Diketopyrrolopyrrole Columnar Liquid-Crystalline Assembly Directed by Quadruple Hydrogen Bonds

Article abstract of DOI:10.1002/anie.201705137

A diketopyrrolopyrrole (DPP) dye self-assembles via a unique hydrogen-bonding motif into an unprecedented columnar liquid-crystalline (LC) structure. X-ray and polarized FTIR experiments reveal that the DPPs organize into a one-dimensional assembly with t

Full text of DOI:10.1002/anie.201705137

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201705137
German Edition: DOI: 10.1002/ange.201705137
Liquid Crystals
Diketopyrrolopyrrole Columnar Liquid-Crystalline Assembly Directed
by Quadruple Hydrogen Bonds
Bartolome Soberats,* Markus Hecht, and Frank Wꢀrthner*
Abstract: A diketopyrrolopyrrole (DPP) dye self-assembles
via a unique hydrogen-bonding motif into an unprecedented
columnar liquid-crystalline (LC) structure. X-ray and polar-
ized FTIR experiments reveal that the DPPs organize into
a one-dimensional assembly with the chromophores oriented
parallel to the columnar axis. This columnar structure is
composed of two p–p-stacked DPP dimers with mirror-image
configurations that stack alternately through quadruple hydro-
gen bonding by 908 rotation. This exotic packing is dictated by
the complementarity between H-bonds and the steric demands
of the wedge-shaped groups attached at the core. This novel LC
supramolecular material opens a new avenue of research on
DPP dye assemblies with photofunctional properties tailored
by H-bonding networks.
structure of PR 254 reveals an attractive supramolecular
motif based on double hydrogen-bonding between the self-
complementary lactam units combined with p–p-stacking
interactions between the DPP p-scaffolds.[34] However, the
perfect interplay of these non-covalent forces establishes the
pigment character, that is, complete insolubility in any
solvent. Upon alkylation of the lactam nitrogen, well-soluble
dyes are generated, which in contrast to their PBI[25b]
counterparts show little propensity for aggregation owing to
weak p–p interactions between the rather small DPP p-
scaffolds. Herein we report the first example for the self-
assembly of DPP dyes guided by H-bonding between non-
alkylated lactam units, leading to a columnar LC phase with
an unprecedented 1D packing formed by quadruple H-
bonded DPP dimers (Figure 1). Surprisingly, we found that
this assembly is formed through the alternate stacking of two
DPP dimers with mirror image configuration (dimer A and
Self-assembly of organic dyes has emerged as a promising
approach for the rational design of functional materials.[1–8]
This is commonly performed by encoding information in the
molecular building blocks to control their assembly process
and packing arrangement which, in turn, define the proper-
ties. In this regard, among non-covalent interactions partic-
ularly hydrogen bonds (H-bonds) evolved as most useful for
the creation of functional non-covalent architectures, such as
supramolecular polymers,[9–12] gels,[13] and liquid crystals.[14–20]
The control of self-assembly directed by H-bonds has been
widely investigated for many dyes such as, for instance,
porphyrins,[21] BODIPYs,[22] pyrenes,[23] merocyanines,[24]
naphthalene, and perylene bisimides (PBIs),[25] giving access
to assemblies with desired functional properties and charac-
teristic optical signatures such as H- and J-type exciton
couplings.[26–28]
Interestingly, despite the wide application of diketopyrro-
lopyrrole (DPP) dyes as the most-brilliant red color pig-
ments[29,30] (for example, Pigment Red (PR) 254, nicknamed
“Ferrari Red”) and semiconductors in organic electronics and
photovoltaics,[31,32] the self-assembly of DPPs in solution and
in the liquid-crystalline (LC) state has been barely
explored.[33] This is particularly surprising as the crystal
[*] Dr. B. Soberats, Prof. Dr. F. Wꢀrthner
Center for Nanosystems Chemistry (CNC) & Bavarian Polymer
Institute (BPI), Universitꢁt Wꢀrzburg
Theodor-Boveri-Weg, 97074 Wꢀrzburg (Germany)
E-mail: bartolome.soberats_reus@uni-wuerzburg.de
wuerthner@uni-wuerzburg.de
Figure 1. a) The chemical structure of DPP 1. b) Illustration of the
molecular self-assembly of DPP 1 into a columnar hexagonal LC
phase, where the DPP molecules organize in stacked dimers forming
H-bonds along the columnar axis. This assembly mode requires the
stacking of two DPP dimers (dimer A and dimer B), which are mutual
enantiomers. The alternate stacking by 908 rotation of dimer A and
dimer B induces quadruple H-bonding interactions. The stacking by
908 rotation of one type of dimers would lead to H-bonding mismatch.
M. Hecht, Prof. Dr. F. Wꢀrthner
Institut fꢀr Organische Chemie, Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201705137.
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dimer B; Figure 1b) that show a complementary H-bonding
topology when they stack by 908 rotation (Figure 1b).
The molecular design of DPP 1 (Figure 1a) bearing two
3,4,5-tridodecyloxyphenyl wedge-shaped moieties at the 3-
À
and 6-position of the core and two free N H groups at the
lactam positions was guided by our previously established
design principles for the H-bond-directed self-assembly of
PBI dyes.[35] The wedge-shaped part is expected to provide
solubility and mesogenic features to the compound, while the
À
free N H and the carbonyl groups are supposed to direct the
self-assembly by H-bonding interactions (Figure 1). DPP
1 was obtained in a straightforward way as a red waxy solid by
slight modifications of a previously described procedure
(Supporting Information).[36] The UV/Vis spectrum of DPP
1 in chloroform showed a characteristic absorption band at
519 nm with vibronic progressions and a weaker band at
352 nm,[29–34] while the spectrum is broadened in the solid
state (Supporting Information, Figure S2).
The LC properties of DPP 1 were examined by polarizing
optical microscopy (POM), differential scanning calorimetry
(DSC), and middle- and wide-angle X-ray scattering (MAXS
and WAXS) experiments. POM observation and DSC experi-
ments revealed the formation of two LC phases, a low-
temperature phase from 0 to 458C and a high-temperature
phase from 45 up to 3368C, at which the material decomposes
(Supporting Information, Figures S3 and S4). The X-ray
diffraction pattern of DPP 1 at 308C shows three broad
reflections. owing to the broadness of the signals, this phase
was not clearly identified, but the two intense reflections in
the small-angle region are consistent with the 200 and 110
signals of a columnar rectangular (Colr) phase (Supporting
Information, Figure S6). In contrast, the X-ray pattern at
808C clearly shows the 100, 110, 200, 210, and 310 character-
istic reflections of a columnar hexagonal (Colh) lattice (a =
28.5 ꢀ) (Figure 2a). In both cases, the presence of a diffused
halo indicates liquid-like disordered aliphatic chains.
Figure 2. a) Integrated intensities along the equator of the WAXS
pattern of DPP 1 at 808C. b),c) WAXS patterns of an extruded fiber of
DPP 1 at 808C with the fiber axis lying (b) and standing (c). Direction
of the fiber is indicated with yellow arrows. Layer lines (hkl) are
indicated along the meridian.
The relative orientation of the DPP cores in both
columnar assemblies was studied by polarized FTIR experi-
ments and POM on aligned samples of DPP 1 at 30 and 808C
(Figure 3; Supporting Information, Figures S5 and S7). The
sample for IR studies was aligned by friction transfer of an
extruded fiber of DPP 1 on a KBr plate, which produced the
orientation of the columns parallel to the substrate and the
shearing direction. This was confirmed in the POM experi-
ments of the aligned samples on glass plates (Supporting
Information, Figure S5).[14,15,17] Importantly, the FTIR spectra
of DPP 1 show the NH stretching vibration at 3170 cmÀ1,
which indicates that the NH groups are H-bonded,[35,37] as it is
expected for a DPP with unsubstituted lactams.[34] The
polarized FTIR spectra of the sheared sample show that the
NH peak is more intense when the columnar phase is
homogeneously aligned in the parallel direction of the
polarizer, and practically disappears when the polarizer is
oriented perpendicularly to the aligned sample (Figure 3a;
Supporting Information, Figure S7). These experiments
revealed that the lactam functionalities and subsequently
the p-surfaces are oriented parallel to the columnar axis
(Figure 3b).
Figure 3. a) Polarized FTIR spectra of an aligned sample of DPP 1 at
808C with the polarizer parallel (red line) and perpendicular (black
line) to the direction of the alignment. b) Representation of the
homogenously aligned DPP 1 columnar assembly and the relative
orientation of the DPP molecules.
To gain deeper insights into the self-assembled structure,
we analyzed in detail the WAXS patterns of lying and
standing fibers of DPP 1 at 808C (Figure 2b,c). The diffuse
signal at 4 ꢀ on the equator (Figure 2b) suggests low
correlated core–core interactions orthogonal to the columnar
axis.[35b] This is in accordance with the orientation of the DPP
cores parallel to the columnar axis as it is also confirmed by
FTIR experiments (Figure 3). Moreover, the observation of
reflections in the quadrants and the meridian of the X-ray
pattern allowed us to get more details about the organization
of the molecules into the columnar assembly. A strong signal
on the meridian at 7 ꢀ matches well with the length of a DPP
from NH to NH (6.0 ꢀ; Supporting Information, Figure S8).
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This reflection was assigned to the layer line hk2 (Figure 2c),
while the off-meridional reflections appearing at a distance
corresponding to 14 ꢀ (twice the length of the DPP) were
assigned to layer line hk1 (Figure 2c). The layer line hk4
could also be observed on the meridian in the wide-angle
region. These observations indicate a correlation of every
second molecule along the columns, which implies a rotation
of 908 between DPPs of adjacent strata.[38] The number of
molecules per columnar slice (h = 7 ꢀ) was estimated to be
two, considering the cell parameters (a = 28.5 ꢀ) and assum-
ing a reasonable density of 1 gcmÀ3 (Supporting Informa-
tion).[35b] Accordingly, the columnar assembly is composed of
p-stacked DPP dimers that are in turn stacked along the
column by 908 rotation (Figure 1b). A similar arrangement of
DPP dyes was observed in low temperature (308C) Colr phase
as revealed by WAXS and polarized FTIR experiments
(Supporting Information, Figures S6 and S7). We assume that
this rotation of the dimeric DPP axial subunits is imposed by
the steric demand of the wedge-shape groups and space filling
effects.[38,39]
According to the parameters obtained from X-ray experi-
ments, we modeled a packing structure for DPP 1 in the Colh
phase using the software package Materials Studio. We
assume that the DPP cores are in the center of the columns,
connected by H-bonds and surrounded by the alkyl chains
(Figure 1). For the modeling studies, we focused on the DPP
part and therefore the peripheral dodecyl chains were omitted
to simplify the system. The model assembly was constructed
starting from a co-facial DPP dimer where the carbonyl and
NH groups are alternated to minimize repulsions (Supporting
Information, Figure S9). The next two DPPs were rotated by
908, according to the X-ray data, and connected by H-bonds
to the initial dimer. The structure was completed with the
stacking of eight molecules, paired in four stacked DPP
dimers with the cores parallel to the columnar axis. The
geometry was optimized by the force-field COMPASS using
the Ewald summation method. The optimized structure shows
a negative non-bonding interaction energy, which suggests
a stable assembly. Figure 4a depicts the resulting minimized
structure that shows a columnar stack of four DPP dimers
connected via quadruple H-bonds. More interestingly, our
modeling studies reveal that this supramolecular assembly via
complementary H-bonds entails the 1D stack of two stereo-
chemically distinct DPP dimers,[40] dimer A and dimer B
(Figure 4b). Each dimer presents a different topology of the
H-bonding units (carbonyl and NH groups). Thus, the perfect
H-bond matching to form the columnar structure from dimers
rotated by 908 (according to X-ray data) is only achieved by
the alternation of dimer A and dimer B (Figure 4c). In
contrast, the formation of an H-bonded network is not
possible by the stack of one type of dimers (Figure 4c). The
steric demands of DPP 1, which imposes the 908 rotation of
dimers, and the complementarity of the H-bonding between
dimeric units determine this unprecedented columnar
arrangement. Retrostructural analysis of the minimized
structure was performed utilizing the program CLEARER
(Supporting Information, Figure S10).[41] Pleasingly, the simu-
lated X-ray pattern shows a very good match with the
Figure 4. a) Geometry-optimized structure of the DPP 1 assembly
showing the alternate stacking of dimers. b) Structure of dimer A and
dimer B found in the assembled structure of DPP 1. c) Illustration of
the H-bonding mismatch (left) and match (right) in B–B and A–B
stacked dimers, respectively. The H-bonds are illustrated as double
arrows, while the H-bond mismatches with red crossed double arrows.
The tridodecyloxyphenyl groups of the DPPs were omitted for clarity.
À
DPP cores red, O blue, N H green. d) Superposition of the simulated
diffraction pattern obtained by CLEARER and the MAXS pattern of DPP
1 at 808C.
reflections of the experimental MAXS pattern, supporting
the proposed structure (Figure 4d).
In summary, we discovered an unprecedented supra-
molecular motif that guides the formation of a liquid-
crystalline 1D assembly of DPP 1 by quadruple hydrogen
bonding between p-stacked DPP dimers. This complex
assembly is formed by the stack of two complementary
dimers where the diketopyrrolopyrrole p-cores lie parallel to
the columnar axis. This uncommon organization in columnar
liquid crystals warrants further exploration, which includes
research on the growth of functional supramolecular fibers in
solution and the application of such novel diketopyrrolopyr-
role self-assemblies in functional materials.
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Keywords: diketopyrrolopyrroles · dyes/pigments ·
hydrogen bonds · liquid crystals · self-assembly
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Manuscript received: May 18, 2017
Accepted manuscript online: July 6, 2017
Version of record online: && &&, &&&&
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Communications
Liquid Crystals
A carefully designed diketopyrrolopyrrole
dye self-assembles via quadruple hydro-
gen bonds into columnar liquid-crystal-
line phases. The 1D assembly is formed
by the stack of two enantiomerically
related dimers (A and B) with comple-
mentary hydrogen-bonding topology. As
a result of this stacking mode, the dyes
are oriented parallel to the columnar axis.
B. Soberats,* M. Hecht,
F. Wꢁrthner*
&&&& — &&&&
Diketopyrrolopyrrole Columnar Liquid-
Crystalline Assembly Directed by
Quadruple Hydrogen Bonds
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www.angewandte.org
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