Processible conjugated polymers: from organic semiconductors to organic metals and superconductors

Abstract

Since conjugated polymers, i.e. polymers with spatially extended π-bonding system offer unique physical properties, unobtainable for conventional polymers, significant research efforts directed to better understanding of their chemistry, physics and engineering have been undertaken in the past two and half decades. In this paper we critically discuss synthetic routes to principal conjugated polymers such as poly(acetylene), polyheterocyclic polymers, poly(p-phenylene vinylene)s, aromatic poly(azomethine)s and poly(aniline) with special emphasis on the preparation of solution (and in some cases thermally) processible polyconjugated systems. In their neutral (undoped) form conjugated polymers are semiconductors and can be used as active components of ‘plastics electronics’ such as polymer light-emitting diodes, polymer lasers, photovoltaic cells, field-effect transistors, etc. Due to its strongly non-linear I=f(V) characteristics in high electric fields, undoped poly(aniline) can be used as stress grading material for high voltage cables. In the next part of the paper we describe redox and acid–base doping of conjugated polymers and its consequences on structural, spectroscopic and electrical transport properties of these materials. Special emphasis is put on dopant engineering, i.e. on the design of the dopants which not only increase electronic conductivity of the polymer but also induce desired properties of the doped polymer system such as improved processibility, special catalytic properties or special optical or spectroscopic properties. Selected examples of technological applications of doped conjugated polymers are presented such as their use as conductive plastics, optical pH sensors, heterogeneous catalysts, gas separation membranes, etc. The paper is completed by the description of the recent discovery of the first organic polymer superconductor.

Introduction

Polymers with spatially extended π-bonding system, here abbreviated as ‘conjugated polymers’, although known for many years, did not draw significant research attention till the mid 1970s. This was caused by the fact that in their vast majority they were intractable and, in many respects, showed inferior properties as compared to already developed polymers. Before 1977, papers dealing with polyconjugated systems were rare and the research devoted to these materials was not systematic. Neither molecular nor electronic structures of conjugated polymers in their undoped state were elucidated. Moreover, the chemical nature of the doping reactions which render these polymers conductive was not known similarly as the mechanism of their conductivity, despite the fact that few papers describing unusually high conductivity of some conjugated polymers were published. In 1977 Heeger, MacDiarmid and Shirakawa showed that poly(acetylene), which is the simplest polyconjugated system, can be rendered conductive by the reaction with bromine or iodine vapors [1]. Spectroscopic studies that followed demonstrated without any ambiguity that this reaction is redox in nature and consists of the transformation of neutral polymer chains into polycarbocations with simultaneous insertion of the corresponding number of Br₃⁻ or I₃⁻ anions between the polymer chains in order to neutralize the positive charge imposed on the polymer chain in course of the doping reaction [2]. This important discovery initiated an extensive and systematic research devoted to various aspects of the chemistry and physics of conjugated polymers both in their neutral (undoped) and charged (doped) states. According to SCIFINDER almost 40,000 scientific papers were published in this field of research since 1977. This previously underestimated family of macromolecular compounds turned out to be extremely interesting both from the basic research and application points of view. As a result, in 2000, Heeger, MacDiarmid and Shirakawa — the founders of the ‘conjugated polymer science’ — were granted Nobel Prize in chemistry [3], [4], [5]. Another exciting event followed shortly after the attribution of the Noble Prize for conducting polymers. Schön et al. [6] demonstrated that regioregular poly(3-hexylthiophene) if used in a field-effect transistor (FET) configuration becomes superconducting at 2.35 K. This is the first case of superconductivity in an organic polymer, albeit not the first example of a superconducting polymer since superconductivity in an inorganic polymer-poly(sulfur nitride): (SN)_x– was discovered more than two decades ago [7].

In the past four years few review papers on various aspects of conjugated and conductive polymers were published in ‘Progress in Polymer Science’ [8], [9], [10], [11], [12], [13]. For this reason, in our paper we will concentrate on problems, which either were not discussed in previous contributions or, despite their importance, mentioned only briefly. Special emphasis will be put on the development in the conjugated polymer field observed in the last two years. The organization of this paper is as follows. In Section 2 we describe the synthesis of conjugated polymers in their undoped (semiconducting state), their basic chemical and physical properties and their applications, as semiconductors, in molecular electronics devices. In Section 3, we discuss the doping process which transforms ‘polymeric semiconductors’ into ‘polymeric metals’ with special emphasis on processes improving the processibility of these materials. Similarly as in Section 2 we describe basic chemical and physical properties of these polymeric metals and their application. In Section 4 we discuss electric field-induced superconductivity in poly(acene)s and poly(thiophene)s. In general, with the exception of poly(acene)s, in our paper we focus on truly polymeric systems. It should be however noted that equally intensive research is presently carried out for conjugated oligomers.

The selection of papers quoted is purely subjective and reflects our research interests. We are aware of the fact that the number of papers cited (321) constitute significantly less than 1% of all papers devoted to electroactive polyconjugated systems. Several important contributions had to be omitted due to space limits.