Multi-colored electrochromic polymer with enhanced optical contrast

European Polymer Journal 46 (2010) 2199–2205

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Multi-colored electrochromic polymer with enhanced optical contrast Simge Tarkuc a, Elif Kose Unver a, Yasemin Arslan Udum b, Levent Toppare a,⇑ a b

Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey Institute of Science and Technology, Department of Advanced Technologies, Gazi University, 06570 Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 15 June 2010 Received in revised form 16 July 2010 Accepted 4 August 2010 Available online 20 August 2010 Keywords: Electrochemical polymerization Multichromism Low band gap polymer Optoelectronic properties Spectroelectrochemisty

a b s t r a c t A new p-conjugated monomer was synthesized which contains an electron-donating unit 3,4-ethylenedioxythiophene and electron withdrawing quinoxaline-based heterocycle to examine the effects of imine unit on the optoelectronic and redox properties of the resulting polymer. Electroactivity of monomer and electrochemical redox behavior of its polymer were investigated by cyclic voltammetry. An irreversible anodic wave at +0.85 V vs Ag wire reference electrode corresponding to the monomer oxidation was observed. Spectroelectrochemical analysis revealed that the neutral polymer has an absorbance at 820 nm. The band gap of the polymer was determined as 1.0 eV from the onset of the p–p* transition. The polymer shows multi-colored electrochromic behavior with five distinct states: brick red (0.3 V), orange (+0.4 V), brown (+0.7 V), green (+0.85 V), gray (+1.2 V). The polymer revealed 34% optical contrast at 460 nm and an excellent optical contrast of 99% in the NIR region. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction A number of different p-conjugated polymers have been synthesized with the goal of enhancing electronic and optical properties such as low band gap, fast switching time, high optical contrast [1,2]. The use of p-conjugated polymers in organic light emitting diodes [3,4], photovoltaics [5], field effect transistors [6,7] and electrochromic devices [8,9] has increased due to the flexibility in tailoring specific optoelectronic properties of these polymers. An electrochromic material is the one where a reversible color change takes place upon an electron transfer (redox) process [10]. The value of the band gap of a neutral polymer is one of the most important factors for controlling the optical properties [11–13]. The donor–acceptor–donor (D–A–D) route is one of the most convenient methods in structural modification. Arranging the sequence of donor and acceptor moieties in polymer backbone helps band gap control. The structure of D–A–D interaction in the polymer backbone resulted in a red-shift in the absorption spectrum which is the main cause for a low band gap (1.0 eV) [14,15]. Polymers with intermediate gaps have distinct optical changes ⇑ Corresponding author. Tel.: +903122103251; fax: +903122103200. E-mail address: [email protected] (L. Toppare). 0014-3057/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2010.08.002

throughout the visible region hence, can exhibit several colors. In case more than two redox states are available, the electrochromic material presents multichromism [16,17]. Multi-colored polymers can be used as active layers in many device applications especially in the information systems. The structures of the p-conjugated monomers are easily tailored by extending conjugation over fused ring systems, by changing the nature and the position of the heteroatom on the donor or acceptor units [18–20]. The presence of heteroatom in the p-conjugated monomers plays an important role in controlling the properties of the polymers due to their intrinsic electron-donating or electron-withdrawing capabilities, hydrogen-bonding and polarizability [21]. P-Conjugated polymers with heteroaromatic units containing nitrogen have been attracting considerable interest since their properties can be modulated by metal binding, protonation, or quaternization at the N-position. P-Conjugated polymers containing 1,10-phenanthroline that acts as a ligand for metals are expected to be promising functional materials in molecular electronics [22–24]. The route described in this contribution is the combination of an electron rich donor 3,4-ethylenedioxythiophene (EDOT) with an electron acceptor quinoxaline-based heterocycle in a donor–acceptor–donor architecture. The motivation is to examine the influence of imine (C@N) functional

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as the counter electrode, and Ag wire as the pseudo-reference electrode. The spectroelectrochemical studies of the polymers were done via Varian Cary 5000 spectrophotometer. Minolta CS-100 spectrophotometer was used to perform colorimetry measurements. Mass Analysis was carried out on a Bruker time of flight (TOF) mass spectrometer with an electron impact ionization source.

group on the optical and electronic properties of the polymer. The metal-complex forming property of the polymer will be reported in a future study. The intramolecular charge transfer between the donor and the acceptor groups leads to a considerable improvement in the optical contrast of the resulting polymer; 99% at 1460 nm. 2. Material and methods

2.3. Synthesis

2.1. Materials

Synthetic route used for the other quinoxaline derivatives [30,31] was modified to synthesize the monomer in concern. The corresponding dibromoquinoxaline acceptor unit was prepared with bromination, reduction and condensation steps. Yet the resulting compound has low solubility in the common solvents such as THF and DMF which are the usual solvents used in Stille coupling reaction. To overcome these drawbacks, firstly D–A–D type p-conjugated monomer backbone (5) was prepared. Subsequent reduction of this compound gave donor substituted diamino compound which undergoes condensation reaction with dicarbonyl compound 1,10-phenanthroline-5,6-dione. The general synthetic strategy towards the p-conjugated monomer, PHEN is outlined in Scheme 1. Bromination of 2,1,3-benzothiadiazole (1) was achieved using a mixture of HBr/Br2 in accordance with a previously reported procedure [25]. 3, 4-Ethylenedioxythiophene (3) was converted to its stannyl derivative (4) by treating it with n-BuLi and Bu3SnCl [26]. A Stille coupling reaction between 4,7-dibromo benzo[c][1,2,5]thiadiazole (2) and 2-tributylstannyl-3,4-eth-

All commercially available reagents were purchased from Aldrich, Merck and Acros. They were used without further purification. 4,7-Dibromo-2,1,3-benzothiadiazole [25], tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane [26], 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7-(2,3-dihydrothieno[3,4-b][1,4]dioxin-7-yl)benzo[c][1,2,5] thiadiazole [27], 1,10-phenanthroline-5,6-dione [28] were synthesized according to the literature. 2.2. Equipments 1 H-NMR spectra were recorded on a Bruker Spectrospin Avance DPX-400 Spectrometer at 400 MHz and chemical shifts (d) were determined relative to tetramethylsilane as the internal standard. The FT-IR spectrum was recorded on a Varian 5000 FT-IR Spectrometer. Electropolymerizations were performed with a Voltalab 50 potentiostat in a three-electrode cell consisting of indium tin oxide doped glass slide (ITO) as the working electrode, platinum wire

N

S

N

S

N

HBr

Br

Br

Br2

1

N

2 O

Pd(PPh3)2Cl2

+ O

O

N

S

N

S

THF O

O

O

O

S

5

O

n-BuLi S

SnBu3Cl

O

SnBu3

4

3

O

S

N

S

N

S

O

O Zn/AcOH O

O H 2N

NH 2 O

S

S

O

N

N

N

N

S

6

5

EtOH

+

O

O S

N

N

H2SO4/HNO3

N

N

KBr

7

O

8

O

Scheme 1. Synthetic route to monomer, PHEN (9).

O S

9

O

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ylenedioxythiophene (4) gave the compound 5 [27]. Zinc was used to reduce the resulting compound to the corresponding diamine (6), [29]. The dicarbonyl compound; 1,10-phenanthroline-5,6-dione (8) was synthesized by a procedure similar to that described previously [28]. Condensation reaction of diamine (6) and 1,10-phenanthroline-5,6-dione (8) was performed in ethanol. At the end of the reaction a violet colored cloudy mixture was observed. Filtration of the reaction mixture was followed by washing with cold ethanol. The residue was purified by column chromatography over silica gel; eluting with 1:1 (dichloromethane:hexane) gave a violet solid. The resulting compound PHEN (9) was characterized via 1HNMR spectroscopy. 1H-NMR (400 MHz, CDCl3): 9.71 (dd, 2H, J = 8.1, 1.8 Hz), 9.20 (dd, 2H, J = 4.4, 1.8 Hz), 8.64 (s, 2H), 7.75 (dd, 2H, J = 8.1, 4.4 Hz), 6.60 (s, 2H), 4.39 (m, 4H), 4.27 (m, 4H). FTIR spectrum of PHEN shows the following absorption peaks: 2925 and 2872 cm1 (aromatic C–H stretching of EDOT and benzene), 1616 and 1558 cm1 (conjugated cyclic C@N stretching), 1073 cm1 (C–O–C stretching), 888 cm1 (C–H out of plane bending), and 740 cm1 (aromatic C@C out of plane antisymmetric stretching). MS (m/z): 562 [M+].

3.2. Spectroelectrochemistry Spectroelectrochemical analysis was performed to evaluate the optical properties of the electrochromic polymer, PPHEN. Fig. 3 shows the spectroelectrochemical series for p-doping between 0.3 V and +1.2 V. The neutral form of PPHEN is brick red in color, with an absorbance peak centered at 820 nm. Stepwise oxidation of the polymer shows that the intensity of p–p* transition decreased as the color changes from brick red to gray. This process is accompanied by three intermediate states; orange, brown and green. Upon oxidation, new absorption bands correspond-

3. Results and discussion 3.1. Electrochemistry The new quinoxaline analogue, PHEN exhibits redox behavior typical of the previous quinoxaline derivatives [30,31]. The electrochemical oxidation of PHEN was carried out in a solution of 102 M monomer and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in dichloromethane (DCM) at a scan rate of 100 mV/s. PPHEN was synthesized on indium tin oxide (ITO) coated glass slides by oxidative electropolymerization. The cyclic voltammogram of PHEN exhibits an irreversible oxidation wave at 0.85 V vs Ag wire (Fig. 1). Following the monomer oxidation, an electroactive polymer film quickly grows on the indium tin oxide (ITO)-coated glass slide revealing a new reversible redox couple with an accompanying increase in the current intensity. PPHEN has oxidation and reduction peaks centered at +0.75 and +0.37 V (Scheme 2). To investigate the long-term switching stability between the doped and neutral states, PPHEN film was deposited on an ITO electrode by repeated scanning in TBAPF6/DCM. The polymer film was washed with DCM and switched between its redox states for 1000 cycles in 0.1 M TBAPF6/DCM electrolyte/solvent couple. The polymer film showed high stability since 93% of the electroactivity remains intact even after 1000 cycles (Fig. 2).

O

O

N

N

N

N

O

O

+ S

S

n

nx PF6

-

Fig. 1. Repeated potential scan electropolymerization of PHEN at 100 mVs–1 in 0.1 M TBAPF6/DCM on an ITO electrode.

Fig. 2. Chronoamperometry experiment for PPHEN on ITO glass in 0.1 M TBAPF6/DCM while switching between reduced and oxidized states. Each interval on the x axes stands for 5 s.

+ 0.75 V

O

+ 0.37 V Scheme 2. p-Doping of PPHEN.

O

N

N

N

N

O

S

O

x PF6-

+x S

n

+ nxe

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Fig. 3. p-Doping: spectroelectrochemistry of a PPHEN film on an ITO-coated glass slide in monomer-free, 0.1 M TBAPF6/DCM electrolyte–solvent couple at applied potentials between 0.3 V and 1.2 V.

ing to polaron and bipolaron species appear at 1250 and 1600 nm, respectively. The band gap (Eg), defined as the onset of the p–p* transition of the neutral PPHEN film, was calculated to be 1.0 eV. In the neutral state, PPHEN showed an absorption maxima at 820 nm which is red-shifted by about 90 nm compared to an analogous polymer, PDPEQ (kmax = 732 nm) [13]. This suggests that optical properties of PPHEN are controlled by inductive effects rather than by delocalization along the polymer backbone. A considerable longer wavelength reflects a higher order of contribution of the charge transfer in the electronic state of PHEN due to its strong electron-accepting property. Electron-accepting property of the p-system of the central aromatic group for compound IV was compared with those of compounds I [19], II [13] and III [20] in terms of the number of electron-withdrawing imine(C@N) groups (Table 1). It can be concluded that the increase in the number of imine groups leads to a bathochromic shift. The bathochromically shifted absorption maxima reflects a more planar backbone structure for IV due to the presence of 1,10-phenanthroline unit. The presence of a planar conformation (which enhances intermolecular p–p interaction) and the p-orbital overlap within the polymer backbone result in a decrease in the band gap of the resulting polymer. The difference in the absorption maxima for II and III shows that the introduction of phenyl units on the skeleton of conjugated polymers leads to a departure from coplanarity. This deviation may result in an increase in Eg. Also the presence of imine units on the fused ring system (IV) rather than on the conjugated main chain (III) makes a difference in lowering the band gap of the polymer. 3.3. Electrochromic switching studies Electrochromic switching studies were performed to monitor the switching ability of the polymer. A square wave potential step method coupled with optical spectroscopy was used to determine the switching times and the contrast. In this double potential step experiment, the po-

tential was set at an initial potential (0.3 V) for 5 s, and was switched to a second potential (+1.2 V) for 5 s before being switched back to the initial potential (0.3 V) again. The percentage of transmittance was then monitored while the polymer was switched between 0.3 V and + 1.2 V with a switching interval of 5 s in 0.1 M TBAPF6/DCM at two different wavelengths (Fig. 4). The optical contrast, measured as the difference in percent T between the reduced and oxidized forms, was determined as 34% at 460 nm. An excellent optical contrast of 99% in the NIR region (1460 nm) is found which a very significant property is since the polymer can be utilized as an active electrochromic material for NIR applications. Response time upon electrochromic switching of the polymer film between its neutral and oxidized states was also monitored. PPHEN has a switching time of 1.8 s at 460 nm. The polymer achieves the above mentioned optical contrast in the NIR region in only 1 s. 3.4. Colorimetry Significant changes in absorption characteristics upon p-doping demonstrate the electrochromic properties of PPHEN. Colorimetry analysis identify the color changes observed upon oxidation; the polymer is brick red at the reduced state (0.3 V), gray at the oxidized state (+1.2 V) along with three distinct intermediate states; orange (+0.4 V), brown (+0.70 V), green (+0.85 V). The Commission Internationale de l’Eclairage (CIE) system was used as the quantitative scale to define and compare colors. The color coordinates; Y, x, y values were measured and summarized in Table 2. The coloration efficiency (CE) is an important characteristic for electrochromic materials and obtained for a certain amount of the charge injected in the polymer as a function of the change in optical density. CE is the ratio between the change in optical density (DOD) and the injected/ejected charge per unit area of the electrode at a specific dominant wavelength (kmax) as illustrated by the following equation

CE ¼ DOD=Q d

2203

732 1.01

Fig. 4. Electrochromic switching, optical absorbance change monitored for PPHEN in 0.1 M TBAPF6/DCM.

CE was measured as 174 cm2 C1 by chronoamperometry while the polymer was switched between 0.3 V and + 1.2 V. This value is quite an improvement in compare to a similar quinoxaline derivative with a coloration efficiency of 115 cm2 C1 [32].

478 1.9

4. Conclusion

kmax(nm) Eg (eV)

Table 1 Effects of changes in p-system of the central aromatic group on the absorption onset and electronic band gap of the resulting polymers.

750 1.2

820 1.0

S. Tarkuc et al. / European Polymer Journal 46 (2010) 2199–2205

The use of electron-donating 3,4-ethylenedioxythiophene and electronwithdrawing quinoxaline-based heterocycle unit on a conjugated backbone leads to a new electrogenerated low band gap polymer, PPHEN. The optoelectronic behavior of the polymer was greatly affected by the properties of quinoxaline-based acceptor unit. The band gap of the polymer was found to be 1.0 eV. The polymer revealed nearly 34% optical contrast at 460 nm and 99% in the NIR region. The polymer exhibits multi-color electrochromism via easily accesible p-type doping, and can be switched between its redox states: brick red (reduced state) – gray (oxidized state) with three intermediate states; orange, brown, green. Optical properties were compared with those of previously reported donor–acceptor–donor type p-conjugated polymers. The p–p* absorption band of the polymer was observed at a longer wavelength than those of similar D– A–D type polymers of quinoxaline derivatives. These data are considered to reflect a stronger charge transfer interac-

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Table 2 Electrochromic properties of PPHEN. Brick red (0.3 V)

Orange (+ 0.4 V)

Brown (+0.7 V)

Green (+ 0.85 V)

Gray (+1.2 V)

Y: 22.3 x: 0.421 y: 0.352

Y: 32.2 x: 0.464 y:0.424

Y: 32.6 x: 0.438 y:0.432

Y: 32.6 x: 0.365 y: 0.423

Y: 31.5 x: 0.312 y: 0.335

tion between 3,4-ethylenedioxy thiophene and electron acceptor quinoxaline-based heterocycle, which has a higher electron-withdrawing ability than pyridine and the quinoxaline derivatives. Acknowledgments

[13]

[14]

Authors gratefully thank TUBA grant. Two of us (S.T, E.K.U) acknowledge TUBITAK-Department of Science Fellowships and Grant Programs.

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