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A Pure Blue to Highly Transmissive Electrochromic Polymer Based on Poly(3,4-propylenedioxyselenophene) with a High Optical Contrast Ratio Baris Karabay, Lutfiye Canan Pekel, and Atilla Cihaner* Department of Chemical Engineering and Applied Chemistry, Atilim Optoelectronic Materials and Solar Energy Laboratory (ATOMSEL), Atilim University, TR-06836 Ankara, Turkey S Supporting Information *

ABSTRACT: A new derivative of 3,4-propylenedioxyselenophene bearing naphthalenylmethyl appeandages on the bridge, called 3,4-dihydro-3,3-bis((naphthalen-2-yl)methyl)-2H-selenopheno[3,4-b][1,4]dioxepine (ProDOSNp2), was synthesized and polymerized via potentiostatic and potentiodynamic methods. The electrochemically obtained polymer film (PProDOS-Np2) is pure blue at the neutral state and highly transparent at the oxidized state. An increase in the size of the substituents on the bridge resulted in an increase in the optical contrast ratio. Upon moving from naked bridge to benzyl and to naphthalenylmethyl substituents on the bridge center, the optical contrast changed from 51% to 65% and finally to 84%, which is the second highest reported optical contrast ratio in polyselenophene family. When compared to polythiophene analogue, the PProDOS-Np2 has lower oxidation potential and band gap, higher optical contrast ratio, coloration efficiency, robustness, and stability. The polymer film preserved its properties even after thousands of cycles under ambient conditions.

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INTRODUCTION It is well-known that different heteroatoms in similar systems (e.g., thiophene, pyrrole, furan, and selenophene) have an important role in the polymerization process and the properties of the corresponding polymers. For example, among the organic conjugated polymers, polythiophene and its derivatives have attracted great attention due to their optical and electronic properties. These promising properties make them potential canditates in industrial applications such as organic lightemitting diodes,1−3 photovoltaic cells,4−7 and transistors.8−11 Also, their high optical contrast ratio in a short time under external potential and high stability under ambient conditions make them favorable in optical displays, smart windows, mirrors, and camouflage materials.12−15 These properties, for example, can be improved and controlled conveniently by the replacement of the S atom with the Se atom in the ring since Se atom has a larger atomic size and lower electronegativity, more metallic character, and polarizability. Therefore, the obtained polyselenophenes can have some advantages over their polythiophene analogues such as lower aromaticity, lower redox potentials, lower band gaps, more rigid, robust, and quinoid structures, and better interchain charge transfers.16−18 These advantages can open up a door to a new vibrant class in conjugated polymer family.19−30 Unfortunately, polyselenophenes received only scant attention due to lack of reasonable synthesis methods. In order to overcome this problem, capitalizing on the 3,4ethylenedioxythiophene (EDOT) unit,31−35 Cava et al. and © XXXX American Chemical Society

Bendikov et al. synthesized 3,4-ethylenedioxyselenophene (EDOS) successfully using different methods.29,36 EDOS was polymerized successfully via electrochemical and chemical polymerization methods. Electrochemically obtained insoluble poly(3,4-ethylenedioxyselenophene) (PEDOS) film changed its color from pure blue to transmissive gray upon oxidation. Also, the band gap of PEDOS film (1.4 eV) was found to be lower than that of poly(3,4-ethylenedioxythiophene) (PEDOT) (1.6−1.7 eV). Furthermore, PEDOS exhibited a higher contrast ratio of 55% coloration efficiency (CE) of 212 cm2/C at 100% of full switching25 when compared to PEDOT (54% at 585 nm and a CE of 137 cm2/C at 100% of a full switch).37 After these pioneering works and by the inspiration of the electrochemical and optical properties of PEDOS, a new series of EDOS derivatives substituted by alkyl chains with various lengths to the bridge were reported.25,26 Among them, hexylsubstituted PEDOS exhibited unprecendented properties; for example, it has a maximum of 88−89% of transmittance change, which is one of the highest reported percent transmittance changes (Δ%T) for electrochromic polymers.38 Furthermore, the film is highly robust and stable. It exhibited 48% of its optical contrast ratio when switched between redox states for 10 000 cycles in the presence of air (without purging with an inert gas). Received: January 6, 2015 Revised: February 1, 2015

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DOI: 10.1021/acs.macromol.5b00022 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules In order to achieve soluble and processable poly(3,4alkylenedioxyselenophene)s, Cihaner and colleagues39,40 have focused studies on a series of dialkyl-substituted 3,4propylenedioxyselenophenes (ProDOS-Cn). The polymerization of ProDOS-Cn gave regioregular soluble polymers bearing low oxidation potentials, high optical contrast ratios, and high transparencies when oxidized. When compared to dialkyl-substituted poly(3,4-propylenedioxythiophene) (PProDOT-Cn) and PEDOS, PProDOS-Cn films have quite stable and robust natures. For example, didecyl-substituted PProDOS film retained 97% of its electroactivity even after 40 000 cycles under ambient conditions.39 A change in bridge size or rigidity of substituents on the bridge can lead to an interchain separation between polymer chains and an improvement of the doping/dedoping ability of the polymers during redox switching.41 Thus, it can be safely concluded that bridge size and substituents on the bridge of the selenophene ring can affect the electrochemical and optical properties of the obtained electrochromic polymers. By using this approach with considering the advantages of polyselenophenes over polythiophenes, Icli-Ozkut et al. compared the effect of heteroatoms and the kind of the substituents like alkyl and aromatic substituents on the propylenedioxy bridge in the similar system on the electrochromic properties.42 They reported that dibenzyl poly(3,4-propylenedioxyselenophene) (PProDOS-Bz2) film has a lower band gap of 1.62 eV than its thiophene analogue PProDOT-Bz2 (1.85 eV).42 The red-shift in the optical spectrum of neutral PProDOS-Bz2 gave rise to a color change from pure blue to transparent gray during oxidation. Also, PProDOS-Bz2 film exhibited the highest CE of 992 cm2/C at 95% of the full contrast in polyselenophene derivatives. Unfortunately, the optical contrast of PProDOS-Bz2 film was calculated as 67%, which is lower than the value of PProDOT-Bz2 reported in the literature (89% at 632 nm). Herein, in order to increase the interchain separation in the PProDOS-Bz2 polymer chains and to achieve a high optical contrast ratio (Δ%T), we extended our work to replace benzyl groups with naphthalenylmethyl groups on the bridge center of the ProDOS system. Therefore, the effect of the heteroatoms and the size of bulky groups on the electrochemical and optical properties of the parent PProDOS and its polythiophene analogue (PProDOT) can be examined systematically. Thus, to achieve this aim, dinaphthalenylmethyl-substituted PProDOS (PProDOS-Np2) as a new member of polyselenophene was synthesized. The monomer was polymerized via electrochemical oxidation, and its properties were compared with polyselenopehene and polythiophene analogues (Scheme 1): 3,4-propylenedioxythiophene (ProDOT) and its polymer PProDOT, 3,4-propylenedioxyselenophene (ProDOS) and its polymer PProDOS, 3,3-dibenzyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine (ProDOT-Bz2) and its polymer PProDOT-Bz2, 3,3-dibenzyl-3,4-dihydro-2H-selenopheno[3,4-b][1,4]dioxepine (ProDOS-Bz2) and its polymer PProDOS-Bz2, 3,4-dihydro-3,3bis((naphthalen-2-yl)methyl)-2H-thieno[3,4-b][1,4]dioxepine (ProDOT-Np2) and its polymer PProDOT-Np2, and 3,4dihydro-3,3-bis((naphthalen-2-yl)methyl)-2H-selenopheno[3,4-b][1,4]dioxepine (ProDOS-Np2) and its polymer PProDOS-Np2.

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Scheme 1. Chemical Structures of 3,4Propylenedioxythiophene (ProDOT) and 3,4Propylenedioxyselenophene (ProDOS) and Their Benzyland Naphthalenylmethyl-Substituted Derivatives

toluenesulfonic acid (PTSA). 3,4-Dimethoxyselenophene (100 mg, 0.523 mmol, 1.0 equiv), 2,2-bis((naphthalen-2-yl)methyl)propane-1,3diol (374 mg, 1.05 mmol, 2.0 equiv), and PTSA (8.9 mg, 0.0523 mmol, 0.1 equiv) were mixed in dry toluene (15 mL) under an argon atmosphere. The reaction mixture was heated under reflux for 2 days. After cooling to room temperature, the solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel (eluent CH2Cl2−hexane, 1:5). 145 mg of the product ProDOS-Np2 was obtained in 58% yield as a white solid with a mp of 141 ± 1 °C (Scheme 2). 1H NMR (400 MHz, CDCl3) δ (ppm): 7.80 (m, 3H), 7.64 (s, 1H), 7.46 (m, 2H), 7.33 (dd, J = 8.4, 1H), 7.01 (s, 1H), 3.93 (s, 1H), 3.07 (s, 1H). 13C NMR (100 MHz, CDCl3) δ (ppm): 151.40, 134.24, 133.29, 132.19, 129.28, 129.11, 127.73, 127.59, 126.12, 125.64, 109.36, 77.22, 60.41, 45.79, 40.03. FTIR (v, cm−1): 2918, 2849, 1459, 1370, 1258, 1013, 790. HRMS: 485.10 g/mol (calculated: 484.09 g/mol) in −2.1 ppm (see Figures S1−S4).

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RESULTS AND DISCUSSION The redox behavior of ProDOS-Np2 was investigated via cyclic voltammetry in an electrolyte solution of 0.1 M TBAH dissolved in CH2Cl2. As expected, ProDOS-Np2 has a lower oxidation potential of 1.82 V than its thiophene analogue ProDOT-Np2 (2.02 V) (Figure 1a). This lower oxidation potential indicates the lower aromaticity of the selenophene ring.17 As shown in Figure 1b, the polymerization was performed between −0.3 and 1.5 V vs Ag/AgCl. During first reverse scan, a nucleation loop was observed, which confirmed the formation of highly conductive species formed on the electrode surface, and one reduction peak at 0.18 V appeared due to the oligomeric species coated on the electrode. These species are oxidized at 0.28 V, and then after each successful repetitive cycle, the intensities of the peak currents started to increase. After a certain number of cycles, two well-defined reversible redox couples were observed at 0.43 V/0.06 V and 1.02 V/0.75 V, which could be attributed to the formation of polaron and bipolaron charge carriers in the obtained polymer film called PProDOS-Np2 (Scheme 2). The polymer film was washed with CH2Cl2 to remove unreacted monomers and oligomeric species from the coated electrode surface. The insoluble PProDOS-Np2 film has a welldefined reversible redox couple at a half-wave potential of 0.24 V, which is lower than PProDOT-Np2 (Figure 2a). Both anodic and cathodic peak currents changed linearly as a function of scan rate, which confirmed that the redox process is nondiffusional and the polymer film was well adhered to the electrode surface (Figure S5). Also, the amount of charge and discharge became constant even at high scan rates, confirming

SYNTHESIS OF ProDOS-Np2

The ProDOS-Np2 was synthesized by the transetherification of 3,4dimethoxyselenophene with 2,2-bis((naphthalen-2-yl)methyl)propane-1,3-diol in the presence of a catalytic amount of pB

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Macromolecules Scheme 2. Synthesis and Electrochemical Polymerization of ProDOS-Np2

Figure 1. (a) Voltammograms of 8 × 10−3 M of ProDOS-Np2 and ProDOT-Np2. (b) Repetitive cyclic voltammogram of ProDOS-Np2 (0.05 M) in order to get PProDOS-Np2 on Pt electrode in an electrolyte solution of 0.1 M TBAH/CH2Cl2 with a scan rate of 100 mV/s between −0.3 and 1.5 V vs Ag/AgCl.

Figure 2. (a) Cyclic voltammograms of PProDOS-Np2 and PProDOT-Np2 films on the Pt electrode in 0.1 M TBAH/CH2Cl2 solution a scan rate of 100 mV/s vs Ag/AgCl. (b) Absorption spectra of the neutral state PProDOS-Np2 and PProDOT-Np2 films on ITO in CH2Cl2.

separation between polymer chains, so the doping/dedoping ability can be improved during redox switching.This intriguing feature makes the polymer film a good candidate to be amenable for use in supercapacitors as an electrode with a broad potential window. The stability of PProDOS-Np2 was tested under ambient conditions in the presence of air (without purging with an inert gas). It exhibited highly stable and robust behavior after thousands of switching between −0.2 and 0.9 V via a squarewave potential method. As expected, the naphthalenylmethyl groups on the bridges will surround the polymer chains and can preserve it from the environment during switching. For example, it retained 84% of its electroactivity even after 5000 cycles (Figure 3). Also, PProDOS-Np2 has higher stability than its thiophene analogue PProDOT-Np2 (63% after 5000 switching).42 Without breaking traditions, the polymer films

charged and discharged (doping/dedoping) ability of the polymer. It is well-known that superior p-doping process of the film can make it a promising candidate for battery and capacitor applications. For example, a conjugated polymer film with a nearly rectangle shape of cyclic voltammogram with a wide potantial window can amenable for use in electrochemical capacitors. In order to show the utility of PProDOS-Np2 film as a capacitor, flat current responses as a fingerprint of the capacitors were observed when the film was cycled between 0.45 and 1.2 V (Figure S6). The polymer film preserved its rectangle shape of cyclic voltammogram even at high scan rates with no appreciable change in voltammogram and also the amount of charge (Qa) and discharge (Qc) did not change as a function of scan rate, indicating a fast charge process (doping/ dedoping). This behavior can be concluded that the presence of naphthalenylmethyl groups can result in an interchain C

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as a member of polyselenophenes.25,26 This high percent transmittance value makes PProDOS-Np2 an attractive candidate for electrochromic applications. When compared to the naked PProDOS (51%) and PProDOS-Bz2 (65%), an increase in the size of the bulky groups in PProDOS-Np2 causes a higher percent transmittance change. However, this value decreases to 64% at 630 nm for PProDOT-Np2 film. On the other hand, the response time (or switching time), which is the time necessary to change the color of the polymer, is a key parameter for electrochromic materials to be found an application in optical devices and displays. In order to find the response time, the percent transmittance changes were recorded in situ via a square-wave potential method. The polymer film on ITO was switched between its redox states under external potentials of −0.3 and 0.7 V with a switching time of 10 s, and the response time was found as about 1 s. The transmissivity of the PProDOS-Np2 film was also confirmed by using the relative luminance measurement (Δ% Y). The polymer film exhibited high luminance over 90% during oxidation, and Δ%Y was calculated to be as 19% between the colored and bleached states (Figure 4).

Figure 3. Stability test for the PProDOS-Np2 (159 mC/cm2) film in 0.1 M TBAH/CH2Cl2 by using a square-wave potential method between −0.2 and 0.9 V for 3 s under ambient conditions. The voltamograms of the PProDOS-Np2 (159 mC/cm2) film at a scan rate of 100 mV/s by cyclic voltammetry as a function of the number of cycles: A: 1; B: 1000; C: 2500; D: 5000 cycles; Qa: anodic charge stored; ipa: anodic peak current; ipc: cathodic peak current.

based on ProDOS units are exceptionally more stable than their ProDOT analogues.39,40,42

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SPECTROELECTROCHEMISTRY Like PProDOT-Np2 films, the optical spectrum of the neutral PProDOS-Np2 film showed vibronic couplings at 595, 641, and 705 nm in its π−π* transition band (Figure 2b). These splittings in the optical spectrum indicated a well-organized (regioregular) and more rigid structure like ProDOT-Np2 (529, 566, and 616 nm). Also, the optical spectrum of PProDOS-Np2 films was red-shifted between 66 and 89 nm (Figure 2b). The band gap (Eg) value was determined from the lowest energy end of the π−π* transition and found as 1.67 eV, which is 0.24 eV smaller than PProDOT-Np2 film (1.91 eV) and somewhat higher than PProDOS (1.65 eV) (see Table 1).30

Figure 4. Relative luminances of PProDOS-Np2 (au 1.1 at 705 nm) and PProDOT-Np2 films (au 0.8 at 630 nm) on ITO in 0.1 M TBAH/ CH2Cl2 at various applied potentials.

Table 1. Optical Properties of PProDOS and PProDOT Derivatives polymers

Eg (eV)

λmax (nm)

PProDOS30 PProDOT43 PProDOS-Bz242 PProDOT-Bz242 PProDOS-Np2 PProDOT-Np242

1.65 1.7 1.62 1.86 1.67 1.91

626, 567, 645, 575, 641, 566,

a

686 625 710 630 705 630

Δ%T

CEa (cm2/C)

color at neutral state

51 54 67 75 84 64

273 255 992 551 755 337

blue violet blue violet blue purple

As shown in Figure 5a, during oxidation of PProDOS-Np2 film on ITO the π−π* transition band started to decrease with a concomitant increase after 750 nm attributed the formation of charge carriers. Upon further oxidation, the color of the polymer films changed from pure blue to transmissive state/ colorless and the π−π* transition band disappeared completely. The absorption band intensified after 750 nm received a maximum level and then decreased, which is the conversion of polaron to bipolaron charge carriers as a signature of a doped conjugated polymer during oxidation. The low absorption intensity beyond 750 nm for oxidized PProDOS-Np2 indicated a weak interaction between polymer chains, which can be attributed to the naphthalenylmethyl groups on the bridges.25 On the other hand, this interaction is stronger between the PProDOT-Np2 chains as shown in Figure 5b. Also, the similar changes in optical spectrum under external potentials were observed for PProDOT-Np2 film, which is purple when neutralized and transmissive purple when oxidized. The PProDOS-Np2 film changed its color reversibly many times. Its stability, robustness, neutral state pure blue color, and transparency in the oxidized state make it a promising candidate for electrochromic displays and devices.

At 95% of the full contrast.

The red-shift in ProDOS-Np2 is also reflected in the colors of the polymers. PProDOT-Np2 was purple at the neutral state (L = 61.86, a = 10.35, b = −36.98), whereas PProDOS-Np2 film was pure blue (L = 75.14, a = −27.85, b = −35.34) when neutralized due to the absence of absorbance between 400 and 500 nm. Both polymer films are highly transmissive at their oxidized states: L = 92.70, a = −2.84, b = 2.93 for PProDOSNp2 and L = 93.88, a = −1.43, b = 0.84 for PProDOT-Np2. Upon moving from neutral state to oxidized state under various applied external potentials, PProDOS-Np2 exhibited 84% of the percent transmittance change, to our best knowledge (Figure S7), which is the second highest reported optical contrast ratio D

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Figure 5. Optical absorption spectra of (a) PProDOS-Np2 and (b) PProDOT-Np2 on ITO in 0.1 M TBAH/CH2Cl2 at various applied potentials.

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The coloration efficiency (CE) is an important parameter for optical and display applications. The CE measures the power efficiency of the electrochromic materials and can be calculated by the equation

*Tel +903125868304; Fax +903125868091; e-mail atilla. [email protected] (A.C.).

CE = ΔOD/Q d and ΔOD = log(Tcolored /Tbleached)

Notes

The authors declare no competing financial interest.

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where ΔOD is the optical density, Qd is the injected/ejected charge during a redox step, and Tcolored and Tbleached are the percent transmittance in the neutral and oxidized states, respectively. The CE of PProDOS-Np2 was calculated as 755 cm2/C, which is one of the highest reported CE for polyselenophene derivatives.

ACKNOWLEDGMENTS We express our thanks to the Scientic and Technical Research Council of Turkey (TUBITAK-111T976) and Atilim University (ATU-BAP-A-1314-01) for their financial support.

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CONCLUSIONS A new derivative of 3,4-propylenedioxyselenophene (ProDOS) bearing naphthalenylmethyl appeandages on the center of the propylenedioxy bridge was sysnthesized and polymerized electrochemically. The corresponding polymer film exhibited one of the highest optical contrast ratios among the polythiophene and polyselenophene derivatives. Electrochemically obtained insoluble polymer film is pure blue at the neutral state and highly transmissive in the oxidized state. Unlike the polythiophene analogue, the results of optical experiments indicated that an increase in the size of the substituent on the bridge gave rise to an increase in the optical contrast of the related ProDOS-based polymers. For example, while the parent PProDOS film has 51% optical contrast, dibenzyl-substituted PProDOS-Bz2 has 65% and dinaphthalenylmethyl-substituted PProDOS-Np2 has 84% optical contrast. Also, the PProDOSNp2 film is highly stable and robust under ambient conditions. In addition, the film reversibly repeated its electrochemical and optical properties. When compared to polythiophene analogue, the obtained polyselenophene-based polymer PProDOS-Np2 has lower oxidation potential, lower band gap, higher optical contrast, coloration efficiency, and stability. The electrochemical and optical studies indicated that unlike selenophene system, there is no direct relationship between electrochromic properties and the rigidity of substituents in thiophene system. Therefore, to support our conclusion, it is necessary to investigate similar molecules with larger bulky groups, like antharacene and pyrene. Work in this line is currently underway in our laboratories.

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AUTHOR INFORMATION

Corresponding Author

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; Figures S1−S7. This material is available free of charge via the Internet at http://pubs.acs.org. E

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