The self-assembly
process of a cobalt-porphyrin derivative (Co-TCNPP)
containing cyanophenyl substituents at all four meso positions on
Au(111) was studied by means of scanning tunneling microscopy (STM)
and low energy electron diffraction (LEED) under ultrahigh vacuum
conditions. Deposition of Co-TCNPP onto Au(111) gave rise to the formation
of a close-packed H-bonded network, which was independent of coverage
as revealed by STM and LEED. However, a coverage-dependent structural
transformation took place upon the deposition of Co atoms. At monolayer
coverage, a reticulated long-range ordered network exhibiting a distinct
fourfold Co coordination was observed. By reduction of the molecular
coverage, a second metal–organic coordination network (MOCN)
was formed in coexistence with the fourfold Co-coordinated network,
that is, a chevron structure stabilized by a simultaneous expression
of H-bonding and threefold Co coordination. We attribute the coverage-dependent
structural transformation to the in-plane compression pressure exerted
by the molecules deposited on the surface. Our study shows that a
subtle interplay between the chemical nature of the building blocks
(molecules and metallic atoms) and molecular coverage can steer the
formation of structurally different porphyrin-based MOCNs.
We report the formation
of one- and two-dimensional metal–organic
coordination structures from para-hexaphenyl-dicarbonitrile
(NC–Ph6–CN) molecules and Cu atoms on graphene
epitaxially grown on Ir(111). By varying the stoichiometry between
the NC–Ph6–CN molecules and Cu atoms, the
dimensionality of the metal–organic coordination structures
could be tuned: for a 3:2 ratio, a two-dimensional hexagonal porous
network based on threefold Cu coordination was observed, while for
a 1:1 ratio, one-dimensional chains based on twofold Cu coordination
were formed. The formation of metal–ligand bonds was supported
by imaging the Cu atoms within the metal–organic coordination
structures with scanning tunneling microscopy. Scanning tunneling
spectroscopy measurements demonstrated that the electronic properties
of NC–Ph6–CN molecules and Cu atoms were
different between the two-dimensional porous network and one-dimensional
molecular chains.
In recent studies,
porphyrin derivatives have been frequently used
as building blocks for the fabrication of metal–organic coordination
networks (MOCNs) on metal surfaces under ultrahigh vacuum conditions
(UHV). The porphyrin core can host a variety of 3d transition metals,
which are usually incorporated in solution. However, the replacement
of a pre-existing metal atom in the porphyrin core by a different
metallic species has been rarely reported under UHV. Herein, we studied
the influence of cyanophenyl and pyridyl functional endgroups in the
self-assembly of structurally different porphyrin-based MOCNs by the
deposition of Fe atoms on tetracyanophenyl (Co-TCNPP) and tetrapyridyl-functionalized
(Zn-TPPyP) porphyrins on Au(111) by means of scanning tunneling microscopy
(STM). A comparative analysis of the influence of the cyano and pyridyl
endgroups on the formation of different in-plane coordination motifs
is performed. Each porphyrin derivative formed two structurally different
Fe-coordinated MOCNs stabilized by three- and fourfold in-plane coordination
nodes, respectively. Interestingly, the codeposited Fe atoms did not
only bind to the functional endgroups but also reacted with the porphyrin
core of the Zn-substituted porphyrin (Zn-TPyP), i.e., an atom exchange
reaction took place in the porphyrin core where the codeposited Fe
atoms replaced the Zn atoms. This was evidenced by the appearance
of molecules with an enhanced (centered) STM contrast compared with
the appearance of Zn-TPyP, which suggested the formation of a new
molecular species, i.e., Fe-TPPyP. Furthermore, the porphyrin core
of the Co-substituted porphyrin (Co-TCNPP) displayed an off-centered
STM contrast after the deposition of Fe atoms, which was attributed
to the binding of the Fe atoms on the top site of the Co-substituted
porphyrin core. In summary, the deposition of metal atoms onto organic
layers can steer the formation of structurally different MOCNs and
may replace pre-existing metal atoms contained in the porphyrin core.
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