Pronounced supramolecular order in discotic donor-acceptor mixtures

Article abstract of DOI:10.1002/anie.200500669

Mixing a hexa-peri-hexabenzocoronene (HBC) derivative and a perylene- or terrylenediimide results in the formation of complex helical arrangements in the columnar superstructures. Weak donor-acceptor interactions between the electron-rich HBC and the electron-poor rylene dye lead to strictly alternating and long-range-ordered stacks. The mixtures display homeotropic orientation on surfaces (picture shows an image from an optical microscope). (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200500669

Angewandte
Chemie
Supramolecular Chemistry
DOI: 10.1002/anie.200500669
Pronounced Supramolecular Order in Discotic
Donor–Acceptor Mixtures**
Wojciech Pisula, Marcel Kastler, Daniel Wasserfallen,
Joseph W. F. Robertson, Fabian Nolde,
Christopher Kohl, and Klaus Müllen*
Dedicated to the memory of Tadeusz Pakula
An approach to tuning the properties of materials is the
blending of two different discotic compounds. In analogy to
the mixing of two linear polymers, various effects can be
expected, ranging from a phase separation in the blend to the
formation of a homogenous one-phase system. In the case of
two polymers, the blending enthalpy determines whether the
mixture will be homogenous or phase separated.[1] The mixing
of two macromolecules that self-assemble into superstruc-
tures is complex, since additional interactions between the
different components complicate the description of the
system.[2] As illustrated in Figure 1, several different supra-
molecular assemblies can be formed in the mixture. For
discotic molecules, which self-assemble in columnar super-
Figure 1. Different possible supramolecular organizations in a mixture
of two discotic compounds with different molecular architectures.
[*] Dr. W. Pisula, M. Kastler, D. Wasserfallen, Dr. J. W. F. Robertson,
F. Nolde, Dr. C. Kohl, Prof. Dr. K. Müllen
Max-Planck Institut für Polymerforschung
55128 Mainz (Germany)
Fax: (+49)6131-379-350
E-mail: muellen@mpip-mainz.mpg.de
[**] This research was supportedby the EU through the NAIMO
(integratedproject no. NMP4-CT-2004-500355) andby the
Deutsche Forschungsgemeinschaft (Schwerpunkt Feldeffekttran-
sistoren). M.K. thanks the Fonds der Chemischen Industrie and the
Bundesministerium für Bildung und Forschung for financial sup-
port.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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structures by p stacking, this ranges from a heterogeneous
phase to micro- and even nanoseparation.
blended molecules are compatible in size, permitting matched
intracolumnar packing and leading to a different self-assem-
bly on surfaces.
Intensive investigations of mixtures containing triphenyl-
ene as the donor compound have been carried out, whereby
trinitrofluorenone,[2,3] azatriphenylenes,[4,5] and mellitic trim-
ides[6] have been used as acceptor molecules. For equimolar
mixtures, alternating stacking of the acceptor and donor
molecules has been observed, leading to an enhanced
columnar stability. Time-of-flight experiments verified an
enhanced columnar organization and revealed considerably
higher charge-carrier mobilities for the blends.[7–9]
In this study, the effect of blending two discotic materials,
which differ significantly in their electronic properties, was
examined. While hexa-peri-hexabenzocoronene substituted
by long, branched alkyl chains (HBC-C10,6, 1; Figure 2) is an
electron-donating agent, perylenediimide[10] (PDI, 2) and
terrylenediimide derivatives (TDI, 3) are strong acceptors.
Semiemperical calculations (AM1) indicate that there is a
weak electronic interaction between the two components,
which should lead to alternating stacking in the columnar
structure and therefore to increased stabilization and a higher
level of order in this donor–acceptor mixture. In addition, the
The substitution of the HBC aromatic core with bulky,
branched side chains led to a considerable lowering of the
isotropization temperature (Ti) to 938C for 1 in comparison to
those of HBC derivatives with linear side chains. Two-
dimensional wide-angle X-ray diffraction (2D-WAXS)
experiments indicated a herringbone arrangement (Fig-
ure 3a) of the HBC discs in the room-temperature plastic
crystalline phase, whereby heating beyond the phase tran-
sition at 248C (second differential calorimetry scan at a rate
of 108CminÀ1) resulted in a disordered columnar liquid-
crystalline (LC) phase. Highly birefringent dendritic struc-
tures appeared when the material was cooled from the
isotropic phase between glass slides, implying defect struc-
tures with columns predominately parallel to the surface
(Figure 2a). During crystallization of 2 from the isotropic
phase (Ti = 1308C), optical textures with pseudofocal conical
fan shapes (Figure 2b) were observed, indicating the forma-
tion of a mesophase in which the molecules are orthogonally
arranged with respect to the stacking axis. Such intracolumnar
Figure 2. Chemical structures andpolarizedoptical microscopic images of a) 1, b) 2, andc) 3 (the arrow indicates the columnar orientation); in
the inset the same section of the sample was rotated relative to the direction of polarization of the light, and it indicates a high level of optical
anisotropy. The crossed arrows indicate the orientation of the polarization filters in the optical microscope. The images were recorded during
crystallization from the isotropic phase at the cooling rate of 18CminÀ1
.
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repeating distance between single building blocks is 0.34 nm.
This distance is in relation to the (hkl) reflection lines
indicating strong correlations over long ranges between ten
molecules per helical pitch (helical pitch distance: 3.4 nm).
The suggested model for the supramolecular arrangement is
illustrated in Figure 4. It is assumed that an alternating
molecular packing results in the helical organization. In this
Figure 3. Room-temperature 2D-WAXS patterns of a) 1 b) 2, and
c) andan equimolar mixture of 1 and 2 (inset shows the sample
directly after extrusion). The Miller’s indexes describe the periodicity
along the columnar structures. All samples were preparedby filament
extrusion, andthe filament axis was orientedvertically in the diffrac-
tion experiment.
Figure 4. Schematic packing model of an alternating intracolumnar
arrangement for the binary mixture. Molecules of 2 are intercalated
between units of 1, which are displaced by 128 with respect to each
other, resulting in a helical pitch of 3.4 nm.
packing of 2 was also confirmed by the 2D-WAXS experi-
ments.
Mixtures of 1 with 2 or 3 were first prepared at set molar
ratios in solution and afterwards blended in the isotropic
phase to avoid demixing due to different solubilities. The
investigation of the thermal behavior by differential scanning
calorimetry (DSC) revealed that all prepared mixtures
formed a macroscopic, homogeneous phase.
Photoluminescence excitation spectra for an equimolar
mixture of 1 and 2 in solution showed a superposition of the
spectra of the pure components. In contrast, drop-cast films of
the blend showed a shift of the fluorescence signal of 2 by
60 nm accompanied by a strong quenching, which was
expected because of energy-transfer processes. The signifi-
cant change of the electronic environment of the chromo-
phore was supported by the differential pulse voltammetry
measurements. These displayed an additional peak for the
mixture which was shifted by 0.18 eV in comparison to
measurements with pure 2.
The 2D-WAXS pattern of the equimolar mixture of 1 and
2 indicated a considerable difference of the supramolecular
arrangement in the blend (Figure 3c). The X-ray pattern
changed significantly when the extruded sample was annealed
for 48 h at ambient conditions, whereby the number of distinct
reflections increased dramatically. A set of new higher order
reflections appeared, indicating an exceptional long-range
order and a complex helical arrangement of the two discotic
species within the columnar stacks. The transformation of the
first equatorial reflection, which converted from an isotropic
shape (Figure 3c) to three sharp peaks, was particularly
impressive. This implied not only a simple intracolumnar
reorganization but also a rearrangement of entire columnar
segments.
structure, PDI (2) is intercalated between the HBC discs (1),
which are rotated with respect to each other by 128 thus
providing the necessary repeat unit for the helical arrange-
ment. This rotation is induced by the space demand of the
alkyl side chains in both derivatives. In 2003 helical columnar
arrangements were observed for HBC derivatives with rigid
substituents.[12] Electronic donor–acceptor interactions were
thought to cause the observed pronounced alternating intra-
columnar ordering. A similar situation was reported for single
crystals consisting of hexafluorobenzene and fullerene; the
interactions were confirmed by fluorescence and differential
pulse voltammetry measurements.[13]
To gain more detailed insight into the supramolecular
arrangement of other mixtures, complementary compositions
with ratios of 1:2and 2:1 were investigated. All extruded
samples showed significantly higher order after annealing.
The 2D-WAXS pattern of both blends displayed reflections at
positions identical to those observed for the 1:1 blend, but
with different reflection intensities, which depended on the
composition ratio. The additional reflections reached their
maximum intensity for the 1:1 molar ratio, possibly due to
sequences of identical molecules of the excess components in
the nonequimolar mixtures.
The new supramolecular structure in the blends signifi-
cantly affected the thermal behavior and the morphology.
When a 2:1 mixture of HBC (1) and PDI (2) was cooled down
from the isotropic phase between two glass slides, a homo-
genous film was obtained that did not display significant
birefringence in polarized light; this would be characteristic
for a homeotropic phase (Figure 5a). The transmission 2D-
WAXS pattern of the film confirmed the proposed orienta-
tion (Figure 5b), whereby the lateral arrangement of the
lattice differed considerably between the domains. This
The layered distribution of the reflections in the meri-
dional direction in the WAXS pattern in Figure 3c was
identified by the Millerꢀs indices (hkl), which were assigned to
the intracolumnar packing.[11] The smallest intracolumnar
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Figure 6. Example of a homeotropically alignedsample basedon the
2:1 mixture of 1 and 3. a) Image from the optical microscope (polar-
izer andanalyzer at a 45 8 angle with respect to each other; lower inset
recorded with crossed polarizers, upper inset image recorded during
crystallization of the dendritic structure). b) Characteristic 2D-WAXS
pattern of the film with homeotropic order.
Figure 5. Example for a homeotropically alignedsample basedon the
2:1 mixture of 1 and 2. a) Image from the optical microscope (polar-
izer andanalyzer at a 45 8 angle with respect to each other; inset
recorded with crossed polarizers). b) Characteristic 2D-WAXS pattern
of the film with homeotropic order (identical hexagonal lattice in
different small domains leads to the appearance of multiple reflec-
tions).
morphology stands in contrast to those of the individual
components. A similar morphology was also obtained for
mixtures of other compositions; however, the number of
birefringent defects increased significantly.
photoluminescence and differential pulse voltammetry, lead
to a significantly higher level of order within the self-
assembled columnar stacks. The molecules in the mixture
assume a homeotropic orientation, due to an auxiliary effect
of the smaller aromatic molecule, while forming the first
monolayer of the sterically demanding HBCs on the surface.
In general, HBCs with nonbulky alkyl substituents align
homeotropically when cooled down from the isotropic phase.
When the p-stacking interactions between the aromatic cores
are significantly reduced in the isotropic phase, the molecules
arrange with their planes parallel to the surface. This
arrangement is assumed to be the most thermodynamically
favored, since interactions (e.g. van der Waals interactions)
between the molecule and the surface are maximized. On the
other hand, the high steric demand of the alkyl chains close to
the aromatic core might hinder the approach the disc to the
Experimental Section
The blends were first prepared in tetrachlormethane and were then
used in the isotropic phase. To ensure an intermixture of both
compounds, the blends were processed in an ultrasonic bath under
nitrogen atmosphere.
The 2D-WAXS experiments were performed by means of a
rotating anode (Rigaku 18 kW) X-ray beam with a pinhole collima-
tion and a Siemens 2D detector. A double graphite monochromator
for the CuKa radiation (l = 0.154 nm) was used. The samples were
oriented by filament extrusion. The optical textures were investigated
using a Zeiss microscope with polarizing filters equipped with a
Hitachi KP-D50 color digital CCD camera. The samples were
sandwiched between two glass slides and then thermally treated
(first heated to the isotropic phase and then slowly cooled down) on a
Linkam hotstage regulated with a Linkam TMS 91 temperature
controller.
Electrochemical measurements were performed on a voltametric
analyzer (AutoLab PGSTAT-30, potentiostat/galvanostat) in a three-
electrode cell with a gold working electrode (3 mm diameter), a silver
quasi-reference electrode (AgQRE, calibrated with the Fc/Fc+ redox
couple E8 = 4.8 eV) and a platinum counterelectrode. Films were
dropcast from a solution of toluene. Tetrabutylammonium perchlo-
rate (TBAClO4, 0.1m) and acetonitrile were used as electrolyte and
solvent, respectively. Differential pulse voltammetry was measured
with 15-mV, 50-ms pulses at 100-ms intervals.
surface, as seen for HBC-C10,6 1 and previously for HBC-
[14]
C14,10
.
In both cases, the high degree of rotational freedom
at the b position of the side chains results in the increase of the
steric requirements, which lead the formation of defect
structures
The steric influence of the substituents on the columnar
packing can be reduced synthetically[15] or, as described in this
work, by packing effects of the donor–acceptor molecules.
The electronic interaction between 1 and 2 resulted in the
strictly alternating stacking of the donor and acceptor species.
The distance between the HBC discs in the columns
increased, whereby the steric influence of the long, branched
C10,6 alkyl chains on the stacking was significantly reduced.
Consequently, the mixture formed a homeotropic phase.
Additionally, the intracolumnar packing was enhanced by the
helical arrangement of the molecules. Analogous behavior
was observed for mixtures of 1 and 3 with molar ratios of 1:1
and 2:1 (Figure 6). Both blends revealed a homeotropic
alignment suggesting an identical intracolumnar packing of
the two discotics as described above.
In conclusion, we have shown that supramolecular
organization in a binary mixture differs strongly from that
of the individual components. Strictly alternating stacks
formed spontaneously when the mixture was cooled from
the isotropic state, because of the weak donor–acceptor
interactions between the electron-rich 1 and the electron-
poor rylene dyes 2 and 3. These interactions, proved by
The photoluminescence was recorded on a SPEX Fluorolog 2
type 212 steady-state fluorometer at a concentration of 10À3 m and
dropcast films on quartz substrates. DSC was measured with a Mettler
DSC 30 at a heating rate of 10 KminÀ1 from À1008C to 2 008C. The
electronic potential density distribution was calculated by means of
Spartan for Windows (AM1 of ground state). The synthesis of HBC-
C10,6 1 has been described elsewhere;[12] the syntheses of PDI 2 and
TDI 3 are described in the Supporting Information.
Received: February 22, 2005
Revised: July 7, 2005
Published online: December 19, 2005
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Keywords: dyes/pigments · helical structures ·
.
homeotropic phases · supramolecular chemistry
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