Chemistry Reference
In-Depth Information
Figure 5.5. Inversion-related dimers of hydrogen-bonded pairs of EDT-TTF-
COOH. C, S and O atoms are represented by black, medium grey and light grey
balls, respectively. H atoms are omitted.
···
with such sulfur-rich molecules and stabilized by S
S van der Waals interac-
tions, as observed in the structures of the neutral forms of EDT-TTF or BEDT-TTF.
In the solid state these pairs of dimers are further organized in a herringbone pattern,
with the ethylene end groups pointing toward the sulfur atoms of a neighbouring
EDT-TTF moiety, as will be shown later.
When sublimated onto
ex situ
cleaved alkali halide (100) surfaces in high vacuum
10
−
6
mbar) with the substrates held at RT, the obtained films are polycrystalline,
with typical microcrystal long dimensions of
c
.7
µ
m, and exhibit no texture, as
evidenced by the bright field images given in Fig. 5.6(d-f). The relatively large
sizes of the microcrystals (something quite common for vapour-deposited neutral
TTF-derivatives, where lengths of 15-20
µ
m are routinely found) allow selected
area electron diffraction (SAED) to be performed, whose patterns are shown in
Fig. 5.6(a-c). Alkali halide substrates are ideal for this kind of experiment since
they can be removed by dissolving them in water.
The analysis of such patterns reveals that the microcrystals are preferentially
oriented with their (021) planes, the contact planes, parallel to the substrate's sur-
face. The interesting point is that, in order to satisfy such orientation, the hydrogen
bonds of the dimers at the interface have to be broken and in addition some reorga-
nization of the molecules is needed (see Fig. 5.6(g)). In conclusion, the molecule-
substrate interactions are sufficiently strong (larger
(
∼
γ
ns
and
γ
nv
values) to induce
COO
Alk bonds, where Alk represents sodium and potassium, but the growing
crystals adapt their structure in order to crystallize in the known monoclinic bulk
phase.
···
5.4
In situ
studies of growth
Let us start this section with real-time measurements of the crystallization process
of highly ordered thin films of the molecular organic radical
p
-NPNN grown from
the vapour phase on glass substrates (Caro
et al.
, 2000). These films exhibit 2D
