Yamabe, Qin, Goto

Futurum
Tsukuba Science Journal

Conductive Polymer

Electrochemical preparation of poly(1,3-dinaphthyl isothianaphthene)
Kohei Yamabe, Qin Zhiyong, Hiromasa Goto*
Division of Materials Science, Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba Ibaraki 305-8573, Japan*Correspondence: gotoh@ims.tsukuba.ac.jp

ABSTRACT
Isothianaphthene is one of the promising monomer units of conjugated polymers because of isothianaphthene
based polymers show small bandgap. In this research, isothianaphthene based new monomer unit wassynthesized. Theoretical discussion of the monomer unit with density functional theory was carried out.
Electrochemical polymerization of the monomer yielded electroactive polymer thin films. UV-vis opticalabsorption spectra and electron spin resonance evaluated optical bandgap and magnetic property of theresultant piolymer. Bandgap of the polymer was to be 1.58 eV.
Keywords: bandgap, isothianaphthene, electrosynthesis.

prepared with previous reported method [14].

Introduction
Recently,
researchesonlow-bandgap
polymers and applications have been carried out[1,2]. Thiophene based conjugated polymers havebeen received much attention for applications
possibility. Polythiophene have bandgap to be
2.0-2.2 eV [3]. Conjugated polymer materials basedorganic electric luminescence devices (OEL) emit
light in visible range.
Polyisothianaphthene (Figure 1) is a unique
conjugated polymer because the dedoped form has
small bandgap of 1.0 eV [4,5], and the doped formshows doping band at infrared region, resulting
colorless. Film forming property and stability
against oxidation of oxygen in the air are important
issue to overcome for applications [6,7].
In this study, new monomer based onisothianaphthene is synthesized, and polymerizedwith electrochemical method [8-10]. Sandwich cellelectropolymerization method which we developed
allows liquid crystal electrochemical polymerizationand liquid crystal magnetic electrochemical
polymerization [11-13]. Isothianaphthene was

Figure1.
Chemicalpolyisothianaphthene.

structureofExperimental
Material
Tetrabutylammonium perchlorate (TBAP),
1-bromonaphthol, hydrazine anhydrous,
tetrakis(triphenylphosphine)palladium(0),propylene carbonate, trimethylstannyl chloride andN,N,N',N'-tetramethylethylenediamine werepurchased from Tokyo Chemical Industry, Toyo,Japan, and used as received.
Ethanol, dichloromethane, hexane, ethyl acetate,tetrahydrofuran, n-butyllithium in hexane anddiethyl ether were purchased from Nacalai Tesque,
Kyoto, Japan, and used as received. Toluene wasLow-bandgap copolymer
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Futurum - Tsukuba Science Journal
was added slowly to the mixture at −78 °C. themixture was stirred vigorously at room temperaturefor 1 h. The mixture was cooled to −78 °C again,
and a solution of trimethylstannyl chloride (1.15 g,5.77 mmol) in THF (6.6 mL) was added to themixture. The mixture was slowly warmed up toroom temperature and stirred for 12 h. The mixture
was poured into a saturated sodium hydrogencarbonate aqueous solution and separated. The
aqueous layer was extracted with ether twice and thecombined organic layer was dried over magnesium
sulfate. After evaporation of the solvent, recrystallization from petroleum ether (30–70 °C) gave a paleyellow solid (0.667 g, 1.45 mmol, yield = 14.6 %).
1H-NMR (TMS (ppm), CDCl3): δ 0.49 (s, 18H,–CH3), 7.07 (dd, 2H, 5,6-H), 7.67 (dd, 2H, 4,7-H).

purchased from Nacalai tesque, Kyoto, Japan, andpurified by distillation.
1,2-Bischloromethylbenzene and sodium periodate
were purchased from Aldrich, United States, andused as received. Sodium sulfide nonahydrate,
magnesium sulfate, sodium hydrogen carbonate andaluminum oxide powder were purchased from Wako
Chemical, Tokyo, Japan, and used as received.
Monomer synthesis
1,3-Dihydro-benzo[c]thiophene (1).
Sodium sulfide nonahydride (26.14 g, 109mmol) was added in ethanol (250 mL)/distilledwater (50 mL) and dissolved with stirring.
1,2-Bischloromethylbenzene (12.31 g, 70.3 mmol)
was added through a soxhlet extractor and themixture was refluxed at 110 °C. After evaporation of
the solvent, the residual tan-colored oil wasextracted with dichloromethane and water. The
organic layer was dried over magnesium
sulfate. Evaporation of the solvent in vacuo gave ayellow solid (8.55 g, 62.7 mmol, yield = 89.7 %).
1H-NMR (TMS (ppm), CDCl3): δ 4.27 (s, 4H,–CH2–), 7.18–7.27 (m, 4H, 3,4,5,6-H).

1,3-Dinaphthyl isothianaphthene (5, DN-ITN)Compound 4 (115.0 mg, 0.25 mmol),
1-bromonaphthol (231.3 mg, 1.12 mmol) andtetrakis(triphenylphosphine)
palladium(0)(Pd(PPh3)4) (0.023 g) were added to toluene (2 mL),and the mixture was refluxed at 90 °C under an argon.
After 24 h, the solution was extractedwith dichloromethane and water. The organic layer
was dried over magnesium sulfate. Evaporation of
the solvent in vacuo afforded a crude product. The
crude product was purified by silica gel column
chromatography (eluent: hexane/ethyl acetate = 1/1)
to afford a reddish brown solid (23.4 mg, 0.061mmol, yield = 24.2 %). 1H-NMR (TMS (ppm),CDCl3): δ 7.17 (t, 2H), 7.25 (t, 2H), 7.33 (t, 2H),
7.53–7.60 (m, 4H), 7.76–7.83 (m, 4H), 7.93 (dd,2H), 8.22 (dd, 2H).

1,3-Dihydro-benzo[c]thiophene 2-oxide (2).
Sodium periodate (13.7 g, 64.0 mmol) was
dissolved in distilled water (177 mL). Compound 1(8.55 g, 62.7 mmol) was dissolved in ethanol (213mL). The ethanol solution was dropping into watersolution at 0 °C with vigorous stirring. After stirringfor 3 h at room temperature, a white inorganic salt inthe solution was filtered off. After evaporation of
the solvent, the residual oil was extractedwith dichloromethane and water. The organic layer
was dried over magnesium sulfate. Recrystallization
with hexane and diethyl ether afforded colorless
crystal (4.23 g, 28.2 mmol, yield =45.0 %). 1H-NMR (TMS (ppm), CDCl3): δ 4.16 (d,2H, C–H), 4.30 (d, 2H, C–H), 7.31–7.34 (m, 2H,4,5-H), 7.38 (dd, 2H, 3,6-H).

ElectropolymerizationDN-ITN as a monomer and TBAP as asupporting salt were dissolved in propylene
carbonate. The electrolyte solution was injected into
sandwiched cell consisting with indium tin oxide
glass electrode and Teflon spacer [12]. Direct
current of 4.0 V was applied to the electrolyte for 10min. After electropolymerization, the cell wasdisassembled and washed with THF and hexane.
The polymer film poly(DN-ITN) has been preparedon ITO glass.

1,3-Di(trimethylstannyl)isothianaphthene (3).
Compound 2 (1.51 g, 9.90 mmol) andaluminum oxide powder (2.41 g, 23.6 mmol) weremixed and finely crushed in a mortar. Sublimation ofthe mixture under reduced pressure and gradually
heating at 110 °C gave isothianaphthene (4) as awhite solid (0.302 g, 2.25 mmol). Freshly
sublimated compound 4 (0.302g, 2.25 mmol) and
N,N,N',N'-tetramethylethylenediamine (0.660 g, 5.68mmol) were dissolved in THF (6.6 mL) under anargon atmosphere.
15 % n-butyllithium in hexane (3.4 mL, 5.70 mmol)

Result and discussion
DFT calculation
Electronic and conformational details of theDN-ITN were obtained with density functionaltheory (DFT) calculations. Electric charge
distribution model is shown Figure 2(a). This result
indicates propagation positions of the monomer in

Low-bandgap copolymer
35

Yamabe, Qin, Goto
the electropolymerization. Possible polymer
structure according to the charge distribution wasshown in Figure 2(b). Figure 2(c,d) revealed highest

occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) models of
the DN-ITN monomer from the DFT calculation.

(a)

(b)

(c)

(d)

Figure 2. Density functional theory (DFT) calculations of DN-ITN: (a) electronic and conformational model,(b) possible polymer structure, (c) HOMO, (d) LUMO.
UV-vis absorption spectra
Optical bandgap can be obtained by UV-vis
absorption spectra [4]. UV-vis optical absorption
spectra of poly(DN-ITN) were measured, Figure 3.
As prepared polymer film was oxidized in theelectro-chemical process. The broad absorption
bands derived from polarons and bipolarons in theoxidized polymer were observed at >900 nm.
Reduced state poly(DN-ITN) film was obtained bytreatment with hydrazine vapor. The reduced statefilm shows no absorption band at long wavelengths
because the polarons and bipolarons bands weredisappeared by the hydrazine reduction. Both theoxidized and the reduced forms show * transitionof the main chain. The polymer shows characteristic
absorption bands at 400 nm and 450 nm. Bandgap
energy of polymer can be calculated from band edgeof the * transition of the main chain in thereduced state [4]. Figure 4 shows absorbance as afunction of photon energy (eV), indicating bandgapof the polymer is to be 1.58 eV. This bandgap value
can be improved compared with 2.0 eV ofpolythiophenes reported in the previous research[15].

Absorbance (Normalized)

Oxidized (doped) film
Neutral (dedoped) film
Bipolarons bandPolarons band

300

400

500600700
Wavelength/nm

800900Figure 3. UV-vis
poly(DN-ITN).

absorption

spectraof

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Futurum - Tsukuba Science Journal

Absorbance (Normalized)

Conclusions
Isothianaphthene combined with naphthyl unit
was synthesized. DN-ITN as a monomer waspolymerized with electrochemical method. Resultantpolymer thin film was obtained on ITO glass.
Optical absorption and electron spin resonancemeasurements reveal the oxidation (doping) process.
Bandgap energy of poly (DN-ITN) in the neutral
state is to be 1.58 eV.
Acknowledgments
This research was supported by ChemicalAnalysis Division Research Facility Centre for
Science and Technology of University of Tsukuba.
1.5
Photon energy (eV)

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Electron spin resonance
Magnetic property of poly(DN-ITN) is
obtained with electron spin resonance (ESR)
spectroscopy measurement. The ESR observes
unpaired electron as a radical of polarons.
Reduced state of poly(DN-ITN) was peeled
off from the ITO glass and set in the ESR tube. The
samples were measured every 5 min after iodine
addiion to carrie out in-situ ESR measurements withiodine vapor doping. No ESR signal was observed at
0 min because of undoped, indicating the polymer isconmpletely dedoped by the hydrazine vapor before
the iodine doping, Figure 5. After the doping (> 5min), unpaired electron density was rapidly
increased due to progress of the doping forpoly(DN-ITN), which generated polarons as radicalcations in the main chain. Subsequently, the ESR
intensity was slowly decreasing. This is because
occurance of transition of polarons to bipolarons(dications) by the heavy doping.
[1016]

Polarons

5Doping
Porarons

Bipolarons

Spin/mg4321005
10Doping time (min)15

Figure 5. In-situ electron spin resonance (ESR)
spectroscopy measurements of the polymer withiodine vapor doping.

Low-bandgap copolymer
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Yamabe, Qin, Goto
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Received: 6 January, 2018.
Accepted: 12 January, 2018.
*E-mail: gotoh@ims.tsukuba.ac.jp© University of Tsukuba Library

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