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Electropolymerized Polythiophenes Bearing Pendant Nitroxide Radicals Fei Li, Yanpu Zhang, Se Ra Kwon, and Jodie L. Lutkenhaus* Artie McFerrin Department of Chemical Engineering, Texas A&M University, 3122 TAMU, College Station, Texas 77843-3122, United States S Supporting Information *
ABSTRACT: We report a facile way to synthesize polythiophenes carrying pendant 2,2,6,6-tetramethylpiperidinyl-1oxyl (TEMPO) radicals, here called PTATs, by electropolymerization in boron trifluoride diethyl etherate (BFEE). The spacing between the TEMPO radical and the polythiophene backbone is varied by an alkyl spacer (n = 2, 4, 6), and the electronic and electrochemical properties are examined using UV−vis spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. Film morphologies are also studied via scanning electron microscopy (SEM) and atomic force microscopy (AFM), which show that the longer octyl chain placed between thiophene and TEMPO effectively suppresses aggregation. The highest conductivity and electroactivity are observed for n = 4 and n = 6, respectively. Such morphology differences provide an opportunity to better understand the charge transport and energy storage properties in electronic materials.
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Poly(3-hexylthiophene) (P3HT), popularly studied in organic solar cells,34,35 has recently shown good electrochemical performance as an additive in hybrid Li-ion battery electrodes with LiFePO436 and V2O5.37 P3HT has good mobility (up to 0.1 cm2 V−1 s−1)38,39 and electrical conductivity (as high as 50 ± 20 S cm−1 upon doping).40,41 It is modestly redox active with a reversible doping level or capacity of 82 mAh/g.42,43 Importantly here, the polythiophene chemistry is synthetically accessible for organic radical functionalization. To date, polythiophene-organic radical polymers are not well understood, and only a handful of reports exist.31−33 This is perhaps because competing redox activity of the thiophene and the radical unit challenges polymerization. Compared with various methods developed to synthesize PTMA,19,44−47 the synthesis of its conjugated counterpart polymers seems to be less explored. Traditional transition-metal-catalyzed coupling polymerizations do not work with such monomers due to interference from the TEMPO radical in the catalytic cycle. One feasible way to synthesize polythiophene-organic radical polymers is through FeCl3-aided oxidative polymerization; however, polymerization is uncontrolled, and the products are intractable, preventing solution processing and characterization.31 Electropolymerization of thiophene monomers bearing pendant nitroxide radicals has been reported;33,48,49 however, only one type of such polymer was reported due to the limited commercial availability of monomers.
n recent times, organic radical polymer batteries have emerged as promising alternatives to conventional Li-ion batteries. 1−5 The organic radical polymer battery has demonstrated mechanical flexibility, extraordinary cyclability, high power, and fast charge transfer kinetics.6−9 The most commonly used organic radical functional groups used include 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO),10−14 2,2,5,5tetramethyl-1-pyrrolidineoxyl (PROXYL),15 galvinoxyl,16,17 nitronylnitroxide,17 and nitroxylphenyls.18 Poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA), which was first reported by Nakahara10 and Nishide11 as cathode materials, has been intensively studied. The theoretical capacity of PTMA is 111 mAh g−1 based on a one-electron transfer process between PTMA’s neutral nitroxide radical and its p-doped oxoammonium cation.11 One major challenge for PTMA, as well as other organic radical polymers, lies in the insulating nature of the polymer backbone. The electronic conductivity of nitroxide radical-based polymers such as PTMA is in the range of 10−6 S cm−1.19 To accommodate the polymer’s low electronic conductivity, organic radical polymer electrodes often contain excessive amounts of carbon additives (upward of 60%),20−26 which proportionally reduce the overall electrode capacity. One way to bypass this issue is to design intrinsic conductivity into the organic radical polymer itself by way of a conjugated backbone. Various conjugated polymers including polyacetylene,27,28 polypyrrole,29,30 and polythiophene31−33 with pendant TEMPO radicals were reported over the years. However, these polymers showed even worse capacity and cycling stability as compared to the PTMA homopolymer. © 2016 American Chemical Society
Received: December 21, 2015 Accepted: February 8, 2016 Published: February 17, 2016 337
DOI: 10.1021/acsmacrolett.5b00937 ACS Macro Lett. 2016, 5, 337−341
Letter
ACS Macro Letters The physical properties of polythiophene-organic radical polymers is even less well understood and remains an intriguing topic. With two redox-active sites per repeat unit, the polymer may display unusually high doping levels or capacity. The conductivity may mimic that of a polythiophene or of PTMA. Further, these properties may be tuned by the spatial proximity of the backbone and the radical group. Here, we report a facile synthetic route to prepare a series of polythiophene-organic radical polymers bearing TEMPO radicals. The nitroxide radical’s proximity to the polythiophene’s backbone is varied using alkyl spacer groups of varying lengths (n = 4, 6, 8). Possible interactions between the backbone and radical group are examined. Finally, the electronic and basic electrochemical properties of the polymer are presented and discussed. The preparation of polymers 1a−1c, poly(2,2,6,6-tetramethyl-4-(4-(thiophen-3-yl)alkoxy)piperidin-1-yloxy) (PTATs), is shown in Scheme 1. Starting from 3-bromothiophene, we
Figure 1. Electropolymerization of 0.01 M (a) 2a, (b) 2b, and (c) 2c monomers in BFEE by cyclic voltammetry. Ten successive cycles at a scan rate of 25 mV·s−1 are shown. Ag wire was used as a quasireference electrode, Pt plate as the counter electrode, and ITO-coated glass as the working electrode. The legend in (a) applies to all panels. (d) Digital images of 1a, 1b, and 1c from electropolymerzation on ITO-coated glass in BFEE.
Scheme 1. Synthetic Route for Polymers 1a−1c
little film growth (
