Chemistry Reference
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depending on the number and distribution of substituents. The experimentally found
structures can easily be predicted from the geometric and chemical nature of the
porphyrin substituents, whichmediate the interactions between individual adsorbed
molecules. The porphyrins used in this study are based on H
2
-TBPP, which has a
free-base porphyrin core and four ditertiarybutylphenyl substituents (Fig. 4.8(a)).
Isolated H
2
-TBPP single molecules are found on the reconstructed gold surfaces.
CTBPP is formed by replacing a ditertiarybutylphenyl group of H
2
-TBPP by a
cyanophenyl substituent (Fig. 4.8(b)). At low coverage, most CTBPP molecules
assemble into triangular clusters on Au(111) (Fig. 4.8(e)). Substituting one more
cyano group in CTBPP gives rise to BCTBPP, which forms two types of isomers
(
cis
and
trans
) with respect to the configuration of two cyanophenyl substituents
(Fig. 4.8(c) and (d), respectively). The
cis
-BCTBPPmolecules are aggregated into a
supramolecular tetramer (Fig. 4.8(f)). In this structure, the antiparallel intermolec-
ular connections of all the cyanophenyl substituents lead to a macrocyclic arrange-
ment of the porphyrin molecules, forming a molecular ring. Sequential aggregation
is achieved for the
trans
-BCTBPP molecules (Fig. 4.8(g)). In this structure, the
antiparallel configuration between the cyanophenyl substituents results in a linear
arrangement of the
trans
-BCTBPP molecules, forming supramolecular wires. The
maximum observed length of the straight wires is above 100 nm.
Linear chains have also been obtained on Ag(111) surfaces by OMBD-growth of
trans
-PVBA (Barth
et al.
, 2000). In this case hydrogen bonding is the main factor
responsible for this 1D configuration. PVBA is a planar molecule composed of a
pyridyl group as the head and a carboxylic acid group as the tail. The molecular
stripes are oriented along the [112] direction of the silver lattice. These 1D su-
perstructures consist of two chains of PVBA. In each chain the molecular axis is
oriented along the chain direction, with a head-to-tail distribution. The orientation
of the molecular chains reflects a good match between the
trans
-PVBA subunits
and the high-symmetry lattice positions.
Hydrogen bonding is again at the origin of the striking 2D supramolecular clus-
ters and chains observed upon submonolayer deposition of NN onto reconstructed
Au(111) (Bohringer
et al.
, 1999). NN molecules, prepared from naphthalene and a
mixture of nitric and sulfuric acids at 320 K, adsorb planarly on Au(111) and it is
interesting to note that the 2D confinement of NN introduces a chirality not present
in the gas phase: the molecule and its mirror image cannot be superimposed by
translation and rotation within the surface plane. This is a case of surface-induced
chirality. In a wide range of low coverages (0.05-0.2 ML) NN molecules self-
assemble mainly in decamers and to a lesser extent in tetramers and undecamers.
1 ML is defined here as a close-packed molecular layer. The decamers consist of an
eight-molecule ring surrounding a two-molecule core. Manipulation experiments
at decreased tunnelling resistances show that the decamers are surprisingly robust
