Facile Synthesis of [6,6]-Phenyl-C_61/71-Butyric Acid Methyl Esters via Sulfur Ylides for Bulk-Heterojunction Solar Cell

Abstract

A one-step, mild synthesis of methanofullerenes as electron acceptors for solution-processed bulk-heterojunction solar cells was developed. [6,6]-Phenyl-C₆₁-butyric acid methyl ester {[60]PCBM} was directly synthesized in good yields, by the reaction of fullerene with sulfur ylide derivatives in the presence of 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) at room temperature. This method was also successfully applied to the preparation of [6,6]-phenyl-C₇₁-butyric acid methyl ester {[70]PCBM}.

Key words

methanofullerene - PCBM - photovoltaics - sulfur ylide - addition–elimination reaction

• References and Notes

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• 13 Methyl 5-Bromo-5-phenylpentanoate (1) Commercially available methyl 5-phenylpentanoate (3.65 g, 19.0 mmol) was dissolved in CCl₄ (50 mL) and treated with NBS (3.68 g, 20.7 mmol) and a trace amount of dibenzoyl peroxide. The mixture was refluxed for 2 h and then cooled and filtered, followed by solvent removal in vacuo. After the residue was purified by silica gel chromatography, product 1 was obtained in 89% yield (4.61 g). ¹H NMR (300 MHz, CDCl₃): δ = 7.40–7.25 (m, 5 H), 4.95 (t, J = 6.9 Hz, 1 H), 3.65 (s, 3 H), 2.37–2.14 (m, 4 H), 1.92–1.56 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 173.42, 141.80, 128.71, 128.40, 127.20, 54.72, 51.57, 39.20, 33.12, 23.60. IR (neat): 2951, 1737, 1455, 1436, 1204, 1174, 759, 697 cm^–1. ESI-MS: m/z calcd for C₁₂H₁₅BrO₂Na: 293.0; found: 292.8 [M + Na]⁺. Dimethyl (5-Methoxy-5-oxo-1-phenylpentyl) Sulfonium Triflate (2a) To a solution of methyl 5-bromo-5-phenylpentanoate (1, 271 mg, 1.0 mmol) and Me₂S (186 mg, 3.0 mmol) in CH₂Cl₂ (1 mL) was added AgOTf (257 mg, 1.0 mmol) at 0 °C. The reaction mixture was stirred and gradually warmed to r.t. for 4 h. After filtration through Celite to remove the precipitate, the filtrate was concentrated under reduced pressure. The residue was then washed twice by decantation with hexane (10 mL). The solvent was removed in vacuo and the sulfonium salt 2a was obtained in 81% yield (324 mg). ¹H NMR (300 MHz, CDCl₃): δ = 7.49–7.42 (m, 5 H), 4.94 (t, J = 7.8 Hz, 1 H), 3.63 (s, 3 H), 3.03 (s, 3 H), 2.64 (s, 3 H), 2.36 (dt, J = 6.9, 2.1 Hz, 2 H), 2.25 (sext, J = 7.5 Hz, 2 H), 1.73–1.50 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 173.05, 130.50, 130.10, 129.74, 129.11, 120.44 (q, J _C–F = 317 Hz) 59.82, 51.43, 32.45, 29.52, 23.64, 22.30, 21.53. IR (neat): 3021, 2935, 1731, 1436, 1258, 757, 712, 638 cm^–1. ESI-MS: m/z calcd for C₁₄H₂₁O₂S: 253.1; found: 253.0 [M – CF₃SO₃]⁺. Dimethyl (5-Methoxy-5-oxo-1-phenylpentyl) Sulfonium Tetrafluoroborate (2b) The procedure was similar to that described above, except for the use of AgBF₄ (195 mg, 1.0 mmol). The sulfonium salt 2b was obtained in 90% yield (306 mg). ¹H NMR (300 MHz, CDCl₃): δ = 7.51–7.44 (m, 5 H), 4.83 (t, J = 8.4 Hz, 1 H), 3.63 (s, 3 H), 3.01 (s, 3 H), 2.62 (s, 3 H), 2.38 (dt, J = 7.2, 3.2 Hz, 2 H), 2.26 (sext, J = 7.6 Hz, 2 H), 1.72–1.52 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 173.19, 130.74, 130.20, 129.99, 129.26, 60.38, 51.63, 32.56, 29.78, 23.70, 22.39, 21.69. IR (neat): 3030, 2951, 1734, 1456, 1437, 1203, 1177, 1061, 701 cm^–1. ESI-MS: m/z calcd for C₁₄H₂₁O₂S: 253.1; found: 252.9 [M – BF₄]⁺.
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• 15 HPLC conditions were as follows: column, CHEMCOSORB5-ODS-UH (4.6 × 150 mm); column temperature, 40 °C; flow rate, 1.2 mL/min; detection, UV at λ = 340 nm; eluent toluene–MeOH (60:40, v/v). The absolute calibration method was used for the quantification of analytes.
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• 18 Preparation of [6,6]-Phenyl-C₆₁-butyric Acid Methyl Ester {[60]PCBM}^7b To a stirred mixture of sulfonium salt 2b (85 mg, 0.25 mmol) and fullerene C₆₀ (100 mg, 0.14 mmol) in ODCB (7 mL) was added DBU (46 mg, 0.30 mmol) in one portion at r.t. After stirring for 6 h, AcOH (ca. 3 equiv) was added. The reaction mixture was concentrated to ca. 3 mL under reduced pressure. The residue was purified by silica gel column chromatography {eluent, stepwise gradient of CH₂Cl₂ (0–80%) in toluene}. The obtained product was dissolved in a small amount of toluene and poured into MeOH. The resulting suspension was centrifuged, and the supernatant was decanted. The product was dried in vacuo at 60 °C. [60]PCBM was obtained in 45% isolated yield (57 mg). Unreacted C₆₀ (22 mg, 22%) and bisadducts (8 mg, 5%) were isolated in the same manner. ¹H NMR (300 MHz, CDCl₃): δ = 7.92 (d, J = 8.7 Hz, 2 H), 7.60–7.44 (m, 3 H), 3.68 (s, 3 H), 2.94–2.88 (m, 2 H), 2.52 (t, J = 7.5 Hz, 2 H), 2.23–2.13 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 173.40, 148.79–136.71, 132.06, 128.43, 128.24, 79.87, 51.85, 51.64, 33.87, 33.67, 22.37. IR (KBr): 2921, 2849, 1735, 699, 573, 550, 526, 482, 453 cm^–1. MS (MALDI): m/z calcd for C₇₂H₁₄O₂: 910.1; found: 910.1 [M]⁺.
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• 20 [6,6]-Phenyl-C₇₁-butyric Acid Methyl Ester {[70]PCBM}^{7b 1}H NMR (300 MHz, CDCl₃): δ = 7.92–7.40 (m, 5 H), 3.74 (minor isomer, s, 0.26 H), 3.67 (major isomer, s, 2.55 H), 3.51 (minor isomer, s, 0.19 H), 2.53–2.42 (m, 4 H), 2.25–1.99 (m, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 173.38, 149.43–128.20, 69.82, 51.68, 33.80, 21.70. IR (KBr): 2941, 2923, 1737, 1429, 795, 726, 699, 674, 643, 579, 534, 459 cm^–1. MS (MALDI): m/z calcd for C₇₂H₁₄O₂: 1030.1; 1030.2 [M]⁺.
• 21 Bis[70]PCBM To a stirred mixture of sulfonium salt 2b (51 mg, 0.15 mmol) and fullerene C₇₀ (25 mg, 0.030 mmol) in ODCB (12 mL) was added DBU (23 mg, 0.15 mmol) in one portion at r.t. After stirring for 7 h, AcOH (ca. 3 equiv) was added and treatments in the same manner of ref. 18, bis[70]PCBM was obtained in 64% isolated yield (23 mg). Unreacted C₇₀ (0.6 mg, 2%) and monoadducts (7.5 mg, 24%) were isolated.¹H NMR (300 MHz, CDCl₃): δ = 7.93–7.37 (m, 5 H), 3.67–3.65 (m, 3 H), 2.50–2.10 (m, 6 H). MS (MALDI): m/z calcd for C₇₂H₁₄O₂: 1220.2; found: 1220.2 [M]⁺. Spectroscopic data were in good agreement with the literature: Lenes M, Shelton SW, Sieval AB, Kronholm DF, Hummelen JC, Blom PW. M. Adv. Funct. Mater. 2009; 19: 3002