The trans-cis isomerization makes azobenzene (AB) a robust molecular switch. Once adsorbed to a metal, however, the switching is inefficient or absent due to rapid excited-state quenching or loss of the trans-cis bistability. We find that tris-[4-(phenylazo)-phenyl]-amine is a rather efficient switch on Ag(111). Using scanning tunneling and atomic force microscopy at submolecular resolution along with density functional theory calculations, we show that the switching process is no trans-cis isomerization but rather a reorientation of the N-N bond of an AB unit. It proceeds through a twisting motion of the azo-bridge that leads to a lateral shift of a phenyl ring. Thus, the role of the Ag substrate is ambivalent. While it suppresses the original bistability of the azobenzene units, it creates a new one by inducing a barrier for the rotation of the N-N bond.

Trioctyl-functionalized triazatriangulenium (trioctyl-TATA) deposited on Au(111) and Ag(111) surfaces by electrospray ionization was investigated using low-temperature scanning tunneling microscopy. The molecule surprisingly adsorbs with gauche rather than anti conformations of the octyl groups. We observed chiral amplification in the islands. Only one of the eight possible configurations of the octyl groups was found in homochiral hexagonal networks. Quantum-chemical calculations confirmed and explained the preference for the gauche conformations of adsorbed trioctyl-TATA.

The conductances of molecular model junctions comprising a triazatriangulenium platform with or without an ethynyl spacer and an upright Zn-porphyrin are probed with a low-temperature scanning probe microscope. The platform alone is found to be highly conductive. The ethynyl-linked Zn-porphyrin moiety reduces the conductance by three orders of magnitude and leads to an unexpected, non-monotonous variation of the force that was measured simultaneously at the tip of the microscope. Density functional theory calculations show that this variation results from an induced tilting of the porphyrin. ExperimentExperiments were performed in a combined STM/AFM in UHV at 5 K with a base pressure of × − 5 10 mbar 11 . The AFM was operated in a non-contact frequency-modulation mode with a tuning fork probe [11]. The free OPEN ACCESS RECEIVED

At low sample voltages, Cl ions may be bidirectionally transferred between a Fe tetraphenylporphyrin (FeTPP) molecule on Au(111) and the tip of a low-temperature scanning tunneling microscope that also operates as an atomic force microscope. Via this effect, Cl-terminated gold tips are prepared. Chlorinated and pristine metal tips are used to probe the forces and conductances of contacts to Au(111), FeTPP, and chlorinated FeTPP. The interaction between the substrate and the Cl ion at the tip may be modeled by a point charge of ≈−0.3 e interacting with its image. The maximal attraction experienced by the tip in junctions with one Cl ion is much larger when the ion is located at the molecule. Junctions with two Cl ions exhibit two maxima of the attractive force. Simultaneous imaging of the tunneling current and the frequency shift of the force sensor with Cl tips reveals intriguing contrast changes that are tentatively interpreted in terms of tip-induced distortions of the molecules. Density functional theory calculations reproduce important aspects of the experimental data.

The structure of molecular adlayers is of great interest for surface functionalization. As molecular complexity increases, the subtle interplay of the relevant interactions becomes more difficult to unravel. Here, we present a scanning tunneling microscope (STM) and atomic force microscope study along with free-energy calculations using density functional theory on two closely related NDI-cyclophane molecules. These three-dimensional double-decker molecules are designed to attach to the substrate with one subunit while the other functional moiety is exposed to the environment. The molecular arrangements obtained on Ag(111) are rationalized by the inclusion of adatoms from the substrate into the structure. The presence of adatoms is identified by a drastic change in corrugation of the STM images that takes place at moderate bias voltages. Our calculations using density functional theory of the system’s free-energy yield that two adatoms favorably coadsorb with the molecules.