Synthesis and electropolymerization of 5,12-dihydrothieno[3′,4′:2,3][1,4]dioxocino[6,7-b]quinoxaline and its electrochromic properties

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Synthesis and electropolymerization of

5,12-dihydrothieno[3

0

,4

0

:2,3][1,4]dioxocino[6,7-b]-quinoxaline and its electrochromic properties

Mine Sulak Ak

a

, Metin Ak

b

, Mustafa Gu¨llu¨

a

, Levent Toppare

b,*

a

Ankara University, Department of Chemistry, 06100 Ankara, Turkey b

Middle East Technical University, Department of Chemistry, 06531 Ankara, Turkey Received 12 February 2007; accepted 21 May 2007

Available online 2 June 2007

Abstract

Synthesis of a new thiophene-based monomer; 5,12-dihydrothieno[30_,40_{:2,3][1,4]dioxocino[6,7-b]quinoxaline (DDQ),}

was realized. The chemical structure of the monomer was characterized by1H NMR, FTIR and mass spectroscopy tech-niques. Electrochemical polymerization of DDQ and characterization of the resulting polymer [P(DDQ)] was performed. Moreover, the spectroelectrochemical and electrochromic properties of the polymer ﬁlm were investigated. P(DDQ) has a low oxidation potential (0.9 V) and low band gap (1.73 eV) compared to polythiophene. In addition, dual-type polymer electrochromic device (ECD) based on P(DDQ) with poly(3,4-ethylenedioxythiophene) (PEDOT) was constructed. Spec-troelectrochemistry, electrochromic switching, stability and open-circuit stability of the device were studied. It was observed that polymer have good switching time, reasonable contrast and optical memory.

Keywords: Conducting polymers; Dioxocino quinoxalines; Electrochemical polymerization; Electrochromic devices; Electrochromism; Spectroelectrochemistry

1. Introduction

Thiophene is one of the most studied heterocyles: it is easy to process, chemically stable, and its syn-thetic applications have been a constant matter of investigation for the last several decades. The

inter-est in this heterocycle has spread from dye chemistry

[1]to modern drug design [2], electronic and opto-electronic devices [3,4], biodiagnostics [5]and con-ductivity based sensory devices [6,7].

Electrochromism is the reversible change in opti-cal property which can occur when the electrochro-mic material is electrocheelectrochro-mically oxidized or reduced, and is ﬁrstly found in the transition-metal oxide ﬁlms such as tungsten oxide [8]and iridium oxide [9]. Commonly, the electrochromic material can display distinct visible color change which is between a transparent (bleached) state and a colored

* _{Corresponding author. Tel.: +90 3122103251; fax: +90} 3122103200.

E-mail addresses:minsu@pau.edu.tr (M.S. Ak), metinak@ pau.edu.tr (M. Ak), Mustafa.Gullu@science.ankara.edu.tr (M. Gu¨llu¨),toppare@metu.edu.tr(L. Toppare).

European Polymer Journal 43 (2007) 3452–3460

www.elsevier.com/locate/europolj

POLYMER

JOURNAL

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state, or between two colored states. This deﬁnition can be extended to more extensive spectral range. Electrochromism has been applied to the ultrafast color display [10] and many other commercial applications.

Conducting polymers have gained a considerable attention since they oﬀer superior properties due to their relative ease in molecular engineering to achieve desired colors for electrochromic systems with respect to inorganic materials[11]. In addition to these, independence to the angle of vision, good UV stability, high temperature range of operation and low cost make them suitable candidates for the electrochromic devices.

A major focus in the study of electrochromic polymeric materials is to control the stability and the color of the material by modiﬁcation of the main-chain and pendant group structures. Polythi-ophene (PTh) shows good conductivity and high stability in its oxidized form but suﬀers from struc-tural defects arising from b couplings, resulting in conjugation breaks. Blocking of the 3- and 4-posi-tions, forces polymerization through the 2- and 5- positions, leading a linear polymer chain with no a–b coupling with fewer structural defects than most heterocyclics[12].

The result yields in changes in the polymer’s absorption characteristics and ultimately its color in its neutral form. During oxidation to form charge carriers, new electronic states are introduced into the polymers which exhibit absorptions at lower energies with a concomitant loss of absorption due to the neutral polymer. As such, these materials exhibit strong electrochromism during redox switch-ing[13].

In this work, we report the synthesis and charac-terization of a new thiophene monomer: 5,12-dihy-drothieno[30_,40_{:2,3][1,4]dioxocino[6,7-b]quinoxaline}

(DDQ). Electrochemical polymerization of DDQ was performed using TBAFB as the supporting elec-trolyte. The resultant products were characterized via the cyclic voltammetry (CV), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and conductivity measure-ments. Second part of the study was devoted to investigate the interesting property of conducting polymers, the ability to switch reversibly between the two states of diﬀerent optical properties, ‘‘elec-trochromism’’. In recent years there has been a growing interest in application of conducting poly-mers in electrochromic devices. Thus, for this reason we investigated the optoelectrochemistry and

mea-sured the color coordinates of the polymer coated on ITO via constant potential electrolysis. We also successfully established and characterized the utili-zation of dual-type complementary colored polymer electrochromic devices using P(DDQ)/poly(3,4-ethylenedioxythiophene) (PEDOT) in sandwich conﬁguration.

2. Experimental 2.1. Materials

Chloroform (Merck), dichloromethane (DCM) (Merck), methanol (Merck), acetonitrile (ACN) (Merck), acetone (Merck), HCl (Merck), NaOH (Merck), acetic acid (Merck), potassium hydroxide (Merck), N,N-dimethylformamide (DMF) (Merck), toluene (Sigma), dry ether (Merck), pro-pylene carbonate (PC) (Aldrich) and poly(methyl metacrylate) (PMMA) (Aldrich) were used without further purification. Tetrabutylammonium tetra-fluoroborate (TBAFB) (Aldrich), tetrabutylammo-nium hexafluorophosphate (TBAFP) (Aldrich), diethyl oxalate (Aldrich), phenylenediamine (Merck), Na2S (Aldrich), ethyl chloroacetate (Aldrich),

sodium bicarbonate (Aldrich), potassium carbonate (Fluka) and 3,4-ethylenedioxythiophene (EDOT) (Aldrich) were used as received.

2.2. Equipments

Voltalab PST50 potentiostat was used for the cyclic voltammetry studies. NMR spectra of the monomers were recorded on a Bruker-Instrument-NMR Spectrometer (DPX-400) using CDCl3 as

the solvent and tetramethylsilane as the internal standard relative to which the chemical shifts (d) are given. The FTIR spectrum was recorded on a Nicolet 510 FTIR spectrometer. Electrical conduc-tivity of the polymer materials was measured at room temperature by using four probe technique with a home made instrument. The surface morphologies of the polymer ﬁlm was analyzed by using JEOL JSM-6400 scanning electron micro-scope. Solatron 1285 potentiostat/galvanostat was used to supply a constant potential during electro-chemical synthesis. Agilent 8453 UV–vis spectro-photometer was used in order to perform the spectroelectrochemical studies of copolymers and the characterization of the devices. Colorimetry measurements were done via Minolta CS-100 spec-trophotometer.

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2.3. Synthesis of 5,12-dihydrothieno[30_,40

:2,3]-[1,4]dioxocino[6,7-b]quinoxaline (DTQ) 2.3.1. Diethyl 2,20_{-thiodiacetate}_[14]

To 248 g (2.02 mol) of ethyl chloroacetate in 1250 ml of acetone, 311 g (1.29 mol) of sodium sul-ﬁde nonahydrate were added slowly with stirring. Heat was applied to initiate the reaction and was discontinued when the reaction started. The reac-tion was allowed to proceed for three hours with stirring at which time it had subsided and cooled. After removal of the sodium chloride by ﬁltration, the acetone layer was separated and the aqueous layer extracted with ether. After drying the com-bined acetone and ether layers, and removal of the acetone and ether by distillation under reduced pressure, the product was distilled further under reduced pressure. The product was a colorless, oily, unpleasant smelling liquid 57% (119.5 g).

2.3.2. Diethyl 3,4-dihydroxythiophene-2,5-dicarboxylate

This compound was prepared according to the procedure of Hinsberg [15]. Using 3.5 g (0.153 mol) sodium in 35 ml of alcohol, 14.6 g (0.1 mol) ethyl oxalate and 10.3 g (0.05 mol) ethyl thiodiacetate, the yield was 10.2 g. Recrystallization from benzene or methanol gave the product: m.p. 134.5C (78.5%).

2.3.3. Synthesis of 2,3-bis(bromomethyl) quinoxaline Phenylenediamine (3 g, 0.022 mol) and 1,4-dib-romo-2,3-butanedione (6.4 g, 0.022 mol) were dis-solved separately in ethanol and mixed, then stirred for 1 h at 0C. After cooling the solution in deep freeze, crystalline 2,3-bis(bromomethyl)qui-noxaline (5.8 g) was obtained in 93% yield.

A mixture of 2,3-bis(bromomethyl)quinoxaline (1.5 g, 4.74 mmol) and K2CO3 (1.9 g, 13.76 mmol)

were added into diethyl 3,4-dihydroxythiophene-2,5-dicarboxylate (1.2 g, 4.61 mmol) solution in 20 ml DMF. The mixture was heated at 120C by vigorously stirring for 2 h, and then cooled solution was poured into cold water. The light brown solid was ﬁltered and dried. The yield was 85% (1.62 g).

Resultant cyclic product (0.5 g) was heated for one night with 5 ml 2.5 N NaOH solution in etha-nol, into which 3–5 drops of water were added. The excess alcohol was distilled. The resulting oily mixture was dissolved in water and washed with chloroform. The ice cooled water phase was acidi-ﬁed with concentrated HCl. The precipitated gray

solids were ﬁltered, dried and the dicarboxylic acid derivative was added into boiling quinoline (2 ml) and copper dust (0.05 g). The mixture was poured into cold water and extracted with dichlorometh-ane. The organic phase was dried and the solvent was removed. Crystalline solid with 25% yield (0.13 g) was obtained. Route for synthesis of the monomer is given inScheme 1a–c.

1

H NMR (CDCl3) = (d, ppm): 8.2 (dd, benzene),

7.8 (dd, benzene), 5.6 (s, CH2–O), 6.65 (s,

thio-phene), 7.2 (s, CDCl3) (Fig. 1).

2.4. Cyclic voltammetry (CV)

CV was used in order to investigate the electroac-tivity of the polymers and to obtain the oxidation– reduction peak potentials of the monomers. The system consists of a potentiostat, a cell containing ITO coated glass working, platinum wire counter and Ag/Ag+ reference electrodes. The measure-ments were carried out at room temperature under nitrogen atmosphere. The oxidation/reduction behavior of DDQ was investigated by using 0.1 M TBAFB/DCM:AN (1:4) solvent–electrolyte couple. 2.5. Electrochemical polymerization of DDQ

Electrochemical polymerization of DDQ via con-stant potential electrolysis was performed in a single compartment cell, in the presence of 50 mg DDQ, 0.1 M TBAFB in DCM/AN (1:4) with the applica-tion of 1.5 V, equipped with Pt working and counter electrodes and a Ag/Ag+ reference electrode. Lim-ited solubility of the monomer in AN was enhanced by adding DCM in electrolysis cell. After electroly-sis, the free standing ﬁlm was washed with AN sev-eral times to remove unreacted monomer. Similar procedure was applied for the synthesis of P(DDQ) on ITO by using a UV-cuvette as a single compartment cell. The electrochromic measure-ments; spectroelectrochemistry, and switching stud-ies of the polymer ﬁlm deposited on ITO coated glass were carried out in same media in the absence of monomer.

2.6. Preparation of the gel electrolyte

Gel electrolyte was prepared by using TBAF-B:ACN:PMMA:PC in the ratio of 3:70:7:20 by weight. After TBAFB was dissolved in ACN, PMMA was added into the solution. In order to

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dis-solve PMMA, vigorous stirring and heating was required. PC, as a plasticizer, was introduced to the reaction medium when all of the PMMA was

completely dissolved. The mixture was stirred and heated until the highly conducting transparent gel was produced[16]. Fig. 1. NMR spectrum of DDQ. COOC2H5 COOC2H5 _C 2H5OOCH2C S C2H5OOCH2C 1. NaOC2H5 2. H+ + S HO OH C C O O OC2H5 C2H5O NH2 NH2 Br Br O O + N N Br Br EtOH 0oC Phenylenediamine 1,4-dibrom-2,3-butanedione

diethyl oxalate diethyl 2,2'-thiodiacetate

diethyl 3,4-dihydroxythiophene-2,5-dicarboxylate 2,3-bis(bromomethyl)quinoxaline 1 2 1 + 2 N N O O S C C O O OC2H5 C2H5O N N O O S K2CO3, DMF 120 o_{C, 1h} 1) hydrolysis 2) decarboxylation

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2.7. Construction of electrochromic devices

P(DDQ) was utilized as the anodically, and PEDOT as the cathodically coloring electrochromic materials. P(DDQ) was potentiostatically deposited on ITO in 0.1 M TBAFB/DCM:AN (1:4 v/v) at +1.5 V. 0.01 M solution of EDOT in 0.1 M TBAFB/AN was used to deposit the PEDOT ﬁlm onto ITO electrode at +1.5 V vs. Ag/Ag+. It is important to balance the charge capacities of the devices prior to assembling the devices. Otherwise, there would be incomplete electrochromic reaction and residual charges would remain during the redox process[17]. Therefore, redox charges of the anodi-cally and cathodianodi-cally coloring polymers were matched by chronocoulometry. In order to obtain the complementary operating conditions, anodically coloring polymers were fully reduced and the cathodically coloring polymer was fully oxidized. By sandwiching the gel electrolyte between the anodically and the cathodically coloring polymers, the device was constructed.

3. Results and discussion 3.1. Cyclic voltammetry

Cyclic voltammogram of DDQ indicated an oxidation peak at 0.60 V and a reduction peak at 0.30 V. When the range between 0.30 V and +1.8 V (Fig. 2) was scanned, electroactivity increased with increasing scan number. This process promotes an electrochromic change of the ﬁlm into a plum-gray color, while a gray cloud is formed around the electrode.

3.2. FTIR spectra

In FTIR spectrum of the DDQ monomer (Fig. 3a) the following absorption bands arise: 3086 cm1 (aromatic C–H), 2924 cm1 (aliphatic C–H), 1558 cm1 (C@N stretching), 1473– 1365 cm1 (aromatic C@C stretching), 1134, 1017 (C–H in plane bending of benzene and thio-phene), 763 cm1 (C–Ha out of plane bending of

thiophene).

FTIR spectra of electrochemically synthesized P(DDQ) showed the presence of the characteristic peaks of the monomer (Fig. 3b). The new broad band at around 1640 cm1was due to polyconjuga-tion. The strong absorption peak at 1084 cm1was attributed to the incorporation BF₄ ions into the polymer ﬁlm during doping process.

Fig. 2. Cyclic voltammogram of DDQ.

Fig. 3. FTIR spectra of the (a) DDQ (b) electrochemically prepared P(DDQ).

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3.3. Conductivity measurements

Electrical conductivity measurement was carried out by using four-point probe technique. The con-ductivity of P(DDQ) ﬁlm was measured as 2· 102S/cm.

3.4. Morphology of ﬁlm

The surface morphology of electrochemically synthesized P(DDQ) was investigated by Scanning Electron Microscope. The solution side of the P(DDQ) ﬁlm revealed globular morphology (Fig. 4) that completely diﬀerent than polythioph-ene surface morphology[18].

3.5. Spectroelectrochemistry of P(DDQ)

Spectroelectrochemisty experiments reveal key properties of conjugated polymers such as band gap (Eg) and the intergap states that appear upon

doping. Spectroelectrochemical analysis of the P(DDQ) ﬁlm was studied in order to elucidate elec-tronic transitions upon doping of the polymer (Fig. 5). The ﬁlm was deposited on ITO via poten-tiostatic electrochemical polymerization of DDQ (0.1 M) in the TBAFB/DM:AN (1:4) at +1.5 V. P(DDQ) coated ITO glass electrodes was investi-gated by UV–vis spectroscopy in the same but monomer free electrolytic system via switching

between 1.4 and +1.4 V with incremental increases in applied potential. Upon applied volt-age, decrease in the intensity of the p–p*_transition

(556 nm) and formation of charge carrier bands were observed. There was a gradual decrease in the peak intensity at around 556 nm upon increase in the applied potential, which was accompanied by the increase in the intensity of peaks at around 950 nm due to the formation of polaron bands at interval potentials. The onset energy for the p–p*

transition (electronic band gap) was found to be 1.73 eV.

3.6. Electrochromic switching of P(DDQ)

The spectroelectrochemistry experiments showed the ability of P(DDQ) to switch between its neutral and doped states with a change in transmittance at a ﬁxed wavelength. A square-wave potential step method coupled with optical spectroscopy, known as chronoabsorptometry was used to probe switch-ing times and contrast in these polymers. In this double potential step experiment, the potential was set at an initial potential for a set period of time (5 s) and was stepped to a second potential for another set period of time (5 s), before being switched back to the initial potential again. The polymer ﬁlms are synthesized on ITO-coated glass slides using constant potential. The transmittance, %T, was then monitored at kmax of the polymer

while the polymer was switched from 1.4 to +1.4 V. For P(DDQ) maximum contrast was

Fig. 4. Surface morphology of electrochemically synthesized P(DDQ).

Fig. 5. Spectroelectrochemistry of the P(DDQ) ﬁlm in 0.1 M TBAFB/AN.

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measured at 556 nm, and found as 20% with a switching time of 1.1 s (Fig. 6).

3.7. Colorimetry

According to CIE system, there are three attri-butes to describe the color; luminance L, hue a, and saturation b. Luminance of material is the transmittance of light through a sample as seen by the human eye. Hue represents the wavelength of maximum contrast. Saturation is the intensity of a certain color. Color changes and L, a, b values were given inTable 1. These values were measured at the fully oxidized and reduced states of P(DDQ). 3.8. Spectroelectrochemistry of ECDs

In this study, we successfully constructed a dual-type complementary colored polymer electro-chromic device (ECD), using P(DDQ)/PEDOT in sandwich conﬁguration with a gel electrolyte consist-ing of low molecular weight PMMA and TBAFB. Spectroelectrochemical studies were carried out to examine the optical properties of the ECD that occur

upon doping or dedoping.Fig. 7shows the spectro-electrochemical data of the P(DDQ)/PEDOT device at voltages varying between1.5 and +1.7 V. When a negative voltage applied to the PEDOT layer, the polymer was in its neutral state with a p–p*

transi-tion kmax at around 605 nm which was responsible

for the blue color. At this voltage, P(DDQ) was in oxidized state (blue) and device reveals the blue color. When a positive voltage was applied to the PEDOT layer, the polymer ﬁlm was oxidized and it exhibited transmissive sky blue color. At this stage, P(DDQ) layer had an absorptive plum color with the a p–p*_{transition at 567 nm. The relative luminance}

L and the a, b values of the devices (Table 1) were measured at the1.5 and +1.7 V.

3.9. Switching of ECDs

One of the most important characteristics of ECDs is the response time needed to perform switching between the two colored states. For this purpose we used chronoabsorptometry by stepping the potential between 1.5 and 1.7 V with a resi-dence time of 5 s. During the experiment, the % transmittance (%T) at the wavelength of maximum contrast was measured by a UV–vis spectrophoto-meter at 605 nm. For P(DDQ)/PEDOT device maxi-mum contrast was obtained at potentials between 1.5 and 1.7. Hence, optical contrast of the P(DDQ)/PEDOT device was measured at these potentials, and found as 15% with a switching time of 1.3 s (Fig. 8).

Fig. 6. Electrochromic switching, optical absorbance change for P(DDQ) monitored at 556 nm between1.4 and 1.4 V.

Table 1

Electrochromic properties

Redox state Color L a b

P(DDQ) Oxidized Blue 68 1 43

Neutral Plum 73 31 22

P(DDQ)/PEDOT Oxidized Plum 72 33 23

Neutral Blue 34 11 55

Fig. 7. Spectroelectrochemical analysis of the P(DDQ)/PEDOT device.

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3.10. Open-circuit stability

The color persistence in the electrochromic devices is an important feature since it is directly related to aspects involved in its utilization and energy consumption during use. The optical memory of an EC material is deﬁned as the time during which this material retains its color without applying potential. We applied a pulse (0.0 V or1.5 V) for 1 s and held the cell in an open-circuit condition for 200 s while the transmittance was monitored as a function of time. Simultaneously, the optical spec-trum at 605 nm as a function of time at open-circuit conditions was monitored (Fig. 9).

4. Conclusion

A new thiophene monomer, 5,12-dihydrothi-eno[30_,40_{:2,3][1,4]dioxocino[6,7-b]quinoxaline (DDQ),}

was synthesized and characterized. Conducting polymer of DDQ was synthesized potentiostatically in (1:4 v/v) dichloromethane (DCM):acetonitrile (ACN)/tetrabutylammonium tetraﬂuoroborate (TBAFB) solvent–electrolyte couple and character-ized via CV, FTIR, SEM and UV–vis spectroscopy. Spectroelectrochemistry analysis of P(DDQ) reﬂected electronic transitions at 556 nm and 950 nm, revealing p–p*_{transition and polaron band}

formation, respectively. Band gap of the P(DDQ) was found as 1.73 which is lower than other com-mon conducting polymers such as polypyrrole, polythiophene[19].

Switching ability of the homopolymer was evalu-ated by a kinetic study upon measuring the %T at the maximum contrast point. We also successfully established the utilization of dual-type comple-mentary colored polymer electrochromic devices using P(DDQ)/poly(3,4-ethylenedioxythiophene) (PEDOT) in sandwich conﬁguration. The switching ability and optical memory of the electrochromic device were investigated by UV–vis spectrophotom-etry and cyclic voltammspectrophotom-etry. This device exhibits moderate switching voltages (1.5 to 1.7 V) and reasonable switching times (1.3 s). Device switches between plum and blue with optical contrast (% DT) of 15%.

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