Chemistry Reference
In-Depth Information
sublimation temperature of DMe-DCNQI, about 320 K, and the fact that DMe-
DCNQI polymerizes at 340 K, which forces the UHV chamber to be pumped for a
prolonged period of time without being baked. During film growth three structural
transitions are observed. At the beginning, the DMe-DCNQI film causes a diffuse
diffraction pattern. At a higher dose, sharp diffraction spots occur corresponding to
a single ML (structure I). 1 ML is defined here as 1
10
14
cm
−
2
. In matrix nota-
.
×
1
tion this commensurate structure is described by
31
, with
a
oI
=
.
0
958 nm,
−
22
100
◦
. Before the diffuse LEED images change to that
of structure I, the LEED images show that the development of structure I starts
from an incommensurate phase with unit vectors
a
o
=
b
oI
=
1
.
000 nm and
α
oI
=
1
.
02 nm,
b
o
=
1
.
2 nm and
87
◦
and transformation matrix
31
. The compression of this struc-
.
3
α
o
=
−
22
.
6
ture results in phase I. After the first ordered structure is complete, a second type
of structure forms (structure II), which is totally different from the first one. The
transition occurs at 1.18 ML. The transformation matrix is described by
30
12
,
5
◦
. Structures I and II exist
in two domain orientations. After structure II is completed, it undergoes another
structural change upon increase of coverage. Structure II becomes compressed in
the substrate [110] direction. The final structure ends with a smaller unit cell with
unit cell vectors
a
o
with
a
oII
=
0
.
867 nm,
b
oII
=
0
.
866 nm and
α
oII
=
70
.
5
◦
and transformation
=
0
.
78 nm,
b
o
=
0
.
86 nm and
α
=
72
.
matrix
2
.
o
.
71
0
.
92
Interface-induced homochiral assemblies
We saw before for the NN/Au(111) system how the surface can induce chirality
on non-chiral molecules because of the confinement to two dimensions. Chirality
is an extremely interesting phenomenon with significant implications in biology
and pharmacology and chiral recognition still remains mysterious (not for Flatland
inhabitants!). The interactions involved in chiral recognition are indeed the same
as those involved in the formation of crystalline phases of MOMs: hydrogen bond-
ing,
π
−
π
and dipole-induced interactions. In addition, crystalline surfaces may
induce segregation of enantiomers by adding molecule-substrate interactions, thus
inducing homochiral assemblies, either in the form of dimers or as SAMs. We are
going to discuss next two relevant examples of vapour-deposited organic layers on
metallic substrates involving chirality: cysteine/Au(110) (Kuhnle
et al.
, 2002) and
heptahelicene/Cu(111) (Fasel
et al.
, 2003).
Cysteine, HS-CH
2
-CH(NH
2
)-COOH, binds covalently to gold due to the thiol
group.When depositing enantiomeric pure (L-cysteine or D-cysteine) onAu(110) at
