Letter pubs.acs.org/OrgLett
Tricyclohexylphosphine-Catalyzed Cycloaddition of Enynoates with [60]Fullerene and the Application of Cyclopentenofullerenes as n‑Type Materials in Organic Photovoltaics An-Ju Wu, Po-Yen Tseng, Wei-Hsin Hsu, and Shih-Ching Chuang* Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, R.O.C. S Supporting Information *
ABSTRACT: The tricyclohexylphosphine-catalyzed [3 + 2] cycloaddition of (E)-alkyl 5-substituted phenylpent-4-en-2ynoates with [60]fullerene was studied. This reaction undergoes an initial 1,3-addition of phosphines toward the α-carbons of enynoates. Subsequent cycloaddition of the generated 1,3dipoles with [60]fullerene and elimination of tricyclohexylphosphines resulted in cyclopentenofullerenes in 20−43% yields. The isolated cyclopentenofullerenes were observed to serve as n-type materials in organic photovoltaics, providing a maximum average power conversion efficiency of 3.79 ± 0.29% upon embedding with P3HT in the active layer.
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Scheme 1. Reactivity of Ynenoates A and Enynoates 1 toward the Conjugate Addition of Phosphines
onjugate addition is an important methodology in the field of applied synthetic chemistry.1 Organophosphines behave as nucleophiles to initiate chemical reactions via addition reactions to electron-deficient alkenoates, allenoates, and alkynoates, thereby generating reactive 1,3-dipolar species in situ for further reactions with electrophiles.2 In this context, conjugate 1,n-addition (n = even number), such as 1,2-,3 1,4- or (β),4 and 1,6-addition5 patterns, have been observed because of resonance stabilization. The evolution of new addition patterns governing conjugated systems toward phosphines inspired us to explore this topic. One unusual three-component reaction was demonstrated to undergo addition of phosphines at the α(δ′)carbon of enyndioates followed by addition to aldehydes and cyclization to generate lactones.6 Furthermore, the addition pattern was shown to be substrate-dependent because swapping the functionality on the conjugated π-systems resulted in different addition patterns. For instance, ynenoates A proceeded through 1,4-addition reactions, evidenced by formation of cyclopentenofullerenes A′ through cycloaddition of the generated dipoles Ia with C60 (Scheme 1).7 Recently, we demonstrated a unique conjugate 1,3-addition reaction for oligoynoates toward phosphines.8 Our continuing interest in finding new conjugated π-systems susceptible to α-addition led us to evaluate the regioselectivity of enynoates 1 with phosphines. Two possible addition pathways, 1,4- or 1,3- (α) addition, toward phosphines were envisioned via dipolar species Ib or Ic. These two 1,3-dipoles can undergo addition to C60 followed by cyclization and elimination of phosphines, thereby generating isomeric cyclopentenofullerenes A′ and 2, respectively. Unequivocally distinguishing between these two addition patterns is not possible without solid evidence for the structures of the isolated products. Furthermore, to our knowledge, the 1,3-addition (α) pattern is less common compared to other © XXXX American Chemical Society
existing 1,n-addition (n = even number) patterns.4 In a report, Nagao et al. demonstrated that PBu3 catalyzed the anticarboboration of an enynoate with organoboranes via 1,4addition of phosphine.9 In the context of using fullerenes in organic photovoltaics, light harvesting with greener and renewable approaches is essential in materials science research for the development of solar technology. Thin-film organic photovoltaics (OPVs) with a conducting polymer, poly(3-hexylthiophene) (P3HT), and a fullerene derivative, [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM),10 as the active layer have been demonstrated to efficiently generate power under illumination.11 Several other representative functionalized fullerene derivatives have been utilized for OPV applications.12 However, the disclosed Received: November 24, 2015
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DOI: 10.1021/acs.orglett.5b03293 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
°C provided approximately half the yield obtained using the best reaction conditions. The formation of 1,3-dipoles became notably sluggish at room temperature, leading to a nearly full recovery of the starting material 1a (entries 12−13). We also observed that the reaction was less efficient when the source of the catalyst was switched to tricyclohexylphosphonium tetrafluoroborate and sodium hydride (HPCy3+BF4−/NaH, entry 14). Our attempts to use other organophosphine catalysts such as PBu3 or PMe3 yielded trace amounts of the desired products at room temperature (entries 15−16); however, PBu3 produced a substantial amount of 2a under standard conditions (9%, entry 17). Finally, reactions with amino catalysts did not yield the desired products (entries 18−19), but instead gave polymeric baseline materials, with a C60 recovery greater than 80%. With the optimal reaction conditions in hand, we next examined the scope of PCy3-catalyzed cycloaddition reactions of [60]fullerene with various enynoates 1a−l (Table 2).
fullerene materials for OPVs with higher power conversion efficiencies (PCEs) exceeding that of P3HT/PC61BM have been constructed using bisadducts of fullerenes13 because of the demand for n-type materials with higher LUMO energy levels.14 By combining synthetic methodologies and applications, we here report the formal catalytic 1,3-addition of phosphines to enynoates through reaction scope analyses and solid evidence of product crystal structures. Furthermore, the isolated cyclopentenofullerenes (CPFs) are demonstrated to be a new class of efficient n-type fullerene materials upon being embedded with P3HT as the active layer for organic photovoltaics, providing PCEs comparable to those of P3HT/PC61BM. To commence this study, we first investigated the reaction conditions with enynoate 1a and C60 catalyzed by PCy3 using variable reactant molar ratios (Table 1). Preliminary screening Table 1. Optimization of the Reaction Conditionsa
Table 2. Study of the Reaction Scopea
entry
C60:1a:K (equiv)
catalyst (K)
temp (°C)
time (h)
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
1:1:0.1 1:1:0.2 1:1:0.3 1:1:0.4 1:1:1 1:1.2:0.4 1:1.8:0.4 1:2.4:0.4 1:1.2:0.4 1:1.2:0.4 1:1.2:0.4 1:1.2:0.4 1:1.2:0.4 1:1.2:0.4 1:1.2:0.4 1:1.2:0.4 1:1.2:0.4 1:1.2:0.4 1:1.2:0.4
PCy3 PCy3 PCy3 PCy3 PCy3 PCy3 PCy3 PCy3 PCy3 PCy3 PCy3 PCy3 PCy3 PCy3c PMe3 PBu3 PBu3 DBU DMAP
120 120 120 120 120 120 120 120 130 110 100 rt rt 120 rt rt 120 120 120
6 6 6 6 6 6 6 6 6 6 6 6 24 6 24 24 6 6 6
16(77) 22(81) 30(73) 34(80) 33(79) 38(89) 19(60) 16(28) 19(80) 36(79) 17(93) trace trace 11(31) trace trace 9(32) 0 0
entry
1
R1
R2
2
yield [%]b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
1a 1b 1c 1d 1e 1f 1g 1h 1i 1j 1k 1l 1m 1n 1o 1p 1q
Ph 4-Me-Ph 4-C4H9-Ph 4-C6H13-Ph 4-C8H17-Ph 4-C10H21-Ph 4-C12H25-Ph 4-tert-butyl-Ph 4-OMe-Ph 3-OMe-Ph 4-F-Ph C6H13 Ph Ph Ph Ph Ph
Me Me Me Me Me Me Me Me Me Me Me Me Et n-Bu iso-Bu n-hexyl n-octyl
2a 2b 2c 2d 2e 2f 2g 2h 2i 2j 2k 2l 2m 2n 2o 2p 2q
38(89) 27(73) 33(74) 35(75) 42(83) 33(73) 33(61) 35(71) 43(81) 36(73) 27(73) 20(54) 31(59) 27(57) 29(59) 30(69) 26(66)
a
a
(entries 1−5) showed that a reaction with 40 mol % of PCy3 at 120 °C for 6 h (entry 4) provided one of the best yields (34%). Slightly increasing the molar amount of enynoate 1a to 1.2 equiv increased the yield (38%; 89% based on consumed C60, entry 6); however, those with 1.5 and 2 equiv of 1a gave lower yields of monoadducts because of the increased formation of bisadducts (entries 7−8). Furthermore, the reaction performed poorly when carried out at a higher temperature (130 °C), likely because of the competing reaction of 1a with 1,3-dipoles, as evidenced by dark-colored baseline materials (entry 9). Reactions conducted at a lower temperature (110 °C) also resulted in a comparable yield (entry 10); reactions run at 100
Enynoates bearing different substituted phenyl or alkyl groups (R1) all underwent PCy3-catalyzed cycloaddition with C60 to afford cyclopenteno[60]fullerenes (2a−l) in 20−43% isolated yields (54−89% based on recovered C60, entries 1−12). The relatively poor yields resulted from the substrates 1 with an R1 group consisting of a 4-fluorophenyl group (entry 11) or alkyl functionality (n-hexyl, entry 12). The less-reactive substrate 1k may have been incurred through resonance of the fluorosubstituent on the phenyl moiety toward the alkynyl α-carbon. Because of electronic effects, the alkynyl α-carbon of 1l became less positively charged and was thus less electrophilic toward phosphines. Furthermore, variations of the R2 alkyl function-
Reaction conditions: C60 (72 mg, 0.10 mmol), enynoates 1 (0.12 mmol), PCy3 (0.04 mmol) in 10 mL of anhydrous o-DCB. bIsolated yields (%) after column chromatography; values in parentheses were based on conversion of C60.
Reaction conditions: C60 (72 mg, 0.10 mmol) in 10 mL of anhydrous 1,2-dichlorobenzene (o-DCB). bIsolated yields after column chromatography; values in parentheses were based on conversion of C60. c PCy3 was generated from HPCy3+BF4−/NaH.
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DOI: 10.1021/acs.orglett.5b03293 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
of a phosphine to the β-carbon of enynoates 1 was excluded. Thus, an initial α-addition (1,3-addition) of PCy3 to enynoates 1 was proposed to generate the 1,3-dipolar species Ia′. Addition of Ia′ to C60 and subsequent 5-endo-trig cyclization provided intermediates Ic′. After proton transfer and elimination of PCy3 from Id′, products 2 were formed. We attempted to isolate unstable intermediate Ic′ from the reaction in entry 9 (Table 2) and observed that it produced the product in ca. 4% yield. The retrieved flash mass data with m/z = 1217.4 (M++1) by atmospheric pressure chemical ionization APCI, as well as 1H and 31P NMR data (Figures S92−S93), supported the possible existence of proposed intermediate Ic′ during the course of the reaction. Cyclic voltammetry (CV) studies revealed that the first halfwave reduction potentials (1E1/2) of 2 were similar in value; these reduction potentials ranged from −1.16 to −1.20 V (Table S2). Among the various cyclopentenofullerenes, compound 2h, which was substituted with a 4-tert-butylphenyl group, exhibited the lowest 1E1/2 value of −1.20 V and thus exhibited the highest LUMO energy level. We fabricated photovoltaic cells with layer configurations of glass/ITO/ PEDOT:PSS/P3HT:CPFs/Ca/Al in a preliminary application study; these cells were prepared by spin-coating the blends of P3HT and CPFs in an o-DCB solution as the active layer (P3HT:CPFs, 15 mg mL−1). Table S3 lists the summary of the device performance parameter data for each CPF in the active layer with an active layer composition ratio for P3HT:CPFs of 1:0.9 (w/w) for 2b−g and 1:1 (w/w) for 2o−q. In our experience, the devices incorporating CPFs 2d and 2e with 4-nhexyl- and 4-n-octylphenyl solubilizing moieties exhibited relatively higher PCEs of the studied CPFs (entries 3 and 4), with average PCEs of 3.71 ± 0.37% and 3.79 ± 0.29%, respectively. In particular, the devices with 2d and 2e in the active layer reached PCEs of up to 4.1%. However, devices with CPFs with much shorter or longer solubilizing groups (R1 = 4methyl- and 4-n-butylphenyl, entries 1−2; R1 = 4-n-decylphenyl and 4-n-dodecylphenyl, entries 5−6) did not exhibit better PCEs. Furthermore, substituting the alkyl functionalities on the alkyl esters with longer alkyl chains resulted in PCEs > 3.0% (entries 7−9). Among the devices fabricated with 2o−q, superior performance was displayed by the device with 2p, which exhibited a PCEave of 3.57 ± 0.10%. These OPV results indicate that these compounds compose another new class of fullerene derivatives to be utilized as efficient n-type materials in thin-film bulk heterojunction organic photovoltaic studies. The preparation of CPFs required four synthetic steps from corresponding aryl halide precursors (see Supporting Information). In summary, we demonstrated the tricyclohexylphosphinecatalyzed [3 + 2] cycloaddition of (E)-alkyl 5-substituted phenylpent-4-en-2-ynoates with [60]fullerene. This reaction proceeded through an initial nucleophilic 1,3-addition of phosphines toward the α-carbon of enynoates to generate reactive 1,3-dipole species. The structures of these fullerene derivatives were supported by single-crystal X-ray diffraction analyses. Furthermore, the isolated cyclopentenofullerene derivatives acted as efficient n-type materials in organic photovoltaics, providing power conversion efficiencies comparable to that of P3HT/PC61BM when embedded with P3HT in the active layer.
alities of the alkyl ester also resulted in the formation of 2m−q in 26−31% yield (57−69% based on recovered C60, entries 13− 17). Among the prepared fullerene derivatives, compounds 2c− g and 2n−q were much more soluble in chloroform or dichloromethane, rendering them good candidate materials for solution-processed bulk heterojunction organic photovoltaic (OPVs) applications. The cyclopentenofullerenes (2) were characterized by MS, 1H, and 13C NMR and infrared spectroscopy. Cyclopentenofullerene 2i (Figure 1) was
Figure 1. Solid-state structure of compound 2i.
successfully crystallized from a CS2/toluene solution through slow evaporation of CS2,15 affirming that the product structure was formed through a [3 + 2] cycloaddition reaction. Additionally, X-ray single-crystal diffraction analysis confirmed that the new covalent bond with C60 was formed through the βand δ-carbons of enynoates 1. A plausible catalytic cycle is presented in Scheme 2. Because of the new covalent bonds of β- and δ-carbons of enynoates 1 with C60, the reaction pathway via an initial nucleophilic attack Scheme 2. Proposed Catalytic Cycle
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DOI: 10.1021/acs.orglett.5b03293 Org. Lett. XXXX, XXX, XXX−XXX
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Huber, R. C.; Ferreira, A. S.; Tolbert, S. H.; Schwartz, B. J.; Rubin, Y. J. Mater. Chem. A 2016, 4, 416. (c) Chen, C.-P.; Lin, Y.-W.; Horng, J.-C.; Chuang, S.-C. Adv. Energy Mater. 2011, 1, 776. (13) (a) He, Y.; Chen, H.-Y.; Hou, J.; Li, Y. J. Am. Chem. Soc. 2010, 132, 1377. (b) Lenes, M.; Wetzelaer, G.-J. A. H.; Kooistra, F. B.; Veenstra, S. C.; Hummelen, J. C.; Blom, P. W. M. Adv. Mater. 2008, 20, 2116. (c) Miller, N. C.; Sweetnam, S.; Hoke, E. T.; Gysel, R.; Miller, C. E.; Bartelt, J. A.; Xie, X.; Toney, M. F.; McGehee, M. D. Nano Lett. 2012, 12, 1566. (d) Wong, W. W. H.; Subbiah, J.; White, J. M.; Seyler, H.; Zhang, B.; Jones, D. J.; Holmes, A. B. Chem. Mater. 2014, 26, 1686. (e) Backer, S. A.; Sivula, K.; Kavulak, D. F.; Fréchet, J. M. J. Chem. Mater. 2007, 19, 2927. (14) Baran, D.; Erten-Ela, S.; Kratzer, A.; Ameri, T.; Brabec, C. J.; Hirsch, A. RSC Adv. 2015, 5, 64724. (15) X-ray crystallographic data for compound 2i: black bricks; crystal size: 0.30 × 0.30 × 0.06 mm3; formula: C73H12O3; crystal system: monoclinic; space group: P21/c; d = 1.529 mg m−3, V = 5071 (2) Å3; a = 12.4010(4) Å; b = 19.0870(5) Å; c = 17.2580(5) Å; α = 90°; β = 94.747(6)°; γ = 90°; R1 = 0.0885; Rw = 0.2317. CCDC 1432221 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.5b03293. Procedures and spectroscopic data for all compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan for their financial support of this research (MOST1042113M009014MY3) and the Research Center for New Generation Photovoltaics of Taiwan for providing instruments for measuring the photovoltaic performance of the fabricated devices. We also thank Ms. Y. Cheng, Precision Instruments Center of National Chiao Tung University, for assistance with retrieving the MS and NMR data of intermediate Ic′.
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DOI: 10.1021/acs.orglett.5b03293 Org. Lett. XXXX, XXX, XXX−XXX
