Self-assembly of supramolecular fullerene ribbons via hydrogen-bonding interactions and their impact on fullerene electronic interactions and charge carrier mobility

Article abstract of DOI:10.1021/ja104750f

The anisotropy of the electronic interactions between fullerenes in crystalline solids was examined using a confocal fluorescence microscope by probing the polarization of the fluorescence emission arising from fullerene excimer-like emitting states. Crystals of C₆₀ obtained by vacuum-sublimation or from chloroform solution exhibited no or little polarization (p = 0 or 0.11, respectively), as expected from the high symmetry of the C₆₀ fcc lattice or the low degree of anisotropy induced by included solvent molecules. The use of hydrogen-bonding to supramolecularly control interfullerene electronic interactions was explored using a fullerene derivative (1) combining a solubilizing 3,4-di-tert-butylbenzene group and a barbituric acid hydrogen-bonding (H-B) moiety. The crystal structure of 1 establishes the existence of fullerene H-B tapes along which interfullerene electronic interactions are expected to be large. In agreement with this, we observe very strong polarization of the fullerene excimer-like emission (p = 0.78), indicative of a high degree of anisotropy in the fullerene interactions. The charge-carrier mobility of 1 as determined from OFET devices was found to be lower than that of C₆₀ (1.2 × 10^-4 vs 1.2 × 10^-2 cm²/s V), which is rationalized on the basis of the reduced dimensionality of 1 as a wire-like semiconductor and variations in the morphology of the device active layer revealed by AFM measurments.

Full text of DOI:10.1021/ja104750f

Published on Web 08/23/2010
Self-Assembly of Supramolecular Fullerene Ribbons via
Hydrogen-Bonding Interactions and Their Impact on Fullerene
Electronic Interactions and Charge Carrier Mobility
Cheng-Che Chu,† Guillaume Raffy,† Debdas Ray,† Andre´ Del Guerzo,†
Brice Kauffmann,‡ Guillaume Wantz,§ Lionel Hirsch,§ and Dario M. Bassani*,†
Institut des Sciences Mole´culaires, CNRS UMR 5255, UniVersite´ Bordeaux 1, 351, Cours de la
Libe´ration, 33405 Talence, France, Institut Europe´en de Chimie et Biologie, UMS 3033 CNRS,
UniVersite´ de Bordeaux, 2 rue Robert Escarpit, 33607 Pessac, France, and Laboratoire
d’Inte´gration des Mate´riaux aux Syste`mes, CNRS UMR 5218, UniVersite´ de Bordeaux, Institut
Polytechnique de Bordeaux, ENSCBP, 16 AV. Pey Berland, 33607 Pessac, France
Received June 1, 2010; E-mail: d.bassani@ism.u-bordeaux1.fr
Abstract: The anisotropy of the electronic interactions between fullerenes in crystalline solids was examined
using a confocal fluorescence microscope by probing the polarization of the fluorescence emission arising
from fullerene excimer-like emitting states. Crystals of C60 obtained by vacuum-sublimation or from chloroform
solution exhibited no or little polarization (p ) 0 or 0.11, respectively), as expected from the high symmetry
of the C60 fcc lattice or the low degree of anisotropy induced by included solvent molecules. The use of
hydrogen-bonding to supramolecularly control interfullerene electronic interactions was explored using a
fullerene derivative (1) combining a solubilizing 3,4-di-tert-butylbenzene group and a barbituric acid hydrogen-
bonding (H-B) moiety. The crystal structure of 1 establishes the existence of fullerene H-B tapes along
which interfullerene electronic interactions are expected to be large. In agreement with this, we observe
very strong polarization of the fullerene excimer-like emission (p ) 0.78), indicative of a high degree of
anisotropy in the fullerene interactions. The charge-carrier mobility of 1 as determined from OFET devices
was found to be lower than that of C60 (1.2 × 10-4 vs 1.2 × 10-2 cm2/s V), which is rationalized on the
basis of the reduced dimensionality of 1 as a wire-like semiconductor and variations in the morphology of
the device active layer revealed by AFM measurments.
investigated,3 the choice of molecular components for n-type
1. Introduction
materials has mostly focused on C60 derivatives.4
Besides being a major goal in supramolecular chemistry,
designing molecular components possessing the necessary
chemical information to self-assemble into well-defined archi-
tectures represents a major route toward devising materials with
molecular precision that can bridge the gap between top-down
and bottom-up engineering while providing improved perfor-
mance.1 Compared to other intermolecular forces, such as van
der Waals, π-stacking, or Coulombic interactions, hydrogen
bonds (H-B) are particularly attractive as they are directional
and do not possess electronic energy levels that interfere with
those in materials for organic electronics applications.2 It is no
surprise, therefore, that great effort has been expended toward
the preparation and characterization of photo- and electro-active
noncovalent assemblies based on H-B. However, whereas a
variety of electron donors (e.g., porphyrins, oligothiophenes,
oligophenylenevinylenes) possessing H-B units have been
(3) For representative examples, see: (a) Hoeben, F. J. M.; Pouderoijen,
M. J.; Schenning, A.; Meijer, E. W. Org. Biomol. Chem. 2006, 4,
4460. (b) D’Souza, F.; Venukadasula, G. M.; Yamanaka, K.; Sub-
baiyan, N. K.; Zandler, M. E.; Ito, O. Org. Biomol. Chem. 2009, 7,
1076. (c) Chen, Z. J.; Lohr, A.; Saha-Moller, C. R.; Wurthner, F. Chem.
Soc. ReV. 2009, 38, 564. (d) Kang, S. C.; Umeyama, T.; Ueda, M.;
Matano, Y.; Hotta, H.; Yoshida, K.; Isoda, S.; Shiro, M.; Imahori, H.
AdV. Mater. 2006, 18, 2549. (e) Imahori, H.; Liu, J. C.; Hotta, H.;
Kira, A.; Umeyama, T.; Matano, Y.; Li, G. F.; Ye, S.; Isosomppi,
M.; Tkachenko, N. V.; Lemmetyinen, H. J. Phys. Chem. B 2005, 109,
18465. (f) Yoon, S. M.; Hwang, I. C.; Kim, K. S.; Choi, H. C. Angew.
Chem., Int. Ed. 2009, 48, 2506. (g) Drain, C. M.; Batteas, J. D.; Flynn,
G. W.; Milic, T.; Chi, N.; Yablon, D. G.; Sommers, H. Proc. Natl.
Acad. Sci. U.S.A. 2002, 99, 6498. (h) Hu, J. S.; Guo, Y. G.; Liang,
H. P.; Wan, L. J.; Jiang, L. J. Am. Chem. Soc. 2005, 127, 17090. (i)
Hill, J. P.; Jin, W. S.; Kosaka, A.; Fukushima, T.; Ichihara, H.;
Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science
2004, 304, 1481. (j) Bayard, E.; Hamel, S.; Rochefort, A. Org.
Electron. 2006, 7, 144. (k) Yagai, S.; Monma, Y.; Kawauchi, N.;
Karatsu, T.; Kitamura, A. Org. Lett. 2007, 9, 1137. (l) Hoeben,
F. J. M.; Zhang, J.; Lee, C. C.; Pouderoijen, M. J.; Wolffs, M.;
Wurthner, F.; Schenning, A.; Meijer, E. W.; De Feyter, S. Chem.sEur.
J. 2008, 14, 8579. (m) Fang, F. C.; Chu, C. C.; Huang, C. H.; Raffy,
G.; Del Guerzo, A.; Wong, K. T.; Bassani, D. M. Chem. Commun.
2008, 6369.
† Institut des Sciences Mole´culaires.
‡ Institut Europe´en de Chimie et Biologie.
§ Laboratoire d’Inte´gration des Mate´riaux aux Syste`mes.
(1) (a) Grozema, F. C.; Siebbeles, L. D. A. Int. ReV. Phys. Chem. 2008,
27, 87. (b) Chu, C. C.; Bassani, D. M. Photochem. Photobiol. Sci.
2008, 7, 521.
(2) (a) El-Ghayoury, A.; Schenning, A.; van Hal, P. A.; van Duren, J. K. J.;
Janssen, R. A. J.; Meijer, E. W. Angew. Chem., Int. Ed. 2001, 40,
3660. (b) Huang, C. H.; Bassani, D. M. Eur. J. Org. Chem. 2005,
4041.
(4) Mallik, A. B.; Locklin, J.; Mannsfeld, S. C. B.; Reese, C.; Roberts,
M. S.; Senatore, M. L.; Zi, H.; Bao, Z. In Organic Field-Effect
Transistors; Bao, Z., Locklin, J., Eds.; CRC Press: Boca Raton, FL,
2007.
9
10.1021/ja104750f  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 12717–12723 12717

A R T I C L E S
Chu et al.
solution was degassed by three cycles of freeze-pump-thaw) was
added dropwise. After refluxing for 2 days, the solvent was removed
under reduced pressure. The crude product was dry-loaded onto a
silica column and washed with toluene to remove C60 prior to
elution with a mixture of 5% ethyl acetate in toluene to afford the
desired product. The solvent was dried under vacuum to afford a
brown powder product (0.32 g, yield ) 37%). MALDI-MS m/z
(%) 1063.95 ([M + H]+, 83), 1085.98 ([M + Na]+, 100), 1101.93
([M + K]+, 30), 1314.08 ([M + DCTB]+, 44), 1337.10 ([M +
DCTB + Na]+, 20).
Since initial reports of fullerene adducts containing H-B
moieties,5 the number of H-B substituted-fullerene derivatives
has increased steadily6,7 Our initial efforts led to a fullerene-
barbituric acid adduct that opened up access to a variety of well-
defined supramolecular C60 architectures,8 based on the melamine-
cyanuric acid motif, that display enhanced electronic properties
of interest in organic photovoltaics9 and for the study of through-
space electronic interactions.10 Despite the strong interest in
developing H-B supramolecular fullerene-based materials, there
is still no direct evidence as to whether the self-assembly
properties of such materials are indeed controlled by the H-B
interactions rather than by the strong aggregation properties of
fullerene, dominated by entropy loss due to solvation.11
Furthermore, it is not yet clear whether the presence of H-B
units can indeed direct interfullerene electronic interactions, nor
to what degree they are compatible with charge transport in
n-type fullerene-based semiconductors. Herein, we provide
experimental evidence for the self-assembly of a H-B fullerene
derivative into a one-dimensional tape and its effect on
determining the directionality of through-space interfullerene
electronic interactions probed using the polarization of the
fluorescence emission from fullerene excimer-like emitting states
in single crystals. Compared to fullerene nanowires obtained
by Langmuir-Blodgett deposition of organogellators,12 the
crystalline materials show very high molecular ordering.
Furthermore, we establish that the presence of H-B motifs is
compatible with n-type charge transport in OFET devices.
2.2. Confocal Fluorescence Microscopy. Measurements were
performed on a Picoquant Microtime 200 inverted confocal
microscope, using a PicoHarp 300 multichannel single photon
counter and two MPD SPADs. The excitation originated from a
frequency doubled Ti-Sa laser (Coherent) tuned at 385 nm with
picosecond pulses (4-6 ps) at 4.76 MHz repetition rate. The laser
beam was injected by 90° reflection on a 80%T/20%R spectrally
flat beam splitter into the microscope objective (100× UPLSAPO,
N.A. 1.4). Linearly polarized excitation light was obtained with a
Babinet Soleil compensator. The emission was collected by
transmission through the same beam splitter and a suitable
interferential filter before being focused on a 50 µm pinhole. Parallel
and perpendicular components of the emitted light were split using
a polarizing beam splitter and two Glan-Thompson polarizers. The
instrumental G-factor is measured on the pertinent spectral range
(650-900 nm) using emission from the crystals and the relation G
) (IVVIHV/IVHIHH)
1/2, where the indexes H and V refer to the
horizontal and vertical position of the excitation (first index) or
emission (second index) polarizers, rotating the crystal by 90°
around the optical axis when the excitation polarization is changed
from V to H. After the pinhole, light can be diverted into an Andor
SR300i spectrometer equipped with a Newton EM-CCD for
spectroscopy measurements.
2. Experimental Section
2.1. Synthesis of 1. A chlorobenzene solution (200 mL) contain-
ing C60 (1.223 g, 1.62 mmol) was degassed by three freeze-pump-
thaw cycles. The solution was brought to reflux, and azido di-tert-
butylbenzyl barbiturate (276 mg, 0.81 mmol) in chlorobenzene (this
2.3. Crystal Structure Determination. Single crystals of 1 were
grown by the slow diffusion of chloroform into an o-dichloroben-
zene solution of 1 and belong to the triclinic space group with cell
parameters C79H25N3O3 ·CHCl3; MW )1183.4, a ) 9.952(2) Å, b
) 15.876(3) Å, c ) 15.877(3) Å, R ) 77.41(3)°, ꢀ ) 83.69(3)°, γ
) 88.42(3)°, V ) 2433(8) Å3, Z ) 2 and Fcalc ) 1.615 g Å cm-3
.
(5) (a) Gonzalez, J. J.; Gonzalez, S.; Priego, E. M.; Luo, C. P.; Guldi,
D. M.; de Mendoza, J.; Martin, N. Chem. Commun. 2001, 163. (b)
Rispens, M. T.; Sanchez, L.; Knol, J.; Hummelen, J. C. Chem.
Commun. 2001, 161. (c) Diederich, F.; Echegoyen, L.; Gomez-Lopez,
M.; Kessinger, R.; Stoddart, J. F. J. Chem. Soc., Perkin Trans. 2 1999,
1577.
Data were collected at 100(2) K on Proxima1 beamline at a
wavelength λ ) 0.8550 Å, with 2θmax ) 26.74° using ꢁ scans.
21128 reflections, 5465 independent, R(int) ) 0.0416. Data were
processed using the XDS package.13 The positions of non-H atoms
were determined by the program SHELXD, and the position of
the H atoms were deduced from coordinates of the non-H atoms
and confirmed by Fourier synthesis. H atoms were included for
structure factor calculations but not refined. The structure was
refined using SHELXL.14 Refinement statistics: goodness-of-fit on
F2 ) 1.657, final R indices [I > 2σ(I)], R1 ) 0.1122, wR2 ) 0.3345;
R indices (all data): R1 ) 0.1250, wR2 ) 0.3490. CCDC 742195
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.
(6) (a) Shi, Z. Q.; Li, Y. L.; Gong, H. F.; Liu, M. H.; Xiao, S. X.; Liu,
H. B.; Li, H. M.; Xiao, S. Q.; Zhu, D. B. Org. Lett. 2002, 4, 1179. (b)
Zhuang, J. P.; Zhou, W. D.; Li, X. F.; Li, Y. J.; Wang, N.; He, X. R.;
Liu, H. B.; Li, Y. L.; Jiang, L.; Huang, C. S.; Cui, S.; Wang, S.; Zhu,
D. B. Tetrahedron 2005, 61, 8686. (c) Wessendorf, F.; Gnichwitz,
J. F.; Sarova, G. H.; Hager, K.; Hartnagel, U.; Guldi, D. M.; Hirsch,
A. J. Am. Chem. Soc. 2007, 129, 16057. (d) Figueira-Duarte, T. M.;
Gegout, A.; Nierengarten, J. F. Chem. Commun. 2007, 109. (e)
D’Souza, F.; Gadde, S.; Islam, D. M. S.; Pang, S. C.; Schumacher,
A. L.; Zandler, M. E.; Horie, R.; Araki, Y.; Ito, O. Chem. Commun.
2007, 480. (f) Sanchez, L.; Rispens, M. T.; Hummelen, J. C. Angew.
Chem., Int. Ed. 2002, 41, 838. (g) Eckert, J. F.; Nicoud, J. F.;
Nierengarten, J. F.; Liu, S. G.; Echegoyen, L.; Barigelletti, F.;
Armaroli, N.; Ouali, L.; Krasnikov, V.; Hadziioannou, G. J. Am. Chem.
Soc. 2000, 122, 7467. (h) Sary, N.; Richard, F.; Brochon, C.; Leclerc,
N.; Leveque, P.; Audinot, J. N.; Berson, S.; Heiser, T.; Hadziioannou,
G.; Mezzenga, R. AdV. Mater. 2010, 22, 763.
2.4. Preparation of OFET Devices. The SiO2/Si substrates were
cleaned in acetone and isopropyl alcohol for 30 min each and
transferred to a Jelight Company Inc. UVO Cleaner 42-220 for
surface cleaning for 5 min. C60 or 1 was thermally evaporated under
vacuum (ca. 1 × 10-6 mbar) onto the cleaned SiO2/Si substrates
heated at 44 °C for C60 and 57 °C for 1 during evaporation on an
Edwards Auto 306 evaporator. Film thicknesses were controlled
with a Temcor Alpha Step IQ tactile profilometer. The coated SiO2/
Si substrates were then transferred to an evaporator in a glovebox,
and calcium electrodes were deposited by thermal evaporation under
vacuum (1 × 10-6 mbar) through a shadow mask to define a round
device of 3 mm in diameter possessing a central gap of 25 µm in
width. The characterization was carried out using a Keithley 4200
(7) For a review of supramolecular fullerene chemistry, see: Sanchez, L.;
Martin, N.; Guldi, D. M. Angew. Chem., Int. Ed. 2005, 44, 5374.
(8) McClenaghan, N. D.; Absalon, C.; Bassani, D. M. J. Am. Chem. Soc.
2003, 125, 13004.
(9) (a) Huang, C. H.; McClenaghan, N. D.; Kuhn, A.; Hofstraat, J. W.;
Bassani, D. M. Org. Lett. 2005, 7, 3409. (b) Huang, C. H.;
McClenaghan, N. D.; Kuhn, A.; Bravic, G.; Bassani, D. M. Tetrahe-
dron 2006, 62, 2050.
(10) McClenaghan, N. D.; Grote, Z.; Darriet, K.; Zimine, M.; Williams,
R. M.; De Cola, L.; Bassani, D. M. Org. Lett. 2005, 7, 807.
(11) Korobov, M. V.; Smith, A. L. In Fullerenes: Chemistry, Physics, and
Technology; Kadish, K. M., Ruoff, R. S., Eds.; Wiley: London, 2000.
(12) Tsunashima, R.; Noro, S. I.; Akutagawa, T.; Nakamura, T.; Kawakami,
H.; Toma, K. Chem.sEur. J. 2008, 14, 8169.
(13) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
(14) Kabsch, W. J. Appl. Crystallogr. 1993, 26, 795.
9
12718 J. AM. CHEM. SOC. VOL. 132, NO. 36, 2010

Supramolecular Fullerene Ribbons
A R T I C L E S
Scheme 1. Synthesis of 1a
a Reagents and conditions: (i) barbituric acid, Pt/C, H2, RT, 85%; (ii) NBS, DMF, then NaN3, 92%; (iii) C60, PhCl, reflux, 37%.
semiconductor analyzer system in the glovebox, where [H2O] and
[O2] < 0.1 ppm.
3. Results and Discussion
The synthesis of 1 is based on the addition of the corre-
sponding azide onto C60,15 as outlined in Scheme 1. Briefly,
reductive alkylation of barbituric acid with 3,5-bis-tert-butyl-
benzaldehyde proceeds smoothly to afford a barbituric acid
intermediate that is freely soluble even in nonpolar hydrocarbon
solvents such as pentane. Bromination in the C-5 position of
the barbiturate, followed by substitution with sodium azide in
DMF, gives the required azido precursor. Addition to C60 in
refluxing chlorobenzene followed by purification by column
chromatography (toluene/ethyl acetate 95:5) gives 1 in 37%
isolated yield. Thanks to the presence of the di-tert-butylbenzene
solubilizing group, the solubility of 1 attains ca. 8 mg/mL in
neat o-dichlorobenzene (o-DCB), and 30 mg/mL in o-DCB/
DMSO (5%). Thermal gravimetric analysis of solid samples of
1 indicates that it is thermally stable up to ca. 100 °C.
Single crystals suitable for X-ray diffraction analysis were
Figure 1. (Top) Solid-state crystal structure of 1 and crystal packing viewed
along the a axis (hydrogens omitted for clarity). (Bottom) Crystal packing
viewed along the c axis highlighting the close van der Waals contact between
fullerenes in the H-B ribbon (di-tert-butylbenzyl groups omitted for clarity).
grown by slow diffusion of chloroform into a solution of 1 in
o-DCB (10% DMSO). The small platelets that formed proved
too thin for measurements on a conventional diffractometer, and
the data was collected using synchrotron radiation on the
Proxima 1 Beamline at SOLEIL. Two molecules of 1 crystallize
in a triclinic (P-1) unit cell along with two molecules of
chloroform, as shown in Figure 1. The barbituric acid is located
in the plane of the fullerene cage, whereas the di-tert-butylbenzyl
residue is pseudoaxial with respect to the C-N bond linking
the barbiturate to the C60. Interestingly, the relative orientation
and distance of the barbituric acid residues in the crystal lattice
of 1 are similar to those observed for one of the polymorphs
(C2/c) of 5,5′-diethylbarbituric acid (DEB).16 In both cases, the
formation of infinite linear H-B tapes is observed because of
the presence of four intermolecular H-B between barbiturates.
The distance between the carbonyl oxygen and the nitrogen of
a vicinal barbiturate is 2.77 Å for 1 (Figure 1) versus 2.87 Å
for DEB,16 and this similarity confirms previous results8
suggesting that the intermolecular spacing in barbituric acid and
cyanuric acid-melamine architectures is well adapted to accom-
modating the rigidly tethered fullerene unit.
where the H-B ribbon draws the fullerenes into van der Waals
contact (Figure 1). The H-B ribbon assembly induced by the
barbituric acid residues thus leads to the formation of pairs of
parallel rows of fullerenes along which charge transport is
expected to be particularly efficient. In contrast, charge transport
along the b or c axis would require tunneling through a barrier
caused by the separation between adjacent sites (ca. 5.9 Å
assuming a van der Waals radius of 10 Å for C60).
The fluorescence emission from single crystals of fullerene
C60 is bathochromically shifted with respect to what is observed
in fluid solution and has been attributed to the formation of
excimer-like electronically excited states.18 Although C60 does
not form excimers in solution, the close packing of the fullerene
molecules in the solid is responsible for the augmented electronic
coupling in the excited state. Halas and co-workers further
showed that the structural features present in the solid-state
emission spectra of C60 crystals could be accurately modeled
using excimer theory.18b Fluorescence emission from crystalline
solids is often polarized as a result of the alignment of the
transition dipole moments. In contrast to locally excited C60,
The distribution of fullerene cages in crystals of 1 is
anisotropic, contrary to what is observed for pristine C60 that
crystallizes in a highly symmetric fcc lattice in which the
fullerenes are separated by a center-to-center distance of 10.0
Å.17 In the unit cell of 1, this distance is 15.88 Å along the b
and c axis, but only 9.95 Å along the direction of the a axis,
(17) (a) Liu, S. Z.; Lu, Y. J.; Kappes, M. M.; Ibers, J. A. Science 1991,
254, 408. (b) Burgi, H. B.; Blanc, E.; Schwarzenbach, D.; Liu, S. Z.;
Lu, Y. J.; Kappes, M. M.; Ibers, J. A. Angew. Chem., Int. Ed. Engl.
1992, 31, 640.
(15) Prato, M.; Li, Q. C.; Wudl, F.; Lucchini, V. J. Am. Chem. Soc. 2002,
115, 1148.
(18) (a) Sun, Y. P.; Wang, P.; Hamilton, N. B. J. Am. Chem. Soc. 1993,
115, 6378. (b) Pippenger, P. M.; Averitt, R. D.; Papanyan, V. O.;
Nordlander, P.; Halas, N. J. J. Phys. Chem. 1996, 100, 2854.
(16) Craven, B. M.; Vizzini, E. A.; Rodrigues, M. M. Acta Crystallogr.,
Sect. B: Struct. Sci. 1969, 25, 1978.
9
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A R T I C L E S
Chu et al.
Figure 2. Comparison of the normalized fluorescence emission (λex ) 385
nm) of a single crystal of fullerene derivative 1 (-) and from a single crystal
of fullerene C60 obtained by either vacuum sublimation (- - -) or from
chloroform solution (- · - ). The emission of 1 in o-DCB solution is also
shown (· · ·, λex ) 365 nm).
whose spherical symmetry excludes polarization of the fluo-
rescence emission, the excimer-like emission from crystalline
C60 can be polarized because of the delocalization of the
excitation energy over two (or more) fullerenes. For example,
excimer (or dimer) emission from π-stacked aromatic chro-
mophores such as pyrene19 and perylene20 is polarized along
one of the molecular axes. However, because C60 crystallizes
in a highly symmetric fcc lattice, we may expect excimer-like
emission from C60 single crystals to be averaged and not
polarized because each locally excited fullerene can interact with
one of twelve neighboring fullerenes isotropically located within
van der Waals contact. We experimentally verified this by
measuring the fluorescence polarization from fullerene single
crystals grown by vacuum sublimation using a confocal
fluorescence microscope. The emission is very similar to that
previously reported for fullerene single crystals and exhibited
no detectable polarization.
Figure 3. (Top) Confocal fluorescence polarization image of a crystal of
1 (λex ) 385 nm) color-coded according to the polarization of the emission.
Double-arrow indicates direction of polarization of the excitation beam,
scale bar is 10 µm. (Bottom) Polar plot of the polarization of the fluorescence
emission with respect to the crystal orientation.
depolarization effects.21 The maximum value of p ) 0.78
obtained for a particular crystal orientation is very large yet
underestimated because of the depolarization effects of the large
numerical aperture objective. This clearly indicates that the
excimer-like emission is highly oriented along a specific
direction of the molecular crystal. Because electronic interactions
in the solid are controlled by the through-space electronic
coupling between adjacent fullerene cages, we deduce that the
latter are likewise preferentially oriented within the crystal.
Examination of the crystal structure of 1 shows that each
fullerene moiety is in close contact with three other fullerenes,
two of which are located on each side along the molecular H-B
ribbon while the third belongs to a proximal in-plane ribbon.
These pairs of adjacent fullerenes are very likely the origin of
the observed orientation of the polarized emission as they are
the closest in space. Further investigation showed that the
polarization of the emission is independent of the orientation
of the linearly polarized excitation beam relative to the crystal
and that it shows a symmetric distribution of positive as well
as negative p values. Such behavior suggests that the crystal’s
absorption is isotropic, which may be due to the high symmetry
The fluorescence emission from single crystals of 1 is shown
in Figure 2, where it is compared to the emission in fluid solution
and to the emission from single crystals of C60 grown by vacuum
sublimation. It can be seen that the emission of 1 in the
crystalline solid is considerably shifted with respect to emission
from fluid solution (by 25 nm) and resembles the emission from
the single crystals of pure C60. We therefore attribute the
emission of 1 in the crystalline phase to arise from the formation
of excimer-like emitting states, analogously to what is observed
for C60 crystals. Fluorescence polarization from single crystals
of 1 was then used to investigate whether the anisotropy induced
by the H-B motifs is reflected in the interfullerene electronic
interactions as compared to pristine C60. In contrast to single
crystals of pristine C60, which exhibited zero polarization, the
emission from single crystals of 1 is very uniformly and highly
polarized, as shown in Figure 3. The polarization (p) is defined
by eq 1:
I|| - I⊥
I|| + I⊥
p )
(1)
(19) (a) Chaudhuri, M. K.; Ganguly, S. C. J. Phys. C: Solid State Phys.
1970, 3, 1791. (b) Hochstrasser, R. M.; Malliaris, A. J. Chem. Phys.
1965, 42, 2243. (c) See also: Birks, J. B. Photophysics of Aromatic
Molecules; Wiley: New York, 1970.
where I|| and I⊥ are the fluorescence emission intensity compo-
nents oriented parallel and perpendicular to the polarization of
the excitation beam, respectively, corrected for instrumental
response except for the large numerical aperture objective
(20) Fergusson, J. J. Chem. Phys. 1966, 44, 2677.
(21) (a) Bahlmann, K.; Hell, S. W. Appl. Phys. Lett. 2000, 77, 621. (b)
Ha, T.; Laurence, T. A.; Chemla, D. S.; Weiss, S. J. Phys. Chem. B
1999, 103, 6839.
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Supramolecular Fullerene Ribbons
A R T I C L E S
(see Figure 2). The polarization of the emission from single
crystals of 1 and C60 ·CHCl3 are compared in Figure 4, where
it can be seen that the maximum polarization induced by the
solvent molecules alone is much lower (p ) 0.11 vs. 0.78).
This is indicative of a much weaker anisotropy in the inter-
fullerene electronic interactions induced by the solvent mol-
ecules alone compared to the anisotropy induced by the H-B
motifs. In line with this, we observe the fluorescence emission
from C60 ·CHCl3 single crystals to be only weakly polarized.
Charge carrier mobility in molecular single crystals has been
found to be strongly correlated to the through-space electronic
coupling between molecules, and is highest along the direction
of the largest intermolecular orbital overlap integral.24 In crystals
of 1, this would correspond to the direction of the H-B ribbon,
along the a axis. However, although self-assembly provides a
convenient route to long-range ordering in molecule-based
materials, the use of ancillary fragments to induce self-
organization must not interfere with the charge injection and
transport processes. To date, this critical point has only been
addressed for a limited number of supramolecular interactions.25
To test whether the H-B interactions in 1 are compatible with
the fabrication of n-type electronic devices, we investigated the
charge-carrier mobility in organic field-effect transistors (OFETs)
prepared using 1 and compared the results with identical devices
prepared using C60. Based on the anisotropic interfullerene
electronic interactions revealed by the fluorescence polarization
experiments, we would expect a lower value of the charge carrier
mobility in the case of devices prepared from 1 with respect to
C60, which possesses isotropic electronic interactions that render
it a good bulk semiconductor. The devices were prepared by
thermal evaporation under secondary vacuum of C60 (device
A) or 1 (device B) onto a heated SiO2/Si (n+-doped) substrate
at 44 and 57 °C respectively. Calcium metal electrodes were
then evaporated through a shadow mask to produce contacts
that were 3 mm in width (W) and spaced 25 µm apart (L). Care
was taken to maintain the evaporator crucible and substrate
temperatures below the decomposition temperature of 1 deter-
mined from TGA measurements. Additionally, IR spectroscopy
confirmed that the material deposited onto the substrate in device
B possessed a spectral signature identical to that of 1.
Figure 4. (Top) Comparison of polarization (p) as a function of angle of
rotation (with respect to long axis of crystal) for 1 (b) and C60 ·CHCl3 (O).
Solid lines represent best fit through the points according to y ) A sin(x - x0)
+ c. (Bottom) Confocal fluorescence polarization image of a crystal of
C60 ·CHCl3 color-coded according to the polarization of the emission (scale
bar )10 µm, λex ) 385 nm, polarization of excitation beam is horizontal with
respect to image). Inset: crystal packing of C60 cages in the molecular crystal
of C60 ·CHCl3 (solvent molecules are disordered and omitted for clarity).
The I/V curves at different gate voltages for devices A and
B are shown in Figure 5. In the saturation region of a plot of Ids
versus drain-source voltage (Vds), Ids follows eq 2:26
of the fullerene chromophore and to rapid energy transfer of
the initial higher energy molecular excitation to the lowest
energy excimer-like emitting state.
The inclusion of solvent molecules can also lead to fullerene
crystals in which the separation of the fullerenes is anisotropic,
as is also the case for C60-PCBM crystallized from solution.22
Because single crystals of 1 show the inclusion of chloroform,
we proceeded to investigate the effect of cocrystallized solvent
on the fluorescence polarization in the solid state. To this end,
crystals were grown by slow diffusion of chloroform into a
toluene solution of C60. The crystals thus obtained possessed
the same crystal structure as that previously determined by
Solovyov et al. from powder diffraction studies.23 They are
composed of hexagonally packed C60 planes separated by
interstitial solvent molecules (space group P6/mmm; see inset
in Figure 4). The fluorescence emission from these crystals also
resembles that of the excimer-like emission observed from
crystals of 1 or crystals of C60 prepared by vacuum sublimation
WC
Ids
)
iµ(Vgs - Vt)2
(2)
2L
where W and L represent the channel width and length between
source and drain electrodes, respectively, Vt is the threshold
voltage, Ci stands for the capacitance per unit surface of the
insulating layer (SiO2 200 nm, Ci ) 17.24 nF/cm2), and µ is
the charge mobility. From the linear portion of a plot of the
square root of the drain-source current (Ids1/2) versus source-
gate voltage (Vgs), the charge mobility and threshold voltage
can be deduced. In the case of device A, a value of µ ) 1.2 ×
10-2 cm2/s V and Vt ) 31 V is found, which are comparable to
or somewhat lower than previously reported values of electron
mobility in C60 OFET devices.27 As expected, the charge
mobility devices prepared from 1 (device B) is ca. 2 orders of
magnitude lower, with µ ) 1.2 × 10-4 cm2/s V. The lower
(22) Rispens, M. T.; Meetsma, A.; Rittberger, R.; Brabec, C. J.; Sariciftci,
N. S.; Hummelen, J. C. Chem. Commun. 2003, 2116.
(23) Solovyov, L. A.; Bulina, N. V.; Churilov, G. N. Russ. Chem. Bull.
2001, 50, 78.
(24) Podzorov, V. In Organic Field-Effect Transistors; Bao, Z., Locklin,
J., Eds.; CRC Press:Boca Raton, FL, 2007.
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A R T I C L E S
Chu et al.
1/2
Figure 5. I/V characterization of OFET device B prepared using 2. (A) Ids/Vds at different Vgs. (B) Corresponding plot of Ids at saturation versus Vgs for
1 (device B, solid line) and for C60 (device A, dashed line).
threshold voltage, Vt ≈ 3 V,28 suggests better matching of the
energy level of the work function of the Ca electrode (2.87 eV)
with the electron affinity of 1 versus C60, which is expected to
facilitate charge injection.29
It has been previously shown that charge mobility in OFET
devices is strongly influenced by grain size,30 and this may also
contribute to the difference in µ observed between the two
devices. For example, in pentacene OFETs, which possess a
similar granular morphology as devices A and B, the charge
carrier mobility is found to decrease rapidly with decreasing
grain size, especially for grain sizes <2 µm.31 Di Carlo et al.
modeled the device properties and proposed that the existence
of barriers between grains is mainly responsible for the decrease
in mobility in devices with smaller grains.32
The difference in the surface morphology between devices
A and B was investigated by AFM (Figure 6). In both cases,
the surface is granular, with grains averaging ca. 80 and 40 nm
in device A and B, respectively. The smaller grain size in device
B compared to A is somewhat surprising, considering that it
was prepared at a slightly higher substrate temperature (57 vs
Figure 6. AFM topography (left) and phase (right) images of devices A
and B prepared by thermal evaporation of C60 or 1, respectively. Images
are 1 µm × 1 µm.
44 °C) and that higher substrate temperatures favor the formation
of larger grains.33 During the thermal evaporation process,
individual molecules are deposited on the substrate and dissipate
(25) Fullerene derivatives possessing H-B substituents have been used in
part of their excess thermal energy as translation motion on the
surface. Heating the substrate can favor this mobility, leading
to the growth of larger domains. The smaller grains obtained
from the thermal depostition of 1 may therefore indicate that it
possesses a lower surface mobility compared to C60, which may
be due to H-B interactions between molecules on the surface.
Since the thickness of the active layer in the devices (40-100
nm) is of the same order of magnitude as the size of the grains
observed by AFM, it is likely that the granular morphology
extends into the bulk of the active layer, reaching the SiO2/
semiconductor interface.
solution-based devices. For examples, see refs 6e, 9 and Liu, Y.; Xiao,
S.; Li, H.; Li, Y.; Liu, H.; Lu, F.; Zhuang, J.; Zhu, D. J. Phys. Chem.
B 2004, 108, 6256. For an example of a H-B fullerene-porphyrin
photoelectrochemical device, see: Kira, A.; Tanaka, M.; Umeyama,
T.; Matano, Y.; Yoshimoto, N.; Zhang, Y.; Ye, S.; Lehtivuori, H.;
Tkachenko, N. V.; Lemmetyinen, H.; Imahori, H. J. Phys Chem. C
2007, 111, 13618. Wessendorf, F.; Gnichwitz, J. F.; Sarova, G. H.;
Hager, K.; Hartnagel, U.; Guldi, D. M.; Hirsch, A. J. Am. Chem. Soc.
2007, 129, 16057. An OFET device based on a fullerene-amide
dendron has been reported by Kusai:Kusai, H.; Nagano, T.; Imai, K.;
Kubozono, Y.; Sako, Y.; Takaguchi, Y.; Fujiwara, A.; Akima, N.;
Iwasa, Y.; Hino, S. Appl. Phys. Lett. 2006, 88, 173509.
(26) Horowitz, G. AdV. Mater. 1998, 10, 365.
(27) Haddon, R. C.; Perel, A. S.; Morris, R. C.; Palstra, T. T. M.; Hebard,
A. F.; Fleming, R. M. Appl. Phys. Lett. 1995, 67, 121.
(28) The value of Vt is difficult to determine with precision because of the
low charge carrier mobility of 1.
4. Conclusion
(29) The electron affinity of 1 is expected to be ca. 0.2 eV higher than that
of C60 on the basis of the difference of the onset of their reduction
potentials.
(30) Nakamura, M.; Ohguri, H.; Goto, N.; Tomii, H.; Xu, M. S.; Miyamoto,
T.; Matsubara, R.; Ohashi, N.; Sakai, M.; Kudo, K. Appl. Phys. A:
Mater. Sci. Process. 2009, 95, 73.
We have successfully developed a fullerene derivative allying
the presence of multiple H-B sites and solubility in organic
media. Its solid-state crystal structure bears great similarity to
that of the barbituric acid H-B molecular recognition unit,
indicating that it is the latter that directs association in the solid
as opposed to aggregation of the C60 moieties. The barbituric
acid residues direct the self-assembly of 1 into infinite H-B tapes
oriented along one of the crystal axis, which then stack along
the b-c direction. One may therefore expect strong anisotropy
in the electrical conductivity of the crystals of 1, which would
(31) (a) Chen, J. H.; Tee, C. K.; Shtein, M.; Anthony, J.; Martin, D. C.
J. Appl. Phys. 2008, 103, 114513. (b) Cho, S. M.; Han, S. H.; Kim,
J. H.; Janga, J.; Oh, M. H. Appl. Phys. Lett. 2006, 88, 071106.
(32) Di Carlo, A.; Piacenza, F.; Bolognesi, A.; Stadlober, B.; Maresch, H.
Appl. Phys. Lett. 2005, 86.
(33) Pratontep, S.; Nu¨esch, F.; Zuppiroli, L.; Brinkmann, M. Phys. ReV.
B: Condens. Matter 2005, 72, 085211.
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12722 J. AM. CHEM. SOC. VOL. 132, NO. 36, 2010

Supramolecular Fullerene Ribbons
A R T I C L E S
also be reflected in lowered bulk charge carrier mobility with
respect to C60 if nanocrystalline domains are formed during the
deposition process. The use of H-B interactions therefore
complements other approaches to obtaining fullerene nanostruc-
tures of controlled dimensionality that rely on nondirective
hydrophobic forces during deposition from fluid solutions.12,34,35
Fluorescence polarization of fullerene emission in the solid was
found to be a useful technique to characterize anisotropy in the
electronic interactions between adjacent fullerenes that may find
further applicability. The strongly polarized fluorescence orig-
inating from fullerene excimer-like emitting states in single
crystals of 1 is in agreement with the ordered orientation of the
fullerenes in the solid. Concerning the use of H-B fullerenes
for the fabrication of electronic devices, we find that the bulk
charge carrier mobility in OFET devices prepared using 1 is
ca. 2 orders of magnitude lower than that measured for C60.
We attribute this to the lower dimensionality of 1 as a conductor,
although variations in grain size in the active layer of the OFET
devices may also contribute. This indicates that, despite the
greater sensitivity of n-type versus p-type organic semiconduc-
tors, the presence of imide H-B units is not deleterious to charge
transport. The fabrication of extended linear fullerene polymers
through H-B self-assembly is especially interesting for the
construction of molecular-level devices in which the molecular
units are designed to self-organize into the principal components,
e.g., wires, junctions, etc. Investigations of the formation and
electronic properties of such nanowires on semiconducting and
insulating surfaces are currently in progress.
Acknowledgment. We warmly thank Dr. Pierre Legrand at
SOLEIL for beamtime and his help during data collection on
PROXIMA1 Beamline and Mrs. O. Babot for the TGA measure-
ments. Financial support for this work was provided by the
Advanced Materials in Aquitaine foundation, the Agence Nationale
de la Recherche (ANR-08-BLAN-016101), and the Re´gion Aquitaine.
Supporting Information Available: General methods and full
experimental details for the synthesis of 1, fluorescence lifetime
image of a single crystal of C60 obtained by vacuum-sublimation
and histogram of emission polarization, I/V characterization of
device A, thermal gravimetric analysis, IR spectra, and crystal
structure of 1 in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
(34) Piot, L.; Silly, F.; Tortech, L.; Nicolas, Y.; Blanchard, P.; Roncali, J.;
Fichou, D. J. Am. Chem. Soc. 2009, 131, 12864.
(35) Geng, J. F.; Zhou, W. Z.; Skelton, P.; Yue, W. B.; Kinloch, I. A.;
Windle, A. H.; Johnson, B. F. G. J. Am. Chem. Soc. 2008, 130, 2527.
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