From branched polyphenylenes to graphite ribbons

Article abstract of DOI:10.1021/ma0257752

This article presents the synthesis of graphitic nanoribbons (～1 nm wide), containing extended conjugated all-benzenoid segments. These were obtained by intramolecular oxidative cyclodehydrogenation of soluble branched polyphenylenes 6, which were prepared by repetitive Diels-Alder cycloaddition of 1,4-bis(2,4,5-triphenylcyclopentadienone-3-yl)benzene (1) and diethynylterphenyl (5) in good yield. While insolubility of the obtained graphite ribbons 7 precluded standard spectroscopic structure elucidation, the electronic and vibrational properties were probed by solid-state UV-vis, Raman, and infrared spectroscopy. A wide and unstructured absorption band covering the visible range of the electronic spectrum (λ_max ～ 800 nm) is observed, confirming the highly extended conjugated framework. The structure proof of the ribbon-type polymer is supported by the inclusion of appropriate model compounds. The profile of the visible Raman spectrum of the material is similar to that of a discrete polycyclic aromatic hydrocarbon (PAH) C₂₂₂H₄₂, characterized by two strong bands (at 1603 and 1322 cm^-1), corresponding to the G and D bands of graphite. The obtained graphite ribbons are not linear but rather contain kinks due to the structural design of the polyphenylene precursor. High-resolution transmission electron microscopy (HRTEM) images of the graphite ribbons 7 disclose two different domains: one is an ordered graphite layer structure with a layer distance of ca. 3.8 A, and one is disordered due to the existence of kinks in the obtained polymers and/or random stacking of graphite ribbons. Attempts to make linear analogues are so far unsuccessful, emphasizing the critical importance of the geometry of the polyphenylene scaffold to successful oxidative cyclodehydrogenation.

Full text of DOI:10.1021/ma0257752

7082
Macromolecules 2003, 36, 7082-7089
From Branched Polyphenylenes to Graphite Ribbons
J ish a n Wu ,† Lileta Gh er gh el,† Ma r k D. Wa tson ,† J ixu e Li,‡ Zh a oh u i Wa n g,†
Ch r istop h er D. Sim p son ,† Ute Kolb,‡ a n d Kla u s Mu1 llen *,†
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany, and
Institute of Physical Chemistry, J ohannes Gutenberg Universita¨t, D-55128, Mainz, Germany
Received October 25, 2002; Revised Manuscript Received J une 16, 2003
ABSTRACT: This article presents the synthesis of graphitic nanoribbons (∼1 nm wide), containing
extended conjugated all-benzenoid segments. These were obtained by intramolecular oxidative cyclode-
hydrogenation of soluble branched polyphenylenes 6, which were prepared by repetitive Diels-Alder
cycloaddition of 1,4-bis(2,4,5-triphenylcyclopentadienone-3-yl)benzene (1) and diethynylterphenyl (5) in
good yield. While insolubility of the obtained graphite ribbons 7 precluded standard spectroscopic structure
elucidation, the electronic and vibrational properties were probed by solid-state UV-vis, Raman, and
infrared spectroscopy. A wide and unstructured absorption band covering the visible range of the electronic
spectrum (λmax ∼ 800 nm) is observed, confirming the highly extended conjugated framework. The structure
proof of the ribbon-type polymer is supported by the inclusion of appropriate model compounds. The
profile of the visible Raman spectrum of the material is similar to that of a discrete polycyclic aromatic
hydrocarbon (PAH) C222H42, characterized by two strong bands (at 1603 and 1322 cm-1), corresponding
to the G and D bands of graphite. The obtained graphite ribbons are not linear but rather contain “kinks”
due to the structural design of the polyphenylene precursor. High-resolution transmission electron
microscopy (HRTEM) images of the graphite ribbons 7 disclose two different domains: one is an ordered
graphite layer structure with a layer distance of ca. 3.8 Å, and one is disordered due to the existence of
“kinks” in the obtained polymers and/or random stacking of graphite ribbons. Attempts to make linear
analogues are so far unsuccessful, emphasizing the critical importance of the geometry of the polyphenylene
scaffold to successful oxidative cyclodehydrogenation.
Ch a r t 1. Lin ea r Hom ologou s Ser ies of Gr a p h ite
Ribbon
In tr od u ction
Ladder or ribbon-type polymers possessing two-
dimensionally planar conformation, high electron delo-
calization, and functional optic-electronic properties
attract great scientific interest considering their poten-
tial applications in areas such as electroluminescence,
plastic lasers, and molecular electronics.1 The synthesis
and characterization of such materials is an ongoing
challenge in synthetic chemistry, with only sparse
examples of defect-free, soluble hydrocarbon ribbon
polymers.1b,f As an allotrope of diamond, fullerenes, and
carbon nanotubes, graphite has only sp2 hybridized
carbons forming planar sheets with high electronic
conductivity. Discrete nanoscale two-dimensional graph-
ite subunits with different shapes and sizes have been
prepared in our group.2 The synthetic route involves two
key steps: (i) synthesis of three-dimensional branched
oligophenylenes by repetitive Diels-Alder cycloaddition
of tetraphenylcyclopentadienones with arylethynylenes
or by cobalt-catalyzed cyclotrimerization of diaryleth-
ynylenes; (ii) oxidative cyclodehydrogenation of the
obtained three-dimensional polyphenylene precursors
with FeCl3 or Cu(OTf)2-AlCl3 to give two-dimensional
graphite segments in high yield. When peripherally
substituted, these graphite segments self-assemble to
columnar liquid crystalline superstructures. These have
demonstrated high one-dimensional charge carrier
mobility,2i promising for organic semiconductors in field-
tistep procedure.2a In analogy to the study of an effective
conjugation length in conjugated polymers,3 the elec-
tronic absorption maximum red-shifted with molecular
aspect ratio due to the ever-increasing delocalized
π-surface. This fits into a larger study2a,r of discrete all-
benzenoid PAHs, in which excellent correlation was
found between the lowest energy absorption band and
the size of the aromatic graphitic sheet, regardless of
molecular shape. An obvious extension of this work,
oxidative planarization of discrete polyphenylenes, to
polydisperse, high molecular weight linear species is
reported here. We describe the synthesis and charac-
terization of the first ribbonlike graphite framework
from branched polyphenylenes by oxidative cyclodehy-
drogenation.
effect transistors (FETs) and photovoltaic cells.2c
homologous series of discrete graphite nanosheets (Chart
1) were synthesized, each molecule by a different mul-
A
Resu lts a n d Discu ssion
* Corresponding author. E-mail: muellen@mpip-mainz.mpg.de.
† Max Planck Institute for Polymer Research.
‡ J ohannes Gutenberg Universita¨t.
Syn th esis. The molecular design of branched poly-
phenylenes suitable for full cyclodehydrogenation is a
10.1021/ma0257752 CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/19/2003

Macromolecules, Vol. 36, No. 19, 2003
From Branched Polyphenylenes to Graphite Ribbons 7083
Sch em e 1. Rep or ted Diels-Ald er Cycloa d d ition
yl-phenyl single bonds along the main chain further
complicates the conformation. The obtained branched
polyphenylenes after cycloaddition as shown in Scheme
1 also contains gaps, i.e., spaces where there are no
phenyl rings between the repeat units, which rules out
the possibility to obtain a continuous ribbon structure
with regular width after planarization. Intramolecular
oxidative cyclodehydrogenation yielded only partial
planarization, with incoherent and relatively small
polycyclic aromatic hydrocarbon (PAH) segments along
the polymer backbone.
Rou te to Br a n ch ed P olyp h en ylen es
We report here that replacement of 1,4-diethynylben-
zenes with 1,4-diethynyl-2,5-di(4′-tert-butylphenyl)ben-
zenes (5) in the Diels-Alder polymerization produces
branched polyphenylene frameworks which can be pla-
narized over much greater length scale. As shown in
Scheme 2, selective Suzuki coupling5 between 1,4-
dibromo-2,5-diiodobenzene (2)6 and commercially avail-
able 4-tert-butylbenzoboronic acid successfully gave the
dibromoterphenylene 3 in 92% yield. 1,4-Bis(trimeth-
ylsilylethynyl)-2,5-bis(4′-tert-butylphenyl)benzene (4) was
then prepared by standard Hagihara-Sonogashira cou-
pling7 of 3 with trimethylsilylacetylene in 89% yield.
Desilylation of 4 under basic conditions afforded the
diethynylterphenylene 5 in 92% yield. Diels-Alder
polymerization between compound 5 and 1 in refluxing
diphenyl ether gave the branched polyphenylenes 6,
which are soluble in common solvents such as dichlo-
romethane and THF, facilitating structural character-
critical step to make 2-D graphite ribbons with extended
conjugation. In our previous work, repetitive Diels-
Alder cycloaddition of bis(tetraphenylcyclopentadi-
enoneyl)benzene (1) and 1,4-diethynylbenzene resulted
in high molecular weight branched polyphenylenes as
shown in Scheme 1.4 As the [4 + 2] cycloaddition
between asymmetrically substituted teraphenylcyclo-
pentadienone and monosubstituted acetylene always
gives two structural isomers, the cycloaddition product
between 1 and 1,4-diethynylbenzene contains three
structural isomeric repeat units. Rotation about phen-
Sch em e 2a
a
Key: (a) Pd(PPh3)4, K2CO3, toluene, 95 °C, 92%; (b) trimethylsilyl acetylene, Pd(PPh3)4, CuI, piperidine, 80 °C, 89%; (c) K2CO3,
methanol/dichloromethane, 92%; (d) Ph2O, refluxing, 88%; (e) FeCl3, nitromethane/dichloromethane, 24 h.

7084 Wu et al.
Macromolecules, Vol. 36, No. 19, 2003
Sch em e 3a
a
Key: (a) Ph2O, reflux; (b) FeCl3, CH2Cl2, CH3NO2, 20 h.
ization. 1H NMR spectra show a broad peak from 7.5 to
6.4 ppm, assigned to the aryl protons, and a single peak
at 1.28 ppm assigned to the tert-butyl side groups. A
matrix-assisted laser desorption ionization mass spec-
trum (MALDI-MS) of the polymer consists of a series
of peaks up to 10 000 with nominal masses correspond-
ing to integer multiples of the repeating structure. Size-
exclusion chromatography (SEC) with polystyrene stan-
dards indicated a number-average molecular weight,
Mn ) 2.5 × 104 (Mw/Mn ) 2.5). It should be noted that
the actual molecular weight is generally overestimated
by SEC when comparing more rigid polymers to poly-
styrene coils. As previously mentioned, structural iso-
mers must also exist in polymer 6 after the Diels-Alder
cycloaddition polymerization. As depicted in Scheme 2,
three isomeric polymeric units are possible; i.e., the
“head” and “tail” bonds in each repeat unit could be on
the same line (“linear”), 120° crossing angle (“cis”) or
antiparallel (“trans”). All “gaps”, which were present in
the precursor polymer from our previous work (Scheme
1), are now filled. Conformation isomers arising from
rotation along the backbone to give structures not like
those depicted could also exist, but they could not be
locked into the final planarized ribbon due to overlap-
ping phenyl rings. The last step toward the graphite
ribbon 7 is cyclodehydrogenation of precursor 6 with
FeCl3 in nitromethane and dichloromethane. The reac-
tion was quenched after 24 h with methanol, and the
black precipitate was washed repeatedly with methanol.
The powder was stirred in concentrated hydrochloridic
acid for 24 h to remove residual iron compounds and
further purified by stirring in hydrazine for 1 day to
reduce persisting radical cations. The final product was
a black, insoluble powder, graphitelike in appearance.
The feasibility of a high yielding fusion of the precur-
sor polymer 6 to graphite ribbons finds support in the
successful conversion of polyphenylene precursors of
ever-increasing size to well-defined disks or ribbon-type
graphite segments,2,9 a relevant example of which is
shown in Scheme 3. The Diels-Alder cycloaddition of 1
with 2 equiv of 1-ethynyl-2,3,4,5-tetraphenylbenzenes
F igu r e 1. MALDI MS spectrum for C114H34
: calculated,
1403.54; found, 1403.30 (100%). Isotope distribution is in good
agreement with the simulated results (black bar). In addition,
some chloronation took place during the cyclodehydrogenation
with Lewis acid iron(III) chloride.
standard column chromatography. The isomer mixture
9a -c was fused with iron(III) chloride under similar
cyclodehydrogenation conditions to give C114H34 graph-
ite segments in quantitative yield. The MALDI mass
spectrum of the obtained product is shown in Figure 1
and indicates a quantitative dehydrogenation process.
The ribbon-type graphite segments C114H34 have a simi-
lar structure to that of polymer 7 and thus can be con-
sidered as a model compound of the graphite ribbons 7.
Ch a r a cter iza tion of th e Gr a p h ite Ribbon s 7. The
poor solubility of the graphite ribbons 7 limits the role
of conventional techniques in structure elucidation.
Alternatively, infrared, solid-state Raman and UV-vis
spectroscopy are standard tools for the characterization
of insoluble PAHs and nanosized graphite domains. The
optical characterization of the product was achieved by
UV-vis spectroscopy of a thin film, prepared mechani-
cally by simply smearing samples on a quartz substrate.
As shown in Figure 2, a broad absorption, with a
continuum-like appearance in the visible range and an
approximate maximum of 800 nm was observed. This
2e
(8) gave three structural isomers 9a -c, which were
distinguished by high performance liquid chromatog-
raphy (HPLC) as three peaks while inseparable by

Macromolecules, Vol. 36, No. 19, 2003
From Branched Polyphenylenes to Graphite Ribbons 7085
F igu r e 3. Infrared spectra of branched polyphenylene precur-
sor 6 (solid line) and graphite ribbon 7 (dot line).
F igu r e 2. Solid-state UV-vis spectrum of graphite ribbon 7,
C114H34, and C222.
broad and unstructured profile is similar to that ob-
served for a recently reported2n giant PAH with 222
carbon atoms (C222), included in Figure 2 for comparison.
This compound, so far the largest discrete all benzenoid
PAH, contains 37 so-called “full” aromatic rings. Ac-
cording to the π-sextet model of Armitt and Robinson,
which was subsequently further developed by Clar, the
π electrons of the all benzenoid hydrocarbons tend to
group into disjoint sextets, which coincide with isolated
“full” benzene rings.2o,p,q These rings, marked with the
Robinson circle, are connected together by C-C single
bonds. Therefore, some benzene rings are viewed as the
seat of π-sextets, some as “empty”. As described in the
Introduction, there is excellent correlation between the
optical spectra of all-benzenoid PAHs and their size with
little influence of the shape of the molecules. Because
of its similar graphite-related model structure and its
size, C222 can be used as a model for the optical
characterization of graphite ribbons.2a,r The UV-vis
spectrum of C222 showed an absorption over the com-
plete visible range of the electronic spectrum, with a
maximum of the band at approximately 765 nm.2n The
similarity between the UV-vis spectra of these two
systems suggests that the extended ribbon segments
within polymer 7 consist of at least 200 carbons (Figure
2). On the other hand, the model compound C114H34
shows a continuum-like absorption with an approximate
maximum of 675 nm (Figure 2). The red shift of the
absorption band in the polymer 7 compared to this
model compound also suggests a higher conjugation in
the graphite ribbons 7.
Relevant information about the polymer 7 has been
obtained by comparison of the most characteristic
spectroscopic features of the infrared spectrum of the
branched polyphenylene precursor 6 and that of the
cyclodehydrogenated product 7. It has to be pointed out
that while the vibrational spectrum of 6 exhibited strong
absorptions the vibrational spectrum of 7 showed only
weak peaks, thus indicating that the final product has
reached a sizable dimension and complexity. Of par-
ticular diagnostic importance are the traditional group
frequencies normally used in spectroscopic analysis of
monosubstituted benzene rings (at 698, 750, and around
3060 cm-1) and the combination peak at 4050 cm-1. The
last is characteristic of the “free” rotating benzene rings
and can indicate the existence of non condensed benzene
rings in the molecule.10 All these bands were observed
in the spectrum of the branched polyphenylene precur-
sor 6, but have disappeared in the spectrum of the
cyclodehydrogenated product 7 (Figure 3). A new set of
F igu r e 4. Raman spectra of graphite ribbon 7, C114H34, and
C222.
weak bands in the spectral region related to C-H out-
of-plane motion suggests the formation of multiply
substituted benzene rings and of complete fused aro-
matic rings.
The successful application of Raman spectroscopy to
the determination of extended π-conjugation and order-
ing of cyclodehydrogenated products has been demon-
strated and is utilized here in a similar fashion.4 Spectra
were recorded from solid samples of polymer 7 using
514 nm excitation laser wavelength. The first-order
Raman spectrum was characterized by two relevant
signature features, a first band situated at 1322 cm-1
,
with a shoulder at 1254 cm-1 and a second band located
at 1603 cm-1 (Figure 4). The investigations of vibronic
frequencies and Raman intensities of several all-ben-
zenoid PAHs of different, but well-defined structure and
size revealed as common features a line near 1600 cm-1
and two or more lines near 1300 cm-1 11
The number of
.
the peaks in the region around 1300 cm-1, their relative
intensities, and positions changed according to the size
and the symmetry of the molecules. On the other hand,
the Raman spectra of polymer 7, the model compound
C114H34 and very large PAHs as C222 could be correlated,
on analytic grounds, to those observed in graphitic
materials with different degree of crystalline order, so-
called disordered graphite. These materials exhibit some
defects which break the conjugation of the ideal, highly
crystalline, two-dimensional graphite lattice and lead
to the formation of nanosized graphitic islands. Two
Raman peaks at 1580 and 1330 cm-1 appear in the
spectrum of disordered graphite, usually referred to as
the G and the D bands, respectively. The first band (G)
is the only feature observed in the first-order Raman
spectrum of highly ordered, crystalline graphite. The D

7086 Wu et al.
Macromolecules, Vol. 36, No. 19, 2003
F igu r e 5. (A) (a) TEM images of the graphite ribbon samples 7, where an ordered graphite layer structure is marked by a
square; (b) high-resolution image (HRTEM) of marked area exhibiting a layer distance of 3.8 Å; (c) selected area electron diffraction
(SAED) pattern from marked area showing 002 reflections. (B) Another extended highly ordered graphite layer structure.
peak originates from imperfections and small conju-
gated domains with finite size. Given the similarity of
the expected structure of the graphite ribbon reported
here with “nanocrystalline” or disordered graphite and
all-benzenoid PAHs, the line centered at 1603 cm-1
could be assigned to the G band and the lines located
around 1322 and 1254 cm-1 to the D bands.
Structural defects of the obtained graphite ribbons,
arising from the incomplete cyclodehydrogenation, could
contribute to the width of the D band. The broadening
of this band is correlated to the convolution of the
Raman transitions from a distribution of aromatic
“domains” with different structures and dimension. For
disordered graphite, the ratio of the intensities of the
D and G peaks [I(D)/I(G)] varies inversely with La and
the number of ordered aromatic rings, according to12
tert-butyl side groups as well as the “kink” structures
in the polymer 7, this larger distance appears reason-
able. Another kind of graphite layer structure over tens
of nm to µm was also observed in some areas as shown
in Figure 5B. These ordered structures obviously arise
from the ordered stacking of linear graphite ribbons.
Direct observation of single graphite molecules is un-
likely because of their strong aggregation. The appear-
ance of the large disordered areas could be the conse-
quence of random stacking of the insoluble graphite
materials during the precipitation from solution and/
or the existence of “kink” microstructures in the polymer
chain. We should point out that, as we found in previous
work,2 structural defects arising from incomplete cyclo-
dehydrogenation can occur, especially when preparing
insoluble giant graphite segments. Here, the structural
defects could also exist and full structural proof is
impossible due to the very poor solubility of polymer 7.
I(D)/I(G) ) C(λ)/La
Attem p t tow a r d Lin ea r Gr a p h ite Ribbon s. Ef-
forts to make precursor polymers (Scheme 4) like
polyterphenylenes (10) or branched polyphenylenes (12)
by Yamamoto coupling13 of compound 3 or Suzuki
coupling between 1,4-dibromo-2,3,5,6-tetrabenzene (11)
and 1,4-benzenediboronic acid all failed most likely
because of severe steric hindrance arising from the
phenyl-phenyl repulsion. To minimize the steric prob-
lem, we designed an alternative route in which a
terphenyl could be copolymerized with an unhindered
monomer. 1,4-Dibromo-2,5-di(2′-biphenyl)ylbenzene (13)
was prepared by selective Suzuki coupling between 1
and 2-biphenylboronic acid in 77% yield. Compared with
11, compound 13 has two biphenyl substituents with
high rotational freedom, and its reactive sites are less
hindered. Polymerization of 13 with 1,4-benzenedibo-
ronic acid under standard Suzuki coupling conditions
only gave oligomers detected by MALDI-MS. The low
molecular weight could be ascribed to the poor solubility
of the product as well as to, albeit reduced, steric
hindrance during the reaction. The oxidative cyclode-
hydrogenation of the oligomers 14 was also attempted
with excess FeCl3 in the mixed solvent dichloromethane/
CS2; however, only partially fused and chlorinated
products instead of ribbon oligomer 15 were observed
from MALDI-MS. This prompted a model study to
check the reactivity of the new polyphenylene scaffold-
ing present in the repeating units of precursor 14. The
where La is the molecule diameter or in-plane correla-
tion length and C(λ) is a parameter dependent on
excitation laser wavelength (for λ ) 514 nm, C(λ) ∼ 44
Å). This relation yields La ) 6.2 nm, corresponding to
ribbon segments with ∼42 conjugated “full” aromatic
rings along the polymer main chain. These results and
the similarity to the Raman spectrum of C222 (Figure
4) support the upper size of the aromatic π-system
determined by the absorption maximum from UV-vis
spectroscopy. Compared with polymer 7 and C222, the
model compound C114H34 has a similar Raman spectrum
with a better-resolved D band and the relative intensity
of D band to that of G band is higher. These features
are ascribed to the smaller conjugated size in the model
compounds.
The morphology of the obtained graphite ribbon
samples was probed by high-resolution transmission
electron microscopy (HRTEM) (Figure 5). In the larger
areas, disordered graphite structure was observed.
However, an ordered graphite layer structure sur-
rounded by amorphous regions was also found. A typical
example shown in Figure 5A discloses an ordered
graphite structure which extends approximately over
6.5 nm width and 35 nm length. Selected area electron
diffraction (SAED) on this region reveals a layer dis-
tance of 3.8 Å, which is larger than that of crystalline
graphite (3.45 Å). Considering the existence of bulky

Macromolecules, Vol. 36, No. 19, 2003
From Branched Polyphenylenes to Graphite Ribbons 7087
Sch em e 4a
a
Key: (a) Ni(COD)2, bipyridine, COD, toluene/DMF, 80 °C; (b) 1,4-benzenediboronic acid, Pd(PPh3)4, K2CO3, 95 °C; (c) Pd(PPh3)4,
K2CO3, 95 °C, 77%; (d) 4-tert-butylbenzoboronic acid, Pd(PPh3)4, K2CO3, 75 °C, 75%; (e) FeCl3, nitromethane/DCM.
model compound 16 was prepared by similar Suzuki
coupling between 13 and excess 4-tert-butylbenzoboronic
acid in 75% yield. Attempted cyclodehydrogenations of
16 with FeCl3, Cu(OTf)2-AlCl3, or SbCl5 all gave a
complicated mixture but not the desired hexabenzocoro-
nene derivative 17. As we have reported,2k hexaphenyl-
benzenes can be easily fused to hexabenzocoronenes
under the same cyclodehydrogenation conditions. The
success of the reaction is acutely dependent on the
nature of substituents attached at the periphery, that
is electron-withdrawing or electron-donating power. The
difference of compound 16 from the hexaphenylbenzenes
lies only in the position of the single bonds, which serve
to hold the benzene rings in a scaffold for planarization.
Higher conformational freedom, e.g., where the biphenyl
group can twist out of the plane of the center phenyl
ring, could impede the planarization. At the same time,
the less symmetrical structure of 16 could promote
selective radical formation with reduced reactivity, thus
giving only partially fused product.
correlation. Particularly interesting is their formal
similarity with disordered graphite, which can provide
further information as to their structural and spectro-
scopic properties. The HRTEM measurement gave a
direct observation of this ribbonlike polymer. In this
context, the 2-D ribbon-type polymers reported here
could be used as models for polydisperse nanosized
graphitic domains in several disordered carbon materi-
als. Efforts to make isomer-free precursors for linear
graphite ribbons by Yamamoto or Suzuki coupling
reaction were precluded by steric hindrance, a problem
that will often be encountered when making extended
ribbons. This problem can be alleviated somewhat by
copolymerization of monomers which define the width,
along with a nonsterically hindered monomer. However,
an oligophenylene precursor obtained in this way failed
to undergo planarization under our cyclodehydrogena-
tion conditions, revealing the strong dependence of this
reaction on subtleties. In any case, a successful precur-
sor route to graphite ribbons paves the way to novel
carbon-rich ribbonlike nanomaterials, which could be
further studied by surface characterization tools such
as scanning tunneling microscopy (STM), atomic force
microscopy (AFM), etc. Poor solubility brings practical
processing difficulties, and our next steps will include
investigations of the cyclodehydrogenation on a metal
surface by heat treatment14 or other oxidants. Postsyn-
thesis noncovalent modification of rigid molecules is
becoming a proven way of engineering processability
and self-assembly,15 even in the case of inorganic
nanocrystals.16 In this light, we shall prepare ribbon
Con clu sion
By rational synthetic design, branched polyphen-
ylenes 6 prepared by repetitive Diels-Alder cycloaddi-
tion could be transformed into graphitelike ribbons
under oxidative cyclodehydrogenation conditions. The
obtained materials were most likely not structurally
linear; however, they gave the first case of two-
dimensional highly conjugated graphite structures with
high aspect ratio. Estimations of the extent of conjuga-
tion by UV-vis and Raman spectroscopy find excellent

7088 Wu et al.
Macromolecules, Vol. 36, No. 19, 2003
MHz): δ, ppm 7.59 (s, 2H), 7.53 (d, J ) 8.5 Hz, 4H), 7.38 (d,
J ) 8.5 Hz, 4H), 3.16 (s, 2H), 1.29 (s, 18H, tert-butyl). 13C NMR
(CD2Cl2, 125 MHz): δ, ppm 151.2, 142.4, 135.8, 135.5, 129.0,
125.4, 120.9, 83.3, 82.4, 34.9, 31.7. Anal. Calcd for C30H30: C,
92.26; H, 7.74. Found: C, 92.21; H, 7.73.
structures with pendant functionality that can act as a
handle for noncovalent attachment of processing aids.
Exp er im en ta l Section
Gen er a l In for m a tion . Except where noted, all starting
materials were purchased from Aldrich, Acros, and ABCR and
used as received. 1H NMR and 13C NMR spectra were recorded
in CD2Cl2, and C2D2Cl4 on a Bruker DPX 250 spectrometer
and referenced to residual proton solvent. MALDI TOF mass
spectra were obtained with TCNQ as matrix according to the
reported sample-preparing method.17 The optical absorption
measurements were performed at ambient temperature on a
quartz substrate with a UV/vis/NIR Perkin-Elmer Lambda 900
spectrometer. IR spectroscopy was performed on transmission
using a Nicolet FT-IR 730 spectrometer. Raman spectra were
examined in KBr pellets and recorded on a Dilor XY 800
spectrometer with a liquid-nitrogen cooled CCD detector and
Raman microscopically unit. As excitation source Ar+ laser has
been used. Powder wide-angle X-ray diffraction (WAXD)
experiments were performed using a Siemens D 500 Kristal-
loflex with graphite-monochromatized Cu KR X-ray beam,
emitting from a rotating Rigaku RV-300 anode. SEC measure-
ment was done in THF with polystyrene as standard and UV
Soma S-3702 as detector. Elemental analyses were performed
at the Institute of Organic Chemistry, J ohannes-Gutenberg
University, Mainz, Germany. Melting points were measured
on Bu¨chi B-545 with a gradient rate of 2 °C/min. HRTEM
measurements were done on a Philips Tecnai F30 analytical
TEM instrument, operated at an accelerating voltage of
300 kv.
Syn th esis. 1,4-Dibr om o-2,5-d i(4′-ter t-bu tylp h en yl)ben -
zen e (3). The mixture of 11.2 g of 1,4-dibromo-2,5-diiodoben-
zene (2) (22.98 mmol), 9.0 g of 4-tert-butylbenzoboronic acid,
795 mg of Pd (PPh3)4, and 31.71 g of K2CO3 in 230 mLof toluene
and 115 mL of water was deoxygenated by two “freeze-pump-
thaw” cycles and heated to 95 °C for 18 h. After cooling, the
mixture was extracted with 100 mL toluene. The organic layer
was washed with water three times, dried over magnesium
sulfate. The solvent was removed under reduced pressure and
the residue was purified by column chromatography (silica gel,
PE) to give 10.6 g of pure product as white crystals (92%).
Mp: 264.4 °C. FD-MS (8 keV): 500.2 (M+). 1H NMR (CD2-
Cl2, 250 MHz): δ, ppm 7.64 (s, 2H), 7.49 (d, J ) 8.2 Hz, 4H),
7.39 (d, J ) 8.2 Hz, 4H), 1.37 (s, 18H). 13C NMR (CD2Cl2, 125
MHz): δ, ppm 151.6, 142.9, 136.9, 135.7, 129.3, 125.5, 121.6,
34.9, 31.5. Anal. Calcd for C26H28Br2: C, 62.42; H, 5.64.
Found: C, 62.40; H, 5.61.
P olyp h en ylen e P r ecu r sor s 6. In a Schlenk tube, 250.0
mg of 5, 442.2 mg of 1,4-bis(2,4,5-triphenylcyclopentadienone-
3-yl)benzene (1), and 5 mL of diphenyl ether were deoxygen-
ated and then heated to reflux under argon for 15 h. After
cooling, methanol was added and the precipitate was collected.
The crude product was purified by repeated (3×) reprecipita-
tion from THF solution with methanol to give 580 mg of a
white powder (88%). 1H NMR (CD2Cl2, 250 MHz): δ, ppm 7.5-
6.4 (br, 43H), 1.28 (s, 18H, tert-butyl). 13C NMR (CD2Cl2, 125
MHz): δ, ppm 149.6, 142.5-124.7 (m), 34.7, 31.5. Anal. Calcd
for (C80H64)n: C, 93.71; H, 6.29. Found: C, 93.69; H, 6.30.
Cyclod eh yd r ogen a tion of P r ecu r sor 6 To Give Gr a p h -
ite Ribbon 7. A 205 mg sample of polyphenylene precursor 6
in 100 mL of dichloromethane was deoxygenated by spurging
with argon, which was continued throughout the reaction.
Then, 4.54 g of iron(III) chloride in 16 mL of degassed
nitromethane was added dropwise. The mixture was stirred
for 24 h followed by quenching with 100 mL of methanol. The
black precipitate was collected and washed with methanol
until the filtrate was colorless. The powder was stirred in 6
M HCl for 24 h to remove residual ferrous impurities. The
sample was then stirred with 25% hydrazine (aqueous) for 24
h to reduce any persistent radical cations. The powder was
collected, washed with water and then methanol, and dried
under vacuum to give 185 mg of black powder.
Syn th esis of Com p ou n d s 9a -c. A solution of ethynyltet-
raphenylbenzene (8, 301.6 mg, 0.742 mmol) and compound 1
(250.0 mg, 0.362 mmol) in diphenyl ether (10 mL) was
deoxygenated and heated at reflux under argon atmosphere
for 15 h. After cooling, the product was precipitated from
ethanol (100 mL), washed by ethanol, and dried under vacuum.
Thus, 434.1 mg (83%) of pure compound was obtained.
MALDI-MS: m/z ) 1447.48; m/z(calculated) ) 1447.76 for
C
114H78.
1H NMR (250 MHz, C2D2Cl4): δ, ppm 7.4-6.6,
multiplet.
Syn th esis of th e Mod el Com p ou n d C114H34. A 50 mg
sample of compound 9a -c was dissolved in 40 mL of fresh
dichloromethane deoxygenated by bubbling argon for 10 min,
and then 739 mg of iron(III) chloride dissolved in 3 mL of
nitromethane was added by syringe over 5 min. The reaction
was quenched after 20 h by 100 mL of methanol. The black
precipitate was washed by methanol and stirred in 25%
hydrazine for 24 h. The powder was washed by methanol and
dried under vacuum to provide 37 mg product. MALDI-MS:
1,4-Di(4′-ter t-bu tylph en yl)-2,5-di(tr im eth ylsilyleth yn yl)-
ben zen e (4). A 4.0 g sample of 3 (0.08 mmol), 277 mg of Pd-
(PPh3)4, and 150 mg of copper(I) iodide in 30 mL of piperidine
were deoxygenated by sparging with argon for 20 min, and
then 2.36 g of trimethylsilylacetylene (24 mmol) was added.
The mixture was heated to reflux under argon overnight. The
solvent was removed under reduced pressure and the residue
was purified by column chromatography (silica gel, PE/DCM
) 10:1) to give 3.8 g of pure product as a white powder (89%).
m/z ) 1403.296; m/z(calculated) ) 1403.54 for C114H34
.
1,4-Dibr om o-2,5-bis(2′-bip h en yl)ylben zen e (13). A 3.52
g sample of 2 (7.21 mmol), 3.0 g of 2-biphenylboronic acid (15.2
mmol), 416 mg of Pd(PPh3)4, 9.96 g of K2CO3 in 140 mL of
toluene, and 70 mL of water were degassed by three “freeze-
pump-thaw” cycles and heated to reflux overnight. The
organic layer was washed with water and dried over magne-
sium sulfate, then concentrated under reduced pressure and
the residue was purified by column chromatography (silica gel,
PE/DCM ) 10:1) to give 3.0 g of a white powder (77%). Mp:
224.2 °C. FD-MS (8 keV): 540.28 (M+). 1H NMR (CD2Cl2, 250
MHz): δ, ppm 7.11-7.53 (br, m). 13C NMR (CD2Cl2, 125
MHz): δ, ppm 143.2, 141.4, 140.9, 138.5, 135.7, 131.0, 130.5,
1
Mp: 211.7 °C. FD-MS (8 keV): 534.92 (M+). H NMR (CD2-
Cl2, 250 MHz): δ, ppm 7.60 (d, J ) 8.5 Hz, 4H), 7.58 (s, 2H),
7.46 (d, J ) 8.5 Hz, 4H), 1.37 (s, 18H, tert-butyl), 0.14 (s, 18H,
TMS). 13C NMR (CD2Cl2, 125 MHz): δ, ppm 151.3, 142.7,
136.4, 134.3, 129.2, 125.2, 122.1, 104.7, 99.8, 34.8, 31.4. Anal.
Calcd for C36H46Si2: C, 80.83; H, 8.67. Found: C, 80.81; H,
8.64.
129.7, 128.9, 128.3, 127.4, 127.1, 122.5. Anal. Calcd for C30H20
Br2: C, 66.69; H, 3.73. Found: C, 66.65; H, 3.72.
-
1,4-Dieth yn yl-2,5-d i(4′-ter t-bu tylp h en yl)ben zen e (5). A
3.1 g sample of 4 (5.8 mmol) was dissolved in 20 mL of DCM
and 15 mL of methanol, and then 4.8 g of K2CO3 (34.8 mmol)
was added. The mixture was vigorously stirred for 3 h at room
temperature and poured into 100 mL of water. The mixture
was extracted with 50 mL of DCM, and the organic layer was
washed with water three times and dried over magnesium
sulfate. The solvent was removed under reduced pressure and
the residue was purified by column chromatography (silica gel,
PE/DCM ) 10:1) to give 2.08 g of pure product (92%). Mp:
252.6 °C. FD-MS (8 keV): 390.62 (M+). 1H NMR (CD2Cl2, 250
Oligop h en ylen es 14. A 500 mg sample of 11(0.926 mmol),
153.4 mg of p-phenyldiboronic acid (0.926 mmol), and 64 mg
of Pd(PPh3)4 were placed in 20 mL toluene, and then 10 mL of
1 M K2CO3(aq) was added. The mixture was degassed by three
“freeze-pump-thaw” cycles and then heated to reflux for 48
h. During the reaction, a white precipitate formed because of
the poor solubility of the product. After cooling, the solid was
collected and the organic layer was poured into 100 mL of
methanol to complete the precipitation. The combined crude
product was dissolved in THF/CS2 and reprecipitated with
methanol two times. The white powder was washed with

Macromolecules, Vol. 36, No. 19, 2003
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M.; Mu¨llen, K. Macromolecules 2000, 33, 3525-3529. (b)
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methanol and dried under vacuum to give 320 mg of oligo-
1
mers 12. H NMR (CD2Cl2, 250 MHz): δ, ppm 6.50-8.72 (br,
m). 13C NMR (CD2Cl2, 125 MHz): δ, ppm 140.9, 129.32-122.82
(multi). Anal. C, 94.10; H, 5.12.
1,4-Bis(4′-ter t-bu tylp h en yl)-2,5-bis(2′-bip h en yl)ylben -
zen e (16). See synthetic protocol for 13. First 250 mg of 13
(0.463 mmol), 329 mg of 4-tert-butylbenzoronic acid (1.85
mmol), 53.4 mg of Pd(PPh3)4, 638.5 mg K2CO3 in 6 mL of
toluene, and 3 mL of water were heated to 95 °C overnight.
After standard workup and column chromatography (silica gel,
PE/DCM ) 5:1) 227 mg of pure product as white powder was
obtained (75%). FD-MS (8 keV): 646.90 (M+). 1H NMR (CD2-
Cl2, 250 MHz): δ, ppm 7.49-6.98 (br, 18H), 6.46 (d, J ) 8.2
Hz, 4H), 6.62 (d, J ) 8.2 Hz, 4H), 1.28 (s, 18H). 13C NMR (CD2-
Cl2, 125 MHz): δ, ppm 149.3, 144.9, 141.4, 141.3, 140.2, 139.9,
139.4, 137.8, 133.7, 132.0, 130.2, 129.4, 128.9, 127.8, 127.6,
126.3, 124.8, 34.6, 31.4. Anal. Calcd for C50H46: C, 92.83; H,
7.17. Found: C, 92.80; H, 7.16.
Ack n ow led gm en t. We acknowledge financial sup-
port from the Zentrum fu¨r Multifunktionelle Werkstoffe
und Miniaturisierte Funktionseinheiten (BMBF 03N
6500), EU-TMR Project SISITOMAS, the Deutsche
Forschungsgemeinschaft (Schwerpunkt Feldeffekt-tran-
sistoren), and the EU Project DISCEL (G5RD-CT-2000-
00321) and the help from Ms. Hansjo¨g Menges for the
Raman spectroscopy measurement.
(5) (a) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981,
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Refer en ces a n d Notes
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diethynyl-2,5-di(4′-dodecylphenyl)benzene (a structure analo-
gous to 5) and 2 equiv of tetra(4-dodecylphenyl)cyclopenta-
dienones provides a soluble branched oligophenylenes, which
can be fused to give a soluble, dodecyl-substituted C78
graphite segment (as shown in Chart 1). Simpson, C. D.;
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