Tricyclohexylphosphine-Catalyzed Cycloaddition of Enynoates with [60]Fullerene and the Application of Cyclopentenofullerenes as n-Type Materials in Organic Photovoltaics

Article abstract of DOI:10.1021/acs.orglett.5b03293

The tricyclohexylphosphine-catalyzed [3 + 2] cycloaddition of (E)-alkyl 5-substituted phenylpent-4-en-2-ynoates 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 1,3-dipoles 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.

Full text of DOI:10.1021/acs.orglett.5b03293

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-2-
ynoates with [60]fullerene was studied. This reaction under-
goes an initial 1,3-addition of phosphines toward the α-carbons
of enynoates. Subsequent cycloaddition of the generated 1,3-
dipoles with [60]fullerene and elimination of tricyclohexyl-
phosphines 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.
Scheme 1. Reactivity of Ynenoates A and Enynoates 1
toward the Conjugate Addition of Phosphines
onjugate addition is an important methodology in the
C
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, respec-
tively. 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
existing 1,n-addition (n = even number) patterns.4 In a report,
Nagao et al. demonstrated that PBu3 catalyzed the anti-
carboboration of an enynoate with organoboranes via 1,4-
addition 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
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03293
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Organic Letters
Letter
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 applica-
tions, 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.
°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%.
+
−
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
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).
a
Table 1. Optimization of the Reaction Conditions
a
Table 2. Study of the Reaction Scope
b
entry C60:1a:K (equiv) catalyst (K) temp (°C) time (h) yield (%)
1
1:1:0.1
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
120
120
120
120
120
120
120
120
130
110
100
rt
6
16(77)
22(81)
30(73)
34(80)
33(79)
38(89)
19(60)
16(28)
19(80)
36(79)
17(93)
trace
b
entry
1
R1
R2
Me
2
yield [%]
2
1:1:0.2
6
1
1a
1b
1c
1d
1e
1f
Ph
2a
2b
2c
2d
2e
2f
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)
3
1:1:0.3
6
2
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
Me
4
1:1:0.4
6
3
Me
5
1:1:1
6
4
Me
6
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
6
5
Me
7
6
6
Me
8
6
7
1g
1h
1i
Me
2g
2h
2i
9
6
8
Me
10
11
12
13
14
15
16
17
18
19
6
9
Me
6
10
11
12
13
14
15
16
17
1j
Me
2j
6
1k
1l
Me
2k
2l
rt
24
6
trace
c
C6H13
Me
120
rt
11(31)
trace
1m
1n
1o
1p
1q
Ph
Et
2m
2n
2o
2p
2q
PMe3
PBu3
24
24
6
Ph
n-Bu
iso-Bu
n-hexyl
n-octyl
rt
trace
Ph
PBu3
120
120
120
9(32)
0
Ph
DBU
DMAP
6
Ph
6
0
a
a
Reaction conditions: C60 (72 mg, 0.10 mmol), enynoates 1 (0.12
Reaction conditions: C60 (72 mg, 0.10 mmol) in 10 mL of anhydrous
1,2-dichlorobenzene (o-DCB). Isolated yields after column chroma-
tography; values in parentheses were based on conversion of C60.
PCy3 was generated from HPCy3 BF4 /NaH.
b
b
mmol), PCy3 (0.04 mmol) in 10 mL of anhydrous o-DCB. Isolated
yields (%) after column chromatography; values in parentheses were
based on conversion of C60.
c
+
−
(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 fluoro-
substituent 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-
B
DOI: 10.1021/acs.orglett.5b03293
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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
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
1
atmospheric pressure chemical ionization APCI, as well as H
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 half-
wave 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
1
group, exhibited the lowest E1/2 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-n-
hexyl- and 4-n-octylphenyl solubilizing moieties exhibited
relatively higher PCEs of the studied CPFs (entries 3 and 4),
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 β-
and 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
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 = 4-
methyl- 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 Informa-
tion).
Scheme 2. Proposed Catalytic Cycle
In summary, we demonstrated the tricyclohexylphosphine-
catalyzed [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 com-
parable to that of P3HT/PC61BM when embedded with P3HT
in the active layer.
C
DOI: 10.1021/acs.orglett.5b03293
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Organic Letters
Letter
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.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03293.
■
S
Procedures and spectroscopic data for all compounds
(PDF)
2014, 26, 1686. (e) Backer, S. A.; Sivula, K.; Kavulak, D. F.; Frec
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.
́
het, J.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jscchuang@faculty.nctu.edu.tw.
(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.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Ministry of Science and Technology of Taiwan
for their financial support of this research (MOST1042113-
M009014MY3) 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
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