Regio- and stereo-selective intermolecular [2+2] cycloaddition of allenol esters with C60 leading to alkylidenecyclobutane-annulated fullerenes

Article abstract of DOI:10.1039/c6cc07320d

The intermolecular [2+2] cycloaddition of allenol esters, which were in situ generated by Pt-catalyzed 1,3-acyloxy migration of propargylic esters, with C₆₀ proceeded regio- and stereo-selectively to give a novel class of alkylidenecyclobutane-annulated fullerenes. The cyclobutane-annulated fullerene derivatives have high-lying LUMO levels, which gave a high open-circuit voltage in organic solar cell applications. The observed high electron mobility provided a good fill factor compared with the PCBM-based devices.

Full text of DOI:10.1039/c6cc07320d

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Regio- and stereo-selective intermolecular [2+2]
cycloaddition of allenol esters with C60 leading to
alkylidenecyclobutane-annulated fullerenes†
Mitsuhiro Ueda,*a Tsukasa Sakaguchi,a Miho Hayama,a Takafumi Nakagawa,b
Yutaka Matsuo,*bc Aiko Munechika,a Shunsuke Yoshida,d Hiroshi Yasudad and
Ilhyong Ryu*a
Cite this:DOI: 10.1039/c6cc07320d
Received 7th September 2016,
Accepted 17th October 2016
DOI: 10.1039/c6cc07320d
www.rsc.org/chemcomm
The intermolecular [2+2] cycloaddition of allenol esters, which were
in situ generated by Pt-catalyzed 1,3-acyloxy migration of propargylic
esters, with C60 proceeded regio- and stereo-selectively to give a novel
class of alkylidenecyclobutane-annulated fullerenes. The cyclobutane-
annulated fullerene derivatives have high-lying LUMO levels, which
gave a high open-circuit voltage in organic solar cell applications. The
observed high electron mobility provided a good fill factor compared
with the PCBM-based devices.
Scheme 1 Transition-metal catalyzed 1,3-acyloxy migration of propargylic
esters.
In the first investigation, when a mixture of propargylic
ester 1a and C60 was treated with 0.5 equiv. of AuCl(PPh)3 in
1,2-dichlorobenzene (ODCB) at 80 1C for 6 h, no reaction took
place (Table 1, entry 1).10 However, when we used PtCl2 as a
catalyst for 1,3-acyloxy migration of 1a under similar conditions,
the [2+2] cycloaddition product 2a was obtained in 21% yield as a
single isomer (Table 1, entry 2). Prolonging the reaction time to
18 h resulted in a significant improvement in the yield of 2a
to 47% (Table 1, entry 3). The amount of PtCl2 was reduced to
0.1 equiv. without a decrease in the yield (Table 1, entries 4
and 5). In entry 4, the isolated yield of 2a was 31%, whereas the
yield based on the amount of C60 consumed was 67%. In the
case of entries 2–5, multiple cycloaddition products were also
Ring annulated fullerenes have attracted considerable attention
because PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), which
is a cyclopropane-annulated fullerene, displays outstanding per-
formance as a material for organic photovoltaics (OPVs).1 In this
research field, we are particularly interested in functionalized
cyclobutane-annulated fullerenes. Despite the tremendous
efforts devoted to the synthesis of cyclobutane annulated full-
erenes based on [2+2] cycloaddition reactions,2,3 application
studies as a material for OPVs were still insufficient to discover
a cyclobutane-annulated fullerene that exhibits a high perfor-
mance as a material. This is in sharp contrast to the fact that
studies on cyclopropane- or cyclopentane-annulated fullerenes
have been pursued vigorously.4
Table 1 Intermolecular [2+2] cycloaddition of in situ generated allenol
a
In this study, encouraged by the recent progress in the in situ
generation of allenol esters from propargylic esters by transition-
metal catalysts (Scheme 1),5–9 we envisaged the [2+2] cycloaddition
of in situ generated allenol esters with C60, which would give a
novel class of alkylidenecyclobutane-annulated fullerenes.
esters (from 1a) with C60
a Department of Chemistry, Graduate School of Science, Osaka Prefecture
University, Sakai, Osaka 599-8531, Japan. E-mail: ueda@c.s.osakafu-u.ac.jp,
ryu@c.s.osakafu-u.ac.jp
b Department of Mechanical Engineering, School of Engineering,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
E-mail: matsuo@photon.t.u-tokyo.ac.jp
c Hefei National Laboratory for Physical Sciences at the Microscale,
University of Science and Technology of China, Hefei, Anhui 230026, China
d Institute for Advanced & Core Technology, Showa Denko K.K., Chiba 267-0056,
Japan
Entry
Catalyst
x equiv.
Time (h)
Yield of 2ab (%)
1
2
3
4
5
AuCl(PPh3)
PtCl2
PtCl2
PtCl2
PtCl2
0.5
0.5
0.5
0.1
0.05
6
6
18
18
18
0
21
47
45 (31, 67)
37
a
Reaction conditions: C60 (0.15 mmol), 1a (10 equiv.), transition-metal
b
(x equiv.) in ODCB (7.5 mL) at 80 1C. Determined by the HPLC area
ratio. The values in parentheses are the isolated yield of 2a (left) and the
yield of 2a on the basis of the amount of C60 consumed (right).
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cc07320d
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Table 2 Intermolecular [2+2] cycloaddition of in situ generated allenol
a
esters (from 1) with C60
Yield of 2b (%)
Entry
1
1
Scheme 2 Intermolecular [2+2] cycloaddition of in situ generated allenol
40 (88)
31 (69)
ester (from 1f and 1g) with C60
.
2
3
35 (57)
33 (68)
Scheme 3 Reaction of C60 with carbonate 4 to give 5 and 6.
4
Using a similar protocol, the propargylic carbonate 4 was
successfully reacted with C60 to give the corresponding adduct 5
in 41% yield, which was further converted into the ketone 6 by
acidic treatment with TsOHꢀH2O (Scheme 3).
a
Reaction conditions: C60 (0.15 mmol), 1 (10 equiv.), PtCl2 (0.1 equiv.)
b
in ODCB (7.5 mL) at 80 1C for 18 h. Isolated yield of 2. The values in
parentheses are based on the amount of C60 consumed.
To confirm the stepwise reaction via the formation of
an allenol ester, we carried out the reaction of C60 with the
isolated allenol ester 1a0, which was obtained by the Pt-catalyzed
1,3-acyloxy migration of 1a and purified by silica filtration to
remove PtCl2. As we expected, the product 2a was obtained in
30% yield (Scheme 4).
detected as side products by LC-MS analysis. The structure of 2a
was characterized using HRMS and NMR techniques (1H, 13C,
DEPT, HMQC, HMBC, and NOESY) after purification using
silica-gel column chromatography (see the ESI†). The geometry
of the product 2a was the Z-isomer, which was presumably
because of the steric repulsion between the Bu group and C60.
Having adopted the optimized conditions mentioned in
entry 4, we next explored the scope of the intermolecular
[2+2] cycloaddition of 1 with C60. The reaction of 1b with C60
gave the corresponding [2+2] cycloaddition product 2b in 40%
yield (Table 2, entry 1). The propargylic ester 1c bearing a
p-chlorophenyl group also reacted with C60 to afford 2c in 31%
yield (entry 2). The propargylic ester 1d bearing a 2-thienyl group
displayed higher reactivity than 1a, and in this case, a large
amount of multiple cycloaddition products was also formed
(entry 3). The terminal acetylene 1e tolerated the reaction con-
ditions to furnish the corresponding [2+2] cycloaddition product
2e in 33% yield. The geometry of the product 2e was E-isomer
due to the steric repulsion of the OPiv group towards the
fullerene ring (entry 4).
A plausible mechanism for the present cycloaddition is illu-
strated in Scheme 5. The activation of the propargylic ester 1a by
the platinum catalyst would induce a 1,3-acyloxy migration to
generate the allenol ester 1a0. A single-electron transfer (SET) from
the aryl group of allenol ester 1a0 to C60 would then occur to give a
radical ion pair11,12,13,14 consisting of the radical anion C60 ꢂ and a
ꢁ
ꢂ
ꢁ
radical cation of 1a0. The radical anion C60 would react regio-
selectively with the radical cation of 1a0 to give the [2+2] cyclo-
addition product 2a via the zwitterionic intermediate A (Scheme 5,
path 1 for 2a). In the case of 1f, which gave a regioisomer, a similar
zwitterionic intermediate B would be formed via a SET pathway
from the aryl group of the allenol ester 1f0 to C60 to give 3a (path 2
for 3a). We deem that in both cases SET with the benzene ring is
crucial for the regioselective reaction.
To determine the physical properties of the alkylidenecyclo-
butane-annulated fullerenes prepared in this study, we chose 2a
We also examined the propargylic ester 1f having a phenyl
acetylene moiety (Scheme 2-1). Surprisingly, in this case, the
fullerene 3a, which was derived via [2+2] cycloaddition with the
pivaloxyalkene portion, was obtained in 42% yield regio- and
stereo-selectively. The 1,3-diphenyl-substituted propargylic ester 1g
also afforded the similar cyclobutanol-annulated fullerene 3b,
albeit in low yield. In this case, the conversion of C60 was low
(HPLC analysis: 3b/multiple cycloaddition products/C60 = 12/13/70)
and a [2+2] dimerization product of the in situ generated allenol
ester was observed (Scheme 2-2).8
Scheme 4 Control experiments.
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Fig. 1 J–V characteristics of the P3HT:fullerene derivatives and PTB7:6
organic solar cells; dotted line, P3HT:2a, dashed line, P3HT:6, and solid
line, PTB7:6.
of the device using 2 is attributable to the large volume of the Piv
group, which hampers fullerene–fullerene interactions.16 With 6
exhibiting the best performance, we tried to increase the PCE of
the P3HT:6 device by modifying the solution concentration of the
active layer and the annealing temperature after spin-coating.
The PCE reached 2.96 ꢃ 0.05% (the best performance was 3.00%)
with a good fill factor (FF, 0.65) and short-circuit current density
Scheme 5 Plausible mechanism.
and the hydrolyzed ketone 6 as model probes. Optical bandgaps
were derived from the UV-visible absorption spectra of 2a and 6
(see Fig. S1 and S2 in the ESI†), which were 1.71 eV for both
compounds. The electrochemical properties were investigated
by cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) (see Fig. S3 and S4 in the ESI†). The lowest unoccupied
molecular orbital (LUMO) levels were estimated from the DPV
data. The LUMO levels of 2a and 6 (ꢂ3.71 and ꢂ3.69 eV,
respectively) were higher than those of PC61BM (ꢂ3.74 eV for
PCBM, determined by CV in dichloromethane).15 These results
promise to provide a high open-circuit voltage (VOC) in organic
solar cell applications.
We investigated the photovoltaic performance of the obtained
fullerene derivatives as electron acceptors in standard bulk hetero-
junction organic solar cells using P3HT as an electron donor
(Table 3, Fig. 1). Solubilities of these electron acceptors in organic
aromatic solvents were respectable, which were similar to PCBM,
enabling thin-film fabrication with active layer solvents at similar
concentrations for the standard P3HT:PCBM bulk heterojunction
devices. First of all, we examined donor : acceptor ratios in the
photoactive layer from 5 : 3 to 2 : 3. The best donor : acceptor ratio
for devices using 2a was 1 : 1, whereas that for devices employing 6
was 5 : 4. Notably, these ratios are almost the same as the ratio in a
P3HT : PCBM device. The devices were thermally annealed at
120 1C. The obtained VOC of devices using 2a (0.60 ꢃ 0.004 V)
was slightly higher than that of a device using PCBM (0.58 V)
because of the high-lying LUMO levels of this fullerene derivative.
The power conversion efficiencies (PCEs) of P3HT:2a and P3HT:6
were 0.95 ꢃ 0.015% and 2.64 ꢃ 0.020%, respectively. The low PCE
( JSC, 8.60 mA cmꢂ2) when the concentration was 30 mg mLꢂ1
,
and the annealing temperature was 150 1C. We fabricated devices
employing these conditions with other fullerene derivatives, but
the PCE decreased because their solubility was lower than that of
6. Finally, we used the low band-gap polymer PTB7 as an electron
donor instead of P3HT and obtained a PCE of 3.78% with a VOC of
0.74 V, JSC of 8.28 mA cmꢂ2, and FF of 0.62 without using additives
(Table 3). The PCE was higher than that of a device using
PC61BM17 (PCE = 3.6%, VOC = 0.71 V, JSC = 9.5 mA cmꢂ2, and
FF = 0.53) with a good VOC and FF.
We hypothesized that the relatively high FFs compared with
those of PCBM devices were derived from the high electron
mobility of 6 or good film morphology. The electron mobility of
the electron-only devices was measured using a space-charge-
limited current (SCLC) method (see Fig. S5 and S6 in the ESI†).
The device structures were Al/active layer/LiF (0.6 nm)/Al.
The electron mobilities of the P3HT:6 and PTB7:6 devices were
1.3 ꢄ 10ꢂ4 and 4.1 ꢄ 10ꢂ4 cm2 Vꢂ1 sꢂ1 with the active-layer film
thickness of 310 and 150 nm, respectively. These electron
mobilities were higher than those obtained from P3HT:PCBM
and PTB7:PCBM electron-only devices.18 The morphologies of
the P3HT:6 devices were investigated by atomic force micro-
scopy (see Fig. S7 in the ESI†). In agreement with the perfor-
mance obtained for the devices annealed at 120 and 150 1C,
high temperature annealing increased the surface roughness,
which probably led to a good FF and mobility. On the other
hand, the PTB7:6 film had a smoother surface compared with
the P3HT:6 film. This implies the high miscibility of PTB7
and 6. We ascribe this to the high solubility of 6.
Table 3 Summary of the device performances of the bulk heterojunction
organic solar cells
In conclusion, we have developed the regio- and stereo-selective
intermolecular [2+2] cycloaddition of in situ generated allenol
esters with C60. For the regioselective cycloaddition of allenol
esters with C60, we proposed pathways involving a radical ion-
pair formed by a SET process. Measuring the physical properties of
2a and ketone 6 for comparison with PCBM, we obtained a
respectable PCE of 3.78% using PTB7 with ketone 6 with a good
VOC (0.74 V) and FF (0.62), which were derived from the high-lying
Jsc
Acceptor Donor Voc [V]
[mA cmꢂ2
]
FF [ꢂ] PCEa [%]
2a
6
6
P3HTb 0.60 (0.595 ꢃ 0.005) 3.45
0.47
0.65
0.62
0.97 (0.95)
3.00 (2.96)
3.78 (3.73)
P3HTb 0.53 (0.534 ꢃ 0.003) 8.60
PTB7c 0.74 (0.737 ꢃ 0.004) 8.28
a
b
The average values are in the parentheses. Poly(3-hexylthiophene-
c
2,5-diyl). Poly(4,8-bis[(2-ethylhexyl)benzo[1,2-b:4,5-b0]dithiophene-2,6-
diyl]{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}).
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decrease.
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and THF, respectively. Comparison is meaningful only when the
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