Electrochemical polymerization of an electron deficient fluorene derivative bearing ethylenedioxythiophene side groups

Electrochimica Acta 55 (2010) 779–784

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Electrochemical polymerization of an electron deficient fluorene derivative bearing ethylenedioxythiophene side groups Buket Bezgin, Ahmet M. Önal ∗ Department of Chemistry, Middle East Technical University, I˙ nönü Bulvarı, 06531 Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 6 July 2009 Received in revised form 19 August 2009 Accepted 19 August 2009 Available online 25 August 2009 Keywords: Donor–acceptor polymers Electrochromism Fluorescence

a b s t r a c t A new low band gap polyfluorene derivative, poly(2,7-bis-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5yl)-fluoren-9-one) (PEFE), containing ethylenedioxythiophene as donor and fluorenone (FO) as an acceptor groups was electrochemically synthesized. Electrochemical polymerization of 2,7-bis-(2,3dihydro-thieno[3,4-b][1,4]dioxin-5-yl)-fluoren-9-one (EFE) was achieved in dichloromethane with 0.1 M tetrabutylammonium-hexafluorophosphate both via and potentiostatic methods. The polymer was characterized by cyclic voltammetry, UV–vis, FT-IR and NMR spectroscopic techniques. Spectroelectrochemical and electrochemical analysis revealed that the polymer film is both p- and n-dopable and can be successfully cycled and switched between its neutral and oxidized/reduced states. Furthermore, PEFE shows electrochromic behavior by a color change from brown to blue with a switching time of 1.65 s during oxidation with a high coloration efficiency (250 cm2 /C). Fluorescence studies were also performed. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Conjugated polymers have become materials of choice for various opto-electronic applications including solar cells [1,2] and light-emitting diodes (LEDs) [3–5]. Most of the organic electronic materials used in these devices are highly conjugated polymers that support the injection and allow the mobility of the charge carriers (holes in the case of p-doped materials and electrons in the case of n-doped materials) [6]. Polyfluorene (PF), which is synthesized via anodic oxidation of fluorene [7–9] for the first time, is a well-known blue-emitting material for LED applications [10,11]. Furthermore, PF and its derivatives exhibit high photoluminescence efficiency and good photostability [12–14]. These properties made PFs a material of interest and a large number of PF derivatives were reported. However, the poor solubility in common organic solvents and high energy barrier for hole injection limit their LED applications [15–18]. Molecular tailoring at this point is inevitable and C-9 position of fluorene monomer offers a large number of possibilities. For example, replacement of labile H atom at C-9 position of fluorenes with alkyl groups enhances the solubility and thus processibility of PFs [19]. On the other hand, an electron withdrawing substituent at C-9 position will lower the conduction band of the polymer and enhance the electron injection [20,21]. Fluorenone (FO) is one of the

∗ Corresponding author. Tel.: +90 312 2103188; fax: +90 312 2103200. E-mail address: [email protected] (A.M. Önal). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.08.030

limited number of examples of fluorenes bearing an electron withdrawing group at C-9 position and its polymer, polyfluorenone (PFO), was expected to improve the properties of LEDs via both electron-injection enhancement and hole-migration blocking [22]. Zecchin et al. synthesized PFO films by cathodic coupling from dibromo derivatives [23]. Uckert et al., on the other hand, utilized a polyketal precursor to obtain PFO films [22]. There are also few reports on PFO synthesis via anodic oxidation of fluorenone [24,25]. However, functionalization of fluorene at C-9 position is not the only way used to enhance the optical properties of PFs. Especially for color tuning purposes, copolymerization is also widely employed. The effects of the incorporation of aromatic amines, in terms of color tuning, were investigated by Park et al. [26]. Poly(fluorene-co-3-methylthiophene) films with tunable fluorescence properties were reported by Wei et al. [27]. Loganathan et al. reported that copolymers of fluorenone with benzene, furan and thiophene exhibit relatively lower band gap (1.9 eV) [28,29]. Since an alternating sequence of donor–acceptor units along the polymer chain reduces the band gap [30–32] interest on the preparation of such copolymers increased and more recently various donor–acceptor type copolymers based on fluorene were reported [33–40]. Wu et al. investigated the effect of various acceptors (quinoxaline, 2,1,3-benzothiadiazole and thieno[3,4-b]-pyrazidine) on color tuning and reported that the resulting polymers emit color in the range of blue to red depending on the acceptor strength [41]. In the light of these informations, we aimed to synthesize a new fluorene derivative containing both donor and acceptor units. The new derivative was

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Scheme 1. Synthetic route of EFE.

synthesized via incorporating 3,4-ethylenedioxythiophene (EDOT) units to 2,7 positions of fluorenone. It was thought that the external EDOT units will not only function as donor groups but also lower the oxidation potential thus, avoiding any side reactions during the electrochemical polymerization. In this paper, we wish to report our results concerning the synthesis and electrochemical polymerization of this new monomer, 2,7-bis(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl (EFE). Electrochemical, electro-optical and electrochromic properties of the corresponding polymer poly(2,7-bis-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl) PEFE were investigated using cyclic voltammetry (CV) and in situ spectroelectrochemical techniques. 2. Experimental 2.1. General information All chemicals were purchased from Aldrich Chemical. 0.1 M tetrabutylammonium-hexafluorophosphate (TBAPF6 ) used as electrolyte, dissolved in dichloromethane (DCM) which was freshly distilled over CaH2 prior to its use. A platinum disc (0.02 cm2 ) and platinum wire were used as working and counter electrodes, respectively. Ag/AgCl in 3 M NaCl (aq) solution was also used as reference electrode. Polymer was obtained via electroanalytical methods CV and constant potential electrolysis. Electrochemical and optical behaviors of the film were also investigated. For electrooptical studies, platinum and silver (calibrated externally using 5 mM solution of ferrocene/ferrocenium couple) wires were used as counter and reference electrodes, respectively. An indium-tin oxide

(ITO, Delta Tech. 8–12 , 0.7 cm × 5 cm) coated by the polymer film was used as the working electrode. Prior to spectroelectrochemical investigations, the polymer films were switched between neutral and doped states several times in order to equilibrate its redox behavior in monomer-free electrolytic solution. In situ spectroelectrochemical studies were performed using Hewlett–Packard 8453A diode array spectrometer. Electroanalytical measurements were performed using a Gamry PCI4/300 potentiostat-galvanostat. Fourier Transform Infrared spectroscopy (FT-IR) spectra of the monomer and its polymer were recorded with a Bruker Vertex 70 spectrophotometer. NMR spectra were recorded on a Bruker NMR Spectrometer (DPX-400) in CDCl3 , and fluorescence measurements were recorded on a Varian Cary Eclipse Fluorescence Spectrophotometer. Scanning electron microscope (SEM) images were recorded on Quanta 400 FEI FESEM. 2.2. Synthesis of 2,7-bis-(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl (EFE) The synthesis route of EFE was shown in Scheme 1. A mixture of 2,7-dibromofluoren-9-one (0.73 mmol) [42], PdCl2 (PPh3 )2 (0.2 mmol) and trimethyl-2-ethylenedioxythienylstannane 1 (1.46 mmol) in dry toluene (50 ml) was refluxed under an argon atmosphere until the starting materials were consumed. The reaction mixture was poured into ice the red precipitate was collected by filtration. The precipitate was washed with hexane/CHCl3 (1:1). Recrystallization from DCM-ethanol gave the compound:

Fig. 1. (a) CV of EFE and FO. (b) Electrodeposition of PEFE from a 2 mM solution of EFE in 0.1 M TBAPF6 /DCM at 100 mV/s onto a Pt disc (area = 0.02 cm2 ).

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Fig. 3. CV of PEFE film demonstrating long term redox switching stability after repeated cyclic (up to 150 cycles) at 100 mV/s onto a Pt disc (area = 0.02 cm2 ).

Fig. 2. (a) Cyclic voltammograms of p- and n-doped PEFE film in TBAPF6 /DCM at scan rate 100 mV/s. Inset: differential pulse voltammetry of p- and n-doped PEFE film in TBAPF6 /DCM. (b) Scan rate dependence study for PEFE (film coated with 25 cycles) in 0.1 M TBAPF6 /DCM at applied scan rates between 20 mV/s and 200 mV/s. Inset: relationship of anodic and cathodic current peaks as a function of scan rate between neutral and (a) oxidized and (b) reduced states of PEFE film in 0.1 M TBAPF6 /DCM.

60% yield, red solid; 1 H NMR (400 MHz, CDCl3 ) ı (ppm): 7.98 (s, 2H), 7.73 (d, J = 9.6 Hz, 2H), 7.4 (d, J = 8 Hz, 2H), 6.29 (s, 2H), 4.3 (dt, J = 8 Hz, 4H), 4.2 (dt, J = 8 Hz, 4H), 13 C NMR (100 MHz, CDCl3 ) ı (ppm): 64.45, 64.87, 98.323, 116.52, 120.42, 121.86, 131.57, 134.14, 134.89, 138.95, 142.04, 142.31, 187.0. MS m/z: 460 (M+ ). 3. Results and discussion 3.1. Electrochemical polymerization of EFE Prior to electrochemical synthesis, electrochemical behavior of monomer, EFE, was investigated using CV. The cyclic voltammogram of FO was also recorded in the same solvent-electrolyte medium for comparison purposes and the results are depicted in Fig. 1. During the first anodic scan EFE exhibits an irreversible oxidation peak at around 1.10 V vs. Ag/AgCl which is much lower than that of FO clearly indicating the effect of external donor units. EFE also exhibits a reversible reduction peak at about −1.29 V due to the formation of stable radical anion (Fig. 1a). Upon repetitive

cycling, in the range of 0.0–1.25 V, a new redox couple (Eox = 0.91 V, Ered = 0.76 V) with a concomitant increase in current intensities was also observed indicating formation of an electroactive film on the working electrode surface (Fig. 1b). After electrodeposition, the polymer film coated electrode was washed with DCM to remove any unreacted monomer and oligomeric species. The redox behavior of polymer film (PEFE) was investigated by recording the cyclic voltammograms in monomer free electrolytic solution and the results are given in Fig. 2. As seen from Fig. 2a, PEFE exhibited a well-defined reversible redox couple due to p-doping (Eox = 0.75 V). Apart from p-doping process, the PEFE film was also found to exhibit n-doping (Ered = −1.40 V). Scan rate dependence experiments showed that anodic and cathodic peak currents increase linearly with increasing scan rate, both for p- and n-doping process, indicating a well-adhered polymer film and a non-diffusional redox process, which was shown in the inset of Fig. 2b. It is also noteworthy to mention that the polymer film was stable upon cycling between its neutral and oxidized states for over hundreds of time at a voltage scan rate of 100 mV/s (Fig. 3). 3.2. Polymer characterization The preparative formation of PEFE was achieved in the same solvent/electrolyte medium using 1.0 cm2 Pt-plate working electrode via constant potential electrolysis at 1.1 V. During electrolysis, a polymer film is rapidly formed on the working electrode surface. At the end of electrolysis, the product was washed with DCM and then dried. The FT-IR spectra of EFE and its polymer PEFE obtained via electrochemical polymerization was investigated. 1,2-Disubstituted and 1,2,4-trisubstituted benzene absorption peaks are characteristic peaks, which arise from the in-plane and out-of-plane CH bending modes observed in the range of 730–850 cm−1 . The peak at 1710 cm−1 is due to the carbonyl group of fluorenone moiety and the one at 3110 cm−1 is due to C–H stretching (␣-hydrogen) of external EDOT units. The noteworthy differences in the FT-IR spectrum of PEFE are as follows: the presence of 1710 cm−1 peak in the spectrum of the polymer film, indicating the presence of carbonyl group in the polymer chain and disappearance of 3110 cm−1 peak indicating that polymerization proceeds via C-5 position of external EDOT units. The peak at 835 cm−1 is due to the dopant anion, PF6 − . Surface morphology of the polymer was also investigated. Fig. 4 shows the SEM micrograph of electrochemically grown PEFE on

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Fig. 4. SEM image of PEFE.

ITO electrode. The micrograph designates a well-adhered smooth polymer film on the surface and growth of aggregates shaped as blossoms (Fig. 4). 3.3. Spectroelectrochemical properties of PEFE In situ spectroelectrochemical measurements were conducted on PEFE film coated ITO working electrode in order to get information about the charge carriers and electrochromic behavior upon doping. For this aim, PEFE polymer film (25 mC/cm2 ) was deposited on ITO and its transition between the neutral (dedoped), oxidized (p-doped) and reduced (n-doped) states was studied in monomerfree electrolyte solution. The changes in electronic absorption spectra recorded in situ at various applied potentials are depicted in Fig. 5. As seen from Fig. 5, the electronic absorption spectrum of the neutral form of the polymer film consists of a broad band

Fig. 5. Electronic absorption spectra of the PEFE on ITO in 0.1 M TBAPF6 at various applied potentials between −0.2 V and 1.2 V. Inset: electronic absorption spectra of the PEFE on ITO in 0.1 M TBAPF6 at various applied potentials between −0.5 V and −1.7 V.

at about 466 nm due to ␲–␲* transition followed by a shoulder at 555. This shoulder might be due to internal charge transfer between donor (EDOT) and acceptor (fluorenone) groups [43]. The evolution of the spectra upon electrochemical doping shows a simultaneous decrease of the absorbance at 466 nm which is accompanied by the formation of a new ill-defined absorption band at 800 nm in the potential range −0.20 V to 0.40 V indicating the formation of charge carriers. Upon further doping, this band undergoes a blue shift to 620 nm and a new broad band beyond 1006 nm starts to intensify due to further formation of charge carriers. The polymer film also exhibits a color change from brown (neutral) to blue (oxidized). All spectra recorded during potential cycling between −0.2 V and 1.2 V passes through a clear isosbestic points at 520 nm, indicating that polymer film was being interconverted between its neutral and oxidized states. The n-doping behavior of the PEFE film was also confirmed by recording the changes in the electronic absorption spectrum during reduction. When the polymer film was reduced its color changes from brown to yellow. These color changes are accompanied with the corresponding changes in the absorption bands. The band at 570 nm began to decrease and the band at 454 nm started to intensify. A new intensifying absorption band centered at 728 nm also noted with a concomitant appearance of an isosbestic point at 533 nm which indicates the presence of only two phases during the n-doping process (see inset of Fig. 5). It is interesting to note that the changes in the electronic absorption spectrum of PEFE recorded during n-doping are not symmetrical as the one recorded during pdoping. Although this observation seems to be unexpected, there are also examples in which spectral changes for p- and n-doping are not symmetrical [6,44]. One of the reasons for such contradictory observations might be the change in the band gap [45]. Since the band gap (Eg ) of conjugated polymers is one of the most important property in determining both their opto-electrical and conducting properties we have also determined Eg of PEFE in its neutral state. The Eg was found to be 1.68 eV from the commencement on the low energy end of 555 nm band. In order to confirm this value we have also measured the band gap utilizing the electrochemical data. To improve the accuracy, differential pulse voltammetry was preferred for recording the voltammogram of PEFE film in monomer-free electrolytic solution (Fig. 2a). As seen from the inset of Fig. 2a, the Eg of PEFE is 1.48 eV which is much lower than that of PFO [24] and other PF derivatives (see Table 1). This is expected due to the Eg lowering effect of donor–acceptor units. The electrochemical data was also used to estimate the HOMO/LUMO energy levels of PEFE for comparison purposes. Ionization potential was calculated using the empirical relation [46] Ip = (Eox + 4.4) eV where Eox is the onset of oxidation potential vs. Ag/AgCl. Since the oxidation onset of polymer film is 0.50 V (see inset of Fig. 2a) Ip of PEFE was estimated as 4.9 eV. Electron affinity of PEFE was estimated to be −3.4 eV by subtracting the band gap energy from Ip . This value is very close to the one reported by Becker et al. for PFO (−3.3 eV) indicating that donor side groups did not affect the LUMO level of PEFE [47]. On the other hand, donor side groups increased the HOMO level, making it closer to the work function of ITO (∼5.0 eV) which enhances hole injection. Due to its importance in electrochromic applications, switching times and optical contrast of the polymer film on ITO were also determined utilizing square wave potential step method coupled with optical spectroscopy. The maximum transmittance difference (%T) in the neutral state of PEFE was found as 16.63%T at max = 466 nm. The coloration efficiency (CE) for polymer is calculated from Eq. (1) as described previously [48,49]:

CE =

OD Qd

(1)

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783

Table 1 Spectroelectrochemical and electrochemical data for various fluorene derivatives. Monomer

Polymer

Chemical structure of monomer

Ref.

o

E oxidation

Band gap Egap (V) (electrochemically)

24

2.40 V

29

1.34 V

2.00 V

–

21

1.42 V

1.50 V

–

21

0.60 V

0.38 V

–

–

1.10 V

1.48 V

1.68 eV

where OD is the change in optical density and Qd is the charge (C/cm2 ) passed during this process. OD is determined from the percent transmittance (%T) before and after a full switch and is calculated using Eq. (2): OD = log

Egap (eV) (optically)

 %T of bleached state 

2.40 eV

compounds like irridium dioxide or tungsten trioxide and most of the conducting polymers reported including PEDOT (195 cm2 /C at 90% of full switch at 585 nm) [50]. 3.4. Fluorescence study

(2)

%T of colored state

The charge passed at 95% of the optical switch was used to evaluate CE since beyond which the naked eye cannot sense the color difference. The polymer film coated on ITO (25 mC/cm2 ) was switched between −0.20 V and 1.12 V vs. Ag/AgCl and the time required to attain 95 % of the total transmittance difference was found to be 1.65 s for fully oxidized state and 1.40 s for fully neutral state at 466 nm (Fig. 6). Also, the CE of PEFE at 466 nm was found as 250 cm2 /C (see Table 2) which is much higher than that of inorganic

Since the electrochemically synthesized PEFE film was found to be partially soluble in tetrahydrafuran its fluorescence property was also investigated and its absorption/emission spectrum recorded in this solvent is depicted in Fig. 7. As it seen from Fig. 7, excitation at 420 nm resulted in the formation of a weak and relatively intense emission bands at about 500 nm and at 580 nm. The higher wavelength emission band corresponds to orange color indicating that this partially soluble polymer is an orange light emitter. Zhang et al. reported that electrochemically synthesized PFO is a

Table 2 Voltammetric and spectroelectrochemical data for PEFE in 0.1 M TBAPF6 /DCM. /nm at charge 25 mC/cm2

Tbleached

Tcolored

%T

OD

CE (cm2 /C)

466

10.09

26.72

16.63

0.423

250

Reduced state

Neutral state

Oxidized state

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orescence property PEFE may find applications in various fields, such as organic lasers and electroluminescent materials. Acknowledgements The authors gratefully acknowledge the financial support of the Scientific and Technical Research Council of Turkey (TUBITAK109T012) and METU-BAP2009-01-03-02. References

Fig. 6. Electrochromic switching study of PEFE as a function of time between −0.2 V and 1.2 V vs. Ag/AgCl in 0.1 M TBAPF6 /DCM.

Fig. 7. Emission (excited at 420 nm) and absorption spectra of PEFE in THF.

blue light emitter with an emission band at 517 nm [25]. The red shift in the case of PEFE might be due to the Eg lowering effect of EDOT side groups (see Table 1). 4. Conclusions In summary a new electron donor/acceptor ␲-conjugated polymer composed of a bi-EDOT and fluorenone repeat unit (PEFE) was successfully synthesized both via potentiodynamic and potentiostatic methods. The PEFE film has relatively low band gap (1.48 eV) which is lower than those previously reported PF derivatives. The film was found to exhibit a well-defined and reversible redox process. Spectroelectrochemical and electrochemical analyses revealed that the polymer film is both p- and n-dopable and can be successfully cycled and switched between blue colored oxidized state, brown colored neutral state and yellow colored in the reduced state. Due to its HOMO level being closer to ITO work function, PEFE expected to have enhanced hole injection. On the other hand its LUMO energy (−3.4 eV) correlates well with the work function of Mg indicating facile electron injection [47]. These properties make PEFE a promising candidate as an ambipolar charge transport material in LEDs. Furthermore, due to its orange light-emitting flu-

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