Star-shaped D-π-A oligothiophenes with a tris(2-methoxyphenyl)amine core and alkyldicyanovinyl groups: Synthesis and physical and photovoltaic properties

Article abstract of DOI:10.1039/c6tc01530a

Synthesis of a series of star-shaped oligomers having a novel electron donating tris(2-methoxyphenyl)amine (m-TPA) core, which is linked through a bithiophene or terthiophene π-bridge with electron-deficient alkyldicyanovinyl (alkyl-DCV) groups, is descri

Full text of DOI:10.1039/c6tc01530a

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Chvalun, C. J. Brabec and S. Ponomarenko, J. Mater. Chem. C, 2016, DOI: 10.1039/C6TC01530A.
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Page 1 of 39
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Starꢀshaped D–π–A oligothiophenes with tris(2ꢀmethoxyphenyl)amine core and
alkyldicyanovinyl groups: synthesis, physical and photovoltaic properties
1
,
,†
2, ,†
1
Yuriy N. Luponosov, * Jie Min, * Alexander N. Solodukhin, Artem V.
1,3
1,3
1,4
Bakirov,
Petr V. Dmitryakov,
Maxim A. Shcherbina,
Svetlana M.
1
,5
1
1,3
2,6
Peregudova, Georgiy V. Cherkaev, Sergei N. Chvalun, Christoph J. Brabec,
1,7
Sergei A. Ponomarenko
1
Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of
Sciences, Profsoyuznaya st. 70, Moscow 117393, Russia
2
Institute of Materials for Electronics and Energy Technology (IꢀMEET),
FriedrichꢀAlexanderꢀUniversity ErlangenꢀNuremberg, Martensstraße 7, 91058
Erlangen, Germany
3
National Research Centre "Kurchatov Institute", 1, Akademika Kurchatova pl.,
Moscow, 123182, Russia
4
Moscow Institute of Physics and Technology, 4 Institutsky line, Dolgoprudny,
Moscow region, 141700 Russian Federation
5
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
Vavilova St. 28, Moscow, 119991, Russia
6
Bavarian Center for Applied Energy Research (ZAE Bayern), Haberstraße 2a, 91058
z, Germany
7
Chemistry Department, Moscow State University, Leninskie Gory 1ꢀ3, Moscow
1
19991, Russia
*
Corresponding authors: Eꢀmail: luponosov@ispm.ru (Yu. N. Luponosov)
Min.Jie@fau.de (J. Min)
†
These authors contributed equally to this work.
1

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Abstract
Synthesis of a series of starꢀshaped oligomers having novel electron donating
tris(2ꢀmethoxyphenyl)amine (mꢀTPA) core, which is linked through bithiophene or
terthiophene πꢀbridge with electronꢀdeficient alkyldicyanovinyl (alkylꢀDCV) groups,
is described. Comprehensive study of the oligomers revealed significant dependence
of their physical properties, including absorption, molecular frontier energy levels,
crystal packing, melting and glass transition temperatures upon the chemical structure.
Comparison of their photophysical properties to the nearest analog having common
dicyanovinyl (DCV) groups demonstrated a number of benefits to use alkylꢀDCV
units for the design of donorꢀacceptor small molecules: higher solubility, increased
electrochemical stability, better photovoltaic performance, possibility to control the
relative physical and photovoltaic properties by simple adjustment of alkyl and
πꢀbridge lengths. Modification of wellꢀknown triphenylamine (TPA) core in the
starꢀshaped oligomers by 2ꢀmethoxy groups increases not only solubility, but also
crystallinity of the oligomers, whereas their photovoltaic performance stays on the
similar level to their analogs with TPA core. The study demonstrates that these design
strategies represent interesting and simple tools for effective modulation of properties
of starꢀshaped molecules.
1
. Introduction
Solar cells based on organic semiconductors are developing as a promising
costꢀeffective alternative to siliconꢀbased solar cells due to easy fabrication by
[
1 ꢀ 3 ]
solution processing, lightweight, and compatibility with flexible substrates.
Despite conjugated polymers have dominated the field of bulk heterojunction (BHJ)
[4]
organic solar cells (OSCs), small molecules are received increasing attention due to
their wellꢀdefined molecular structures, ease of functionalization, amenability to
standard organic purification and characterization methods, and reduced
2

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batchꢀtoꢀbatch variability. There has been significant recent progress in the
[5ꢀ7]
development of small molecules for use in BHJ OSCs.
The dynamic development
in solutionꢀprocessed small molecule OSCs has recently led to high power conversion
[
8,9]
efficiency (PCEs) of over 10%. Despite that, development of the cheap and stable
photoactive materials as well as novel design strategies for small molecules is still a
key to boost potential for commercial applications.
Oligothiophenes and their derivatives with wellꢀdefined structures possess extensive
πꢀelectron delocalization along the molecular backbone and are well known as good
[10]
holeꢀtransporting materials. Despite their excellent semiconducting properties, they
do not absorb strongly in the red and nearꢀinfrared part of the spectrum where most
solar photons are concentrated. Inclusion of electronꢀwithdrawing units to the
oligothiophene backbone can extend their absorption spectrum toward longer
wavelengths by an intramolecular charge transfer (ICT) and thus leading to a better
match with the solar spectrum. For this purpose, dicyanovinyl (DCV) unit is one of
[
11, 12]
widely used acceptor groups due to its strong electronꢀaccepting character.
main drawback of the DCV group is presence of an active vinyl hydrogen,
A
[
13, 14]
which decreases stability of the materials. A replacement the active hydrogen by an
alkyl was found to be an efficient tool not only to increase the electrochemical
[
15,16]
stability of donorꢀacceptor small molecules,
but also to control their solubility
and a number of important solidꢀstate physical properties by simple adjustment of
[17,18]
alkyl chain length at DCV substitute.
To fineꢀtune the energy levels in small molecules, some additional electronꢀdonating
[19ꢀ22]
[23,24]
[25, ]
blocks such as dithienosilole,
benzodithiophene,
dithienopyrrole,
etc.,
are often used. For design of starꢀshaped molecules, a triphenylamine (TPA) core is
one of the widely used electronꢀdonating block due to its low cost, high
[
12, 26 , 27 ]
electrochemical stability and hole transporting ability.
Moreover, the
propellerꢀlike structure of TPA prevents strong π–π interactions between neighboring
groups, so that the vast majority of TPA derivatives are amorphous and highly soluble
materials.In order to optimize photophysical properties of TPAꢀbased starꢀshaped
oligomers, a few attempts to modify TPA block were done, which were based mainly
3

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[28ꢀ31]
on its planarization strategy.
Nevertheless, an increase in performance of organic
[
28]
[29]
fieldꢀeffect transistors (OFET),
organic lightꢀemitting diodes (OLEDs)
and
[
30,28]
OSCs
as compared to the devices based on nonꢀmodified analogs, revealed a
great potential of TPA modification to moderately and effectively tune the properties
of starꢀshaped molecules.
Meanwhile, alkoxy substituents at phenyl rings of TPA have been successfully used in
3
2,
design of unsymmetrical DꢀA molecules for organic and hybrid organic solar cells.[
3
3
]
Alkoxy substituents at TPA core can increase solubility, change intraꢀ and
intermolecular interactions as well as energy levels of the molecules, and thus
influence the solidꢀstate properties of pristine oligomers and their blends. However,
the first example of DꢀA starꢀshaped molecule with alkoxy groups at TPA, namely
34
[ ]
with tris(2ꢀmethoxyphenyl)amine (mꢀTPA) core, has been reported recently.
In this work we report on the synthesis and comprehensive study of a series of four
DꢀπꢀA oligomers having tris(2ꢀmethoxyphenyl)amine (mꢀTPA) core as donor and
either alkylꢀDCV or DCV as acceptor units, as well as either biꢀ or terthiophene as
πꢀbridges (Scheme 1). Comparison of properties of these four molecules with those of
[15ꢀ17, 35]
their full analogs with TPA core
reveals the benefits and drawbacks of this
chemical design strategy. Furthermore, variation of πꢀbridge and alkyl chain length as
well as type of substituent at DCV group allows elucidating the structureꢀproperty
relationships in this series of starꢀshaped molecules systematically.
4

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H
NC
NC
NC CN
CN
S
NC CN
CN
S
S
S
H
S
S
O
N
S
O
N
S
O
O
O
O
N(Ph-OMe-2T-DCV)3
N(Ph-OMe-2T-DCV-Me)3
S
S
S
S
CN
NC
H
CN
NC
CN
S
CN
CN
CN
S
S
S
S
CN
S
S
S
CN
S
NC
S
NC
O
N
S
O
N
S
O
O
O
O
N(Ph-OMe-3T-DCV-Et)3
N(Ph-OMe-3T-DCV-Hex)3
S
S
S
S
S
S
NC
NC
NC
NC
Scheme 1. Chemical structures of the starꢀshaped DꢀπꢀA oligomers investigated.
2
2
. Results and discussion
.1 Synthesis and chemical characterization
Synthesis of starꢀshaped oligomers with mꢀTPA and alkylꢀDCV groups,
[
35 ]
N(PhꢀOMeꢀ2TꢀDCV)
N(PhꢀOMeꢀ3TꢀDCVꢀHex)
for their analogs with TPA core.
include several key reaction stages: preparation of the initial aldehyde or ketones
Scheme 2a and 2b), synthesis of organoboron derivative of biꢀ or terthiophene
3
,
N(PhꢀOMeꢀ2TꢀDCVꢀMe)
3
,
3
N(PhꢀOMeꢀ3TꢀDCVꢀEt) ,
3
, was carried out using our recently developed approach
[15,16]
The synthetic routes outlined in Scheme 2
(
precursors (Scheme 2c), and preparation of starꢀshaped precursors with protective
groups followed by deprotection reaction and substitution of carbonyl functions by
DCV groups (Scheme 2d). However, in this work, several important optimizations of
the initial steps were elaborated to simplify the synthesis. First, preparation of ketones,
5

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1
bꢀc, was carried out by FriedelꢀCrafts acylation of 2,2'ꢀbithiophene in the presence
of tin chloride in 80ꢀ86 % yields. This optimization allows using less expensive
precursor and reagents as compared to manganeseꢀcatalyzed acylation of magnesium
[
15,16]
derivative of 5ꢀbromoꢀ2,2'ꢀbithiophene.
Nevertheless, it should be noted, that
FriedelꢀCrafts acylation reaction was found to proceed with low reaction yields in the
case of terthiophenes, probably, due to an oiling effect. Therefore, the extension of
oligothiophene length (preparation of 4bꢀc) was carried out via Suzuki cross coupling
between organoboron derivatives of dioxaneꢀprotected bithiophene ketones (3bꢀc) and
2
ꢀbromothiophene in 71ꢀ74% yields. Second, protection of carbonyl functions of the
ketones was found to be more convenient with 5,5ꢀdimethylꢀ1,3ꢀdioxane groups as
compared to previously used 1,3ꢀdioxolane ones, due to higher stability and better
isolated yields of the protected compounds.
Note that for the synthesis of the model starꢀshaped molecule with DCV groups,
3
N(PhꢀOMeꢀ2TꢀDCV) , we have used for the first time the similar strategy as for the
molecules with alkylꢀDCV groups, whereas their common preparation is based on
Vilsmeier–Haack formylation of starꢀshaped oligothiophenes followed by Knövenagel
[
12,16]
condensation.
The main difference in the synthetic route for
, as compared to the molecules with alkylꢀDCV groups, is
N(PhꢀOMeꢀ2TꢀDCV)
3
preparation of the aldehyde precursor (1a) instead of the ketone one, which was done
by lithiation of commercially available 2,2'ꢀbithiophene followed by its reaction with
dimethylformamide (DMF) in 94 % yield. In addition, there were also variations in
the rates of protection, deprotection and Knövenagel reactions due to different activity
of ketones and aldehydes (see Experimental Section for details).
As a result, all target starꢀshaped oligomers under discussion were prepared in good
1
reaction yields, while their chemical structures and high purity were proved by Hꢀ,
1
3
and
Electronic Supplementary Information (ESI), Figures S1 – S35). The low reaction
yield of N(PhꢀOMeꢀ3TꢀDCVꢀEt) can be explained by losses during purification by
CꢀNMR spectroscopy, elemental analysis, and massꢀspectroscopy (see
3
column chromatography, because of its insufficient solubility in dichloromethane,
which was used as eluent. All these oligomers obtanied demonstrate reasonable
6

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solubility in common organic solvents such as THF, chloroform, 1,2ꢀdichlorobenzene
ODCB), etc.
(
RCOCl
BuLi
DMF
O
O
(
b)
S
SnCl4
(
a)
S
S
S
S
S
S
Toluene
ꢀ20° C, 3h
THF/hexane
78 °C, 2 h
yield: 94%
H
S
1b-c
R
0
1a
ꢀ
yield: 80 - 86%
a:R=H
b:R=C H5
where:
2
c:R=C H
13
6
(
c) 1
HO
OH
BuLi
IPTMDOB
O
O
a-c
S
O
R
O
S
O
R
benzene, pꢀTosH
reflux, 7 ꢀ 20 h
yield: 71-97%
B
S
2
S
THF / hexane
78° C, 2h
S
a-c
O
Br
ꢀ
3a-c
yield: 95-99%
yield:
1-74%
toluene / ethanol,
Pd(PPh3)4
7
aq. 2 M Na CO
reflux, 8ꢀ12h
BuLi
2
3
O
O
IPTMDOB
THF/hexane
78° C, 2 h
yield: 98 %
O
S
S
O
R
O
R
B
S
S
n
O
S
b-c
S
ꢀ
5
4b-c
X
(
d)
S
X
Br
O
N
Br
O
S
n
X =
O
R
O
1M HCl, THF
3
a, 5b-c
* yield:
91-95%
N
6a-c
reflux
4ꢀ6h
Pd(PPh3)4
O
O
O
O
Toluene/ethanol
Na CO3 2M
O
2
reflux, 8ꢀ12 h
X =
R
* yield: CH (CN)
yield:
2
2
Br
7a-c
NC
X =
70-89%
S
18-70% pyridine
9
0 ꢀ 105°С
CN
6
ꢀ 8 h
a: n=2, R=H
n
X
where: b: n=3, R=C H
R
2
5
*
c: n=3, R=C H
N(Ph-OMe-nT-DCV-R)3
6
13
3 3
Scheme 2. Synthesis of N(PhꢀOMeꢀ2TꢀDCV) , N(PhꢀOMeꢀ3TꢀDCVꢀEt) , and
N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
.
2
.2 Solubility and Thermal Properties.
The exact values of solubility for all starꢀshaped oligomers prepared were measured in
ODCB at room temperature (Table 1). One can see that the solubility in this series of
molecules depends significantly on the chemical structure and tends to decrease with
elongation of the πꢀbridge length and to increase with elongation of the alkyl chain
length. The lowest solubility was found for the model molecule without terminal alkyl
groups, N(PhꢀOMeꢀ2TꢀDCV) and for N(PhꢀOMeꢀ3TꢀDCVꢀEt) having the longest 3T
3
,
3
arms and short ethyl groups. Comparison of the solubility values for the series of
mꢀTPAꢀbased molecules to those obtained for their full analogs with TPA core
revealed that usage of the former core can increase solubility by ca. 2ꢀ5 times (Table
1
).
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Table 1. Solubility and thermal properties of the starꢀshaped oligomers
First heating
, °C ∆H , J/g T
Second heating
2
TGA (air)TGA (N )
Solubility,
mg/mL
Compounds
a
T
m
m
g
, °C dC
p
, J/(g·K) T
d
, °C
d
T , °C
N(PhꢀOMeꢀ2TꢀDCV)
3
9
14
8
200
248
243
173
276
32
54
50
30
28
147
130
119
76
0.33
0.23
0.27
0.33
0.17
403
415
403
386
405
408
402
391
400
405
N(PhꢀOMeꢀ2TꢀDCVꢀMe)
3
N(PhꢀOMeꢀ3TꢀDCVꢀEt)3
N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
20
2
b
N(Phꢀ2TꢀDCV)
3
145
b
N(Phꢀ2TꢀDCVꢀMe)
3
3
270
ꢀ
82
ꢀ
136
75
0.26
0.29
416
388
416
388
b
N(Phꢀ3TꢀDCVꢀHex)
3
12
a
b
Notes: Measured in ODCB; data from reference [16]; dC
p
– heat capacity change at
d
the glass transition; T corresponds to 5% weight losses.
Thermal properties of the oligomers synthesized were investigated by
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). One
can see that all these oligomers possess very high stability both in the air and under
d
inert atmosphere (Figure 1). Decomposition temperatures (T ), corresponding to 5%
weight losses, were found to be above 380°C in the air and under the inert atmosphere,
as exhibited in Table 1. The residual weights vary from 50ꢀ75% at the heating up to
7
00 ºC in the nitrogen. Comparison of the weight losses at very high temperatures
indicates that it is caused mostly by decomposition of the aliphatic groups. Influence
of the terminal hydrogen at DCV unit on the thermal stability of the oligomers cannot
be evaluated by this technique, since the weight loss due to hydrogen elimination is
around 0.2%, which is close to the experimental error.
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Figure 1. TGA of starꢀshaped DꢀπꢀA oligomers (a) in air and (b) under nitrogen.
DSC analysis (Figure 2a and Table 2) reveals that all these oligomers as received are
crystalline materials with the relatively high values of melting temperature (T
m
) and
melting enthalpy (∆H ). However, the crystal structure was not restored after the first
m
melting and subsequent cooling to the room temperature. One can observe a glass
transition on the second heating scans only (Figure 2b), where a glass transition
temperature (T ) lies in the range of 74ꢀ147ºC. This peculiarity was used to simplify
g
dissolution of these compounds, since materials in the amorphous state dissolve much
m m
better as compared to the crystalline one. Comparison of T and ∆H values for the
compounds studied allows concluding that elongation of the oligothiophene πꢀbridge
or shortening of the alkyl chains lead to improved ordering in the bulk.
Figure 2. DSC first (a) and second (b) heating scans of starꢀshaped DꢀπꢀA oligomers
with mꢀTPA core.
It is interesting to compare phase behavior of the molecules with mꢀTPA core to their
[15,16,34]
full analogs having TPA core (see Table 1).
N(Phꢀ3TꢀDCVꢀHex) , being the full
3
9

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analog of N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
but with TPA core, is characterized by a
[
34]
hexagonal
columnar
mesophase.
Meanwhile,
N(Phꢀ2TꢀDCV)3
and
N(Phꢀ2TꢀDCVꢀMe)
3
can be also crystalline as received similar to their analogs with
and
N(PhꢀOMeꢀ2TꢀDCVꢀMe) are rather similar, while the melting temperature of
mꢀTPA core. Melting temperature and enthalpy in the pair of N(Phꢀ2TꢀDCVꢀMe)
3
3
3 3
N(Phꢀ2TꢀDCV) is substantially higher than that of N(PhꢀOMeꢀ2TꢀDCV) , albeit their
melting enthalpies being also close to each other.
2
.3 XꢀRay diffraction data
Xꢀray studies of asꢀreceived samples of the compounds with mꢀTPA core revealed
that all of them are highly crystalline materials with substantially different crystal
lattices (Figure 3). However, smallꢀangle region of diffractograms is dominated by a
strong reflection corresponding to the periodicity of ~ 30 Å. Moreover, its fourth
order reflection is also one of the strongest in the wideꢀangle region for all the
compounds investigated. As the position of the mentioned reflection does not depend
on the arm length of the specific starꢀshaped molecule, one can conclude, that this
periodicity is due to the stacking of the molecules one above the other. Molecular
modelling of the compounds with mꢀTPA core revealed that such discotic motifs form
pairs in which methoxy groups of both partners are oriented outwards, the distance
between the molecules being 5.8 Å (see discussion and Figures S38, S39 in the ESI).
The same pairs are also the most specific feature of the crystal structure of mꢀTPA
[
36 ]
compound.
calculated crystalline ones allows to suggest that molecular pairs form the columns
with an inner 3 or 3 helical ordering which is rather common for the compounds
with the molecules possessing rotary axis of symmetry of the third order.
Comparison of the experimental macroscopic densities with the
1
2
1
0

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Figure 3.
N(PhꢀOMeꢀ2TꢀDCVꢀMe)
N(PhꢀOMeꢀ3TꢀDCVꢀHex) (4).
XꢀRay
scattering
patterns
for
N(PhꢀOMeꢀ2TꢀDCV)
(3)
3
(1),
and
3
(2)
,
N(PhꢀOMeꢀ3TꢀDCVꢀEt)
3
3
Excluding reflections corresponding to the periodicity along the columns, one can
obtain parameters of the columns ordering in lateral direction. Here two main criteria
of the choice correctness were used – first, comparison of calculated and experimental
densities and the arm length. We should though notify in advance that the
macroscopic density was measured using the bulk samples after their melting and
subsequent cooling, i.e. amorphous ones (Table 2). Thus, we allowed the calculated
crystal density to be 10ꢀ15% higher than the experimental one. Second, all the
reflections observed should have been indexed (see Tables S1 – S4 in ESI).
1
1

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Table 2. Crystal lattice parameters calculated and experimentally measured densities
for compounds with mꢀTPA cores.
ρcalc
g/cm
,
ρ ,
g/cm
arm
L ,
Å
exp
Compounds
Type
a, Å b, Å c, Å
3
3
N(PhꢀOMeꢀ2TꢀDCV)
3
Orthorombic 25.3 20.9 30.7 1.296 1.322
Orthorombic 20.7 18.8 33.3 1.693 1.303
Orthorombic 32.9 17.7 28.4 1.557 1.319
Orthorombic 33.9 31.2 27.7 1.411 1.227
14.9
16.1
22.1
26.5
N(PhꢀOMeꢀ2TꢀDCVꢀMe)
N(PhꢀOMeꢀ3TꢀDCVꢀEt)
N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
3
3
Figure 4. Packing of the molecules in crystal lattices calculated based on Xꢀray
scattering patterns for N(PhꢀOMeꢀ2TꢀDCV) (1), N(PhꢀOMeꢀ2TꢀDCVꢀMe) (2)
N(PhꢀOMeꢀ3TꢀDCVꢀEt) (3) and N(PhꢀOMeꢀ3TꢀDCVꢀHex) (4).
3
3
,
3
3
Figure 4 shows 001 cleaves of calculated crystal lattices. One should remember that
every molecule on such a cleave represents the whole 3 helical column. However,
1
every hand of the molecule has a faceꢀtoꢀedge πꢀπ stacking with the molecule in the
neighboring column, thus providing a helical pathway for charge carriers. The length
of star’s arm varies from 14.9 to 26.5 Å. All the lattices, with the exception of that
formed by N(PhꢀOMeꢀ3TꢀDCVꢀHex) contain two columns with different type of
3
,
rotation (one in the middle and four quarters in the corners). Thus, one can observe
1
2

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the layers of the columns of particular orientation alternating with the layers of the
columns of an opposite one. Crystal lattice formed by the compound
N(PhꢀOMeꢀ3TꢀDCVꢀHex)3, contains three columns (two in the middle and four
quarters in the corners) and correspondingly three different types of column layers.
This fact could be due to a substantial length of hands of particular starꢀshaped
molecule, so the packing of two different layers still leaves too much free space.
Summing up structural information concerning the starꢀshaped compounds with and
without methoxy groups in their TPA cores, one can conclude that methoxy groups
play definitive role in formation of pair aggregates inside the supramolecular columns,
which are not observed in TPAꢀbased analogs. Such aggregates are responsible for
substantially higher ordering and formation of crystal structures in the materials with
methoxy groups in the TPA cores. In comparison, formation of ordered structures in
the compounds without methoxy groups in TPA cores is considerably hindered.
2
.4. Optical and electrochemical properties
The UVꢀvis absorption spectra of the starꢀshaped oligomers in dilute ODCB solutions
and thin films are shown in Figure 5. Samples used for the measurements of film
absorption spectra were prepared by doctorꢀblading from ODCB solutions. The
corresponding optical data are summarized in Table 3. In ODCB, all compounds
demonstrate similar spectra: the weak UV absorption peak (370ꢀ412 nm) as generally
accepted corresponds to the πꢀπ* transition and the intensive visible peak (520ꢀ540
nm) is attributed to the ICT transition between the mꢀTPAꢀoligothiophene donor and
[12]
the alkylꢀDCV or DCV acceptor units. However, it has been shown that in fact both
electronic excitations have a mixed character and the intensity of the highꢀenergy
absorption feature is a direct measure of the degree of conformational disorder in the
[37]
starꢀshaped molecule.
Indeed, with increasing of the πꢀconjugated bridge length
from 2T to 3T the shorter wavelength region peaks are redshifted from 375 nm to 412
nm, which is attributed to the stronger localized πꢀπ* transition for 3T πꢀbridge.
Interestingly, that the expected redshifts of low energy bands for molecules with 3T
πꢀbridges were not observed, since the extension of the conjugation cannot
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compensate a decrease in ICT between the core and terminal DCV units caused by
longer distance between them and intramolecular distortions in 3T chain. Comparison
to the absorption bands of the starꢀshaped molecules based on alkylꢀDCV units as
acceptor groups in films, the low energy absorption band is redꢀshifted by 16 nm for
3
N(PhꢀOMeꢀ2TꢀDCV) due to somewhat stronger electronꢀwithdrawing ability of DCV
[
15]
groups. In thin films, the optical spectra of all these starꢀshaped molecules become
broader and maxima of their absorption peaks (λmax) shift to longꢀwave region as
compared to those observed in solutions (Figure 5b).
Figure 5. UVꢀvis absorption spectra of mꢀTPAꢀbased oligomers in (a) dilute ODCB
solutions and in (b) films casted from ODCB.
Table 3. Optical and electrochemical properties of the investigated
mꢀTPAꢀbased oligomers.
UVꢀvis absorption
cyclic voltammetry
a
b
solution
film
oxidation
reduction
opt
d
d
EC
λ
max
λ
onset
E
g
φ
оx /HOMO
φ
red /LUMO E
g
Compounds
λ
max (nm)
c
(
nm)
(nm) (eV)
(V)/(eV)
(V)/(eV) (eV)
N(PhꢀOMeꢀ2TꢀDCV)
3
377/540 456/562 707 1.75
376/524 452/542 688 1.80
412/524 428/531 688 1.80
412/524 432/535 688 1.80
0.80/ꢀ5.20 ꢀ0.94/ꢀ3.46 1.74
0.80/ꢀ5.20 ꢀ1.02/ꢀ3.38 1.82
0.74/ꢀ5.14 ꢀ1.03/ꢀ3.37 1.77
0.74/ꢀ5.14 ꢀ1.03/ꢀ3.37 1.77
N(PhꢀOMeꢀ2TꢀDCVꢀMe)
N(PhꢀOMeꢀ3TꢀDCVꢀEt)
N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
3
3
a
b
c
Measured in ODCB solution. Cast from ODCB solution. Bandgap estimated from the onset
opt
g
d
wavelength (λedge) of the optical absorption: E
= 1240/λedge; standard formal reduction (φred) and
oxidation (φ ) potentials vs. SCE obtained for films of the oligomers.
ox
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The electrochemical properties of the four starꢀshaped molecules and their highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels were examined by cyclic voltammetry (CV) (see Table 3 and
Figure 6). From these data, it is clearly seen that all molecules demonstrate similar
oxidation behavior, which appears as two consecutive and fully reversible waves. A
decreased value of the first standard formal oxidation potentials (φox) for
N(PhꢀOMeꢀ3TꢀDCVꢀEt)
3
and N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
,
as compared to
2
Tꢀcontaning analogs, revealed that elongation of πꢀbridge length facilitates the
oxidation process. However, influence of πꢀbridge length on the first standard
reduction potential (φred) was not observed, because in this process the reduction of
DCV groups occurs firstly. Therefore, somewhat different φred value (ꢀ0.94 V vs. ꢀ1.03
V relatively to SCE) was observed for the N(PhꢀOMeꢀ2TꢀDCV) having common
3
DCV groups, which indicates its hindered reduction. Another important difference
between the molecules having either DCV or alkylꢀDCV groups is irreversibility of
the reduction process for the former due to presence of active vinyl hydrogens in their
structure. In Figure 6, one can see that the reduction of all compounds with
alkylꢀDCV groups proceeds with the first reversible and the second irreversible waves,
3
whereas reduction of N(PhꢀOMeꢀ2TꢀDCV) goes with the first irreversible wave only.
These observations indicate less stability of anion radicals for oligomers having active
hydrogen at DCV groups, whereas replacement of the vinyl hydrogens to alkyls
stabilizes the anion radicals and increases their electrochemical stability.
HOMO and LUMO energy levels were calculated using first standard formal
oxidation and reduction potentials according to the following equation: LUMO =
[38]
e(φred+4.40) (eV) and HOMO = ꢀe(φox+4.40) (eV). For N(PhꢀOMeꢀ3TꢀDCVꢀEt)
3
and N(PhꢀOMeꢀ3TꢀDCVꢀHex) the LUMO was found to be ꢀ3.37 eV, which is similar
3
to N(PhꢀOMeꢀ2TꢀDCVꢀMe)
gives a bit lowered bandgap of 1.77 eV as compared to 1.82 eV for their
Tꢀcontaining analogue. The lowest electrochemical bandgap value (1.74 eV) was
observed for N(PhꢀOMeꢀ2TꢀDCV)3, which is in good agreement with its optical
3
, whereas the HOMO was found to be ꢀ5.14 eV, which
2
[
16]
bandgap (see Table 1). Compared to their analogues with TPA core, this series of
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starꢀshaped molecules shows somewhat higher HOMO levels due to the presence of
additional electron donating methoxy substitutes in the mꢀTPA core.
Figure
6.
Cyclic
voltammograms
of
N(PhꢀOMeꢀ2TꢀDCV)
(3)
3
(1),
and
N(PhꢀOMeꢀ2TꢀDCVꢀMe)
3
(2), N(PhꢀOMeꢀ3TꢀDCVꢀEt)
3
N(PhꢀOMeꢀ3TꢀDCVꢀHex) (4). Insets (dashed curves) correspond to the cyclic
3
voltammogram of the first wave.
2
.5 Charge transport
Since the intended application of the starꢀshaped molecule under study is BHJ OSC as
a donor component in their mixtures with the acceptor component PC70BM, a space
charge limited current (SCLC) method was used to map out hole and electron only
mobilities of the oligomers/PC70BM blended films. The JꢀV characteristics of both
holeꢀonly and electronꢀonly diodes can be excellently fit to the MottꢀGurney model
h e
(Figure 7). We calibrated the hole (ꢀ ) and electron (ꢀ ) mobility by fitting the
currentꢀvoltage curves over six diodes. The related average values of hole and
electron only mobilities of the blended films are given in Table 4. All devices based
on the starꢀshaped molecules with alkylꢀDCV groups demonstrated slightly higher
hole only mobilities, but lower electron only mobilities as compared to their analogue
with DCV groups, N(PhꢀOMeꢀ2TꢀDCV)
3
. Comparison of the devices based on
N(PhꢀOMeꢀ2TꢀDCVꢀMe) and N(PhꢀOMeꢀ3TꢀDCVꢀEt) showed that elongation of the
3
3
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oligomer πꢀbridge length from 2T to 3T does not influence significantly the mobility
in blends. While the increase of alkyl chain length from ethyl to hexyl in
N(PhꢀOMeꢀ3TꢀDCVꢀEt)
decrease of both the hole and electron mobility. This observation, on the one hand,
can be explained by the poorer blend morphology for
N(PhꢀOMeꢀ3TꢀDCVꢀHex) :PC70BM system as shown below. On the other hand,
3
/N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
systems leads to obvious
3
elongation of alkyl chain results in weakening of the intraꢀ and intermolecular
interactions in the solid state, which is in agreement with our previous results as well
[
34]
as the thermal properties of pristine oligomers.
Comparison of the hole only
to the values
mobility for N(PhꢀOMeꢀ2TꢀDCVꢀMe) and N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
3
[34]
obtained for their full analogs with TPA core, revealed a bit higher mobility for the
blends based on the molecules with mꢀTPA core. These results in charge transport
properties of blends will further influence the device performance as discussed below.
Table 4. Hole and electron only mobilities determined from the SCLC measurements.
a
a
Thickness
nm)
81
ꢁ
h
Thickness ꢁ
e
Semiconductor layers
e/ h
ꢁ ꢁ
2
−1 −1
2
−1 −1
(
[cm V s ] (nm)
[cm V s ]
ꢀ4
ꢀ4
ꢀ4
ꢀ4
ꢀ3
N(PhꢀOMeꢀ2TꢀDCV)
N(PhꢀOMeꢀ2TꢀDCVꢀMe)
N(PhꢀOMeꢀ3TꢀDCVꢀEt) :PC70BM (1:2)
:PC70BM (1:2.5)
3
:PC70BM (1:2.5)
4.58×10
6.07×10
5.87×10
4.86×10
70
75
75
74
1.27×10 2.77
ꢀ4
3
:PC70BM (1:2)
86
78
85
8.15×10 1.34
ꢀ4
3
9.91×10 1.69
ꢀ4
N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
6.73×10 1.38
a
The reported mobility data are average values of the blended films over six devices.
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Figure 7. The dark JꢀV characteristics of devices based on the mꢀTPA core oligomers
including hole (a) and electron (b) only devices. The solid lines represent the best
fitting using the SCLC modified MottꢀGurney model.
2
.6. Photovoltaic properties of BHJ devices
To study the effects of donor core, acceptor group, oligothiophene πꢀbridge length and
alkyl chain length on the photovoltaic properties, BHJ OSCs with a configuration of
ITO/PEDOT:PSS/oligomer:PC BM (wt%)/ZnO(25nm)/Al(100nm) were fabricated,
70
and the detailed device fabrication process is described in the Experimental Section.
The corresponding parameters of devices with the best blending ratios are shown in
Figure S40, while optimized performances of these devices were achieved with the
weight ratio of 1:2 or 1:2.5 without any treatments, as shown in Figure 8a and
summarized in Table 5.
3 3
As expected, OSCs based on N(PhꢀOMeꢀ2TꢀDCV) and N(PhꢀOMeꢀ2TꢀDCVꢀMe)
show relatively high V up to 0.9 V, due to their lowꢀlying HOMO energy levels.
oc
With increasing of the πꢀconjugated oligothiophene bridge length, OSCs based on
3 3
N(PhꢀOMeꢀ3TꢀDCVꢀEt) and N(PhꢀOMeꢀ3TꢀDCVꢀHex) showed lower Voc of 0.82 V
and 0.86 V, respectively. These results are attributed to their HOMO level variations
[16, 35]
(Figure 8c) and also consistent with our previous results.
In addition, the slightly
higher
V
oc values (see Figure 8c) for (N(PhꢀOMeꢀ2TꢀDCVꢀMe)
as compared to N(PhꢀOMeꢀ2TꢀDCV)
3
3
and
and
N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
N(PhꢀOMeꢀ3TꢀDCVꢀEt)3 may be explained by presence of longer terminal alkyls
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8

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for the former, leading to a weakening of the intermolecular interactions between the
[
17, 39 ]
donor and acceptor moieties.
N(PhꢀOMeꢀ2TꢀDCVꢀMe) exhibit the highest Jsc values (see Figure 8d), resulting
from the high and balanced charge transport properties. N(PhꢀOMeꢀ3TꢀDCVꢀHex)
Among these four systems, OSCs based on
3
3
system in contrast exhibits the poorest J value, probably due to the longest alkyls at
sc
DCV, suppressing intramolecular and intermolecular interactions.[17] The
corresponding Jsc values are consistent with the external quantum efficiency (EQE)
measurements (see Figure 8b). In addition, these devices exhibited similar FF values
of ca. 50% (see Table 5). Finally, the PCEs of these devices are 3.73% for
N(PhꢀOMeꢀ2TꢀDCV)
3
,
4.38% for N(PhꢀOMeꢀ2TꢀDCVꢀMe)
3
,
3.55% for
N(PhꢀOMeꢀ3TꢀDCVꢀEt)
3
and 3.18% for N(PhꢀOMeꢀ3TꢀDCV) , respectively. In
3
general, the PCEs of OSCs based on molecules with mꢀTPA core and alkylꢀDCV
groups are slightly lower but comparable with those obtained for their analogs with
[
16, 35]
TPA core.
However, obvious improvement of the photovoltaic performance from
2
.31% for N(Phꢀ2TꢀDCV) system to 3.73% for N(PhꢀOMeꢀ2TꢀDCV) system was
3
3
found, which is attributed to better molecular solubility and blend morphology of the
latter. In addition, compared to the photovoltaic performance of the molecules with
bithiophene as πꢀbridges, further extending of πꢀbridges to terthiophene did not lead
to any improvement of the device performance. Nonetheless, the results of
3 3
N(PhꢀOMeꢀ3TꢀDCVꢀEt) and N(PhꢀOMeꢀ3TꢀDCVꢀHex) in devices demonstrated that
the longer alkyl terminal chains resulted in the worse photovoltaic parameters due to
the poor charge transport and worse blend morphology, which is in a good agreement
[17]
with the previous research for TPAꢀbased starꢀshaped molecules.
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Figure 8. Current densityꢀvoltage (IꢀV) curves (a) and EQE spectra (b) of solar cells
based
N(PhꢀOMeꢀ2TꢀDCVꢀMe)
3
1:2, wt%), and N(PhꢀOMeꢀ3TꢀDCVꢀHex) :PC70BM(1:2.5, wt%), respectively, under
on
N(PhꢀOMeꢀ2TꢀDCV)
3
:PC70BM
(1:2.5,
wt%),
3
:PC70BM (1:2, wt%), N(PhꢀOMeꢀ3TꢀDCVꢀEt)
3
:PC70BM
(
ꢀ2
the illumination of AM 1.5, 100 mWcm ; Correlation between: (c) V and
oc
experimental HOMO values of mꢀTPA based molecules and (d) Voc and Jsc values
of the OSCs based on these four systems.
Table 5. Photovoltaic Properties of oligomer:PC70BM OSCs, under the illumination
ꢀ2
of AM 1.5, 100 mW cm .
a
D:A
Voc
Jsc
FF
[%]
PCE( )
[%]
Donor
ꢀ2
[
wt%] [V] [mAcm ]
N(PhꢀOMeꢀ2TꢀDCV)
3
1:2.5
0.90
0.92
0.82
0.86
8.52
9.01
8.65
7.78
48.6
52.8
50.0
47.6
3.73(3.62)
N(PhꢀOMeꢀ2TꢀDCVꢀMe)
3
1:2
1:2
4.38(4.18)
3.55(3.44)
3.18(3.02)
N(PhꢀOMeꢀ3TꢀDCVꢀEt)
3
N(PhꢀOMeꢀ3TꢀDCVꢀHex)
3
1:2.5
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a
the average values of PCEs calculated from over six devices.
2
.7 Film morphology
Absorption spectra and electronic structure of the components as well as their charge
transport properties in blends are obviously important for device operation, but the
[20]
role of film morphology cannot be ignored. The topographic AFM images of the
blended films determined for the optimized OSCs are given in Figure 9.
2
Figure 9. Tapping mode AFM surface scans (5×5 µm ) of blended films
3 3
N(PhꢀOMeꢀ2TꢀDCV) :PC70BM (1:2.5 wt%) (a), N(PhꢀOMeꢀ2TꢀDCVꢀMe) :PC70BM
(
1:2 wt%) (b), N(PhꢀOMeꢀ3TꢀDCVꢀEt) :PC BM (1:2 wt%) (c), and
3
70
3
N(PhꢀOMeꢀ3TꢀDCVꢀHex) :PC70BM (1:2.5 wt%) (d).
All the blended films are rather smooth with an average surface roughness (RMS) of
.61 nm, 0.35 nm, 0.46 nm and 0.54 nm for N(PhꢀOMeꢀ2TꢀDCV) :PC BM,
0
3
70
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N(PhꢀOMeꢀ2TꢀDCVꢀMe)
3
:PC70BM,
N(PhꢀOMeꢀ3TꢀDCVꢀEt)
3
:PC70BM,
and
N(PhꢀOMeꢀ3TꢀDCVꢀHex) :PC BM films, respectively. However, in all cases, the
3
70
blended film morphologies were slightly different. It is evident from AFM studies that
the variations in the molecular size, πꢀbridge and alkyl length can cause changes in
the microstructure of the blended films. Among these four blended films,
N(PhꢀOMeꢀ2TꢀDCVꢀMe)
addition, the most pronounced nanoscaleꢀaggregation of donor and acceptor phase
was also found for the best performing N(PhꢀOMeꢀ2TꢀDCVꢀMe) :PC70BM system,
3
:PC70BM system exhibited the smallest RMS value. In
3
indicating that these nanoscale domains are beneficial to the exciton charge separation,
charge transportation and in turn enhance the device performance. Thus, judicious
choice of the core, oligothiophene πꢀbridge and alkyl chain length in starꢀshaped
molecules is of crucial importance for fineꢀtuning the structureꢀ–morphologyꢀproperty
relationships.
3
. Conclusions
In summary, we synthesized a series of starꢀshaped oligomers with DꢀπꢀA structure
and demonstrated the first comprehensive attempt to study the structureꢀproperty
relationships depending on different substituents at donor and acceptor units as well
as the length of either oligothiophene πꢀbridge or terminal alkyl chain length. All
these oligomers with a novel mꢀTPA core exhibit good solution processability and
high thermal and thermooxidation stability. Various properties, such as solubility in
organic solvents, phase behavior, crystal packing, HOMO/LUMO levels,
chargeꢀtransport ability in blends, morphological characteristics and photovoltaic
performance can be fineꢀtuned simply by changing the πꢀbridge and alkyl terminal
chain length. On the one hand, the main benefits of usage of the alkylꢀDCV acceptor
groups to design starꢀshaped molecules are confirmed significantly by their higher
solubility, electrochemical stability and photovoltaic performance as compared to
normal DCV units. On the other hand, the introduction of mꢀTPA core, as compared
to TPA, not only leads to higher solubility, but also possesses the possibility to obtain
molecules in a crystalline state without significant changing of other photophysical
2
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and photovoltaic properties. Bearing the widespread usage of TPAꢀderivatives, we
envisage that tris(2ꢀmethoxyphenyl)amine can find their own niche as a donor unit to
design highly crystalline but still soluble semiconducting small molecules for organic
and hybrid electronics.
4
. Experimental Section
.1. Characterization
4
1
Н NMR spectra were recorded at a “Bruker WPꢀ250 SY “spectrometer, working at a
signal (7.25 ppm) as the internal
frequency of 250.13 MHz and utilising CDCl
3
13
standard. C NMR spectra were recorded using a “Bruker Avance II 300”
1
spectrometer at 75 MHz. In the case of Н NMR spectroscopy, the compounds to be
1
3
3
analysed were taken in the form of 1% solutions in CDCl . In the case of C NMR
spectroscopy, the compounds to be analysed were taken in the form of 5% solutions in
CDCl . The spectra were then processed on the computer using the ACD Labs
3
software.
Massꢀspectra (MALDI) were registered on the Autoflex II Bruker (resolution FWHM
1
8000), equipped with a nitrogen laser (work wavelength 337 nm) and timeꢀofꢀflight
massꢀdetector working in reflections mode. The accelerating voltage was 20 kV.
Samples were applied to a polished stainless steel substrate. Spectrum was recorded in
the positive ion mode. The resulting spectrum was the sum of 300 spectra obtained at
different points of the sample. 2,5ꢀdihydroxybenzoic acid (DHB) (Acros, 99%) and
αꢀcyanoꢀ4ꢀhydroxycinnamic acid (HCCA) (Acros, 99%) were used as matrices.
Elemental analysis of C, H and N elements was carried out using CHN automatic
analyzer CE 1106 (Italy). The settling titration using BaCl was applied to analyze
2
sulphur. Experimental error for elemental analysis is 0.30ꢀ0.50%. The Knövenagel
condensation was carried out in the microwave “Discovery”, (CEM corporation,
2
3

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USA), using a standard method with the open vessel option, 50 watts.
Thermogravimetric analysis was carried out in dynamic mode in 30 ÷ 900°C interval
using Mettler Toledo TG50 system equipped with M3 microbalance. Heating/cooling
rate was chosen to be 10°C/min. Every compound was studied twice: in air and in
nitrogen flow of 200 ml/min. DSC scans were obtained with Mettler Toledo DSC30
system with 20°C/min heating/cooling rate in temperature range of +20 ÷ 250°C for
all compounds. Nitrogen flow of 50 ml/min was used.
Cyclic voltammetry measurements were carried out using solid compact layers of the
oligomers, which in turn were made by electrostatically rubbing the materials onto a
3 6 4 2
glassy carbon electrode. Measurements were made in CH CN:oꢀC H Cl =1:4
solution using 0.1 M Bu NPF as supporting electrolyte and using IPCꢀPro M
4
6
ꢀ1
potentiostat. The scan rate was 200 mVs . The glassy carbon electrode was used as a
work electrode. Potentials were measured relative to a saturated calomel electrode
(SCE).
Absorption profiles were recorded with a Perkin Elmer Lambdaꢀ35 absorption
spectrometer from 350 to 1100. AFM measurements were performed with a Nanosurf
Easy Scan 2 in contact mode.
The structures of hole only and electron only devices were as
Glass/ITO/PEDOT:PSS/semiconductor layer/MoO
3
(15 nm)/Ag (100 nm) and
Glass/ITO/ ZnO/semiconductor layer/Ca (15nm)/Ag (100 nm).
Smallꢀangle diffraction patterns of high resolution were recorded using SAXSꢀ and
WAXS camera S3ꢀMicropix, manufactured by Hecus (CuKα, λ = 1.542 Å). Two
detectors were used: twoꢀdimensional Pilatus 100K and linear gas position sensitive
detector PSD 50M. For shaping of Xꢀray beam the Fox 3D vacuum optics were used,
the slits in the Kratky collimator were set to 0.1 and 0.2 mm correspondingly. The
ꢀ1
ꢀ1
angular scale was between 0.003 Å and 1.9 Å . The exposure varied from 600 to
5
000 s while the Joule heater hot stages allowed heating the samples up to 300 ˚С.
The wide scattering angles were recorded at Protein crystallography station at
2
4

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Journal of Materials Chemistry C
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DOI: 10.1039/C6TC01530A
Kurchatov Synchrotron with a 2D detector MarCCD165 at wavelength of 1.0402 Å
ꢀ3
(
dE/E = 2E ). Station optics include a bending magnet with a 1.7 Tl (E = 7.1 keV),
c
2
twoꢀcrystal monochromator with a focusing mirror resulting in a 2x1 mm beam with
11
12
2
a 10 ꢀ10 fotons per mm flux.
All devices were fabricated in the normal architecture. Photovoltaic devices were
made by doctorꢀblading on indium tin oxide (ITO)ꢀcovered glass substrates (from
Osram). These substrates were cleaned in toluene, water, acetone, and isopropyl
alcohol. After drying, the substrates were bladed with 40 nm PEDOT:PSS (Heraeus
Deutschland, PEDOT PHꢀ4083). Photovoltaic layers, consisting of four different
small molecules were dissolved in ODCB with PC70BM as acceptor with various
weight ratios, and bladed on top of PEDOT:PSS layer. The thicknesses of these
blends are ca. 80ꢀ90 nm. After that, a ZnO layer (25 nm) was doctorꢀbladed on top of
the active layer. Finally, an aluminum top electrode of 100 nm thickness was
2
evaporated. The typical active area of the investigated devices was 10.4 mm . The
currentꢀvoltage characteristics of the solar cells were measured under AM1.5G
2
irradiation on an OrielSol 1A Solar simulator (100 mW/cm ). The EQE was detected
with cary 500 Scan UVꢀVisꢀNIR Spectrophotometer under monochromatic
illumination, which was calibrated with a monoꢀcrystalline silicon diode. Absorption
profiles were recorded with a Perkin Elmer Lambdaꢀ35 absorption spectrometer from
3
50 to 1100.
4
.2 Materials
Tetrakis(triphenylphosphine)palladium(0)Pd(PPh ) , 2,2'ꢀbithiophene, nꢀbutyl lithium
3
4
(1.6 M solution in hexane), isopropoxyꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolane
(IPTMDOB), pꢀtoluenesulfonic acid (pꢀTosH), heptanoyl chloride, acetyl chloride,
DMF, 2,2ꢀdimethylpropaneꢀ1,3ꢀdiol, 2ꢀbromothiohene malononitrile were obtained
from SigmaꢀAldrich Co. and used without further purification. Pyridine, THF, toluene,
benzene, were dried and purified according to the known techniques and then used as
a solvent. Tris(4ꢀbromoꢀ2ꢀmethoxyphenyl)amine was obtained as described in
[40]
reference.
All reactions, unless stated otherwise, were carried out under an inert
2
5

Journal of Materials Chemistry C
Page 26 of 39
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DOI: 10.1039/C6TC01530A
atmosphere.
4
.3. Synthesis of the oligomers.
2,2'ꢀbithiopheneꢀ5ꢀcarbaldehyde (1a). 1.6 M solution of butyllithium (13.27 mL,
21.2 mmol) in hexane was added dropwise to a solution of 2,2'ꢀbithiophene (7.98 g,
48.0 mmol) in 220 mL of dry THF at ꢀ78 ºC. Afterwards the reaction mixture was
stirred for 60 min at ꢀ78 ºC and then anhydrous DMF (5.26 mL, 72.0 mmol) was
added in one portion. The reaction mixture was stirred for 1h at –78 ºC, then the
cooling bath was removed, and the stirring was continued for 1h. After completion of
the reaction, 50 ml 1 M HCl to the reaction mixture were added to the reaction
mixture followed by the addition of 300 mL of freshly distilled diethyl ether and 150
mL of distilled water. The organic phase was separated, washed with water, and dried
over sodium sulfate and filtered to give product (8.8 g, 94%) as a red solid. M.p.:
5
5ꢀ56 ºC. The product was used in the subsequent synthesis without further
1
purification. H NMR (250 MHz, CDCl
3
,
δ, ppm): 7.05 (dd, 1H, J
1
= 4.0 Hz, J
2
=4.0
Hz), 7.23 (d, 1H, J = 4.0 Hz), 7.34 (d, 2H, J = 4.3 Hz), 7.65 (d, 1H, J = 4.0 Hz), 9.85
1
3
(
s, 1H). C NMR (125 MHz, CDCl
3
): δ [ppm] 124.26, 126.17, 127.12, 128.38,
1
3
36.03, 137.38, 141.68, 147.18, 182.59. Calcd (%) for C OS : C, 55.64; H, 3.11; S,
9
H
6
2
3.01. Found: C, 55.79; H, 3.24; S, 32.97. MALDI MS: found m/z 194.27; calculated
+
for [M] 194.21.
1
ꢀ(2,2'ꢀbithienꢀ5ꢀyl)propanꢀ1ꢀone (1b). SnCl
mixture of 2,2'ꢀbithiophene (19.85 g, 119.4 mmol) and propionyl chloride (11.05 g,
19.4 mmol) in toluene (120 mL) at 0 °C. After stirring at 0 °C for about 2 h, an ice
was added and the reaction mixture was diluted with CH Cl . The mixture was
washed successively with water and a saturated aqueous solution of NaHCO , then
dried over Na SO . Solvent was evaporated under vacuum, and the crude product was
4
(33.43 g, 128.3 mmol) was added to a
1
2
2
3
2
4
purified by recrystallization from hexane affording 1b as a white solid (26.2 g, 86%
1
yield). M.p.: 96 °C. H NMR (250 MHz, CDCl
3
, d, ppm): 1.23 (t, 3H, J = 7.3 Hz),
= 3.7 , J = 1.1 Hz), 7.15 (d, 1H, J =
.3 Hz), 7.28–7.33 (overlapping peaks, 2H), 7.58 (d, 1H, J = 4.3 Hz). C NMR (75
2
.90 (m, 2H, M = 4, J = 7.3 Hz), 7.05 (dd, 1H, J
1
2
13
4
2
6

Page 27 of 39
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DOI: 10.1039/C6TC01530A
MHz, CDCl
3
): d [ppm] 8.59, 32.16, 124.04, 125.46, 126.30, 128.16, 132.34, 136.38,
1
41.98, 145.11, 193.49. Calcd (%) for C H OS : C, 59.43; H, 4.53; S, 28.84. Found:
11
10
2
+
C, 59.25; H, 4.44; S, 28.74. MALDIꢀMS: found m/z 222.43; calculated for [M]
2
22.33.
ꢀ(2,2'ꢀbithienꢀ5ꢀyl)heptanꢀ1ꢀone (1c). This compound was obtained by the method
1
described above for compound 1b using 2,2'ꢀbithiophene (10 g, 60.1 mmol), SnCl
4
(16.83 g, 64.6 mmol), heptanoyl chloride (8.94 g, 60.1 mmol). Purification of crude
product by column chromatography on silica gel (eluent hexane) gave compound 1c
1
(
13.56 g, 80% yield) as a colorless liquid. H NMR (250 MHz, CDCl
3
, δ, ppm): 0.88
(
t, 3H, J = 6.7Hz), 1.27ꢀ1.42 (overlapping peaks, 6H), 1.73 (m, 2H, M = 5, J = 7.3
= 3.7 Hz, J = 1.1 Hz), 7.15 (d, 1H, J =
.3 Hz), 7.28ꢀ7.33 (overlapping peaks, 2H), 7.58 (d, 1H, J = 4.3 Hz). C NMR (125
Hz), 2.85 (t, 2H, J = 7.3 Hz), 7.05 (dd, 1H, J
1
2
13
4
MHz, CDCl
3
): δ [ppm] 14.03, 22.48, 24.85, 28.99, 31.58, 39.02, 124.06, 125.48,
1
6
2
2
26.33, 128.18, 132.48, 136.38, 142.32, 145.24, 193.27. Calcd (%) for C15H18OS : C,
2
4.71; H, 6.52; S, 23.03. Found: C, 64.87; H, 6.58; S, 22.97. MALDIꢀMS: found m/z
78.49; calculated for [M]+ 278.44.
ꢀ(2,2'ꢀbithienꢀ5ꢀyl)ꢀ5,5ꢀdimethylꢀ1,3ꢀdioxane (2a). 2,2ꢀdimethylꢀ1,3ꢀpropanediole
(9.32 g, 89.6 mmol) and pꢀTosH (0.596 g, 3.1 mmol) were added to a solution of
compound 1a (8.7 g, 44.8 mmol) in dry benzene (90 mL). Afterwards, the mixture
was stirred at reflux for 10 hours using water separator. After that, the mixture was
extracted with toluene and washed with distilled water. The combined organic phases
were dried over sodium sulfate and filtered. The solvent was evaporated in vacuum
and the residue was dried at 1 Torr. This crude product was purified by column
chromatography on silica gel (eluent toluene) to give pure product (12.15 g, 97%) as a
1
white solid. M.p.: 61ꢀ62 ºC. H NMR (250 MHz, CDCl
3
,
δ, ppm): 0.79 (s, 3H), 1.28
(
s, 3H), 3.61 (d, 2H, J = 11 Hz), 3.74 (d, 2H, J = 11 Hz), 5.61 (s, 1H), 6.98ꢀ7.05
overlapping peaks, 3H), 7.14 (dd, 1H, J = 1.0 Hz, J = 3.7 Hz), 7.19 (dd, 1H, J
= 3.7 Hz). C NMR (125 MHz, CDCl ): δ [ppm] 21.84, 22.99, 30.23,
7.54, 98.17, 123.12, 123.87, 124.53, 125.70, 127.79, 137.34, 137.72, 140.19. Calcd
(
1
2
1
=
13
1
.0 Hz, J
2
3
7
2
7

Journal of Materials Chemistry C
Page 28 of 39
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DOI: 10.1039/C6TC01530A
(%) for C14
H
16
O
2
S
2
: C, 59.97; H, 5.75; S, 22.87. Found: C, 60.11; H, 5.67; S, 22.80.
+
MALDI MS: found m/z 280.15; calculated for [M] 280.41.
5
,5ꢀdimethylꢀ2ꢀ[5'ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)ꢀ2,2'ꢀbithienꢀ5ꢀyl]
ꢀ1,3ꢀdioxane (3a). 2.5 M solution of nꢀbutyllithium (15.59 mL, 39.0 mmol) in hexane
was added dropwise to a solution of compound 2a (10.93 g, 39.0 mmol) in 270 mL of
dry THF at ꢀ78 ºC. Afterwards the reaction mixture was stirred for 60 min at ꢀ78 ºC
and then isopropoxyꢀ4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolane (7.3 mL, 39.0 mmol)
was added in one portion. The reaction mixture was stirred for 1h at –78 ºC, then the
cooling bath was removed, and the stirring was continued for 1h. After completion of
the reaction, 400 mL of freshly distilled diethyl ether and 150 mL of distilled water
and 21.2 mL of 1 M HCl were added to the reaction mixture. The organic phase was
separated, washed with water, and dried over sodium sulfate and filtered. The solvent
was evaporated to give 15.68 g (99 %) of the pure product as a gray solid. M.p.:
1
64ꢀ166 ºC. The product was used in the subsequent synthesis without further
1
purification. H NMR (250 MHz, CDCl
3
,
δ, ppm): 0.79 (s, 3H), 1.27 (s, 3H), 1.33 (s,
1
2H), 3.61 (d, 2H, J = 11 Hz), 3.73 (d, 2H, J = 11 Hz), 5.60 (s, 1H), 7.01 (d, 1H, J =
.7 Hz), 7.09 (d, 1H, J = 3.7 Hz), 7.20 (d, 1H, J = 3.7 Hz), 7.49 (d, 1H, J = 3.7 Hz).
3
1
3
C NMR (125 MHz, CDCl
3
): δ [ppm] 21.83, 22.95, 24.75, 30.21, 77.51, 84.16, 98.09,
1
23.79, 125.07, 125.81, 137.54, 137.91, 140.81, 144.03. Calcd (%) for C20H27BO S :
4 2
C, 59.11; H, 6.70; S, 15.78. Found: C, 59.25; H, 6.82; S, 15.70. MALDIꢀMS: found
+
m/z 406.43; calculated for [M] 406.37.
2
ꢀ(2,2'ꢀbithienꢀ5ꢀyl)ꢀ2ꢀethylꢀ5,5ꢀdimethylꢀ1,3ꢀdioxane (2b). This compound was
obtained by the method described for compound 2a using 1b (26.1 g, 117.3 mmol),
,3ꢀdimethylꢀ1,3ꢀpropanediol (73.36 g, 704.8 mmol) and pꢀTosH (1.56 g, 8.2 mmol).
1
Purification by column chromatography on silica gel (eluent toluene : hexane :
triethylamine 49.95: 49.95 : 0.1) gave pure compound 1b (31.4 g, 81%) as a colorless
1
liquid. H NMR (250 MHz, CDCl
3
, δ, ppm): 0.65 (s, 3H), 0.9 (t, 3H, J = 7.3 Hz),
1
.22 (s, 3H), 1.84 (m, 2H, M = 4, J = 7.3 Hz), 3.69 (d, 2H, J = 11 Hz), 3.70 (d, 2H, J
=
11 Hz), 6.85 (d, 1H, J = 3.7 Hz), 6.98 (dd, 1H, J
1
= 3.7Hz, J
2
= 3.7 Hz), 7.05 (d, 1H,
): δ [ppm]
1
3
J = 3.7 Hz), 7.12ꢀ7.21 (overlapping peaks, 2H). C NMR (75 MHz, CDCl
3
2
8

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7
1
2
.66, 21.96, 22.81, 29.88, 37.77, 71.92, 100.56, 123.33, 123.56, 124.32, 126.97,
27.80, 137.49, 137.64, 143.33. Calcd (%) for C H O S : C, 62.30; H, 6.54; S,
1
6
20
2 2
0.79. Found: C, 62.45; H, 6.65; S, 20.72. MALDIꢀMS: found m/z 308.56; calculated
+
for [M] 308.46.
2
ꢀethylꢀ5,5ꢀdimethylꢀ2ꢀ[5'ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)ꢀ2,2'ꢀbithi
enꢀ5ꢀyl]ꢀ1,3ꢀdioxane (3b). This compound was obtained by the method described for
compound 3a using 2b (31.1 g, 101 mmol), 1.6 M solution of butyllithium (62.01 mL,
1
01 mmol) in hexane, IPTMDOB (18.76 g, 101 mmol) to give 41.5 g (95%) of the
1
pure product (purity was 98% according to H NMR) as a blue solid. M.p.: 119ꢀ120
1
ºC. The product was used in the subsequent synthesis without further purification. H
3
NMR (250 MHz, CDCl , δ, ppm): 0.65 (s, 3H), 0.9 (t, 3H, J = 7.3 Hz), 1.22 (s, 3H),
1
.34 (s, 12H), 1.84 (m, 2H, M = 4, J = 7.3 Hz), 3.39 (d, 2H, J = 11 Hz), 3.69 (d, 2H, J
=
11 Hz), 6.85 (d, 1H, J = 3.7 Hz), 7.10 (d, 1H, J = 3.7 Hz), 7.19 (d, 1H, J = 3.7 Hz),
1
3
7
2
1
.50 (d, 1H, J = 3.7 Hz). C NMR (75 MHz, CDCl
4.62, 24.81, 29.88, 37.78, 71.93, 84.22, 100.54, 124.06, 124.78, 127.12, 137.51,
37.98, 144.03, 144.19. Calcd (%) for C22 : C, 60.83; H, 7.19; S, 14.76.
3
): δ [ppm] 7.67, 21.96, 22.80,
H
4 2
31BO S
Found: C, 60.49; H, 7.25; S, 14.70. MALDIꢀMS: found m/z 434.27; calculated for
+
[
M] 434.49.
2
ꢀethylꢀ5,5ꢀdimethylꢀ2ꢀ(2,2':5',2''ꢀterthienꢀ5ꢀyl)ꢀ1,3ꢀdioxane (4b). In an inert
atmosphere, degassed solutions of 2ꢀbromthiopene (3.75 g, 23 mmol) and compound
b (8 g, 18 mmol) in toluene/ethanol mixture (100/10 mL) and 2M solution of aq.
Na CO (14 mL) were added to Pd(PPh ) (399 mg, 0.35 mmol). The reaction mixture
3
2
3
3 4
was stirred under reflux for 20 h, and then it was cooled to room temperature and
poured into 100 mL of water and 100 mL of toluene. The organic phase was separated,
washed with water, dried over sodium sulfate and filtered. The solvent was evaporated
in vacuum and the residue was dried at 1 Torr. The product was purified by column
chromatography on silica gel (eluent toluene : triethylamine 99.9 : 0.1) to give pure
1
compound 4b (5,39 g, 74%) as a yellow solid. M.p.: 93ꢀ94 ºC. H NMR (250 MHz,
CDCl
3
, δ, ppm): 0.66 (s, 3H), 0.92 (t, 3H, J = 7.3 Hz), 1.22 (s, 3H), 1.86 (m, 2H, M =
2
, J = 7.3 Hz), 3.41 (d, 2H, J = 11 Hz), 3.71 (d, 2H, J = 11 Hz), 6.86 (d, 1H, J = 3.7
2
9