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Abstract

We present investigations on noncovalent bonding and
supramolecular self-assembly of two related molecular building
blocks at a noble metal surface: 4-[trans-2-(pyrid-4-yl-
vinyl)]benzoic acid (PVBA) and 4-[(pyrid-4-yl-ethynyl)]benzoic
acid (PEBA). These rigid, rodlike molecules comprising the same
complementary moieties for hydrogen bond formation are
comparable in shape and size. For PVBA, the ethenylene moiety
accounts for two-dimensional (2-D) chirality upon confinement
to a surface; PEBA is linear and thus 2-D achiral. Molecular
films were deposited on a Ag(111) surface by organic molecular
beam epitaxy and characterized by scanning tunneling
microscopy. At low temperatures (around 150 K), both species
form irregular networks of flat lying molecules linked via
their endgroups in a diffusion-limited aggregation process. In
the absence of kinetic limitations (adsorption or annealing at
room temperature), hydrogen-bonded supramolecular assemblies
form which are markedly different. With PVBA, enantiomorphic
twin chains in two mirror-symmetric species running along a
high-symmetry direction of the substrate lattice form by
diastereoselective self-assembly of one enantiomer. The
chirality signature is strictly correlated between neighboring
twin chains. Enantiopure one-dimensional (1-D) supramolecular
nanogratings with tunable periodicity evolve at intermediate
coverages, reflecting chiral resolution in micrometer domains.
In contrast, PEBA assembles in 2-D hydrogen-bonded islands,
which are enantiomorphic because of the orientation of the
supramolecular arrangements along low-symmetry directions of
the substrate. Thus, for PVBA, chiral molecules form 1-D
enantiomorphic supramolecular structures because of mesoscopic
resolution of a 2-D chiral species, whereas with PEBA, the
packing of an achiral species causes 2-D enantiomorphic
arrangements. Model simulations of supramolecular ordering
provide a deeper understanding of the stability of these
systems.