Chemistry Reference
In-Depth Information
Figure 4.17. Two perspective views of heptahelicene corresponding to the mono-
clinic
P
2
1
/
c
polymorph (Hark & Beurskens, 1976). Heptahelicene belongs to the
low symmetry
C
1
point group (only
E
applies).
low coverages (
1ML) cysteine molecules form dimers exhibiting the same enan-
tiomeric form: either LL or DD. When depositing a racemic mixture (L-cysteine
and D-cysteine) dimers are only of the LL or DD forms, but never heterochiral
(LD). Dimerization of cysteine molecules on Au(110) surfaces is thus highly stere-
oselective.
The second example concerns heptahelicene. Figure 4.17 shows the structure of
left-handed M-heptahelicene, where M stands for minus. On Cu(111) surfaces the
M-heptahelicene molecules are found to adsorb in a geometry with their terminal
phenanthrene group (the first three carbon rings) oriented parallel to the (111)
faces and to successively spiral away from the surface from the fourth ring on, as
determined by X-ray photoelectron diffraction experiments (Fasel
et al.
, 2001).
Figure 4.18 shows STM images of the two close-packed structures observed for
M-heptahelicene. Each bright dot represents one molecule. At
95ML a long-
range ordered structure, apparently built-up from clusters containing six molecules
and from clusters containing three molecules (6&3-structure), is observed. The
six-membered clusters appear as an anticlockwise pinwheel, thus showing hand-
edness. At
ϑ
=
0
.
1 ML the unit cell of the adsorbate lattice contains a group of three
molecules (3-structure) exhibiting a particular cloverleaf shape. Two rotational do-
mains, rotated by 180
◦
with respect to each other, are observed in both structures.
The observed adsorbate lattice structures show enantiomorphism, that is, adsorp-
tion of the right-handed P-heptahelicene (P stands for positive) leads to structures
which are mirror images of those observed for M-heptahelicene. This effect can
be clearly observed in the high-resolution STM images of Fig. 4.19. Furthermore,
the enantiomeric lattices form opposite angles with respect to the [110] substrate
surface direction. The combined molecule-substrate systems thus exhibit extended
ϑ
=
