Subphthalocyanine Platform for Single-Molecule Machines on Surface: Ligand-Directed Adsorption on Au(111)

TL;DR

Researchers developed a platform for single-molecule machines using subphthalocyanine and found that ligand design controls how molecules adsorb on surfaces.

Contribution

A ligand-directed strategy to control the adsorption geometry of subphthalocyanine-based single-molecule machines on Au(111).

Findings

Long axial ligands induce reverse adsorption geometry, forming one-dimensional chains.

Short ligands allow the SubPc platform to adsorb flat and enable rotation via STM.

Adsorption orientation and packing are determined by the axial ligand's structure.

Abstract

For the development of single-molecule machines on surfaces,
a
vertical molecular geometry based on a simple, common platform is
a promising design approach. This could allow decoupling of the active
unit from the supporting surface and obtaining of a flexible modular
system. An ideal platform for this purpose is subphthalocyanine with
its bowl-shaped geometry and axial functionalization. We functionalized
SubPcs with a series of vertical, axial ligands with varying conjugation
lengths. Their adsorption on the Au(111) surface was studied by low-temperature
scanning tunneling microscopy, supported by simulations. We found
that increasing the conjugation length of the axial ligand induces
a distinct transition in the adsorption geometry. Long ligands, such
as azobenzene and naphthalene derivatives, adopt a reverse adsorption
geometry with the ligand adsorbed flat on the surface and the SubPc
platform pointing upward. These reverse molecules further interact,
forming one-dimensional chains. The intermolecular arrangement and
distances in the chains are determined by the orientation of the axial
ligand on the surface. In contrast, the shortest ligand, which is
formed by a single phenyl ring derivative, predominantly adsorbs with
the SubPc platform on the surface and allows rotation by the STM tip.
Our findings reveal a clear structure–adsorption relationship
and offer a rational strategy to control the orientation and packing
of SubPc-based single-molecule machines on surfaces through the design
of the axial ligands.

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