Regiocontrolled Electrosynthesis of [60]Fullerene Bisadducts

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Regiocontrolled Electrosynthesis of [60]Fullerene Bisadducts: Photovoltaic Performance and Crystal Structures of C o-Quinodimethane Bisadducts 60

Zong-Jun Li, Sisi Wang, Shu-Hui Li, Tao Sun, Wei-Wei Yang, Kazutaka Shoyama, Takafumi Nakagawa, Il Jeon, Xiaoniu Yang, Yutaka Matsuo, and Xiang Gao J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01732 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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The Journal of Organic Chemistry

Regiocontrolled Electrosynthesis of [60]Fullerene Bisadducts: Photovoltaic Performance and Crystal Structures of C60 o-Quinodimethane Bisadducts Zong-Jun Li†, Sisi Wang‡, Shu-Hui Li†, Tao Sun†, Wei-Wei Yang†, Kazutaka Shoyama§, Takafumi Nakagawa§, Il Jeon§, Xiaoniu Yang*‡, Yutaka Matsuo*§#¶, and Xiang Gao*† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China ‡

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China §

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,

Tokyo 113-0033, Japan #

Department of Mechanical Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo,

Bunkyo-ku, Tokyo 113-8656, Japan ¶

Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology

of China, Hefei, Anhui 230026, China *[email protected], *[email protected], *[email protected]

1

ACS Paragon Plus Environment


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ABSTRACT C60 o-quinodimethane bisadducts (C60(QM)2) are promising electron acceptors for the bulk heterojuction (BHJ) organic solar cells (OSCs). However, previous production of C60(QM)2 often resulted in excessive regioisomers, which were difficult to purify and might consequently obscure the structure-performance study of the organofullerene acceptors. Herein, the electrosynthesis of C60(QM)2 is reported. The reaction exhibits a strong regiocontrol with generation of less regioisomers. Single regioisomers of cis-2, trans-3 and e C60(QM)2 are obtained, and the structures are solved with single crystal X-ray diffraction. Interestingly, the cis-2 and trans-3 regioisomers exhibit better photovoltaic performance than the e regioisomer in the OSCs based on poly(3-hexylthiophene) (P3HT), which can be correlated with their structural difference.

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INTRODUCTION Organofullerenes are a special type of electron acceptors that can promote the charge separation at the interface due to the existence of low lying excited states in their anions,1 making them promising electron acceptors for application in BHJ OSCs.2,3 In order to have a high open-circuit voltage (VOC) and therefore a high power conversion efficiency (PCE), an elevated LUMO energy level is preferential,4,5 which can be achieved by cleaving the electron-deficient fullerene π-system via putting addends to the carbon cage.6 Consequently, fullerene bisadducts have a better performance than the monoadducts, and fullerene bisadducts such as bis-PC61BM (bis-[6,6]-phenyl-C61-butyric acid methyl ester),7 indene-C60 bisadduct (IC60BA),8 indene-C70 bisadduct (IC70BA)9 and C60(QM)2 (also known as NC60BA, dihydronaphthyl-based [60]fullerene bisadducts)10 are among the most successful fullerene acceptors for OSCs so far. However, these derivatives are typically composed of up to eight types of regioisomers for C60 bisadducts11‒14 and much more regioisomers for C70 bisadducts,15 which may deteriorate the short circuit current density (JSC) and fill factor (FF) of the devices due to the disordered packing of the regioisomeric mixture. The pursuit of pure regioisomeric fullerene bisadducts for applications in OSCs has aroused a great interest recently.14,16−20 The results are quite encouraging with the pure regioisomers improving the PCE of the OSC devices in most cases, with the exception of IC60BA due to unfavorable phase separation.17b However, the acquisition of pure regioisomers from a less regioselective reaction is painstaking, even with the high performance liquid chromatography (HPLC), as evidenced by the availability of limited single crystal structures for the fullerene bisadduct acceptors so far,16,17b,18 which are typically grown out of a solution of high purity and are important in establishing the structure-performance relationship of the organofullerene acceptors. The development of a more regioselective method for fullerene bisfunctionalization is therefore of 3



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interest. The tether-directed remote multifunctionalization introduced by Diederich et al.21 is widely used in reducing the number of regioisomers for bisfunctionalization of fullerene acceptors.16,18b,19,22,23 However, the bulky tethered functional groups in the fullerene acceptors would likely lessen the intermolecular fullerene–fullerene contact, lowering the performance of the devices. The reactions involving anionic fullerene species have shown a good regiocontrol for multifunctionalization via the charge-directed mechanism.25−28 It is therefore of interest to investigate the regioselectivity of C60 bisfunctionalization by controlled potential bulk electrolysis (CPE), which generates anionic fullerene species electrochemically.26,28‒31 Herein, we report the electrosynthesis of C60(QM)2 starting from the dianion of C60QM (C60 o-quinodimethanemonoadduct). In contrast to the traditional Diels–Alder approach,10,12a,14 the CPE method resulted in less regioisomers, facilitating the purification of the C60(QM)2 isomers. The obtained pure C60(QM)2 regioisomers were characterized by X-ray single crystal diffraction analyses, and were examined for their photovoltaic performance in the P3HT-based OSCs.

RESULTS AND DISCUSSION Synthesis and Characterization of C60QM. C60QM was obtained by the reaction of

α,α’-dibromo-o-xylene with C602–, which was generated by bulk electrolysis at a potential of ‒1.1 V vs the saturated calomel electrode (SCE) in freshly distilled DMF solution containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) under argon at rt. The potentiostat was switched off after the electrolytic formation of C602− was completed, and a 10-fold excess of α,α’-dibromo-o-xylene was added into the solution under Ar. The reaction was allowed to proceed for about 30 min with stirring. The mixture was dried with a rotary evaporator under reduced pressure, and the residue was washed with methanol to remove TBAP and excessive α,α’-dibromo-o-xylene. The obtained crude mixture of 4


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C60QM was put into toluene, and the soluble portion was purified by eluting with toluene over a semi-preparative Buckyprep HPLC column (10 ID × 250 mm) (Figure S1), and an isolated yield of 68.3% was achieved. Single crystal of C60QM was obtained by slow evaporation of a solution of purified C60QM in

n-hexane. Figure 1 shows the single-crystal structure of C60QM with the labeling of partial carbon atoms and selected bond lengths and angles. Even though C60QM and its analogues have been synthesized and characterized,32,33 to our best knowledge, no single crystal structure of this type of compounds has ever been reported. As shown in Figure 1, the o-xylene group is added to the C60 cage at a [6,6]-bond across two hexagons, where the dihydronaphthalene ring is positioned toward one side of C60 with an angle of approximate 110° for the C1–C61–C62 and C2–C68–C67 bonds. The 1H NMR spectrum of the compound at rt (Figure S4) exhibits two broad resonances at 4.84 and 4.45 ppm due to the methylene protons located over the five- and six-membered rings of C60, respectively, suggesting a rigidity nature of the dihydronaphthalene group in C60QM.

Figure 1. Single crystal structure of C60QM with 30% thermal ellipsoids. Hydrogen atoms and solvent molecules were omitted for clarity. Selected bond lengths (Å) and bond angles (deg): C1–C2, 1.606(7); C1–C61, 1.576(7); C61–C62, 1.505(8); C2–C68, 1.568(8); C67–C68, 1.492(8); C2–C1–C61, 110.8(4); C1–C61–C62, 110.3(5); C61–C62–C67, 116.2(5); C62–C67–C68, 115.7(5); C67–C68–C2, 111.2(5); 5



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C68–C2–C1, 110.4(4).

Electrosynthesis and Isolation of the Pure cis-2, trans-3 and e C60(QM)2 Regioisomers. The electrosynthesis of C60(QM)2 was achieved with procedures similar to those for synthesis of C60QM, except C60QM2‒ was used instead of C602‒. C60QM2‒ was generated by electrochemically reducing C60QM at a potential of ‒1.2 V vs SCE. The reaction afforded three major products of the cis-2, trans-3 and e regioisomers with an isolated yield of 13.8%, 32.4% and 20.1%, respectively. The reaction also resulted in a very small amount of trans-2 regioisomer, which however, was not evaluated for the OSC performance due to low yield and had little effect on the purification of other regioisomers. Compared to the traditional Diels–Alder approach, where regioisomers of trans-1, trans-2, trans-3, trans-4, e, cis-3 and cis-2 were typically produced with the yield of more the trans-2, trans-3, trans-4 and e isomers,12a,14,17a the electrosynthetic method produces less regioisomers and more cis-2 isomer. Figure 2a shows the HPLC trace of the crude reaction mixture eluted with toluene over a semi-preparative Buckyprep column. The purification yielded Fraction-I, cis-2 C60(QM)2 regioisomer and unreacted C60QM with retention time of 5.2, 5.9 and 6.7 min, respectively. A broad small fraction is shown at around 4.5 min in the HPLC trace, which is likely associated with the C60(QM)3 (trisadducts) regioisomeric mixture judging from the retention time and MS spectrum (Figure S6). The obtainment of the pure cis-2 C60(QM)2 regioisomer is quite unusual, as the purification of the cis-2 C60(QM)2 from the Diels–Alder reaction mixture was often obscured by the co-formed cis-3 regioisomer due to their structural resemblance.14 Fraction-I was further purified by eluting over a semi-preparative Buckyprep-M column with a mixture of toluene and n-hexane (v:v = 1:1) as shown in Figure 2b, which afforded the pure trans-3 and e regioisomers along with a slight amount of the trans-2 regioisomer.

6


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trans-3 C60(QM)2

Fraction-I

(a)

(b) e C60(QM)2 cis-2 C60(QM)2

C60QM

trisadducts

trans-2 C60(QM)2

0

2

4

6

8

0

10

5

10

15

20

Time (min)

Time (min)

Figure 2. (a) HPLC trace of the C60(QM)2 crude product mixture eluted by toluene over a semi-preparative Buckyprep column. (b) HPLC trace of Fraction-I eluted by a mixture of toluene/n-hexane (v:v = 1:1) over a semi-preparative Buckyprep-M column.

Previous structural elucidations of C60(QM)2 regioisomers were mainly based on the spectroscopic characterizations.12a,14 However, the 1H and

13

C NMR spectral characterization of the compounds at

room temperature is often complicated by the peak broadening due to the rigidity of the hydronaphthalene rings. Fortunately, single crystals of the cis-2, trans-3 and e C60(QM)2 regioisomers were obtained, and the structures of the compounds were solved explicitly by X-ray diffraction analyses. Figure 3 shows the X-ray single crystal structures of the cis-2, trans-3 and e C60(QM)2regioisomers. The crystals of cis-2 and trans-3 C60(QM)2 are composed of a 50:50 mixture of two mirror-image enantiomers.

(a)

(b)

(c)

Figure 3. Single crystal structures of (a) cis-2, (b) trans-3 and (c) e C60(QM)2 with 30% thermal 7



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ellipsoids. Hydrogen atoms and solvent molecules were omitted for clarity.

As shown in Figure 3, the individual hydronaphthalene ring in the three regioisomers remains essentially the same orientation similar to that in C60QM, with the quinodimethane plane bent by about 110º toward one side of the C60 cage, suggesting that there is little interaction between the two hydronaphthalene rings in the molecules. Further characterizations with HRMS (Figures S7, S11 and S15) and UV-vis (Figures S8, S12 and S16) are consistent with the structural assignment of the compounds, where the respective molecular ions and UV-vis absorptions characteristic of the cis-2,

trans-3 and e isomers are shown.11–14 However, the 1H and 13C NMR spectra at room temperature of the compounds (Figures S9, S13, S17, S10, S14 and S18) cannot provide much information for the structural elucidation. The signals for the methylene and C6H4 protons of the three isomers are rather broad, and the number of peaks for the carbons is inconsistent with the expected molecular symmetry of the isomers (cis-2: Cs; trans-3: C2; e: Cs), likely associated with the rigidity of the hydronaphthalene rings. The slight fraction eluted before the trans-3 isomer in the HPLC trace (Figure 2b) is recognized as a C60(QM)2 regioisomer by the HRMS (Figure S19), 1H and

13

C NMR (Figures S21 and S22)

characterizations. The fraction is further identified as the trans-2 isomer on the basis of the HPLC elution order (Figure 2b) and UV-vis absorptions (Figure S20), because the regioisomeric C60 bisadducts are generally eluted in the sequence of trans-1, trans-2, trans-3, trans-4, e, cis-3, cis-2, and cis-1 over a normal-phase column according to the increasing order of polarity, and the UV-vis absorptions are characteristic of the structure of the compound.11–14,34 Notably, the regioselectivity (cis-2, trans-3 and e) exhibited by the reductive o-xylenation of C60QM is in agreement with the reductive tetraprotonation of C60, where C60H4 regioisomers of trans-3, e and one unidentified configuration were generated.24 Mechanistic Consideration. Previous work has shown that the reaction of C602‒ with alkyl 8


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bromide (RBr) undergoes via single-electron transfer (SET) from C602‒ to RBr, affording C60‒• and alkyl radical (R•), which subsequently reacts with each other by radical coupling to form RC60‒, and eventually produces organo[60]fullerene by SN2 reaction with another RBr.35 A similar mechanism is expected for the formation of C60(QM)2 through the reaction of C60QM2‒ with α,α’-dibromo-o-xylene as shown in Scheme 1. Scheme 1. Formation of C60(QM)2 from the reaction of C60QM2‒ with α,α’-dibromo-o-xylene

Consequently, understanding of the electron spin density distribution in C60QM‒• is crucial in rationalizing the regiocontrol exhibited by the reaction. The reaction with o-BrCH2(C6H4)CH2• would preferentially occur at the C60 carbon atom with a large electron spin density in C60QM‒•, resulting in the [BrCH2(C6H4)CH2C60QM]– intermediate. Once the [BrCH2(C6H4)CH2C60QM]– intermediate is formed, the structure of the C60(QM)2 product is essentially finalized as the subsequent SN2 reaction would happen only at the [6,6]-ortho carbon atom with respect to the BrCH2(C6H4)CH2–C60QM bond due to the restraint of the o-quinodimethane structure. Figure 4 illustrates the eight types of [6,6]-double bonds in C60QM and labels the carbon atoms 9



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with the larger the electron spin density value in the eight types of double bonds in C60QM‒• calculated with Gaussian09 software package at B3LYP/6-31G level (see Figure S23 for the labeling of the carbon atoms with smaller electron spin density in the eight types of double bonds in C60QM‒•). The plot of the spin density distribution generated with the Multiwfn program36 is also shown in Figure 4. The calculation predicts that the carbon atoms at e, cis-2 and trans-3 double bonds bear the largest three electron spin densities of 0.078, 0.070 and 0.052, consistent with the preferential formation of the e,

cis-2 and trans-3 C60(QM)2 bisadducts. A relatively large electron spin density of 0.030 is predicted to localize at the cis-1 double bond, however, the formation of the cis-1 C60(QM)2 is unlikely due to the potential large steric hindrance between the two closely positioned bulky o-quinodimethane addends.12a,14 The result indicates that the distribution of the electron spin density plays a key role in achieving the regiocontrol of the reaction, demonstrating a promising potential for the electrosynthesis in fullerene bisfunctionalization.

(b)

(a)

(c)

cis-1

e

cis-2 cis-3 trans-4

0.030 0.070 0.078

0.002 0.017 0.052

trans-3 trans-1

0.014

trans-2

-0.001

Figure 4. (a) Relative positional relationship of the eight types of [6,6]-double bonds with respect to the existing hydronaphthalene addend in C60QM. (b) Labeling of the carbon atoms with larger electron spin density in the eight types of double bonds in C60QM‒•. The largest three values are marked in red. (c) Spin density distribution in C60QM‒• at an isosurface value of 0.00008. 10


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Photovoltaic Performance of the cis-2, trans-3 and e C60(QM)2 Regioisomers and Correlation with the Single Crystal Packing Structure. Figure 5 shows the cyclic voltammograms (CV) of the

cis-2, trans-3 and e C60(QM)2 regioisomers in PhCN containing 0.1 M TBAP (tetra-n-butyl ammonium perchlorate), while Table 1 lists the respective half-wave potentials and LUMO energy levels of the compounds. The isomers exhibit essentially the same cyclic voltammograms by showing three quasi-reversible one-electron transfer processes with similar redox potentials, indicating that the addition pattern has little effect on the electrochemical property of C60(QM)2. The half wave potential for the first reduction (E1/2red1) of the cis-2, trans-3 and e C60(QM)2 regioisomers is –1.18, –1.17 and –1.16 V vs Fc/Fc+ (ferrocene/ferrocenium couple), respectively, indicating that the LUMO level of the respective regioisomer is –3.62, –3.63 and –3.64 eV (LUMO level = –(4.80 + E1/2red1) eV),37 consistent with the result reported previously for the C60 bisadducts.38 For comparison, the cyclic voltammograms of C60QM and PC61BM under the same conditions are also shown, which exhibit the E1/2red1 at –1.03 and –0.98 V vs Fc/Fc+, indicating a much lower LUMO level of –3.77 and –3.82 eV.

cis-2

tran-3 e

C60QM

PC61BM

-0.8

-1.2

-1.6

-2.0

-2.4

+

E (V vs Fc /Fc)

Figure 5. Cyclic voltammograms of PC61BM, C60QM, cis-2, trans-3 and e C60(QM)2. 11



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Table 1. Redox Potentials (V vs Fc/Fc+) and LUMO Energy Levels (eV) of cis-2, trans-3 and e C60(QM)2, C60QM, and PC61BMa compounds

E1/2red1

E1/2red2

E1/2red3

LUMO level

cis-2

–1.18

–1.59

–2.24

–3.62

trans-3

–1.17

–1.58

–2.25

–3.63

e

–1.16

–1.56

–2.26

–3.64

C60QM

–1.03

–1.44

–1.99

–3.77

PC61BM

–0.98

–1.39

–1.94

–3.82

a

Cyclic voltammetry measurements were performed in benzonitrile containing 0.1 M TBAP as

supporting electrolyte. Glassy carbon, platinum wire and saturated calomel electrodes (SCE) were used as the working, counter, and reference electrodes, respectively.

OSCs with a configuration of ITO/PEDOT:PSS/P3HT:acceptor/Ca/Al were fabricated to examine the photovoltaic performance of the pure C60(QM)2 regioisomers. Regioisomeric C60(QM)2 mixtures with specific composition were used for comparison in order to eliminate the uncertainty caused by the composition difference from batch to batch.39 The weight ratio of the P3HT to the fullerene acceptor of active layer was 1:0.8 for all devices. Reference devices, P3HT:C60QM and P3HT:PC61BM, were also prepared under the identical conditions for comparisons. Figure 6 displays the current density–voltage (J–V) curves of the devices under the illumination of AM 1.5G with an intensity of 100 mW/cm2 and external quantum efficiency (EQE) curves of the P3HT-based OSCs, while Table 2 lists the device characteristics (on the basis of eight independent devices) including VOC, JSC, FF, PCE and electron mobility (µe). The result shows a VOC of 0.84 V for the devices with acceptor of either the cis-2, trans-3,

e C60(QM)2 pure regioisomer or isomeric C60(QM)2 mixture, which is 220 or 270 mV higher than that of the reference device with acceptor of C60QM or PC61BM, consistent with the similar but elevated LUMO energy level of the cis-2, trans-3 and e C60(QM)2 pure regioisomers compared with C60QM and 12


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PC61BM, as the LUMO energy level is related to VOC of the devices.5 In addition, the EQE curves show a high incident photon-to-current efficiency (IPCE) for devices with acceptors of cis-2 and trans-3 C60(QM)2, C60QM and PC61BM, consistent with the large JSC observed in these devices. 5 cis-2 trans-3 e cis-2/trans-2 trans-3/e cis-2/e cis-2/trans-3/e

0

cis-2 trans-3 e cis-2/trans-3 trans-3/e cis-2/e cis-2/trans-3/e

0.6

EQE%

-2

Current density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C60QM PC61BM

-5

0.4

C60QM PC61BM

0.2

0.0 300

-10 0.0

0.4

0.8

400

500

600

700

800

Wavelength (nm)

Voltage (V)

Figure 6. (a) J–V curves and (b) EQE curves of the P3HT-based OSCs. Table 2. Photovoltaic Performancea of the P3HT-based OSCs with Fullerene Acceptors of the cis-2,

trans-3 and e Pure C60(QM)2 Isomers, Isomeric C60(QM)2 Mixture, C60QM and PC61BM under the Illumination of AM 1.5G, 100 mW/cm2 acceptors

Jsc(mA•cm–2)

Voc(V)

FF (%)

PCE (%)

µe (cm2•V−1•s−1)

cis-2

0.84 ± 0.00

8.21 ± 0.01

72.1 ± 0.5

4.97 ± 0.04

2.2 × 10–4

trans-3

0.84 ± 0.00

8.33± 0.06

71.3 ± 0.3

4.99 ± 0.02

3.1 × 10–4

e

0.83 ± 0.01

7.53 ± 0.05

65.4 ± 0.8

4.09 ± 0.02

1.8 × 10–4

cis-2:e (1:1)

0.84 ± 0.00

7.28 ± 0.01

63.6 ± 0.7

3.89 ± 0.05

1.8 × 10–4

trans-3:cis-2 (1:1)

0.84 ± 0.01

7.72 ± 0.17

61.2 ± 1.6

3.95 ± 0.07

1.7 × 10–4

trans-3:e (1:1)

0.84 ± 0.01

7.60 ± 0.11

70.4 ± 1.3

4.51 ± 0.03

2.3 × 10–4

trans-3:cis-2:e (1:1:1)

0.84 ± 0.00

7.38 ± 0.06

63.9 ± 0.8

3.96 ± 0.05

2.1 × 10–4

C60QM

0.62 ± 0.01

7.93 ± 0.02

69.4 ± 0.8

3.43± 0.01

–

PC61BM

0.57 ± 0.01

7.75 ± 0.13

72.0 ± 0.5

3.16 ± 0.02

–

a

The average and standard deviation of the photovoltaic characteristics were obtained from eight independent devices.

13



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As shown in Table 2, devices with the C60(QM)2 acceptors, irrespective of the pure isomer or isomeric mixture, all exhibit a better performance with a higher PCE compared to devices with C60QM and PC61BM due to the improved VOC. Impressively, devices with the single cis-2 and trans-3 C60(QM)2 regioisomers exhibit a better PCE performance (4.97% and 4.99%) than the OSCs with the isomeric mixture by having a larger JSC and FF, consistent with the performance improvement by using a single regioisomeric fullerene bisadduct acceptor.16,17a,18a However, the device with the pure e isomer exhibits a significantly lower PCE of 4.09%, which is even lower than that of the OSC with 1:1 mixture of trans-3 and e isomers (4.51%). Previous work on the indine-C60 bisadducts has shown a lower PCE for the devices with single trans-3, e1 and e2 regioisomer acceptors due to the excessive phase separation in the blend films caused by the more self-aggregation tendency of the pure isomers.17b While the situation is different for the e C60(QM)2 single regioisomer, as no excessive phase separation was observed, suggesting that the low PCE performance of e C60(QM)2 is likely related to the intrinsic property of the molecule, and not all of the single regioisomers of fullerene bisadducts are suitable for OSC applications. The charge-carrier mobility of the organofullerene acceptors was measured by the space charge limited current (SCLC) method (Figures S24 and S25). The trans-3 and cis-2 isomers show a higher electron mobility of 3.1× 10−4 and 2.2 × 10−4 cm2•V−1•s−1, respectively, while the e isomer exhibits a lower electron mobility of 1.8 ×10–4 cm2•V−1•s−1, in agreement with the larger JSC exhibited by the devices of the trans-3 and cis-2 C60(QM)2 and smaller JSC exhibited by the device of the e isomer. The mixing of different regioisomers generally causes a decrease of the electron mobility compared with the original single regioisomers, suggesting that the electron mobility is likely related to the molecular ordering of organofullerenes, which could be disturbed when different molecules are involved. The 14


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obtained values(1.7~3.1 ×10–4 cm2•V−1•s−1) are comparable to that reported previously for the C60(QM)2 mixture (5.5 ×10–4 cm2•V−1•s−1).38 Previous work proposed that the ability of organofullerene molecules to form fullerene channels comprising extended structure of electronically coupled fullerene cages is crucial for a high electron mobility of active layer and therefore a large photocurrent of the devices.17a,40 However, the relationship of such conductive fullerene channels with the performance of OSCs still remains elusive due to the lack of single crystal structures of regioisomeric fullerene bisadducts, which are critical in identifying the fullerene channels and the packing difference caused by the subtle structural variation of the regioisomers. The availability of the three single-crystal structures of C60(QM)2 regioisomers may therefore reveal such information, providing a better understanding on the performance of fullerene acceptors from the perspective of molecular structures. The LUMO distributions in cis-2, trans-3 and e C60(QM)2 were calculated with Gaussian09 software package at B3LYP/6-31G level in order to validate the conductive nature of the fullerene cage, and the result is shown in Figure 7. The calculations predict that the LUMO is localized mainly at the sp2 carbon atoms of the C60 cage for the three C60(QM)2 regioisomers, suggesting that electrons could be conducted via the π–π interacted fullerene cages, consistent with the proposal that the formation of extended structure of electronically coupled fullerene cages is critical for a high electron mobility of active layer.17a,40 In addition, the result indicates that the involvement of the addend or the sp3 C60 carbon atoms in the organofullerene ordering would likely decrease the electron transport even though the molecules are packed tightly, consistent with the small JSC exhibited for OSCs using pentaaryl[60]fullerenes as the acceptors, where the pentaaryl addends are involved in the stacking of the organofullerenes.41,42 15



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Figure 7. Calculated LUMO (0.02 isosurface) distributions in the cis-2, trans-3 and e C60(QM)2.

Figures 8a‒c display the diagrams of the three-dimensional single-crystal packing structures of the

cis-2, trans-3 and e C60(QM)2 regioisomers, while Figures 8d‒l show the more detailed packing structures of the respective regioisomers along the a-, b- and c-axis. An overview of Figures 8a‒c indicates that both the cis-2 and trans-3 C60(QM)2 molecules are better organized into well structured molecular arrays compared to the e isomer. Further examination reveals that two closely packed fullerene channels composed of strongly π–π interacted fullerene cages are formed along the a- and

b-axis for cis-2 C60(QM)2 (Figures 8d,e) and along the b- and c-axis for trans-3 C60(QM)2 (Figures 8g,h), where the hydronaphthalene addends are not involved in the stacking and the fullerene cages are separated by a short centroid-centroid distance of 10.158, 9.902,10.281 and 10.172 Å, respectively, which is even shorter than the estimated outer diameter of C60 (10.34 Å).43 The cis-2 and trans-3 C60(QM)2 molecules are also well ordered along the c- and a-axis, respectively. However, the hydronaphthalene addends are involved in this type of packing, which push the fullerene cages away by a longer centroid-centroid distance of 11.982 and 12.296 Å, suggesting this channel is less conductive compared to the others.

16


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(a)

(b)

(c)

a-axis

(d)

(g)

(j)

b-axis

(e)

c-axis

(f)

(h)

(i)

(k)

(l)

Figure 8. Three-dimensional single-crystal packing diagrams of (a) cis-2, (b) trans-3 and (c) e C60(QM)2. The fullerene cages and hydronaphthalene addends are represented with the grey and green 17



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color, respectively. Single crystal packing diagrams along the a-, b-, and c-axis for cis-2 (d‒f), trans-3 (g‒i) and e C60(QM)2 (j‒l). The centroid-to-centroid distance of the neighboring fullerene cages within a well ordered molecular alignment is labeled. The solvent molecules are omitted for clarity.

In contrast, the e C60(QM)2 molecules exhibit a much less ordered packing tendency. The neighboring molecules all seem to have different orientations with no molecular ordering being achieved when viewed along the a- and b-axis (Figures 8j,k). A molecular alignment is formed when the molecules are packed along the c-axis (Figure 8l). However, a long centroid‒centroid distance of 12.146 Å between the neighboring fullerene cages is shown due to the interaction of the hydronaphthalene addends in the molecular ordering, indicating that this channel is unlikely effective for electron-transportation. The different packing tendency of the cis-2, trans-3 and e C60(QM)2 regioisomers is likely related to the intrinsic molecular structures. The cis-2 and trans-3 C60(QM)2 molecules can be considered as pseudo-one-dimensional molecules, where the hydronaphthalene addends are positioned either closely together at one end of C60 cage or far apart at the two ends of the molecule, facilitating the formation of conductive fullerene channels by having the other two dimensions of the C60 cage available for molecular packing. However, the e C60(QM)2molecules are essentially two-dimensional molecules due to the equatorial configuration of the hydronaphthalene addends, where only one dimension of the C60 cage is available for molecular packing. Such an arrangement would decrease the intermolecular fullerene-fullerene contact, promoting molecular disordering as shown in Figures 8j and 8k, and resulting in poor fullerene channel structure. The single crystal packing structures of cis-2, trans-3 and e C60(QM)2 show unambiguously that the cis-2 and trans-3 regioisomers have better conductive fullerene channels than the e isomer. The 18


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result is well correlated with the better electron mobility and JSC exhibited by the devices comprising the single cis-2 and trans-3 regioisomers with respect to the e isomer, indicating that the C60(QM)2 acceptor molecules in the BHJ layer of the OSCs likely have similar packing tendency to those in the crystalline structure.

CONCLUSION A compelling electrosynthetic approach for regiocontrolled preparation of C60(QM)2 is reported. The reaction leads to the preferential formation of the cis-2, trans-3 and e regioisomers, which is highly regioselective compared to the reported approaches, and facilitates the purification with the obtainment of three single-crystal structures for the cis-2, trans-3 and e isomers. Computational calculations predict that the regioselectivity is controlled by the electron spin density distribution in C60QM–•, indicating a promising potential of electrosynthesis for regioselective preparation of C60 bisadducts. Photovoltaic examination reveals a good performance for the cis-2 and trans-3 C60(QM)2, but a poor performance for the e regioisomer. The photovoltaic performance of cis-2, trans-3 and e C60(QM)2 is well correlated with the conductive fullerene channels, which can be identified explicitly in the single crystal packing structure of the respective regioisomer and are closely related to the intrinsic molecular structure of the compound, shedding light on understanding the structure-performance relationship of organofullerene acceptors.

EXPERIMENTAL SECTION General Methods. All reactions were carried out under an atmosphere of Ar. All reagents were obtained commercially and used without further purification, unless otherwise noted. DMF was distilled over P2O5 under vacuum at 300 K prior to use. Electrochemical grade TBAP was recrystallized from 19



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absolute ethanol and dried under vacuum at 313 K prior to use. Controlled-potential bulk electrolysis (CPE) was carried out on a potentiostat/galvanostat using an “H” type cell. Two platinum gauze electrodes (working and counter electrodes) were separated by a sintered glass frit. For cyclic voltammetry (CV) measurements, a three-electrode cell was used and a glassy carbon, a platinum wire were used as working electrode and counter electrode. In both CPE and CV measurements, a saturated calomel electrode (SCE) was used as reference electrode. A fritted-glass bridge of low porosity which contained the solvent/supporting electrolyte mixture was used to separate the SCE from the bulk of the solution. Synthesis and Separation of C60QM. Typically, C60 (360 mg, 0.5 mmol) was electrolyzed at ‒1.1 V versus SCE in freshly distilled DMF solution containing 0.1 M TBAP under an argon atmosphere at rt. The potentiostat was switched off after the electrolytic formation of C602− was complete, and a 10-fold excess of α,α’-dibromo-o-xylene (1.32 g, 5.0 mmol) was added into the solution under inert atmosphere. The reaction was allowed to proceed for 30 min under stirring. The mixture was dried with a rotary evaporator under reduced pressure, and the residue was washed with methanol to remove TBAP and excessive α,α’-dibromo-o-xylene. The obtained crude mixture of C60QM was put into toluene, and the soluble part was purified using Buckyprep column (10 mm × 250 mm) eluted with toluene at a flow rate of 4.0 mL/min with the detector wavelength set at 380 nm. An amount of 281.4 mg of C60QM was obtained (68.3% isolated yield). Spectral Characterization of C60QM. Positive ESI FT-ICR MS, m/z calcd for C68H9+ ([M + H]+) 825.06988, found 825.07127; 1H NMR (600 MHz, CS2/CDCl3) δ 7.72-7.65 (m, 2H), 7.61-7.52 (m, 2H), 4.83 (d, J = 12 Hz, 2H), 4.44 (d, J = 12 Hz, 2H);

13

C NMR (150 MHz, CS2/CDCl3) δ 156.5, 156.3,

147.4, 146.3, 146.0, 145.6, 145.4, 145.2, 144.9, 144.4, 142.9, 142.4, 142.0, 141.9, 141.5, 141.4, 140.1, 20


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139.9, 137.8, 136.0, 135.2, 127.9, 127.8, 65.6, 45.0; UV-vis (in o-DCB), λmax: 409, 435, 467, 641 and 708 nm. X-ray Single-Crystal Diffraction of C60QM. Black crystals of C60QM were obtained by slow diffusing n-hexane into a carbon disulfide solution of C60QMat room temperature. Single-crystal X-ray diffraction data were collected on an instrument equipped with a CCD area detector using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) in the scan range 1.76° < θ < 25.21°. The structure was solved with the direct method of SHELXS-97 and refined with full-matrix least-squares techniques using the SHELXL-97 program within WINGX. Crystal data of C60QM•2CS2; C70H8S4, Mw = 977.00, Monoclinic, space group P2(1)/c, a = 16.130(3) Å, b = 10.2406(18) Å, c = 23.601(4) Å, α = 90°, β = 102.039(3), γ = 90°, V = 3812.8(11) Å3, Z = 4, Dcalcd = 1.702 Mg·m–3, µ = 0.308 mm–1, T = 188(2) K, crystal size 0.23 × 0.15 × 0.11 mm; reflections collected 21581, independent reflections 6804; 4947 with I > 2σ (I); R1 = 0.0830 [I > 2σ(I)], wR2 = 0.2133 [I > 2σ(I)]; R1 = 0.1097 (all data), wR2 = 0.2273 (all data), GOF (on F2) = 1.114. Synthesis and Separation ofC60(QM)2. The procedures were similar to those for preparation of C60QM except C60QM was used as the starting material instead of C60. C60QM (200 mg, 0.24 mmol) was electrolyzed at ‒1.2 V versus SCE and formed C60QM 2−. The potentiostat was switched off after the electrolytic formation of C60QM2− was complete, and a 10-fold excess of α,α’-dibromo-o-xylene (0.64 g, 2.4 mmol) was added into the solution under inert atmosphere. The reaction was allowed to proceed for 30 min under stirring. The crude mixture of C60(QM)2 was put into toluene, and the soluble part was subjected to a two-stage HPLC purification. At the first stage, the crude mixture was eluted with toluene over a Buckyprep column (10 mm × 250 mm) at a flow rate of 4.0 mL/min with the detector wavelength set at 380 nm, which afforded Fraction-I and pure cis-2 C60(QM)2. Fraction-I was then further purified 21



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by eluting with a mixture of toluene and n-hexane (v:v = 1:1) over Buckyprep-M column (10 mm × 250 mm), which produced pure trans-3 and e C60(QM)2 regioisomers along with a small amount of trans-2 C60(QM)2 regioisomer. Compounds trans-3, e and cis-2 C60(QM)2 were obtained with an isolated yield of 32.4% (73.0 mg), 20.1% (45.3 mg) and 13.8% (31.1 mg), respectively. Spectral Characterization of cis-2 C60(QM)2. Positive ESI FT-ICR MS, m/z calcd for C76H17+ ([M + H]+) 929.13248, found 929.13044; 1H NMR (600 MHz, CDCl3) δ 7.82-7.28 (m, 8H), 4.78-3.69 (m, 8H); 13C NMR (150 MHz, CS2/CDCl3) δ 160.7, 160.2, 158.2, 149.2, 148.9, 148.6, 147.3, 146.8, 146.5, 146.0, 145.4, 145.0, 144.7, 144.5, 144.2, 144.0, 143.7, 142.5, 141.3, 140.7, 137.7, 137.4, 135.7, 133.1, 132.7, 128.2, 127.8, 127.5, 127.2, 126.9, 126.6, 62.7, 61.2, 44.6, 44.2, 42.8, 42.3; UV-vis (in o-DCB),

λmax: 450, 639 nm. X-ray Single Crystal Diffraction of cis-2 C60(QM)2. Black crystals of cis-2 C60(QM)2 were obtained by slow diffusing n-hexane into a carbon disulfide solution of cis-2 C60(QM)2at room temperature. Single crystal X-ray diffraction data were collected on an instrument equipped with a CCD area detector using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) in the scan range 1.35°
2σ (I); R1 = 0.0689 [I > 2σ(I)], wR2 = 0.1627 [I > 2σ(I)]; R1 = 0.1171 (all data), wR2 = 0.1934 (all data), GOF (on F2) =1.019. Spectral Characterization of trans-3 C60(QM)2. Positive ESI FT-ICR MS, m/z calcd for C76H17+ 22


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([M + H]+) 929.13248, found 929.13355; 1H NMR (600 MHz, CDCl3) δ 7.93-7.30 (m, 8H), 4.84-3.95 (m, 8H); 13C NMR (150 MHz, CDCl3) δ 160.5, 157.8, 157.4, 156.6, 149.5, 149.4, 149.2, 149.0, 148.7, 148.6, 148.4, 148.3, 148.2, 145.5, 145.4, 145.0, 144.8, 143.9, 143.8, 142.9, 142.7, 141.9, 141.51, 141.3, 140.8, 139.8, 139.6, 138.1, 136.5, 135.8, 135.1, 134.4, 129.0, 128.2, 127.9, 127.6, 125.3, 65.1, 64.8, 45.4, 45.3, 44.3, 43.6; UV-vis (in o-DCB), λmax: 420, 462, 487, 646, 713 nm. X-ray Single Crystal Diffraction of trans-3 C60(QM)2. Black crystals of trans-3 C60(QM)2 were obtained in chloroform solution of trans-3 C60(QM)2 at room temperature. Single crystal X-ray diffraction data were collected on an instrument equipped with a CCD area detector using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) in the scan range 1.85° 2σ (I); R1 = 0.0685 [I > 2σ(I)], wR2 = 0.1764 [I > 2σ(I)]; R1 = 0.0791 (all data), wR2 = 0.1469 (all data), GOF (on F2) = 1.028. Spectral Characterization of e C60(QM)2. Positive ESI FT-ICR MS, m/z calcd for C76H16+ (M+) 928.12465, found 928.12588; 1H NMR (600 MHz, CDCl3) δ 7.66-7.31 (m, 8H), 4.54-3.73 (m, 8H); 13C NMR (150 MHz, CDCl3) δ 161.6, 155.5, 155.0, 154.6, 154.2, 150.3, 149.2, 148.2, 147.4, 147.4, 146.9, 146.5, 146.3, 146.0, 145.2, 144.77, 144.75, 144.6, 144.0, 143.7, 143.0, 142.4, 141.6, 141.2, 141.0, 140.5, 138.1, 137.9, 137.8, 137.7, 136.5, 135.2, 134.4, 129.0, 128.2, 127.8, 127.7, 125.3, 64.8, 64.5, 44.9, 44.6; UV-vis (in o-DCB), λmax: 425, 549, 620 nm. 23



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X-ray Single Crystal Diffraction of e C60(QM)2. Black crystals of e C60(QM)2 were obtained by slow diffusing n-hexane into a carbon disulfide solution of e C60(QM)2at room temperature. Single crystal X-ray diffraction data were collected on an instrument equipped with a CCD area detector using graphite-monochromated Cu Kα radiation (λ = 1.54184 Å) in the scan range 2.181° < θ < 45.201°. The structure was solved with the direct method of SHELXT-2014 and refined with full-matrix least-squares techniques using the SHELXL-2014 program within Yadokari-XG 2009. Crystal data of e C60(QM)2•C6H6; C82H22, Mw = 1007.00, Monoclinic, space group P2(1)/c, a = 22.5926(6) Å, b = 17.1813(4) Å, c = 24.2882(6) Å, α = 90°, β = 116.2910(10), γ = 90°, V = 8452.7(4) Å3, Z = 8, Dcalcd = 1.583 Mg·m–3, µ = 0.699 mm–1, T = 190(2) K, crystal size 0.17 × 0.14 × 0.06 mm; reflections collected 110767, independent reflections 6895; 5275 with I > 2σ(I); R1 = 0.0431 [I > 2σ(I)], wR2 = 0.0994 [I > 2σ(I)]; R1 = 0.0672 (all data), wR2 = 0.1108 (all data), GOF (on F2) = 1.045. Spectral Characterization of trans-2 C60(QM)2. Positive ESI FT-ICR MS, m/z calcd for C76H16+ (M+) 928.12465, found 928.12631; 1H NMR (600 MHz, CDCl3) δ 7.95-7.46 (m, 8H), 5.06-4.28 (m, 8H); 13

C NMR (150 MHz, CS2/CDCl3) δ 160.7, 154.4, 154.2, 154.0, 147.9, 147.2, 147.0, 146.8, 146.7, 146.3,

145.8, 145.7, 145.4, 145.2, 145.0, 144.5, 144.0, 143.8, 143.6, 143.3, 142.7, 142.3, 142.2, 141.1, 139.3, 138.7, 138.1, 137.7, 137.6, 128.6, 127.9, 127.8, 127.5, 125.0, 64.0, 63.9, 44.9, 44.8, 44.2; UV-vis (in

o-DCB), λmax: 476, 626, 655, 689, 724 nm. Device Fabrication and Characterization. The glass-indium tin oxide (ITO) substrates were ultrasonically washed by detergent, de-ionized water, acetone and isopropanol sequentially for 25 min/each

and

subsequently

exposed

to

UV–ozone

for

25

min.

Poly(3,4-ethylenedioxy-thiophene):poly(styrene-sulfonate) (PEDOT:PSS) was passed through a 0.45 µm filter before being deposited on ITO with a thickness around 30 nm by spin coating at 3000 rpm and 24


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dried at 150 °C for 20 min under air. The weight ratio of P3HT to the fullerene acceptor was fixed at 1:0.8 and the concentration of P3HT and fullerene acceptor was kept at 10 and 8 mg/mL in o-DCB. The solution was kept stirring at 60 °C for 12 h. The P3HT and fullerene acceptor were then spin-coated at a speed rate of 300 rpm for 5 min on the top of PEDOT:PSS layer under nitrogen atmosphere. After that, the film was thermally annealed at 150 °C for 15 min. The device was completed by thermal evaporation of Ca (20 nm) and Al (100 nm) under < 1 × 10–5 Torr. All the active area of the device is 9 mm2. The current-voltage (J-V) measurement of the polymer photovoltaic cells was conducted immediately in glovebox. The electron mobility was measured by the space charge limited current (SCLC) method onan electron-only device with astructure of ITO/TIPD/Active layer/Ca/Al. The measurement of electron mobility was conducted in the dark on a computer-controlled Keithley 2400 Source Measure Unit (SMU). The SCLC electron mobility was estimated following the Mott-Gurney square law J = 9(ε0εrµ)/8 × ( V2/d3 ), where J is current density, ε0 is the permittivity of vacuum, εr is dielectric constant of the fullerene derivatives, µis electron mobility, V=Vappl–Vbi–Vr, Vappl is the applied potential, Vbi is the built-in potential resulting from workfunction difference between two electrodes, Vr is the voltage drop due to the resistance, and d is ﬁlm thickness. Computational Methods. The geometry optimizations and frequency calculations of C60QM‒•,

cis-2, trans-3 and e C60(QM)2 were performed using DFT at B3LYP/6-31G level with Gaussian09 program package. The spin density distribution calculation of C60QM‒• and LUMO distributions in cis-2,

trans-3 and e C60(QM)2 were carried out at B3LYP/6-31G level.

ASSOCIATED CONTENT

25



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Supporting Information. Spectral data for compounds C60QM, cis-2, trans-3 and e C60(QM)2, X-Ray crystal data for of C60QM, cis-2, trans-3 and e C60(QM)2, and calculation details. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected]; *[email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Dr. Shentang Wang and Prof. Wuping Liao at Changchun Institute of Applied Chemistry for the single-crystal measurement of the e isomer of C60(QM)2. This work was supported by the National Natural Science Foundation of China (21472183, 21574132, 201504090, 21302179 and 21602192), Jilin Provincial Science & Technology Department (20160520128JH, 2016052118JH and 20170101172JC), Guangdong Provincial Department of Science and Technology (2015B020239004), the Grants-in-Aid for Scientific Research (15H02219 and 16H04187), and the Strategic Promotion of Innovative Research and Development from the Japan Science and Technology Agency.

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