A ferrocene functionalized multichromic p and n dopable donor–acceptor–donor type conjugated polymer

Journal of Electroanalytical Chemistry 648 (2010) 184–189

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A ferrocene functionalized multichromic p and n dopable donor–acceptor–donor type conjugated polymer Sß erife Özdemir, Abidin Balan, Derya Baran, Özdemir Dog˘an, Levent Toppare * Middle East Technical University, Department of Chemistry, 06531 Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 28 April 2010 Received in revised form 18 June 2010 Accepted 19 July 2010 Available online 27 July 2010 Keywords: Ferrocene Multichromism Donor–acceptor–donor Conjugated polymer

a b s t r a c t Ferrocene substituted donor–acceptor–donor type polymer; poly(5,8-bis(2,3-dihydrothieno[3, 4-b][1,4]dioxin-5-yl)-2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline) (PDEFNQ) was synthesized and its electrochromic properties were investigated. Cyclic voltammetry and spectroelectrochemistry studies for PDEFNQ showed that it is a multichromic green to transmissive polymer with high tendency to be both p and n doped. PDEFNQ is foreseen as a promising candidate for various applications. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Growing importance of conducting polymers [1] lies in the fact they can be exploited in a wide field of applications. Sensors [2], light emitting diodes [3], solar cells [4], field effect transistors [5], and electrochromic devices [6] are some of the important industrial applications of conducting polymers. The reversible and visible change in transmittance or reflectance as a result of an applied voltage is called as electrochromism [7,8]. Conducting polymers can be used for electrochromic devices due to their fast switching times [9], high contrasts [10], processibility [6] and easy tuning of electronic and optical properties via small structural alterations [11]. The neutral state color of these conducting polymers is very important for electrochromic devices. Most of the electrochromic polymers in the literature reflect mainly blue and red colors in their reduced state because they absorb only one dominant wavelength. The requirement for reflecting mainly green color is having two simultaneous absorption bands in the red and blue regions of the visible spectrum. In addition these bands must be controlled with the same applied potential. For completion of color space, neutrally green colored polymer should reveal also a transmissive state. Recently, donor–acceptor–donor type polymers satisfy these requirements [12]. Resonance which allows a stronger double bond character between the donor and acceptor units can cause a significant * Corresponding author. Tel.: +90 3122103251; fax: +90 3122103200. E-mail address: [email protected] (L. Toppare). 1572-6657/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2010.07.010

decrease in band gap [13]. Band gap determines the conductivity and color of the neutral polymer. The logic behind the donor– acceptor approach is having a high HOMO of the donor and a low level of the LUMO moieties incorporated into the monomer [14]. The materials with low band gaps usually reveal a transmissive oxidized state and colored reduced state as a consequence of lower energy transitions in the doped state. In addition, these types of materials usually have two absorption maxima [15]. In literature different type of conducting polymers containing metal complexes on polymer backbone was synthesized. The versatility of approach to yield conductive thin films in which the metal is present in the polymer backbone is illustrated in the range of complexes that were polymerized. Polythiophene-RuðbpyÞnþ hy3 brids [16], ruthenium oligothienyl acetylide complexes [17], oligothienylferrocene complexes [18] were reported. Recently, attention on conducting polymers functionalized by ferrocene is aroused since these type of polymers show the redox properties of both groups [19]. As a result, these are useful in a range of applications and studies such as sensors [20,21], electro active Langmuir/Blodgett films [22], free-standing redox-active films [23]. Conducting polymers containing ferrocene on main conjugation path of polymer backbone were also synthesized [24,25]. Electrochemical experiments revealed that the ferrocene clicked PEDOT films have fast electron transfer ability and the spectroelectrochemical studies show that the conducting polymer can switch from opaque purple to red then blue [18]. We report here the synthesis and electrochemical properties of a ferrocene containing donor–acceptor type green electrochromic

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2.2.1. Synthesis of 5,8-dibromo-2-(naphthalen-2-yl)-3-ferrocenyl4a,8a-dihydroquinoxaline A solution of 3,6-dibromo-1,2-phenylenediamine (168.6 mg, 0.64 mmol) and 1-ferrocenyl-2-naphthylethanedione (231 mg, 0.63 mmol) in EtOH (40.6 mL) was refluxed overnight with a catalytic amount of p-toluene sulfonic acid (PTSA). The mixture was cooled to 0 °C. The precipitate was isolated by filtration and washed with EtOH several times to afford the desired compound in 89% yield (140 mg, 0.279 mmol). 1H NMR (CDCl3) (r) 8.15 (s, 1H), 7.87–7.76 (m, J = 14.4 and 7.1 Hz, 5H), 7.63 (dd, J = 8.5 and 1.6 Hz, 1H), 7.51–7.45 (m, 2H), 4.60–4.57 (m, 2H), 4.28–4.25 (m, 2H), 3.93 (s, 5H). 13C NMR (CDCl3) (r) 156.05, 153.75, 139.70, 138.24, 135.95, 133.66, 133.00, 132.94, 131.96, 129.58, 128.71, 127.81, 127.76, 127.07, 127.04, 126.49, 123.72, 123.14, 71.72, 70.52, 70.23.

polymer 5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline (DEFNQ) which has multi colored states and a highly transparent oxidized state.

2. Experimental 2.1. Materials and equipments All chemicals were purchased from Aldrich except anhydrous THF and chloroform, which were purchased from Merck. 1H NMR and 13C NMR spectra were recorded at 25 °C in CDCl3 at 400 MHz and 101 MHz respectively, with Me4Si as the internal standard. A Voltalab potentiostat was used for all electrochemical studies. Electropolymerization was performed in a three-electrode cell consisting of an Indium Tin Oxide doped glass slide (ITO) as the working electrode, platinum wire as the counter electrode, and Ag wire as the reference electrode. Tetrabutylammonium tetrafluoroborate (Bu4NBF4) in ACN/DCM (5:95, v:v) was used as the electrolytic medium. 1H and 13C NMR spectra were recorded in CDCl3 on Bruker Spectrospin Avance DPX-400 Spectrometer. Chemical shifts were given in ppm downfield from tetramethylsilane. Varian Cary 5000 UV–Vis spectrophotometer was used to perform the spectroelectrochemical studies of the polymer. Colorimetry measurements were done via Minolta CS-100 Spectrophotometer.

2.2.2. Synthesis of 5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline 5,8-Dibromo-2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline (50 mg, 0.0836 mmol) and tributyl (2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane (478.8 mg, 0.753 mmol) were dissolved in dry THF (25 mL). The solution was purged with argon for 30 min. and PdCl2(PPh3)2 was added under argon atmosphere. The mixture was stirred at 100 °C under argon atmosphere for 15 h, cooled and concentrated on the rotary evaporator. The residue was subjected to column chromatography (silica gel, CHCl3:hexane, 2:1) to afford a red solid in 88.0% yield (53 mg, 0.074 mmol). 1H NMR (CDCl3) (r) 8.63–8.49 (m, 2H), 8.23 (s, 1H), 7.89–7.83 (m, 3H), 7.75 (dd, J = 8.5 and 1.6 Hz, 1H), 7.52–7.44 (m, 2H), 6.61 (s, 1H), 6.42 (s, 1H), 4.76–4.70 (m, 2H), 4.40–4.35 (m, 2H), 4.34–4.30 (m, 2H), 4.30–4.26 (m, 2H), 4.25 (dd, J = 4.9 and 2.9 Hz, 2H), 4.24– 4.19 (m, 2H), 3.91 (s, 5H). 13C NMR (CDCl3) (r) 151.99, 150.10, 141.49, 141.32, 140.32, 140.23, 137.48, 136.90, 135.95, 133.39, 133.10, 129.63, 128.90, 128.66, 128.01, 127.81, 127.71, 127.67, 127.56, 127.01, 126.67, 126.26, 113.47, 103.10, 102.78, 82.32, 77.21, 71.87, 70.06, 70.04, 65.01, 64.96, 64.39, 64.35, 53.40, 29.69.

2.2. Synthesis 2-Hydroxy-1-(naphthalen-2-yl)-2-ferrocenylethanone [26], 1-ferrocenyl-2-naphthylethanedione [26], 4,7-dibromo-2,1,3-benzothiadiazole [27], 3,6-dibromo-1,2-phenylenediamine [28], 5, 8-dibromo-2-(naphthalen-2-yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline [29] and tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5yl)stannane [30] were synthesized according to the previously reported methods.

O

OH

O

O

H Fe

H

+ 1

N

KCN

Fe

2

S

N

O

Fe

O 4

3

N

HBr Br2

MnO2

S

N

Br

5

H2N

NaBH4 Br

NH2

Br

Br

6

7

Fe

Fe PdCl2(PPh3)2 N

N

N

S

O

S

N

O Br

O

O

O DEFNQ

O

S

9

SnBu3

Scheme 1. Synthetic route of monomer DEFNQ.

Br 8

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HRMS (EI) for C40H26FeS2N2O4 calculated 721.0919, found 721.0918. 3. Results and discussion 3.1. Synthesis The synthetic route to the monomer is shown in Scheme 1. The reagent 2-hydroxy-1-(naphthalen-2-yl)-2-ferrocenylethanone was synthesized by benzoin condensation of ferrocene carboxaldehyde and b-naphthaldehyde [26] and oxidized by MnO2 to obtain 1-ferrocenyl-2-naphthylethanedione as a red solid [26]. The reagent 2,1,3-benzothiadiazole was brominated with a HBr/Br2 mixture in high yields [27] and reduced with NaBH4 to afford 3,6-dibromo-1,2-phenylenediamine as described previously [28]. The purified compound was condensed with 1-ferrocenyl-2-naphthylethanedione in ethanol to afford 5,8-dibromo-2-(naphthalen-2yl)-3-ferrocenyl-4a,8a-dihydroquinoxaline in high yields [29]. Stannylation of EDOT was achieved in two steps according to previously reported methods [30]. The Stille coupling reaction of dibromoquinoxaline with tributyl(2,3-dihydrothieno[3,4b][1,4]dioxin-5-yl)stannane was performed in dry THF with Pd(PPh3)2Cl2 as the catalyst to give the desired compound DEFNQ.

0.01 M DEFNQ solution at a scan rate of 100 mV/s in ACN/DCM (5:95, v:v) solvent mixture (Fig. 2). An oxidation peak at 0.82 V and its reverse cathodic peak at 0.45 V corresponding to PDEFNQ appeared with increasing number of cycles (Fig. 2). The polymer also revealed n-doping property where a reversible redox couple at 1.7 V and 1.35 V, versus the Ag wire pseudo reference electrode, was observed for the n-type doping and dedoping processes, respectively (Fig. 3). The anodic and cathodic peaks of PDEFNQ were proportional to each other even at different scan rates, which is an indication of a well adhered polymer film and a non diffusion controlled charge transfer process [31].

3.3. Spectroelectrochemistry To probe the optical behaviors of PDEFNQ upon doping, spectral changes were investigated by UV–Vis–NIR spectrophotometer in a monomer free, 0.1 M TBAPF6, ACN solution with gradually increasing potentials between 0.3 V and 1.2 V. Since PDEFNQ is a donor– acceptor type polymer and this type of materials usually show two distinct absorption maxima, peaks at 450 nm and 760 nm were observed (Fig. 4). The second peak at longer wavelength corresponds

3.2. Cyclic voltammetry Reversible redox couple of ferrocene was recorded in 0.1 M TBAPF6/ACN mixture on ITO. Characteristic oxidation and reduction peaks of Fc (ferrocene) were observed at 0.78 V and 0.17 V respectively, versus Ag wire pseudo reference electrode. The cyclic voltammogram (CV) of acceptor unit containing Fc group was also recorded in order to observe the effect of acceptor unit on redox behavior of Fc. The oxidation potential of Fc was shifted to a slightly higher potential due to electron deficient quinoxaline unit. Fig. 1 summarizes the CV of Fc, the acceptor unit and the first run of the electropolymerization of DEFNQ in ACN/DCM (5:95, v:v) medium with TBAPF6 electrolyte. During stepwise oxidation between 0.3 V and +1.2 V, first peak (Fc/Fc+) was observed at 0.72 V and the monomer peak was appeared at 1.1 V (Fig. 1). After the monomer oxidation, a reduction appears at 0.63 V as a result of deposition of polymer chains on ITO. This reduction leads to an insulator electrode surface hence; ferrocene reduction peak cannot be seen. The repetitive cycles of electropolymerization of the monomer on ITO was performed in a 0.1 M TBAPF6 and

Fig. 1. Single scan voltammograms of (a) Fc solvent–electrolyte couple.

, acceptor unit

Fig. 2. Multiple scan voltammogram for polymerization of DEFNQ in DCM/ACN/ TBAPF6 solution.

in ACN and (b) the first run of electropolymerization of DEFNQ in a DCM/ACN (5:95, v:v)/TBAPF6

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Fig. 3. Single scan voltammogram of PDEFNQ upon both p and n-type doping.

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which is a rarely seen property among conjugated polymers. These peaks were decreased simultaneously upon stepwise oxidation and the polymer revealed a highly transmissive state. As a result of charge carrier formation upon oxidation, the absorption in the visible region reached a minimum value and new absorption band evolved at 1000 nm. Bipolaron formation was tracked at 1750 nm in NIR region. Up to date almost all neutral state green polymers were accompanied by only a transmissive oxidized state when oxidized [12]. Introduction of ferrocene units into the polymer backbone makes PDEFNQ a promising multichromic neutral state green to transmissive electrochromic polymer during p-type doping. PDEFNQ film is saturated green ( Y: 17.2, x: 0.30, y: 0.39) in its neutral state, the color changes to olive (Y: 35.83, x: 0.39, y: 0.45) when partially oxidized and finally highly transparent (Y: 57.2, x: 0.31, y: 0.36) in its fully oxidized state. Although conducting polymers have tendency to exhibit both pand n-doping ability, only a few of them show this notable property [32]. The n-type polymers can be used in the fabrication of many different types of polymer based electronic devices such as LEDs [33] and ambipolar field effect transistors [5]. True n-type doping process can be proved by a reversible redox couple at negative potentials and spectroscopic changes. We achieved the reduction of a Fc containing conjugated polymer, PDEFNQ by inserting Fc units to a material with donor–acceptor–donor units [12a,12b]. Despite the common belief that the spectra for n-doping cannot be recorded under ambient conditions [34], charge carrier formation upon reduction of PDEFNQ was observed in open air medium

Fig. 4. Electronic absorption spectra for PDEFNQ upon p doping between 0.3 V and 1.2 V with 0.1 V potential intervals in a monomer free solution.

Fig. 5. Electronic absorption spectra for PDEFNQ upon n doping between 0.5 V and 2.1 V with 0.2 V potential intervals.

to the lowest energy p–p transition of the neutral polymer. The band gap calculated from the onset of this transition was 1.3 eV. Two absorption peaks in the red and blue regions of the visible spectrum resulted in a neutral state green polymer, PDEFNQ,

Fig. 6. Percent transmittance changes as a function of time at 730 nm and 1750 nm for PDEFNQ.

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[4]

[5]

Fig. 7. Relative luminance (Y%) values for PDEFNQ upon applied potential.

[6]

(Fig. 5). Polymer revealed purple color when totally reduced (Y: 24.7, x: 0.34, y: 0.33). 3.4. Kinetic studies Kinetic measurements were done in a monomer free, ACN/ TBAPF6 solvent–electrolyte system. The polymer film was switched between its reduced and oxidized states with 5 s time intervals. Percent transmittance changes in the visible and NIR regions and the switching times (the time required for one full switch) were determined. In the visible region PDEFNQ revealed 33% optical contrast with a switching time of 0.8 s. The percent transmittance change and the switching time in the NIR region for the polymer film were measured as 40% and 1.6 s, respectively (Fig. 6). Relative luminance studies which measure the film lightness or darkness (the amount of transmitted light through the polymer film) were performed on electrochemically polymerized PDEFNQ films (Fig. 7). The values ranged from 17% i.e. absorbed state, to 55% when the polymer film is transmissive.

[7] [8] [9] [10] [11] [12]

[13]

4. Conclusion

[14]

PDEFNQ, a ferrocene functionalized, both p and n dopable donor–acceptor–donor type conjugated polymer was synthesized and characterized. PDEFNQ is a neutrally green polymer but it has multicolored states with a transmissive oxidized state. According to the electrochemical and spectroscopic results PDEFNQ is an interesting candidate for optoelectronic applications.

[15] [16] [17] [18] [19] [20]

Acknowledgments Authors thank TUBA and METU grants.

[21] [22] [23]

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