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.
Plate et al. (Tue,) studied this question.