Synthesis and characterization of a tetrathiafulvalene-based polymer

Article abstract of DOI:10.5012/bkcs.2012.33.5.1451

A novel tetrathiafulvalene (TTF)-based main-chain polymer (6TTF-polymer) was successfully synthesized via a condensation polymerization between a newly synthesized dihydroxy TTF derivative and a malonyl chloride, and its chemical structure was characteriz

Full text of DOI:10.5012/bkcs.2012.33.5.1451

Synthesis and Characterization of a Tetrathiafulvalene-Based Polymer
Bull. Korean Chem. Soc. 2012, Vol. 33, No. 5 1451
http://dx.doi.org/10.5012/bkcs.2012.33.5.1451
Articles
Synthesis and Characterization of a Tetrathiafulvalene-Based Polymer
Sehyun Lee, Lei Wang,† Seok-Ho Hwang,‡ Myong-Hoon Lee,* and Kwang-Un Jeong*
Department of Polymer-Nano Science Technology and Polymer Materials Fusion Research Center,
Chonbuk National University, Jeonju, Jeonbuk 561-756, Korea
*E-mail: kujeong@jbnu.ac.kr (K.-U. Jeong); mhlee2@chonbuk.ac.kr (M.-H. Lee)
†Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China
‡Department of Polymer Science and Engineering, Dankook University, Yongin 448-701, Korea
Received May 10, 2011, Accepted November 22, 2011
A novel tetrathiafulvalene (TTF)-based main-chain polymer (6TTF-polymer) was successfully synthesized via
a condensation polymerization between a newly synthesized dihydroxy TTF derivative and a malonyl chloride,
and its chemical structure was characterized by spectroscopic techniques. Molecular weight of the 6TTF-
polymer (9,030 g/mol by gel permeation chromatography) was large enough to form the ductile film. The
electrochemical and optical properties of the 6TTF-polymer were further estimated by cyclic voltammetry,
ultraviolet and photoluminescence spectroscopes. The highest occupied molecular orbital level (EHOMO = −4.79
eV) and band-gap energy (Eg = 1.91 eV) of the 6TTF-polymer suggested that TTF-based polymer could act as
a good electron donating material for the optoelectronic applications.
Key Words : Polymer, Tetrathiafulvalene, Fullerene, Synthesis
Introduction
separations and defects such as cracks may occur during and
after the solution coating, which results in a poor energy
conversion.9 Therefore, it is desirable to synthesize a new
polymer containing electron donor or/and acceptor. To satisfy
this purpose, a new polymer (6TTF-polymer) containing
electron donating (TTF) moiety was designed and synthe-
sized and its electro-optical properties were investigated.
Tetrathiafulvalene (TTF)-based molecules have been of
great interests because of their unique photophysical and
electrochemical behaviors, which makes it possible to apply
them as organic charge transfer materials in the field of
molecular switches,1 sensors,2 self-assembled redox-active
supramolecules3 and organic field-effect transistors (OFETs).4
Especially, TTF derivatives have been considered as pro-
mising active materials for organic photovoltaics (OPV) due
to their excellent electron donating properties.5
Result and Discussion
A new polymer (6TTF-polymer) containing electron-rich
tetrathiafulvalene (TTF, electron donor) was successfully
synthesized via coupling reactions. The detailed synthetic
strategies were described in Scheme 1 and 2. To synthesize
the targeted TTF-based monomer (8), the key starting com-
pound 4 was prepared by the three-step reactions according
to the literature.10 The hydroxyl group of 4 was protected by
the reaction with tert-butyldiphenylsilyl chloride in the
presence of imidazole to afford compound 5. Then, TTF
derivative 7 was obtained by the reaction of compound 6 in
the presence of triethylphosphate. Deprotection of compound
7 with tetra-n-butyl-ammonium fluoride resulted in the
targeted TTF-based monomer having two hydroxyl groups
(8). All the synthesized compounds were purified by repeat-
ed column chromatographies and/or recrystallizations, and
their chemical structures and purities were confirmed by
proton (1H) nuclear magnetic resonance (NMR, JNM-EX
400) in deuterated chloroform (CDCl3), as shown in Figure
1. Chemical structure of compound 2 was verified by the
Due to the several advantages of bulk heterojunction
(BHJ) OPVs, such as low cost and good processability, many
BHJ materials containing electron donating and/or accepting
moieties have been synthesized and characterized.6 Even
though the charge separation in the BHJ materials is success-
fully achieved by introducing the interpenetrated network
morphology, the total energy conversion efficiency of the
solar cell is fairly limited in the consequence of the poor
current collection which originates from the poorly organiz-
ed path for charge flows and the mismatches between the
absorption of the conjugated materials and the solar irradia-
tion.7
In this aspect, utilizing the organic materials with ex-
cellent processability and good mechanical, chemical and
thermal stabilities, the photo-efficiencies of solar cells can
be significantly improved by controlling the structure and
morphology of BHJ organic materials.8 However, in the
low molecular weight organic systems, macroscopic phase

1452 Bull. Korean Chem. Soc. 2012, Vol. 33, No. 5
Sehyun Lee et al.
Scheme 1. Reagents and conditions: (i) Br(CH2)2CN, acetone, reflux, overnight; (ii) CsOH·H2O, dimethylformamide, methanol, iodo-
methane, 20 oC → 60 oC; (iii) CsOH·H2O, methanol, then, Br(CH2)6OH, 20 oC, 12 h; (iv) t-BuPh2SiCl, imidazole, N,N-dimethylformamide,
20 oC; (v) Hg(OAc)2, 0.5 h; (vi) P(OEt)3, 130 oC, 3 h; (vii) TBAF in THF, tetrahydrofuran, 20 oC, 24 h.
proton chemical shift at -S-CH2- (2.8 ppm). Protons of the
methyl group exhibited a resonance signal (S-CH3) at 2.5
ppm. After the substitution reaction of 6-bromohexan-1-ol to
the compound 3, new peaks corresponding to the alkyl
protons of compound 4 appeared in the range of 1-2 ppm,
and the peak at 3.7 ppm was identified as the methylene
protons adjacent to hydroxyl group. Protection and deprotec-
tion reactions were evidenced by the chemical shifts of
aromatic protons appearing in the range of 7-8 ppm.
As schematically illustrated in Scheme 2, the reaction of
TTF-based dihydroxy monomer (8) with malonyl chloride
resulted in the 6TTF-polymer (9). The number average
molecular weight of 6TTF-polymer estimated by gel per-
meation chromatography (GPC) was 9,030 g/mol, which
corresponded to the average degree of polymerization (n) =
8. The chemical structure of 6TTF-polymer was also identi-
fied by 1H NMR, as represented in Figure 1(b), The
chemical shift at 12 ppm corresponding to the carboxylic
acid in the 6TTF-polymer indicated that the 6TTF-polymer
was terminated mainly with carboxylic acid groups probably
generated from the reaction of malonyl chloride with
residual water molecules. Therefore, it was suggested that
6TTF-polymer with a higher molecular weight could be
achieved if the water were completely removed from the
Scheme 2. Reagents and conditions: (i) malonyl chloride, triethyl-
amine, dichloromethane; (ii) DBU, tetrabromomethane, toluene,
70 °C, 40 h.

Synthesis and Characterization of a Tetrathiafulvalene-Based Polymer
Bull. Korean Chem. Soc. 2012, Vol. 33, No. 5 1453
To obtain 6TTF-C60 polymer (10) containing both electron
donor (TTF) and acceptor (C60) moieties in the polymer
chain, we attempted to introduce the fullerene moiety (C60)
to the 6TTF-polymer (9) main chain by the malonate-C60
coupling reaction. However, the resulting 6TTF-C60 polymer
was not soluble in any of organic solvents, and hence,
precipitated out during the polymer reaction (Scheme 2).
The poor solubility of 6TTF-C60 polymer should be originat-
ed from the chemical crosslinking at the C60 moiety. It was
reported that the reaction between C60 and malonate moiety
resulted in only 40% of mono-substituted product, and the
rest of C60 was substituted by more than two malonate
groups.11 Therefore, it is believed that a large number of C60
molecules have participated in the chemical crosslinking
during the polymer reaction of 6TTF-polymer. To avoid the
insolubility of 6TTF-C60 polymer, more sophisticated reac-
tion conditions should be taken into account, which will be
investigated further in the future.
To explore the electro-optical properties of the resulting
6TTF-polymer as a candidate material for OPV, electro-
chemical, optical and photoelectric properties were investi-
gated by utilizing cyclic voltammetry (CV), ultraviolet-
visible (UV-vis) spectroscopy and photoluminescence (PL)
spectroscopy, respectively. Electrochemical properties of the
6TTF-polymer were first studied by CV, and their results
were represented in Figure 3 and tabulated in Table 1.
Additionally, the CV data for TTF itself were also included
in Table 1 for the comparison.12-14 It was found that redox
1
Figure 1. (a) H NMR spectra of the synthesized compounds
from 2 to 9. (b) 1H NMR spectrum of the 6TTF-polymer.
Figure 3. Cyclic voltammogram of the 6TTF-polymer (0.01 mM
in CH2Cl2). Here, Bu4ClO4 (0.1 M) worked for a supporting
electrolyte. A glass carbon electrode, an Ag/AgCl electrode and a
platinum wire were also used as a working electrode, a reference
electrode and a counter electrode, respectively.
Figure 2. Photographic images of the free (a) and bent (b) 6TTF-
polymer films.
Table 1. Electrochemical potentials and energy levels of the 6TTF-
polymer
reaction system before and during the polymerization.
6TTF-polymer was soluble in common organic solvents,
such as toluene, hexane, dichloromethane and tetrahydro-
furan, which allowed us to fabricate the robust thin films by
a simple solution casting. As shown in Figure 2, the cast
a
b
E1a/V E2 /V λUV/nm Eg /eV HOMOc/eV
TTF
0.34
0.73
0.75
-
-
-
6TTF-polymer 0.47
381
2.27
4.79
6TTF-polymer films were bendable and ductile, indicating
that the molecular weight of synthesized 6TTF-polymer was
high enough.
b
aE1 and E2 stand for the half wave oxidation potentials. Eg means the
optical band gap energy. cEHOMO is defined as −(Eoxonset + 4.4 eV). Here
Eoxonset means the onset potential for the oxidation.14

1454 Bull. Korean Chem. Soc. 2012, Vol. 33, No. 5
Sehyun Lee et al.
the UV result and CV analysis, the HOMO level and band-
gap energy (Eg) of the 6TTF-polymer were estimated to be -
4.79 and 1.91 eV, respectively. Compared with the HOMO
levels of typical p-type organic semiconductors (4.9-5.5 eV),
the 6TTF-polymer showed slightly higher energy indicating
that the TTF moiety is more stable to air/light oxidation as
mentioned in the discussions of optical properties based on
the UV spectral results.15 Additionally, it is worth to note
that the 6TTF-polymer possesses a relatively low band gap
which is promising as a good electron donor for the OPV
applications.
As shown in Figure 4(b), PL spectra were obtained from
the 6TTF-polymer/PCBM blends with various weight ratios,
indicating that the 6TTF-polymer can be used as a good
electron donating polymer having a low band-gap for photo-
voltaic applications. For PL spectra of the 6TTF-polymer/
PCBM blends, dilute solutions were prepared in chloro-
benzene with 0.01 wt % concentration, and excited by
pumping a 560 nm wavelength laser which was determined
by UV-vis spectrum of 6TTF-polymer (Figure 4(a)). Further-
more, BHJ solar cell devices (ITO/PEDOT:PSS/6TTF-poly-
mer:PCBM/Al) were fabricated by using the 6TTF-polymer/
PCBM blends and the photo-conversion efficiencies were
compared with respect to the 6TTF-polymer/PCBM ratio.
Even though 6TTF-polymer was fairly well quenched by
PCBM, the photo-conversion efficiency of the BHJ solar
cell was low (0.01%). Therefore, to improve the photo-
conversion efficiency, the HOMO level of 6TTF-polymer
should be tuned by varying the conjugation length of TTF
moiety and the molecular packing structure, and the morpho-
logy of 6TTF-polymer/PCBM blends should be optimized,
which are under the investigations.
Figure 4. (a) UV-vis absorption spectrum of the 6TTF-polymer in
dichloromethane. (b) PL spectra of the 6TTF-polymer/PCBM/
chlorobenzene solutions at room temperature when they were
excited with the 560 nm pumping light.
properties of the 6TTF-polymer did not change significantly
from those of TTF.12 Since TTF-derivative salts keep the
high symmetric planar conformation and undergo oxidation
at mild potentials to generate a stable radical cation, TTF-
derivative salts can exhibit high electrical conductivity.12-14
Two reversible oxidation peaks appearing at 0.47 (cation
radical TTF+) and 0.75 V (dication TTF2+) for TTF itself
were detected at 0.62 and 0.90 V for the 6TTF-polymer,
respectively. The difference of electrochemical potentials
could be due to the combined effects of the electro-donating
effect from the disulfide groups and the molecular aggre-
gation.13 Slight increase in the oxidation potentials implied
that the TTF moiety becomes more stable to the air/light
oxidation reaction which is one of the main drawbacks of
TTF molecules for the practical application.14
To investigate optical properties of the 6TTF-polymer,
UV-vis absorption was also monitored in dichloromethane
(Figure 4(a)), and the results were tabulated in Table 1. The
6TTF-polymer exhibited two absorption bands between 270
and 350 nm and a wide absorption band in the range of 450-
550 nm. Absorptions between 270 and 350 nm corresponds
to the absorptions of the malonyl group in the 6TTF-
polymer, and a wide absorption in the range of 450-550 nm
is contributed by the TTF moiety in the polymer.15 Based on
Conclusion
A novel 6TTF-polymer containing TTF moiety in the
main chain was successfully synthesized via a condensation
polymerization between dihydroxy TTF derivative and mal-
onyl chloride, and its chemical structure was characterized
by spectroscopic techniques. The resulting 6TTF-polymer
was further functionalized by the reaction of the malonate
group in the polymer chain with C60 to obtain 6TTF-C60
polymer, in which an electron donor (TTF) and electron
acceptor (C60) were covalently bound in the polymer main
chain. While the 6TTF-polymer showed excellent solubility
in common organic solvents, the 6TTF-C60 polymer was
insoluble in any of common organic solvents due to the
chemical crosslinking caused by multiple substitutions at the
C60 moiety during the introduction of C60 onto 6TTF-
polymer. Number average molecular weight of the 6TTF-
polymer was estimated to be 9,030 g/mol by GPC, and the
resulting film was ductile to be bent. The electrochemical
and optical analyses of the 6TTF-polymer by CV, UV-vis
and PL spectroscopes revealed that the HOMO level
(EHOMO) and band-gap energy (Eg) of the 6TTF-polymer
were −4.79 and 1.91 eV, respectively. The high HOMO level
and low band-gap energy suggested that the 6TTF-polymer

Synthesis and Characterization of a Tetrathiafulvalene-Based Polymer
Bull. Korean Chem. Soc. 2012, Vol. 33, No. 5 1455
could act as a good electron donor for the OPV applications.
The BHJ solar cells fabricated with 6TTF-polymer/PCBM
blends, however, displayed relatively low photo-conversion
efficiencies.
rated. The residue was purified by a column chromato-
graphy with dichloromethane/hexane as an eluent to obtain
compound 7 as a red orange oil (2.05 g, 88%). H NMR
(CDCl3, δH): 7.67-7.62 (8H, m), 7.41-7.36 (12H, m), 3.63
(4H, t), 2.83 (4H, t), 2.46 (6H, s), 1.68 (4H, m), 1.56 (4H,
m), 1.36 (8H, m), and 1.05 (18H, s).
1
Experimental Section
4-(6-Hydroxyhexylthio)-2-(4-(6-hydroxyhexylthio)-5-
(methylthio)-1,3-dithiol-2-ylidene)-5-(methylthio)-1,3-di-
thiole (8). Tetrabutylammonium fluoride (1 M in THF, 11
mL, excess) was added to the solution of compound 7 (2.05
g, 1.97 mmol) under the argon atmosphere at room temper-
ature. The reaction mixture was stirred for 12 h. After the
evaporation of solvent, the residue was purified by column
chromatography with ethyl acetate as an eluent to afford
compound 8 (0.9 g, 81%). 1H NMR (CDCl3, δH): 3.63 (4H,
t), 2.87 (4H, t), 2.50 (6H, s), 1.66 (4H, m), 1.58 (4H, m), and
1.42 (8H, m).
6TTF-Polymer (9). Malonyl chloride (0.66 g, 4.7 mmol)
was added to the solution of compound 8 (2.47 g, 4.82
mmol), trietylamine (0.52 g, 5.1 mmol) and dichloro-
methane (100 mL). The reaction mixture was stirred at room
temperature for 20 h under the argon atmosphere. The
residue was precipitated in hexane, and the resulting solid
was dried in vacuum oven to obtain compound 9 (reddish
power, 1.5 g, 48%).
Materials. Column chromatography was performed on
Merck silica gel (70-230 mesh). All reagents were of com-
mercial qualities and used as supplied, otherwise we stated
the purification processes. The compound 3 was prepared in
our laboratory by the two-step reactions according to the
reference 10.
4-(6-Hydroxyhexylthio)-5-(methylthio)-1,3-dithiole-2-
thione (4). Under the nitrogen atmosphere at room temper-
ature, compound 3 (3.0 g, 9.8 mmol) and cesium hydroxide
monohydrate (3.8 g, 22 mmol) were dissolved in methanol
(50 mL). After stirring for 2 h, 6-bromohexan-1-ol (2.22
mL, 16.5 mmol) was added into the mixture, and the
solution was stirred for 12 h at room temperature. After the
evaporation of solvent, the residue was purified by column
chromatography by using ethyl acetate/methylene chloride
as an eluent to obtain compound 4 (yellowish oil, 3 g, 76%).
FT IR spectrum (KBr, cm−1): 1060 (C=S), and 3270 (OH);
1H-NMR (CDCl3, δH): 3.65 (2H, t), 2.87 (2H, t), 2.50 (3H,
s), 1.66 (2H, m), 1.58 (2H, m), and 1.42 (4H, m).
4-(6-(2,2-Dimethyl-1,1-diphenylpropoxy)hexylthio)-5-
(methylthio)-1,3-dithiole-2-thione (5). To the solution of
compound 4 (2.60 g, 8.33 mmol) in dimethylformamide (50
mL), the mixture of imidazole (6.0 g, 88 mmol) and tert-
butyldiphenylsilyl chloride (2.29 g, 8.33 mmol) was added
under the argon atmosphere at room temperature. The
solution was stirred at room temperature for 12 h. After the
evaporation of solvent, the residue was dissolved in di-
chloromethane, washed with water, and dried with magne-
sium sulfate. Column chromatographic purification with
dichloromethane afforded compound 5 as an orange oil (4.0
g, 87%). 1H NMR (CDCl3, δH): 7.68-7.64 (4H, m), 7.42-7.38
(6H, m), 3.65 (2H, t), 2.83 (2H, t), 2.47 (3H, s), 1.68 (2H,
m), 1.56 (2H, m), 1.36 (4H, m), and 1.05 (9H, s).
6TTF-C60 Polymer (10). Under the argon atmosphere,
C60 fullerene (0.11 g, 0.152 mmol) was added to a mixture of
6TTF-polymer 9 (0.055 g), tetrabromomethane (0.26 g, 0.76
mmol) and 1,8-diazabicyclo[5,4,0]-undec-7-ene (DBU, 23
μL, 1.52 mmol) in toluene (100 mL). The mixture was
stirred at 70 °C for 40 h. During the polymer reaction, 6TTF-
C60 polymer (10) was precipitated from the mixture as a dark
brown power (0.05 g, 46%). The obtained compound 10 was
washed with methanol several times and dried in a vacuum
oven. The dark brown powder was not soluble in any of
common organic solvents.
1
Measurements. H NMR spectra were recorded with a
Jeol JNM-EX 400 spectrometer using CDCl3. Here, Me4Si
was used as the internal standard. Gel permeation chromato-
graphy (GPC) was performed on a Shimadzu Prominence
system equipped with a UV detector using CHCl3 as the
eluent at 40 °C. The sample solutions were filtered with a
PTFE filter (pore size: 0.2 μm) before injection. UV-vis ab-
sorption spectra were measured in dichloromethane with a
Hewlett-Peakard 8452A diod array spectrophotometer.
Photoluminescence spectra were obtained using a Perkin-
Elmer LS55 luminescence spectrometer. Cyclic voltam-
metry (CV) experiment was performed utilizing the Autolab
PGSTAT 30 workstation. The electrolyte solution (0.1 M)
was prepared from tetrabutylammonium perchlorate (Bu4ClO4).
Platinum wire (2 mm in a diameter) was employed as the
counter electrode. Here, an aqueous Ag/AgCl electrode was
used as the reference and ferrocene (Fc) was added as an
internal reference. A glassy carbon electrode was also used
as the working electrode. The HOMO level (EHOMO) of the
4-(6-(2,2-Dimethyl-1,1-diphenylpropoxy)hexylthio)-5-
(methylthio)-1,3-dithiol-2-one (6). Mercuric acetate (4.54
g, 14.3 mmol) was added to the solution of compound 5 (3.5
g, 6.54 mmol) in dichloromethane (50 mL). After stirring for
0.5 h at room temperature, the reaction mixture was filtered,
and the solvent was evaporated. Column chromatographic
purification of the residue with dichloromethane provided
1
compound 6 as a pale green oil (3.0 g, 88%). H NMR
(CDCl3, δH): 7.67-7.64 (4H, m), 7.42-7.38 (6H, m), 3.65
(2H, t), 2.83 (2H, t), 2.47 (3H, s), 1.68 (2H, m), 1.56 (2H,
m), 1.36 (4H, m), and 1.05 (9H, s).
4-(6-(tert-Butyldiphenylsilyloxy)hexylthio)-2-(4-(6-(tert-
butyldiphenylsilyloxy)hexylthio)-5-(methylthio)-1,3-dithiol-
2-ylidene)-5-(methylthio)-1,3-dithiole (7). The solution of
compound 6 (2.5 g, 4.67 mmol) in triethylphosphate (20
mL) was stirred at 130 °C for 5 h. After gradually cooling
the mixture to room temperature, the solvent was evapo-
6TTF-polymer was obtained from the equation, EHOMO
=
− (Eoxonset + 4.4 eV), where Eoxonset are the onset potentials for

1456 Bull. Korean Chem. Soc. 2012, Vol. 33, No. 5
Sehyun Lee et al.
the oxidation in CV.14 The band-gap energy (Eg) of the
6TTF-polymer was calculated from the equation, Eg = hc/
λ0.1max, where h, c and λ0.1max stand for the Planck constant,
the speed of light and the wavelength (the absorption
coefficient drops to 10% of the peak value in UV spectrum),
respectively.14 For the measurement of the photo-conversion
efficiency, PEDOT:PSS (Baytron PH 500) was first spin-
coated to get the thin film (thickness = 30 nm) on the ITO
surface. The film was baked at 120 °C for 20 min in a nitro-
gen-filled glovebox. Then, 6TTF-polymer/PCBM composites
layer (ca. 100 nm thick) was applied on the top of PEDOT:
PSS film. Finally, as an electrode Al layer was deposited
with the thickness of 80 nm. The active area of all devices
was 6 mm2. All current-voltage (I-V) characteristics of the
devices were measured utilizing Keithley 2400 sourcemeter
under the AM 1.5G-filtered irradiation (100 mW cm−2).
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