Three-dimensional photochemical microfabrication of poly(3,4-ethylene- dioxythiophene) in transparent polymer sheet
Introduction
Certain conducting polymers, such as polypyrrole (PPy) and polythiophene, begin to absorb in the visible region when the transition from the monomer to polymer form occurs, and these polymers can therefore be used for image formation. To form images, local polymerization of the monomers and shape processing of the polymer regions are required. However, many conducting polymers are insoluble in most solvents, making these materials difficult to process. Several studies have already been carried out on two-dimensional (2D) patterning of conducting polymer films using a variety of techniques [1], [2], [3], [4], [5], [6], [7]. For the purpose of the image formation, we and other researchers have reported that 2D micropatterns of polyaniline or PPy can be produced on a substrate using the local oxidation power associated with photo-induced electron transfer from an excited state of a tris(2,2′-bipyridyl)ruthenium complex [Ru(bpy)32 +] to an electron acceptor [8], [9]. Electron transfer between molecules containing Ru(bpy)32 + can be achieved not only by a one-photon process using an ordinary light source, but also by a multi-photon process using a femtosecond pulse laser [10], [11], [12], [13].
We have previously reported a local photopolymerization method for conducting PPy [14]. In this method, polymerization occurred under the influence of the spatially localized oxidation power associated with two-photon absorption by a Ru(bpy)32 + photosensitizer and successive photo-induced electron transfer between the excited state of the Ru(bpy)32 + and a methylviologen [MV2 +] electron acceptor. In practice, polymerization occurred only at the focal point of the near-infrared femtosecond pulse laser, so that 3D PPy micropatterns could be produced in a transparent polymer sheet by suitable scanning of the focal point [15]. The lateral resolution of the patterning process was less than 500 nm, which was better than the diffraction-limited spot diameter of the optical system, which is given by 1.22 × λ/NA, where the wavelength λ was 850 nm and the numerical aperture NA was 1.2 [16]. These results clearly indicate that multi-photon processes can achieve higher patterning resolution than one-photon processes.
The poly(3,4-ethylenedioxythiophene) (PEDOT) is known to exhibit high electrical conductivity and chemical stability, and has been used in electrochromic devices, the hole transfer layer in electroluminescence devices and in modern thermoelectric applications [17], [18], [19]. Similar to other conducting polymers, the film and 2D pattern were examples of the processed form of PEDOT [20]. Since the electrical conductivity of PEDOT is expected to be at least as high as that of PPy (~ 200 S/cm), and possibly as high as ~ 1000 S/cm, 3D micropatterns formed from PEDOT have the potential to be used in a wide range of fields including switchable metamaterials.
In the present study, an attempt was made to fabricate 3D PEDOT micropatterns using a dimeric compound of 3,4-ethylenedioxythiophene (bis-EDOT) as the starting material.
Section snippets
Experiment
Bis-EDOT and all other chemicals were purchased from the AZUMA Corporation and Aldrich Chemicals, respectively. These chemicals were used without further purification. The photopolymerization solution comprised 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4), 25 mM bis-EDOT, 1 mM MV2 + and 1 mM Ru(bpy)32 + dissolved in acetonitrile.
A 10 mm × 10 mm Nafion 117 sheet (from DuPont, 200 μm thickness) was used as the substrate for PEDOT deposition. The sheet was immersed in the polymerization solution, and
Results and discussion
After the formation of the excited state [Ru(bpy)32 + ⁎] in the photosensitizer, electron transfer occurs from Ru(bpy)32 + ⁎ to MV2 +, which is the oxidative deactivation process. This leads to the formation of the oxidation state Ru(bpy)33 +, and photopolymerization of the monomer occurs due to the oxidation power of Ru(bpy)33 +. The requirement for downhill electron transfer is that the monomer has a more negative oxidation potential than the redox potential of the photosensitizer. Therefore, the
Summary
Using bis-EDOT as the starting material, 3D micropatterns of PEDOT were produced using a multi-photon photopolymerization method. Electrochemical measurements were carried out on both EDOT and bis-EDOT, and it was determined that their oxidation peak potentials were + 1.15 and + 0.55 V vs. Ag/Ag+, respectively. Since the value for EDOT is more positive than the redox potential for the ruthenium complex used as the photosensitizer (+ 0.62 V vs. Ag/Ag+), photopolymerization of EDOT is difficult to
Acknowledgment
This study was carried with the support of Shinkawa, Ltd.
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