Structure property relationships based on phenyl-1H-pyrrole end-capped thiophene semiconductors

Article abstract of DOI:10.1071/CH12112

Thiophene oligomers end-capped with phenyl-1H-pyrrole were used as active layers of organic field-effect transistors. Two types of cores that vary in size and shape of-linked thiophenes and fused-ring thiophenes were chosen to study the structureproperty

Full text of DOI:10.1071/CH12112

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2012, 65, 1252–1256
Communication
http://dx.doi.org/10.1071/CH12112
Structure Property Relationships Based on Phenyl-1H-
]
Pyrrole End-Capped Thiophene Semiconductors
A B
,
A
A C
,
A
B
Ying Liu,
Ji Zhang, Yunqi Liu,
Gui Yu, and Ziyi Ge
A
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
Ningbo Institute of Material Technology and Engineering, Chinese Academy
of Sciences, Ningbo 315201, China.
B
C
Corresponding author. Email: liuyq@iccas.ac.cn
Thiophene oligomers end-capped with phenyl-1H-pyrrole were used as active layers of organic field-effect transistors.
Two types of cores that vary in size and shape of a-linked thiophenes and fused-ring thiophenes were chosen to study the
structure–property relationship. The crystal structures, optical, electrochemical and electronic properties were investigated,
and the heterocyclic compounds exhibited clear p-type semiconducting behavior.
Manuscript received: 22 February 2012.
Manuscript accepted: 2 April 2012.
Published online: 29 June 2012.
Introduction
pyrrole-phenylene is more favorable to conjugation of the whole
system relative to C–C bond formation. This structural motif is
designed to lower the highest occupied molecular orbit
(HOMO) levels by introducing pyrrole rings to increase air
stability.[11] We describe in the present paper the preparation
and photophysical properties of five thiophene oligomers end-
capped with phenyl-1H-pyrrole. Two phenyl-1H-pyrroles are
separated by the spacers of thiophene oligmers. Furthermore, we
discuss the structure–property relationship of phenyl-1H-
pyrrole end-capped thiophene semiconductors. To the best of
our knowledge, this contribution represents the first investiga-
tion of these phenyl-1H-pyrrole end-capped thiophene oligo-
mers for OTFT. Two types of cores that vary in size and shape
(molecular configuration) to a-linked thiophenes and fused-ring
thiophenes are chosen to study the structure–property relation-
ship. They are characterized in detail, and their optoelectronic
properties are studied by UV–vis/fluorescence spectroscopy and
cyclic voltammetry. Thiophene oligomers end-capped with
phenyl-1H-pyrrole are an attractive way to tailor the optical and
electronic characteristics of these materials, and these semi-
conducting materials are easily synthesized and can be purified
in large quantities.
A great number of organic semiconducting materials in organic
thin-film transistors (OTFT) have been proposed and developed
during the last two decades.[1] OTFT materials with not only
high performance but also good stability in the operational
environment are necessary for commercial applications. It is an
important part of the OTFT research to design and synthesize
new derivatives. Oligophenylenes and oligothiophenes have
been a subject of considerable interest, due to their good per-
formance for p-type semiconductors.[2] Thiophene oligomers, in
particular, have been studied as model compounds for OTFT to
interpret charge transfer mechanisms, since these heterocyclic
materials exhibit both interesting electronic and optical prop-
erties and possess thermal and chemical stability.[3] Many
research groups have reported extensive results related to the
effect of structure and/or packing properties of single crystals of
thiophene oligomers on their electrical properties.[4] The strat-
egy of introducing different terminal groups to cap oligothio-
phenes or fused thiophenes has been found to produce hole
transporting materials in OTFT.[5] The synthesis of new con-
jugated thiophene oligomers by introducing functionalizing
organic subunits in thiophene cores via co-oligomerization is
used in an attempt to obtain high mobilities. Phenyl or biphenyl
as end groups constitute an important class of materials that have
been studied extensively and used as organic semiconductors.[6]
The results on the charge transport experiments indicate that the
thin films of such co-oligomers exhibit high mobilities of
Methods
The target compounds PPTT, PPDTT, PP2T, PP3T, and PP4T
were synthesized via a Suzuki cross-coupling reaction as illus-
trated in Scheme 1. The synthetic method of 1-(4-bromo-
phenyl)-1H-pyrrole (1) was reported in the literature under the
condition via copper iodide catalyzed N-arylation reaction.[12]
In this reaction, the selective reaction of 1-bromo-4-iodo-
benzene in the present method was iodine before bromine. In
the next step, 1 was treated with n-BuLi and borylation to afford
the desired aryl boronate ester 2. As a general procedure, the
0.17 cm2 Vꢀ1 sꢀ1 [7] The oligofluorene end-capped thiophenes,
.
also introduced as a high-performance p-type active material,
had better air and UV stability.[8] Some pentacene-like hetero-
acenes containing sulfur[9] or nitrogen[10] atoms have been
developed to improve the solubility, stability and co-facial
packing in the solid state. In this study, we use the phenyl-1H-
pyrrole end-capped thiophenes because the C–N bond in
Journal compilation Ó CSIRO 2012
www.publish.csiro.au/journals/ajc

Thiophene Semiconductors
1253
CuI, nBu4NBr
NH
ꢀ
I
Br
N
Br
NaOH, Toluene
reflux, 22 h
1
O
(1) nBuLi, THF, ꢁ78ꢂC
N
B
O
O
O
O
(2)
B B
O
2
S
Br
Pd(PPh3)4, 2 M K2CO3
THF, reflux
2
ꢀ
Br
m
S
N
S
N
S
m
m ꢃ 1, PPTT
m ꢃ 2, PPDTT
Pd(PPh3)4, 2 M K2CO3
THF, reflux
2
ꢀ
Br
Br
n
S
N
N
S
n
n
n
n
ꢃ 2, PP2T
ꢃ 3, PP3T
ꢃ 4, PP4T
Scheme 1. Preparation of thiophene oligomers.
(a)
Table 1. Optical and electrochemical properties
Compound
Td
labs
[nm]
lem
[nm]
Eogpt
EHOMO
[8C]
[eV]
[eV]
PPTT
PPDTT
PP2T
PP3T
PP4T
378
400
367
414
474
333
367
338
351
369
498
2.76
2.63
2.56
2.38
2.25
ꢀ5.59
ꢀ5.49
ꢀ5.50
ꢀ5.54
ꢀ5.56
525
532/570
557/606
591/634
(b)
reaction was performed using 5 mol % tetrakis(triphenylpho-
sphine)palladium [Pd(PPh3)4] as catalyst in tetrahydrofuran.
Using this approach, a wide variety of phenyl-1H-pyrrole end-
capped thiophene oligomers were synthesized. They were
purified three times by gradient sublimation under vacuum and
slightly soluble in hot toluene or chlorobenzene. The thermal
properties of the target compounds shown in Table 1 were
evaluated by thermal gravimetric analysis with good thermal
stability.
Fig. 1. (a) Molecular structure of PP3T. Top view of the molecule and side
view along the molecular long axis. (b) The stacking structures of PP3T.
˚
˚
SꢁꢁꢁH contacts (2.97 A) and CHꢁꢁꢁp contacts (2.85 A).
single crystals that were grown in a vapor phase sublimation in a
temperature-gradient furnace. Single crystals of PP2T have
been reported in the literature.[13] X-ray diffraction (XRD) study
was performed on the crystal PP3T to determine the solid-state
structures. The crystal structure of PP3T was determined by a
single crystal XRD study and the ORTEP drawing is depicted in
Fig. 1. The molecular backbone is nearly planar, and has a very
Results
We have determined crystallographic structures of the PP3T
molecular shape. The structure analysis was carried out using

1254
Y. Liu et al.
slight twist along its long axis. The two phenylene-1H-pyrrole
end groups are both deviated 10.98 from the main a-linked
thiophenes plane. Intermolecular interactions are short CHꢁꢁꢁp
and SꢁꢁꢁH contacts in neighboring molecular layers. The crystal
packing of PP3T is classified into a herringbone geometry
manner, similar to pentacene.[14] The tilt angel between two
molecular planes is 46.868.
substrate temperature (Tsub) of ,158C were recorded and the
results are summarized in Table 1. The absorption spectra (labs
)
of the oligomers were red-shifted as the extent of thiophene ring
fusion or the number of thiophene rings increased. The highest
occupied molecular orbital (EHOMO)–lowest unoccupied molec-
ular orbital (ELUMO) energy gap (Eg) (Table 1), determined by
extrapolating the long-wavelength absorption edge, decreased
with increasing molecular length for both series of oligomers.
The oligomers of fused-thiophene cores had slightly broader
bandgaps than the oligomers of oligo-thiophene cores.
The UV-vis absorption spectra of vacuum-evaporated thin
films of the oligomers deposited on a quartz substrate with a
To obtain the HOMO and LUMO energy levels, the oligo-
mers were investigated in the film state by electrochemical
cyclic voltammetry. All oligomers were oxidized irreversibly in
the positive potential region. The HOMO energy level can be
calculated from the onset potential of the oxidative wave
according to literature reports,[15] and was in the range of
ꢀ5.59 to ꢀ5.49 eV, as summarized in Table 1. The results show
that the introduction of different central cores had little influence
on the HOMO energy levels. For all oligomers, we did not
observe any redox processes in the negative potential region;
however, the LUMO level could be estimated by subtracting the
optical energy gap from the HOMO level.
PPTT
PPDTT
PP2T
PP3T
PP4T
To better understand the relationship between the film
morphology and the mobility, the thin films (,50 nm in
thickness) of these oligomers on n-octadecyltrichlorosilane
(OTS)-modified SiO2/Si substrate surfaces were investigated
by thin-film XRD and atomic force microscopy (AFM). Fig. 2
shows the XRD patterns of the five films at Tsub of 158C. In all
films, the first-order reflection was very intense. The XRD
5
10
15
20
25
30
2θ Angle [ꢂ]
Fig. 2. The thin-film X-ray diffraction diagrams of the Tsub ¼ 158C.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Fig. 3. 2 ꢂ 2 mm2 atomic force microscopy images at different substrate temperatures: (a) PPTT (Tsub ¼ 158C),
(b) PPDTT (Tsub ¼ 158C), (c) PP2T (Tsub ¼ 158C), (d) PP4T (Tsub ¼ 158C), (e) PP3T (Tsub ¼ 158C), (f) PP3T
(Tsub ¼ 608C) and (g) PP3T (Tsub ¼ 1008C).

Thiophene Semiconductors
1255
(a)
Table 2. Thin-film transistor characteristics
The carrier mobility: mTFT [cm2 Vꢀ1 sꢀ1], on/off current ratio: Ion/off
threshold voltage: VT [V] deposited at different substrate temperatures
V
DS ꢃ
ꢁ25
,
ꢁ60 V
ꢁ20
ꢁ15
ꢁ10
ꢁ5
0
Tsub [8C]
Material
Tsub
VT
Ion/off
mTFT
ꢁ40 V
PPTT
15
15
ꢀ36
ꢀ30
ꢀ35
ꢀ50
ꢀ28
ꢀ40
ꢀ20
ꢀ25
ꢀ37
ꢀ50
ꢀ35
105
106
105
105
105
105
104
107
106
106
107
5.3 ꢂ 10ꢀ5
6.8 ꢂ 10ꢀ3
3.7 ꢂ 10ꢀ3
3.5 ꢂ 10ꢀ3
1.7 ꢂ 10ꢀ3
6.6 ꢂ 10ꢀ4
1.1 ꢂ 10ꢀ5
6.4 ꢂ 10ꢀ2
2.4 ꢂ 10ꢀ2
4.5 ꢂ 10ꢀ3
4.0 ꢂ 10ꢀ2
ꢁ20 V
PPDTT
0 V
60
5
100
15
PP2T
PP3T
PP4T
10
10
0
ꢁ10 ꢁ20 ꢁ30 ꢁ40 ꢁ50 ꢁ60 ꢁ70
VDS [V]
60
100
15
60
10ꢁ4
(b)
100
15
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0
10ꢁ5
10ꢁ6
10ꢁ7
10ꢁ8
10ꢁ9
10ꢁ10
10ꢁ11
V
DS ꢃ ꢁ60 V
PP3T exhibited a mTFT almost 10 times as high as that of
PPDTT (6.4 ꢂ 10ꢀ2 versus 6.8 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1). Further
extension of the conjugation of oligothienoacenes from PP3T to
PP4T did not increase the transistor performance. It is well
known that the morphology of thin-film influences the device
performance dramatically. This is consistent with thin-film
XRD and AFM images of the oligomers as aforementioned,
which indicates better film morphology and molecular order in
oligothiophene compounds than in fused thiophene compounds.
PP3T devices showed a best hole mobility of 0.064 cm2 Vꢀ1 sꢀ1
and an on/off ratio of 107 in saturation regime fabricated at
Tsub ¼ 158C. When the substrate temperature was increased to
1008C, the charge transport mobility decreased to
0
ꢁ10 ꢁ20 ꢁ30 ꢁ40 ꢁ50 ꢁ60
VG [V]
Fig. 4. The (a) output and (b) transfer characteristics of organic thin-film
transistor devices based on PP3T at Tsub ¼ 158C at different gate voltages.
0.0045 cm2 Vꢀ1 sꢀ1
.
patterns of PP3T and PP4T exhibited strong and sharp diffrac-
tion peaks with multiple orders of reflection, which indicated a
preferred orientation of molecules in the films with high
crystalline on their substrates. For PPTT and PP2T films, the
other higher peaks disappeared. AFM images in Fig. 3 show
complementary information about these films. All the AFM
images at Tsub of 158C consisted of small grains of size 200 nm.
For the PP3T film, when the temperature of the substrate was
increased further, the grains grew larger on the substrate but
some boundaries between the grains were formed simultaneous-
ly, which related to the drop in mobility. This evolving trend of
morphology was also consistent with the fluctuation trend of the
value of mTFT, which could be further confirmed by the follow-
ing OTFT study.
The OTFT devices fabricated in a top-contact configuration
showed typical p-channel transistor characteristics under ambi-
ent conditions. The typical output and transfer characteristics of
PP3T on OTS/SiO2/Si at Tsub of 158C are shown in Fig. 4. The
main parameters of device performance, i.e. mobility (mTFT),
threshold voltage (VT), Ion/off at different Tsub, are summarized
in Table 2.
Conclusion
We have discussed the optical, electrochemical and OTFT
properties for a series of thiophene semiconductors with two
types of spacer. The oligomers of fused-thiophene cores have
slightly broader optical bandgaps than the oligomers of oligo-
thiophene cores. Investigation of their electrochemical proper-
ties indicates that different thiophenes make little difference to
the HOMO energy levels. Under the same device fabrication
conditions, the phenyl-1H-pyrrole end-capped oligothiophenes
exhibited superior device performance than the corresponding
fused thiophenes. The best performance of a mobility of
0.064 cm2 Vꢀ1 sꢀ1 and an on/off ratio of 107 was obtained at
Tsub ¼ 158C. These results suggest that the phenyl-1H-pyrrole
systems, as good prototypes for organic semiconductors, are
promising candidates for OTFT.
Supplementary Material
The Supplementary Material is available online on the Journal’s
website. It contains a description of obtained materials, fabri-
cation of OTFT devices, the UV-vis absorption spectra, cyclic
voltammetry, and further crystallographic details for PP2T and
PP3T.
Discussion
For data analysis, it is clear that the OTFT performance strongly
depends on substrate temperature value. The phenyl-1H-pyrrole
end-capped oligothiophenes exhibited superior device perfor-
mance than the corresponding fused thiophenes under the same
device fabrication conditions. For example, at Tsub ¼ 158C, the
best OTFT of PP2T exhibited a mTFT higher than that of PPTT
(1.7 ꢂ 10ꢀ3 versus 5.3 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1), while the OTFT of
Acknowledgements
This work was supported by the National Natural Science Foundation of
China (No. 21102156 and 2011A610205), the Major State Basic Research
Development Program (2009CB623603), and the Chinese Academy of
Sciences.

1256
Y. Liu et al.
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