Chemistry Reference
In-Depth Information
Figure 1.15. Crystal structure of TTF-TCNQ: perspective view along the stack-
ing
b
-axis.
P
2
1
/
46
◦
.
C, S and N atoms are represented by black, medium grey and light grey balls,
respectively. H atoms are not represented for clarity. Crystallographic data from
Kistenmacher
et al.
, 1974.
c
,
a
=
1
.
230 nm,
b
=
0
.
382 nm,
c
=
1
.
847 nm,
β
=
104
.
metal-semiconductor (also known as metal-insulator) transition occurs at
c
.54K,
decreasing
σ
b
, which is interpreted in terms of a Peierls transition.
It soon became apparent that high conductivity is associated with crystal struc-
tures in which the donor and acceptor molecules form seggregated stacks, with
considerable
-overlap and delocalization along the stacks. Therefore, research
was focused on the synthesis of new donor and acceptor molecules with planar or
near-planar geometries. For the TMTSF-TCNQ salt two crystallographic phases are
known, with either mixed or segregated donor-acceptor stacks (see Fig. 1.16). The
first one is semiconducting and the second is metallic, exhibiting a metal-insulator
transition at 57 K (Jacobsen
et al.
, 1978), thus closely related to TTF-TCNQ.
TTF-TCNQ continues to be a unique compound because large single crystals
can be grown, the crystals are stable in air and only one crystallographic phase is
known (monoclinic). Concerning TTF and TCNQmolecules, both have similar size
and identical weight (204 amu, TTF being C
6
S
4
H
4
and TCNQ, C
12
N
4
H
4
), belong
to the same point group
D
2
h
and their ionization potential/electron affinity values
π
