One-way rotation of a chemically anchored single molecule-rotor.

We present the chemical anchoring of a DMBI-P molecule-rotor to the Au(111) surface after a dissociation reaction. At the temperature of 5 K, the anchored rotor shows a sequential unidirectional rotational motion through six defined stations induced by tunneling electrons. A typical voltage pulse of 400 mV applied on a specific location of the molecule causes a unidirectional rotation of 60° with a probability higher than 95%. When the temperature of the substrate increases above 20 K, the anchoring is maintained and the rotation stops being unidirectional and randomly explores the same six stations. Density functional theory simulations confirm the anchoring reaction. Experimentally, the rotation shows a clear threshold at the onset of the C-H stretch manifold, showing that the molecule is first vibrationally excited and later it decays into the rotational degrees of freedom.

STM induced manipulation of azulene-based molecules and nanostructures: the role of the dipole moment.

Among the different mechanisms that can be used to drive a molecule on a surface by the tip of a scanning tunneling microscope at low temperature, we used voltage pulses to move azulene-based single molecules and nanostructures on Au(111). Upon evaporation, the molecules partially cleave and form metallo-organic dimers while single molecules are very scarce, as confirmed by simulations. By applying voltage pulses to the different structures under similar conditions, we observe that only one type of dimer can be controllably driven on the surface, which has the lowest dipole moment of all investigated structures. Experiments under different bias and tip height conditions reveal that the electric field is the main driving force of the directed motion. We discuss the different observed structures and their movement properties with respect to their dipole moment and charge distribution on the surface.

Transmitting Stepwise Rotation among Three Molecule-Gear on the Au(111) Surface.

The realization of a train of molecule-gears working under the tip of a scanning tunneling microscope (STM) requires a stable anchor of each molecule to the metal surface. Such an anchor can be promoted by a radical state of the molecule induced by a dissociation reaction. Our results, rationalized by density functional theory calculations, reveal that such an open radical state at the core of star-shaped pentaphenylcyclopentadiene (PPCP) favors anchoring. Furthermore, to allow the transmission of motion by STM manipulation, the molecule-gears should be equipped with specific groups facilitating the tip-molecule interactions. In our case, a tert-butyl group positioned at one tooth end of the gear benefits both the tip-induced manipulation and the monitoring of rotation. With this optimized molecule, we achieve reproducible and stepwise rotations of the single gears and transmit rotations for up to three interlocked units.

Dodecacene Generated on Surface: Reopening of the Energy Gap.

The acene series represents a model system to investigate the intriguing electronic properties of extended π-electron structures in the one-dimensional limit, which are important for applications in electronics and spintronics and for the fundamental understanding of electronic transport. Here, we present the on-surface generation of the longest acene obtained so far: dodecacene. Scanning tunneling spectroscopy gives access to the energy position and spatial distribution of its electronic states on the Au(111) surface. We observe that, after a progressive closing of the gap and a stabilization to about 1 eV at the length of decacene and undecacene, the energy gap of dodecacene unexpectedly increases to 1.4 eV. Considering the acene series as an exemplary general case, we discuss the evolution with length of the single tunneling resonances in comparison with ionization energy, electronic affinity, and optical gap.

On-surface synthesis of nitrogen-doped nanographenes with 5-7 membered rings.

We report on the formation of nitrogen-doped nanographenes containing five- and seven-membered rings by thermally induced cyclodehydrogenation on the Au(111) surface. Using scanning tunneling microscopy and supported by calculations, we investigated the structure of the precursor and targets, as well as of intermediates. Scanning tunneling spectroscopy shows that the electronic properties of the target nanographenes are strongly influenced by the additional formation of non-hexagonal rings.

Tuning the conductance of a molecular wire by the interplay of donor and acceptor units.

We investigate the conductance of optimized donor-acceptor-donor molecular wires obtained by on-surface synthesis on the Au(111) surface. A careful balance between acceptors and donors is achieved using a diketopyrrolopyrrole acceptor and two thiophene donors per unit along the wire. Scanning tunneling microscopy imaging, spectroscopy, and conductance measurements done by pulling a single molecular wire at one end are presented. We show that the conductance of the obtained wires is among the highest reported so far in a tunneling transport regime, with an inverse decay length of 0.17 Å-1. Using complex band structure calculations, different donor and acceptor groups are discussed, showing how a balanced combination of donor and acceptor units along the wire can further minimize the decay of the tunneling current with length.

Electronic Resonances and Gap Stabilization of Higher Acenes on a Gold Surface.

On-surface synthesis provides a powerful method for the generation of long acene molecules, making possible the detailed investigation of the electronic properties of single higher acenes on a surface. By means of scanning tunneling microscopy and spectroscopy combined with theoretical considerations, we discuss the polyradical character of the ground state of higher acenes as a function of the number of linearly fused benzene rings. We present energy and spatial mapping of the tunneling resonances of hexacene, heptacene, and decacene, and discuss the role of molecular orbitals in the observed tunneling conductance maps. We show that the energy gap between the first electronic tunneling resonances below and above the Fermi energy stabilizes to a finite value, determined by a first diradical electronic perturbative contribution to the polyacene electronic ground state. Up to decacene, the main contributor to the ground state of acenes remains the lowest-energy closed-shell electronic configuration.

Hexacene generated on passivated silicon.

On-surface synthesis represents a successful strategy to obtain designed molecular structures on an ultra-clean metal substrate. While metal surfaces are known to favor adsorption, diffusion, and chemical bonding between molecular groups, on-surface synthesis on non-metallic substrates would allow the electrical decoupling of the resulting molecule from the surface, favoring application to electronics and spintronics. Here, we demonstrate the on-surface generation of hexacene by surface-assisted reduction on a H-passivated Si(001) surface. The reaction, observed by scanning tunneling microscopy and spectroscopy, is probably driven by the formation of Si-O complexes at dangling bond defects. Supported by density functional theory calculations, we investigate the interaction of hexacene with the passivated silicon surface, and with single silicon dangling bonds.

Unimolecular Logic Gate with Classical Input by Single Gold Atoms.

By a combination of solution and on-surface chemistry, we synthesized an asymmetric starphene molecule with two long anthracenyl input branches and a short naphthyl output branch on the Au(111) surface. Starting from this molecule, we could demonstrate the working principle of a single molecule NAND logic gate by selectively contacting single gold atoms by atomic manipulation to the longer branches of the molecule. The logical input "1" ("0") is defined by the interaction (noninteraction) of a gold atom with one of the input branches. The output is measured by scanning tunneling spectroscopy following the shift in energy of the electronic tunneling resonances at the end of the short branch of the molecule.

On-Surface Annulation Reaction Cascade for the Selective Synthesis of Diindenopyrene.

We investigated the thermally induced on-surface cyclization of 4,10-bis(2'-bromo-4'-methylphenyl)-1,3-dimethylpyrene to form the previously unknown, nonalternant polyaromatic hydrocarbon diindeno[1,2,3-cd:1',2',3'-mn]pyrene on Au(111) using scanning tunneling microscopy and spectroscopy. The observed unimolecular reaction involves thermally induced debromination followed by selective ring closure to fuse the neighboring benzene moieties via a five-membered ring. The structure of the product has been verified experimentally as well as theoretically. Our results demonstrate that on-surface reactions give rise to unusual chemical reactivities and selectivities and provide access to nonalternant polyaromatic molecules.

Decacene: On-Surface Generation.

Acenes are intriguing molecules with unique electronic properties. The difficulties in their preparation owing to low stability under ambient conditions are apparent because successful syntheses of long unsubstituted acenes are still scarce, in spite of the great attention they have attracted. Only unsubstituted acenes up to heptacene have been isolated in bulk, with nonacene being the largest acene detected to date. Herein we use on-surface assisted reduction of tetraepoxy decacene precursors on Au(111) as the key step to generate unprecedented decacene which is visualized and its electronic resonances studied by scanning tunneling microscopy (STM) and spectroscopy (STS).

Imaging the electronic structure of on-surface generated hexacene.

Surface-assisted reduction of specially designed air-stable precursors allows us to study single hexacene molecules on Au(111) by scanning tunneling microscopy and spectroscopy, mapping with intramolecular resolution their extended electronic eigenstates.