Synthesis of phenylenevinylene oligothiophene derivatives with and without cyano side substitution and evaluation of optoelectronic characteristics

Article abstract of DOI:10.1246/cl.150274

Two types of phenylenevinylene terthiophene with and without cyano side substitution were synthesized to investigate the effect of the molecular structure on optoelectronic properties. The oligothiophene with cyano side substitution exhibited large bathoc

Full text of DOI:10.1246/cl.150274

CL-150274
Received: March 25, 2015 | Accepted: April 25, 2015 | Web Released: July 5, 2015
Synthesis of Phenylenevinylene Oligothiophene Derivatives with and without Cyano Side
Substitution and Evaluation of Optoelectronic Characteristics
Mizuho Kondo,*1 Yukihiro Inoue,1 Yuki Koeduka,1 Masahiro Funahashi,2 Akira Heya,1 Naoto Matsuo,1 and Nobuhiro Kawatsuki*1
1Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo,
2167 Shosha Himeji, Hyogo 671-2280
2Department of Advanced Materials Science, Faculty of Engineering, Kagawa University,
2217-20 Hayashi-cho, Takamatsu, Kagawa 761-0396
(E-mail: mizuho-k@eng.u-hyogo.ac.jp)
Two types of phenylenevinylene terthiophene with and
without cyano side substitution were synthesized to investigate
the effect of the molecular structure on optoelectronic properties.
The oligothiophene with cyano side substitution exhibited large
bathochromic effect and behaved as organic electron acceptors
with decrease in field effect conductivity due to increase in dipole
moment.
Figure 1. Chemical structures, nomenclature, and thermal properties
of oligothiophenes used in this study. K: crystal; LC: liquid crystal;
I: isotropic.
Oligothiophenes have attracted much attention in the fields of
organic semiconductors and optoelectronics due to their good
processability and tunability of their solid and crystalline state.
These compounds have significant potential for use as molecular
semiconductors in organic electronics and device applications such
as organic field effect transistors (OFETs),1 photovoltaic solar
cells,2 and lasers.3 There is a wide range of possibilities for
chemical modification; therefore, various oligothiophene deriva-
tives have been synthesized.
In particular, cyanovinyl oligothiophene has a strong elec-
tronic push-pull structure with good processability and redox
properties,4 and has been used as a dye sensitizer in Grätzel-type
solar cells.2b Both symmetric and asymmetric cyano-based
oligothiophenes have also been used as OFETs with tetra-
cyanoquinodimethane analogs that switch high electron mobility
(circles) solvents (1.2 © 10¹5 M) under exposure to 365 nm light. The
Figure 2. UV absorption (black) and PL spectra (colored) of 4TTh
(top) and 4CnTTh (bottom) dissolved in THF (triangles) and DMSO
and ambipolar transport properties by changing the incorporation
amount of electron-withdrawing (EW) substitution.5 Recently,
Funahashi and Kato have prepared one cyano-substituted oligo-
thiophene that has liquid-crystalline nanostructures consisting of
ionically conductive and electronic charge transport layers by the
association of π-conjugated molecules with ionic moieties.6 The
use of cyano-based terthiophene as a mesogenic unit was reported,
although the field effect conductivity of cyanovinyl oligothiophene
has not been evaluated to date.
In this work, we prepared two types of phenylenevinylene
terthiophene with and without cyano side substitution and
evaluated the effect of the molecular structure on optoelectronic
properties. To design the molecular structure, enhancement of the
thermal properties is necessary in order to utilize wet processes
such as spin-casting and drop-casting to make it suitable for
industrial applications. Alkyl chain substitution is an effective way
to solubilize aromatic compounds; therefore, to utilize substituted
compounds for electronic devices, it is necessary to add substitu-
ents in an appropriate manner, i.e., without disturbing the electronic
properties of the original conjugated core.7 In addition, asymmetric
structure along the conjugated structure is useful for enhancing the
solubility and liquid crystallinity. Consequently, the asymmetric
oligothiophene shown in Figure 1 was designed. The synthesis
route for the compounds, experimental details, and thermal
insets show photographs of the THF (left) and DMSO (right) solutions
upon exposure to 365 nm light.
properties are given in the Supporting Information (SI). Figure 2
shows absorption and photoluminescence (PL) spectra of the
oligothiophenes dissolved in tetrahydrofuran (THF) and dimethyl
sulfoxide (DMSO) at a concentration of ca. 1.2 © 10¹5 M excited at
365 nm light. The peak wavelengths for absorbance (-abs), photo-
luminescence (-PL), maximum absorption coefficients (¾), and
quantum yields (Φf) for the solutions under UV irradiation are
summarized in Table 1, and excitation spectra are provided in
Supporting Information S4 (SI). It has been reported that cyano-
based oligothiophene molecules typically exhibit intramolecular
charge transfer.4 A cyano group located in the α position causes a
bathochromic shift; the more polar DMSO stabilizes a strongly
charge polarized ground electronic state so that a very large amount
of planar conformers should be present in the DMSO solution. As a
result, the absorbance is slightly broad due to its intramolecular
charge-transfer characteristics, because the planar conformation
optimizes the conjugation of thiophene and phenylacetonitryl
moieties.4 These results suggest that intramolecular charge transfer
(ICT) occurred from the conjugated backbone to the cyano branch.
The energy of the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) were estimated
1010 | Chem. Lett. 2015, 44, 1010–1012 | doi:10.1246/cl.150274
© 2015 The Chemical Society of Japan

Table 1. Wavelengths of absorption and photoemission spectral
peaks, maximum absorption coefficients, and quantum efficiencies
for the oligothiophene solutions
Table 2. Measured reduction potentials and HOMO and LUMO
energies of the oligothiophenes estimated from DFT calculations
a)
a)
Eox
Ered
¦Ea)
/eV
HOMOb) LUMOb) ¦EDFT
a)
abs
b)
-
-
¾
Φf
/%
/V
/V
/eV
/eV
/eV
em
Sample
Solvent
¹1
/nm
/nm
/M¹1 cm
4TTh
4CnTTh
0.33
0.5
®
®
¹4.88
¹5.20
¹1.95
¹2.43
2.93
2.77
4TTh
THF
DMSO
THF
412
418
415
427
495
501
443
543
83000
58000
59000
40000
3.6
3.5
0.14
0.025
¹1.95 ¹2.45
aMeasured in CH2Cl2 containing 0.1 M n-Bu4NClO4 as a supporting
electrolyte at room temperature. bCalculations were performed using
DFT with the restricted B3LYP functional and the 6-31G(d) basis set,
as implemented in Gaussian 09.
4CnTTh
DMSO
b
aConcentration: 1.2 © 10¹5 mol L¹1. Excitation at 365 nm.
from density functional theory (DFT) calculations. The dipole
moment of 4CnTTh is 3.97 D, which is approximately five times
larger than that of 4TTh (0.78 D), but the electron clouds in the
conjugated backbone are delocalized for the HOMO and LUMO
level (Figure S5, SI). The energies of both the HOMO and LUMO
of 4CnTTh are lower than those of 4TTh, and the reduction of the
LUMO is larger than that of the HOMO, which reduces the energy
band. Additionally, the effect of solvent polarity on -abs was also
estimated by time-dependent (TD)-DFT method, and the result
is provided in ST1 (SI). Red shift in -abs is observed and
bathochromic effect became larger in 4CnTTh, supporting the
experimental results although the change was slight. On the other
hand, difference in the photoemission behavior of the oligothio-
phens was more prominent; the PL spectrum peak of 4CnTTh
showed large red shift when it was dissolved in DMSO, while
4TTh showed a slight change. In addition, the photoemission
intensity of the 4CnTTh solution is weaker than that of the 4TTh
solution, as shown in the inset of Figure 2 and Table 1. Sun et al.
reported that ICT was induced in oligothienylene derivatives with
electron-withdrawing substitution by polar solvents,8 followed by
geometric twisting (TICT).9 They also indicated that the PL of the
compounds in nonpolar solvents is emitted from the fluorescent
states with planar conformations that showed small difference
between the compound with and without EW substitution,
presuming that the same mechanism occurred in our compounds
due to the similarity of chemical structure and the effect of solvent
polarity on absorbance and photoemission.
Cyclic voltammetry measurements of the compounds were
conducted in CH2Cl2 in the presence of n-Bu4NClO4 (0.1 M) using
Fc/Fc+ as an internal standard (Table 2). For compound 4CnTTh,
quasi-reversible one-electron oxidation and one-electron reduction
were observed at the half-wave potentials of 0.50 and ¹1.95 V vs.
Ag+/Ag, respectively. The cyclic voltammograms for compound
4TTh indicate only an ambiguous one-electron oxidation at 0.33 V,
without any reversible reduction peaks within the electrochemical
window of the electrolyte solution (Figure S6, SI). The introduc-
tion of the cyano groups at the ethylene linkages reduced the
LUMO level, which leads to the expected positive shift of the
reduction potential and stable anion species are observed.
Figure 3. Double logarithmic plots of transient photocurrent curves
for hole transport in 4TTh (A) and 4CnTTh (B) under various applied
voltages in a 4 ¯m cell irradiated with 356 nm light. Applied voltage:
50 V (red); 40 V (blue); 30 V (green); 20 V (purple); 10 V (pink); 0 V
(black). Black arrows indicate kink point of the photocurrent when the
applied voltage was 50 V.
of dipolar moment orientation was generated in LC phase and it
remained in the thiophene solid, causing distribution of the HOMO
levels. In amorphous organic semiconductors, this distribution of
the energy levels of the π-orbitals leads to a decrease in the carrier
mobilities and their field- and temperature-dependence.11
Thin films of the oligothiophenes were prepared by subli-
mation onto quartz substrates, and their surface profiles were
investigated. Figure 4A shows an X-ray diffraction (XRD) pattern
for 4TTh, which corresponds to a d-spacing of d = 2.38 nm. The
4TTh film had a small angle reflection at 2ª = 3.71°, consistent
with the molecular length of the 4TTh molecule (2.39 nm)
estimated from DFT calculations. An atomic force microscopy
(AFM) image for the 4TTh film is shown on the right side of
Figure 4A, where small circular grains (< 0.2 ¯m) and a smooth
surface (Rq µ 2.0 nm) were observed. To further evaluate the
surface profile of the 4TTh film, the angular dependency of a probe
beam was measured using polarized absorption spectroscopy. The
detailed geometry of the optical setup is shown in Figure S1 (SI),
and angular-dependent absorption spectra are summarized in
Figure S7A (SI). The absorption peak for the film is approximately
343 nm, which is shorter than that for the THF solution, and the
absorbance increased with the tilt angle. In addition, the PL
spectrum of the substrate was similar to that of solution with low
intensity, as displayed in Figure S8 (SI), which indicates that 4TTh
aligned normal to the substrate and formed H-aggregation.
The carrier transport characteristics of the oligothiophenes
were examined using the time-of-flight (TOF) technique.10 Transit
times were determined from kink points in double logarithmic
plots of the transient photocurrent curves. The arrows in Figure 3
indicate the transit times for 4TTh and 4CnTTh. The hole
¹2
mobilities at 50 V are calculated to be 2.1 © 10 and 4.6 ©
¹1
10¹3 cm2 V¹1 s for 4TTh and 4CnTTh, respectively. The lower
field-dependent hole mobility of 4CnTTh should be attributed to
the local electric field produced by dipole moments of the cyano
groups of 4CnTTh molecules. Unlike molecular crystals, disorder
Chem. Lett. 2015, 44, 1010–1012 | doi:10.1246/cl.150274
© 2015 The Chemical Society of Japan | 1011

region for 4TTh that is lower than TOF measurement due to the
existence of disordered regions and the smaller size of grains in
the film.1c In contrast, a decrease in output was observed in the
saturated region for 4CnTTh. The field effect mobility was
¹1
estimated to be 1.6 © 10¹4 cm2 V¹1 s in the (quasi) saturated
region, which was one order lower than that for 4TTh, which is
consistent with the TOF measurement results (Figure 5B). It can
be speculated that the decrease of ID is due to the lower molecular
crystallinity and larger dipole moment of 4CnTTh molecules. In
comparison to the previous oligothiophene derivatives with cyano
substitution,5 the incorporation amount of EW is not enough to
induce electron mobility, causing reduction of the LUMO level
and hole mobility.
In summary, two types of phenylenevinylene terthiophene
were prepared. Oligothiophene with the cyano side group
exhibited large bathochormic effect with low quantum yields and
behaved as organic electron acceptors in solution state, while it
showed p-type conductivity when an OFET was fabricated. The
oligothiophenes examined in this study exhibited liquid-crystalline
properties; therefore, improvement of the hole mobility could be
expected with the use of an alignment layer,12 and this is currently
under investigation.
Figure 4. XRD patterns (left) and AFM images (right) of (A) 4TTh
and (B) 4CnTTh films sublimed on quartz substrates. Scale bar: 1 ¯m.
This work was supported in part by a Grant-in-Aid for Young
Scientists B (No. 25810079) from the Japan Society for the
Promotion of Science.
Supporting Information is available electronically on J-STAGE.
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1012 | Chem. Lett. 2015, 44, 1010–1012 | doi:10.1246/cl.150274
© 2015 The Chemical Society of Japan