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 357

(a) (b) (c) (d) (e)

Figure 9.19 Qualitative orbital energy diagrams for (a) the SOMO of a single π radical, (b) the HOMO and

LUMO of a radical dimers, (d) a half-filled

conductionbandforastackofuniformlyspacedradicals,and(e)aMottinsulatorforastackofuniformlyspaced
radicals.

π dimer, (c) valence and conduction bands for a stack of radical π

At the time of Haddon's proposal, no such stacked radical structures existed; in fact, the precious few
examples of stable neutral radicals in existence did not have the flat, delocalized, sterically unencumbered
structure to form any sort of stack at all (Haddon's proposal focused on the phenalenyl radical as

the building block of choice for other reasons - see below - but it took over twenty years for stable
versions of these radicals to be realized 218 ). The development of several classes of thiazyl radicals in the
1970s and 1980s set the stage for some of these to be explored as candidates for neutral radical-based
conducting materials. The 1,2,3,5-dithiadiazolyl ( 35 ) and diselenadiazolyl ( 45 ) radicals were the first to be
seriously pursued in this regard, owing to their relative ease of synthesis, stability, and preponderance of
heteroatoms (sulfur and nitrogen) available for intermolecular interactions. The latter feature was believed
to be of possible use in providing additional orbital overlap which could aid in decreasing the band gap
(Peierls distortions are prevalent for low dimensional (anisotropic) structures). 213,219
Extensive investigations into the solid state structures and properties of a wide range of 1,2,3,5-
dithiadiazolyl 35 and diselenadiazolyl radicals 45 produced several small band gap semiconductors (the
sulfur-based radicals are insulators (conductivity

π

>

10 − 8 S/cm) but several of the selenium variants have

10 − 5 S/cm). These efforts also lead to a greater appreciation
of molecular and solid state design principles which guided subsequent efforts. With few exceptions these
radicals formed

room temperature conductivities of up to ∼

dimer structures in the solid state, precluding the possibility of the half-filled band archi-
tecture of Figure 9.19d. However, many of these derivatives adopt

π

π
dimers; the distance between dimers are typically close to the van der Waals contact distances between
sulfur or selenium atoms. The interdimer interactions can, and often do, lead to appreciable band for-
mation in the solid state. Band gaps for these materials range from values as high as 2.0 eV (in cases
where intermolecular overlap is very poor) down to as little as

π -stacked structures of cis -cofacial

0.5 eV. The 5-cyano-2-furyl substituted
diselenadiazolyl radical (Figure 9.20) is one of the best examples - EHMO band calculations reveal that
valence and conduction bands are due solely to

∼

π stacked structure (i.e., one-dimensional in nature), which