Tetrathiahexacene as building block for solution-processable semiconducting polymers: Exploring the monomer size limit

Article abstract of DOI:10.1021/ma101183b

A larger fused monomer in a polythiophene chain which can improve stability and device performance has been reported. The largest building block in a soluble polythiophene so far is tetrathienoacene (P3). This polymer is very stable toward oxidation in an organic field-effect transistor and also shows high charge-carrier mobilities. The polymerization using dibromodithiophene as comonomer was performed using Pd(PPh₃)₄ as catalyst in toluene. The reaction mixture was heated for 40 minutes in the microwave to 150 °C with a power density of 70 W/mL. In a field-effect transistor, a good device characteristic, which is low contact resistance and low hysteresis, have been measured. The field-effect mobility does not yet reach values needed for applications but is likely to be achieved after more processing optimizations.

Full text of DOI:10.1021/ma101183b

6264 Macromolecules 2010, 43, 6264–6267
DOI: 10.1021/ma101183b
Tetrathiahexacene as Building Block for
Solution-Processable Semiconducting Polymers:
Exploring the Monomer Size Limit
Ralph Rieger, Dirk Beckmann, Wojciech Pisula,
€
Marcel Kastler, and Klaus Mullen*
Max Planck Institute for Polymer Research, Ackermannweg 10,
55128 Mainz, Germany
Received May 27, 2010
Revised Manuscript Received June 25, 2010
The search for organic semiconductors suitable to produce
flexible, lightweight, and cost-effective devices is the focus of
many research groups around the globe.1,2 There is a particular
interest in organic field-effect transistors which may be used in
various applications such as RFID tags.3,4 Great advances have
been achieved, pushing upfield-effect mobilities almost to the
regime of amorphous silicon.5-7 Especially thiophene-contain-
ing polymers are highly promising toward applications due to
their good mobility obtained by simple solution-based process-
ing techniques.8,9 Poly(3-hexylthiophene) (P3HT) is among the
most intensively studied semiconducting polymers.10-12 How-
ever, it does not meet all requirements for device production
such as, for instance, operational stability under ambient con-
ditions.13
To improve the properties of P3HT, comonomers con-
sisting of fused polycycles have been introduced into the
polythiophene chain. Benzodithiophene (P1), for example,
increases the ionization potential of the resulting polymer,
thus also increasing its stability.14,15 In addition, the struc-
tural order of the polymer in the film is enhanced by the high
aggregation tendency of the large monomer. Fused oli-
gothiophenes have also been used, such as dithienothiophene
(P2), with remarkable stability and high charge-carrier
mobility.16 The largest building block in a soluble polythio-
phene so far is tetrathienoacene (P3).17 This polymer is very
stable toward oxidation in an organic field-effect transistor
and also shows high charge-carrier mobilities.
The aim of the present work is to show whether an even
larger fused monomer in a polythiophene chain can further
improve stability and device performance. The crucial point
is the question if a sufficiently soluble polymer can be
obtained yielding highly ordered films upon processing.
Tetrathiahexacene (2) was chosen as building block to extend
the monomer size beyond the current limit. It allows the
attachment of alkyl chains in a way that no steric repulsion
between neighboring chains disturbs the packing and that
the polymer can benefit from the high aggregation tendency
of extended polycyclic aromatic hydrocarbons.18,19
The synthesis is outlined in Scheme 2. It starts with routine
thiophene chemistry: bromination of 3-iodothiophene (1),
selective Sonogashira coupling with tetradecyne, and Stille
coupling with bis(trimethylstannyl)thieno[2,1-b]thiophene (4),
which is made according to the literature procedure.20 The key
step to make the desired molecule is a base-induced cycliza-
tion of two triple bonds of the thieno[2,1-b]thiophene core in
molecule 5, forming benzene rings with attached alkyl chains.
When using the strong nonionic base diazabicycloundecene
Figure 1. UV-vis absorption spectra of PTTH in o-dichlorobenzene at
a concentration of 10-4 M with regard to the repeat unit. Extinction
coefficients are calculated for the repeat unit of the polymer.
(DBU) in N-methylpyrrolidone (NMP), the target molecule 6
can be obtained in high yield. An initial attempt to form a
polymer out of this building block targeted a homopolymer.
For this reason, structure 6was brominated with NBS to allow a
nickel-catalyzed dehalogenation polymerization. However, no
soluble polymer was obtained. The polymer chain is obviously
too stiff to be solubilized by the alkyl chains. To increase the
solubility, additional dithiophenes have been introduced to yield
poly(tetrathiahexacene-alt-ditetradecyldithiophene) (PTTH).
This goal is reached by a Stille copolymerization. Compound 6
was doubly lithiated by tert-butyllithium and subsequently
treated with trimethyltin chloride, yielding the bis-stannylated
compound 7. The polymerization using dibromodithiophene
(8) as comonomer was performed using Pd(PPh3)4 as catalyst in
toluene. The reaction mixture was heated for 40 min in the
microwave to 150 °C with a power density of 70 W/mL. The
product which precipitated in the reaction medium could be
dissolved in dichlorobenzene. After reprecipitation and Soxhlet
extraction to remove low molecular weight material, the pure
polymer was obtained with a molecular weight of Mn=18 kg/
mol and Mw=42 kg/mol as determined by size-exclusion chro-
matography in reference to polystyrene as standard. The latter
value corresponds to a degree of polymerization DP=32. It is
known from other polymers that the charge-carrier mobi-
ity drops with very low molecular weights, but the value
reached for PTTH should be sufficient to explore the device
performance.21,22
The UV-vis absorption spectrum of PTTH (Figure 1) in
a dichlorobenzene solution shows three resolved bands
around 500 nm at room temperature. The spectrum almost
resembles the absorption of the film which means that the
chains are close to each other in solution similar to the
situation prevailing in the film, indicating high intermolecu-
lar forces of the fused system in solution. When the polymer
solution is heated to about 100 °C, the bands at 520 and 560
nm vanish. Only one broad peak at 450 nm remains, which is
typical for polythiophenes like P3HT and structurally re-
lated polymers with a tendency to aggregate in solution.23,24
At elevated temperatures, the thermal motion reduces the
aggregation and the absorption of a singly solvated polymer
chain is detected.
*Corresponding author. E-mail: muellen@mpip-mainz.mpg.de.
pubs.acs.org/Macromolecules
Published on Web 07/14/2010
r 2010 American Chemical Society

Communication
Macromolecules, Vol. 43, No. 15, 2010 6265
Scheme 1. Development of Polythiophenes from P3HT to More and More Extended Monomer Building Blocks
Scheme 2. Synthesis of PTTHa
a Conditions: (i) NBS, DMF, 1 h, 95%; (ii) 1-tetradecyne, Pd(PPh3)4, CuI, triethylamine, THF, RT, overnight, 98%; (iii) Pd2(dba)3, P(o-tol)3,
o-dichlorobenzene, 120 °C, 3 h, 64%; (iv) DBU, NMP, reflux, 78%; (v) 1: tert-butyllithium, THF, 0 °C; 2: Me3SnCl, 42%; (vi) Pd(PPh3)4, toluene,
MW, 150 °C, 80%.
The absorption onset at 550 nm is at about the same value as
for P3HT. This means that PTTH possesses almost the same
effective conjugation length despite the much stiffer polymer
backbone. For P3HT, about 15 thiophene units have been deter-
mined at room temperature to constitute the effective conjuga-
tion length.25,26 Given a similar conjugation within and between
the repeat units in both systems, this translates into less than two
repeating units for PTTH, an amazingly low value.
To investigate the bulk morpholgy of the polymer, fibers
have been extruded to measure two-dimensional wide-angle
X-ray scattering. The diffraction pattern is shown in Figure 2.
The well-resolvedpeaks indicate a pronouncedordering of the
polymer. The equatorial reflections reveal lamellar order as
typically observed for polythiophenes.
thiophene unit. It is thus very likely that the alkyl chains of
adjacent polymer chains interdigitate as depicted in Figure 2b.
This explains the high order of the polymer in the bulk in
addition to the aggregation of the large π-system. The latter is
reflected by the very low π-π distance (0.37 nm) as deter-
mined by the position of the equatorial reflections of the
diffraction pattern. This value is assmallas for other polymers
showing field-effect mobilities well above 0.1 cm2/(V s). A low
stacking distance is important for high charge-carrier mobi-
lities to reduce the potential barrier of a charge-carrier hop-
ping from one chain to the neighboring one.27
The polymer PTTH has been tested in a field-effect transistor
using a silicon substrate which has been treated with HMDS to
avoid charge-carrier trapping due to the acidic silicon dioxide
dielectric surface. The polymer was spin-cast on top from a
dichlorobenzene solution and annealed for 5 min at 100 °C.
The lamellar distance was determined as 2.38 nm, which is
about the length of the attached tetradecyl chain plus a

6266 Macromolecules, Vol. 43, No. 15, 2010
Communication
These parameters are known to be very important for
reliable applications.30
In summary, it could be demonstrated that a mono-
mer consisting of six fused rings can still form a soluble
polymer when copolymerized with alkylated dithiophenes.
The strong interchain aggregation leads to highly ordered
bulk material with low π-π distance, showing the effec-
tivity of introducing monomers of this size. In a field-effect
transistor, good device characteristics, i.e., low contact
resistance and low hysteresis, have been measured. The
field-effect mobility does not yet reach values needed for
applications but is likely to be achieved after more process-
ing optimizations. The present results strongly support the
potential of conjugated polymers containing large fused
π-systems.
Figure 2. (a) Two-dimensional wide-angle X-ray diffraction pattern of
an extruded fiber of PTTH. (b) Cartoon suggesting the packing of the
polymer. The polymer structures are obtained by an MMFF optimiza-
tion and fit to the X-ray diffraction data.
Supporting Information Available: Experimental details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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