C-H/π-interaction-guided self-assembly in π-conjugated oligomers

Article abstract of DOI:10.1002/chem.201102670

We report CH/π hydrogen-bond-driven self-assembly in π-conjugated skeletons based on oligophenylenevinylenes (OPVs) and trace the origin of interactions at the molecular level by using single-crystal structures. OPVs were designed with appropriate pendants in the aromatic core and varied by hydrocarbon or fluorocarbon tails along the molecular axis. The roles of aromatic π-stack, van der Waals forces, fluorophobic effect and CH/π interactions were investigated on the theromotropic liquid crystallinity of OPV molecules. Single-crystal structures of hydrocarbon OPVs provided direct evidence for the existence of CH/π interactions between the π-ring (H-bond acceptor) and alkyl C-H (H-bond donor). The four important crystallographic parameters, d_c-x=3.79 A, θ=21.49°, =150.25° and d_Hp-x=0.73 A, matched in accordance with typical CH/π interactions. The CH/π interactions facilitate the close-packing of mesogens in x-y planes, which were further protruded along the c axis producing a lamellar structure. In the absence of CH/π interactions, van der Waals interactions drove the assembly towards a Schlieren nematic texture. Fluorocarbon OPVs exhibited smectic liquid-crystalline textures that further underwent Smectic A (SmA) to Smectic C (SmC) phase transitions with shrinkage up to 11 %. The orientation and translational ordering of mesogens in the liquid-crystalline (LC) phases induced H- and J-type molecular arrangements in fluorocarbon and hydrocarbon OPVs, respectively. Upon photoexcitation, the H- and J-type molecular arrangements were found to emit a blue or yellowish/green colour. Time-resolved fluorescence decay measurements confirmed longer lifetimes for H-type smectic OPVs relative to that of loosely packed one-dimensional nematic hydrocarbon-tailed OPVs. Weak-force-driven self-assembly: CH/π hydrogen-bonding-driven diverse molecular self-assembly in π-conjugated molecules is reported (see figure). The self-organisation of π-conjugated mesogens in three-dimensional crystal lattices is proven to be vital for tuning the thermotropic liquid-crystalline phases and their emission colour. Copyright

Full text of DOI:10.1002/chem.201102670

FULL PAPER
DOI: 10.1002/chem.201102670
À
C H/p-Interaction-Guided Self-Assembly in p-Conjugated Oligomers
Mahima Goel and Manickam Jayakannan*[a]
Abstract: We report CH/p hydrogen-
bond-driven self-assembly in p-conju-
gated skeletons based on oligophenyle-
nevinylenes (OPVs) and trace the
origin of interactions at the molecular
level by using single-crystal structures.
OPVs were designed with appropriate
pendants in the aromatic core and
varied by hydrocarbon or fluorocarbon
tails along the molecular axis. The
roles of aromatic p-stack, van der
Waals forces, fluorophobic effect and
CH/p interactions were investigated on
the theromotropic liquid crystallinity of
OPV molecules. Single-crystal struc-
tures of hydrocarbon OPVs provided
direct evidence for the existence of
CH/p interactions between the p-ring
bond donor). The four important crys-
tallographic parameters, dcÀx =3.79 ꢀ,
liquid-crystalline textures that further
underwent Smectic A (SmA) to Smec-
tic C (SmC) phase transitions with
shrinkage up to 11%. The orientation
and translational ordering of mesogens
in the liquid-crystalline (LC) phases in-
duced H- and J-type molecular ar-
rangements in fluorocarbon and hydro-
carbon OPVs, respectively. Upon pho-
toexcitation, the H- and J-type molecu-
lar arrangements were found to emit a
blue or yellowish/green colour. Time-
resolved fluorescence decay measure-
ments confirmed longer lifetimes for
H-type smectic OPVs relative to that
of loosely packed one-dimensional
nematic hydrocarbon-tailed OPVs.
q=21.498, f=150.258 and dHpÀx
=
0.73 ꢀ, matched in accordance with
typical CH/p interactions. The CH/p
interactions facilitate the close-packing
of mesogens in x–y planes, which were
further protruded along the c axis pro-
ducing a lamellar structure. In the ab-
sence of CH/p interactions, van der
Waals interactions drove the assembly
towards a Schlieren nematic texture.
Fluorocarbon OPVs exhibited smectic
Keywords: conducting materials
·
·
liquid crystals
self-assembly
chemistry
· nanomaterials
·
supramolecular
À
(H-bond acceptor) and alkyl C H (H-
Introduction
stabilizing their secondary and tertiary structures.[3] CH/p in-
teractions were found to be responsible for the specific ori-
entation of the alkyl chain towards the phenyl group of
amino acid residues, complexation of proteins with cofac-
tors, carbohydrates, and so on.[4] CH/p interactions were
also observed in a few small organic molecules and inorgan-
ic complexes.[5] p-Conjugated oligomers (or polymers) are
emerging as an important class of materials and they were
typically self-organized by weak non-covalent interactions,
such as hydrogen bonding,[6] metal-ion interactions,[7] aro-
matic p stacking,[8] and van der Waals forces,[9] etc. High mo-
lecular weight conducting polymers and oligomers employed
in the optoelectronic devices typically consist of a rigid p-
conjugated backbone with flexible alkyl chains anchored on
the periphery for solubility purposes.[10] Therefore, it is rea-
sonable to assume that these p-conjugated systems could
also have weak hydrogen bonding through CH/p interac-
À
The interactions between C H bonds and p systems belongs
to an important type of weak hydrogen bond (non-cova-
À
lent), with the C H group and p ring acting as hydrogen
donor and acceptor, respectively.[1] The energy associated
with CH/p interactions are much smaller (0.8 kcalmolÀ1) rel-
ative to that of typical strong hydrogen-bonding interactions
between OH···H or NH···H (7.8 to 3.5 kcalmolÀ1).[2] Interest-
ingly, it was found to be strong enough to stabilize mole-
cules in a particular conformation to produce higher-order
self-assembly.[2] The CH/p interactions in protein folding
was recognized as one of the main non-covalent forces in
[a] M. Goel, Dr. M. Jayakannan
Department of Chemistry, Indian Institute of Science Education
and Research, Dr. Homi Bhabha Road, Pune 411008
Maharashtra (India)
Fax : (+91)20-2589-8022
E-mail: jayakannan@iiserpune.ac.in
À
tions between the p ring and C H in alkyl chains. To our
knowledge, there is no report on the CH/p-interaction-in-
duced molecular self-assembly in the p-conjugated systems.
Thus, the study of CH/p interactions in p-conjugated materi-
als is an important task as it will provide in depth insight
into the self-organization phenomena.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102670. It contains syntheses
and characterization of all compounds, GPC, TGA and DSC traces,
plots of enthalpies against different pendent groups, temperature-de-
pendent PLM textures, crystallographic images, XRD plots for HC
and FC-OPVs, absorption and excitation spectra, intensity decay pro-
files for OPV-HC-C8, OPV-FC-C8, OPV-HC-EH and OPV-FC-EH,
1H, 13C and 19F NMR spectra and MALDI spectra. Additionally,
Tables (ST1–ST4) are also provided.
The present work reports, for the first time, CH/p interac-
tion driven mesoscopic self-organization in the p-conjugated
building blocks, more specifically based on oligophenylene-
vinylenes (OPV)s (see Scheme 1). Single-crystal structures
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hexyl and norbornane methyl
are referred to as EH and NB
instead of Cx.
Thermal transitions in OPVs
were studied by differential
scanning calorimetry (DSC) at
heating and cooling rates of
108minÀ1 (see Figures SF3 and
SF4 in the Supporting Informa-
tion). OPVs with fluorocarbon
tails showed two thermal transi-
tions in heating and cooling
cycles confirming their thermo-
tropic liquid-crystalline nature.
The change in the chain length
or cyclic nature of the middle
alkoxy unit (from C1 to C11, C8
or NB) decreased the liquid-
crystalline
(LC)-transition
window by almost 60–808C.
Among the three C8 carbon
atom pendants, the OPV-con-
Scheme 1. Weak-force-driven diverse self-assembly in p-conjugates.
taining branched side chain
unit—ethylhexyl—was found as
of the OPVs were resolved and the origin of the molecular
self-assembly in p-conjugates was traced at the molecular
level. The design of the OPVs was chosen in such a way
that the skeletons could only self-organize by either aryl p-
stack, interdigitations of flexible alkyl units or a combina-
tion of both. The aromatic core was fixed as a trans-distyrl
unit (p-ring, H-bond acceptor) in all the OPV molecules
a crystalline solid (see Figure SF3 in the Supporting Infor-
mation). The comparison of the enthalpies of thermal transi-
tions (see Tables ST1 and ST2 in the Supporting Informa-
tion) and the peak intensities in DSC profiles indicated that
the fluorocarbon OPVs, in general, underwent less energetic
crystallization processes initially (isotropic-to-LC) followed
by larger crystallization during the LC-to-crystalline phase
change (completely reversible in the subsequent heating
cycle). The trend was almost identical in all fluorinated
tailed OPVs irrespective of the pendant groups attached in
the middle aromatic core. Unlike in the fluorocarbon-tailed
OPVs, the hydrocarbon-tailed OPV series was characterized
by a strong isotropic-to-LC phase transition followed by
weak LC-to-crystalline phase change. The enthalpies of
melting and crystallization transitions were plotted against
the type of pendent groups (see Figure SF5 in the Support-
ing Information). The enthalpies (DH) of the transitions are
directly associated with the packing of the chromophores:
highly packed structures need more energy for melting and
to expel larger amounts of heat exothermally during the
crystallization process. Based on the enthalpy values (see
Tables ST1 and ST2 and Figure SF5 in the Supporting Infor-
mation), it may be concluded that the hydrocarbon tails pro-
duced high crystalline samples relative to that of the fluoro-
carbon tails. For a particular series, the enthalpies of melting
(or crystallization) followed the order for pendants: NB <
C1 =EH <C8 =C11 (which was found to be similar in both
OPV series). Although the cycloaliphatic system was capa-
ble of inducing LC-mesophase in OPVs, the chromophore
packing was relatively weak relative to their linear chain an-
alogues.[12]
À
and the alkyl chains (C H, H-bond donor) in the skeleton
are varied at two positions: 1) in the central aromatic ring
by using linear, branched and cycloaliphatic alkyl units with
C8 or C11 carbon atoms and 2) at the longitudinal position
by either a hydrocarbon or fluorocarbon tail containing C11
carbon atoms. Semi-perfluorinated chains were introduced
in the OPV structures to vary rigidity,[11] which further facili-
tated the study on the role of the tail on the self-assembly.
Thus, the present building blocks have three weak non-cova-
lent forces, namely, van der Waals forces, aromatic p–p
stacking and fluorophobic effects in combination with CH/p
weak hydrogen bonds. These forces drove diverse molecular
self-assembly in these OPV units.
Results and Discussion
Two series of OPVs consisting of hydrocarbon and fluoro-
carbon tails were synthesized by multi-step reactions and
their structures are shown in Scheme 2. The detailed syn-
thetic procedures and structural characterization are provid-
ed in the Supporting Information. The OPVs are referred as
OPV-HC-Cx or OPV-FC-Cx, in which HC and FC stand for
hydrocarbon and fluorocarbon tails, respectively. Cx =C1, C8
or C11 represent the number of carbon atoms present in the
linear chains methyl, n-octyl and n-undecyl units. 2-Ethyl-
OPVs were subjected to polarized light microscopic anal-
ysis with a programmable hot stage to study their tempera-
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Self-Assembly in p-Conjugated Oligomers
FULL PAPER
ical droplets as nucleating sites
and these droplets quickly im-
pinged on neighbouring drop-
lets to give rise to Schlieren
nematic liquid-crystalline tex-
tures (see Figure SF10 in the
Supporting Information). The
shape and nature of the nuclei
in OPV-HC-C11 was very differ-
ent and the oval-shaped nuclei
resulted in the formation of a
leaf-like LC texture at 70–658C
(see Figure SF11 in the Sup-
porting Information). On subse-
quent cooling below 608C in
the
LC-active
temperature
window, these leaf-like textures
transformed into well-ordered
two-dimensional lamellar tex-
tures (see image at 408C in
Figure 1). OPV-HC-NB showed
leaf-like nucleation that further
grew into flower paddle-like
textures (see SF12 in the Sup-
porting Information). OPV-HC-
C8 and OPV-HC-EH did not
show any LC characteristic tex-
tures and their images showed
crystalline solids. Unlike their
Scheme 2. Synthesis of hydro- and fluorocarbon-tailed OPVs: i) R-OTs, NaOH, EtOH, H2O, 80 8C or R-Br,
KOH, DMSO, 308C; ii) HBr in AcOH, glacial acetic acid, paraformaldehyde, 808C; iii) triethylphosphite,
1258C; iv) K-tOBu, dry THF, 308C.
ture-dependent LC textures. The LC textures of fluorocar-
bon and hydrocarbon-tailed OPVs are shown in Figure 1
(full temperature window PLM images for each sample are
provided in Figures SF6 to SF14 in the Supporting Informa-
tion). The nucleation of the FC-OPVs from the isotropic
phase appeared as smectic batonnets (see OPV-HC-C11)
that subsequently grew both in lateral and longitudinal di-
rections as Smectic A (SmA)-type images (at 1458C, see
Figure SF6 in the Supporting Information). On further cool-
ing (or holding at this temperature isothermally), the mor-
phology transformed into fan-shaped Smectic C (SmC) LC
mesophases (see image at 1408C).[13] The nucleation and
crystal growth pattern of all the fluorinated molecules were
found to be almost identical. The slight differences in the
orientation of the crystalline vectors in the smectic phases
and variation in the optical birefringence among fluorinated
OPVs are attributed to their difference in the pendant units
attached in the central aromatic ring. Thus, the formations
of identical SmC LC mesophases irrespective of the pendent
groups attached in the middle aromatic core suggested that
in the FC-OPV series the molecular self-assembly was pre-
dominately directed by the fluorocarbon tail rather than
pendent units. The LC textures of hydrocarbon-tailed OPVs
were found to be completely different and they showed var-
iation in the textures depending on the nature of middle
pendent groups (see Figure 1). OPV-HC-C1 exhibited spher-
Figure 1. PLM textures of fluorocarbon and hydrocarbon-tailed liquid-
crystalline OPVs.
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2869

M. Jayakannan and M. Goel
fluorocarbon counterparts, the nature of pendent groups in
the middle aromatic p core determined the nature of the nu-
cleation and the types of the LC textures in the hydrocarbon
tail OPVs. Two important points could be derived from the
LC textures: 1) in hydrocarbon-tailed OPVs, the pendent
geometry in the middle aromatic core determined the mo-
lecular self-organization and 2) in fluorocarbon-tailed OPVs,
the stiff and rigid nature of the tail dominated the self-or-
ganization and the nature of pendants has less influence.
Single crystals for OPV molecules were grown and sub-
jected to X-ray analysis for elicitation of their molecular
packing in the solid state. Among all the OPVs, only OPV-
HC-C11 and OPV-HC- C1 could be grown as good quality
crystals for single-crystal X-ray structure analysis. The crys-
tal structures of OPV-HC-C11 and OPV-HC-C1 in the CPK
model are given in Scheme 3. Both molecules crystallized in
sions along the crystallographic axes. The 3D arrangements
of OPV-HC-C1 molecules were found as inseparable tightly
packed bundles of p-conjugated sheets. On the other hand,
OPV-HC-C11 showed layer-like packing in the x–y plane,
which further protruded along the c axis by the long undecyl
chains in the middle aromatic ring. Each layer was separated
by 16.04 ꢀ, which was almost identical to the c-axis length
in the unit cell.
To get more insight into the layer-like formation of OPV-
HC-C11 molecules, the close-contact between the adjacent
molecules was analyzed and the details are shown in
Figure 2. The central OPV molecule A showed two non-co-
Figure 2. CH/p weak hydrogen bonding in an OPV single crystal.
valent interactions each with molecule B and C on either
side (total of four interactions above and below the mole-
cule A). The undecyl pendent groups in the middle benzene
ring were not involved in any type of intermolecular interac-
tions with the surroundings; as a result the molecules pro-
jected away along the c axis (see Scheme 3). The close-
packed structure indicated the existence of CH/p interac-
tions among the molecules (g). Four CH/p interactions
per molecule were found between the middle aromatic ring
with hydrogen atoms H11A of the undecyl alkyl chains that
lay along the molecular axis. There are four important geo-
Scheme 3. X-ray crystal structure of OPVs and their three-dimensional
packing.
the triclinic lattice in the space group of PÀ1 and their unit-
cell parameters are given in the Supporting Information.
Side views of these molecules revealed that all three ben-
zene rings in the aromatic backbone lay in the same plane.
The torsion angles from the central ring to either side were
obtained as 1.39 and À1.398, which confirmed their copla-
narity. The two undecyl chains present in the terminal aryl
rings along the longitudinal position lay on the same plane
of the three aromatic rings. The two other alkoxy units (un-
decyl or methoxy) present in the middle ring projected per-
pendicular on either side of the plane defined by the aro-
matic rings. Based on crystallographic bank data, the earlier
reported OPV molecules were found to have non-planar ge-
ometry in the aromatic rings and were found not to show
any type of supramolecular structures.[14] In the present
study, appropriate substitution of alkyl chains in the aromat-
ic rings resulted in the formation of a planar OPV-conjugat-
ed backbone that leads to molecular self-assembly in the
thermoptropic LC mesophase. To further understand the
molecular packing, the unit cells were built in three dimen-
metric parameters that determine the CH/p interactions,
[4]
which are dcÀx, q, f and dHpÀx
.
The parameter dcÀx is the
distance between the donor carbon atom to the centre of
the acceptor p ring, q is the angle between the ring normal
and a vector connecting the donor carbon atom to the
À
centre of the ring, f is the angle between C H with the
vector connecting H to the ring centre and dHpÀx is the pro-
[4]
À
jection of the C H on the p ring. To determine these pa-
rameters in the present case, vectors were drawn as shown
in the inset of Figure 2 and the values were calculated as:
dcÀx =3.79 ꢀ; q=21.498; f=150.258 and dHpÀx =0.73 ꢀ.
These values are within the range typically acceptable for
CH/p interactions (literature values are: dcÀx <4 ꢀ; q<258;
f>1208 and dHpÀx <1.20 ꢀ).[2,4] This provided direct evi-
dence for the existence of strong CH/p interactions in the
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Self-Assembly in p-Conjugated Oligomers
FULL PAPER
OPV-HC-C11 molecules. The CH/p interactions facilitated
the alignment of the OPV chromophores in the x–y layer,
which were further separated by the undecyl units that were
protruded perpendicular in the c axis. A similar attempt to
determine the secondary interactions in OPV-HC-C1 mole-
cules indicated the absence of any CH/p interactions. In
OPV-HC-C11, two out of the three benzene rings overlapped
with molecules on either side along the p planes in the mo-
lecular axis and the intermolecular distance was obtained as
3.64 ꢀ (see Figure SF17 in the Supporting Information). The
adjacent molecules in OPV-HC-C1 were not oriented along
the plane and they were separated by 3.78 and 7.08 ꢀ.
These intermolecular distances were much larger than that
of 3.41 ꢀ that is observed for strong p–p interactions in
graphite.[15] Therefore, the existence of strong aromatic p–p
interactions among the OPV molecules were not possible. It
is very important to note that despite both OPV molecules
being planar, the large intermolecular distance made them
less favourable to self-organize by aromatic p–p interac-
tions. The lack of the direct face-to-face p–p stacking be-
tween the aromatic planes confirmed the orientation of mol-
ecules along the b axis as in J-type aggregates. Hence, CH/p
interactions are the predominant secondary interactions in
the three-dimensional self-assembly of hydrocarbon-tailed
OPVs.
Single-crystal X-ray structures were determined for the
crystals (in the solid state) that were grown by using sol-
vent-assisted processes. On the other hand, liquid-crystalline
mesophases were obtained by molecular self-assembly
under a solvent-free crystallization process (in the solid
state). Therefore, it is very important to establish correlation
between the single-crystal X-ray structures with those crystal
lattices generated in the LC phases. To identify the liquid-
crystalline crystal lattices, the OPVs were subjected to varia-
ble-temperature wide-angle powder X-ray diffraction analy-
sis. Variable-temperature wide-angle XRD patterns for
OPV-HC-C11 are shown in Figure 3a. The sample was com-
pletely melted at 1108C and upon cooling crystalline peaks
started to appear at 808C. These peaks became sharp and
intense at 708C. The intense peaks were found at 2q=8.2
(d=10.76 ꢀ, two close peaks), 10.2, 11.6, 17.6, 20.1, 23.5 and
25.78. At much lower temperatures 60–408C, the two close
peaks at 2q=8.208 were completely shifted to the low-angle
region at 2q=5.6 (d=16.04 ꢀ) and 5.98 (d=15.4 ꢀ) without
affecting other peaks in the region from 2q=10–258. The
comparison of PLM images at 81, 57 and 458C along with
the X-ray diffraction pattern revealed that the shift in the
low-angle peaks from 2q=8.2 to 5.68 were due to the forma-
tion of higher-order lamellar textures. Powder X-ray diffrac-
tion patterns were computationally simulated based on the
single-crystal structure (for OPV-HC-C11) and plotted along
with experimental XRD patterns in Figure 3b.[16] The miller
indice values for each peak in the simulated XRD plot
matched very well with that of the variable-temperature ex-
perimental plots. This confirmed that the crystalline lattices
in the single crystal were identical to those present in the
LC-phases. The comparison of the PLM images and WXRD
Figure 3. Variable-temperature powder X-ray diffraction patterns of
OPV-HC-C11 a) Simulated X-ray diffraction pattern for OPV-HC-C11
based on the single-crystal data. b) PLM images of OPV-HC-C11 cap-
tured while cooling from isotropic (molten state).
analysis suggested that the peaks at 2q=5.68 were corre-
sponding to the 001 fundamental lamellae layer in the la-
mellar LC textures of OPV-HC-C11 (see PLM picture at
458C). The interplannar distance 16.04 ꢀ in the single crys-
tal (Scheme 3) is matched with that of the d-spacing of the
001 plane at 2q=5.68 (d=16.04 ꢀ) in the XRD plots (see
Figure 3a). This further proved that the formation of the 001
fundamental peak in the XRD pattern of LC texture is cor-
responded to the layer arrangements of OPV-HC-C11 as
seen in Scheme 3. Similarly, OPV-HC-C1 was also subjected
to XRD analysis; however, no sharp peaks in the low-angle
regions were observed below 2q<108 for self-organized
structures (see Figure SF18 in the Supporting Information).
The perfect matching of single-crystal structural data with
the powder XRD proved that the crystal lattices in the LC-
phases were organized through strong CH/p interactions
while cooling from isotropic melt. Therefore, for the first
time, such good correlation between the single-crystal struc-
ture and LC mesophase ordering has been established in p-
conjugated skeletons.
Fluorocarbon-tailed OPVs in smectic LC phases were
subjected to variable-temperate powder X-ray diffraction
analysis (growing single crystals for FC-OPVs have not been
successful so far). The X-ray diffraction patterns of OPV-
FC-C11 in the respective LC temperature range are shown in
Figure 4. Upon cooling from the isotropic melt, intense crys-
talline peaks appeared in the wide-angle region in a regular
periodicity corresponding to layer packing (see Figure 4a).
To determine smectic layer thickness and the optical tilt
angle of mesophases, OPV-FC-C11 was further subjected to
variable-temperature small-angle X-ray diffraction analysis.
The two-dimensional electron density diagram at 408C (see
Figure 4b) and the corresponding XRD patterns at 150, 125,
70 and 408C are shown in Figure 4c.[17] In Figure 4c, the fun-
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M. Jayakannan and M. Goel
Both hydrocarbon and fluorocarbon-tailed OPVs were
subjected to photophysical studies in solution (toluene) and
in the LC frozen films. The emission spectra of a few repre-
sentative OPVs in toluene are shown in Figure 5a. The ab-
Figure 4. Variable-temperature wide-angle X-ray patterns (a), 2D elec-
tron-density diagram (b) and small-angle X-ray diffraction patterns (c) of
OPV-FC-C11. A mesogen packing model describes the SmA to SmC
transformation (d).
damental peak corresponding to the 001 lattice was ob-
served at 2q=2.52 (dA =35.01 ꢀ, at 1508C). At 1258C, the
peak was shifted to 2q=2.80 (dc-spacing=31.51 ꢀ), which
indicated that the molecules underwent a transition from
SmA to a more ordered SmC. Upon further cooling, the
peak position did not change much, which confirmed the
formation of stable layer-like arrangements. According to
the classic rigid-rod model, the SmA-to-SmC phase transi-
tion is accompanied by a contraction of the layer spacing ꢂdꢃ
that scales with the cosine of the tilt angle q (see Fig-
ure 4d).[13,18] Comparing the SmC and SmA layer spacing,
the tilt angle of the molecules in the Sm-C mesophase was
calculated as q=cosÀ1 dC/dA, with dC and dA being the layer
thickness in the SmC and SmA phases (see Figure 4d). The
phase transition was accompanied by 11% reduction in the
interlayer shrinkage ((dA-dC)/dA)ꢄ100) which is typically ob-
served in a SmA to SmC transition. The tilt angle was calcu-
lated as q=25.188, which was in accordance to the range of
q (ꢀ25 to 358) typically reported in earlier examples.[12] This
confirmed that the peak shift in the small angle patterns is
corresponding to the phase transition from SmA to SmC.
Based on the 001 fundamental peak at 2q=2.52 (dA =
35.01 ꢀ, in small angle diffraction), the wide angle XRD
patterns peaks were assigned for 002, 003···007 layers, re-
spectively (see Figure 4a). This trend was almost identical in
the variable-temperature WXRD patterns of all other fluori-
nated OPVs (see Figure SF20 in the Supporting Informa-
tion). Based on the above analysis, it may be summarized
that all the fluorocarbon-tailed OPV mesogens arranged in
two-dimensional layers (as H-type organization) leading to
the formation of SmA textures, which transformed into a
SmC mesophase at lower temperatures.
Figure 5. Emission spectra of OPVs in toluene (a) and LC films (b).
Time-resolved fluorescent decay profiles of the OPVs (c, d and e) with
lifetime values. The photographs showed the emission from hydrocarbon
and fluorocarbontailed OPVs with respect to their J- and H-aggregates.
sorbance, excitaion and emission spectra (see Figure SF21 in
the Supporting Information) of OPVs in solution were
found to be almost identical, which suggested that the chro-
mophores did not possess any intermolecular ordering in so-
lution. The quantum yields of the samples were determined
by using quinine sulfate as the reference and the values are
reported in Table ST4 in the Supporting Information. The
quantum yields of the OPV samples were obtained 0.40–
0.50 and these values matched with earlier reports.[6f,9] LC
films of OPVs were cast on the glass substrates described
for PLM studies and their absorbance and excitation spectra
were recorded. The onset of the absorbance spectra of the
samples were found 20–35 nm redshifted for hydrocarbon-
tailed OPVs relative to that of their fluorocarbon counter-
parts (see Figure SF23 and Table ST5 in the Supporting In-
formation). The excitation spectra of the OPVs were found
very broad (see Figure SF24 in the Supporting Information,
for excitation spectra) as similar to observed in earlier re-
ports.[9] The emission spectra of the OPVs were recorded by
exciting at 380 nm and their spectra are given in Figure 5b.
Unlike in solution, the emission spectra of the hydrocarbon-
tailed OPVs were 20–40 nm redshifted relative to their fluo-
rocarbon counterparts. The reason for the shift in the emis-
sion colour is attributed to the difference in the OPV molec-
ular packing. All fluorinated OPV molecules exhibited
layer-like self-assembly in the smectic LC phase and the
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Chem. Eur. J. 2012, 18, 2867 – 2874

Self-Assembly in p-Conjugated Oligomers
FULL PAPER
molecules are packed parallel as in H-type aggregates. On
the other hand, the hydrocarbon-tailed OPV molecules have
orientated in slip-through arrangements as in J-type aggre-
gates (see Figure SF17 in the Supporting Information). The
presence of H- and J-aggregates could be further confirmed
by the blue- and redshift in the absorbance spectra of fluo-
rocarbon and hydrocarbon-tailed OPVs, respectively. The
photographs of the OPV samples in vials are shown for the
emission colour obtained by exciting the samples with a
hand-held UV lamp at 365 nm. The blue emission from the
fluorocarbon OPVs and yellowish/green from the hydrocar-
bon OPVs directly provide evidence of their difference in
their H- and J-type aggregates in the solid state. Time-de-
pendent fluorescence decay measurements were carried out
by the TCSPC technique by using a 375 nm diode laser as
the excitation source and the fluorescence intensity decays
were collected at emission maxima. The fluorescent decay
profiles of OPVs in solution (monomer emission) were
found to be almost identical with single exponential decay
with lifetimes in the range of 1.2–1.3 ns (like monomer emis-
sion, see Figure SF22 and Table ST4 in the Supporting Infor-
mation).[6f,9] This further confirmed that OPV molecules do
not possess any self-assembly in the solution state.[9] The in-
tensity decays for FC and HC-OPV molecules in the LC
frozen films are shown in Figure 5c–e (see Figure SF25 in
the Supporting Information for other samples). The overall
observation of the decay profiles clearly indicated that the
fluorocarbon-tailed OPVs had more stable emission charac-
teristics relative to those of the hydrocarbon OPVs. The
decay profiles were fit into multi-exponential decay models
and all the OPVs either fitted to bi- or triexponential fits.
The fluorescence lifetime values for the OPVs and their c2-
values are given in Table ST5 in the Supporting Information.
The t1, t2 and t3 values for the fluorocarbon OPVs were
almost twice compared to that of hydrocarbon OPVs. In the
present case, the fluorocarbon tail OPVs are closely packed
in H-aggregate form, and therefore, the possibility for the
orientational relaxation is very less in the solid state. On the
other hand, the slip through arrangements in the J-aggre-
gates of the hydrocarbon tail possessed a high degree of
freedom for molecular orientation in the excited state. As a
consequence, the J-aggregated hydrocarbon-tailed OPVs
showed fast decay in the time-resolved fluorescence meas-
urements.
rangements of mesogens produced smectic LC textures in
fluorocarbon-tailed OPVs. The OPV-HC-C11 molecules with
a large pendent in the middle ring underwent unique CH/p
interactions along the molecular axis to produce layer-like
self-assembly. The x–y layers constituted by CH/p interac-
tions are further protruded along the c axis to make perfect
lamellar-type LC morphology. In the absence of CH/p inter-
actions (changing C11 to C1 carbon in the middle aromatic
core), van der Waals forces emerged as a main secondary
force for the self-organization of OPV chromophores in the
nematic texture.
Conclusion
CH/p hydrogen-bonding-driven self-assembly in the p-con-
jugated skeletons of OPVs was established and the origin of
interactions at the molecular level is traced by using single-
crystal structure. OPVs were designed with appropriate
pendants in the aromatic core with hydrocarbon or fluoro-
carbon tails to self-organize in thermotropic liquid crystal-
line phases. The CH/p interaction has emerged as a main
driving force in the hydrocarbon p-conjugated skeleton and
facilitates the close-packing of mesogens for lamellar struc-
ture. In the absence of CH/p interactions, other non-cova-
lent forces, such as van der Waals interactions or the fluoro-
phobic effect, control the self-organization of OPVs. Upon
photoexcitation, the molecular arrangements of OPV meso-
gens were found to influence emission and their fluores-
cence lifetime. Moreover, these new hydrocarbon and fluo-
rocarbon liquid-crystalline OPVs are potential candidates
for application in opto-electronics, which will be explored in
future work. In a nut shell, we proved that the self-assembly
of aromatic p-conjugated units through weak secondary
forces, such as CH/p interactions and van der Waals forces
are capable of making a diverse thermotropic LC meso-
phase in the solid state. The approach demonstrated here is
not only restricted to OPVs and, in principle, could be ex-
tended to a large number of other p-conjugated systems.
Experimental Section
CCDC-839244 and 839245 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
The mechanism for the molecular self-organization of
OPV chromophores is proposed as follows. OPV chromo-
phores have four non-covalent weak forces—CH/p interac-
tions, van der Waals forces, fluorophobic effect and aromatic
p stacking. Based on the single-crystal analysis, the role of
aromatic p stacking as a main driving force for the molecu-
lar self-assembly of hydrocarbon-tailed OPVs is ruled out.
However, it is very difficult to generalize these behaviours
of hydrocarbon-tailed OPVs to other thermotropic LC ma-
terials. Typically, LC mesogens are arranged in a specific
manner while cooling from the isotropic phase (melt) to
produce nematic and smectic textures. The two-dimensional
ordering through both the orientation and transitional ar-
Acknowledgements
Research grant from the Department of Science and Technology (DST),
New Delhi, INDIA, under nano-mission initiative SR/NM/NS-42/2009 is
acknowledged. M. Goel thanks CSIR, India for SRF fellowship.
[1] a) G. R. Desiraju, Acc. Chem. Res. 1991, 24, 290–296; b) G. R. De-
siraju, Acc. Chem. Res. 1996, 29, 441–449; c) T. Steiner, Chem.
Chem. Eur. J. 2012, 18, 2867 – 2874
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2873

M. Jayakannan and M. Goel
Commun. 1997, 727–734; d) T. Steiner, G. R. Desiraju, Chem.
Commun. 1998, 891–892; e) P. Chakrabarti, U. Samanta, J. Mol.
Biol. 1995, 251, 9–14.
[10] a) L. Akcelrud, Prog. Polym. Sci. 2003, 28, 875; b) T. A. Skotheim,
Handbook of Conducting Polymers, Marcel Dekker, New York,
1986; c) H. S. Nalwa, Handbook of Advanced Electronic and Pho-
tonic Materials and Devices, Academic Press, 2000.
[2] a) G. R Desiraju, T. Steiner, The weak Hydrogen Bond, Oxford
Univ. Press, London, 1999; <lit b>M. Nishio, M. Hirota, Y. Ume-
zawa, The CH-p interaction. Evidence, Nature and Consequences,
Wiley-VCH, NewYork, 1998.
[3] a) M. J. Plevin, D. L. Bryce, J. Boisbouvier, Nat. Chem. 2010, 2, 466–
471; b) C. A. Sacksteder, S. L. Bender, B. A. Barry, J. Am. Chem.
Soc. 2005, 127, 7879–7890.
[11] References for the fluorophobic effect: a) M. Hird, Chem. Soc. Rev.
2007, 36, 2070–2095; b) F. H. Stillinger, Z. Wasserman, J. Phys.
Chem. 1978, 82, 929–940; c) F. Babudri, G. M. Farinola, F. Naso, R.
Ragni, Chem. Commun. 2007, 1003–1022; d) V. Percec, M. Glodde,
G. Johansson, V. S. K. Balagurusamy, P. A. Heiney, Angew. Chem.
2003, 115, 4474–4478; Angew. Chem. Int. Ed. 2003, 42, 4338–4342;
e) S. De Feyter, F. C. D. Schryver, J. Phys. Chem. B 2005, 109, 4290–
4302; f) J. A. Smith, R. A. J. DiStasio, N. A. Hannah, R. W. Winter,
T. J. R. Weakley, G. L. Gard, S. B. J. Rananavare, Phys. Chem. B
2004, 108, 19940–19948.
[12] References on thermotropic LCs: a) F. S. Precup-Blaga, A. P. H. J.
Schenning, E. W. Meijer, Macromolecules 2003, 36, 565–572; b) W.
Pisula, Z. Tomovic, M. Wegner, R. Graf, M. J. Pouderoijen, E. W.
Meijer, A. P. H. J. Schenning, J. Mater.Chem. 2008, 18, 2968–2977;
<lit c>J. F. Hulvat, M. Sofos, K. Tajima, S. I Stupp, J. Am. Chem.
Soc. 2005, 127, 366–372; d) P. Jonkheijm, V. D. P. Schoot, A. P. H. J.
Schenning, E. W. Meijer, Science 2006, 313, 80–83; e) T. Maddux,
W. Li, L. Yu, J. Am. Chem. Soc. 1997, 119, 844–845; f) B. Strehmel,
A. M. Sarker, J. H. Malpert, V. Strehmael, H. Seifert, D. C. Neckers,
J. Am. Chem. Soc. 1999, 121, 1226–1236; g) B. D. Olsen, S. Y. Jang,
J. M. Luning, R. A. Segalman, Macromolecules 2006, 39, 4469–4479;
h) Z. Bao, K. R. Amundson, A. J. Lovinger, Macromolecules 1998,
31, 8647–8649.
[4] M. Brandl, M. S. Weiss, A. Jabs, J. Suhnel, R. Hilgenfeld, J. Mol.
Biol. 2001, 307, 357–377 and references therein.
[5] Reference for CH/p in organic molecules: a) E. Kim, S. Paliwal,
C. S. Wilcox, J. Am. Chem. Soc. 1998, 120, 11192–11193; b) Y. Ko-
bayashi, K. Saigo, J. Am. Chem. Soc. 2005, 127, 15054–15060; c) K.
Dziubek, M. Podasiadlo, A. Katrusiak, J. Am. Chem. Soc. 2007, 129,
12620–12621; d) A. Matsumoto, T. Tanaka, T. Tsubouchi, K. Ta-
shiro, S. Saragai, S. Nakamoto, J. Am. Chem. Soc. 2002, 124, 8891–
8902; e) L. S. Birchall, S. Roy, V. Jayawarna, M. Hughes, E. Irvine,
G. T. Okorogheye, N. Saudi, E. D. Santis, T. Tuttle, A. A. Edwards,
R. V. Ulijn, Chem. Sci. 2011, 2, 1349–1355; references for CH/p in
inorganic molecules: f) M. Yamakawa, I. Yamada, R. Noyori,
Angew. Chem. Int. Ed. 2001, 40, 2818–2821; g) B. W. Gung, B. U.
Emenike, M. Lewis, K. Kirschbaum, Chem. Eur. J. 2010, 16, 12357–
12362; h) K. Kobayashi, K. Ishii, S. Sakamoto, T. Shirasaka, K. Ya-
maguchi, J. Am. Chem. Soc. 2003, 125, 10615–10624.
[6] References for hydrogen bonding: a) P. G. A. Janssen, N. Meeuwe-
noord, G. V. Marel, S. Jabbari-Farouj, P. V. Schoot, M. Surin, Z. To-
movic, E. W. Meijer, A. P. H. J. Schenning, Chem. Commun. 2010,
46, 109–111; b) A. P. H. J. Schenning, P. Jonkheijm, E. Peeters,
E. W. Meijer, J. Am. Chem. Soc. 2001, 123, 409–416; c) S. K. Asha,
A. P. H. J. Schenning, E. W. Meijer, Chem. Eur. J. 2002, 8, 3353–
3361; d) J. V. Herrikhuyzen, S. K. Asha, A. P. H. J. Schenning, E. W.
Meijer, J. Am. Chem. Soc. 2004, 126, 10021–10027; e) A. Ajaya-
ghosh, C. Vijayakumara, R. Varghese, S. J. George, Angew. Chem.
2006, 118, 470–474; Angew. Chem. Int. Ed. 2006, 45, 456–460;
f) S. S. Babu, V. K. Praveen, S. Prasanthkumar, A. Ajayaghosh,
Chem. Eur. J. 2008, 14, 9577–9584; g) Y. Hirai, S. S. Babu, V. K.
Praveen, T. Yasuda, A. Ajayaghosh, T. Kato, Adv. Mater. 2009, 21,
4029–4033.
[13] I. Dierking, Textures of Liquid Crystals, 2nd ed., Wiley-VCH,
Berlin, 2003.
[14] A few important references for OPV-based single crystals: a) G. P.
Bartholomew, G. C. Bazan, X. Bu, R. J. Lachicotte, Chem. Mater.
2000, 12, 1422–1430; b) C. A. Sierra, P. M. Lahti, Chem. Mater.
2004, 16, 55–61; c) C. M. L. Vande Velde, L. Chen, J. K. Baeke, M.
Moens, P. Dieltiens, H. J. Geise, M. Zeller, A. D. Hunter, F. Block-
huys, Crystal Growth & Design 2004, 4, 823–830; Design 2004, 4,
823–830; d) K. A. White, D. A. Chengelis, M. Zeller, S. J. Geib, J.
Szakos, S. Petoud, N. L. Rosi, Chem. Commun. 2009, 4506–4508;
e) R. E. Gill, A. Meetsma, G. Hadziioannou, Adv. Mater. 1996, 8,
212–214.
[15] C. H. Desch, The Chemistry of Solids, Cornell University Press,
1934, 180–181.
[7] References for metal-ion interactions: a) B. Linton, A. D. Hamilton,
Chem. Rev. 1997, 97, 1669–1680; b) F. S. Han, M. Higuchi, D. G.
[16] Comparison of simulated powder XRD patterns from single-crystal
data is used for assigning crystal lattices in experimental powder
XRD plots. This method is well adopted in the area of metal–organ-
ic frameworks; a) S. K. Ghosh, W. Kaneko, D. Kiriya, M. Ohba, S.
Kitagawa, Angew. Chem. 2008, 120, 8975–8979; Angew. Chem. Int.
Ed. 2008, 47, 8843–8847; b) S. K. Ghosh, J.-P. Zhang, S. Kitagawa,
Angew. Chem. 2007, 119, 8111–8114; Angew. Chem. Int. Ed. 2007,
46, 7965–7968.
[17] Small-angle XRD analysis of the hydrocarbon-tailed counterpart
OPV-HC-C11 did not show any peak below 2q < 38 since the 001
fundamental plane was already observed in the wide angle at 2q=
5.68 (see Figure 3a). This confirmed the appearance for the 001
plane in the fluorocarbon-tailed OPV-FC-C11 in small-angle XRD.
[18] a) J. C. Roberts, N. Kapernaum, Q. Song, D. Nonnenmacher, K.
Ayub, F. Giesselmann, R. P. Lemieux, J. Am. Chem. Soc. 2010, 132,
364–370; b) J. P. F. Lagerwall, F. Giesselmann, Phys. Rev. E 2002,
66, 031703/1–031703/11; c) M. Krueger, F. Giesselmann, Phys. Rev.
E 2005, 71, 041704/1–041704/8.
Kurth, Adv. Mater. 2007, 19, 3928–3931; c) D. Knapton,
; S. J.
Rowan, C. Weder, Macromolecules 2006, 39, 651–657; S. J. Rowan,
C. Weder, Macromolecules 2006, 39, 651–657; d) J. B. Beck, S. J.
Rowan, J. Am. Chem. Soc. 2003, 125, 13922–13923; e) S. Ghosh, S.
Ramakrishnan, Angew. Chem. 2005, 117, 5577; Angew. Chem. Int.
Ed. 2005, 44, 5441- 5447; f) S. Ghosh, S. Ramakrishnan, Angew.
Chem. 2004, 116, 3326–3330; Angew. Chem. Int. Ed. 2004, 43, 3264–
3268.
[8] References for aromatic p-stacking: a) R. S. Lokey, B. L. Iverson,
Nature 1995, 375, 303–305; b) A. J. Zych, B. L. Iverson, J. Am.
Chem. Soc. 2000, 122, 8898–8909; c) P. Osswald, D. Leusser, D.
Stalke and F. Wꢅrthner, Angew. Chem. 2005, 117, 254–257; Angew.
Chem. Int. Ed. 2005, 44, 250–253; d) J. Baby, S. K. Asha, Chem.
Mater. 2008, 20, 169–181; e) M. Prehm, F. Liu, X. Zeng, G. Ungar,
C. Tschierske, J. Am. Chem. Soc. 2011, 133, 4906–4916.
[9] References for van der Waals interactions: a) M. Goel, M. Jaya-
kannan, J. Phys. Chem. B 2010, 114, 12508–12519; b) S. R. Amru-
tha, M. Jayakannan, J. Phys. Chem. B 2009, 113, 5083–5091; c) S. R.
Amrutha, M. Jayakannan, Macromolecules 2007, 40, 2380–2391.
Received: August 26, 2011
Revised: December 4, 2011
Published online: February 1, 2012
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Chem. Eur. J. 2012, 18, 2867 – 2874