791 Macromolecular Research, Vol. 17, No. 10, pp 791-796 (2009) Electrochromic Pattern Formation by Photo Cross-linking Reaction of PEDOT Side Chains Jeonghun Kim, Yuna Kim, and Eunkyoung Kim* Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Korea Received Febuary 11, 2009; Revised April 1, 2009; Accepted April 4, 2009 Abstract: An electrochemically and photochemically polymerizable monomer, 2-((2,3-dihydrothieno[3,4-b][1,4]dioxin- 2-yl)methoxy)ethyl methacrylate (EDOT-EMA), was explored for patterning of poly(3,4-ethylenedioxythiophene) (PEDOT) via side chain cross-linking. The polymer from EDOT-EMA was deposited electrochemically to produce polymeric EDOT (PEDOT-EMA), which was directly photo-patterned by UV light as the side EMA groups of PEDOT-EMA were polymerized to give cross-linked EMA (PEDOT-PEMA). Absorption and FTIR studies of the UV-exposed film (PEDOT-PEMA) indicated that the photo-patterning mainly originated from the photo cross-link- ing of the methacrylates in the side-chain. After irradiation of the film, the conductivity of the irradiated area decreased from 5.6×10 S/cm to 7.2×10 S/cm, possibly due to bending of the conductive PEDOT channel as a result of the side chain cross-linking. The patterned film was applied to a solid state electrochromic (EC) cell to obtain micro-patterned EC cells with lines up to 5 μm wide. Keywords: PEDOT, side chain cross-linking, electrochemical, photo-pattern, conductivity, electrochromic. Introduction Patterning of active polymer films at micro scales is important for the development of organic electronics, optics, and bio-engineering. 1,2 In particular, the micro-litho- graphic formation of conducting patterns is a key prerequi- site for various practical applications, such as in integrated circuits, field-effect transistors, multichannel molecular rec- ognition, region-specific cell growth or protein fixation, optical memory storage devices, and electroluminescent and electrochromic displays. 3-11 Several methods, including top-down and bottom-up patterning of polymers, have been suggested for the generation of conducting patterns 12 (e.g., plasma etching, micromolding, photolithography, printing techniques, self-assembly of block copolymers, and insta- bility-induced patterning). These methods lead to increased control over the pattern resolution and conductivity, render- ing them highly valuable methods for various micro- to nano-technological applications. The photolithography and plasma patterning of conductive copolymers have success- fully been used to fine-tune conductive nanostructures; however, these methods require multi-step patterning pro- cesses including an etching process. Thus, tools to pattern conductive polymers in a predictable and simple manner are indispensable. PEDOT has attracted considerable attention because it gives highly stable thin films with sufficient electrical con- ductivity for applications as an electrode in organic elec- tronics after secondary doping. 13-17 The conductivity is highly sensitive to the structural change in PEDOT main chain, and structural distortion by dopants and any external stimuli affects the electrical conductivity of PEDOT. In this context, selective disconnection of the conducting channel by a photon method is of interest because it would provide a new and simple patterning method for PEDOT film. Several different approaches to create photolithographically defined patterns of poly-3-alkylthiophenes on insulating or conducting substrates has been reported. 18 Acrylate-functionalized 3- alkylthiophene copolymer produced negative-tone images by irradiating a spin-coated film of the polymer through a mask and then development of the latent image with chloroform. The photoinduced decrease in the solubility of the acrylate-functionalized 3-alkylthiophene copolymer is due to cross-linking of the acrylate side groups. A poly-3- alkylthiophene containing an acid-labile tetrahydropyranyl (THP) group in the side chain affords photolithography to pattern the material by acid-catalyzed removal of the THP group. 19,20 Photocurable EDOT monomers could be interesting as it can induce high degree of cross-linking and can be com- bined with other monomers to tailor polymer properties. As an example, MDOT (EDOT-methylene metharcrylate) was copolymerized and applied for photolithographic grating- pattern formation. 21,22 However, unlike the side chain func- *Corresponding Author. E-mail: eunkim@yonsei.kr