The use of tethered addends to decrease the number of isomers of bisadduct analogues of PCBM

Article abstract of DOI:10.1002/chem.201000537

Roped in: The number of isomers present in the bisadduct analogue PCBM (bis-PCBM; PCBM=phenyl-C₆₁-butyric acid methyl ester) a recently introduced acceptor for bulk heterojunction organic photovoltaic devices has been reduced to seven by linking two tosylhydrazone addends with an ethylene glycol tether and reacting them with C₆₀.

Full text of DOI:10.1002/chem.201000537

DOI: 10.1002/chem.201000537
The Use of Tethered Addends to Decrease the Number of Isomers of
Bisadduct Analogues of PCBM**
Ricardo K. M. Bouwer[b] and Jan C. Hummelen*[a]
This communication describes the reduction of the possi-
ble isomers that are formed upon double cyclopropanation
of C60 by the use of a bidentate tosylhydrozone in which the
reactive groups are linked by an ethylene glycol tether. One
of the most widely used electron acceptors in bulk hetero-
junction organic photovoltaic devices (OPVs) is phenyl-C61-
butyric acid methyl ester (PCBM), which dramatically in-
creased the efficiency of early OPVs by enabling the fuller-
ene component to be co-soluble with the (typically polymer-
ic) donor molecule.[1] Since the discovery of PCBM, power
conversion efficiencies of more than 5% have been reached
in OPVs that use blends of poly(3-hexylthiophene) (P3HT)
and PCBM.[2] Recently, significantly higher efficiencies were
reached with other polymers.[3] This has been accomplished
in part by improving the bulk-heterojunction morphology of
the composite films these blends form, but there are two
other important factors: 1) the band gap of the polymer,
which is important for efficient light absorption[4] and 2) the
open circuit voltage (Voc) that the OPV produces. The most
direct way to influence the Voc of donor–acceptor-based
OPV devices is to alter the offset between the HOMO of
the donor and the LUMO of the acceptor. The attachment
of electron-donating groups or the saturation of multiple
double bonds on the fullerene cage raises the LUMO
energy of the resulting fullerene moiety, thereby increasing
Voc without negatively impacting the band gap of the donor
(and hence the ability of the OPV device to absorb light).[5]
Previous work from our group has shown that introducing
electron-donating groups on the phenyl ring of PCBM
yields a small increase in the Voc of OPVs with P3HT as the
donor.[6] Recently, higher adducts of PCBM were used to
further increase the Voc of P3HT-based OPVs by approxi-
mately 100 meV per saturated double bond (i.e., adduct).[7]
The drawback to this approach is the concomitant increase
in the formation of regioisomers when compared with the
synthesis of mono-adducts such as PCBM. Even bis-adducts
with two identical, symmetric addends (the functional group
that forms a bond to the fullerene cage) can be formed as
eight different regioisomers.[8]
Figure 1C shows these possible regioisomer positions;
bonds that are highlighted in bold indicate the two attach-
ment points of the addends. Bis-PCBM (PCBM that has a
second identical addend added to the fullerene cage), how-
ever, has non-symmetric addends, raising the number of pos-
sible stereo- and regio-isomers to 22, of which 15 are dis-
cernable in an HPLC chromatogram obtained by using a
silica gel column (Figure 1A). The presence of multiple iso-
mers of bis-PCBM decreases electron mobility in OPVs by
reducing the p–p interactions between fullerene cages.[9]
Different isomers can also have different reduction poten-
tials[10] and isomers with less-negative reduction potentials
than the average will act as (shallow) electron traps.[11]
Moreover, the overall morphology of OPVs is negatively
impacted by the presence of a mixture of isomers.[9]
[a] Prof. Dr. J. C. Hummelen
Stratingh Institute for Chemistry and
Zernike Institute for Advanced Materials
University of Groningen, Nijenborgh 4
9747AG Groningen (Netherlands)
Fax : (+31)50-3638751
Herein, we report our efforts to reduce the number of iso-
mers of bis-PCBM that are formed by the double cyclopro-
panation of C60 by tethering the two addends with an ethyl-
ene glycol linker. The eight possible regio isomers of bis-
PCBM, depicted in Figure 1C, exist as a mixture of up to
four stereoisomers (depending on the symmetry of the re-
gioisomer); the two methyl esters can point away from each
other, towards each other, or one in either direction. The
cis-1 isomer, however, is sterically prohibited by the bulk of
the addends.[8] The number of regioisomers is reduced fur-
ther by tethering the addends together with a linker that is
E-mail: J.C.Hummelen@rug.nl
[b] R. K. M. Bouwer
Stratingh Institute for Chemistry
University of Groningen/Dutch Polymer Institute
Nijenborgh 4/P.O. Box 902
9747 AG Groningen/5600 AX Eindhoven (The Netherlands)
[**] PCBM=Phenyl-C61-butyric acid methyl ester.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000537.
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COMMUNICATION
hydrazone addends together. In addition to decreasing the
number of possible regioisomers that can form, our ap-
proach—which employs diazo additions[13] with asymmetric
addends instead of Bingel additions with symmetric ones—
reduces the number of possible stereoisomers by restricting
the orientation of the ester moiety; only the regioisomers
that are close together can have the ester groups pointing in
opposite directions due to the short tether.
Scheme 1 shows the synthesis of the ethylene-tethered
bis-(4-benzoylbutyric acid tosylhydrazone) 3 and the addi-
tion to the fullerene moiety (see the Supporting Information
for more details). 4-Benzoyl butyric acid (1) was reacted in
a Dean–Stark apparatus with ethylene glycol in the presence
of a catalytic amount of concentrated sulfuric acid. The
treatment of the resulting ester, 2, with tosylhydrazide in
toluene and using a Dean–Stark apparatus afforded the bis-
tosylhydrazone 3 in good yield. C60 and 3 were then dis-
solved in ortho-dichlorobenzene (ODCB) and heated to
1108C. The subsequent addition of tBuOK induced the
transformation of the tosylhydrazone moieties into highly
reactive diazo groups, which reacted in situ with C60 to give
the tethered adducts mixture 4. The crude product was puri-
fied by column chromatography (SiO2/toluene). Compound
4 was readily transesterified to bis-PCBM by methanol in
ODCB in the presence of a catalytic amount of concentrat-
ed sulfuric acid.
The yield of this tethered double cyclopropanation reac-
tion is not as high as would be expected from the analogous
standard bis-PCBM double cylopropanation reaction be-
cause of the formation of cross-linked side products and the
partial hydrolysis of the tethered ester by trace amounts of
hydroxide in the tBuOK, resulting in the insoluble acid (car-
boxylate) analogue of PCBM. However, since fewer isomers
are formed, the overall yield of the purified isomer fraction
is higher (16% compared with ꢀ11% without using the
tether).
Figure 1. HPLC chromatograms of the mixtures of isomers of bis-PCBM
obtained by standard bis-addition (A) and from tethered addends (B)
showing that less isomers are present in the latter. C) All possible re-
gioisomers of bis-adducts of C60.
shorter than the distance between every possible combina-
tion of two reactive sites on the fullerene cage. Diederich
and co-workers[8] used a tether-directed approach for syn-
thesising different regioisomers and substitution patterns in
Bingel additions (addition to a fullerene cage by nucleophil-
ic attack[12]) of bis-malonates to C60 by tethering them with
diol linkers. We applied a similar approach in which ethyl-
ene glycol was used to link two 4-benzoylbutyric acid tosyl-
Scheme 1. Synthesis of the ethylene-tethered bis-(4-benzoylbutyric acid tosylhydrazone)
3 and the addition to the fullerene moiety. i) AlCl3,
ClCH2CH2Cl; ii) ethylene glycol, H+(cat.), toluene, Dean–Stark; iii) tosylhydrazide, toluene, Dean–Stark; iv) C60, ODCB, tBuOK; v) MeOH, H+, ODCB.
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J. C. Hummelen and R. K. M. Bouwer
Figure 2. The 13C NMR spectrum of the carbonyl (left) and bridgehead (right) area of the isomers obtained from tethered addends (a colour version of
the figure is available in the Supporting Information).
Analysis of the tether-directed bis-PCBM (abbreviated
tbis-PCBM) by HPLC revealed that only the isomers lead-
ing to the last five peaks of the HPLC chromatogram (Fig-
ure 1B) of the full isomer mixture (i.e., bis-PCBM synthes-
ised by the un-tethered method; Figure 1A) are formed.
However, due to the broadness of the peaks, we cannot
state with certainty that these peaks belong to single iso-
mers. High-resolution 13C NMR spectroscopy (Figure 2)
showed eight resonances in the carbonyl region at d=
171.84, 171.68, 171.60, 171.51, 171.47 (two peaks), 171.42
and 170.82 ppm. If all of the isomers were symmetric, this
would correspond to eight different isomers. However, the
similar intensities of the peaks at d=170.82 and 171.51 ppm
are indicative of stemming from a single isomer, as are the
peaks at d=171.42 and 171.60 ppm. There are similar group-
ings of intensities in the area d=73–81 ppm, where the
bridgehead carbon atom peaks are found; two peaks for the
symmetric isomers and four for the asymmetric ones.[14]
From this analysis, we are tempted to conclude that there
are two symmetric and three non-symmetric major isomers.
There are also two minor isomers, one symmetric and one
non-symmetric, as indicated by resonances for their bridge-
head carbon atoms, but not their carbonyl carbon atoms,
bringing the total to seven isomers—five major and two
minor.
Semi-empirical molecular modelling (PM3, Hyper-
chem 8.0) of tbis-PCBM isomers reveals that the length of
the ethylene tether limits the possible regioisomers to the
cis-2, cis-3, E and trans-3 bis-adducts. Including the stereo-
isomers, there are seven possible isomers; one non-symmet-
ric E isomer (with its second ester group pointing towards
the first addend), three cis-2 isomers (two symmetric, both
ester groups pointing in the same direction, one non-sym-
metric, the ester groups pointing in opposite direction), one
non-symmetric cis-3 isomer (the ester groups pointing the
same way), one non- symmetric trans-3 isomer (with the
ester groups pointing towards each other) and one symmet-
ric cis-3 isomer (with both ester groups pointing toward
each other). This is in good agreement with the HPLC and
13C NMR spectroscopy data.
In conclusion, the use of an ethylene tether reduces the
number of isomers of bis-PCBM that are formed from the
double cyclopropanation of C60 from 22 to 7 when com-
pared with standard bis-addition. The isomers were identi-
fied as seven forms of E, cis-2, cis-3 and trans-3 regioisomers
through a combination of HPLC, NMR spectroscopy and
molecular modelling. Of these seven forms, five can be con-
sidered major isomers. Future efforts will focus on testing
the influence of tbis-PCMB on the performance of OPVs,
improving the yield of the tethered double cyclopropanation
and investigating the influence of other tethers on the prod-
uct isomer distribution.
Acknowledgment
This research forms part of the research programme of the Dutch Poly-
mer Institute (DPI), project #524.
Keywords: donor–acceptor systems · fullerenes · organic
solar cells · regioselectivity · synthesis design
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Received: March 1, 2010
Published online: August 20, 2010
Chem. Eur. J. 2010, 16, 11250 – 11253
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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