A Route to Stable Interfaces Between Dissimilarly Doped Conjugated Polymers
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reported using ion implantation,[9] but such a doping methodology can result in graphitization and is not general.[4] Herein, we proposed a general approach to interfaces between conjugated polymers with different doping profiles. The approach utilizes internally charge compensated conjugated polymers. Here, the terminology internally compensated refers to the specific situation of a doped polymer where the π-system charge is compensated by bound counter-ions. In the literature, such internally compensated polymers are often referred to as “self-doped.”[10-15] With such materials, interfacial reactivity is not possible because mobile charge compensating io ns are not available to support bulk chemical reactions. The idea draws strong analogy to inorganic semiconductors where the charge balancing counter-ions – ionized dopant atoms – are fixed in crystalline lattice sites. As a demonstration of the potential of ionically functionalized conjugated polymers, herein referred to as conjugated ionomers, in the fabrication of interfaces between differentially doped samples, we report the enhanced stability of an internally compensated, oxidatively doped form of p olyacetylene interfaced to an undoped neutral substituted polyacetylene. The specific materials chosen for study were functionalized polyacetylenes synthesized from the ring opening metathesis polymerization of monosubstituted cyclooctatetraenes (R-COTs). Specifically, polyacetylenes with cationic [poly(C 8H7CH2CH2NMe3OTf) ≡ PA cat], anionic [poly(C 8H7CH2CH2SO3NMe4) ≡ PAan] and neutral [poly(C 8H7CH(CH3)CH2CH3) ≡ PA butyl] substituents were used in this study. EXPERIMENTAL PA cat, PA an and PA butyl were synthesized as reported previously.[16] PA cat and PA an were deposited from dimethyl sulfoxide (DMSO) solutions onto planar gold electrodes, which consisted of two thin -film regions of gold on glass separated by a 100µm gap. The gap in the gold substrate permitted in situ two -probe conductivity measurements. The polymers were oxidatively doped by electrochemical means in 0.12 M tetrabutylammonium tetrafluoroborate (Bu 4NBF4) / CH2Cl2 with potentiostatic control at 0.73 V vs SCE. Excess Bu 4NBF4 was subsequently extracted from the polymers with CH2Cl2. The conductivities of the oxidized films were estimated to be in the range 0.070.2 Ω -1cm-1 by two probe measurements. RESULTS AND DISCUSSION From the results presented below and literature precedence,[11] it is presumed that the electrochemical oxidation of PA an followed by rinsing with CH2Cl2 results in the formation of an internally compensated form of the polymer. Studies on related anionically substituted OCPs have demonstrated that electrochemical o xidation occurs primarily with the loss of cations rather than the incorporation of anions.[11] Furthermore, any BF 4- incorporated during electrochemical oxidation would occur with the formation of salt within the polymer, which is presumably removed during rinsing with CH2Cl2. Schematic 1A illustrates the central experiment designed to show the feasibility
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