Tuning packing and solubility of donor (D)-acceptor (A) polymers by cis - Trans isomerization within alkenyl side chains

Article abstract of DOI:10.1021/cm5021355

The impact of alkenyl substituents on the behavior of cyclopentadithiophene-benzothiadiazole (CDT-BTZ) donor (D)-acceptor (A) polymers in organic field-effect transistors (OFETs) and on the supramolecular organization was investigated. Linear cis- and trans-alkenes were attached to the donor unit of CDT-BTZ polymers to demonstrate the dependence of supramolecular ordering and solubility in organic solvents on chemical conformation. The layer interdigitation of the substituents differed due to shape disparities between cis- and trans-alkenes. While trans-alkenes exhibit zigzag structures that are beneficial for close packing, cis-alkenes are curved and thus possess a less regular shape that is disadvantageous to thin film ordering. This was proven by grazing incidence wide-angle X-ray scattering (GIWAXS) studies, which revealed shorter intermolecular distances for the polymer with trans-alkene substituents even in comparison to analogous polymers with saturated alkyl substituents. Furthermore, the isomerization of the cis-substituents toward their trans-conformers allowed improvement of the polymer crystallinity in thin films and was investigated in transistor devices and solubility studies.

Full text of DOI:10.1021/cm5021355

Article
pubs.acs.org/cm
Tuning Packing and Solubility of Donor (D)−Acceptor (A) Polymers
by cis−trans Isomerization within Alkenyl Side Chains
Felix Hinkel, Tomasz Marszalek, Wojciech Zajaczkowski, Sreenivasa Reddy Puniredd, Martin Baumgarten,
̈
Wojciech Pisula,* and Klaus Mullen*
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
*
S Supporting Information
ABSTRACT: The impact of alkenyl substituents on the
behavior of cyclopentadithiophene−benzothiadiazole (CDT−
BTZ) donor (D)−acceptor (A) polymers in organic field-
effect transistors (OFETs) and on the supramolecular
organization was investigated. Linear cis- and trans-alkenes
were attached to the donor unit of CDT−BTZ polymers to
demonstrate the dependence of supramolecular ordering and
solubility in organic solvents on chemical conformation. The
layer interdigitation of the substituents differed due to shape
disparities between cis- and trans-alkenes. While trans-alkenes
exhibit zigzag structures that are beneficial for close packing, cis-alkenes are curved and thus possess a less regular shape that is
disadvantageous to thin film ordering. This was proven by grazing incidence wide-angle X-ray scattering (GIWAXS) studies,
which revealed shorter intermolecular distances for the polymer with trans-alkene substituents even in comparison to analogous
polymers with saturated alkyl substituents. Furthermore, the isomerization of the cis-substituents toward their trans-conformers
allowed improvement of the polymer crystallinity in thin films and was investigated in transistor devices and solubility studies.
are π-conjugated polymers are rigid-rod type macro-
1
B
molecules possessing limited solubility in organic
solvents. Therefore, alkyl substituents must be attached at the
2
periphery of the polymer backbone in order to apply them in
3
solution-processed organic electronics such as organic field-
4
effect transistors (OFETs) and bulk heterojunction (BHJ)
5
solar cells. As positive effects, solubilizing groups can promote
6
phase formation and supramolecular ordering through self-
7
Figure 1. Chemical structures of CDT−BTZ polymers P1 and
targeted polymers P2 and P3.
assembly (hard core, soft chain) of the polymeric semi-
8
conductor in thin-films and in bulk. In regioregular P3HT, for
instance, sequential interdigitation of the precisely positioned
hexyl substituents leads to lamellar packing and high
crystallinity. Regarding commonly employed alkyl substituents,
branched alkanes ensure high solubility, whereas long linear
ones favor lamellar packing through interdigitation of the
6
(I /I = 10 ) have been reported for this polymer in
on off
−1
11
dependence on a high molecular weight (M = 35 kg mol ).
n
P1 showed good order and excellent film forming abilities.
However, the question of the contribution of the hexadecyl
substituent remained unanswered. The steric demand plays a
key role, whereby the prevention of backfolding and curvature
of the substituents is of high importance. Thus, the first
synthesis of D−A polymers (P2 and P3) bearing stereo-
isomeric alkenyl substituents was accomplished. The zigzag
shape of trans-alkenes attached at the periphery of CDT−BTZ
2
b
aliphatic chains. Recent literature has presented a myriad of
donor (D)−acceptor (A) polymers whose building blocks are
3c,9
varied and adjusted
but has often disregarded chemical or
stimuli-induced modification of the alkyl substituents. While
methods to drastically change the chemical structure of
10
functional side chains (i.e., ketones and esters ) have been
well established in organic electronic devices, there has only
been limited attention paid toward manipulating alkyl
substituents that are still important in tuning thin film
morphology.
(
P3) enforces interdigitation of the solubilizing groups in the
layer structure and prevents them from backfolding. cis-
Substituents in P2, however, are curved and less regularly
shaped, allowing no room for interdigitation of the substituents.
The D−A polymer P1 (Figure 1) consisting of cyclo-
pentadithiophene and benzothiadiazole (CDT−BTZ) revealed
a close relation between morphology in thin films and transistor
Received: June 12, 2014
Revised: August 5, 2014
Published: August 5, 2014
2
−1 −1
performance. Charge-carrier mobilities of up to 3.3 cm V
s
©
2014 American Chemical Society
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Chemistry of Materials
Article
The impact of the carbon−carbon double bond configuration
in the solubilizing groups on the organization was studied for
thin films by grazing incidence wide-angle X-ray scattering
cis-monomer, the alkenyl chain was obtained by a nucleophilic
substitution reaction of 1,6-dibromohexane with lithiodec-1-yne
followed by hydrogenation catalyzed by Lindlars catalyst. After
the alkenyl chain was introduced into the CDT system, the 2-
positions were finally brominated with NBS at low temperature.
The second monomer CDT(trans) was achieved by a pathway
(
GIWAXS), while the charge carrier transport in the
corresponding layers was monitored by transistor measure-
ments. By comparing P2 and P3, we were able to correlate
parameters such as chain-to-chain and π-stacking distances with
OFET performance.
13
consisting of three reactions, as the trans-double bond was
obtainable by a more convenient metathesis reaction. 1-Bromo-
8-octene was attached to the CDT unit, and the 2-positions
were subsequently brominated. After bromination, the trans-
carbon−carbon double bond was generated by a metathesis
reaction with Grubbs second generation catalyst. All com-
Furthermore, cis−trans-isomerization studies of P2 focused
on the modification of the thin film organization and
subsequent solubility differences elucidated via UV−vis spec-
troscopy. These phenomena were in full agreement with the
hypothesis that altering the configuration in the alkenyl
substituent leads to a shape variation toward trans-isomers
which is beneficial for the interlayer ordering. It has been
reported that irradiation with UV−C light (wavelength of <200
nm) leads to cis−trans isomerization of long aliphatic alkenes
1
13
pounds were characterized by H and C NMR spectroscopy,
FD mass spectrometry, and high-resolution electrospray
ionization (HR-ESI) mass spectrometry, which were in full
agreement with the molecular structures.
CDT(cis) and CDT(trans) were employed in the synthesis
of D−A polymers with BTZ diboronic ester 8 (Scheme 3)
12
such as fatty acids. However, π−π* transitions of the aromatic
backbone at similar wavelengths and polymer degradation
caused by the high energy of the light demonstrated the need
for a better way to isomerize cis-alkenes toward their trans-
conformers in P2. Therefore, treatment of the cis-alkenyl
substituents with diphenyl sulfide under light irradiation was an
optimal chemical solution, since common isomerization
reagents like iodine or benzophenone caused either a strong
irreversible doping or polymer degradation.
a
Scheme 3
The synthetic pathways for the two stereoisomeric
monomers, 2,6-dibromo-4,4-di((Z and E)-hexadec-7-en-1-yl)-
4
H-cyclopenta[1,2-b:5,4-b′]dithiophene CDT(cis) and
CDT(trans), are shown in Schemes 1 and 2. In case of the
a
Scheme 1. Synthesis of CDT(cis)
a
(
i) 8, Pd(PPh ) , K CO , Aliquat 336, toluene, H O, 1 d, 100 °C,
3 4 2 3 2
4
5−55%.
under Suzuki conditions. During that procedure, both polymers
precipitated from the reaction solution after 1 day (P2) and 5 h
(
P3). Stirring was continued for 24 h before end-capping with
benzene boronic acid and bromobenzene. The resulting
polymers were poured into methanol, filtered off, precipitated
from a 1,2,4-trichlorobenzene/methanol mixture three times,
and subjected to Soxhlet extraction with hexane, acetone, and
ethyl acetate, respectively. Finally, characterization was carried
out by GPC analysis in 1,2,4-trichlorobenzene (130 °C, PS
a
(
i) n-BuLi, THF, Ph CH, TMEDA, 1,6-dibromohexane, −78 °C, 2 h,
3
8
5
7%; (ii) Lindlars catalyst, quinoline, hexane, H , r.t., 12 h, quant.; (iii)
, DMF, KOH, 50 °C, 16 h, 70%; (iv) NBS, AcOH, chloroform, −10
2
°C, 5 h, 78%.
a
Scheme 2. Synthesis of CDT(trans)
1
standard), H NMR, and FT-IR spectroscopy. A clear
difference in molecular weights (MWs) was measured with
−
1
−1
M of 22 kg mol (PDI = 2.6) for P2 and 10 kg mol (PDI =
n
−1
2
.5) for P3. The polymer solubility of 1.18 mg mL (P2) and
1
0
.51 mg mL− (P3) in THF was determined by UV−vis
absorption. The Lambert−Beer equation was used to calculate
the solubility of the polymers in a saturated solution among a
series of defined concentrations. This result pointed out the
lower solubility for the polymers with alkenyl substituents in
trans-conformation (P3) with respect to lower MWs and
explained its early precipitation during polymerization.
Thermogravimetric analysis (TGA) showed an initial weight
loss of the polymers at 400 °C, whereas differential scanning
calorimetry (DSC) did not reveal any phase transition.
a
(
i) 1-Bromo-8-octene, KOH, DMF, 50 °C, 16 h, 70%; (ii) Grubbs
second generation catalyst, 1-decene, DCM, reflux, 10 h, 64%; (iii)
NBS, AcOH, chloroform, −10 °C, 5 h, 55%.
The supramolecular organization in thin P2 and P3 films has
been investigated by grazing incidence wide-angle X-ray
scattering (GIWAXS). GIWAXS measurements provide val-
4
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Chemistry of Materials
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uable information about the film organization of the polymers
including their interlayer and π-stacking distances as well as
Figure 3. The presented patterns for the isomerized P2 films
did not show any significant difference in the polymer packing
14
relative arrangement toward the surface. Films for both
polymers were deposited by drop-casting a 2 mg mL− o-
dichlorobenzene (DCB) solution on the transistor substrates
heated at 100 °C. The GIWAXS patterns presented in Figure 2
1
Figure 3. GIWAXS patterns of the films of P2 for which the
isomerization of the carbon−carbon double bond has been performed
in (a) solution (iv) and (b) solid state (iii).
and interlayer distance in comparison to the pristine P2. The
previous HPLC results based on the monomeric unit
CDT(cis), with a maximum ratio between trans:cis of 77:23,
showed the possible reason for such a behavior. This indicated
that 25% of the alkenyl chains with cis conformation remained
unchanged, preventing a decrease of the interlayer distance.
Figure 2. GIWAXS patterns of (a) P2 and (b) P3.
revealed significant differences in molecular organization
between P2 and P3. In the meridional plane of the pattern
of P2 (Figure 2a), reflections at small-angles were ascribed to a
layer structure with a real-space periodicity of 2.66 nm as
17
However, the full width of half-maximum (fwhm) values for
−1
determined from the main peak position at q = 0.24 Å and
z
the main interlayer reflection on the meridional plane
correspond to the coherence length of the layer structures
and indicated changes in polymer order. Table 1 summarizes
qxy = 0 Å− (Supporting Information Figure S7a). Reflections
up to the third order confirmed long-range ordering over the
entire film thickness. The preferential intensity distribution of
the described reflections on the meridional plane (Supporting
Information Figure S7b) suggested a tendency for an edge-on
organization of the polymer backbone on the silicon substrate.
The rather isotropic wide-angle reflection at equatorial plane
1
Table 1. Comparison of the FWHM for the Interlayer
Reflection of Thin Films with Varied Substituent
Isomerization
−1
−1
for q = 1.7 Å and q = 0 Å was assigned to the π-stacking
(iii) P2 + Ph
+ UV
(365 nm)
S
2
(iv)
isomerization
in solution
xy
z
(
ii) P2 +
Ph2S
distance of 0.37 nm. The GIWAXS pattern obtained for P3
showed a similar long-range ordering in the film. An interlayer
distance of 2.48 nm and an unchanged π-stacking distance of
(
i) P2
fwhm,
1.60 ± 0.02
1.49 ± 0.02
1.48 ± 0.02
1.36 ± 0.02
−2
Å−1
1
0
0.37 nm were extracted. Interestingly, P3 clearly exhibited a
shorter interlayer distance (2.48 nm) as compared to the
CDT−BTZ polymer (2.56 nm) with saturated hexadecyl chains
the fwhm parameters calculated for samples with various
isomerization procedures: (i) pristine P2, (ii) P2 treated with
−
1
11
(
M = 35 kg mol ).
n
Furthermore, the low solubility and therefore lower MWs of
diphenyl sulfide Ph S via drop casting in film, (iii) P2 after
2
P3 increased the importance of chemical modification of the
alkene substituents in P2. Through shape adaption via cis−trans
isomerization of the carbon−carbon double bonds in the highly
soluble P2, we aimed for improving its thin film organization
toward dimensions of P3. This was the first attempt ever to
chemically modify an alkenyl group in D−A polymers and
presented a new perspective in tuning thin film morphology.
For a better comparison, the cis−trans isomerization was first
investigated for the monomeric CDT(cis) unit. The con-
formation of the cis-alkene substituents was altered at ambient
conditions in the presence of diphenyl sulfide and light
treatment with Ph S and irradiation with light at 365 nm in
2
film, and (iv) P2 isomerized in solution before film deposition
(GIWAXS pattern presented in Supporting Information, Figure
S8). The highest fwhm value was observed for pristine P2.
Interestingly, this value decreased gradually with progressive
isomerization (starting from P2, P2 film + Ph S, P2 film + Ph S
2
2
+ UV irradiation to P2 isomerized in solution before solution
deposition), suggesting an improvement in organization.
The charge carrier transport of P2, P3, and P2 isomerized to
trans-conformation was studied in field-effect transistors. The
devices were based on a hexamethyldisilazane (HMDS)
modified substrate with bottom Au contacts and bottom-gate
configuration. The films were deposited according to the same
procedure described in the structural part. All fabricated devices
exhibited unipolar field-effect behavior with maximum charge
15
irradiation at 365 nm. This led to a mixture of cis:trans-
isomers in a ratio of 23:77, which is in full agreement with
1
6
expected values. The isomerization was quantitatively
corroborated by HPLC analysis and followed by FT-IR
spectroscopy (see Supporting Information). The role of the
isomerization process on the supramolecular structure of P2
has been investigated by two independent approaches: in
solution and in thin film. The procedures of substituent
isomerization are described in detail in the Supporting
Information. The GIWAXS results for P2 films based on
isomerized carbon−carbon double bonds are presented in
2
−1 −1
5
carrier mobility of 0.61 cm V
s
(I /I = 1 × 10 ) for P2
on off
2
−1 −1
5
and 0.39 cm V
s
(I /I = 6 × 10 ) for P3. The slight
on off
variation in the charge carrier mobility was most probably
caused by the difference in molecular weight. Hexadecyl
substituted CDT−BDT polymers revealed enhanced mobilities
11
for higher molecular weights. The differences of the off-
current and turn-on voltage between P2 and P3 were obvious
4
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and were related to the isomerization. As shown in Figure 4
small differences in off-current were evident in the transfer
which could have confirmed an improvement of interlayer
organization.
UV-irradiation at 365 nm induced changes to the trans-
conformation resulting in tighter interlayer packing and
increased conherence length of crystalline domains. fwhm
parameters confirmed higher polymer order and improved
order uniformity of the layer structures, which lowered trapping
via midgap states at the interlayer boundaries and reduced both
the off-current and turn-on voltage in the transistor. On the
other hand, the turn-on voltage decreased only when both Ph2S
and UV-irradiation were used for the film treatment.
Surprisingly, a dramatic solubility decrease in organic solvents
was observed during the isomerization of P2. This effect was
due to the strong dependence of solubility on polymer
interactions causing aggregation. At first, the reaction of P2
with diphenyl sulfide was carried out in 1,2-dichlorobenzene
under light irradiation at wavelength of 365 nm (as described as
method iv before). After precipitation of the isomerized
polymer from methanol, the differences in solubility in hexane,
dichloromethane (DCM), toluene, and THF (Figure 6) were
Figure 4. Transfer characteristics of bottom-contact, bottom-gate
transistors based on P2 (black) and P3 (red).
characteristics. This effect was explained by higher intra-
molecular organization of the P3 polymer in comparison to P2.
Improving the polymer organization in the thin film led to
smaller charge trapping through midgap states presented at
1
8
layer boundaries. In other words, the charge transport in the
low voltage area was limited by additional traps created by
isolating alkyl chains. This effect was also confirmed by a lower
turn-on voltage of 2 V for P3 and 6 V for P2.
The influence of the improved polymer organization was
even more evident for P2, in which the carbon−carbon double
bond in alkyl chain was isomerized, as reflected by the fwhm
parameter, turn-on voltage, and off-current. Figure 5a shows the
Figure 6. Quantitative determination of solubility after calibration
(concentration dependent absorbance extrapolated for a saturated
solution using Lambert−Beer equation) in hexane, THF, dichloro-
methane, and toluene before and after isomerization with diphenyl
sulfide and light irradiation.
19
determined by UV−vis absorbance after calibration. The data
strongly suggested a solubility decrease after cis to trans-
isomerization by more than 30% in DCM. However, the error
of these measurements was quite high, as residual amounts of
Figure 5. Transfer characteristics of P2 after isomerization in solid film
(
a) by UV irradiation and (b) by adding Ph S reagent and UV
2
irradiation.
transfer characteristics obtained for the same device based on
P2 before and after UV irradiation. Interestingly, only the off-
current was significantly improved, resulting in a lower value
after applying the UV irradiation. These changes in device
operation confirmed that the slight order improvement
apparently influences the interlayer transport leading to a
decreased off-current and thus higher on/off ratio. Figure 5b
presents the same device in which the isomerization of the
Ph S might falsify the values. Therefore, an error in
measurements of roughly 10% was estimated.
2
In summary, the first ever synthesis of D−A polymers
bearing stereoisomeric alkenes was accomplished yielding two
CDT−BTZ polymers, P2 and P3. Due to shape disparities in
alkene substituents the layer interdigitation differed. The zigzag
shape of the trans-alkenes enforced interdigitation in the
lamella, while curvature in cis-alkenes was found to be
disadvantageous to thin film ordering. Thus, P3 organized in
higher ordered layer structures than P2. Interlayer distances of
2.48 nm were found for P3 in comparison to 2.66 nm for P2.
Interestingly, this distance for P3 was even smaller than for P1
substituted by saturated hexadecyl chains, confirming the great
influence of the zigzag shape of the trans-substituent. As a
disadvantage, P3 exhibited limited solubility in organic solvents,
which increased the meaningfulness of isomerization studies for
P2. Configuration changes in the alkenyl substituent led to
shape adaption being beneficial for the interlayer order. The
improvement of ordering in thin films was confirmed by the
decrease in interlayer coherence length represented by the
carbon−carbon double bond has been performed by Ph S with
2
−2
−1
UV irradiation. The fwhm parameters of 1.49 × 10
Å
for P2
−2
−1
with Ph S and 1.48 × 10
Å
for UV isomerized P2 with Ph2S
2
were on the same level but were smaller in comparison to
pristine P2, proving again larger coherence lengths along the
interlayer periodicity after isomerization. The transfer charac-
teristics obtained for the nonmodified P2 thin film and after
adding the Ph S reagent showed the same off-current value.
2
Applying the high boiling point reagent, which probably was
not fully removed from thin film, slightly altered the shape of
the transfer curve. Therefore, the comparison of the off-current
between both films did not provide any valuable information
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corresponding fwhm parameters. The transistor behavior of P2,
P3, and isomerized P2 (solution, solid state) showed lower off-
currents for the latter ones. This phenomenon was explained by
the higher order and smaller spacings of the layer structures
which reduced the charge trapping through midgap states
presented at the interlayer boundaries. The hole mobilities,
however, were in the same range of 0.3−0.6 cm V s . This
finding suggested the conclusion that the interlayer distance is
not as crucial for the charge carrier transport as the molecular
weight. Additionally, the solution-processed isomerization of
P2 led to a decrease in solubility up to 30% in organic solvents
as determined by the UV−vis absorbance after calibration.
Future work could apply the introduction of such stereo-
isomeric alkenes into liquid discotics like hexa-peri-hexabenzo-
coronene to study their influence on the self-assembly behavior.
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ASSOCIATED CONTENT
Supporting Information
Additional information including synthetic procedures and
characterization of materials, decoupled NMR spectra, FT-IR
spectra, and HPLC. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
*
S
̌
́
Z.; Simpson, C.; Kastler, M.; Pakula,
̈
Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539. (c) Tashiro, K.; Sasaki,
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AUTHOR INFORMATION
Corresponding Authors
(W.P.) E-mail: pisula@mpip-mainz.mpg.de.
(K.M.) E-mail: muellen@mpip-mainz.mpg.de.
■
A.; Langford, J. I.; Le Bail, A.; Louer
N.; Popa, N. C.; Stephens, P. W.; Toby, B. H. J. Appl. Crystallogr.
004, 37, 911.
̈
, D.; Masson, O.; McCowan, C.
*
2
*
(18) (a) George, G. M.; Jack, M. B.; Alex, C. M. Organic Field-Effect
Transistors; CRC Press: Boca Raton, FL, 2007; p 341. (b) Henning, S.
Organic Field-Effect Transistors; CRC Press: Boca Raton, FL, 2007; p
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
1
03. (c) Hoichang, Y. Organic Field-Effect Transistors; CRC Press: Boca
Raton, FL, 2007; p 371. (d) Hong, Z.; Michelle, L. S.; Mark, E. R.;
Colin, R.; Abhijit Basu, M.; Stefan, C. B. M.; Zhenan, B.; Jason, L.
Organic Field-Effect Transistors; CRC Press: Boca Raton, FL, 2007; p
Funding
1
59. (e) Michael, G. K. Organic Field-Effect Transistors; CRC Press:
Boca Raton, FL, 2007; p 551.
19) Stangenberg, R.; Saeed, I.; Kuan, S. L.; Baumgarten, M.; Weil,
Notes
The authors declare no competing financial interest.
(
T.; Klapper, M.; Mu
̈
llen, K. Macromol. Rapid Commun. 2014, 35, 152.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the DELTA electron storage ring in
Dortmund for providing synchrotron radiation and technical
support for GIWAXS. The authors thank also Dr. Adam
Kiersnowski for helpful discussion and technical support for
GIWAXS.
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