Predictability of thermal and electrical properties of end-capped oligothiophenes by a simple bulkiness parameter

Article abstract of DOI:10.1021/cm400702t

The branching topology of end groups attached to several series of oligothiophenes has a systematic effect on thermal and electrical properties of the oligomers. The series were synthesized in a modular approach and show a distinct drop of the melting point T_m on increasing bulkiness of the substituents. The same trend can be found for the dissociation temperatures T_dis of aggregates in solution. Similarly, monolayer OFET mobilities μ_FET are significantly decreasing with increasing bulkiness of the substituents. A simple geometric model is presented quantitatively correlating the transition temperatures and mobilities with the substituents' structure based on a bulkiness parameter P, which allows predicting T_m, T _dis, and μ_FET of corresponding not yet synthesized oligomers with branched substituents. This model might be generally applicable for end-capped rod-like conjugated oligomers.

Full text of DOI:10.1021/cm400702t

Article
pubs.acs.org/cm
Predictability of Thermal and Electrical Properties of End-Capped
Oligothiophenes by a Simple Bulkiness Parameter
Andreas Kreyes,†,‡ Ahmed Mourran, Zhihua Hong, Jingbo Wang, Martin Moller,§
§
§
§
̈
∥
∥
‡
,†
Fatemeh Gholamrezaie, W. S. Christian Roelofs, Dago M. de Leeuw, and Ulrich Ziener*
†
Institute of Organic Chemistry III, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany
‡
Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
§
Interactive Materials Research, DWI an der RWTH Aachen e.V. & Institute of Technical and Macromolecular Chemistry,
Forckenbeckstraße 50, D-52056 Aachen, Germany
∥
Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
*
S Supporting Information
ABSTRACT: The branching topology of end groups attached
to several series of oligothiophenes has a systematic effect on
thermal and electrical properties of the oligomers. The series
were synthesized in a modular approach and show a distinct
drop of the melting point Tm on increasing bulkiness of the
substituents. The same trend can be found for the dissociation
temperatures Tdis of aggregates in solution. Similarly, monolayer
OFET mobilities μFET are significantly decreasing with increasing
bulkiness of the substituents. A simple geometric model is presented quantitatively correlating the transition temperatures and
mobilities with the substituents’ structure based on a bulkiness parameter P, which allows predicting T , T , and μ of
m
dis
FET
corresponding not yet synthesized oligomers with branched substituents. This model might be generally applicable for end-
capped rod-like conjugated oligomers.
KEYWORDS: transition temperature, oligothiophene, branched substituents, structure−property relationship, OFET mobility
INTRODUCTION
compounds. It would be of utmost interest to tailor the
molecules not just on a phenomenological basis but specifically
in a predictable way. It dates back already to the 1940s that a
model was established to predict the boiling points of pure4
paraffins by a simple geometric approach (Wiener Index).
Further developments led to the prediction of melting points of
■
In recent years much effort was put on the molecular design of
organic field effect transistors (OFETs) by chemical tailoring
the active oligomers or polymers in order to control thei1r
supramolecular arrangement and finally the functional device.
Two design strategies were followed, i.e., band gap engineering
by adjusting the chemical structure of the conjugated moiety
and steering the molecular packing by a broad variety of
intermolecular interactions like van der Waals, hydrogen
bonding, π−π interactions, etc. While a correlation between
the supramolecular arrangement like thin film or bulk
morphology of the electronic materials with the electrical
5
alkanes, and many more approaches were worked out to
predict melting points of various compounds via molecular
6
descriptors. The melting point for a given class of organic
materials can be steered most simply by introducing branched
substituents as they offer a high structural variability to adjust
the intermolecular interactions, molecular symmetry, and
conformational degree of freedom of the molecules. Many
reports are found in the literature where branched substituents
were attached to the functional cores of the compounds like in
2
properties of the devices is demonstrated in several cases, a
direct line between molecular structure and electrical perform-
1a,3
ance is difficult to reveal. Even more, so far only qualitative
structure−functionality correlations are reported for such
materials but no quantitative relationships. On the other side,
simple physical properties like solubility or phase transition
temperatures determine the processability for manufacturing
devices and the thermal working area of the final devices.
Furthermore, only well-controlled fabrication processes guar-
antee reproducible and good electrical properties like charge
carrier mobilities. In order to control those processes via
molecular parameters of organic materials, substituents are
introduced enhancing solubility and lowering thermal tran-
sitions like melting compared to the unsubstituted parent
7
8
oligothiophenes, phenylene thiophene co-oligomers, hexa-
9
10
benzocoronenes, perylene tetracarboxdiimides, and isoindi-
11
go-based conjugated polymers to steer solubility and thermal
properties while still maintaining the desired materials
properties like charge carrier mobility.
7
b,d,e,12
Despite the
wealth of those reports to find the golden way between
processability and materials properties there is no systematic
Received: March 1, 2013
Revised: April 15, 2013
Published: April 30, 2013
©
2013 American Chemical Society
2128
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Chemistry of Materials
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investigation of thermal transitions and charge carrier (OFET)
mobility of conjugated oligomers on the geometry of branched
substituents to the best of our knowledge. Recently, the
dependence of hole mobilities on the branching position of the
alkyl substituents in polymer thin film transistors was
Temperature dependent absorption spectroscopy was performed on
a Perkin-Elmer UV/vis spectrometer Lambda 16. The sample
solutions were prepared from stock solutions, heated up, and stored
at 5 °C for at least 12 h before measuring. Temperature dependent
fluorescent spectroscopy was performed on a Horiba FluoroMax 3.
The sample solutions were diluted from stock solutions, heated up,
and stored at −20 °C for at least 12 h before measuring. DSC
measurements were performed on a Perkin-Elmer DSC 7 under
nitrogen atmosphere and with a heating rate of 10 K/min.
1
3
described.
In the present contribution we report on in total five series of
mono-end-capped R-4T-H, bis-end-capped R-7T-R and R-9T-
R, and precursor oligothiophenes R-7T*-R and R-9T*-R,
respectively, and their thermal behavior in solid and solution
state as well as monolayer OFET mobility with respect to the
number of thiophene units and the chemical structure of the
substituents R (Chart 1). For comparison, thermal transition
Transistors were fabricated by direct spin coating the solution onto
the electrodes at concentrations that give full monolayer surface
coverage. Heavily doped silicon wafers, acting as the common bottom
gate covered with a 200 nm layer of thermally grown SiO , were used.
2
The gold source and drain contacts were defined by conventional
photolithography. Ti (10 nm) was used as an adhesion layer. To
remove any organic contaminants, prior to spin-coating the surface
was exposed to UV-irradiation under flux of oxygen for 12 min. The
monolayer was applied by spin-coating from a toluene or
chlorobenzene solution with spinning speed between 500 and 2000
rpm for 3 min. The solution was heated up to 60 °C to improve the
solubility of the oligomer. The electrical characterization of the field
effect transistors was carried out in a probe station under high vacuum
Chart 1. Chemical Structures of the Oligothiophenes and the
Carbosilane Substituents I−IX and the Corresponding
Structural Parameters a and b −b and Bulkiness Parameter
1
1
3
P (see also Table 1)
−6
(
10 mbar) with a Keithly 4200-SCS Semiconductor Characterization
System. A ring geometry was chosen to prevent parasitic leakage
currents. The pristine transistors exhibited very little hysteresis. Scaling
of the mobility with channel length shows only a small contact
resistance, which is confirmed by the linear mobility that is comparable
to the saturated mobility.
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of some of the compounds in
7
b,d,e,12b
Chart 1 was already described previously.
display the reaction pathways including the long oligomers R-
1T-R (124) and R-13T-R (125) for completeness. In the first
Schemes 1−3
1
part of the synthesis the substituents were constructed and
attached to thiophene. For this purpose, thiophene was
alkylated with allylbromide, 6-bromohex-1-ene, and 11-
bromoundec-1-ene, respectively, to obtain olefins 1−3, in
which the distance between branching point and oligomer core
is predefined. For substituents I−VII, which bear one or two
long alkyl chains, alkylsilanes 4−7 and dialkylsilanes 8 and 9
were synthesized by coupling in situ generated alkyl Grignard
reagents to chlorodimethylsilane (I−V) and dichloromethylsi-
14
lane (VI, VII), respectively. In the following step these silanes
were added to the olefinic double bonds of 1−3 by
hydrosilylation reactions catalyzed by Karstedt catalyst
(
(
Pd (dvtms) ) to obtain monosubstituted thiophenes 10−16
2
3
see Scheme 1). To get access to substituents with more of the
long alkyl chains (VIII−IX) the strategy had to be slightly
modified (Scheme 1). Monosubstituted thiophenes 19 and 20
could not be synthesized as described above. Hydrosilylations
of 2 and 3 with the corresponding trialkylsilanes were not
successful as no conversion of the starting materials took place.
This we attribute to the steric demand of these silanes that
hinder the coordination to the catalyst. Hence, olefins 2 and 3
were first hydrosilylated with trichlorosilane and subsequently
alkylated with excess of Grignard agents. This worked very well
for substituent VIII, but in the case of IX the undesired
Markovnikov byproduct occurred, which could not be
separated from the desired anti-Markovnikov product. This
problem was overcome by application of carbene catalyst N,N′-
dicyclohexylimidazol-2-yliden-(divinyltetramethylsiloxan)Pt-
data from solution and (OFET) mobility data of even longer
7b
oligomers (R-11T-R, R-13T-R) are included, too. In order to
get access to a wide structural variety, we chose branched
carbosilanes as side chains (Chart 1). The substituents own one
branching point, whose distance to the aromatic core amounts
to 3, 6, or 11 methylene groups. Additionally, the length
(
C H , C H , C H ) and number (1 to 3) of long
6 13 10 21 14 29
peripheral alkyl groups are varied. The substituents can be
ordered according to their bulkiness parameter P, which will be
described below.
EXPERIMENTAL SECTION
■
Details of the synthesis of the oligothiophenes and precursor
molecules is described in the Supporting Information or was reported
1
5
(0), which exclusively produced the anti-Markovnikov
product.
7
b,d,e,12b
earlier.
2
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Scheme 1. Synthesis of Monosubstituted Thiophenes 10−16, 19, 20, and 24 with One (I−V), Two (VI, VII), and Three Long
Alkyl Chains (VIII, IX, see Chart 1) in the Periphery via Alkylation−Hydrosilylation
Scheme 2. Stille-Type Coupling Reactions to Monosubstituted Oligomers R-nT-H
In the following part the monosubstituted oligomers R-nT-H
see Scheme 2) and disubstituted odd-numbered oligomers R-
nT-R (Scheme 3) were built up according to common
stannylation and Stille-type coupling reactions yielding the
(
2
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Scheme 3. Synthesis of Disubstituted Oligothiophenes R-nT-R up to a 13-mer via Stille-Type Coupling to Soluble Diketal
Precursors, Diketones, and Ring Closing
disubstituted diketals 72−89, in which the conjugation length
of the end products is already given. The ketal groups then
were removed with hydrochloric acid so that diketones 90−107
were obtained and converted to the final end-capped
oligothiophenes R-7T-R (108−116), R-9T-R (117−122), R-
1T-R (123, 124), and IX-13T-IX (125) using Lawesson’s
reagent.
Thermal Transitions in Solid State. It can be expected
that the chemical structure of the substituents R also plays a
major role for the thermal properties, besides the influence on
solubility, namely, the melting behavior of the oligothiophenes,
which we investigated by differential scanning calorimetry
The length of the branches as well as the distance between
the aromatic core and the branching point are crucial factors;
e.g., for R-7T-R T varies from 127 to 290 °C (I > V > II > IV
m
>
III ≈ VI > VIII > VII > IX). This effect is expected and can
be in the first instance attributed to the bulkiness of the
substituents partially suppressing intermolecular attractions
between the aromatic cores by steric hindrance (see below
and Chart 1).
1
Interestingly, this substitution dependent order of Tm is
found not only for the symmetric oligomers R-7T-R and R-9T-
R but also for the monosubstituted series H-4T-R and even for
the diketone precursor molecules R-7T*-R and R-9T*-R
(Table 1). This suggests that the effect of the structure of the
substituents on Tm follows a general principle. In order to
obtain a deeper understanding of this principle and a more
quantitative relation between Tm and the structure of R we
introduce a bulkiness parameter P. As the effective bulkiness
depends on both the absolute volume of R and R’s distance
from the oligomeric core P is expressed by the approximated
volume ratio of the chains behind the branching point (see
Charts 1 and 2, red bonds) and the spacer between the
(
DSC). The thermograms of most of the compounds show
several endothermic transitions owing to (thermotropic liquid
crystalline) mesophases as proved by polarizing optical
microscopy and X-ray (data not shown). In the present
contribution, we will concentrate on the transition at the
highest temperature to the isotropic phase, which shall be called
melting temperature Tm for simplicity. Indeed, distinct
differences of T are observed (see Table 1).
m
Chart 2. Schematic Illustration of Substituents and Their
Spatial Demand
Table 1. Melting Points T [°C] of Oligothiophenes R-nT-R
m
and Precursor Oligomers R-nT*-R with Substituents I−IX
a
oligomeric core
bulkiness parameter
R
4T
7T*
7T
9T*
9T
P
I
149
137
242
214
189
199
217
196
133
163
92
290
260
241
254
282
239
177
205
127
1.09
3.67
II
III
IV
V
9.33
6.67
4.00
branching point and the thiophene moieties (see Charts 1 and
Chart 2, blue bonds). Thereby, we approximate the spatial
demand of the red chains as a cuboid with the length of a side
VI
VII
VIII
IX
130
71
289
233
269
176
347
270
319
224
12.00
66.67
42.00
366.67
96
given by the number of backbone atoms per chain (b , b , and
52
1
2
b3, respectively (Chart 1)) where the branching point (silicon
atom) counts for the shortest of the three chains. The spacer’s
volume is similarly approximated by a cuboid with two sides as
a
Missing oligomers could either not be synthesized due to poor
solubility (R-9T*-R, R-9T-R) or were not available with sufficient
purity (R-4T-H).
unit length a (= 1) and the third side as number of backbone
0
2
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Chemistry of Materials
Article
atoms a (Chart 1). Thus, P increases when the ratio of the two
volumes increases, by increasing the length of the branches
and/or decreasing the length of the spacer (eq 1).
anisotropic to the liquid state decreases with increasing P,
which might be attributed to higher flexibility already in the
anisotropic state and presumably decreased packing density
compared to compounds with lower P. Those findings might
apply to the other oligothiophene series as well but before
evaluation a clear assignment of all transitions has to be
undertaken.
1
b b b
1
2 3
2
P =
a a
(1)
1
0
In good agreement with the experimental data, the order of P
I < II ≈ V < IV < III < VI < VIII < VII < IX Chart 1, Table 1)
Comparing Tm,min with Tm in Table 1 reveals that the
minimum is already reached with a bulkiness parameter P of ca.
(
is almost perfectly reverse to the order of T with the
m
3
70. Further reduction of T is not expected at least with the
m
substituents. More quantitatively, Figure 1 shows that there is a
present type of branched substituents. Tm,max corresponds to Tm
of oligomers with nonbranched (linear) substituents. There-
fore, comparison to literature values should prove if the
presented simple geometric approach is valid also for other
analogous compounds. Indeed, H-4T-R with a linear C H
2
0
41
7e
substituent with T = 148 °C matches perfectly the predicted
m
value (149 ± 11 °C, Table 2) while R-7T-R with the same
7e
substituent C H and Tm = 295 °C falls in the expected
20
41
range (277 ± 25 °C, Table 2). Besides the potential of P to
predict the limits of melting transitions Tm,min and Tm,max in a
given series of oligothiophenes we assume that T of not yet
m
synthesized oligothiophenes with substituents with one
branching point can be predicted if the fit parameters of the
series are already known. One example shall illustrate our
assumption. The bulkiness parameter P of a branched pure
hydrocarbon 2-octyldodecyl substituent amounts to 90 (b =
1
Figure 1. Correlation between T of various R-nT-R and bulkiness P;
symbols, experimental data; lines, fit according to eq 2.
10, b = 9, b = 1, a = 1) leading to an expected value of T =
m
2
3
m
158 °C for a corresponding R-7T-R. The literature data is quite
7e
close with T = 171 °C.
m
clear exponential decay of Tm with P displaying good
correlation coefficients between 0.9501 and 0.9905 (Table 2)
following eq 2.
Thermal Transitions in Solution State. The thermal
dissolution of aggregates of π-conjugated molecules in solution
can be regarded as a microscopic analogue to macroscopic
melting. Therefore, besides the melting properties in bulk, the
aggregation and dissociation of aggregates in solution should
also be controlled by the chemical structure of the substituents.
Note that solvent-oligothiophene and solvent−solvent inter-
actions will play a crucial role for the aggregation process in
solution, besides the intermolecular interactions between
oligothiophene molecules governing the bulk melting process.
In the following we try to solve the question if there is also such
a clear structure−property relationship for the aggregation
process as for the melting (see above). Qualitative aspects of
the self-assembly of the presented oligothiophenes were
investigated by temperature dependent UV/vis spectroscopy.
Figure 2 shows exemplary absorption spectra of the four
oligothiophenes II-7T-II (109), II-9T-II (117), VII-9T-VII
(120), and VII-11T-VII (123), and further spectra are found in
Supporting Information (Figure 1S). The spectra were
measured in TCE (1,1,2,2-tetrachloroethane, good solvent) or
TCE/Isopar M mixtures. Isopar M is a nonpolar paraffinic
mixture with C11 to C16 (bad solvent). The different solvents
were used because of substantially different aggregation
+ (Tm,max − Tm,min) × e− m
k P
Tm = T
m,min
(2)
with Tm,min is the minimum melting transition temperature for
P = ∞, expressing the lowest achievable melting transition by a
maximum bulkiness and depression of T ; T
is the
m
m,max
maximum temperature achievable for P = 0; and km is a
dimensionless constant.
In order to elucidate the thermodynamic basis for the clear
dependence between Tm and P we evaluated the transition
enthalpies ΔH and entropies ΔS . Most of the thermograms
m
m
of the oligothiophenes within one series show multiple and
different number of transitions. In order to compare
thermodynamic data of the single compounds we have chosen
the series with the most homogeneous appearance of the DSC
traces, i.e., the diketones R-7T*-R. Both variables ΔH and
m
ΔS decrease strongly with increasing P (see Supporting
m
Information Figure 3S). Thus, this dependency confirms the
assumption above that increasing bulkiness lowers intermo-
lecular attractions (ΔH , e.g., π−π interactions). The decrease
m
of ΔS indicates that the change in chain order from the
m
Table 2. Fit Parameters for the Exponential Decay of T with P for the Different Series of Oligothiophenes R-4T-H, R-nT-R,
m
and R-nT*-R
oligomeric core
4
T
7T*
0.9501
7T
0.9593
9T*
0.9807
9T
0.9652
R2
0.9905
Tm,min/°C
Tm,max/°C
km
51 ± 5
149 ± 11
0.021 ± 0.003
93 ± 12
224 ± 24
0.018 ± 0.008
129 ± 12
277 ± 25
0.018 ± 0.003
173 ± 14
308 ± 34
0.011 ± 0.003
222 ± 18
375 ± 49
0.014 ± 0.004
2
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Chemistry of Materials
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−
5
−1
Figure 2. Temperature dependent absorption spectra of (a) II-7T-II (109) (in TCE:IsoparM = 1:3 (v:v), c = 1 × 10 mol L ), (b) II-9T-II (117),
−5
−1
−5
−1
−6
−1
(
c) VII-9T-VII (120), and (d) VII-11T-VII (123) (in TCE, b: c = 1 × 10 mol L ; c: c = 4 × 10 mol L ; d: c = 5 × 10 mol L in TCE).
behavior of the oligomers. At high temperatures (no
aggregation), broad and unstructured bands are observed.
Upon cooling, the spectral shape changes due to the formation
of aggregates. These changes depend considerably on the
structure of R, the length of the oligomeric core, and to some
extent on the nature of the solvent. While in the molecularly
dissolved state the position of the maxima increases with
In contrast, the corresponding temperature dependent
photoluminescence spectra display a much clearer picture.
The position of the peaks is changed neither by temperature or
concentration nor by the structure of R but the overall intensity
increases substantially upon increasing temperature. Figure 3
shows exemplary photoluminescence spectra of I-7T-I (108).
At high temperatures, when the compounds are molecularly
dissolved, high fluorescence intensity and a distinct vibronic
fine structure (up to 0−2) are observed. For all septithiophenes
the 0−0 band occurs at around 536−538 nm and is red-shifted
increasing number of thiophene units n from R-7T-R (λmax
=
=
457 to 451 nm depending on the solvent) to R-9T-R (λmax
4
65 to 475 nm depending on the substituent), it does not
7b
further grow for R-11T-R and R-13T-R (λmax = 475 nm).
This indicates that the effective conjugation length does not
increase any more due to twisting of the thiophene moieties
around the inter-ring single bond, which reduces intramolecular
π-overlap. Upon aggregation it is found that the slightly
branched substituents I−V (Chart 1) cause a significant
hypsochromic shift of λ up to around 2100 cm− (Δλ
1
≈
max
max
4
0 nm) and a vibronic fine structure. These features are very
similar to those of oligothiophenes with linear alkyl α,ω-
7e
substituents and are typical for H-aggregates. In contrast, the
more bulky substituents VI−IX (Chart 1) lead to bathochromic
−1
shifts of λ up to around 1000 cm (Δλ ≈ 20 nm). A
max
max
uniform trend is found for the 0−0 transition in the aggregated
state with increasing core length shifting from 537 nm (R-7T-
R) to 551−561 nm (R-9T-R), 570−575 nm (R-11T-R), and
585 nm (R-13T-R) fairly independent of the structure of R.
These findings demonstrate that there is a dependence of the
electronic absorption properties on the structure of R but too
complex to be quantitatively correlated as found for the melting
transitions in bulk (see above).
Figure 3. Exemplary temperature dependent fluorescence spectra of
septithiophene I-7T-I. Inset: normalized spectra at lowest and highest
temperature, which show the change of relative intensities of the
emission bands.
2
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Chemistry of Materials
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to 557−561 nm when the conjugated system is expanded to 9-
mers (see Supporting Information Figure 2S). Very similar to
the absorption spectra at high temperatures, no significant
further bathochromic shift is observed when additional
monomer units are incorporated. Upon cooling fluorescence
is quenched, accompanied by a change in relative intensities of
the three vibrational bands (see inset in Figure 3). In a previous
7
b
publication we showed for compounds IX-11T-IX (124) and
IX-13T-IX (125) that at low temperatures the spectra are
superpositions of weakly emitting aggregates and residual free
molecules.
This quenching of fluorescence intensity follows a sigmoidal
7
b
progression, which we found for all oligothiophenes
exemplarily shown for I-7T-I (108) in Figure 4. The inflection
points of these S-shaped curves are defined as dissociation
temperature Tdis of the aggregates, which depend on chemical
structure, solvent, and concentration.
Figure 5. Dependence of the dissociation temperature of aggregates in
solution T , on the bulkiness parameter P in TCE and TCE:Isopar M
dis
−5
−1
9
:1 (v:v), respectively (c = 1 × 10 mol L ). The solid line
represents the fit curve for R-9T-R in TCE according to eq 3.
noted that this order could be derived only from three different
series of measurements in the different solvents (TCE, TCE/
Isopar M, respectively) indicated by the triple slashes because
of the big differences in solubility.
Because of the limited accessible temperature range and
solvent and concentration dependence of Tdis a clear
quantitative relation between Tdis and P can be derived only
for the nonathiophene series R-9T-R according to eq 3 (cf. eq
2
2
=
) delivering the minimum dissociation temperature Tdis,min
=
80.4 ± 5.2 K (for P → ∞), the maximum temperature T
dis,max
351.5 ± 4.9 K (for P → 0), and the constant k = 0.047 ±
dis
2
0
.014 with R = 0.99034.
+ (Tdis,max − Tdis,min) × e−
kdisP
Tdis = T
m,mis
(3)
Figure 4. Concentration dependent sigmoidal progression of photo-
luminescence intensity of I-7T-I (108) in TCE.
Comparing those results with the evaluation of T in bulk
demonstrates that, expectedly, the dissociation temperatures
m
Tdis are much lower than T , in addition to the dependence of
Tdis on the solvent type. The additional interactions of the
solvent molecules are also reflected by both the increased value
Plotting Tdis at a constant concentration (1 × 10−5 mol L−1)
vs the bulkiness parameter P (definition see above) reveals that
there is a monotonic decline of Tdis with increasing P for all
oligomers R-nT-R (Table 3, Figure 5). Qualitatively, the
following order of Tdis with the substituents R can be found: I >
II > III /// V > IV > III > VI (derived from R-7T-R) /// II >
III > VI > VIII > VII > IX (derived from R-9T-R). It has to be
m
of kdis in comparison with k and by the smaller differences
m
ΔT between different oligomer lengths (ca. 50 K for the same
dis
substituent in the identical solvent, see Table 3, Figure 5)
compared with ΔT (ca. 100 K for the same substituent, see
m
Figure 1 and Table 1).
Monolayer Field Effect Mobilities. The ultimate goal of
the presented compounds is to investigate their electrical
properties and correlate them with the substituents’ structures.
Such characteristics can be determined within devices like
organic field effect transistors (OFETs) to evaluate mobilities.
In a previous study we have shown that the influence of the
substituents’ geometry (branching position) on the mobility is
negligible for OFETs processed under standard conditions with
Table 3. Dissociation Temperatures T in TCE or TCE/
Isopar M Mixture at c = 1 × 10 mol L
dis
−5
−1
bulkiness
parameter
a
oligomeric core
b
c
b
b
b
R
7T
7T
9T
11T
13T
P
I
291.1
279.8
1.09
3.67
II
338.7
330.0
1
2b
thick films prepared by physical vapor deposition. Here, only
little control over local variations in film thickness, interfacial
structures, grain boundaries, etc. is given. Therefore, we
decided to determine the mobilities of some oligomers in the
present contribution from monolayer OFETs in accordance
with a recently published protocol restricting the active layer to
just a monolayer (≈ 5 nm) by direct casting the material onto a
III
IV
V
292.4
295.1
303.3
283.8
9.33
6.67
4.00
VI
VII
VIII
IX
320.0
285.0
288.4
12.00
66.67
42.00
366.67
334.7
303.8
344.1
12a
SiO −dielectric surface. The spin coating process reprodu-
2
a
cibly results in the deposition of electrically connected
monolayers. Representative AFM images of compounds II-
7T-II (109) and IX-7T-IX (116) with a small and large
bulkiness parameter P (3.67 and 366.67, respectively) show
Missing oligomers did either show no aggregates at all or not the full
aggregation/dissolution process in the chosen solvents and technically
b
accessible temperature range (ca. −10 to +80 °C). Solvent: 1,1,2,2-
c
tetrachloroethane (TCE). Solvent: TCE/Isopar M (1:9, v:v).
2
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Chemistry of Materials
Article
closed monolayers (Figure 6). AFM images of corresponding
monolayers of the other septithiophenes are shown in
Supporting Information (Figure 4S).
We stress here the difficulties related to electrical contact
between 5 nm thick oligothiophene layers and 100 or 50 nm
thick gold electrodes. In order to get comparable results we had
to choose appropriate oligomers from which reproducibly
defined monolayers can be formed from solution. It turned out
that almost the full R-7T-R series (R = I−IV, VI, VIII, IX
(108−111, 113, 115, 116)) are ideal candidates for this
purpose; for unknown reasons, the missing oligomers did not
show reproducible results, and thus their data were left out. The
mobilities μ show a clear tendency with a drop by 4 orders of
magnitude from 1.3 × 10− to 8 × 10 cm V
2
−7
2
−1 −1
s
with
increasing bulkiness parameter P (Table 4). Exemplary transfer
Table 4. Monolayer FET Mobilities μ vs Bulkiness
Parameter P for a Series of R-7T-R
μ [cm V s−1
2
−1
]
ln(μ/μ0)
a
P
R
8 × 10−
1.3 × 10−
6 × 10−
2 × 10−
1 × 10−
3
−4.83
−4.34
−5.12
−3.91
−9.21
−11.51
−14.04
1.09
3.67
I
2
II
3
III
IV
VI
VIII
IX
9.33
2
6.67
4
12.00
42.00
366.67
−5
1 × 10
8 × 10−
7
a
μ0: unit mobility
curves at a drain bias of −20 V are shown in Supporting
Information (Figure 5S). The drop in the mobility correlates
well with weakened intermolecular π−π interactions of the
oligomeric cores because of increased bulkiness of the
substituents.
Besides the qualitative consideration, quantitative evaluation
is also possible. If the natural logarithm of the mobilities ln(μ/
μ ) (μ : unit mobility) is plotted against P an exponential decay
0
0
according to eq 4 can be fitted (Figure 7).
k P
μ
ln(μ/μ ) = ln(μ /μ ) − ln(μ /μ )e
(4)
0
min
0
max min
Figure 7. Logarithmic saturated mobilities extracted from monolayer
OFETs of R-7T-R oligomers vs bulkiness parameter P. The solid line
represents the fitted curve according to eq 4.
The fit delivers ln(μmin/μ ) = −14.1, ln(μmax/μmin) = 10.7, kμ
0
2
=
0.0347 ± 0.0170 and R = 0.8942 corresponding to a
−
7
2
−1 −1
minimum and maximum mobility μ = 8 × 10 cm V
s
min
and μmax = 3 × 10− cm V s , respectively. It shows that μ
P → ∞) is basically reached with the bulky substituent IX (see
Table 4). μ would be the mobility of a oligothiophene with P
2
2
−1 −1
min
(
Figure 6. AFM images of II-7T-II (109) (top) and IX-7T-IX (116)
max
→
0 or linear substitutents. Indeed, alkyl endsubstituted
(
bottom) showing closed monolayers with the profile taken at the
2
−1 −1
white line and deposited by spin-coating from a solution with c = 0.67
oligothiophenes show mobilities of up to 0.1 cm V
only in exceptional cases slightly higher values. Differences
s
and
−1
3
(
109) and 1 g L (116), respectively. Scan size is 10 μm × 10 μm.
2
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Chemistry of Materials
Article
might be caused by the presence of the silicon atom in our
substituents. It shall be noted that the corresponding
monolayer mobility of a R-7T-R with a pure hydrocarbon 2-
octyldodecyl substituent (P = 90, see above) amounts to 1 ×
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ASSOCIATED CONTENT
Supporting Information
Additional absorption and emission spectra; melting enthalpies
and entropies; AFM images of monolayers; transfer curves of
monolayer OFETs; and synthesis protocols. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
*
S
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́
́
(
AUTHOR INFORMATION
Corresponding Author
E-mail: ulrich.ziener@uni-ulm.de.
■
*
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge support of the present work by the
Deutsche Forschungsgemeinschaft (DFG) projects ZI567/4-1
and MO982/2-1.
2
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