A Pure Blue to Highly Transmissive Electrochromic Polymer Based on Poly(3,4-propylenedioxyselenophene) with a High Optical Contrast Ratio

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 (ProDOSNp 2 ), was synthesized and polymerized via potentiostatic and potentiodynamic methods. The electrochemically obtained polymer film (PProDOS-Np 2 ) 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-Np 2 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. ■ INTRODUCTIONIt 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 3,4-ethylenedioxythiophene (EDOT) unit, 31−35 Cava et al. andBendikov 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...

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