Molecular ordering in bis(phenylenyl)bithiophenes

Article abstract of DOI:10.1039/b701035d

The crystal structure of phenylene-bithiophene oligomers (PTTP) has been investigated by powder X-ray diffraction measurements and ab initio Monte Carlo modelling. Two new hydroxyl-terminated PTTP powders, 5,5′-bis-4-(6- hydroxyhexyloxy)-phenyl-2,2′-bithi

Full text of DOI:10.1039/b701035d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Molecular ordering in bis(phenylenyl)bithiophenes{
Melissa A. Stokes,a Refik Kortan,b Sandrine Rivillon Amy,c Howard E. Katz,d Yves J. Chabal,*ac
Christian Kloce and Theo Siegriste
Received 23rd January 2007, Accepted 24th May 2007
First published as an Advance Article on the web 5th June 2007
DOI: 10.1039/b701035d
The crystal structure of phenylene–bithiophene oligomers (PTTP) has been investigated by
powder X-ray diffraction measurements and ab initio Monte Carlo modelling. Two new hydroxyl-
terminated PTTP powders, 5,59-bis-4-(6-hydroxyhexyloxy)-phenyl-2,29-bithiophene and the
shorter alkyl chain substituted oligomer 5,59-bis(4-hydroxyphenyl)-2,29-bithiophene, are
compared to single crystals of 5,59-bis(4-ethylphenyl)-2,29-bithiophene. The new molecules are
characterized by a close packed, distorted hexagonal array with tilted short axes in a herringbone-
type structure. Neighbouring molecules within layers are displaced along the long axis. Modelling
the electrostatic potential surface suggests that electrostatic interactions are responsible for the
observed herringbone-like ordering. Alkyl chains facilitate a partial mixing through
interdigitation of aromatic and aliphatic moieties.
phenlyene–thiophene aromatic cores has been shown to
Introduction
enhance long-range molecular ordering and thus increase
organic transistor field effect mobility.2,3,5 For this class of
Characterizing and modelling the structure of molecular
ordering is central to understanding conduction mechanisms
molecules, it is important to determine the crystal structure
in organic electronics. Substituted phenylene–thiophene mole-
in order to determine the effect of adding peripheral
cules are interesting candidates for organic electronics, based
substituents to
a core molecule. In general, long-chain
on their stability, solubility, general high field effect mobility,
and surface chemical reactivity.1–4 Phenylene–thiophene oli-
gomers uniquely combine field effect mobility of the order of
0.01–0.1 cm2 V s21, resistance to oxidative doping and
therefore higher on/off ratios relative to zero gate voltage
than all-thiophene oligomers, and the possibility of solution
processing that is relatively rare for molecular solids.1,2 Their
syntheses are facile enough to allow the attachment of polar
end groups, such as the hydroxyalkyl described here. Such
groups enable further surface modification after film
deposition, and controlled interactions with a variety of
overlying substances. The addition of hexyl end groups on
oligothiophenes order in a herringbone arrangement with
¯
triclinic (P1, P1), monoclinic P21, P21/c, P21/a or orthorhom-
bic P212121, or Pbca symmetry.6–9 However, the structural
implications of extensive alkyl and/or polar end group
functionalization have not yet been uncovered, due to the
difficulty involved in preparing single crystals. Structure
analysis is difficult because unlike smaller unsubstituted
molecules, which readily crystallize and can be analyzed by
single crystal X-ray diffraction, longer molecules require
powder X-ray diffraction and a more complex analysis.
Therefore, we utilize powder X-ray methods to solve the
crystal structure of longer molecules not available as single
crystals. To understand the influence of polar and alkoxy
termini on molecular ordering, we have combined synchrotron
powder X-ray diffraction measurements and ab initio
Monte Carlo modelling to determine the ordering of two
new hydroxyl-terminated phenylene–bithiophene powder-
form oligomers, 5,59-bis(4-hydroxyphenyl)-2,29-bithiophene
(HOPTTPOH) and 5,59-bis-4-(6-hydroxyhexyloxy)-phenyl-
2,29-bithiophene (HOC6PTTPC6OH), and compared them
with a simplified single crystal of 5,59-bis(4-ethylphenyl)-
2,29-bithiophene (2PTTP2).
aRutgers University, Department of Biomedical Engineering, 599 Taylor
Road, Piscataway, NJ 08854, U. S. A.
E-mail: yves@rutchem.rutgers.edu; Fax: +1 732 445 4991;
Tel: +1 732 445 8248
bKoc University, Department of Physics, Istanbul, Turkey.
E-mail: rkortan@ku.edu.tr; Fax: +90 212 338 1547;
Tel: +90 212 338 1701
cRutgers University, Department of Chemistry and Chemical Biology,
610 Taylor Road, Piscataway, NJ 08854, U. S. A.
E-mail: rivillon@physics.rutgers.edu; Fax: +1 732 445 4991;
Tel: +1 732 445 7347
dJohns Hopkins University, Department of Materials Science and
Engineering, 3400 North Charles Street, 103 Maryland Hall, Baltimore,
MD 21218, U. S. A. E-mail: hekatz@jhu.edu; Fax: +1 410 516 529;
Tel: +1 410 516 6141
Experimental
eBell Laboratories, Alcatel-Lucent, 600 Mountain Avenue, Murray Hill,
NJ 07974, U. S. A. E-mail: tsi@alcatel-lucent.com; Fax:+1 9085823104;
Tel: +1 908 582 5253
Synthesis
5,59-Bis(4-ethylphenyl)-2,29-bithiophene
was
synthesized
{ Electronic supplementary information (ESI) available: Powder
diffraction data for 5,5’-bis(4-ethylphenyl)-2,2’-bithiophene (2PTTP2)
at 120 and 300 K, 5,5’-bis(4-hydroxyphenyl)-2,2’-bithiophene
(HOPTTPOH), 5,5’-bis-4-(6-hydroxyhexyloxy)-phenyl-2,2’-bithio-
phene (HOC6PTTPC6OH). See DOI: 10.1039/b701035d
from 5,59-bis(tributylstannyl)-2,29-bithiophene and 1-bromo-
4-ethylbenzene, using the Stille-coupling procedure described
by Mushrush et al. for a hexylphenyl analog.3 Single crystals
were grown by physical vapor transport.
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5,59-Bis(4-hydroxyphenyl)-2,29-bithiophene was synthesized
using the Stille-coupling procedure with 1-bromo-4-(tert-butyl-
dimethylsilyloxy) benzene. The starting material was prepared
by stirring 4-bromophenol (6 g), tert-butyldimethylsilyl chloride
(5.3 g), imidazole (2.4 g) and dimethylformamide (50 mL)
overnight at room temperature, and partitioning the reaction
mixture between ethyl ether and aqueous sodium carbonate. The
deprotection of the tert-butyldimethylsilyl-protected four ring
compound (purity verified by NMR after solidifying from the
reaction mixture upon cooling to room temperature) was as
follows: the protected compound (105 mg) was stirred with
methylene chloride (50 mL), acetic acid (0.2 mL), and
tetrabutylammonium fluoride (150 mg) for two days under
nitrogen. The dihydroxy compound was then collected as a
yellow-green solid (60 mg). The compound is susceptible to
surface oxidation in air, evident by the powder colour changing
to green. Infrared absorption spectroscopy of aged-in-air KBr
pellet samples shows loss of peaks at 1570, 1420, and 1030 cm21
and a gain of intensity at 1360 cm21 (not shown). However,
many other peaks are the same in both fresh and aged samples.
Aging does not lead to any increased signal within the 1620–
1800 cm21 range, which would have been indicative of quinoid
formation. The sharpness of the powder diffraction peaks shows
that crystallinity is well preserved in the aged sample.
verify that the beam does not damage the sample. Powder
averaging was achieved by in situ 4u theta rotation of the
sample while each two-theta data point was collected.
Structual modelling analysis
The powder data were analyzed by the CMPR11 and Crysfire12
sub-algorithms to determine the unit-cell. Chekcell13 then
examined the Crysfire solutions and the space groups were
identified. PowderCell14 was used for manual trials of
calculating the diffraction pattern for different unit-cell
decorations and space groups. An ab initio reverse Monte
Carlo structure determination program developed by Favre-
Nicolin and Cerny (Fox software)15 was used to identify the
molecular conformations and unit-cell decorations by real-
space simulations. The initial minimum energy molecular
shapes of fully planar molecules and other structural
parameters of molecules were determined by Ghemical.16
The Mercury program was used to generate real-space images.
Results and discussion
The crystal structures of 2PTTP2, HOPTTPOH and
HOC6PTTPC6OH were determined by single-crystal X-ray
diffraction and powder X-ray diffraction, respectively, using
synchrotron powder X-ray diffraction and ab initio Monte
Carlo modelling. The 2PTTP2 molecule crystallizes in a
monoclinic unit-cell with a symmetry P21/a and unit-cell
5,59-Bis-4-(6-hydroxyhexyloxy)phenyl-2,29-bithiophene was
synthesized using the Stille-coupling procedure with 1-bromo-
4-(6-hydroxyhexyloxy)benzene. This starting material was
prepared by heating 4-bromophenol (9.3 g), 6-bromohexanol
(9.8 g), potassium tert-butoxide (6.1 g) and dimethylforma-
mide (50 mL) for four hours at 70 uC, partitioning between
ethyl ether and water, and purifying the concentrated organic
fraction on a 100 g silica gel column. Purification by
recrystallization from 30–40 mL of dimethylformamide per
gram of crude solid was performed.
˚
˚
˚
parameters a = 9.345A, b = 5.782A, c = 18.393A, and b =
99.97u. The conjugated backbone phenylene–bithiophene–
phenylene is flat, and the alkyl chain has its terminal methyl
group oriented almost perpendicular to the PTTP unit (torsion
angle of 103.57u). The PTTP backbone is flat in contrast to the
PTTP system in diisopropyl–PTTP, where the phenylene rings
are tilted by 32.2u against the bithiophene motif.4 The
molecules pack in a herringbone-type arrangement, with the
molecules aligned approximately along the c-axis, with a
herringbone angle of 53.41u, thus forming 2-dimensional layers
with pseudo-hexagonal packing. This type of ordering is
Single-crystal diffraction
A single crystal of 2PTTP2 is used as a reference material since
ethyl is the shortest alkyl chain possible. The structure was
defined as crystalline-H in liquid crystals.17 The cross-sectional
determined using
a conventional single-crystal Oxford
2
area per molecule is about 23 A . A similar hexagonal packing
˚
Diffraction Xcalibur 2 CCD-equipped diffractometer and
analyzed, using the CRYSTALS program.10 Crystal growth
of 2PTTP2 usually resulted in small, very thin platelets, with
thicknesses of less than 1 mm. Therefore, weak sample
scattering will affect the crystal structure determination. Two
data sets were collected on a 2.5 mm-thick sample, one at room
temperature and the other at 120 K. The 120 K data set gives
an improved refinement over the room temperature data.
However, the number of observed reflections is still severely
limited by the size of the crystal.
was observed in the thin film structure of 5,59-bis(4-hexylphe-
nyl)-2,29-bithiophene (6PTTP6).2
The structure of 2PTTP2 can be explained by electrostatic
and steric interactions. The electrostatic potential surface16 at
the van der Waals distance, as shown in Fig. 1(a), displays the
most electropositive and electronegative parts of the molecule,
indicated in red and blue colours, respectively. In the crystal-
line phase, electrostatic attraction would favour placing
the most electronegative part of the molecule (the centre of
the phenyl rings) near the most electropositive part of the
neighbouring molecule (between the hydrogen atoms of the
thiophene and the phenyl rings). The only solution that will
satisfy this condition for every single molecule is the
herringbone structure shown in Fig. 1(b). Steric interactions,
set by the aromatic groups, determine the orientation and
ordering, while the short alkyl chains determine the herring-
bone-type layer spacing.
Powder diffraction
The HOPTTPOH and HOC6PTTPC6OH powder diffraction
data were collected at the Brookhaven National Laboratories
National Synchrotron Light Source (NSLS), with beamline
˚
X22A and a wavelength of l = 1.2084A, using a four-circle
goniometer at room temperature. The powder sample was in
pellet form and was mounted on a glass coverslip. Diffraction
data acquired at different beam intensities and exposure times
The crystalline structure of HOPTTPOH was determined
by ab initio powder data analysis, yielding an orthorhombic
3428 | J. Mater. Chem., 2007, 17, 3427–3432
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Fig. 1 Energy-minimized configuration of the 2PTTP2 molecule and
value of the electrostatic potential at the van der Waals distance or
electrostatic potential surface (a) and crystal structure of 2PTTP2
determined from single-crystal X-ray diffraction (b).
˚
unit-cell in a P212121 group symmetry with a = 5.558A, b =
3
˚
˚
˚
7.665A, c = 37.188A, and cell volume = 1584.29A . Figs. 2(b)
and (c) show the molecular structure in the unit-cell and the
ab initio diffraction data fit for rigid planar molecules as
determined by Ghemical.16 The long axis of the molecule is
slightly tilted towards the centre of the distorted hexagonal
packing. This type of ordering is defined as crystalline-K in
liquid crystals.17 This structure can also be understood in
terms of electrostatic interactions. As shown in Fig. 2(a),
electropositive and electronegative sections of the molecule
extend over a much larger area along the long axis of the
molecule compared with 2PTTP2. Close examination of the
structure confirms that every molecule is coordinated to
maximize this electrostatic interaction. Without the alkyl
spacer chains, molecules pack closer to each other as evidenced
Fig. 2 Energy-minimized configuration of the HOPTTPOH molecule
and value of the electrostatic potential at the van der Waals distance or
electrostatic potential surface (a), real-space crystal structure (b), and
ab initio powder pattern fit of HOPTTPOH (c).
2
˚
˚
˚
˚
by a small y19.5 A per molecule cross-sectional area.
a = 33.056A, b = 7.486A, c = 5.876A, and b = 96.161u with
figure of merit values of M(20) = 9.0, and F(20) = 14.9.12,18,19
This cell assignment correctly predicts weak peaks at (1,1,1),
For a rigid HOPTTPOH molecular shape, the tilt to the
(101) plane is about 28u or 56u between the neighbouring
molecular short axes, whereas the long axes are inclined by
4.23u to the ,001. direction. The shortest distances between
¯
(5,0,1), (4,0,1), (0,2,1) and (10,0,1) that were originally
¯
omitted. The cross-sectional area per molecule is approxi-
2
mately 20A . The short axis of the molecule is tilted from the
˚
˚
˚
neighbouring molecules are S–H = 2.44 A, C–S = 3.36 A,
˚
and O–O = 2.24 A, which are shorter than observed in
(101) plane by about 38.5u or about 77u between neighbouring
molecule short axes that is characteristic of the crystalline-K
phase.17 The experimental and calculated diffraction patterns
in P21 symmetry are shown in Fig. 3. In the P21/n, P21/a space
˚
2PTTP2, S–H = 3.07A, and C–S = 3.86A (the van der Waals
˚
˚
values for these distances are S–H = 3.05A, C–S = 3.55A, and
˚
˚
O–O = 2.80A). This indicates that there are strong orbital
˚
group symmetries with doubled unit-cell a = 66.112A, b =
interactions between the HOPTTPOH molecules. The unu-
sually short O–O contact distance may indicate the presence of
H-bonds between the molecules. Dimerization is unlikely, due
to the atomic contact values falling within expected bonding
distance. Fits allowing all degrees of freedom between the core
rings determine the thiophene rings to be planar, while phenyl
rings are slightly rotated and translated. Although these
refinements were found not to alter the unusually short
intermolecular distances, the exact magnitude of these slight
displacements cannot be determined reproducibly.
˚
˚
7.486A, c = 5.876A, and b = 96.161u fits improve slightly,
indicating that one of these space group symmetries may be the
correct one. Here, the shortest distances between neighbouring
˚
˚
˚
molecules are H–S = 2.18A, H–C = 2.67A, and C–S = 3.25A,
indicating the possible presence of H-bonds between the
molecules. Another PTTP analogue with shorter alkyl-
terminated end-units, 5,59-bis(4-isopropylphenyl)-2,29-bithio-
phene has been determined as P21/a, (a = 8.387, b = 9.634, c =
13.368, and b = 96.43u), with a similar layered, herringbone-
type arrangement of molecules tilted along the long and the
short axes.4
The HOC6PTTPC6OH structure is similarly identified
by powder data analysis as a monoclinic unit-cell with
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(101) plane, the bithiophene rings remain planar [39u to the
(101) plane], but one of the phenyl rings goes through a slight
rotation [26.5u to the (101) plane]. The corresponding
calculated pattern of this conformation and the measured
data are shown in Fig. 4, along with the difference data and the
expanded range inset that shows that the observed differences
are mainly due to peak profile shapes. The data exhibit a non-
uniform peak broadening, indicating the presence of some
characteristic defects. For instance, the (020) peak at 18.576u is
significantly broader than predicted, suggesting a specific
defect that disrupts the long-range order along this axis.
The structure of HOC6PTTPC6OH consists of layers whose
spacing is defined by the tilted molecular length. Layering is
induced by the aromatic/aliphatic separation of the neighbour-
ing molecules. Within the layers near the (100) plane,
molecules are close packed in a distorted hexagonal array.
For a perfect hexagonal array, b/c = 3K = 1.732. In our case,
b/c = 1.39 in the plane perpendicular to cores. Molecules in
neighbouring (101) planes are shifted along the long axis of the
molecule such that one of the thiophene rings of molecules in
one layer is approximately aligned with one of the phenyl rings
of the neighbouring layers as shown in Fig. 3(b). Molecules
within the layer have alternating offsets along the long
molecule axis such that a phenyl ring aligns with a thiophene
ring. The alternative arrangement produces the interdigitation
of the alkyl chains. Molecules in the (101) planes are all tilted
in one direction with respect to molecules in neighbouring
planes that are tilted in the opposite direction. The magnitude
of the tilt is about 38.5u to the (101) plane, for molecules
modelled by rigid aromatic cores and flexible aliphatic tails.
The HOC6PTTPC6OH structure appears to be interdigi-
tated, with partial mixing of the aliphatic and aromatic parts
of the neighbouring molecules. The calculated electrostatic
potential of a planar molecule, as shown in Fig. 3(a), finds a
charge distribution on the aromatic part similar to that of
2PTTP2. In the ab initio calculated structure, some of the most
Fig. 3 Energy-minimized geometry of the HOC6PTTPC6OH mole-
cule and value of the electrostatic potential at the van der Waals
or electrostatic potential surface (a). Crystal structure of
HOC6PTTPC6OH determined by ab initio Monte Carlo calculations,
using a rigid PTTP core and a single torsional motion on alkyls (b) and
the corresponding powder data fit (c).
To verify the uniqueness of the HOC6PTTPC6OH solution,
we have systematically evaluated the results obtained from
multiple repeat calculations, using different initial conditions,
molecular conformations, unit-cell decorations, and space
group symmetries. We find that overall a herringbone packing
structure is readily reproduced even in fully rigid molecules.
The main findings: molecules are aligned near the a-axis,
neighbouring molecules are shifted along the molecular long
axis, and tilted both along the short axis in opposite directions
with respect to each other forming a herringbone-type
structure. If, however, all of the atoms in the molecule are
unrestrained and allowed to move simultaneously (within
bond length and angle limits), the global optimization leads to
similar solutions for different conformations of the alkyl
chains. This is not surprising, since some degree of alkyl chain
tail disorder should exist. Existing data on similar molecules
have consistently identified a rigid alkyl chain in the trans
configuration. This observation was the basis for our
assumption made in Fig. 3. This assumption has little effect
on the overall quality of the fits, indicating that our molecular
ordering model is accurate.
Fig. 4 Globally optimized Monte Carlo structure determination of
HOC6PTTPC6OH, where six rigid groups, two alkyls and four rings
model the molecule. The bottom curve shows the difference data. The
inset shows the details of the fit. Observed differences are attributed to
dissimilar functional forms of the peak profiles.
In the case of torsional freedom of four distinct rings of the
core, we find that the aliphatic tails again align parallel to the
3430 | J. Mater. Chem., 2007, 17, 3427–3432
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electronegative phenyls are aligned with the hydrogen atoms of
the neighbouring thiophenes, strongly suggesting that the most
electropositive part is now shifted towards the thiophene rings.
This can be explained by the separation of phenyl hydrogen
atoms from the plane of the thiophene hydrogen atoms, or a
rotation of the phenyl rings. Since we have fixed the aromatic
core to be rigid in our ab initio and potential surface models,
we would not be sensitive to such effects. A driving force for
interdigitation would be purely steric, diluting the high mass
density core regions with the extra space of the aliphatic
regions.
surface in analogy to the crystalline-H phase of liquid crystals.
The crystal structures of HOPTTPOH and HOC6PTTPC6OH
powders are determined by synchrotron X-ray powder
diffraction (XRD) to be analogues of crystalline-K phase of
liquid crystals.17 In all cases, crystalline structures are found
with layer spacing defined by the tilted long axes of the
molecules whose common bis(phenylenyl)bithiophene cores
have a similar tendency to be flat as well as similar ordering.
The molecules are close packed in a distorted hexagonal array
with their tilted short axes forming a herringbone-type
structure, while neighbouring molecules within the layers are
displaced along the long axis with respect to each other. In the
case of HOC6PTTPC6OH, alkyl chains appear to facilitate a
partial mixing of the aromatic and aliphatic parts by
interdigitation. Electrostatic interactions alone can account
for the interdigitation of the herringbone-type ordering in this
novel class of molecular compounds. With its hydroxyl (polar)
termination, thin films of HOC6PTTPC6OH deposited with
vacuum sublimation are being considered and currently
studied with infrared absorption spectroscopy as a useful
platform for attaching biomolecules and for the investigation
of its gas sensing properties using organic field effect
transistors.
These findings are similar to the case of the mesomorphic
smectic phase of liquid crystals, where long-range ordering is
governed by the interaction of aromatic and aliphatic parts of
the molecule and steric effects. The strength of the aromatic
core interactions plays a major role in determining the range of
smectic phases in thermotropic liquid crystals. For strong core
interactions, layered phases will be stable at all temperatures
and will not disorder into nematic phases. Within the layers,
the molecules are typically close packed in an hexagonal array.
In liquid crystals, smectic-E phase molecules become ordered
in
a herringbone arrangement within the plane, while
remaining perpendicular to the layer. At low temperatures,
all hindered motions of the molecules gradually disappear,
long-range order sets in, and the molecules crystallize. If the
molecules become tilted with respect to the layers in the
crystalline phase, the two generalized tilt directions (perpendi-
cular to a side or towards a corner of the hexagonal in-plane
underlying structure, respectively) define the crystalline-H and
Acknowledgements
This research was supported by NSF-IGERT (NSF DGE
0333196), NSF (CHE-0415652), DOE NSET (Grant No.
04SCPE389) and the Brookhaven National Laboratories
NSLS Faculty-Student Research Support Program. We also
acknowledge the assistance of Ben Ocko and John Hill at the
NSLS X22A beamline, as well as Dr. V. Favre-Nicolin for his
Fox software.
K
phases.17 For the HOC6PTTPC6OH case, the only
observable phase is crystalline, suggesting that aromatic
interactions are very strong. As the aliphatic tails appear to
be frozen, it is improbable that highly conformable aliphatic
tails would display perfect long-range order. In fact, the frozen
disorder of the alkyl chains may be responsible for our broad
diffraction peaks. In support of this, the HOPTTPOH
(identical molecule without aliphatic tails) structure reveals a
highly ordered crystalline structure, with well-defined Bragg
peaks. While Monte Carlo calculations treat the alkyl chains as
fully crystalline, in essence the optimized atomic coordinates of
the tails are weighted average positions. The fact that these
alkyl-substituted bis(phenylenyl)bithiophenes are planar facil-
itates the pi-bond overlap. Moreover, the electrostatic inter-
actions help reduce the intermolecular distance. Therefore,
these molecules are promising for use as organic semiconduc-
tors. The methods presented for evaluating the structure of
alkyl-substituted PTTP compounds, which cannot easily form
single crystals, will allow further investigation of compounds
whose alkyl substitution would provide advantageous proper-
ties, such as low surface energies, surface sensitivity, or order-
enhancing properties.4
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Conclusions
The molecular structure orderings of phenylene–bithiophene
(PTTP) oligomers have been studied with different end groups.
The structure of 2PTTP2 is determined by single-crystal
diffraction to be layered, with an in-plane herringbone-type
packing. The molecules are tilted with respect to the layer
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3432 | J. Mater. Chem., 2007, 17, 3427–3432
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