Exceptionally long-range self-assembly of Hexa-peri-hexabenzocoronene with dove-tailed alkyl substituents

Article abstract of DOI:10.1021/ja048351r

The substitution of a hexa-peri-hexabenzocoronene by bulky, space-demanding, 2-decyltetradecyl side chains proved to be an effective procedure to influence the thermal and self-aggregation behavior. The extremely large steric requirement of the introduced

Full text of DOI:10.1021/ja048351r

Published on Web 06/04/2004
Exceptionally Long-Range Self-Assembly of Hexa-peri-hexabenzocoronene
with Dove-Tailed Alkyl Substituents
Wojciech Pisula, Marcel Kastler, Daniel Wasserfallen, Tadeusz Pakula, and Klaus M u¨ llen*
Max-Planck Institut f u¨ r Polymerforschung, 55128 Mainz, Germany
Received March 23, 2004; E-mail: muellen@mpip-mainz.mpg.de
Scheme 1a
Electronic devices based on organic semiconductors have gained
increased attention in nanotechnology, especially applicable to the
1
2
field of field-effect transistors and photovoltaics. A promising class
of materials in this research field is discotic liquid crystals. The
disclike molecules arrange into one-dimensional columnar super-
structures due to strong interactions between the aromatic cores.
Hexa-peri-hexabenzocoronenes (HBC) exhibit some of the highest
values for the intrinsic charge carrier mobility for mesogens, which
a
Synthesis of HBC-C14,10 1: i) NBS, PPh3, CH2Cl2, 0 °C; ii) Mg,
3
makes them promising candidates for electronic devices. Prereq-
Cl2Pd[dppf] x CH2Cl2, THF; iii) Co2(CO)8, dioxane, reflux, 24 h; iv) FeCl3,
CH3NO2, CH2Cl2, 25 °C, 45 min.
uisites for efficient charge carrier transport between electrodes are
a high purity of the material to reduce possible trapping sites for
4
5
charge carriers and a pronounced and defect-free, long-range order.
Appropriate processing techniques are required to induce a high
degree of aligned structures in the discotic material. Highly ordered
surface layers of different discotics, in particular of HBC derivatives,
6
have been previously obtained by using Langmuir-Blodgett-,
7
zone-casting techniques, or solution casting onto preoriented PTFE
substrates. The long-range order achieved by the latter protocol
8
proved to be suitable for field effect transistors (FET).8
Herein, a new HBC derivative with six branched, space-
demanding, alkyl side chains is presented. The key feature of the
implemented chain is the close proximity of the branching site to
the aromatic core, which dramatically influences the self-organiza-
tion behavior. This system perfectly fulfils the described de-
mands: extraordinary long-range self-ordering, high purity, and
facile processing from the isotropic state due to its low isotropization
temperature.
a
Scheme 1 shows the previously described,9 straightforward
synthetic route for C6-symmetric HBC derivatives. The alkylbro-
mide 3, obtained from the commercially available alcohol 2, was
converted into the Grignard reagent which was subsequently
coupled in a Kumada-type reaction with 4,4′-dibromodipheny-
lacetylene.10 The alkylated diphenylacetylene 5 was cyclotrimerized
to the hexaphenylbenzene derivative 6, and planarized with iron-
Figure 1. Cross-polarized optical microscopy images of HBC-C14,10 1 (a)
during isothermal crystallization at 35 °C; (b) crystallized at 38 °C; (c)
crystallized at 45 °C; (d) POM image from (c) using a λ-plate. All samples
were crystallized between two glass slides.
cooling procedure (Figure 1a). This crystallization behavior is
atypical for discotic systems, and spherulite formation of discotics
1
2
has been published only for low-molecular mass liquid crystals
or discotics with substantially smaller cores.13
(III)chloride in the final step to yield the HBC-C14,10 1 on a
multigram scale.
The spherulites revealed high anisotropy and therefore coherent
long-range order. This was expressed by the Maltese cross, where
Unlike other HBCs,9a it was possible to purify the substance by
normal preparative column chromatography with hexane as the
eluent. Inorganic and organic impurities were efficiently removed,
so that the desired compound 1 was obtained at high purity.11
Moreover, 1 is the first HBC known to be soluble in nonpolar
solvents, such as pentane or hexane.
12
the isogyres followed the extinction of the analyzer/polarizer
direction, indicating a radial alignment of columns from the center.
Consequently, the columns were oriented in the spherulite growth
direction with an edge-on arrangement of the molecules. Using a
λ-plate and analyzing the yellow-blue distributions in the optical
spherulite image, it was possible to determine that the spherulites
are optically negative; i.e., the refractive index parallel to the radial
direction is smaller than that perpendicular to it (Figure 1d). The
observed textures proved a spontaneous self-assembly of 1 over
several hundreds of micrometers. Upon closer inspection of different
spherulites, a macroscopic periodicity along the radius was
discovered. The periodic contrast, due to birefringent effects,
appears on single fibrous structures, suggesting a helical twist of
crystallites within the fibers,14 as presented in Figure 2. Extinction
The thermal behavior of 1 as monitored by differential scanning
calorimetry (DSC) revealed a phase transition from a “waxy-soft”
crystalline phase to the isotropic melt at 46 °C. Compared to other
n-alkyl-substituted HBCs, the isotropization temperature was shifted
considerably from approximately 420 °C (above decomposition)
to 46 °C.9 To obtain information about the supramolecular
organization of the compound, the phases were investigated using
polarized optical microscopy (POM). Surprisingly, well-ordered
spherulites nucleated randomly over the whole sample during the
8074
9
J. AM. CHEM. SOC. 2004, 126, 8074-8075
10.1021/ja048351r CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
the processing of organic materials. Because of the decreased
interaction, the self-assembly into superstructures is modulated and
therefore leads to low nucleation rates and a formation of larger,
ordered spherulites. The long, flexible, hydrophobic alkyl chains
force the system to minimize contact with polar substrates such as
glass or ITO, which results in an edge-on alignment of the
molecules.
In conclusion, the first discotic system with an exceptional self-
organization behavior over long-range to form spherulite textures
was reported. This system is a promising candidate as an active
component of electronic devices, due to its great processability and
modulated, self-organization behavior.
Figure 2. (a) Example of a cross-polarized optical microscopy image
illustrating the periodic features along the spherulite radius, (b) schematic
illustration of the helical fibrous structure, and (c) schematic presentation
of the twist of the crystallites within the fibers.
Acknowledgment. We acknowledge financial support from the
Zentrum f u¨ r Multifunktionelle Werkstoffe und Miniaturisierte
Funktionseinheiten (BMBF 03N 6500), EU-TMR project SISITO-
MAS, the Deutsche Forschungsgemeinschaft (Schwerpunkt Feld-
effecttransistoren), and the EU project DISCEL (G5RD-CT-2000-
00321). M.K. thanks the Fonds der Chemischen Industrie and the
Bundesministerium f u¨ r Bildung und Forschung for financial support.
Figure 3. (a) 2D-WAXS of the zone-crystallized sample; the arrow
indicates the moving direction of the sample; (b) POM image of the same
sample positioned 45° from the polarizer/analyzer axes (inset: same image
at 0° from the polarizer/analyzer axes); (c) AFM topography of zone-
crystallized 1 revealing long fibrous structures.
Supporting Information Available: Experimental procedures and
characterization for 3, 5, and 6; evaluation of the crystallization kinetics,
zone-crystallization processing parameters, additional AFM data, and
rheology results. This material is available free of charge via the Internet
at http://pubs.acs.org.
occurred when the optical axis of crystallites coincided with the
polarization plane (see Figure 2a,b).
The investigation of the crystallization kinetics revealed that the
size of the spherulites was not significantly temperature dependent
up to 35 °C. Above this temperature, the size increased exponen-
tially, even exceeding 4 mm when crystallized isothermally at 40
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