Chemistry Reference
In-Depth Information
N
N
S
S
S
S
N
N
148
Iodine charge transfer salts of other dithia- and diselenadiazolyl diradicals include those based on the 1,4-
(
148, 150
) and 1,3-phenylene (
149, 151
) linked diradicals,
228,229
heterocyclic (
152, 153, 154
) spacers,
230,231
and the diradical
39
with the two dithiadiazolyls directly attached to one another.
122
The selenium-based
systems are made by electrochemical reduction of the corresponding dications in the presence of iodine,
because the diselenadiazolyl diradicals are too involatile for the cosublimation chemistry to work. The
structure and electronic properties of [
148
][I] are qualitatively reproduced in the iodide charge transfer salts
of
149
-
155
, that is, regularly spaced
10 S/cm)
room temperature conductivities, and metal - insulator transitions between 200 and 270 K. Band structure
calculations suggest conduction bandwidths of 3 - 4 eV, based largely on the “perfect”
π
stacks alongside disordered polyiodide channels, high (
>
stacked structures
(i.e., interstack interactions are, in a relative sense, far less significant in these materials). There appears
to be some degree of flexibility in the degree of charge transfer from the diradicals, based on (i) analyses
of the S-S and S-N bond lengths in the charge transfer salts and (ii) in a few instances, the observation
of other stoichiometries: diradicals
152
and
154
afford iodide salts with a diradical:iodine ratio of 2 : 1
in addition to the more common 1 : 1 ratio.
230
π
10
−
5
S/cm)
than the 1 : 1 materials. Bromide-based charge transfer salts of
148
and
149
have also been made by
a comproportionation reaction between the diradical and its corresponding dication (dibromide) salt.
228
These materials contain fully reduced bromide (Br
−
), which means that the diradicals carry a full positive
charge and are mixed valent. However the oxidized diradicals do not adopt the cofacially aligned
The 2 : 1 salts have lower conductivities (
∼
stacked
structure and in fact the individual CN
2
S
2
rings can be assigned as neutral or cationic based on analysis
of their S-S and S-N bond lengths. Accordingly the bromide materials are poor conductors.
π
N
N
Se
Se
Se
N
N
N
N
S
Se
S
Se
Se
Se
N
N
N
N
S
N
N
S
Se
149
150
151
N
N
N
N
N
N
N
Se
S
Se
Se
S
S
O
S
S
S
N
N
S
N
N
Se
N
N
S
S
154
152
153
The only charge transfer salts based on dithiadiazolyl
mono
radicals are the (non-conducting) “triple-
decker” structures based on the phenyl derivative (
35
,R
=
phenyl; Figure 9.8) and a conducting stacked
structure based on the parent dithiadiazolyl HCN
2
S
2
.
105
This radical has two crystalline phases,
104
one of
which consists of
-dimers clustered into cyclic arrays with small void channels (Figure 9.25a).
Cosublimation of this radical with iodine produces the same stacked arrangement, but now with undimerized
radicals (Figure 9.25b,c) and disordered polyiodide in the channels. This material is noteworthy for two
reasons: (i) it is the only dithiadiazolyl charge transfer salt in which the basic packing pattern of radicals
is retained upon reaction with iodine; and (2) the stoichiometry, (HCN
2
S
2
)
6
I
1
.
1
, is more iodine-poor than
π
-stacked
π
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