From armchair to zigzag peripheries in nanographenes

Article abstract of DOI:10.1021/ja062026h

Synthetic concepts toward the synthesis of large, not-fully benzenoid polycyclic aromatic hydrocarbons (PAHs), decorated with phase-forming and solubilizing n-dodecyl chains, are presented based on the intramolecular cyclodehydrogenation reaction of suitable oligophenylene precursors. The formal addition of successive C2 units into the armchair bays of the parent hexa-per/-hexabenzocoronene extends the aromatic system and leads to PAHs with a partial zigzag periphery. This variation of the nature of the periphery, symmetry, size, and shape has a distinct impact upon the electronic properties and the organization into columnar superstructures. Both computational and experimental UV/vis spectra, which are in good agreement, emphasize the dependence of the characteristic bands α, p, and β upon the overall size and symmetry of the PAHs. While the number and the substitution patterns of attached n-dodecyl chains do not influence the electronic properties, the thermal behavior and supramolecular organization are strongly influenced, which has been elucidated with differential scanning calorimetry (DSC) and 2D wide-angle X-ray diffractometry (2D-WAXS) on mechanically aligned samples. This study provides valuable insight into the future design of semiconducting materials based on extended PAHs.

Full text of DOI:10.1021/ja062026h

Published on Web 07/04/2006
From Armchair to Zigzag Peripheries in Nanographenes
Marcel Kastler, Jochen Schmidt, Wojciech Pisula, Daniel Sebastiani, and
Klaus Mu¨llen*
Contribution from the Max-Planck-Institute for Polymer Research, Postfach 3148,
D-55021 Mainz, Germany
Received March 24, 2006; E-mail: muellen@mpip-mainz.mpg.de
Abstract: Synthetic concepts toward the synthesis of large, not-fully benzenoid polycyclic aromatic
hydrocarbons (PAHs), decorated with phase-forming and solubilizing n-dodecyl chains, are presented based
on the intramolecular cyclodehydrogenation reaction of suitable oligophenylene precursors. The formal
addition of successive C2 units into the armchair bays of the parent hexa-peri-hexabenzocoronene extends
the aromatic system and leads to PAHs with a partial zigzag periphery. This variation of the nature of the
periphery, symmetry, size, and shape has a distinct impact upon the electronic properties and the
organization into columnar superstructures. Both computational and experimental UV/vis spectra, which
are in good agreement, emphasize the dependence of the characteristic bands R, p, and â upon the overall
size and symmetry of the PAHs. While the number and the substitution patterns of attached n-dodecyl
chains do not influence the electronic properties, the thermal behavior and supramolecular organization
are strongly influenced, which has been elucidated with differential scanning calorimetry (DSC) and 2D
wide-angle X-ray diffractometry (2D-WAXS) on mechanically aligned samples. This study provides valuable
insight into the future design of semiconducting materials based on extended PAHs.
columnar mesophases.8 The intermolecular π-contact allows
Introduction
charge carrier migration9 between the stacked discotic molecules,
Polycyclic aromatic hydrocarbons (PAHs) constitute a large
and diverse class of organic molecules.1 The major natural
source on earth for PAHs is crude oil, coal, and oil shale.2 The
spectroscopy of interstellar material even proved the abundance
of large PAHs, which are in fact the largest, detected molecules
in space.3 Some PAHs are widespread environmental pollutants
and relatively potent carcinogens.4
Clar proposed that the properties of PAHs could be best
understood in terms of localization of the aromatic sextets
present in the molecules.5 Fully benzenoid PAHs can formally
be drawn only with Kekule´ rings without isolated “double”
bonds and are known to be kinetically very stable, unreactive
substances. Two important examples of that class of compounds
are triphenylene6 and hexa-peri-hexabenzocoronene (HBC).7
When substituted appropriately, many PAHs form very stable
which opens the possibility to implement these structures in
organic electronics, such as field-effect transistors, hole-injecting
layers, or photovoltaic devices.7,10 Recently, we presented a
concept to synthesize a series of large, all-benzenoid PAHs with
a broad range of sizes and shapes.11
Tetracene, a constitutional isomer of triphenylene, differs
distinctly in its properties: lower resonance energy,12 a lower
ionization potential,13 a smaller HOMO-LUMO gap,13 and
thermally far less stability.5 This emphasizes inter alia the
dependence of properties upon the nature of the periphery,
symmetry, and shape.14 Dias15 discussed different perimeters
for PAHs, where the two most prominent, zigzag and armchair
edge, are shown in Figure 1.
Graphite and many carbonaceous materials consist primarily
of large polyaromatic molecules of varying size, shape, and
periphery. An understanding of the chemistry of graphite and
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Blank, J.; Wegner, H.; de Meijere, A. Science 2002, 295, 1500-1503. (b)
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M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Mu¨llen, K. J. Am. Chem.
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9526
J. AM. CHEM. SOC. 2006, 128, 9526-9534
10.1021/ja062026h CCC: $33.50 © 2006 American Chemical Society

Nanographenes: From Armchair to Zigzag Peripheries
A R T I C L E S
Scheme 1. Synthesis of the Asymmetrically Substituted HBC 1ba
a (i) 93%; (ii) FeCl3, 93%.
substituted tetraphenylcyclopentadienone19 followed by an
oxidative cyclodehydrogenation reaction with iron(III) chloride
(Scheme 1).
Interestingly, the symmetry reduction in the substitution
pattern of 1b enhanced the solubility significantly compared to
that of 1a, although fewer solubilizing chains were attached.
Both 1a and 1b contain only armchair sites and will be used as
reference compounds. In the following, the synthesis of PAHs
with a partial zigzag periphery will be presented.
Figure 1. Different edge types for PAHs.
its physical properties is hampered by the inability to isolate
the molecules for an individual study. Extended PAHs with
different peripheries are required to test theoretical modeling
of the properties of graphite.16
The synthesis of the 44 aromatic carbons containing PAH 2
with one zigzag site started with 4-methoxyphenanthrene (8),
which has been synthesized by transition-metal-catalyzed cy-
cloisomerization according to Fu¨rstner’s protocol (Scheme 2).20
Demethylation with boron tribromide yielded the corresponding
alcohol, which was converted into the nonaflate derivative 9 in
good yields. Compound 9 proved to be more reactive in the
Hagihara-Sonogashira cross-coupling than the tosylate or the
triflate and led to the trimethylsilyl-protected acetylene deriva-
tive. After desilylation, the Diels-Alder conversion of the
4-ethynylphenanthrene (10) with alkyl-substituted tetraphenyl-
cyclopentadienone afforded the precursor 11 in excellent yields.
High field NMR spectroscopy permitted resolution of all proton
resonances for 11, which could be assigned utilizing H,H COSY
and NOESY experiments (Supporting Information). The line
broadening of the phenyl proton signals at room temperature
suggested that the phenyl group rotation was slow on the NMR
time scale. Finally, 11 was oxidatively planarized with iron-
(III) chloride to the 8,11,14,17-tetra-n-dodecyl-tetrabenzo[bc,-
ef,hi,uv]ovalene (2) in good yields.
For the synthesis of the PAH 3 with 46 aromatic carbon atoms
and two zigzag edges, pyrenediketone 1221 was converted in a
double Aldol condensation with 1,3-bis(4-bromophenyl)-2-
propanone22 to the thermally unstable cyclopentadienone deriva-
tive 13, which precipitated during the reaction (Scheme 3). The
analogous condensation with the n-dodecyl-substituted 1,3-
diphenyl-propan-2-one led only to traces of the desired product
because it decomposed already during the reaction due to its
good solubility in ethanol. Despite the thermal instability, in
the presence of 10, 13 underwent at higher temperatures a
Diels-Alder reaction, affording the PAH 14. Hydroboration of
dodecene with 9-BBN and subsequent Suzuki cross-coupling
reaction with the bromo-functions in 14 afforded the bis-
alkylated derivative 15 in good yields. In contrast to the
In this paper, we present novel synthetic concepts toward not
fully benzenoid, phase-forming, extended PAHs. The formal
introduction of C2 units into the armchair sites of the parent
HBC led to a homologous series of four novel PAHs with
varying symmetries, one composed of 44, two which contain
46, and finally, one which includes 48 carbon atoms within the
aromatic moiety. All these extended PAHs, which contain zigzag
sites, were substituted with multiple n-dodecyl chains on the
perimeter to enhance the solubility and to generate a discotic
liquid-crystalline phase behavior, which has been investigated
using differential scanning calorimetry (DSC) and 2D wide-
angle X-ray scattering (2D WAXS) of mechanically aligned
fibers. The experimental and simulated UV/vis spectra, and the
nature of the supramolecular organization for the novel materials
with partial zigzag perimeter, were compared with the ones of
different n-dodecyl-substituted hexa-peri-hexabenzocoronene,
which contain only armchair sites, to explore the influence of
the PAH symmetry, size, and periphery on these properties.
These structural parameters of the molecules allowed a sensitive
tuning of electronic properties, which is an important prereq-
uisite for the successful implementation as semiconducting,
active material in electronic devices.
Results and Discussion
Synthesis. The hexa-n-dodecyl-substituted HBC (1a) with
D
6h symmetry has been synthesized according to the literature.17
Another differently substituted HBC derivative has been pre-
pared, which carried only four n-dodecyl chains in the corona
of the aromatic disk.18
This 2,5,8,11-tetra-n-dodecyl-hexa-peri-hexabenzocoronene
(1b) has been made in two steps via a Diels-Alder cycload-
dition reaction between diphenylacetylene and the n-dodecyl-
(16) (a) Stein, S. E.; Brown, R. L. J. Am. Chem. Soc. 1987, 109, 3721-3729.
(b) Nakada, K.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV.
B 1996, 54, 17954-1796. (c) Takagi, Y.; Fujita, M.; Igami, M.; Kusakabe,
K.; Wakabayashi, K.; Nakada, K. Synth. Met. 1999, 103, 2574-2575.
(17) Herwig, P.; Kayser, C. W.; Mu¨llen, K.; Spiess, H. W. AdV. Mater. 1996,
8, 510-513.
(19) Iyer, V. S.; Yoshimura, K.; Enkelmann, V.; Epsch, R.; Rabe, J. P.; Mu¨llen,
K. Angew. Chem., Int. Ed. Engl. 1998, 37, 2696-2699.
(20) Fu¨rstner, A.; Mamane, V. J. Org. Chem. 2002, 67, 6264-6267.
(21) Young, E. R. R.; Funk, R. L. J. Org. Chem. 1998, 63, 9995-9996.
(22) Ito, S.; Wehmeier, M.; Brand, J. D.; Ku¨bel, C.; Epsch, R.; Rabe, J. P.;
Mu¨llen, K. Chem. Eur. J. 2000, 6, 4327-4342.
(18) Pisula, W.; Tomovic, Z.; Simpson, C.; Kastler, M.; Pakula, T.; Mu¨llen, K.
Chem. Mater. 2005, 17, 4296-4303.
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J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9527

A R T I C L E S
Kastler et al.
Scheme 2. Synthesis of PAH 2 with One Zigzag Sitea
a (i) BBr3, 86%; NfF, NEt3, 93%; (ii) trimethylsilylacetylene, CuI, PPh3, Cl2Pd(PPh3)3, 87%; K2CO3, MeOH, 95%; (iii) 95%; (iv) FeCl3, 79%. The
ovalene moiety is colored in red in structure 2 to better comprehend the IUPAC nomenclature.
Scheme 3. Synthesis of PAH 2 with Two Zig-zag Sitesa
a (i) 1,3-Bis-(4-bromo-phenyl)-propan-2-one, KOH, 47%; (ii) 75%; (iii) dodeca-1-ene, 9-BBN, KOH, Cl2Pd(dppf), 91%; (iv) FeCl3, 90%.
For both the 44 and 46 aromatic carbon atoms containing
PAHs 4 and 5, which include two and three zigzag sites,
respectively, 4-methoxy-phenanthrene-9,10-dione (16) was syn-
thesized in good yield following the published procedure.23 The
reaction of the carbonyl centers with n-dodecylmagnesium
bromide led to the corresponding diol which was reduced with
iodic acid directly to the 9,10-dialkylated phenanthrene (Scheme
4). At the same time, the reaction conditions allowed demethyl-
ating the methoxy function. The separation of the desired
phenanthrenol from side products turned out to be difficult, so
the crude compound was converted with nonafluoro-butane-1-
sulfonyl fluoride to the nonaflate 17, which could be easily
purified. Optimized conditions allowed the conversion of 17 to
the pinacol boronic ester 18 in good yields using tetrakis-
(triphenylphosphino)palladium(0) as catalyst in DMF. The
Suzuki cross-coupling reaction with 1,3,5-tribromobenzene
required a strong base to yield the sterically crowded precursor
molecule 21. Temperature-dependent 1H NMR experiments
proved that the molecule existed as two stable atropisomers
(Supporting Informaiton) in solution at room temperature, and
temperatures of about 180 °C were needed to promote the free
rotation of the phenanthrenyl units around the central benzene
ring (Supporting Information). The oxidative cyclodehydroge-
nation reaction of 21 with iron(III) chloride produced the fully
planarized PAH, 5,6,11,12,17,18-hexa-n-dodecyl-diphenanthro-
[3,4,5,6-uvabc;3′,4′,5′,6′-efghi]ovale ne (5), in high yield.
The Suzuki cross-coupling reaction of 18 with 1,3,5-tribro-
mobenzene gave as a major product the less crowded compound
19, which was isolated and subsequently coupled with com-
Figure 2. Aromatic region of the 1H NMR spectrum of 15, recorded in
1,1,2,2-tetrachloroethane-d2 at 100 °C (500 MHz).
precursor 11, the rotation of the phenyl with the protons m and
n was even at elevated temperatures (<180 °C) slow on the
time scale of the NMR experiment (Figure 2), leading to four
broad resonances.
Analogous to 11, all proton resonances could be assigned
using H,H COSY and NOESY experiments on a high-field
instrument. Finally, 8,17-bis-n-dodecyl-dibenzo[hi,uv]phenan-
thro-(3,4,5,6-bcdef)-ovalene (3) was obtained after the successful
oxidative cyclodehydrogenation. Although hardly soluble, it was
1
possible to obtain a resolved H NMR spectrum of the PAH 3
(23) Cai, X. W.; Brown, S.; Hodson, P.; Snieckus, V. Can. J. Chem. 2004, 82,
195-205.
(Supporting Information).
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Nanographenes: From Armchair to Zigzag Peripheries
A R T I C L E S
Scheme 4. Synthesis of PAHs 4 and 5 with Two and Three Zigzag Sitesa
a (i) RMgBr; HI, AcOH; NfF, NEt3, 90%; (ii) bispinacolatodiborane, AcOK, Pd(PPh3)4, 72%; (iii) 1,3,5-tribromobenzene, Ba(OH)2, Pd(PPh3)4, 70%; (iv)
2-biphenylboronic acid, Ba(OH)2, Pd(PPh3)4, 71%; (v) FeCl3, 91%; (vi) 1,3,5-tribromobenzene, Ba(OH)2, Pd(PPh3)4, 8%; (vii) FeCl3, 83%.
Figure 3. Investigated PAHs written in the Clar nomenclature including the symmetry group of the aromatic core.
mercially available 2-biphenylboronic acid to obtain the precur-
sor 20. Interestingly, the reaction was high yielding and the
compound did not form stable atropisomers because the more
flexible biphenyl unit could not hinder the rotation of the
phenanthrenyl units at room temperature on the time scale of
the NMR. After the planarization reaction, the PAH, 5,6,17,-
18-tetra-n-dodecyldibenzo[ef,hi]phenanthro[3,4,5,6-u,v,a,b,c]-
ovalene (4), could be isolated in high yield.
The previously described oxidation24 of the zigzag sites in
the PAHs 3-5 toward their corresponding o-quinone analogues
was attempted but in these cases no desired product could be
identified by mass spectrometry. Contrary to 2, where the
electron density was polarized toward the zigzag site, the
localization of charge was less pronounced in PAHs with
multiple zigzags, which has been supported by calculations
(Supporting Information).
Spectroscopy. Typically, PAHs reveal three bands with
increasing intensity. Clar25 introduced the nomenclature R-, p-,
and â-band, which originated from empirically determined
correlation between the so-called “ortho” and “para” reactivity
and the absorption spectra. The weak R-band appears usually
at the highest wavelength of the three and corresponds to a
transition from the second highest occupied molecular orbital
(HOMO-1) to the lowest unoccupied molecular orbital (LU-
MO).26 This is also known as the 0-0 transition. The very
intense â-band possesses the lowest wavelength of the three
and corresponds to a transition from the highest occupied
molecular orbital (HOMO) to the second lowest unoccupied
orbital (LUMO+1).26 The p-band is of intermediate wavelength
and intensity and can be assigned to a transition from HOMO
to LUMO.26 However, in the case of symmetric molecules, this
transition can be symmetry forbidden, which is independent of
the normal dipole selection rules.27
The above syntheses of four novel, extended, soluble nan-
ographenes with different peripheries (Figure 3) and symmetry
points groups afford unprecedented model systems for graphite.
In the following, their properties will be investigated by elec-
tronic spectroscopy and their propensity to pack into columnar
superstructures will be elucidated by 2D-WAXS experiments.
The number and the oscillator strength of the transitions in
the usually structure-rich absorption spectra are mainly depend-
ent on the symmetry and the topology of the molecule.5,25,28The
characteristic absorption and photoluminescence (PL) spectra
(25) Clar, E. Polycyclic Hydrocarbons; Academic Press and Springer-Verlag:
London, 1964; Vols. I+II.
(26) Fetzer, J. C. Large (C>)24) Polycyclic Aromatic Hydrocarbons; John
Wiley & Sons: New York, 2000.
(24) Wang, Z. H.; Tomovic, E.; Kastler, M.; Pretsch, R.; Negri, F.; Enkelmann,
V.; Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 7794-7795.
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J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9529

A R T I C L E S
Kastler et al.
Table 1. Comparison between the Calculated Excitation Energies
(E) and Oscillator Strengths (f) and the Experimentally Determined
Energies of the PAHs 1-5
transition
E (eV) exp.
E (eV) calc.
f (calc)
1
2
3
4
5
a
p
b
2.66
3.16
3.42
2.90
3.03a
3.48
0.00
0.00
0.72
a
p
b
2.56
2.62
3.26
2.75
2.79b
3.26
0.0001
0.15
0.31
a
p
b
2.43
2.62
3.12
2.72
2.62c
3.21
0.0003
0.26
0.71
a
p
b
2.53
2.77
3.20
2.66
2.64d
3.26
0.0003
0.18
0.02
a
p
b
2.33
2.92
3.11
2.55
2.67e
3.20
0.00
0.00
0.91
a Wave function composition: -0.49 (H-1 f L+1), 0.49 (H f L).
b Wave function composition: 0.27 (H-1 f L+1), 0.11 (H-1 f L+2),
0.62 (H f L). c Wave function composition: -0.23 (H-1 f L+1), 0.63
(H f L). d Wave function composition: -0.23 (H-1 f L+1), 0.64 (H f
L). e Wave function composition: 0.50 (H-1 f L+1), 0.50 (H f L).
of the novel PAHs 2-5 (Figure 3) will now be compared with
those of the parent compound 1a (Figure 4). The observations
are summarized in the following:
(i) The electronic spectra of 1b, with a different substitution
pattern, were identical with that of 1a. This emphasizes that
the alkyl substituents have no effect upon the electronic
spectroscopy.
(ii) The PAHs with a higher symmetry group such as 1 (D6h)
or 5 (D3h) exhibit narrow UV/vis spectra, with only few
transitions and broader PL spectra compared to their lower
symmetric counterparts 2-4. Furthermore, the energy space
between the absorption and the PL bands is larger for PAHs
with a higher symmetry group.
(iii) Symmetry reduction led to more allowed transitions and
therefore to broader, less structured UV/vis absorption spectra
in the case of the C2V-symmetric PAHs 2 and 4. Even the usually
weak R-bands were more intense.
(iv) The PL spectra of the PAHs 1-5 revealed many sharp
bands, which is typical for a symmetry-forbidden transition from
the S1 to the ground state S0. Therefore, different vibronic levels
of the ground state were reached from the excited state, leading
to a set of distinct peaks.
(v) The 0-0 transition in the PL spectrum for the D6h
symmetric PAH 1 is only weakly allowed, while it becomes
the strongest band when the symmetry was reduced.
(vi) The molar extinction coefficients of the most intense UV/
vis absorption band for the PAHs 1-5 were in the same range.
(vii) The Stokes shift was in all cases very small (<5 nm)
and independent of the solvent used, which indicated no polarity
difference of the ground and the excited state which has been
observed for other PAHs as well.8b
Quantum chemical calculations were carried out to determine
energies and intensities of the excitations, using a density
functional theory (DFT) approach. The structures of PAHs 1-5
were first optimized on the B3LYP/3-21G level and subse-
Figure 4. UV/vis and photoluminescence emission spectra of PAHs 1-5;
the R-bands are magnified for better visualization.
quently a TDDFT (time-dependent DFT)29 calculation (B3LYP/
6-31G(d)) was conducted. The calculations are in good agree-
ment with the experimentally determined peak positions (Table
1), and the error is within the expected range for this type of
(27) (a) Zander, M. Fluorimetrie; Springer-Verlag: Berlin, 1981. (b) Becker,
R. Theory and Interpretation of Fluorescence and Phosphorescence; Wiley-
Interscience: New York, 1969.
(28) Harvey, R. G. Polycyclic Aromatic Hydrocarbons; Wiley-VCH: New York,
1997.
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Nanographenes: From Armchair to Zigzag Peripheries
A R T I C L E S
The PAHs 1 and 5 are the most symmetrical ones (D6h and
D3h), leading to degeneracy of HOMO/HOMO-1 and LUMO/
LUMO+1. The resulting band gap is larger than the ones of
the less symmetrical molecules. In the case of the PAHs 2-4
the HOMO-LUMO transition gives the largest contribution to
the p-band (see the corresponding coefficients in Table 1) and
thus the excitation energy is smaller. The oscillator strengths,
as a measure for the intensity of the bands, indicate that the
p-band in the symmetrical cases (PAH 1 and 5) is symmetry-
forbidden. These results agree with group theoretical examina-
tions of the ground and excited states.31 Despite this, the
corresponding line appears in the experimental spectrum because
at finite temperatures the symmetry is dynamically broken and
the strict selection rule does not hold anymore. As indicated in
Figure 6, the excitation energy of the p-band taken from the
TDDFT calculation does not correspond to the HOMO-LUMO
gap as computed in the electronic ground state. This is a well-
known phenomenon which is due to the interaction of the
excited electron with the remaining valence electrons. However,
since this is a perturbative effect, the ordering of the energies
is preserved in general and thus the above discussion still holds.
Figure 5. Dependence of the absorption bands from the size of the aromatic
core for 1-5.
compound, as shown in calculations for various polycyclic
aromatic hydrocarbons.30
With increasing size of the aromatic system, we expect a
smaller band gap and thus decreasing excitation energies.
Plotting the absorption maximum of the R- and the â-band,
respectively, versus the number of carbon atoms in the aromatic
system, one finds both for the experimental and the calculated
data a linear dependence (Figure 5). The energy of these
transitions exhibits only a dependence upon the size of the
aromatic system and is thus not influenced by the nature of the
periphery or the symmetry of the aromatic system.
In contrast, the p-band, which originally corresponds to the
transition between the HOMO and the LUMO,26 does not reveal
a simple linear dependence upon the size of the aromatic system.
The calculation revealed that this band not only is due to a
HOMO-LUMO transition but also involves the transition
between HOMO-1 and LUMO+1 (Table 1). In the case of
PAH 2 there is even a small contribution of HOMO-1 to
LUMO+2. The nonlinear behavior of the p-band can be
understood qualitatively by the ordering of the orbital energies.
Figure 6 shows the energy of the orbitals HOMO - 1, HOMO,
LUMO, and LUMO + 1 taken from a ground-state calculation.
For all investigated PAHs, the ratio between the peak
wavelength of the R- and the â-band was constant, which has
been empirically reported already by Clar25 for substantially
smaller compounds.
Supramolecular Organization. The substitution of the
aromatic systems with flexible alkyl chains enhanced on one
hand the solubility to obtain a material which could be
characterized by solution analytics. On the other hand, the
flexible substituents imposed on the rigid aromatic cores in many
cases a discotic phase behavior. In the past, discotic materials
have gained great importance as promising, active components
in organic devices.6,32 Compound 1a was successfully imple-
mented in field-effect transistors,7,33 while other derivatives
served as donor component in organic heterojunction photo-
voltaic devices.10 In these applications, long-range and defect-
free organizations of the planar, aromatic molecules are required
to yield high-performance values in the electronic device.
In the following, the thermal behavior and supramolecular
organization of the novel, liquid-crystalline PAHs 1-5 (Figure
Figure 6. Orbital energies in the ground state of the PAHs 1-5. Also given are the excitation energy of the p-band from the TDDFT calculation, the
number of the aromatic carbon atoms, and the symmetry group of the molecule.
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9531

A R T I C L E S
Kastler et al.
Table 2. Phase Behavior of the PAHs 1-5a
phase. This relation was in good agreement with the determined
organization in the respective phases, as described in the next
section.
1
temperature
C)
enthalpy (J g-
)
derivative
(
°
(1st transition during heating)
phase transition
Self-assembly. Both concentration- and temperature-depend-
ent NMR spectroscopy and the electronic spectra from solution
doubtless indicated a pronounced tendency of all investigated
PAHs to self-assemble in solution. The analysis of the respective
spectra revealed a similar association behavior which has already
been previously observed for different HBC derivatives.8c For
the investigation in the bulk phase, specimen for the 2D-WAXS
experiments were prepared by filament extrusion as described
previously.18 All derivatives were extruded in their deformable,
plastic state to ensure an adequate alignment of the columnar
structures along the shear direction. For reasonable comparison
of the supramolecular organization of all derivatives, the patterns
were recorded at room temperature before annealing, which
could influence strongly the molecular packing as observed for
1b.18 Temperature-dependent measurements gave additional
insight into the order in the higher temperature phases. Except
compound 4, all derivatives revealed a characteristic hexagonal
mesophase with a nontilted intracolumnar organization above
the first transition. In contrast, the pronounced asymmetry of 4
resulted in a tilted packing of the disks toward the columnar
axis and a cubic intercolumnar arrangement in the mesophase.
All derivatives, except 3, arranged in a “herringbone”
manner34 with tilted molecules within the columns, whereby
the tilting angle varied only slightly when derived from the
azimuthal angle of the off-meridional reflections. The variation
of the positions of the equatorial reflections implied differences
of the lateral 2D unit cells, which were dependent on the size
of the aromatic core and the number of n-dodecyl side chains.
In general, the intensity and sharpness of the reflections were a
strong indication of the degree of supramolecular order.
Figures 7 and 8 show the patterns for the higher symmetric
compounds 1a, 3, and 5 and the “asymmetrical” derivatives 1b,
2, and 4, respectively. It is obvious that the higher symmetric
aromatic core enhanced the supramolecular order. The reflec-
tions indicating both the inter- and intracolumnar arrangement
for the compounds possessing a higher symmetric design were
sharper and more distinct in comparison to the ones for the less
symmetric PAHs. The highest degree of crystallinity was
observed for 3 due to the small number of only two n-dodecyl
side chains and the relatively high D2h core symmetry. The
resulting structure gave rise to multiple meridional reflections
with a nontilted intracolumnar arrangement, implying an
extraordinary high intracolumnar order. Both symmetrical 1a
and 5 differed in the core size and the position of substitution
of the six n-dodecyl chains. Since the pattern of 1a revealed
more distinct, higher order reflections, one could assume that
in this case the slight core increase did not play any role in the
organization of the molecules. However, the distribution of the
alkyl side chains around the core influenced the packing to a
greater extent.
1a
107 (82)
420*
45.8
Cr - Colho
Colho - I
1b
2
147 (123)
400*
15.7
13.8
28.3
11.5
18.4
Cr - Colho
Colho - I
148 (98)
400*
Cr - Colho
Colho - I
3
226 (216)
>500*
Cr - ?
Colho - I
4
173 (144)
210
Cr - Colco
Colco - I
5
48 (22)
>500*
Cr - Colho
Colho - I
a List of abbreviations: Cr, crystalline phase; Colho, mesophase (hex-
agonal ordered columnar phase); Colco, mesophase (cubic ordered columnar
phase); I, isotropic phase. Phase transitions upon cooling are given in
brackets. (*) Assigned by polarized optical microscopy.
3) will be described to gain a further understanding of how
periphery, symmetry, and substitution pattern of the PAH
influence the nature of the packing in the bulk.
Thermal Behavior. The thermal behavior, which is sum-
marized in Table 2, was determined by differential scanning
calorimetry (DSC) and polarized optical microscopy (POM).
These data give insight into the stability of the respective phases
and transition enthalpies dependent upon the molecular archi-
tecture, which can be correlated to the propensity of the
molecules to establish the supramolecular organizations.
All investigated PAHs were crystalline-like at room temper-
ature. The comparison of the thermal data showed a stronger
dependence on the number and the substitution pattern of the
n-dodecyl side chains than on the shape and symmetry of the
aromatic core. Thus, with increasing number of alkyl chains
the phase transition temperature from the crystalline phase to
the mesophase decreased. Therefore, independent of the sym-
metry of the aromatic core, derivatives 1b, 2, and 4 entered the
mesophase at significantly higher temperatures than 1a and 5
with more alkyl side chains. Moreover, the density of substitu-
tion and consequently the steric hindrance played an additional
effect, as was obvious for the D3h-symmetric compound 5 which
revealed the lowest first transition temperature. On the other
hand, the DSC data suggested that a higher symmetry group of
the aromatic core enhanced the intermolecular forces and thus
affected the isotropization temperature, which is much higher
for the symmetric PAHs 1a, 3, and 5 in comparison to the
derivatives consisting of an asymmetric core like 4 melting
already at 210 °C. The enthalpy of the phase transition (Table
2) is an indication for the change of the organization between
two phases. Therefore, assuming a similar order in the me-
sophase for all derivatives, the enthalpy of the first phase
transition increases with increasing crystallinity of the material
and thus with higher supramolecular order in the crystalline
In the case of 5, the alkyl substituents are paired on two
neighboring carbon atoms in the periphery of the PAH. This
leads due to steric reasons to a worse packing than that of the
HBC 1a, where the substitution point of the n-dodecyl chains
are well-distributed at the corona.
(29) Jamorski, C.; Casida, M. E.; Salahub, D. R. J. Chem. Phys. 1996, 104,
5134-5147.
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(31) Bra¨uchle, C. Chem. Phys. 1982, 67, 97-109.
(32) Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B. J. Mater. Chem.
1999, 9, 2081-2086.
(33) 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, 495-499.
(34) Fischbach, I.; Pakula, T.; Minkin, P.; Fechtenko¨tter, A.; Mu¨llen, K.; Spiess,
H. W.; Saalwachter, K. J. Phys. Chem. B 2002, 106, 6408-6418.
9
9532 J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006

Nanographenes: From Armchair to Zigzag Peripheries
A R T I C L E S
Figure 7. Room-temperature 2D-WAXS patterns of derivatives with a high-symmetry group (a) 1a, (b) 3, and (c) 5.
Figure 8. Room-temperature 2D-WAXS pattern of derivatives with lower symmetry group as (a) 1b, (b) 2, and (c) 4.
The lower symmetry decreased additionally the supramo-
lecular order as in the case of 1b, 2, and 4. In all three 2D-
WAXS patterns both equatorial and meridional reflections
became more diffuse and weak. Especially compound 4,
consisting of a core with a low symmetry and a high number
of directly “neighboring” substituents, revealed the highest
degree of disorder. The equatorial first-order reflections,
characteristic of the intercolumnar arrangement, were almost
isotropic, suggesting a poor alignment of the columnar structures
in the extruded samples. However, sharper off-meridional
reflections might imply a more uniform tilting of the disks than
in the other investigated cases. According to the thermal
behavior of compounds 1b and 2, the additional two carbon
atoms within the core also did not affect the molecular packing
to a great extent. The 2D-WAXS results demonstrate clearly
the influence of the symmetry of the aromatic core, the size of
the aromatic system, and the substitution pattern upon the
supramolecular organization. This emphasizes the importance
of the molecular architecture on the formation of the supramo-
lecular structures, which is important for the future syntheses
of discotic materials for potential application in electronic
devices.
The synthetic concept allowed decoration of the rigid aromatic
systems with multiple n-dodecyl side chains to guarantee
solubility and liquid-crystalline phase behavior.
The change of the symmetry, which goes along with the
partial change of the periphery type, had a pronounced influence
upon the electronic spectra. Both the experimental and com-
putational UV/vis spectra were in good agreement and revealed
that the spectra of highly symmetric molecules were transition
poor compared to the PAHs with lower symmetries. The PAH
typical bands R and â depended linearly upon the overall size
of the aromatic system, while the position of the p-band was
mainly controlled by the symmetry of the PAH. The number
and the substitution pattern of the n-alkyl chains did expectedly
not have any influence upon these molecular properties. In
contrast, the nature of the flexible surrounding on the PAH
imposed its character onto the supramolecular organization of
the synthesized mesogens. All investigated compounds exhibited
a columnar mesophase, whereby the symmetry of the aromatic
part influenced only the stability of the arrangement as measured
by DSC. However, the number and the position of the flexible
alkyl chains controlled both the thermal behavior and the quality
of the supramolecular organization. Fewer chains with a
symmetric arrangement around the aromatic disk caused a
stabilization of the columnar organization due to an unperturbed
aromatic π-stacking. More chains, which were asymmetrically
distributed on the periphery of the PAH, were more difficult to
pack, and hence the columnar arrangement exhibited more
defects and a smaller long-range correlation. The enthalpy of
the first transition in the DSC reflected the degree of crystallinity
in the room temperature phase, which was directly related to
the symmetry of the core and the substitution pattern. A
pronounced higher order in the crystalline phase was thus found
for the higher symmetry compounds, whereby the organization
of the mesophase was comparable for all PAHs in the
investigated homologous series.
Conclusion
The synthesis of a homologous series of extended, not-fully
benzenoid PAHs, which differed in size and symmetry of their
aromatic system, has been described. The synthetic routes
toward these PAHs were all based on final oxidative cyclode-
hydrogenation reactions of precursors with adequate architec-
tures, in which different smaller PAH moieties were introduced.
The introduction of different phenanthrenyl units into the
precursor molecules allowed the periphery of the final PAH to
be successively modified from fully armchair to partially zigzag.
This opened the possibility to investigate the role of symmetry,
size, and periphery in molecular and supramolecular properties.
9
J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006 9533

A R T I C L E S
Kastler et al.
The tuning of electronic properties and the control of the
organization into long-range oriented molecular stacks are a
challenge for materials science in a quest for novel, semicon-
ducting materials with specific properties. The absorption
behavior and the position of the molecular orbitals were two
important parameters that influence the performance of organic
heterojunction photovoltaic devices. It is known that a high
supramolecular order enhances the charge carrier transport along
the one-dimensional structures.35 Therefore, the development
of organic semiconductors which self-assemble into highly
organized arrays is one major goal. The tuning of electronic
levels of the active component in an electronic device is of key
importance to reduce charge carrier injection barriers at the
electrodes or to optimize the exciton polarization in solar cells.
This study elucidated the importance of the molecular design
on the molecular properties and self-organization and gave
important insights into the future development of semiconduct-
ing materials based on discotic PAHs.
Acknowledgment. This work has been financially supported
by the Zentrum fu¨r Multifunktionelle Werkstoffe und Minia-
turisierte Funktionseinheiten (BMBF 03N 6500), the Deutsche
Forschungsgemeinschaft (Schwerpunktprogramm Organische
Feldeffekt-Transistoren) as the EU projects DISCEL (G5RD-
CT-2000-00321), NAIMO (NMP4-CT-2004-500355), and MAC-
Mes (GRD2-2000-30242). M. Kastler thanks the “Fonds der
Chemischen Industrie“ for financial support.
Supporting Information Available: Experimental procedures,
characterization for the described compounds, MALDI spectra,
enlarged electronic spectra of the PAHs 2-5, and additional
computational data. This material is available free of charge
via the Internet at http://pubs.acs.org. See any current masthead
page for ordering information and Web access instructions.
(35) Paraschiv, I.; Giesbers, M.; van Lagen, B.; Grozema, F. C.; Abellon, R.
D.; Siebbeles, L. D. A.; Marcelis, A. T. M.; Zuilhof, H.; Sudholter, E. J.
R. Chem. Mater. 2006, 18, 968-974.
JA062026H
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9534 J. AM. CHEM. SOC. VOL. 128, NO. 29, 2006