Face-to-face C6F5-[60]fullerene interaction for ordering fullerene molecules and application to thin-film organic photovoltaics

Article abstract of DOI:10.1039/c0cc03028g

Treatment of [60]fullerene with potassium methylnaphthalenide and excess C₆F₅CH₂Br afforded 1,4-bis(pentafluorobenzyl) [60]fullerene, the study of which showed that there is a face-to-face interaction between [60]fullerene and a perfluoro aromatic ring, allowing the molecule to be utilized for high-performance organic photovoltaic devices.

Full text of DOI:10.1039/c0cc03028g

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Face-to-face C6F5–[60]fullerene interaction for ordering fullerene
molecules and application to thin-film organic photovoltaicswz
Chang-Zhi Li,a Yutaka Matsuo,*ab Takaaki Niinomi,ac Yoshiharu Satoac and
Eiichi Nakamura*ab
Received 4th August 2010, Accepted 4th October 2010
DOI: 10.1039/c0cc03028g
Treatment of [60]fullerene with potassium methylnaphthalenide
and excess C6F5CH2Br afforded 1,4-bis(pentafluorobenzyl)[60]-
fullerene, the study of which showed that there is a face-to-face
interaction between [60]fullerene and a perfluoro aromatic
ring, allowing the molecule to be utilized for high-performance
organic photovoltaic devices.
Weak molecular interactions play a key role in the ordering of
molecules in molecular science. Among a number of such weak
interactions, the interactions that control molecular ordering
of fullerene are rather rare, despite the fact that the designing
of fullerene-based photovoltaic devices1 and field effect
transistors2 requires rational methods for ordering fullerene
molecules in the solid state—a crucial element to achieve high
carrier mobility.3 We previously exploited the isotropic shape
and large molecular size of fullerene for one-dimensional
ordering,4 and the fullerene–fullerene facial interaction for
vesicle,5 lamellar ordering.6 We report herein that a face-to-
face interaction between a C6F5 group and [60]fullerene causes
molecular ordering, as illustrated in the organization of
1,4-bis(pentafluorobenzyl)[60]fullerene, C60(CH2C6F5)2 (1),
in the solid state. We also noted that perfluorobenzene
(C6F6) caused deaggregation of 1 in solution, suggesting that
this interaction is effective even in solution. The face-to-face
interaction between a C6F5 group and [60]fullerene is formally
related to the well-established face-to-face p–p stacking in a
C6H6/C6F6 pair,7 where a quadrupole interaction results in a
Scheme 1
dark red solution in 3 h. The reaction of the dianion with
10 equiv. of C6F5CH2Br afforded a 1,4-bis(pentafluorobenzyl)
adduct 1 (Scheme 1) in 55% yield from fullerene. Compound 1
was characterized by high resolution APCI–TOF MS, 1H and
13C NMR spectroscopies, as well as elemental analysis.
This compound has a high thermal stability, as determined
by TG–DTA measurement under a nitrogen atmosphere
(T95 = 308 1C, 5% weight loss).
During the characterization study, we made an interesting
observation that molecule 1 in chloroform forms aggregates,
which are converted to a monomer upon addition of 10 equiv.
of C6F6 to the solution. Thus, a dynamic light scattering
(DLS) analysis of the chloroform solution (0.2 M) showed
two peaks with average radii of 190.0 nm and 1.3 nm (Fig. 1a,
top). Addition of 5 equiv. of C6F6 shifted both peaks to the
smaller side, and addition of 10 equiv. caused complete
deaggregation, converting the two broad peaks into one sharp
peak at 0.6 nm (Fig. 1a, middle and bottom, and Fig. 1b). We
surmised that C6F6 has a strong interaction with the fullerene
molecule. We then studied the crystal structure to discover
if such a C6F6–fullerene interaction manifests itself in the
crystal of 1.
stabilization energy gain of 4 to 5 kcal molꢀ1 8
. We utilized
compound 1 as an electron acceptor in a solution-processable,
small-molecule-based thin-film photovoltaic device that showed
a power conversion efficiency of 1.5%.
We synthesized 1 in one pot from [60]fullerene (Scheme 1).9
Reduction of C60 with potassium as mediated by 1-methyl-
naphthalene at room temperature in THF afforded C602ꢀ as a
Crystallographic analysis of a single crystal (containing no
solvent molecules) showed that there is a face-to-face inter-
action between the C6F5 group and the fullerene core, and that
the two fluorinated benzyl groups catch another fullerene
molecule and form an interlocked one-dimensional zigzag
array along the c-axis, as shown in Fig. 2a. Thus, a red
fullerene core is caught by two C6F5 groups of the neighboring
fullerene (space-filling representation). Along the c-axis, the
fullerene’s centroid-to-centroid distance was 10.36 A, which is
comparable to the reported value of [C60]PCBMꢁPhCl
co-crystal (less than 10.13 A)10 and the crystals of C60 (fcc,
10.0 A).11 The one-dimensional zigzag columns further
aggregate in a rectangular fashion to form a densely packed
crystal structure, as shown in Fig. 2b.
a Nakamura Functional Carbon Cluster Project, ERATO,
Japan Science and Technology Agency, Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan
b Department of Chemistry, The University of Tokyo, Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan.
E-mail: matsuo@chem.s.u-tokyo.ac.jp,
nakamura@chem.s.u-tokyo.ac.jp; Fax: +81 3-5800-6889;
Tel: +81 3-5800-6889
c Mitsubishi Chemical Group Science and Technology Research Center,
Inc., 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan
w This article is part of the ‘Carbon Nanostructures’ web-theme issue
for ChemComm.
z Electronic supplementary information (ESI) available: Experimental
procedures and compound characterisation data, XRD, DSC, and
TG-DTA data. CCDC 785439. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0cc03028g
Details of the crystal structure showed that the C6F5–fullerene
interaction occurs on a fullerene’s CQC double bond that
connects two hexagons. The C6F5 group precisely faces toward
c
8582 Chem. Commun., 2010, 46, 8582–8584
This journal is The Royal Society of Chemistry 2010

Fig. 2 Single-crystal analysis of compound 1. (a) A columnar zigzag
array of the fullerene molecule along the c-axis. The red fullerene
molecules interact face-to-face with two C6F5 groups shown as a
space-filling model. (b) Rectangular packing of the zigzag columns
in a crystal. Fluorine atoms are shown in yellow. (c) A close-up view of
the face-to-face interaction between the C6F5 group and a fullerene
CQC bond (red) after removal of disordered atoms. Black prism
crystals of 1 suitable for single-crystal X-ray diffraction were obtained
by slow diffusion of methanol gas into a chloroform solution. The
fullerene core shows disorder. The tetragonal unit cell (Fig. 1a)
with space group P43212 (a = 13.897 A, b = 13.897 A, c = 20.108 A,
a = b = g = 901) and the cell volume (3883.39 A3) were determined.
Fig. 1 C6F5–[60]fullerene interaction of 1 in solution. (a) Aggregation/
deaggregation of 1 in chloroform in the presence of C6F6. Apparent
hydrodynamic radius (Rh) from the DLS study of 1 (0.2 mM) in
chloroform with varying amounts of C6F6. (b) Schematic illustration
of deaggregation process of aggregates.
Fig. 3 Electrochemical properties of 1 [C60(CH2C6F5)2] (upper) and 2
the center of this nonplanar double bond, as illustrated in
Fig. 2c.
[C60(CH2C6H5)2] (lower).
The solid-state morphology, in particular, its temperature
dependence, is important for small organic molecules to be
utilized for organic photovoltaic applications. For instance,
bis(dimethylphenylsilylmethyl)[60]fullerene (SIMEF)12 shows
a transition from amorphous glass to crystal (Tc) at 149 1C,
and it forms a morphologically stable n-layer by annealing
after device fabrication. We found a Tc value of 185 1C
for 1 upon a first heating process in the differential scanning
calorimetric (DSC) measurement (ESIz). The crystalline phase
at 190 1C exhibited a characteristic powder X-ray diffraction
(XRD) pattern, which matches the simulation based on single-
crystallographic data (ESIz), indicating that the crystals
obtained by heating an amorphous solid have the same
structure as those obtained for a single-crystal growth.
As shown in Fig. 3, compound 1 (and the protio compound,
1,4-bisbenzyl[60]fullerene 2)13 was found to be electrochemically
stable, and showed three reversible reduction waves at
red
E1/2 = ꢀ0.93, ꢀ1.57, and ꢀ2.17 V (Fig. 3, black line) as
measured in THF containing tetrabutylammonium perchlorate
as a supporting electrolyte (vs. Fc/Fc+). The LUMO levels
of 1 and 2 were deduced by the established equation
LUMO = ꢀ(4.80+ E1/2red1).14 Thus, the LUMO level of 1
(ꢀ3.87 eV) was found to be 0.1 eV lower than that of 2
(ꢀ3.76 eV) and SIMEF (ꢀ3.74 eV).
We examined the OPV performance of 1 in our own p-i-n
device configuration that showed 5.2% efficiency using
SIMEF as an electron acceptor.12b We fabricated a device
(ITO/PEDOT:PSS/BP/BP:SIMEF/1/BCP/Al) by spin coating
1 (0.8 wt%) in a solvent mixture (C6F6/toluene = 2/3) as an
n-layer. The device exhibited a power conversion efficiency of
1.51% (open circuit voltage, VOC = 0.48 V; short circuit
current density, JSC = 7.0 mA cmꢀ2; fill factor, FF = 0.45;
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 8582–8584 8583

series resistance, Rs = 5.5 O cm2). Although we cannot draw a
solid conclusion from this unoptimized experiment, the VOC
value is expectedly low. This observation is consistent with a
recent proposal that the open circuit voltage (VOC) of OPV is
proportional to the energy difference between the LUMO of
the acceptor and the HOMO of the donor.15 We speculate that
the fluoroaromatic–C60 moieties mediated structure ordering
of 1 discussed above may have contributed to the lowering of
the series resistance.
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In conclusion, we have found that there is a favorable
interaction between a perfluoroaromatic ring and the p surface
of fullerene, and that this interaction contributes to the
self-organization of compound 1 in the solid phase and
perhaps in solution. The attachment of a pentafluorobenzyl
group to fullerene resulted in a thermo- and electrochemically
stable acceptor. Detailed physical studies on the device
performance are the subject of future studies.
9 Experimental procedure: potassium metal (124 mg, 3.19 mmol)
was added in one portion to a freeze–thaw degassed mixture of
C60 (1000 mg, 1.39 mmol) and 1-methylnaphthalene (5.93 mL,
41.7 mmol, 30 equiv.) in 150 mL THF. A dark red solution was
produced after stirring under argon at room temperature for 3 h.
Pentafluorobenzyl bromide (3628 mg, 13.9 mmol) was then added.
After stirring for an additional 8 h, the reaction mixture was
quenched with aq. NH4Cl (0.5 mL). After concentration to a
volume of ca. 10 mL, the desired product was precipitatedby
methanol. Purification on a silica gel column (eluent: first CS2/
hexane = 1/1, then CS2) afforded 1,4-bis(pentafluorobenzyl)[60]-
fullerene 1 in 55% yield (830 mg). Further purification of the
sample for measurements and OPV evaluation was achieved with
preparative HPLC and GPC. The compound was stable under air
as a solid or in solution.
Financial supports from MEXT, Japan (KAKENHI,
#22000008 and Global COE Program for Chemistry
Innovation) and Japan Science and Technology Agency, JST
(Strategic Promotion of Innovative Research and Development)
are gratefully acknowledged.
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This journal is The Royal Society of Chemistry 2010