EMPA Activities 2002
Advanced Materials
and Surfaces
Chirality transfer from single molecules into
self-assembled monolayers
The adsorption of enantiopure heptahelicene
on Cu(111) leads to well-ordered superstructures. Depending on surface coverage, the chirality of the molecules is transferred either into
handed nanoclusters consisting of six molecules or into “long-range-twisted” chiral superstructures.
Manfred Parschau,
Roman Fasel,
Karl-Heinz Ernst
The interest in helical phases composed of chiral
molecules has mainly been determined by their technological applications such as in liquid crystal displays. However, the correlation of molecular chirality
with the helicity of the chiral liquid crystal, i.e. the
mechanism of chiral induction into the mesoscopic
structure, has not yet been fully understood.
b
a
M
P
c
A promising approach for getting more insights into
the process of intermolecular chirality transfer is the
investigation of the self-assembly of chiral molecules
on surfaces where chirality transfer in two dimensions can be studied. This allows the use of scanning
tunneling microscopy (STM), which is an excellent
tool for studying molecular pattern formation. The
adsorption of the helically shaped chiral heptahelicene C30H18 on Ni(111) and Ru(0001) showed no
chiral effects, because of the low mobility of heptahelicene on these surfaces. On Cu(111), on the other hand, the molecules are highly mobile at room
temperature.
In Fig. 1, four examples of ordered superstructures
formed by heptahelicene on Cu(111) are shown. The
adsorbate structures at a coverage of θrel = 0.9 are
built up from clusters of six and three molecules,
while at θrel = 1 only three-molecule clusters are observed. The structures generated by M-heptahelicene are mirror images of those formed from P-heptahelicene, and vice versa. The transfer of chirality is
particularly obvious for the six-molecule cluster. The
P-heptahelicene cluster forms a “clockwise spiral”,
while the M-heptahelicene cluster forms a “counterclockwise spiral”. At θrel = 1, the supramolecular
chirality is expressed by a tilt of the three-molecule
clusters in opposite directions. We currently aim at
developing a microscopic model for this two-dimensional chirality transfer, which is based on theoretical
calculations considering the molecular orientations
within the observed chiral clusters.
d
Fig. 1: STM images acquired from superstructures of M(left) and P-heptahelicene (right) at saturated monolayer
coverage, i.e. θ rel = 1, (a, b); and at 90% of the monolayer
coverage, i.e. θ rel = 0.9 (c, d). The unit-cells of the adsorbate lattices are indicated.
Links: www.empa.ch/abt124
> Surface technologies
Contact: [email protected]
References:
K.-H. Ernst et al., in Complex Mediums III, Proceed.
SPIE Vol. 4806, 248 – 257 (2002)
K.-H. Ernst et al., Surf. Sci., in press (2002)
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