.01−135.04 (two diastereomers, 1C), 139.76− 139.82 (two diastereomers, 1C), 140.11, 140.79−140.82 (two diastereomers, 2C), 141.92, 142.62−142.73 (two diastereomers, 1C), 142.97, 143.17, 144.12 (2C), 144.59, 144.67, 144.79, 145.07 (2C), 145.28−145.32 (two diastereomers, 2C), 145.38, 145.57 (2C), 145.81, 146.49, 146.57−146.63 (two diastereomers, 1C), 146.74, 146.78− 146.84 (two diastereomers, 2C), 147.04 (2C), 147.40, 147.46, 147.51, 147.56−147.57 (two diastereomers, 1C), 147.67, 147.92, 148.29, 148.59, 148.80, 149.31, 149.38, 149.45, 149.49, 149.73, 149.77−49.79 (two diastereomers, 1C), 149.85, 150.12, 150.18−150.22 (two diastereomers, 1C), 150.32, 150.57−150.60 (two diastereomers, 1C), 152.42, 154.11−154.15 (two diastereomers, 1C), 156.23−156.33 2580

dx.doi.org/10.1021/cm301238n | Chem. Mater. 2012, 24, 2572−2582

Chemistry of Materials

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(two diastereomers, 2C), 157.35−157.53 (two diastereomers, 1C), 157.56, 158.22, 159.09, 160.45−160.47 (two diastereomers, 1C). IR (powder, cm−1): 2952 (s), 2924 (s), 1583 (s), 1488 (s), 1458 (s), 1432 (s), 1251 (s), 1027 (s), 746 (s). UV/vis (CH2Cl2): λmax [nm] (ε [L mol−1 cm−1]) = 414 (19 000); HR-APCI-MS (−) for C99H38O4 [M−]: calcd 1290.2776; found 1290.2780. Device Fabrication and Characterization. The ITO layer on the glass substrate was 145 nm thick with a sheet resistance of 8 Ω /sq. The surface roughness, Ra, was 0.7 nm and Rmax was 8.1 nm. A stripe pattern with a 2 mm wide ITO layer was etched with a conventional photolithographic technique. Prior to the formation of the buffer layer, the patterned ITO glass was ultrasonically cleaned using a surfactant, rinsed with water, then given UV−ozone treatment. A conducting poly(3,4-ethylenedioxylenethiophene)-polystyrene sulfonic acid (PEDOT:PSS, AI4083) layer was formed on the ITO substrate by spin coating an aqueous dispersion to obtain a smooth 30-nm-thick film. The PEDOT:PSS coated substrate was dried in air for 10 min at 120 °C, then dried in a nitrogen glovebox for 3 min at 180 °C prior to use. A BP layer was formed on the PEDOT:PSS layer by spin coating (1,500 rpm) a precursor solution that contained 1.0 wt.% of CP in a chloroform/ chlorobenzene (1/2, v/v) solution. Thermal conversion was carried out at 180 °C for 20 min. The thickness of the crystallized BP p-layer was 25 nm. On top of the BP layer, the n-layer, consisting of a fullerene derivative, was spin-coated (3,000 rpm) from a CS2/chlorobenzene (1/1, v/v) solution that contained 0.8 wt.% of the fullerene derivative. After drying, the device was annealed at 120 °C for 5 min and was transferred from the glovebox to the vacuum chamber without exposure to air. After the deposition of the NBphen layer (6−8 nm), the top electrode (Al, 80 nm) was deposited with a metal shadow mask, which defined a 2 mm stripe pattern perpendicular to the ITO stripe. Finally, the fabricated organic photovoltaic cell was encapsulated with backing glasses using a UV-curable resin under nitrogen atmosphere. The encapsulated organic photovoltaic cells were subjected to J−V measurements under both dark and irradiated conditions. Current− voltage sweeps were taken with a Keithley 2400 source measurement unit controlled by a computer. The light source used to determine power conversion efficiency was a solar simulator system (Sumitomo Heavy Industries Advanced Machinery), AM1.5G, with an intensity of 100 mW/cm2.

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ASSOCIATED CONTENT

S Supporting Information *

Spectra data of all new compounds and crystallographic data of 2 and 6 (CIF). This material is available free via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] (Y.M.); [email protected] (E.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was generously supported by the Funding Program for Next-Generation World-Leading Researchers (Y.M.), MEXT (KAKENHI, 22000008), and Strategic Promotion of Innovative Research and Development from Japan Science and Technology Agency, JST.

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REFERENCES

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