Chemistry Reference
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approximately 0.5 eV (EHMO calculated) arise mostly from intrastack overlap; and (ii) intrinsic semicon-
ductive behavior, with room temperature conductivities between 10
−
6
-10
−
5
S/cm and thermal activation
energies of
∼
0.4 eV. A significant outlier among the bis(dithiazoles) in terms of properties is a derivative
of pyrazine-containing radical
101
(R
1
methyl).
182
=
This compound adopts a different type of stacking
π
pattern, one in which the
stacks are not slipped (tilted) but also not cofacially superimposed, the result of
which is a complex network of intra- and interstack overlap. This produces an unusually large bandwidth
(1.5 eV) and exceptionally high conductivity for a thiazyl radical: the room temperature conductivity is
nearly 10
−
3
S/cm and the activation energy is only 0.19 eV, or
half
the value of the other bis(dithiazole)
materials. Incorporation of selenium into either (
97b, 97c
)orboth(
97d
) chalcogen positions of these radi-
cals does not dramatically perturb the molecular features (e.g., spin distributions, ion energetics), but does
lead to substantially better intermolecular interactions, as manifested by larger bandwidths (up to 1 eV),
higher room temperature conductivities (10
−
6
up to 10
−
3
S/cm) and smaller activation energies (generally
0.3 - 0.17 eV).
184 - 189221 - 224
∼
9.4.2 Thiazyl radical-based charge transfer salts
The use of thiazyl radicals as single-component conductors represents an alternative to the more conven-
tional charge transfer (CT) salt based materials. Charge transfer salts consist of donor (D) and acceptor
(A) molecules between which there is
partial
charge transfer from D to A, that is, one or both com-
ponents have non-integral charge states. The solid state structures of these materials consist of
π
stacks
of all D and/or all A (as opposed to alternating stacks DADADA
...
), which gives rise to partially filled
conduction bands. In molecular terms the
stacks are formally
mixed valent
, consisting of a mixture of
neutral (closed shell) molecules and the corresponding radical ions. For example, the prototypical charge
transfer salt TTF - TCNQ (tetrathiafulvalene - tetracyanoquinodimethane) has a degree of charge transfer
of 0.59, that is, on average the TTF and TCNQ
π
π
stacks contain 59 % radical cations and radical anions,
respectively.
Thiazyl radicals have also been explored as the donor component in charge transfer type conducting
materials. Phenomenologically, the use of a neutral radical instead of a closed shell species as donor has
the effect of changing the occupancy of the conduction band (Figure 9.21): the unoxidized stack of closed
shell donors has a filled conduction band (Figure 9.21a), whereas the neutral radical stack has a half
filled band (Figure 9.21c; cf. Figure 9.19d). Partial oxidation of the closed shell species creates
stacks
consisting of a mixture of radical cations and neutral molecules, whereas the mixed valent stacks in the
neutral radical case are combinations of
closed shell
cations and neutral radicals.
In 1984, Wolmershauser reported that the benzo-1,3,2-dithiazolyl radical (
69
) reacts with TCNQ to give
an insoluble material with 1 : 1 stoichiometry and a room temperature conductivity of
π
1S/cm.
155
Based on
the analogy to the TTF - TCNQ family of charge transfer salts, the authors speculated that the structure of
their material consisted of segregated stacks of partially oxidized dithiazolyl radicals and partially reduced
TCNQ stacks as well. This turned out to be the case, as reported by Awaga in 2008:
225
∼
the X-ray structure
indeed consists of the two components in separate
stacks (Figure 9.22), and analyses of the structural
(S-N bond lengths) and spectroscopic (v(CN) in the infrared spectrum) data suggest a degree of charge
transfer of approximately 0.6. This value is remarkably close to the charge transfer in TTF - TCNQ and can
be understood based on the fact that the oxidation potential of
69
is virtually identical to that of TTF. How-
ever,
unlike
TTF - TCNQ the conductivity of
69
:TCNQ is
not
metallic but instead thermally activated. This
behavior is believed to be based on relatively poor ion energetics (high ionic fluctuation energy) associated
with radical
69
.
225
A highly conducting charge transfer complex of
69
with an anionic nickel bis(dithiolene)
has also been reported. In this material the 1,3,2-benzodithiazoles do not
π
π
stack but the metal dithiolene
fragment does, implicating the latter as the major contributor to the charge transport properties.
226
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