Influence of alkyl substituents on the solution- and surface-organization of hexa-peri-hexabenzocoronenes

Article abstract of DOI:10.1021/ja0430696

Three hexa-peri-hexabenzocoronenes (HBCs) with branched, bulky alkyl substituents of different lengths in the periphery of the aromatic core have been synthesized to tune the self-association properties in solution. ¹H NMR and photophysical mea

Full text of DOI:10.1021/ja0430696

Published on Web 03/05/2005
Influence of Alkyl Substituents on the Solution- and
Surface-Organization of Hexa-peri-hexabenzocoronenes
Marcel Kastler, Wojciech Pisula, Daniel Wasserfallen, Tadeusz Pakula, and
Klaus M u¨ llen*
Contribution from the Max-Planck-Institute for Polymer Research,
Postfach 3148, D-55021 Mainz, Germany
Received November 17, 2004; E-mail: muellen@mpip-mainz.mpg.de
Abstract: Three hexa-peri-hexabenzocoronenes (HBCs) with branched, bulky alkyl substituents of different
lengths in the periphery of the aromatic core have been synthesized to tune the self-association properties
1
in solution. H NMR and photophysical measurements were used to probe the solution organization in
comparison to the known hexa-dodecyl-substituted HBC in different solvent systems. Thermodynamic
parameters for the self-association in solution, obtained by curve fitting of the concentration- and temperature-
dependent NMR data using van’t Hoff analysis, indicated that the self-association is an enthalpically driven
process that is entropically disfavored. Photoluminescence and NMR results were both employed to
determine the critical concentration where no self-association for different compounds occurred. The
interactions between the molecules could be controlled by varying the nonsolvent content in the solvent
mixtures, supporting the model of solvophobic effects. The spatial demand of the solubilizing side chains
modulated the self-association in solution. This behavior was translated into the solution casting process,
where the kinetic in addition to the thermodynamic parameters played an essential role for structure
formation. The study illuminates the relationship between the solution association of HBCs and the
morphology, when processed on a surface. These results are essential for the application of these materials
in devices.
Introduction
A prominent class of such compounds utilizes aromatic
stacking to form one-dimensional columnar rods in solution.
The electrostatic driving force of these aromatic interactions is
the result of an attraction between the positively charged
σ-framework and the negatively charged electron cloud of the
The self-assembly of single elements into complex, organized
supramolecular structures via specific intermolecular interactions
is impressively illustrated in nature, for example, by the base-
1
pair stacking in the DNA and tertiary structures of proteins.
7
neighboring units. Moore and co-workers reported the synthesis
This principle transfers the intrinsic information from the
2
of various types of shape-persistent phenylacetylene macrocycles
molecular level onto the superstructure. To exploit this concept
8
(
PAMs). They investigated the influence of substitution,
for material science, it is important to understand the interactions
3
temperature, and solvent on the self-association behavior. The
between the single building blocks. Different noncovalent
9
10
solution-phase organization of other macrocycles, helicenes,
interactions between the molecules play significant roles during
the self-assembly. Inspired by nature, the main challenge of
supramolecular chemistry is to design specific intermolecular
interactions leading to the desired architectures with increased
size and complexity. Different attractive intermolecular forces,
1
1
12
porphyrins, and phthalocyanines has also been reported.
Discotic liquid crystals have attracted considerable interest
due to their self-organization and electronic properties. These
(
6) (a) van Gorp, J. J.; Vekemans, J.; Meijer, E. W. J. Am. Chem. Soc. 2002,
4
5
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1
24, 14759-14769. (b) Gearba, R. I.; Lehmann, M.; Levin, J.; Ivanov, D.
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242. (d) W u¨ rthner, F.; Thalacker, C.; Sautter, A. AdV. Mater. 1999, 11,
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J. AM. CHEM. SOC. 2005, 127, 4286-4296
10.1021/ja0430696 CCC: $30.25 © 2005 American Chemical Society

Self-Organization of Hexa-peri-hexabenzocoronene
A R T I C L E S
mesogens are usually composed of a flat aromatic core
substituted in the periphery with flexible, long alkyl chains,
which provide solubility and influence the association between
the molecules. These aggregates revealed high charge carrier
mobilities along the columnar stacking axis, which make them
Therefore, one major challenge for material science is to tune
the self-association of a material because it translates into the
processing behavior and furthermore into the performance of a
device. Molecules with a pronounced tendency to self-associate
are suitable for processing from solution, because the required
pre-aggregation is given. Materials with a moderate propensity
to aggregate exhibit low isotropization temperatures and become
available for processing from the melt.
1
3
a promising class as active components in electronic devices.
Hexa-peri-hexabenzocoronene (HBC) exhibits one of the highest
1
4
intrinsic charge carrier mobilities for a discotic mesogen
because of its large aromatic core and strong self-association
accordingly into rodlike superstructures. The substitution in the
corona of the disk influences the self-association and conse-
quently the thermal properties, solubility and morphology.
Extending the length of the alkyl chain from 3,7-dimethyl-
octyl to 3,7,11,15-tetramethyl-hexadecyl resulted in a lowering
of the isotropization temperature of about 200 °C.15 Moreover,
we have recently shown that the introduction of sterically
demanding, dove-tailed 2-decyl-tetradecyl chains further reduced
the temperature of transition to the isotropic state to 46 °C.
For device fabrication, it is important to align the active
component in the gap between two electrodes in a way that
charges can migrate effectively.17 Next to adequate electronic
The self-assembly of HBC-based materials in the bulk and
on surfaces has been intensively studied by different techniques
such as X-ray diffraction and scanning probe microscopy.20 The
strong tendency of the material to order suggested a detailed
study of the self-assembly in relation to the observed morphol-
ogies after the processing.
Herein, we describe the synthesis of a homologous series of
HBC derivatives with six branched, space-demanding alkyl side
chains of different lengths. The solution-phase self-association
behavior of this new type of HBC derivative has been compared
to the known hexa-tert-butyl- 1d and hexa-dodecyl-hexa-peri-
hexabenzocoronene 1e. The aggregation properties influenced
by the steric demand of the alkyl chains were analyzed using
1
6
1
temperature- and concentration-dependent H NMR, UV/vis, and
properties, the morphology of the material is a crucial property
in forming distinct columnar pathways over large domains and
thus is closely related to the device performance. Controlling
the interactions between the molecules and therefore the self-
association by changing the chemical composition is the key
factor tailoring the solubility and the thermal properties. The
degree of self-aggregation dominates the processing from
solution. For solution casting, it is required to deposit pre-
aggregated species onto the substrate. It was recently shown
that the solution-based zone-casting processing is highly de-
pendent on the solubility and thus on the solution self-
aggregation of the single building blocks.18 Thereby, HBC-
C12 1e, which is known to possess a limited solubility, was
successfully aligned by the above-mentioned technique resulting
in highly ordered and uniaxially oriented layers. Field-effect
transistors (FETs) of 1e were built exploiting zone-cast films
photoluminescence spectroscopy in different organic solvents.
These measurements included the determination of self-associa-
tion constants and thermodynamic parameters. Furthermore, the
morphology formation from solution of the HBC derivatives
was investigated, revealing the nucleation and growth of these
materials, which is important for the solution processability.
Kinetic investigations of the solution casting process were
undertaken by varying the evaporation speed of the solvent.
Results
Synthesis. Scheme 1 outlines the previously described
2
1
synthetic route for C6-symmetric HBC derivatives. The
2
2
alkylbromides 3b, c were obtained from the corresponding
alcohols 2b,c, whereas 2-ethyl-hexyl-1-bromide 3a was com-
mercially available. The three branched alkylbromides 3a-c
were converted into the corresponding Grignard reagents and
subsequently coupled in a Kumada-type reaction with 4,4′-
-
2
2
17b
and gave charge carrier mobilities up to 10 cm /Vs. On
the other hand, the optimal processing parameters of discotic
materials with an enhanced solubility are more difficult to
determine but lead to highly oriented films as well.1
2
3
dibromodiphenylacetylene. The alkylated diphenylacetylenes
5a-c were cyclotrimerized to the hexaphenylbenzene-deriva-
tives 6a-c and were planarized with iron(III)chloride to yield
the HBC derivatives 1a-c on a multigram scale.
9
(
11) Kano, K.; Fukuda, K.; Wakami, H.; Nishiyabu, R.; Pasternack, R. F. J.
Am. Chem. Soc. 2000, 122, 7494-7502.
The derivatives 1b,c were the first HBCs highly soluble even
in nonpolar solvents, such as pentane or hexane. Moreover, for
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Lui, K. M.; Ng, D. K. P. J. Mater. Chem. 1997, 7, 2063-2067.
1
b,c, the long branched chains in the corona of the aromatic
disks permitted the purification of the compounds, unlike other
(
13) (a) Bao, Z. N.; Lovinger, A. J.; Dodabalapur, A. AdV. Mater. 1997, 9, 42-
24
4
5. (b) Adam, D.; Schuhmacher, P.; Simmerer, J.; Haussling, L.; Siemens-
known HBCs, by normal preparative column chromatography
with hexane as eluent. This efficiently removed all inorganic
meyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371,
1
41-143. (c) Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B.;
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(b) Tchebotareva, N.; Yin, X. M.; Watson, M. D.; Samori, P.; Rabe, J. P.;
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D.; Venturini, A.; M u¨ llen, K.; Rabe, J. P. J. Am. Chem. Soc. 2004, 126,
3567-3575.
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16) Pisula, W.; Kastler, M.; Wasserfallen, D.; Pakula, T.; M u¨ llen, K. J. Am.
Chem. Soc. 2004, 126, 8074-8075.
(
17) (a) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson,
M.; M u¨ llen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. AdV. Mater.
2
003, 15, 495-499. (b) Schmidt-Mende, L.; Fechtenkotter, A.; M u¨ llen,
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122.
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R. J. J. Org. Chem. 1961, 26, 3980-3987.
(
18) (a) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth, L.; Kolb, U.; Tracz,
A.; Sirringhaus, H.; Pakula, T.; M u¨ llen, K. AdV. Mater., accepted. (b) Tracz,
A.; Jeszka, J. K.; Watson, M. D.; Pisula, W.; M u¨ llen, K.; Pakula, T. J.
Am. Chem. Soc. 2003, 125, 1682-1683.
19) Pisula, W.; Tomovic, Z.; Stepputat, M.; Kolb, U.; Pakula, T.; M u¨ llen, K.,
in preparation.
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(
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2001, 57, 3769-3783.
J. AM. CHEM. SOC.
9
VOL. 127, NO. 12, 2005 4287

A R T I C L E S
Kastler et al.
Scheme 1. Synthesis of HBC with Bulky, Branched Alkyl Chains of Different Lengthsa
a
(
i) NBS, PPh3, CH2Cl2, 0 °C, 80%; (ii) Mg, THF, Cl2Pd(dppf), 56-86%; (iii) Co2(CO)8, dioxane, reflux, 12 h, 75-93%; (iv) FeCl3, CH3NO2, CH2Cl2,
room temperature, 30 min, 75-94%.
order of magnitude lower.28 Recording satisfactory spectra at
such low concentrations required high field instrumentation and
very long experimental times. With an inverse probe head,
spectra of the dilute samples were obtained after about 100 000
scans on a 500 MHz instrument.
In 1,1,2,2-tetrachloroethane-d2 (Figure 2A), the aromatic
resonance for HBC-C12 1e at 30 °C reached a plateau at a
-
6
concentration lower than approximately 1 × 10 M, which
indicated monomeric species. Compounds 1a-c revealed a
Figure 1. Investigated HBC derivatives.
1
strong concentration dependence in the H NMR spectra
and organic impurities, obtaining a high level of purity, as
indicated by the change of the absorbance and resolution in the
UV/vis spectrum of a sample (same concentration) before and
after the purification.
The shortest chain a did not permit such a purification due
to its limited solubilizing effect on the HBC disk. Alternatively,
occurring at much higher values than in the case of HBC-C12
-5
1
e. Below 1 × 10 M, the three HBC derivatives with the
branched alkyl chains 1a-c existed as monomers in solution.
HBC-C6,2 1a possessed the lowest solubility among the three
dove-tailed HBC samples. In comparison with HBC-C10,6 1b
and HBC-C14,10 1c, HBC-C6,2 1a appeared to be more strongly
aggregated (chemical shift at higher field at the same concentra-
tion). HBC-C10,6 1b showed the lowest self-association in
1a was repeatedly filtered through a silica plug with hot toluene
to remove inorganic impurities from the cyclodehydrogenation
step.
1
,1,2,2-tetrachloroethane-d2 at 30 °C, followed by HBC-C14,10
Self-Aggregation Behavior. The self-aggregation behavior
of the HBC derivatives (Figure 1) was investigated quantitatively
t
1c with the longest alkyl substituents, while HBC- Bu 1d failed
to exhibit association in solution.
1
by using concentration dependence of the H NMR chemical
Least-squares curve fitting was performed on the concentra-
2
5
shifts and photophysical measurements. Additionally, these
results were compared to those of hexa-tert-butyl- 1d and hexa-
dodecyl-hexa-peri-hexabenzocoronene 1e.26
1
tion-dependent H NMR assuming an indefinite self-association
25
process to elucidate the stacking behavior. The association
processes were thereby described as consecutive equilibrium
states, where monomers were added to the existing aggregate.
Additional parameters, such as the chemical shift of the
monomer, terminal, and internal units, were required to fit
the concentration-dependent NMR data. The commonly used
monomer-dimer model, which could be used when association
into higher aggregates was negligible, was not suitable to
analyze our data. The model of indefinite self-association yielded
The chemical shift data provide insight not only into the
structural arrangement, but furthermore give information about
the stacking in solution. Thus, the aromatic resonance of HBC-
C12 1e is strongly dependent on concentration, which is
considered as a signature for aromatic stacking.2 The ring
current magnetic anisotropy of an aromatic ring induces a
resonance shifting of the nuclei of a molecule, in close vicinity
to it.2 The interacting units are usually in a slightly offset, face-
to-face geometry arrangement that leads to an upfield shifting.
The chemical shift of the aromatic proton of HBC-C12 1e in
7
5
1
the best curve fittings for the concentration-dependent H NMR
data. Table 1 lists the determined association constants. The
solution aggregation of HBC-C12 1e was significantly more
pronounced, as already evidenced by a lower solubility. At a
1
,1,2,2-tetrachloroethane-d2 at 60 °C shifted from 7.9 ppm at a
-
1
-6
concentration of 1.7 × 10 M to 9.0 ppm at 1.0 × 10 M,
-5
concentration of 4.75 × 10 M, the average aggregate of
suggesting stacking beyond dimers.27
HBC-C12 1e consisted of ca. six disks, but with a broad
distribution, while the HBCs with the branched substituents
existed as monomers (not shown).
As compared to other systems, such as PAMs, self-association
for HBCs occurs in a concentration range, which is at least 1
1
Temperature-dependent H NMR spectra were also taken of
(
25) Martin, R. B. Chem. ReV. 1996, 96, 3043-3064.
(
26) Herwig, P.; Kayser, C. W.; M u¨ llen, K.; Spiess, H. W. AdV. Mater. 1996,
the aggregating HBC-derivatives 1a-c,e at a concentration of
8
, 510-513.
(
27) Wu, J.; Fechtenk o¨ tter, A.; Gauss, J.; Watson, M. D.; Kastler, M.;
Fechtenk o¨ tter, C.; Wagner, M.; M u¨ llen, K. J. Am. Chem. Soc. 2004, 126,
(28) Shetty, A. S.; Zhang, J. S.; Moore, J. S. J. Am. Chem. Soc. 1996, 118,
1019-1027.
1
1311-11321.
4288 J. AM. CHEM. SOC.
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VOL. 127, NO. 12, 2005

Self-Organization of Hexa-peri-hexabenzocoronene
A R T I C L E S
Figure 2. (A) Concentration-dependent 1H NMR chemical shifts, recorded in 1,1,2,2-tetrachloroethan-d2 (30 °C, 500 MHz), fitted with the indefinite
1
self-association model including next nearest neighbor relations. (B) Temperature-dependent H NMR of the investigated HBC derivatives in 1,1,2,2-
-
3
tetrachloroethane-d2 and dichloromethane-d2, respectively, at a concentration of 4.75 × 10 M (500 MHz), fitted polynomially (second order).
Figure 3. (A) Solvent-dependent aggregation behavior of HBC-C14,10 1c at 30 °C (500 MHz), fitted with the indefinite self-association model including
next nearest neighbor relations. (B) Chemical shift dependence of HBC-C14,10 1c in a mixture of methanol-d4 and 1,1,2,2-tetrachloroethane-d2 with different
-
3
compositions (concentration: 4.75 × 10 M).
Table 1. Association Constants for Self-Aggregation of the
Investigated HBC Derivative in 1,1,2,2-Tetrachloroethane-d2 at 30
Table 2. Association Constants of HBC-C12 1e and HBC-C14,10
1c in Different Solvents at 30 °C
°
C
/L mol- for 1c
1
K
/L mol- for 1e
1
NMR solvent
K
a
a
K
a
/L mol-
1
compound
cyclohexane-d12
1,1,2,2-tetrachloroethane-d2
THF-d8
23.4
2.9
3.6
not soluble
900
1a
1b
1c
1e
13.4
1.8
2.9
a
5500
benzene-d6
420
14 900
898
a
Recorded at 40 °C.
-
3
1
4
.75 × 10 M in 1,1,2,2-tetrachloroethane-d2. Due to its limited
Figure 3A shows the concentration-dependent H NMR fits
for HBC-C14,10 1c in different solvents, and Table 2 summarizes
the determined association constants of HBC-C12 1e and
HBC-C14,10 1c. The aggregation of these two HBC derivatives
in 1,1,2,2-tetrachloroethane-d2 and THF-d8 was moderate as
compared to the values in cyclohexane-d12 or benzene-d6. The
self-association in benzene-d6 was higher than in other solvents,
although the chemical compatibility to the aromatic HBCs
should be better.
solubility, HBC-C12 1e could not be measured at a temperature
below 70 °C. The aggregation of all investigated structures is
also strongly temperature dependent (Figure 2B), whereby
HBC-C10,6 1b aggregated slightly stronger than HBC-C14,10
1
c above 60 °C. Above 130 °C, both derivatives existed
exclusively as monomer, because the aromatic resonance
reached the value for the monomer. HBC-C12 1e revealed also
the strongest aggregation at high temperatures in relation to the
other examples. The chemical shift decreased by lowering the
temperature as was observed for HBC-C10,6 1b dissolved in
dichloromethane-d2.
Because the investigated polyaromatic hydrocarbons were
insoluble in polar solvents such as acetone-d6, acetonitrile-d3,
or DMSO-d6 at the NMR concentration level, the NMR
J. AM. CHEM. SOC.
9
VOL. 127, NO. 12, 2005 4289

A R T I C L E S
Kastler et al.
1
Figure 4. (A) Temperature- and concentration-dependent H NMR data of HBC-C14,10 1c and HBC-C12 1e in THF-d8 (500 MHz), fitted with the indefinite
self-association model including next nearest neighbor relations. (B) van’t Hoff plot of self-association constants Ka for HBC-C14,10 1c and HBC-C12 1e.
Table 3. Association Constant Ka and Thermodynamic
absorption spectra of HBCs were typical for polycyclic aromatic
hydrocarbons, showing the characteristic groups of bands of
different extinction coefficients according to the Clar nomen-
clature: R, p, â, which correspond to the transitions from the
ground-state S0 into the excited states S1, S2, and S3, respec-
tively. In C6-symmetric HBCs, the R transition was only weakly
allowed and thus difficult to observe, because the transition is
geometrically not allowed. According to Kashas law, the
photoluminescence spectra exhibit only transitions from S1 into
vibrational levels of S0.
Parameters for Self-Aggregation of HBC-C14,10 1c and HBC-C12
1e in THF-d8 at 30 °C
/L mol-
1
∆
H/kJ mol-
1
∆
G/kJ mol-
1 a
∆
S/J mol- K-
1
1
compound
K
a
HBC-C14,10 1c
HBC-C12 1e
3.6
5500
-91.7
-283
-5.82
-36.7
-290
-832
a
At 296 K.
3
0
31
experiments were carried out in a mixed solvent system of
,1,2,2-tetrachloroethane-d2 and methanol-d4, with several dif-
ferent compositions. HBC-C14,10 1c at a concentration of 4.75
1
To gain deeper insight into the self-association behavior,
concentration-dependent UV/vis spectra of the feebly self-
associating HBC-C14,10 1c and the strongly aggregating HBC-
C12 1e were recorded, showing only small differences upon
-
3
×
10 M exhibited an increasing tendency to self-aggregate
with an increasing portion of the nonsolvent, methanol-d4,
relative to 1,1,2,2-tetrachloroethane-d2 (Figure 3B). The aromatic
resonance of 1c shifted upfield linearly, until the substance
precipitated at a methanol-d4 volume fraction of 30%. As a
control experiment, concentration-dependent measurements in
a constant ratio of methanol-d4 and 1,1,2,2-tetrachloroethane-
d2 were carried out (not shown here). This led to the same results
as the previously described experiment, confirming no influence
of the chemical shift upon the solvent polarity in the investigated
concentration range.
-
4
-7
concentration changes in the range from 1 × 10 to 1 × 10
M (not shown). In the same concentration range, the photo-
luminescence (PL) and photoluminescence excitation (PLE)
spectra revealed a more pronounced effect (Figure 5A). The
PL spectra for both compounds were normalized at 480 nm,
because this band was only little influenced by concentration.
While increasing the concentration, the intensity of the fluo-
rescence bands at 469, 489, and 497 nm decreased for both
investigated HBCs. In contrast, the broad band in the region
above 500 nm became higher in intensity, indicating the
formation of aggregates. The PLE spectra turned out to be a
much more sensitive probe for the presence of aggregation. The
bands at 345, 360, and 390 nm were strongly dependent on the
concentration and lost intensity in concentrated solutions. At
high concentrations, a weak, broad band appeared for both
investigated HBC derivatives at a wavelength of 407 nm, which
could be assigned to an emission from the aggregates. The UV/
vis spectra showed an isosbestic point at a wavelength of 398
nm, pointing toward a linear dependence of the two bands at
Thermodynamic parameters of the self-association behavior
were obtained from determination of association constants at
different temperatures in THF-d8 (Figure 4A). Probably either
due to changes of the conformation of the HBC molecules
(deviation from planarity) or due to changes of the disk mobility
in the solvent cage with increasing temperature, the electronic
environment of the protons varied and thus the chemical shifts
for the monomeric HBCs at low concentrations decreased with
increasing temperature.
The van’t Hoff plots for the strongly aggregating HBC-C12
e and the less aggregating HBC-C14,10 1c are shown in Figure
1
390 (p-band) and 407 nm, respectively (Supporting Information).
4
b, while the thermodynamic data are presented in Table 3. At
All PLE spectra in Figure 5A were normalized at the isosbestic
point.
the lowest temperature and highest concentration, the HBC-
C12 1e sample became cloudy during the measurement. Because
the solution NMR measurement detected only aggregates in
solution, the determined association constant was too small to
fit into the linearized van’t Hoff plot.
The strength of the self-association appeared to be sensitive
1
to solvent effects, as was observed in the above-described H
(
29) Prince, R. B.; Saven, J. G.; Wolynes, P. G.; Moore, J. S. J. Am. Chem.
Soc. 1999, 121, 3114-3121.
The electronic spectrum provided a sensitive probe for
interactions between molecules within an aggregate. The
(
30) Clar, E. Polycyclic Hydrocarbons; Academic: New York, 1964.
2
9
(31) Kasha, M. Discuss. Faraday Soc. 1950, 14-19.
4290 J. AM. CHEM. SOC.
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Self-Organization of Hexa-peri-hexabenzocoronene
A R T I C L E S
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4
-7
Figure 5. (A) PL and PLE spectra of HBC-C14,10 1c and HBC-C12 1e in CHCl3 at concentrations between 1 × 10 and 1 × 10 M (black to blue);
normalized at 480 and 398 nm, respectively. Arrows point in the direction of the high concentration. (B) Concentration-dependent PL and PLE spectra of
-
5
-7
HBC-C12 1e and HBC-C14,10 1c in CHCl3:CH3OH ) 7:3 and 5:5, respectively (concentration from 6 × 10 to 4 × 10 M, black to blue).
1
Figure 6. (A) Comparison of PL and H NMR (500 MHz) results of HBC-C12 1e and HBC-C14,10 1c: PL intensity at 523 nm of the at 480 nm normalized
1
spectra, recorded in THF, and the chemical shift from the H NMR, recorded in THF-d8, are plotted against concentration. (B) Fluorescence yields of
HBC-C12 1e and HBC-C14,10 1c. Inset: Determination of the fluorescence quantum yields of the two materials in the linear regime at low concentrations
(no self-association), all measurements (pathway ) 1 cm; 25 °C; absorption and excitation wavelength ) 360 nm; integration range ) 425-600 nm).
NMR results. Due to the aggregation, a non-Lambert-Beer law
behavior was observed for the investigated derivatives. The
extinction coefficient decreased slightly, when the concentration
was increased. A more pronounced effect was observed, when
the polarity of the solvent was changed.
spectrum versus the concentration gave a curve that was
1
comparable to the concentration-dependent H NMR plot (Figure
-
7
6A). At a nominal concentration of 3 × 10 M for HBC-C12
-
4
1e and 2 × 10 M for HBC-C14,10 1c, the concentration-
dependent plots from both measurements leveled out, indicating
the presence of only the monomeric species in solution. Plotting
the values for absorbance at 360 nm determined by UV/vis
spectroscopy versus the area under the corresponding PL curve
(excitation at 360 nm) yielded two different curves for HBC-
C12 1e and HBC-C14,10 1c (Figure 6B). The inner filter effect
became dominant at high absorbances, making an interpretation
difficult. On the other hand, because both derivatives possess
almost identical extinction coefficients at low concentrations,
the difference between the two plots at large absorbances was
remarkable. For HBC-C12 1e, the extinction coefficient was
far more dependent on the concentration, and furthermore the
fluorescence as compared to HBC-C14,10 1c was strongly
quenched, which could be attributed to the higher self-
association of HBC-C12 1e.
In the case of HBC-C14,10 1c, the maximum absorption in
the UV/vis spectrum (â-band) shifted hypsochromically from
3
62 to 358 nm as either the content of methanol or the
concentration increased, whereas the same band for HBC-C12
e did not shift at all (Supporting Information). The photo-
luminescence spectra of HBC-C14,10 1c and HBC-C12 1e,
1
-
4
-7
recorded concentration dependent (1 × 10 to 1 × 10 M) in
a mixture of chloroform and methanol with a ratio of 7:3 and
5
:5, respectively, exhibited a dramatic change with respect to
the polarity of the solvent (Figure 5B). The main band in the
PLE for HBC-C12 1e disappeared completely, whereas the band
at 415 nm intensified. The changes in the PL and PLE spectra
for HBC-C14,10 1c were identical to the concentration-dependent
measurements in chloroform, where the region above 500 nm
pointed toward self-association.
The inset in Figure 6B suggests only a slight difference
in the fluorescence quantum yield between HBC-C12 1e
(3.2%) and HBC-C14,10 1c (3.5%), proving the similar elec-
Plotting the change of the relative intensity of the band at,
for example, 523 nm from the normalized photoluminescence
J. AM. CHEM. SOC.
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A R T I C L E S
Kastler et al.
Figure 7. AFM images in tapping made on HOPG surface of spin-coated films HBC-C14,10 1c from (A) a 10 M THF solution and (C) a 10-4 M solution;
-
7
-
7
-4
HBC-C12 1e from (B) a 10 M THF solution and (D) a 10 M solution.
7
tronic behavior of the two substances in the absence of self-
association.
Temperature-dependent UV/vis and PL spectroscopy was
conducted on samples of differing concentration, exhibiting only
very small effects with temperature changes.
(Figure 7D). When decreasing the concentration down to 10-
M, in the case of HBC-C14,10 1c single 60-80 nm wide blobs
with a height of ca. 3 nm appeared, which were randomly
distributed over the surface (Figure 7A). From a solvent mixture
of methanol and chloroform (1:1) at the same concentration,
single flat pancake-shaped features with a constant thickness
of ca. 2.5 nm were formed (not shown).
Single long nanofibers were found from a 10- M solution
of HBC-C12 1e in THF, which were 1.2 nm high and ca. 20
nm wide, exceeding even several hundreds of nanometers in
length (Figure 7B). These fibers seemed to consist of a
monomolecular layer with several columns lying parallel along
After having gained information about the self-association
of the HBC derivatives in solution, it was important to explore
how the solution behavior translates into the solution processing,
which is a key step in implementing the materials successfully
into electronic devices based on organic materials. To investigate
the effect of self-association at the interface between solvent
and substrate onto the film morphology32 of the HBC deriva-
tives, the organization from solution was investigated by the
AFM technique. Therefore, the samples were first prepared by
spin-coating at room temperature on HOPG surfaces.
7
the fiber axis, whereby the molecules were arranged edge-on.
The morphology of a drop-cast film (10-4 M in toluene)
of HBC-C12 1e (Figure 8) consisted of several hundred
micrometers long, birefringent microfibers. Their dimensions
slightly changed with different solvents. By casting from toluene,
longer, but thinner, microfibers were observed in comparison
to the films prepared from THF or other solvents with lower
evaporation temperatures. In all cases, the morphology was
uniform within the drop-cast film, whereby the columnar
alignment coincided with the microfiber axis as it was previously
-
4
When HBC-C14,10 1c was spin-cast from a 10 M THF
solution, the freshly cleaved graphite surface was covered by
an inhomogeneous ∼100 nm thick film that showed no
nanoscopic order (Figure 7C), whereas the film from HBC-
C12 1e prepared under the same conditions exhibited a more
pronounced anisotropy in the structure, such as fibrous features
(
32) (a) Nguyen, T. Q.; Bushey, M. L.; Brus, L. E.; Nuckolls, C. J. Am. Chem.
Soc. 2002, 124, 15051-15054. (b) Klok, H. A.; Jolliffe, K. A.; Schauer,
C. L.; Prins, L. J.; Spatz, J. P.; Moller, M.; Timmerman, P.; Reinhoudt, D.
N. J. Am. Chem. Soc. 1999, 121, 7154-7155. (c) Jonkheijm, P.; Hoeben,
F. J. M.; Kleppinger, R.; van Herrikhuyzen, J.; Schenning, A.; Meijer, E.
W. J. Am. Chem. Soc. 2003, 125, 15941-15949.
3
3
proven. Indeed, the spin-coated HBC-C12 1e films revealed
highly oriented columnar domains as visualized by transmission
(33) K u¨ bel, C. Dissertation, Johannes Gutenberg Universit a¨ t, 1998.
4292 J. AM. CHEM. SOC.
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Self-Organization of Hexa-peri-hexabenzocoronene
A R T I C L E S
Figure 8. (A) POM image of drop-cast HBC-C12 1e from a 10-4 M toluene solution (right with and left without cross-polarizer); (B) AFM topography
of the microfiber morphology (tapping mode); (C) TEM image of a spin coated HBC-C12 1e film (10- M toluene).
4
electron microscopy (TEM) (Figure 8C). This tendency to create
highly anisotropic structures was even observed within a
polymer matrix.34
Due to this extraordinary nucleation and growth of such large
fibers in solution, a dipping experiment was performed to align
these structures uniaxially as shown in Figure 10A. At a
concentration of 10 M in toluene and a low moving velocity
of the support, a homogeneous film was acquired with indeed
oriented microfibers (Figure 10C,D). Moreover, these fibers
consisted of uniaxially aligned HBC columns as the 2D WAXS
pattern indicated (Figure 10B).
-
1
On the other hand, drop-cast HBC-C14,10 1c and HBC-C10,6
b films from 10 M THF solutions were completely amor-
-
4
1
phous. By using toluene as solvent, the film morphology
changed slightly, revealing birefringent parts that appeared at
the rim of the drop-cast films where most of the material was
deposited.
The drop-cast film of HBC-C6,2 1a (Figure 9) prepared from
Discussion
-
4
a 10 M THF solution also did not show any birefringence in
polarized light, but small particles precipitated randomly from
the solution. When the same material was processed from
toluene with an identical concentration, short crystalline needles
appeared at the rim of the film (Figure 9C). These structures
were highly birefringent and were mainly oriented in the
direction of the drop evaporation, whereas the interior of the
film revealed the same, but shorter, structures (Figure 9A). By
using a comparable solvent with respect to the association such
as xylene, which possesses a significantly higher evaporation
temperature, the length of these crystalline needles within the
film interior increased considerably (Figure 9B). Finally, when
a saturated toluene solution of HBC-C6,2 1a was slowly
evaporated from a small vessel, centimeter-long, crystalline
fibers were obtained on the bottom of the vessel (Figure 9D).
Because the investigated HBC derivatives did not possess
any functional group, which were capable of forming, for
example, hydrogen bonds, one assumed a face-to-face π-stack-
ing of the aromatic cores, based on the concentration dependence
of the aromatic resonance in the NMR. The influence of the
ring current from neighboring molecules caused the shift of the
aromatic protons, decreasing with increasing distance from the
core. The R-carbon of the aliphatic side chain exhibited a much
smaller concentration dependence, whereas the chemical shift
of the next carbon in the chain was almost independent of
concentration.
9d
The observed self-association equilibria for the hexa-alkylated
HBCs were fast on the time scale of the NMR experiment,
leading to only one broadened, averaged peak for all aggregates.
Recently, we described oligophenylene dendronized HBC,
which enhanced the attractive interactions between the mol-
(
34) Tracz, A.; Wostek, D.; Kucinska, I.; Jeszka, J. K.; Watson, M.; M u¨ llen,
K.; Pakula, T. In NATO AdVanced Study Institute Series; Graja, A., Buła,
R., Kajzar, F., Eds.; Kluwer: Dordrecht, 2002; p 315.
27
ecules. There, the equilibrium kinetics of the dendronized HBC
J. AM. CHEM. SOC.
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A R T I C L E S
Kastler et al.
Figure 9. POM of HBC-C6,2 1a drop-cast from (A) toluene and (B) xylene both at a concentration of 10-4 M, (C) optical image of the rim of the drop-cast
film prepared in (a) (left without and right with cross-polarizer), and (D) optical image obtained after evaporation of a HBC-C6,2 solution with a concentration
close to the saturation.
Figure 10. (A) Setup for the dipping experiment of HBC-C6,2 1a; at the deposition zone, a meniscus is formed at which the material crystallized onto the
moving support; (B) 2D WAXS pattern of the oriented HBC-C6,2 1a film (arrow indicates an artifact); (C) POM of the nucleation site, at which the
structures begin to grow in the moving direction of the support; (D) POM of the interior of the film revealing a uniaxial orientation of the microfibers.
was much slower as compared to the time scale of the NMR,
as indicated by the observation of distinct aromatic resonances
for monomeric and dimeric species.
The self-association behavior of the HBCs with branched
alkyl substituents occurred in a different concentration range
as compared to the parent HBC-C12 1e. Bulky tert-butyl-groups
4294 J. AM. CHEM. SOC.
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Self-Organization of Hexa-peri-hexabenzocoronene
A R T I C L E S
in the HBC periphery suppressed aggregation completely and
possessed a low solubilizing effect, which has already been
confirmed by UV/vis and time-resolved fluorescence measure-
ments.3 It is presumed that substituents, exhibiting a large steric
demand in the close vicinity of the disk, prevent the molecules
from closely approaching each other, thereby hindering π-stack-
5
2
8
ing. This observation confirmed the notion that HBCs were
cofacially stacked. In our study, HBC-C12 1e showed the
-
4
Figure 11. Stacking motif for neighboring HBCs: (A) lateral displacement,
strongest aggregation in the concentration range from ∼10
(B) schematic structure of an aggregate with rotational and translational
-
7
to ∼10 M accompanied by the lowest solubility. Depending
on the temperature and the nature of the solvent, the HBC
existed on average as monomer at a concentration at ca. 10-
M, which was at least an order of magnitude lower than that
reported for the other π-systems.12b The solubilizing dodecyl
chains possessed only a small steric demand in the periphery
of the aromatic disk, which allowed the molecules to approach
easily, resulting in a high association constant. The three new
HBC derivatives, which carried space-filling alkyl, dove-tailed
substituents, self-associate only at much higher concentrations,
showing substantially smaller association constants as compared
to HBC-C12 1e. The steric hindrance induced by the side chains
offsets in the lateral direction, and (C) rotational offset along the stacking
axis.
7
gregates. The presence of an isosbestic point at a wavelength
of 398 nm revealed a linear dependence between the monomer
absorption and the absorption of aggregates. In the PL, the broad,
less resolved, concentration-dependent bands above 500 nm
pointed toward the existence of aggregates. The photophysical
changes in the case of HBC-C14,10 1c were less pronounced
due to the reduced self-association in solution or the hindered
electronic coupling between neighboring molecules. The hypso-
chromic shift in the case of HBC-C14,10 1c, which was not
observed for the parent HBC-C12 1e, suggested a different
electronic coupling between the aromatic disks and thus a
different geometrical arrangement in the aggregated stacks. This
is possibly due to a different geometry in the aggregate caused
by the large steric demand of the substituents in the corona.
Quantum mechanical calculations including electron config-
uration interactions were performed for a better understanding
of the spectral changes in the UV/vis of both investigated
compounds (Software: Hypercube, Inc., HyperChem 6.0 for
Windows). After the optimization of the HBC geometry with
an ab initio calculation (basis set: 6-31G*), PM3 calculations
(20 occupied and 20 unoccupied orbitals for electron configu-
ration) were applied to dimeric HBC stacks with a distance
between the cores of 3.6 Å, as was experimentally determined
by powder X-ray diffractometry. Translation and lateral rotation
of the disks with respect to each other, which is energetically
36
perturbed the disks from approaching one another. Comparing
HBC-C6,2 1a to HBC-C10,6 1b, the length of the side chains
increased and led to a diminished self-association. Further
extension to HBC-C14,10 1c did not reduce the aggregation in
solution at room temperature because now the interactions
between the chains became significant. The attractive interac-
tions, initiated by the long chains, enhanced the self-association
slightly, resulting in a higher association for HBC-C14,10 1c as
compared to the derivative 1b. At higher temperatures, the van
der Waals forces decreased and HBC-C14,10 1c exhibited as
expected the lowest self-association among the investigated
HBCs. Thus, the steric hindrance modulated the strength of the
self-association in solution, and temperature affected the ag-
gregation size additionally as can be seen from Figure 2b.
Furthermore, the self-association was strongly dependent on
37
the nature of the solvent. The association constant for HBC-
C14,10 1c in cyclohexane-d12 as solvent was about 10 times higher
than those in THF-d8 and 1,1,2,2-tetrachloroethane-d2, which
could be attributed to the solvophobic effect. It was assumed
that aromatic solvents suppress π-stacking between aromatic
solutes because such interactions between the solute and solvent
would have prevailed over those between solutes. On the
contrary, it was observed that HBCs self-associate stronger in
3
9
favored (Figure 11), influenced significantly the calculated
electronic spectra. Therefore, it is reasonable to assume the
existence of different averaged stacking motifs for the two
HBCs. On the basis of the performed calculations, it is not
possible to identify specific geometrical arrangements of the
HBCs in solution.
For all investigated HBCs, the fluorescence quantum yields
were in the same range, but the behavior of HBC-C12 1e and
HBC-C14,10 1c differed significantly at high concentrations,
indicating either different mechanisms of fluorescence quenching
due to different geometries of the molecules with respect to
each other or a stronger self-association behavior at high
benzene-d6, which was confirmed in previous studies for
macrocycles such as PAMs.9d
Addition of methanol to the solution of HBCs enhanced the
38
solvophobic interactions, leading to stronger aromatic stacking.
At the same concentration, the resonance of the aromatic HBC
protons shifted with increasing methanol content upfield,
indicating a stronger self-association. Additionally, the concen-
tration-dependent UV/vis (Supporting Information) and photo-
luminescence measurements in a constant mixture of chloroform
and the nonsolvent methanol revealed the existence of ag-
1
concentration, which was supported by the H NMR solution
measurements.
NMR and fluorescence spectroscopy allowed the independent
determination of the critical concentrations for self-association
of HBC-C14,10 1c and HBC-C12 1e.
The thermodynamic parameters derived from the temperature-
and concentration-dependent NMR data suggested that there was
an enthalpy gain accompanied by an entropy punishment during
the aggregation process, which was also observed for PAMs.
In contrast to the PAMs, which have a much smaller π-area,
(
(
(
(
35) Biasutti, M. A.; Rommens, J.; Vaes, A.; De Feyter, S.; De Schryver, F.
D.; Herwig, P.; M u¨ llen, K. Bull. Soc. Chim. Belg. 1997, 106, 659-664.
36) Luz, Z.; Poupko, R.; Wachtel, E. J.; Zimmermann, H.; Bader, V. Mol. Cryst.
Liq. Cryst. 2003, 397, 367-377.
37) H o¨ ger, S.; Bonrad, K.; Mourran, A.; Beginn, U.; M o¨ ller, M. J. Am. Chem.
Soc. 2001, 123, 5651-5659.
38) Lahiri, S.; Thompson, J. L.; Moore, J. S. J. Am. Chem. Soc. 2000, 122,
1
1315-11319.
(39) Gauss, J.; Ochsenfeld, C., in preparation.
J. AM. CHEM. SOC.
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A R T I C L E S
Kastler et al.
the association enthalpy for the investigated HBCs was much
higher.28 The Gibbs free energies for HBC-C12 1e and HBC-
C14,10 1c reflect the difference in association between the two
substances.
of the distribution made scattering methods such as dynamic
light scattering or X-ray scattering tenuous. Vapor pressure
osmometry (VPO) and dynamic light scattering experiments did
not provide interpretable results, due to low solubility and small
contrast in the tested solvents.
The investigation of the film morphology obtained through
solution casting revealed additional information about the solu-
tion behavior of the HBC derivatives. It has been found that in
addition to the thermodynamical aspect the organization from
solution is also dominated by kinetics. This has been impres-
sively demonstrated in the case of HBC-C6,2 1a, which was
prepared from different solvents with different evaporation rates.
Comparing the films of HBC-C6,2 1a which were drop-cast
from solvents such as toluene and xylene, in which the self-
association is known to be very similar, one observed larger
micro-objects in the case of the higher boiling xylene. The
morphology of a film obtained from THF differed significantly,
because only an amorphous structure was obtained. In the latter
case, it should be noted that the degree of self-association is
smaller.
Nevertheless, the evaporation time of the solvent in the casting
process influenced significantly the macroscopic self-organiza-
tion on the substrate. A slow evaporating solvent such as xylene
allowed the formation of large anisotropic objects, whereas a
solvent with a lower boiling point did not give enough time to
exhibit a similar organization. The role of the kinetics in the
case of HBC-C6,2 1a is more pronounced as compared to that
of HBC-C12 1e. This effect can be explained by the steric
requirement induced by the dove-tailed alkyl substituents, which
hampered the disks from approaching one another. Therefore,
the self-assembly of these molecules occurs on a relatively large
time scale. This stands in contrast to HBC-C12 1e, which
revealed homogeneously distributed micro-ribbons over the
drop-cast film independent of the used solvent. These extra-
ordinary structures resulted, on one hand, from the strong self-
association tendency and, on the other hand, from the low steric
demand of the attached alkyl chains.
Conclusion
We have described the synthesis of three novel HBC
derivatives with sterically demanding alkyl substituents. The
complementary higher solubility enabled a more straightforward
purification. The materials showed a reduced self-association
behavior in solution, as compared to the parent HBC-C12 1e,
which exhibited, due to the large π-area and unhindered
stacking, one of the highest self-association constants based on
π-stacking. The steric requirement of the substituents allowed
tuning of the self-association in solution, which is a key step
for materials science in designing building blocks for self-
assembly, where the property of the single molecule translates
into the supramolecular architecture. The self-association was
an enthalpically driven process, which was, as expected,
entropically not favored.
In polar solvents, the association of HBCs was enhanced due
to the solvophobic effect. Astonishingly, the association in
benzene, as a potentially HBC compatible solvent, was very
high, what could not be explained until now. NMR and PL
spectroscopy were both suitable to detect the critical concentra-
tion for aggregation.
Morphology studies revealed that both the thermodynamical
properties and the kinetic aspects are important during self-
aggregation in solution. These conclusions should elucidate
whether the solution association propensity can be translated
to the bulk behavior and thus used for the processing of the
material and device fabrication. This result is of fundamental
importance in predicting the bulk properties (e.g., thermal
behavior, solubility) of new materials prior to the synthesis and
thus designing promising candidates for a successful application
in organic devices.
Consequently, HBC-C12 1e was successfully processed by
the zone-casting technique into highly ordered surface layers,
supported by the pronounced size of the pre-aggregates, which
existed already before the deposition from solution. In contrast,
it is difficult to zone-cast HBC-C6,2 1a due to its slow self-
aggregation and the necessity of using high concentrations to
reach a sufficient pre-aggregation in solution. On the other hand,
adequate processing conditions during a dipping experiment
Acknowledgment. This work has been financially supported
by the Zentrum f u¨ 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.K. thanks the “Fonds der Che-
mischen Industrie“ and the “Bundesministerium f u¨ r Bildung und
Forschung“ for financial support. We thank Dr. M. Wagner and
S. Spang for conducting high-resolution, solution NMR experi-
ments, J. Schnee for recording photoluminescence spectra, M.
Harding from the “Johannes Gutenberg Universit a¨ t Mainz” for
compiling and debugging the fitting algorithm, and T. Julius
for graphical support.
(higher concentrations and low solvent evaporation rates)
allowed an uniaxial orientation into surface layers. This solution
processing promises a successful implementation of the material
in a FET device.
Due to the increase of the peripheral steric demand of the
longer dove-tailed side chains, both compounds HBC-C14,10
1c and HBC-C10,6 1b showed a limited tendency to create
anisotropic structures from solution. No enhancement in the self-
organization onto the surface was observed, even though larger
pre-aggregates were observed by the addition of methanol. Both
HBC derivatives only formed small domains after the deposition,
because the association rate decreased extremely as compared
to HBC-C6,2 1a.
Supporting Information Available: Experimental procedures,
characterization for the described compounds, and additional
UV/vis spectra. This material is available free of charge via
the Internet at http://pubs.acs.org.
The small difference of the hydrodynamic radius between
the aggregates and the monomeric species and the polydispersity
JA0430696
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