Small organic molecules, capable of undergoing efficient and reversible photochemical reactions to switch them between (at least) two (meta)stable isomers associated with markedly different properties, continue to impact the materials world. Such photoswitches are being implemented in a variety of materials for applications ranging from optical devices to "smart" polymers. All approaches exploit the photoswitching molecular entities as gates, which translate an incoming light stimulus to trigger macroscopic property changes of the materials. In this progress report, the most promising recent examples in this field are highlighted and put in perspective. Moving from supramolecular systems in solution to surfaces and finally to bulk materials, important design concepts are discussed, emphasizing both the challenges as well as the great promise of such truly advanced materials.

Photoswitches: From Molecules to Materials

Any future artificial transporters and robots operating at the nanoscale are likely to require molecules capable of directional translational movement over a surface. Even the design of such molecules is a daunting task, however, as they need to be able to use light, chemical or electrical energy to modulate their interaction with the surface in a way that generates directional motion. Kudernac et al. now unveil just such a molecule, made by attaching four rotary motor units to a central axis. Inelastic electron tunnelling induces conformational changes in the rotors and propels the molecule across a copper surface. By changing the direction of the rotary motion of individual motor units, the self-propelling molecular 'four-wheeler' structure can follow random or preferentially linear trajectories. This design provides a starting point for the exploration of more sophisticated molecular mechanical systems, perhaps with complete control over their direction of motion. Propelling single molecules in a controlled manner along an unmodified surface remains extremely challenging because it requires molecules that can use light, chemical or electrical energy to modulate their interaction with the surface in a way that generates motion. Nature’s motor proteins1,2 have mastered the art of converting conformational changes into directed motion, and have inspired the design of artificial systems3 such as DNA walkers4,5 and light- and redox-driven molecular motors6,7,8,9,10,11. But although controlled movement of single molecules along a surface has been reported12,13,14,15,16, the molecules in these examples act as passive elements that either diffuse along a preferential direction with equal probability for forward and backward movement or are dragged by an STM tip. Here we present a molecule with four functional units—our previously reported rotary motors6,8,17—that undergo continuous and defined conformational changes upon sequential electronic and vibrational excitation. Scanning tunnelling microscopy confirms that activation of the conformational changes of the rotors through inelastic electron tunnelling propels the molecule unidirectionally across a Cu(111) surface. The system can be adapted to follow either linear or random surface trajectories or to remain stationary, by tuning the chirality of the individual motor units. Our design provides a starting point for the exploration of more sophisticated molecular mechanical systems with directionally controlled motion.

Electrically driven directional motion of a four-wheeled molecule on a metal surface

Quantum chemical calculations of various azobenzene (AB) derivatives have been carried out with the goal to describe the energetics and kinetics of their thermal cis → trans isomerization. The effects of substituents, in particular their type, number, and positioning, on activation energies have been systematically studied with the ultimate goal to tailor the switching process. Trends observed for mono- and disubstituted species are discussed. A polarizable continuum model is used to study, in an approximate fashion, the cis → trans isomerization of azobenzenes in solution. The nature of the transition state(s) and its dependence on substituents and the environment is discussed. In particular for push−pull azobenzenes, the reaction mechanism is found to change from inversion in nonpolar solvents to rotation in polar solvents. Concerning kinetics, calculations based on the Eyring transition state theory give usually reliable activation energies and enthalpies when compared to experimentally determined valu...

Quantum Chemical Investigation of Thermal Cis-to-Trans Isomerization of Azobenzene Derivatives: Substituent Effects, Solvent Effects, and Comparison to Experimental Data

Although the local environment of a molecule can play an important role in its chemistry, rarely has it been examined experimentally at the level of individual molecules. Here we report the precise control of intramolecular hydrogen-transfer (tautomerization) reactions in single molecules using scanning tunnelling microscopy. By placing, with atomic precision, a copper adatom close to a porphycene molecule, we found that the tautomerization rates could be tuned up and down in a controlled fashion, surprisingly also at rather large separations. Furthermore, we extended our study to molecular assemblies in which even the arrangement of the pyrrolic hydrogen atoms in the neighbouring molecule influences the tautomerization reaction in a given porphycene, with positive and negative cooperativity effects. Our results highlight the importance of controlling the environment of molecules with atomic precision and demonstrate the potential to regulate processes that occur in a single molecule.

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Controlling intramolecular hydrogen transfer in a porphycene molecule with single atoms or molecules located nearby

Structures and electronic transport on silicon surfaces

The adsorption and switching behavior of 3,3′,5,5′-tetra-tert-butylazobenzene (meta-TBA) are investigated by low-temperature scanning tunneling microscopy on three different metal substrates: Au(111), Cu(111), and Au(100). The trans state is the most stable configuration after adsorption, displaying similar appearances in the STM images, independent of the substrate. However, the self-assembly and switching behavior is highly dependent on the chemistry and corrugation of the surface. On the Au(111) surface, the tip-induced isomerization is probed successfully and different driving mechanisms are characterized. The experimental images are in good agreement with calculated ones. However, the switching effect is completely suppressed on Cu(111) and Au(100).

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Adsorption and Switching Properties of Azobenzene Derivatives on Different Noble Metal Surfaces: Au(111), Cu(111), and Au(100)

We have studied the growth of ultrathin Ag films on Si(111)7×7 between 80–100 K with low‐temperature scanning tunneling microscopy. Nucleation of Ag in both halves of the 7×7 is observed. Corner holes and dimer sites are not occupied. Percolation occurs at submonolayer coverage through contact between the nucleated 2D Ag islands. This results in a honeycomb random site percolation problem. Upon completion of the first monolayer, 2D layer‐like growth is observed.

Low-temperature scanning tunneling microscopy study of nucleation, percolation, and growth of ultrathin Ag films on Si(111)7 × 7

We have used the pulse injection technique for the preparation of ordered molecular layers of Cu-TBPP molecules, avoiding any heating procedure of the molecules for their deposition. The controlled removal of the pure solvent is characterized by thermal desorption spectroscopy and is proved by the observation of large areas of clean self-ordered molecular layers by scanning tunnelling microscopy at 8 K after pulse injection and subsequent annealing. A comparison with results obtained by conventional deposition by sublimation reveals an equivalent appearance of the molecules, which is dominated by the lateral phenyl groups. The molecular structure of the Cu-TBPP layer is characterized and different domains are found which show the influence of the copper substrate on the growth orientation.

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Surface Science Letters Preparation of self-ordered molecular layers by pulse injection

The authors show in this letter that single metal atoms on a Ni(111) surface can be pushed by electromigration forces from a scanning tunneling microscope tip. This repulsive interaction is observed over a length scale of 6nm. While for voltages above −300mV the atoms are pulled by the microscope tip, the atoms are pushed away below this threshold. This migration is explained by a resonant scattering of strongly correlated electrons. At small voltages chemical forces are pulling the atom, while for larger voltages the atomic manipulation is assisted by the tunneling current.

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Electromigration of single metal atoms observed by scanning tunneling microscopy

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K. H. Rieder

Papers

Adsorption and Switching Properties of Azobenzene Derivatives on Different Noble Metal Surfaces: Au(111), Cu(111), and Au(100)

Low-temperature scanning tunneling microscopy study of nucleation, percolation, and growth of ultrathin Ag films on Si(111)7 × 7

Surface Science Letters Preparation of self-ordered molecular layers by pulse injection

Electromigration of single metal atoms observed by scanning tunneling microscopy

STM as an operative tool: physics and chemistry with single atoms and molecules