The Synthesis and Characterization of Stable Radicals Containing the Thiazyl (SN) Fragment and Their Use as Building Blocks for Advanced Functional Materials - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 356

and particularly those based on radicals. 210 The properties of poly(thiazyl) have been comprehensively
reviewed by Labes 37 and by Banister and Gorrell, 38 and only a short summary of the salient characteristics
of this material is presented here. Poly(thiazyl) is a highly conducting material, with a room temperature
conductivity of approximately 1000 S/cm. Upon lowering the temperature the conductivity increases by as
much as 100-fold and becomes superconducting at 0.26 K (it should be emphasized here that the confirma-
tion of superconductivity in (SN) x predated the discovery of the copper oxide “high T C ” superconductors
by about 15 years; at the time of the (SN) x breakthrough, the record superconducting transition tempera-
ture (T C ) for any material was only 23 K). Extensive experimental and computational investigations reveal
(SN) x to be an anisotropic, but genuine three-dimensional solid state material. This can be understood
most simply by considering the electronic structure of a single (SN) x chain, which is predicted to undergo
a Peierls distortion and hence be at best a semiconductor. The solid state structure of (SN) x (Figure 9.4)
also provides compelling evidence for significant interactions between SN chains. The multidimensional
electronic structure is crucial to understanding this unique material, and this (along with the benefit of
hindsight) makes it possible to understand why attempts to prepare “hybrid” thiazyl conducting polymers
combining SN units and organic components failed. 211
In addition to inspiring number of efforts to make new sulfur - nitrogen-containing polymers, the trans-
port properties of (SN) x had a large impact on the field of inorganic compounds of sulfur and nitrogen,
particularly radical compounds. As a result of nearly three decades of sustained effort, the foundation of
fundamental studies of thiazyl radicals has enabled more targeted studies of intriguing, and possibly tech-
nologically relevant, physical properties such as charge transport and magnetism. This evolution (“from
molecules to materials”) has positioned thiazyl radicals collectively as one of the more important classes
of molecular building blocks for advanced materials. Detailed coverage of the literature in this area would
require a separate (and very large) chapter as a companion to this one. Instead, the following sections pro-
vide a brief introduction to the main physical properties exhibited by thiazyl radicals; subsequent sections
are organized phenomenologically rather than by radical type. Rawson provided a general overview of the
materials properties of thiazyl radicals a few years ago, 212 and specific materials-oriented pursuits have

been covered at various stages by Oakley, 210,213 Awaga, 214 and Rawson. 215

9.4.1 Charge transport properties of thiazyl radicals
The pursuit of electrically conducting materials has been a major thrust of molecular thiazyl radical research
for some time. In addition to the general context established by poly(thiazyl), a more specific rationale
for the construction of conducting materials based on neutral radicals was independently developed by
Haddon. 216,217

stacked array of neutral radicals, with significant inter-
radical orbital overlap, should give rise to a half-filled conduction band (Figure 9.19d). The presence
of charge carriers (unpaired electrons) and a partly filled band fulfill two requirements for metallic type
conduction. This approach contrasts the more established route to molecular metals in which unpaired
electrons are generated by electron transfer from a donor to an acceptor molecule, that is, conducting
π

The basic premise was that a π

stacks of radical ions (Section 9.4.2). However, a uniformly spaced stack of uncharged radicals has
some pitfalls. If intermolecular overlap within the stack is poor, the unpaired electrons can simply localize
on each molecule and render the material a Mott insulator (Figure 9.19e). Even with substantial overlap
(i.e., bandwidth in the solid state), a one-dimensional structure of this type is inherently unstable and can
undergo a Peierls distortion, a structural distortion which opens up a band gap and renders the material a
semiconductor at best. In chemical terms this can be described as radical dimerization, leading to small
gap if there is still sufficient inter dimer overlap (Figure 9.19c) or, in extreme cases, localized dimers with
little to no interaction with neighboring dimers (Figure 9.19b).