Ionic Liquids in Organic Synthesis - American Chemical Society

Chapter 3

Electropolymerization of Benzene in an Ionic Liquid

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on July 27, 2013 | http://pubs.acs.org Publication Date: January 18, 2007 | doi: 10.1021/bk-2007-0950.ch003

F. Endres, N. Borissenko, and S. Zein El Abedin Clausthal University of Technology, D-38678 Clausthal-Zellerfeld, Germany

Due to their wide electrochemical windows ionic liquids are ideally suited to electrochemical studies. It is demonstrated in this short article that in the ionic liquid 1-Hexyl-3methylimidazolium tris(pentafluoroethyl)trifluorophosphate benzene can be well electropolymerized to give polyphenylene. The deposit is electrochemically active and shows a quasireversible electrochemical behavior. It is shortly discussed that ionic liquids might have a certain impact on the fabrication of devices with conducting polymers.

Introduction Electronically conductive polymers have been intensively studied since 1977 (1). Typical examples are polyacetylene, polypyrrole, polythiophene, polyp-phenylene, polyaniline and their derivatives, but also many other heterocyclic or aromatic polymers (2). Often these polymers can be prepared by electrochemical oxidation from aqueous or non-aqueous electrolyte solutions containing the respective monomer. They are characterized by a good electrical conductivity combined with a low weight. The conductivity is based on the conjugated π-electron system, and the formation of charge carriers like polarons or bipolarons by doping. There are more than 5000 papers in the literature that are devoted solely to polypyrrole. One reason is surely that it can be made quite easily in aqueous solutions. A common feature of these conducting polymers is 28

© 2007 American Chemical Society

In Ionic Liquids in Organic Synthesis; Malhotra, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


29 that they show a quasireversible electrochemical behaviour during successive reduction and oxidation cycles. The doping degree of these conducting polymers can be adjusted by proper selection of the electrode potential. The interesting properties of these polymers have led to their use in supercapacitors, sensors, electrochromic devices, light emitting diodes and other ones, to mention a few. Among the conducting polymers poly(para)phenylene (PPP) is very interesting because it is suited for the fabrication of blue organic light emitting diodes (OLED) (3 - 6). However, the electrochemical polymerization of benzene to PPP is still a challenge as water in the solution has to be avoided strictly. Therefore, in the past only solvents like concentrated sulfuric acid (7), liquid S 0 (8) or liquid HF were feasible for the electropolymerization of benzene. In 1993 ionic liquids based on A1C1 were employed for the first time for the electropolymerization of benzene (9). Because of side reactions due to chlorine co-evolution during the electropolymerization the quality of the deposits was not satisfying. Ionic liquids are by definition ionic melts with a melting point below 100 °C (10). During the recent 5 years there has been a remarkable progress in the synthesis of ionic liquids, and there are now already several companies on the market from which these new solvents can be purchased. Especially air and water stable ionic liquids have gained a great interest in the recent years, as depending on the liquid high ionic conductivities, negligible vapour pressures and wide electrochemical windows are obtained. These properties are very important for electrochemical studies, and it has been shown that it is possible to electrodeposit nanoscale light metals and semiconductors (11, 12) in them. It should be mentioned here that ionic liquids have already been employed for the electrodeposition of polypyrrole (13) and poly(3,4-ethylenedioxythiophene) (14) as well as a few other ones. In general it is observed that the electrochemical reversibility of the polymers in ionic liquids is excellent. 2

3

Experimental For synthesis and characterization an ionic liquid from Merck KGaA (EMD) was employed: l-Hexyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate ([HMIm]FAP) was used as a solvent for the electropolymerization of benzene as well as for the electrochemical characterization of the synthesized polymer films. The liquid was dried under vacuum at an elevated temperature of 110 °C under stirring to water contents below 3 ppm prior to use, as determined by Karl-Fischer titration. Benzene (FLUKA > 99,5 %) with a water content below 0,005 % was used without further purification, its concentration in the liquid was 0,2 mol/1 (giving a water content below 5 ppm). All experiments were performed in an argon filled glove box (Vacuum Atmospheres OMNILAB) with water and oxygen below 1 ppm. For the electrochemical experiments a PARSTAT



30 2263 potentiostat/galvanostat controlled by a PowerCV software was employed. The quartz crystal microbalance studies were performed measuring the electrical admittance curve of a quartz (covered on both sides with microcrystalline platinum) in parallel to the electrochemical experiments with a network analyzer (Agilent E5100A). One side of the quartz served as working electrode. A platinum ring as well as a platinum wire served as counter and reference electrodes, respectively. Prior to each experiment the platinum electrodes were cleaned by heating to red heat in a hydrogen flame. The electrochemical cell was cleaned at room temperature in a mixture of 50/50 v-% concentrated H S 0 and aqueous hydrogen peroxide (30 %) followed by refluxing in pyrogene free water (aqua destillata ad iniectabilia). A high resolution field emission scanning electron microscope (Carl Zeiss DSM 982 Gemini) was utilized to investigate the surface morphology of the polymer film. 2

4

Results and Discussion Figure 1 shows the electrochemical window of the ionic liquid on poly- and microcrystalline platinum:

Figure 1. Electrochemical window of l-hexyl-3~methylimidazolium tris(pentafluoroethyl)trifluorophosphate on platinum


31


An electrochemical window of a little more than 4 Volt is observed, which in the cathodic regime is limited by the irreversible reduction of the organic cation leading to less defined oligomers. In the anodic regime the electrochemical window is limited by the decomposition of the organic anion. On this electrode potential scale the peak couple ferrocene/ferrocinium is at +500 mV, demonstrating that the anodic decomposition limit of this ionic liquid is about + 3000 mV vs. NHE. Figure 2 shows 5 successive cycles of potentiodynamic benzene polymerization:

1.D Q.B G.B

Ε G.4 u < 0.2

Ξ

o.o -0.2 -0.4 0.0

0.5

1.0

1.5

2.0

E / V v s . Pt

Figure 2. Potentiodynamic electropolymerization of benzene at a scan rate of 10mV/s, c(benzene) =0.2 mol/l

In the first scan only capacitive currents flow until an electrode potential of 1.7 Volt. At 1.7 Volt the electrooxidation of benzene starts giving rise to a dark deposit. In the first reduction scan there is a rising cathodic current which peaks at about +750 mV. In the following anodic scan an oxidation current is observed with a peak at about +1250 mV before at +1750 mV a further oxidation of benzene sets in. With a rising number of cycles the reduction and oxidation currents rise, which is typical for a conducting polymer. As was shown elsewhere (15) the peak currents vary almost linearly with the square root of the scan rate indicating diffusion control. However, the wide peak separation is indicative of high overvoltages in the polymer. In order to determine the degree of doping in the polymer we performed quartz crystal microbalance experiments. Simultaneously with the electro-



32 chemical investigations the resonance curve of a platinum covered quartz resonator was measured. Details on this experiment will be reported elsewhere (16). As for all conducting polymers, the charge needed for the formation of the oxidized polymer can be related to the charge needed to build the polymer backbone (2F per mole of benzene), and the charge needed to oxidize the polymer, until it reaches the (potential-dependent) degree of oxidation x. The mass change is due to the effective mass of the benzene rings (76 g per mole of benzene) and the amount of counter anions incorporated into the polymer to compensate for the positive charges within the polymer (445 g * χ per mole of benzene). To complicate matters, there can be an additional contribution to mass by trapped ionic liquid. Therefore one can write:

M

dm _ M dQ~

+χ·Μ,

C

Ionic Liquids in Organic Synthesis - American Chemical Society

Recommend Documents