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 365

Figure9.26 Schematic interaction diagram for dimer formation from two cyclic thiazyl radicals. The sulfur

and nitrogen atoms are not indicated in the dimer orbitals because the intermolecular overlap can be between
atomsofthesametypeornot.

π

for thiazyl radical dimerization is presented in Figure 9.26. The SOMO-SOMO interaction involves end-

to-end interaction between the atomic p orbitals which constitute the radical SOMO. The weak, diffuse
bonding nonetheless suggests that

π

dimers should be diamagnetic and, as such, relatively uninteresting
from a magnetism perspective. In fact, this picture of radical association is far from complete. There are a
few examples of thiazyl

π

dimer structures (e.g., a few dithiadiazolyls 115,116 and one thioaminyl radical 241 )

π

3.25 A). The magnetic susceptibility of these
compounds exhibit notable paramagnetism at higher temperatures, which arises from thermal population
of a triplet excited state which becomes accessible when the radical-radical interaction is sufficiently
weak. 115,116
Several

in which the inter-radical distances are significantly longer ( ∼

dimer structures based on 1,2,3,5-dithiadiazolyl polyradicals show paramagnetic behavior
at high temperatures, though the origin of this effect is the thermally induced dissociation of the dimer
“bonds”. Figure 9.27a depicts the magnetic susceptibility for 1,3-phenylene-bridged diradical 149 ,which
adopts a

π

dimer structure (Figure 9.7b). 126 This material is diamagnetic up to nearly 400 K,
above which there is a significant increase in susceptibility. This behavior was ascribed to the extensive
uncoupling of

π stacked, π

dimers to generate radicals: The susceptibility near 500 K corresponds to approximately
one spin per molecule. If the temperature is lowered prior to the onset of decomposition at 500 K the
susceptibility increase is quasi-reversible - the temperature decrease profile does not follow the temperature
increase profile, suggesting that not all spins re-align into

π

dimers (i.e., radical “defect” sites remain in
the lattice upon re-cooling). Interestingly, this phenomenon occurs only for the dithiadiazolyl structures
which adopt

π

stacked (e.g., 148 , 123
see Figure 9.7a, and others 124 ) only show irreversible susceptibility increases at the decomposition point
of the material (Figure 9.27b for 148 ). The dependence of the pseudo-reversible “spin breakout” on the
presence of a

dimers 126 - 128 ; radical dimer structures which are not

π stacks of π π

π -stacked structure implies that cooperative (inter-dimer) effects between π dimers are a

prerequisite for this process.