A fluorine-substituted hexakisdecyloxy-hexa-peri-hexabenzocoronene

Article abstract of DOI:10.1021/ol051972k

(Chemical Equation Presented) The first fluorine-substituted hexabenzocoronene has been synthesized and is much more readily reduced than its nonfluorinated analogues. Thin films of this material show columnar order at room temperature, and the charge-car

Full text of DOI:10.1021/ol051972k

ORGANIC
LETTERS
2005
Vol. 7, No. 22
5019-5022
A Fluorine-Substituted Hexakisdecyloxy-
hexa-peri-hexabenzocoronene
Qing Zhang,† Paulette Prins,‡ Simon C. Jones,† Stephen Barlow,†
Takeshi Kondo,† Zesheng An,† Laurens D. A. Siebbeles,‡ and Seth R. Marder*,†
Center for Organic Photonics and Electronics and School of Chemistry and
Biochemistry, Georgia Institute of Technology, Atlanta 30332-0400, and
Opto-Electronic Materials Section, DelftChemTech, Delft UniVersity of Technology,
Mekelweg 15, 2629 JB Delft, The Netherlands
seth.marder@chemistry.gatech.edu
Received August 15, 2005
ABSTRACT
The first fluorine-substituted hexabenzocoronene has been synthesized and is much more readily reduced than its nonfluorinated analogues.
Thin films of this material show columnar order at room temperature, and the charge-carrier mobility measured by the pulse-radiolysis time-
resolved microwave conductivity technique is comparable to that of other hexabenzocoronene materials.
Hexa-peri-hexabenzocoronene (HBC) and its derivatives,
often called “superbenzenes”, can be regarded as graphene
fragments consisting of 13 fused benzene rings.1a These
compounds often exhibit strong π-π stacking interactions
that promote self-assembly in solution and in films and have
attracted increasing attention because of their electronic and
optoelectronic properties. In measurements performed by van
de Craats and co-workers, the sum of the hole and electron
mobilities was determined to be as high as ca. 1 cm2 V-1
s-1 in solid phases of certain hexakisalkyl derivatives by
pulse-radiolysis time-resolved microwave conductivity (PR-
TRMC).1c,d There have been many studies on the supramo-
lecularself-assemblingpropertiesofthisfamilyofcompounds.2a-d
It has been shown that substituted HBCs can be processed
from solution to form films with uniaxially ordered columnar
stacks, which lie parallel to the substrate.2b,c Aida reported
that an amphiphilic HBC self-assembles to form graphite-
like nanotubes.2d
Mu¨llen’s group and others have functionalized HBC with
extended conjugated structures and alkyl groups on the
periphery.3a-c Alkyl- and aryl-substituted HBCs usually have
very high isotropic clearing points, in some cases higher than
420 °C. To lower the clearing point of HBC compounds,
thereby potentially simplifying processing of the materials
into device structures, HBCs with multiply branched sub-
stituents have been synthesized.3d Recently, Draper reported
† Georgia Institute of Technology.
(2) (a) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.;
Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119 (b) van de Craats,
A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson, M.; Mu¨llen, K.;
Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. AdV. Mater. 2003, 15, 495.
(c) Bunk, O.; Nielsen, M. M.; Solling, T. I.; Van de Craats, A. M,
Stutzmann, N. J. Am. Chem. Soc. 2003, 125, 2252. (d) Hill, J. P.; Jin, W.;
Kosaka, A.; Fukushima, T.; Ichihara, H.; Shimomura, T.; Ito, K.; Hashizume,
T.; Ishii, N.; Aida, T. Science 2004, 304, 1481.
‡ Delft University of Technology.
(1) (a) Watson, M.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV. 2001, 101,
1267. (b) Pisula W.; Kastler, M.; Wasserfallen, D.; Pakula, T.; Mu¨llen, K
J. Am. Chem. Soc. 2004, 126, 8074. (c) van de Craats, A. M.; Warman, J.
M.; Fechtenko¨tter, A.; Brand, J. D.; Harbison, M. A.; Mu¨llen, K. AdV. Mater.
1999, 11, 1469. (d) van de Craats, A. M.; Warman, J. M.; Mu¨llen, K.;
Geerts, Y.; Brand, J. D. AdV. Mater. 1998, 10, 36.
10.1021/ol051972k CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/05/2005

Scheme 1
a hetero-“superbenzene” by changing the chemical makeup
potassium carbonate in tetrahydrofuran to form 2-decyloxy-
5-ethynyl-1,3-difluoro-benzene, 4. Compound 4 was reacted
with 3 under Hagihara-Sonogashira coupling conditions to
make a substituted diphenylacetylene, 5. Cobalt(0)-catalyzed
cyclotrimerization was carried out on 5 to give compound
6.3b This step was followed by intramolecular cyclodehy-
drogenation to yield the final product, 7, in low yield.3b,7
An analytically pure sample was obtained by passing the
product through a silica gel column, eluting with hexane/
dichloromethane. The product was soluble in most common
organic solvents including hexane, acetone, THF, and
dichloromethane. The UV-vis spectrum recorded in THF
(Figure 1) shows a pattern of absorption bands characteristic
of the core to form a new family of nitrogen-functionalized,
graphene-like nanostructures.3e Direct attachment of heter-
atoms such as oxygen, nitrogen, and sulfur to the HBC core
is expected to modify self-assembly and electronic properties.3a
However, the attempted synthesis of hexakisalkoxy-substi-
tuted HBC resulted in an extended quinoidal compound in
96% yield through ether cleavage and oxidation.4
We were interested in the effects of different substituents
on the morphological and electronic properties of HBCs. In
particular, we reasoned that the undesirable oxidation reac-
tions encountered in the synthesis of alkoxy HBCs could
potentially be circumvented by using electron-withdrawing
groups, such as fluorine, to counteract the electron-donating
effects of the alkoxy groups. Here, we report the synthesis
and thermotropic liquid-crystalline (LC) behavior of 7, which
is the first example both of a fluorine-substituted HBC and
of a hexakisalkoxy HBC.
Compound 7 was synthesized in seven steps from com-
mercially available 2,6-difluorophenol, 1. Compound 1 was
alkylated to form 2-decyloxy-1,3-difluoro-benzene, 2;5 this
was then iodinated selectively to give 2-decyloxy-1,3-
difluoro-5-iodo-benzene, 3,6 which was coupled with tri-
methylsilyacetylene using Hagihara-Sonogashira conditions.
After removal of the solvent, the product was stirred with
Figure 1. UV-vis spectrum of 7 in THF.
(3) (a) Wu, J.; Watson, M. D.; Zhang, L.; Wang, Z.; Mu¨llen, K. J. Am.
Chem. Soc. 2004, 126, 177. (b) Fechtenko¨tter, A.; Tchebotareva, N.; Watson,
M.; Mullen, K. Tetrahedron 2001, 57, 3769. (c) Lee, M.; Kim, J.;
Peleshanko, S.; Larson, K.; Yoo, Y.; Vaknin, D.; Markutsya, S.; Tsukruk
J. Am. Chem. Soc. 2002, 124, 9121. (d) Liu, C.; Fechtenko¨tter, A.; Watson,
M. D.; Mu¨llen, K.; Bard, A. J. Chem. Mater. 2003, 15, 124 (e) Draper, S.
M.; Gregg, D. J.; Madathil, R. J. Am. Chem. Soc. 2002, 124, 3486.
(4) Weiss, K.; Beernink, G.; Dotz, F.; Birkner, A.; Mu¨llen, K.; Woll, C.
H. Angew. Chem., Int. Ed. 1999, 38, 3748.
of HBCs, but red-shifted ca. 600 to 1000 cm-1 nm relative
to n-alkyl-substituted HBCs with λmax (ꢀmax) ) 365 (ꢀmax
)
5.67 × 104), 408 nm (2.17 × 104) (M-1cm-1).3a
Previous electrochemical studies of hexa-tert-butyl HBC
show two reversible reductions at E1/2 ) -2.66 and -2.96
(5) Allen, C. F. H.; Gates, J. W. Organic Syntheses; Wiley: New York,
1955; Collect. Vol. III, p 140
(6) Wrobel, J.; Dietrich, A.; Gorham, B. J.; Sestanj, K. J. Org. Chem.
1990, 55, 2694
(7) We also obtained unreacted 6 and a yellow insoluble material from
the reaction mixture, accounting for the low yield.
5020
Org. Lett., Vol. 7, No. 22, 2005

+/0
n
V vs FeCp2 (THF, 0.1 M Bu4NPF6) and two reversible
oxidations at E1/2 ) +0.53 and +0.96 V (CH2Cl2, 0.1 M
nBu4NPF6).8 Compound 7 (Figure 2) shows two quasi-
Figure 3. Polarized optical micrograph of 7 cooled to room
temperature from the mesophase (250 µm × 190 µm).
transition) and of a thin film of 7 (prepared by drop casting
a THF solution onto a quartz substrate at room temperature,
Figure 4) were characteristic of a discotic columnar as-
Figure 2. Cyclic voltammograms (250 mV s-1, 0.1 M nBu4NPF6,
internal FeCp2) of 7 in CH2Cl2 (left) and THF (right).
reversible reductions under the same conditions, but at
considerably less reducing potential: E1/2 ) -1.96 and -2.34
V vs FeCp2+/0. The oxidation is irreversible (Eox ) +1.59
V vs FeCp2+/0 at 250 mV s-1), apparently due to absorption
on the electrode, as previously observed for a hexakis-
dodecyl HBC.9 Evidentally the electron-withdrawing effect
of the 12 fluorine substituents more than outweighs the
electron-donating effect of the six alkoxy groups.
Thermogravimetric analysis (TGA) shows the onset of
weight loss (attributed to decomposition) at 392 °C. Using
differential scanning calorimetry (DSC), the sample shows
two endothermic peaks. The peak at 39 °C is assigned to a
crystalline-to-mesophase transition. Compound 7 exhibits a
texture consistent with a columnar discotic LC mesophase
over temperature range from 39 to 220 °C (the highest
temperature accessible with our microscope); moreover, a
sample cooled from the mesophase to room temperature
shows a similar texture (Figure 3), suggesting that the
columnar structure is retained in the solid phase. The
transition at 310 °C presumably represents the clearing point,
since the compound appears to be a free-flowing liquid above
this temperature. This clearing point is lower than that for
many alkyl- and phenyl-substituted HBC compounds (ca.
420 °C for hexakis-n-alkyl derivatives3d); however, clearing
points as low as 46 °C can be obtained by using branched
alkyl chains in hexa-alkyl HBCs.1b
Figure 4. X-ray diffraction pattern of a thin film sample of 7
prepared on quartz; indexing is to a hexagonal unit cell.
sembly.10 These patterns can each be indexed to a hexagonal
unit cell, indicating an ordered 2D-hexagonal arrangement
of columns in the lateral direction. The unit cell parameters
calculated from the (100) reflections are ahex ) 26.8 Å (bulk
LC) and ahex ) 26.6 Å (thin film), and represent the
intercolumn core-core distances, which are slightly smaller
than that determined for the Colhd mesophase of dodecyl-
substituted HBC (28.6 Å).11 These core-core distances are
smaller than the diameter of 7 with fully extended alkyl
chains (calculated by molecular modeling to be ca. 38 Å),
indicating some chain interdigitation or folding between
columns.
X-ray diffraction patterns of both the bulk LC phase of 7
(prepared by heating the sample past the crystalline-LC
(8) (a) Herwig, P. T.; Enkelmann, V.; Schmelz, O.; Mu¨llen, K. Chem.
Eur. J. 2000, 6, 1834. (b) A similar value for the oxidation is also reported
in: Rathore, R.; Burns, C. L. J. Org. Chem. 2003, 68, 4071. (c) Values
converted from vs SCE to vs FeCp2+/0 using values of +0.46 and +0.56 V
for FeCp2 vs SCE in CH2Cl2/0.1 M nBu4NPF6 and THF/0.1 M nBu4-
+/0
NPF6 respectively, according to Connelly, N. G.; Geiger, W. E. Chem. ReV.
1996, 96, 877.
(10) Levelut, A. M. J. Chim. Phys. Phys.-Chim. Biol. 1983, 80, 149
(11) Fischbach, I.; Pakula, T.; Minkin, P.; Fechtenko¨tter, A.; Mu¨llen,
K.; Spiess, H. W.; Saalwa¨chter, K. J. Phys. Chem. B 2002, 106, 6408.
(9) Stabel, A.; Herwig, P.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int.
Ed. Engl. 1995, 34, 1609.
Org. Lett., Vol. 7, No. 22, 2005
5021

The charge-carrier mobility of 7 was studied with the
PR-TRMC technique,12 at an electromagnetic field frequency
of 30 GHz. Pressed pellets (10 mg) of the material were
irradiated with 10 nanosecond pulses of 3 MeV electrons
from a Van de Graaff accelerator, which results in the
creation of a uniform micromolar concentration of electron-
hole pairs. The resulting change in conductivity was moni-
tored through the microwave power absorbed by the mobile
charge carriers in the material. If one assumes that no charge
recombination occurs during the 3 MeV electron pulse, a
value of 0.028 cm2 V-1 s-1 is obtained as a lower limit for
the sum of the charge-carrier mobilities. However, the
charge-carrier concentration is overestimated in this experi-
ment by the assumption of no charge recombination; if one
takes into account that only a fraction of the charges survives
the recombination during the electron pulse13 one can
estimate the one-dimensional mobility along the stacks of
molecules as 0.50 cm2 V-1 s-1, very similar to the 0.55 cm2
V-1 s-1 reported for hexakis-decyl-HBC.1c
V vs FeCp2+/0).14 There are several literature reports of
fluorinated derivatives of well-known hole-transport materials
that function as electron-transport materials.15
In conclusion, a fluorine-substituted hexakis-decyloxy-
hexa-peri-hexabenzocoronene has been synthesized and
shown to form columnar assemblies in its LC phase and in
thin films. Its solubility in many common organic solvents
suggests facile processibility. The new fluorine-substituted
HBC shows similar mobility characteristics to a nonfluori-
nated analogue, but electrochemical data suggests electron
injection may be possible into this material.
Acknowledgment. We thank Dr. Junsheng Yu for some
polarized microscope images. This material is based in part
upon work supported by the STC Program of the National
Science Foundation, no. DMR 0120967. We also thank the
NSF for funding through award no. CHE-0211419 and the
DOE and the ONR for support.
The PR-TRMC measurements do not enable us to distin-
guish between hole and electron mobilities. HBCs have
previous been found to act as hole-transport agents in
photovoltaics2a and field-effect transistors;2b however, the
changes in energy levels revealed by the electrochemical data
suggest that, at least relative to a hexakisalkyl HBC, electron
injection might be facilitated in 7 and hole injection might
be diminished. Indeed 7 is more readily reduced than the
well-known electron-transport material, Alq3 (E1/2 ) -2.30
Supporting Information Available: Experimental details
and characterizing data; X-ray data for thin film and bulk
LC samples of 7; NMR spectra for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL051972K
(14) Anderson, J. D.; McDonald, E. M.; Lee, P. A.; Anderson, M. L.;
Ritchie, E. L.; Hall, H. K.; Hopkins, T.; Nash, E. A.; Wang, J.; Padias, A.;
Thayumanavan, S.; Barlow, S.; Marder, S. R.; Jabbour, G.; Shaheen, S.;
Kippelen, B.; Peyghambarian, N.; Wightman, R. M.; Armstrong, N. R. J.
Am. Chem. Soc. 1998, 120, 9646.
(15) (a) Bao, Z.; Lovinger, A. J.; Brown, J. J. Am. Chem. Soc. 1998,
120, 207. (b) Facchetti, A.; Mushrush, M.; Katz, H. E.; Marks, T. J. AdV.
Mater. 2003, 15, 33.
(12) Warman, J. M.; de Haas, M. P. In Pulse Radiolysis; Tabata, Y.,
Ed.; CRC Press: Boca Raton, 1991; p 101.
(13) Schouten, P. G.; Warman, J. M.; de Haaas, M. P.; van Nostrum, C.
F.; Gelinck, G. H., Nolte, R. J. M.; Copyn, M. J.; Zwikker, J. W.; Engel,
M. K.; Hanack, M.; Chang, Y. H.; Ford, W. T. J. Am. Chem. Soc. 1994,
116, 6880
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Org. Lett., Vol. 7, No. 22, 2005