Self-crystallization of C70 cubes and remarkable enhancement of photoluminescence

Article abstract of DOI:10.1002/anie.201005076

Good solvent, poor solvent: A simple precipitation method enabled the spontaneous formation of homogeneous C₇₀ cube crystals by self-crystallization in cavities of a good solvent (mesitylene) surrounded by a poor solvent (isopropyl alcohol, IPA; see picture). The enormously increased photoluminescence (PL) intensity of the C₇₀ cube crystals relative to that of C₇₀ powder was mainly attributed to the high crystallinity of the cubes. Copyright

Full text of DOI:10.1002/anie.201005076

Communications
DOI: 10.1002/anie.201005076
Fullerene Crystals
Self-Crystallization of C70 Cubes and Remarkable Enhancement of
Photoluminescence**
Chibeom Park, Eunjin Yoon, Masaki Kawano, Taiha Joo, and Hee Cheul Choi*
Light emission from organic molecular systems is one of the
essential properties for the development of next-generation
cost-effective, light, flexible, and large-scale electronic and
optoelectronic devices.[1,2] Because the light emission medi-
ated by exciton recombination requires a specific energy band
gap, highly conjugated organic molecules in the form of
crystalline structures have been studied intensively for this
purpose,[3,4] and diverse strategies have been developed for
the synthesis of highly conjugated organic and polymeric
molecules with geometrically well-defined crystal shapes,[5,6]
by which their optical properties can be modulated.
Fullerenes, one of the most popular types of highly
conjugated organic molecules, have been studied mainly for
their semiconducting[7] and superconducting[8,9] properties,
which result from the discrete HOMO–LUMO band struc-
ture as well as the facile doping effect on low-lying energy
levels of empty orbitals, into which extra electrons are readily
accommodated. In contrast, their optical emission properties
have scarcely attracted any attention because of the low
exciton population for radiative recombination in fullerene
molecules.[10] However, fullerene molecules are still believed
to have much potential in terms of their optical properties, as
indicated by recent observations of abnormally increased
fluorescence from self-aggregated molecules that have
intrinsically low emitting power when they exist in an
ensemble state in solution or powder phases. For example,
Hara and co-workers reported that an asymmetric disulfide
compound in which a photoisomerizable azobenzene unit was
coupled to a biphenyl fluorophore generally showed negli-
gible fluorescence, but that the photoinduced aggregation of
the fluorophores led to remarkably increased fluorescence
intensity.[11] The Tang and Park research groups have reported
similar phenomena brought about by so-called aggregation-
induced emission (AIE)[12,13] or crystallization-induced emis-
sion enhancement (CIEE).[14,15] In the case of fullerenes,
changes in the photophysical properties of C60 and C70 upon
aggregation have also been investigated: both absorption and
fluorescence spectra were gradually modulated as the degree
of aggregation was systematically changed.[16–21] However,
such changes in fluorescence properties were mainly observed
for random aggregates, no detailed crystal structures of which
are available.
Herein we report that C70 molecules undergo self-
crystallization into high-definition cube-shaped crystals in
the solution phase at room temperature. As a result of their
high crystallinity with sharp edges, C70 cube crystals display
enhanced fluorescence. Such fluorescence has not been
observed from C70 in either powders or bulk crystals under
ambient conditions at room temperature. The synthesis of C70
cube crystals in the solution phase was inspired by recent
discoveries of “geometrically defined” fullerene structures
obtained through 1) precipitation in a mixture of solvents,
2) drop drying, and 3) vapor transport. In the case of the
precipitation method, fullerenes spontaneously aggregate in
two-solvent mixtures containing good and poor solvents for
fullerenes, and their geometrical structures are determined by
the type and shape of the solvents.[22,23] A drop-drying process
with a single good solvent also resulted in various C60
structures, such as spherical particles, wires, and hexagonal
disks, the geometries of which were determined selectively
according to the type of solvent used.[24–26] Both processes in
the solution phase lead to the inclusion of solvent molecules
in the final fullerene crystals; however, solvent-free and
highly crystalline C60 hexagonal disks were also obtained by a
vapor-transport process.[27] In our current study, we synthe-
sized unprecedented C70 cube crystals by precipitation from a
mixture of the good solvent mesitylene and the poor solvent
isopropyl alcohol (IPA).
[*] C. Park, Prof. Dr. H. C. Choi
Department of Chemistry and Division of Advanced Materials
Science, Pohang University of Science and Technology
San 31, Hyoja-Dong, Nam-Gu, Pohang, 790-784 (Korea)
Fax: (+82)54-279-3399
E-mail: choihc@postech.edu
Homepage: http://www.postech.ac.kr/chem/nmrl
E. Yoon, Prof. Dr. T. Joo
Department of Chemistry
Pohang University of Science and Technology
Prof. Dr. M. Kawano
Division of Advanced Materials Science
Pohang University of Science and Technology
[**] This research was supported by National Research Foundation of
Korea (NRF) grants funded by MEST (2010-00285, 2008-04306,
2007-8-1158, 2005-01325), KOSEF (through the EPB center; R11-
2008-052-02000), and the Ministry of Health, Welfare & Family
Affairs (A0900062), and by the Korean Research Foundation
(MOEHRD, KRF-2005-005J13103). H.C.C. thanks the World Class
University (WCU) program (R31-2008-000-10059-0). We thank D. Y.
Noh and K.-J. Kim at the 5C2 beamline of the Pohang Accelerator
Laboratory (PAL) for XRD measurement, N. Won for fluorescence
microscope imaging, and D. M. Kim and Y.-G. Ko for TGA
measurement.
We first attempted the precipitation method for the
synthesis of C70 cube crystals in a solvent mixture composed of
mesitylene and IPA. C70 powder was dissolved in mesitylene,
and IPA was slowly added to the resulting solution to form an
immiscible liquid–liquid interface. The mixture was then
agitated by manual shaking and kept at room temperature
without any further agitation for 24 h. A fine black precipitate
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005076.
9670
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started to appear after approximately 3 h. When we collected
the final black precipitate, we found crystals the shape of
rectangular prisms with high-definition edges. However, the
C
70 cube crystals showed quite a broad size distribution, which
may be due to inhomogeneous nucleation of C70 molecules
during precipitation (see Figure S1 in the Supporting Infor-
mation). To increase the size homogeneity, we applied brief
ultrasonication for the first 30 s after the rapid addition (in 3–
4 s) of IPA to the solution of C70 in mesitylene. The color of
the mixture changed from light reddish brown to dark
blackish brown within the first
Figure 1. Homogeneous C70 cube crystals. a) Low- and b) high-magnifi-
cation SEM images of C70 cube crystals; c) bright-field optical micro-
scope image.
30 s of the reaction, as
observed for the reaction with-
out ultrasonication. This obser-
vation implies that nucleation
for the self-crystallization of
C70 occurs almost instantane-
ously. Although the growth
rate was not affected signifi-
cantly by ultrasound treat-
ment, as indicated by the
color change of the solution,
the size distribution and shape
uniformity of the C70 cubes
were improved enormously,
with an average size of
2.05 mm (Figure 1; the C70 crys-
tals shown were obtained from
a 1:4 (v/v) mixture of the C70/
mesitylene
solution
(0.2 mgmLÀ1) and IPA). This
observation indicates that the
nucleation step occurs more
homogeneously with the assis-
tance
of
ultrasonication,
whereas the process without
ultrasonication allows random
nucleation only at the interface
of two liquids through slow
diffusion (Figure 2). An optical
microscopic image in the
bright-field reflection mode
revealed a shiny and yellowish
Figure 2. Schematic representation of the precipitation method for the self-crystallization of C70 cubes.
The two right-most images were taken from the bottom of the vials; the black rings are precipitates of C70
cubes.
metallike C70 surface, which implies that the C70 surface is
highly populated with free electrons (Figure 1c).
systematically changing 1) the concentration of C70 in mesi-
tylene from 0.1 to 0.4 mgmLÀ1 and 2) the volume ratio of the
C70/mesitylene solution to IPA from 1:1 to 1:4. Similar cube-
shaped crystals with homogeneous size distributions were
obtained under almost all conditions, with the exception of
the stacked pyramidal crystals formed from a 1:1 mixture of
the C70/mesitylene (0.1 mgmLÀ1) solution and IPA (Figure 3).
Meanwhile, the average size of the C70 cubes gradually
decreased as both the amount of IPA and the concentration of
C70 in the starting solution increased. These results support
the hypothesis that the addition of IPA induces local cavities
of mesitylene in which C70 molecules are localized and their
self-crystallization guided. Since the size of the cavity should
become smaller as the amount of IPA is increased, the
average size of the resulting cube crystal is also smaller. The
decrease in the average size as the C70 concentration is
We believe the key factor for C70 cube formation to be the
formation of local mesitylene cavities, which are instanta-
neously generated when IPA is added to the C70/mesitylene
solution. Owing to the low solubility of C70 in IPA, the local
concentration of C70 in the mesitylene cavities is increased,
and therefore instant nucleation of C70 occurs in the
mesitylene cavities. This mixed-solvent-mediated nucleation
phenomenon is similar to the traditional recrystallization
process and the synthesis of various self-crystallized C60
structures. Once C70 molecules are localized in the cavity of
the good solvent mesitylene, which is surrounded by IPA,
mesitylene guides C70 molecules to self-crystallize into cubes.
To evaluate the influences of mesitylene and IPA on the
formation of C70 cubes, we performed control experiments by
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the ratio of mesitylene to C70 is 2.05, as calculated by dividing
the weight change by the molecular weight of each compo-
nent. According to the GC–MS and TGA results, the
chemical formula of a C70 cube crystal is C70·(mesitylene)2,
which agrees well with a previous result for a bulk C70/
mesitylene crystal.[30]
The detailed crystal structure of the C70 cube was modeled
on the basis of single-crystal X-ray diffraction data. To obtain
C70 cube crystals large enough for X-ray crystallography, we
used a classical liquid–liquid diffusion (LLD) process[31,32]
(see the Experimental Section). The size of the crystals
formed by the LLD process varied from 10 to 100 mm;
however, these crystals also had a cube or rectangular-prism
shape. With a selected cube crystal, we first attempted to
collect single-crystal X-ray diffraction data at room temper-
ature, but failed to obtain a reasonable amount of data. This
difficulty seemed to be due to the thermal motion of C70
molecules positioned at lattice points; the motion of these
molecules increases the Debye–Waller factor, as in the case of
single crystals of C60.[33] Hence, the data was collected at 90 K,
and the crystal structure was resolved.[34] The crystal structure
of C70·(mesitylene)2 was determined to have a cubic unit cell
(a = b = c = 10.4774(8) ꢀ). On the basis of a careful Laue
Figure 3. Dependence of the size and morphology of C70 cubes on the
solution composition. The scale bars in the images are 10 mm; M and
I stand for the C70/mesitylene solution and IPA, respectively.
increased can be explained by the increased number of
nucleation sites, as exemplified by various syntheses of
semiconductor quantum-dot particles.
[35]
ꢀ
group check, a space group P43m was selected.
The
To further confirm the role of IPA as a favorable “poor”
solvent during the precipitation process, we attempted the
same reactions with methanol, ethanol, and acetone instead
of IPA. Self-crystallized C70 crystals were also obtained from
these solvent systems, but the edges were not sharp, and the
size distribution was quite broad even with ultrasonication.
Thus, the combination of mesitylene and IPA provided the
most favorable self-crystallization environment for C70 (see
Figure S2 in the Supporting Information). Although IPA
affords cavity room for crystallization, it is mesitylene that
directly causes the crystallization of C70 molecules into cube
crystals, since random C70 crystal morphologies were obtained
when mesitylene was replaced with toluene, m-xylene, and m-
dichlorobenzene (see Figure S3 in the Supporting Informa-
tion), in agreement with previous results.[28,29]
approximate position of C70 was determined by using a
rigid-group model, and it was converged to the center of the
unit cell. Because of 24 symmetry operations, symmetry-
related C70 molecules form spherical electron density. Differ-
ence Fourier synthesis generated several electron-density
peaks corresponding to mesitylene molecules positioned near
each corner of the unit cell. These positions are related to the
interstitial sites available in a C70 cube crystal and are
analogous to the positions occupied by solvent molecules in
1D and 2D C60 and C70 crystals formed through the mediation
of different types of solvent.[22,24,26] Refinement based on the
use of two independent mesitylene molecules converged to a
final R factor of 20%. The C70 molecule is located in the
center of the unit cell, and two mesitylene molecules occupy
each corner with a partial occupancy factor (Figure 4). There
is no particularly short contact between C70 and mesitylene.
Since these results correspond to the crystal structure of
Whether or not mesitylene and IPA were included in the
C
70 crystals was a point of keen interest, so we investigated the
crystals by gas chromatography–mass spectrometry (GC–MS)
and thermal gravimetric analysis (TGA) for the presence of
the solvents. For GC–MS analysis, C70 cubes were dissolved in
CCl4 and injected into the GC–MS instrument.[26] Two
components were separated at 1.82 and 6.91 min by GC
(see Figure S4a in the Supporting Information) and identified
as CCl4 and mesitylene, respectively, from the MS spectra (see
Figure S4b,c in the Supporting Information). Notably, the
poor solvent IPA was not detected, which implies that IPA
indeed only induces local cavities of C70/mesitylene without
direct involvement in the crystallization. The ratio of C70 to
mesitylene in a C70 cube was estimated by TGA conducted in
a nitrogen atmosphere (see the Supporting Information for
the preparation of the sample for TGA). A significant weight
decrease occurred at 808C, and the weight continued to
decrease until the temperature reached about 2308C (see
Figure S4d in the Supporting Information). The final weight
loss of 22.7% due to the removal of mesitylene indicates that
C70 as it exists at low temperature, we analyzed the C70 cubes
again at room temperature by powder X-ray diffraction
(XRD) to confirm that the crystal retains this structure. As
shown in Figure 5a, the diffraction peaks are intense and
perfectly matched to the simple cubic structure (a = 10.59 ꢀ),
in analogy to previously reported C70 microparticles.[36] This
result implies that, at room temperature, C70 cubes have
basically the same crystal structure as that at 90 K, with a
slightly increased lattice constant. The intensity increases of
the (001) and (002) peaks indicate that most C70 cubes make
preferred surface contacts to the substrate through {001}-
family planes. A transmission electron microscope (TEM)
image and selected-area electron diffraction (SAED) data
were also used to correlate each facet of the cube crystal to
the plane of the simple cubic crystal structure. A clear
electron-diffraction pattern appeared upon irradiation with
an electron beam normal to the surface of a cube crystal
(Figure 5b,c). The measured d spacing of the nearest and
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(peak shifts and peak broadening) of C70 aggregates formed
either by simple precipitation or by the use of charge-transfer
linkage molecules,[18,21] no such enormous PL enhancement
has been reported. The C70 cubes were bright red in
fluorescence microscope images (lex = 510–560 nm, lem
=
660–710 nm; Figure 6a), whereas no significant PL was
observed from a C70 powder (Figure 6b). Figure 6c,d shows
transmission optical microscope images of C70 cubes and a C70
powder, respectively.
Figure 4. Crystal structure of C70·(mesitylene)2. a) Overall view of the
unit cell (top left), and the unit cell viewed along the a (top right), b
(bottom left), and c axes (bottom right). b) Schematic representation
of the unit cell of the C70·(mesitylene)2 cube crystal. The red sphere at
the center and gray part spheres at each corner represent one C70 and
two mesitylene molecules, respectively.
Figure 5. a) Powder X-ray diffraction patterns of a C70 cube, as mea-
sured at room temperature (black line) and calculated from single-
crystal data obtained at 90 K (red line). Both spectra were matched to
the wavelength of the CuKa1 line (l=1.54056 ꢀ). b,c) TEM image of a
C70 cube and corresponding electron-diffraction pattern ([001] zone
axis).
Figure 6. a),b) Fluorescence optical microscope images of C70 cubes
and C70 powder, respectively. c,d) Transmission optical microscope
images corresponding to (a) and (b), respectively. The scale bars in
(a–d) are 10 mm. e) PL spectra of C70 cubes (red) and C70 powder
(blue). In the inset, the PL spectrum of the C70 powder has been
increased by a factor of 10 for comparison. f) Time-resolved PL spectra
measured at l=750 nm (lex =400 nm). The time constants are 510 ps
(91%) and 1.6 ns (9%) for the C70 cubes, 560 ps (85%) and 1.1 ns
(15%) for C70 in solution, and 40 ps (45%), 100 ps (50%), and 610 ps
(5%) for the C70 powder. The spectra for the C70 cubes and C70 in
solution are shifted by two orders and one order of magnitude,
respectively, to enable clear comparison.
second-nearest peaks to the center was 10.83 and 15.32 ꢀ,
respectively. These d-spacing values correspond well to (100)
and (110) planes of a simple cubic crystal structure within
2.3% error; hence, each facet of the C70 cube was confirmed
to be a {100}-family plane. This conclusion agrees well with
the results of powder XRD.
A unique characteristic of the C70 cubes is a remarkably
enhanced photoluminescence (PL). Although many studies
have revealed absorption and emission spectral changes
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More-detailed information about the PL enhancement
Experimental Section
was obtained from PL spectra and time-resolved PL (TRPL)
studies. The PL intensity of the C70 cubes was approximately
30 times higher than that of the C70 powder, even though an
excess amount of the C70 powder was examined (Figure 6e).
The PL spectrum of the C70 cubes is red-shifted slightly from
that of the solution, as observed previously for C70 crystals.[37]
The enhancement of PL seems to be induced by the high
crystallinity of the C70 cube, rather than by the influence of
mesitylene molecules included in the crystal. To confirm this
hypothesis, we prepared mesitylene-free C70 cubes by remov-
ing mesitylene molecules in an H2 atmosphere at 3008C for
3 h. The resulting crystals retained the cube geometry as well
as the PL intensity (see Figure S5 in the Supporting Informa-
tion).
The direct influence of the high crystallinity of the C70
cubes on the PL enhancement was confirmed by the TRPL
results. At 750 nm, the TRPL of the C70 cube decayed
biexponentially with time constants of 510 ps (91%) and
1.6 ns (9%; Figure 6 f, red circles). The main component can
be assigned to the S1–S0 transition of a C70 molecule, as its
time constant is close to the lifetime of the S1 state of an
individual C70 molecule in solution.[38,39] That is, interactions
between C70 molecules in the cube crystal may be negligible,
so that fluorescence quenching, which is observed frequently
in molecular aggregates, does not occur. On the other hand,
the TRPL of C70 powder shows much faster decay, which is
consistent with the much weaker fluorescence intensity of the
powder. This faster decay is possibly due to increased
nonradiative relaxation through coupling between C70 mole-
cules. Note that this interpretation is different from the
Synthesis of C70 cube crystals by precipitation: The starting C70/
mesitylene solution was prepared by mixing C70 powder (MER
Corporation, 95%) and mesitylene (Sigma–Aldrich, 98%), followed
by ultrasonication for 30 min. C70/mesitylene solutions with concen-
trations of 0.1, 0.2, and 0.4 mgmLÀ1 were used. Isopropyl alcohol
(IPA) was added to the vial containing the C70/mesitylene solution,
and the mixture was subjected to ultrasonication for the first 30 s. The
final mixture was maintained at room temperature for 24 h. The
initially dark purple color of the C70/mesitylene solution became
lighter after the addition of IPA. As precipitation occurred, the final
solution lost its original color and became transparent. It took several
hours for the precipitates to settle at the bottom of the vial. The
precipitates were transferred onto a precleaned Si or quartz substrate
for characterization.
Synthesis of large C70 cube crystals by liquid–liquid diffusion: IPA
was added slowly along the wall of a vial containing the C70/
mesitylene solution so that the two liquids could form a clear
interface. Over 5 days, both liquid layers slowly diffused into each
other and merged to form a homogeneous mixture, and large C70 cube
crystals were formed on the bottom of the vial. The crystals were
transferred onto a glass slide with a pipette and placed in the single-
crystal X-ray diffractometer by using a microneedle. Full-sphere data
sets were collected by using a Bruker APEX II QUAZAR instrument
in house and the ADSC Quantum 210 detector system in the 6B2
beamline of the Pohang Accelerator Laboratory.
Structure characterization: A JEOL JSM-7401F instrument was
used for SEM imaging. A JEOL JEM3010 instrument was used with
an acceleration voltage of 300 kV for recording TEM images and
electron-diffraction patterns. Powder X-ray diffraction data were
collected by using a synchrotron (Pohang Accelerator Laboratory,
5C2 beamline) X-ray source with
1.23956 ꢀ.
a radiation wavelength of
Received: August 12, 2010
Published online: November 9, 2010
aggregation-induced
(crystallization-induced)
emission
mainly caused by the restriction of intramolecular rota-
tions:[12–15] an effect that is absent in C70.
Keywords: cube crystals · fullerenes · photoluminescence ·
self-crystallization · solvent effects
.
In summary, C70 molecules undergo spontaneous self-
crystallization into cube crystals in a precipitation process in
solvent mixtures containing mesitylene and IPA as a good and
a poor solvent, respectively. After optimization of the
composition of the solvent and the concentration of C70, and
with the use of ultrasonication, we obtained C70 cube crystals
with sharp edges, clean surfaces, and a homogeneous size
distribution. The crystal structure has a simple cubic unit cell
with a lattice constant of 10.47 ꢀ at 90 K and 10.59 ꢀ at room
temperature; thus, the unit cell is composed of one C70 and
two mesitylene molecules. C70 cube crystals emit remarkably
increased PL as a result of the substantially increased
crystallinity of C70 together with the decreased interactions
between C70, so that optical quenching is suppressed. Our
results provide great opportunities for the fundamental
understanding of the formation of various C70 self-crystallized
architectures in simple solvent mixtures with specific combi-
nations of solvents. Moreover, new C70 architectures with
unprecedented optical properties will boost the development
of organic-molecule-based optoelectronic devices for diverse
applications.
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