Azo Supramolecules on Au(111) with Controlled Size and Shape

Robust Single-Supermolecule Switches Operating in Response to Two Different Noncovalent Interactions

Supramolecular electronics provide an opportunity to introduce molecular assemblies into electronic devices through a combination of noncovalent interactions such as [π···π] and hydrogen-bonding interactions. The fidelity and dynamics of noncovalent interactions hold considerable promise when it comes to building devices with controllable and reproducible switching functions. Here, we demonstrate a strategy for building electronically robust switches by harnessing two different noncovalent interactions between a couple of pyridine derivatives. The single-supermolecule switch is turned ON when compressing the junction enabling [π···π] interactions to dominate the transport, while the switch is turned OFF by stretching the junction to form hydrogen-bonded dimers, leading to a dramatic decrease in conductance. The robustness and reproducibility of these single-supermolecule switches were achieved by modulating the junction with Ångström precision at frequencies of up to 190 Hz while obtaining high ON/OFF ratios of ∼600. The research presented herein opens up an avenue for designing robust bistable mechanoresponsive devices which will find applications in the building of integrated circuits for microelectromechanical systems.

Low-Threshold Reversible Electron-Induced and Selective Photoinduced Switching of Azobenzene Derivatives under Ambient Conditions

Mono-carboxyl-functionalized azobenzene and arylazopyrazole have been employed for electron-induced and photoinduced switching under ambient conditions. The microscopic structure and the switching behavior is understood using scanning tunneling microscopy. The carboxyl functional group in these molecules offers low threshold energy for the electron-induced reversible switching compared with nonfunctionalized azobenzene. The low threshold is understood using charged intermediate states during the switching. A selectivity has been observed for the photoinduced switching. Because of strong hydrogen bonding, only the free phenyl groups in the molecules change their configuration.

Robust and Selective Switching of an FeIII Spin-Crossover Compound on Cu2N/Cu(100) with Memristance Behavior

The switching between two spin states makes spin-crossover molecules on surfaces very attractive for potential applications in molecular spintronics. Using scanning tunneling microscopy, the successful deposition of [Fe(pap)₂]⁺ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) molecules on Cu₂N/Cu(100) surface is evidenced. The deposited Fe^III spin-crossover compound is controllably switched between three different states, each of them exhibiting a characteristic tunneling conductance. The conductance is therefore employed to readily read the state of the molecules. A comparison of the experimental data with the results of density functional theory calculations reveals that all Fe(pap)₂ molecules are initially in their high-spin state. The two other states are compatible with the low-spin state of the molecule but differ with respect to their coupling to the substrate. As a proof of concept, the reversible and selective nature of the switching is used to build a two-molecule memory.

Influence of CH···N Interaction in the Self-Assembly of an Oligo(isoquinolyne-ethynylyne) Molecule with Distinct Conformational States

Molecular conformational flexibility can play an important role in supramolecular self-assembly on surfaces, affecting not least chiral molecular assemblies. To explicitly and systematically investigate the role of molecular conformational flexibility in surface self-assembly, we synthesized a three-bit conformational switch where each of three switching units on the molecules can assume one of two distinct binary positions on the surface. The molecules are designed to promote C-H···N type hydrogen bonds between the switching units. While supramolecular self-assembly based on strong hydrogen-bonding interactions has been widely explored, less is known about the role of such weaker directional interactions for surface self-assembly. The synthesized molecules consist of three nitrogen-containing isoquinoline (IQ) bits connected by ethynylene spokes and terminated by tert-butyl (tBu) groups. Using high-resolution scanning tunnelling microscopy, we investigate the self-assembly of the IQ-tBu molecules on a Au(111) surface under ultrahigh-vacuum conditions. The molecules form extended domains of brick-wall structure where the molecular backbones are packed regularly but without selection of specific molecular conformations. However, statistical analysis of the extended network demonstrates alignment/correlation for the orientations of the switching units indicating specific interactions. The primary interaction motifs in the structure are quantified from DFT calculations, showing that the brick-wall structure is indeed stabilized by two types of weak C-H···N bonds, involving either aromatic hydrogens on the IQ groups or nonaromatic hydrogens on the tBu groups. Analysis of the C-H···N interactions in the brick-wall structure explains the observed distribution and alignment of molecular conformations as well as the overall organization of the molecular surface structures.

Thermal and Optical Modulation of the Carrier Mobility in OTFTs Based on an Azo-anthracene Liquid Crystal Organic Semiconductor

One of the most striking features of organic semiconductors compared with their corresponding inorganic counterparts is their molecular diversity. The major challenge in organic semiconductor material technology is creating molecular structural motifs to develop multifunctional materials in order to achieve the desired functionalities yet to optimize the specific device performance. Azo-compounds, because of their special photoresponsive property, have attracted extensive interest in photonic and optoelectronic applications; if incorporated wisely in the organic semiconductor groups, they can be innovatively utilized in advanced smart electronic applications, where thermal and photo modulation is applied to tune the electronic properties. On the basis of this aspiration, a novel azo-functionalized liquid crystal semiconductor material, (E)-1-(4-(anthracen-2-yl)phenyl)-2-(4-(decyloxy)phenyl)diazene (APDPD), is designed and synthesized for application in organic thin-film transistors (OTFTs). The UV-vis spectra of APDPD exhibit reversible photoisomerizaton upon photoexcitation, and the thin films of APDPD show a long-range orientational order based on its liquid crystal phase. The performance of OTFTs based on this material as well as the effects of thermal treatment and UV-irradiation on mobility are investigated. The molecular structure, stability of the material, and morphology of the thin films are characterized by thermal gravimetric analysis (TGA), polarizing optical microscopy (POM), (differential scanning calorimetry (DSC), UV-vis spectroscopy, atomic force microscopy (AFM), and scanning tunneling microscopy (STM). This study reveals that our new material has the potential to be applied in optical sensors, memories, logic circuits, and functional switches.

Kinetically controlled hierarchical self-assemblies of all-trans-retinoic acid on Au(111)

Kinetically controlled hierarchical self-assemblies of all-trans-retinoic acid on Au(111) were investigated via low-temperature scanning tunneling microscopy in ultra-high vacuum. The dominant molecular hierarchical superstructure could be selectively controlled to dimer, tetramer, or pentamer patterns, which were stabilized by hydrogen bonds and van der Waals interactions.

Optically and thermally induced molecular switching processes at metal surfaces

Using light to control the switching of functional properties of surface-bound species is an attractive strategy for the development of new technologies with possible applications in molecular electronics and functional surfaces and interfaces. Molecular switches are promising systems for such a route, since they possess the ability to undergo reversible changes between different molecular states and accordingly molecular properties by excitation with light or other external stimuli. In this review, recent experiments on photo- and thermally induced molecular switching processes at noble metal surfaces utilizing two-photon photoemission and surface vibrational spectroscopies are reported. The investigated molecular switches can either undergo a trans-cis isomerization or a ring opening-closure reaction. Two approaches concerning the connection of the switches to the surface are applied: physisorbed switches, i.e. molecules in direct contact with the substrate, and surface-decoupled switches incorporated in self-assembled monolayers. Elementary processes in molecular switches at surfaces, such as excitation mechanisms in photoisomerization and kinetic parameters for thermally driven reactions, which are essential for a microscopic understanding of molecular switching at surfaces, are presented. This in turn is needed for designing an appropriate adsorbate-substrate system with the desired switchable functionality controlled by external stimuli.

Switching ability of nitro-spiropyran on Au(111): electronic structure changes as a sensitive probe during a ring-opening reaction

Spiropyran is a prototype molecular switch which undergoes a reversible ring-opening reaction by photoinduced cleavage of a C-O bond in the spiropyran (SP) to the merocyanine (MC) isomer. While the electronic states and switching behavior are well characterized in solution, adsorption on metal surfaces crucially affects these properties. Using two-photon photoemission and scanning tunneling spectroscopy, we resolve the molecular energy levels on a Au(111) surface of both isomeric forms. Illumination at various wavelengths does not yield any observable switching rate, thus evidencing a very small upper limit of the quantum efficiency. Electron-induced switching from the SP to the MC isomer via generation of a negative ion resonance can be detected with a quantum yield of (2.2 ± 0.2) × 10(-10) events/electron in tunneling spectroscopy. In contrast, the back reaction could not be observed. This study reveals that the switching properties of surface-bound molecular switches can be very different compared with free molecules, reflecting the strong influence of the interaction with the metal substrate.

A chiral cyclotrisazobiphenyl: synthesis and photochemical properties

A chiral cyclotrisazobiphenyl macrocycle was synthesized conveniently in three steps from the literature known 3,3'-diaminobimesityl in 37-38% overall yield. Irradiation with 302 nm, 365 nm or visible light allows access to different photostationary states (PSSs). These PSSs can be conveniently read out by CD-spectroscopy as each of them exhibits a positive, a negative, or no signal, respectively, at 275 nm.

Functionalization, self-assembly, and photoswitching quenching for azobenzene derivatives adsorbed on Au(111)

We have used scanning tunneling microscopy to investigate the structure and photoswitching behavior of azobenzene molecules functionalized with bulky spacer groups and adsorbed onto Au(111). We find that positioning tert-butyl "legs" in a canted arrangement on the azobenzene phenyl rings quenches photoisomerizability of the molecule on Au(111). Addition of cyano groups at the para positions changes the molecular self-assembly significantly, but does not alter the quenched photoisomerizability. This behavior likely arises from a combination of molecule-surface interactions, molecule-molecule interactions, and alteration of azobenzene electronic structure resulting from the position-specific addition of tert-butyl groups.

An electron induced two-dimensional switch made of azobenzene derivatives anchored in supramolecular assemblies

Supramolecular assemblies of 4-anilino-4'-nitroazobenzene are investigated on the Au(111) surface by low temperature scanning tunneling microscopy and spectroscopy with submolecular resolution. Adsorption at 250 K leads to three different structures that are formed via hydrogen bonds: a star structure and two types of line structures: a meandering and a zigzag line. The formation of these supramolecular assemblies is affected by the available space on the fcc domains of the reconstructed Au(111) substrate as well as by the two-dimensional chirality of the molecules on the surface. The star structure is enantiomerically pure, while both types of lines consist of a racemic mixture. Bonding between homochiral pairs differ from the one between heterochiral pairs in the position of the hydrogen bonds. Inside these supramolecular assemblies two configurations of the molecules are identified: An almost straight trans-configuration and a slightly bent cis*-configuration. The trans-configuration largely reflects the structure of this isomer in gas phase, while the cis*-configuration is two-dimensional on the surface in contrast to the three-dimensional gas phase cis-configuration. The reversible trans-cis* isomerization is induced by electron tunneling through the LUMO+1 state of the molecule, which is located at +2.9 V.

Surface-assisted assembly of discrete porphyrin-based cyclic supramolecules

We employed de novo synthesized porphyrin modules to construct discrete cyclic supramolecular architectures supported on a copper surface. The programmed geometry and functionality of the molecular modules together with their conformational flexibility and substrate interaction yields symmetric discrete assemblies, including dimers and chains as well as three- to six-membered cyclic structures. The area of the molecular cavities is extended by creating bicomponent structures combining building blocks with different symmetry.