Chemistry Reference
In-Depth Information
C
18
H
37
S
S
CN
NC
S
CN
S
S
S
S
S
S
S
S
S
NC
S
SC
18
H
37
SC
18
H
37
C
18
H
37
S
(a)
(b)
Figure 4.12. Isomers of bis-cyanoethylthio-bis-octadecylthio-TTF: (a)
cis
,
C
2
ν
and (b)
trans
,
C
2
h
.
As measured with a low-current STM at RT the determined 2D lattice constants are
a
o
=
62
◦
. The arrangement and dimensions
of the individual macrocycles indicate interdigitation of some butyl side chains in
a regular way. This 2D lattice ordering corresponds to a more compressed molec-
ular arrangement than that found in the bulk material (
a
2
.
46 nm and
b
o
=
2
.
52 nm with
α
o
=
=
2
.
58 nm,
b
=
2
.
58 nm,
60
◦
for the (100) plane). The toroidal macrocycles thus self-assemble in a six-
fold symmetry indicating that the three-fold symmetry of the molecules is increased
in the adsorbate lattice. In fact in the ML coverage regime the crystal structure can
be quite different from that of the bulk. The STM image reveals brighter and darker
parts. The brighter regions correspond to higher
I
t
through the macrocyclic
α
=
π
-
conjugated system. The alkyl side chains are supposed to lie in the dark regions.
Our second selected example concerns the isomers of bis-cyanoethylthio-bis-
octadecylthio-TTF (Gomar-Nadal
et al.
, 2003), shown in Fig. 4.12. The syn-
thesis of bis-cyanoethylthio-bis-octadecylthio-TTF is achieved starting from a
bis-cyanothioethyl dithiole-thione molecule and using the mono-deprotection strat-
egy discussed in Section 2.5 followed by alkylation. After conversion of this inter-
mediate compound to the corresponding ketone and coupling in trimethyl phosphite
(strategy S2) a mixture of both possible isomers is isolated.
The solutions were prepared in 1-octanol or 1-phenyloctane and a drop was
placed on a freshly cleaved HOPG surface. A ML of
trans
-bis-cyanoethylthio-
bis-octadecylthio-TTF in 1-octanol spontaneously physisorbs at the liquid/HOPG
interface as revealed by scanning tunnelling microscopy (Fig. 4.13(a, c)).
Two polymorphs are observed, termed
α
β
, where for both structures the
molecules are organized in a lamellar structure. The TTF cores in the middle of
the lamellae (dark grey arrows in Fig. 4.13(a)) appear as bright bands. In the
and
α
polymorph TTF cores are lying flat on the surface, forming an angle of 31
◦
with
respect to the lamellar axis. The octadecyl chains appear with a darker contrast
and form an angle of 60
◦
with respect to the lamellar axis. The intermolecular
distance within lamellae is 0.83 nm. The
polymorph (Fig. 4.13(b) and (c))
has the molecules organized in alternating double-core (dark grey arrows) and
β
