Materials Characterization and Synthesis of Conductive Aromatic-Bis(Benzothiazoles)
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MATERIALS CHARACTERIZATION AND SYNTHESIS OF CONDUCTIVE AROMATIC-BIS(BENZOTHIAZOLES) Max D. Alexander Jr.*, Balasubramanian Sankaran, B. Robert McKellar, and Douglas S. Dudis Polymer Branch, Materials and Manufacturing Directorate, Air Force Research Laboratory Wright-Patterson Air Force Base, Ohio 45433-7750, USA ABSTRACT Functionalized aromatic-bis(benzothiazoles) have been synthesized by our group and have shown promise as conductive n-dopable polymers and oligomers. When reduced (n-doped) these materials typically exhibit conductivity on the order of tens of S/cm. Here we examine the material properties of this family of derivatized, conductive aromatic-bis(benzothiazoles). A variety of synthetic approaches have been examined to produce these polymers and oligomers, and will be discussed. Material characterization has been accomplished by spectro-electrochemistry, nuclear magnetic resonance (NMR), electron spin resonance (ESR), Fourier Transform Infrared (FTIR) spectroscopy, and direct current (DC) conductivity. INTRODUCTION There are currently very few n-doped organic materials available and even fewer that exhibit long-term environmental stability [1,2]. This can be attributed to the molecular composition of the materials and their corresponding reduction potentials. An n-doped material is in a reduced form, which is often susceptible to re-oxidation by oxygen and/or water when exposed to the environment. Encapsulation is required to preserve the conductivity level of the reduced material. To exhibit stability toward reoxidation by water, n-doped polymers and organic compounds must have an electrode potential greater than –0.658 V vs. Standard Calmel Electrode (SCE). To be stable in the presence of both oxygen and water, the electrode potential must be greater than +0.571 V vs. SCE. Most traditional n-doped polymers and organic compounds exhibit reduction potentials less than –1.0 V vs. SCE, and therefore are not stable in an open environment. In contrast, the aromatic-bis(benzothiazole) materials we have developed have been shown to remain in their conducting state for several months, without the need for encapsulation. Previously, we have shown that poly(p-phenylene-benzobisthiazole) (PBZT) can be protonated at the thiazole imine sites under acidic conditions and subsequently reduced in the presence of metals such as copper, zinc, or iron [3]. The proposed mechanism for this process is illustrated in Figure 1. The resulting polymer is predicted to form an alternating quinoidal / aromatic structure, with a high degree of electron delocalization [4]. The product is a moderately air stable, black, n-doped polymer with d.c. conductivity on the order of 102 S/cm . This is an exceptionally remarkable result as both the unprotonated and the unreduced-protonated polymers have conductivities on the order of 10-12 S/cm. Also of great consequence, is the apparent near stability of the ndoped product toward air. This is an indication that the reduction potential of the C8.35.1
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