Hot Carrier-Induced Tautomerization within a Single Porphycene Molecule on Cu(111)

Anisotropic coupling of individual vibrational modes to a Cu(110) substrate

We present a study on the excitation of individual vibrational modes with ballistic charge carriers propagating along the Cu(110) surface. By means of the molecular nanoprobe technique, where the reversible switching of a molecule-in this case tautomerization of porphycene-is utilized to detect excitation events, we reveal anisotropic coupling of two distinct vibrational modes to the substrate. The N-H bending mode, excited below |E| ≈ 376 meV, exhibits maxima perpendicular to the rows of the Cu(110) substrate and minima along the rows. In contrast, the N-H stretching mode, excited above |E| ≈ 376 meV, displays maxima along the rows and is constant otherwise. This inversion of the anisotropy reflects the orthogonality between the N-H bending and stretching mode. Additionally, we observe an energy-dependent asymmetry in the propagation direction of charge carriers injected into the Cu(110) surface state. Hereby, the anisotropic band structure results in a correlation between the group velocity and the tunneling probability into electronic states of the substrate.

STM studies on porphyrins and phthalocyanines at the liquid/solid interface for molecular-scale electronics

Porphyrins and phthalocyanines are promising candidates for single-molecule electronics. Among the many characterization tools, scanning tunneling microscopy (STM) represents a very powerful one to gain insight into the electronic properties at the molecular level, by correlating the charge transport behaviours of π-conjugated molecules with ultrahigh resolution imaging. In view of the sophistication of molecular self-assembly in the presence of a solution phase, in this frontier, we focus on STM studies on porphyrins and phthalocyanines at the liquid/solid interface, placing emphasis on the electronic and magnetic properties, as well as the switching behaviour of surface-confined or surface-anchored molecules. Furthermore, we have also addressed the topics of potential that can be exploited in this area.

Highly Regioselective Cyclodehydrogenation of Diphenylporphyrin on Metal Surfaces

Exploring the effect of porphin tautomerism on the regioselectivity of its derivatives is a big challenge, which is significant for the development and application of porphyrin drugs. In this work, we demonstrate the regioselectivity of 2H-diphenylporphyrin (H2-DPP) in the planarization reaction on Au(111) and Ag(111) substrates. H2-DPP monomer forms two configurations (anti- and syn-) via a dehydrogenation coupling, between which the yield of the anti-configuration exceeds 90%. Using high-resolution scanning tunneling microscopy, we visualize the reaction processes from the H2-DPP monomer to the final two planar products. Combined with DFT calculations of the potential reaction pathway and comparative experiments on Au(111) and Ag(111) substrates. Using M-DPP (M = Cu and Fe), we confirm that the regioselectivity of H2-DPP is derived from the reaction energy barrier during the cyclodehydrogenation reaction of different tautomers. This work reveals the regioselectivity mechanism of H2-DPP on the atomic scale, which holds great significance for understanding the chemical conversion process of organic macrocyclic molecules.

Portraits of Soot Molecules Reveal Pathways to Large Aromatics, Five-/Seven-Membered Rings, and Inception through π-Radical Localization

Incipient soot early in the flame was studied by high-resolution atomic force microscopy and scanning tunneling microscopy to resolve the atomic structure and orbital densities of single soot molecules prepared on bilayer NaCl on Cu(111). We resolved extended catacondensed and pentagonal-ring linked (pentalinked) species indicating how small aromatics cross-link and cyclodehydrogenate to form moderately sized aromatics. In addition, we resolved embedded pentagonal and heptagonal rings in flame aromatics. These nonhexagonal rings suggest simultaneous growth through aromatic cross-linking/cyclodehydrogenation and hydrogen abstraction acetylene addition. Moreover, we observed three classes of open-shell π-radical species. First, radicals with an unpaired π-electron delocalized along the molecule's perimeter. Second, molecules with partially localized π-electrons at zigzag edges of a π-radical. Third, molecules with strong localization of a π-electron at pentagonal- and methylene-type sites. The third class consists of π-radicals localized enough to enable thermally stable bonds, as well as multiradical species such as diradicals in the open-shell triplet state. These π-diradicals can rapidly cluster through barrierless chain reactions enhanced by van der Waals interactions. These results improve our understanding of soot formation and the products formed by combustion and could provide insights for cleaner combustion and the production of hydrogen without CO₂ emissions.

Chirality-Specific Unidirectional Rotation of Molecular Motors on Cu(111)

Molecular motors have chemical properties that enable unidirectional motion, thus breaking microscopic reversibility. They are well studied in solution, but much less is known regarding their behavior on solid surfaces. Here, single motor molecules adsorbed on a Cu(111) surface are excited by voltages pulses from an STM tip, which leads to their rotation around a fixed pivot point. Comparison with calculations shows that this axis results from a chemical bond of a sulfur atom in the chemical structure and a metal atom of the surface. While statistics show approximately equal rotations in both directions, clockwise and anticlockwise, a detailed study reveals that these motions are enantiomer-specific. Hence, the rotation direction of each individual molecule depends on its chirality, which can be determined from STM images. At first glance, these dynamics could be assigned to the activation of the motor molecule, but our results show that this is unlikely as the molecule remains in the same conformation after rotation. Additionally, a control molecule, although it lacks unidirectional rotation in solution, also shows unidirectional rotation for each enantiomer. Hence, it seems that the unidirectional rotation is not specifically related to the motor property of the molecule. The calculated energy barriers for motion show that the propeller-like motor activity requires higher energy than the simple rotation of the molecule as a rigid object, which is therefore preferred.

Tautomerization of single asymmetric oxahemiporphycene molecules on Cu(111)

We have studied 22-oxahemiporphycene molecules by a combination of scanning tunneling microscopy at low temperatures and density functional theory calculations. In contrast to other molecular switches with typically two switching states, these molecules can in principle exist in three different tautomers, due to their asymmetry and three inequivalent binding positions of a hydrogen atom in their macrocycle. Different tautomers are identified from the typical appearance on the surface and tunneling electrons can be used to tautomerize single molecules in a controllable way with the highest rates if the STM tip is placed close to the hydrogen binding positions in the cavity. Characteristic switching processes are explained by the different energy pathways upon adsorption on the surface. Upon applying higher bias voltages, deprotonation occurs instead of tautomerization, which becomes evident in the molecular appearance.

Spectroscopic investigation of photophysics and tautomerism of amino- and nitroporphycenes

Parent, unsubstituted porphycene and its two derivatives: 2,7,12,17-tetra-n-propylporphycene and 2,7,12,17-tetra-t-butylporphycene were substituted at the meso position with amino and nitro groups. These two families of porphycenes were characterized in detail with respect to their spectral, photophysical, and tautomeric properties. Two trans tautomers of similar energies coexist in the ground electronic state, but only one form dominates in the lowest excited singlet state. Absorption, magnetic circular dichroism (MCD), and emission anisotropy combined with quantum-chemical calculations led to the assignment of S₁ and S₂ transitions in both tautomers. Compared with the parent porphycene, the S₁-S₂ energy gap significantly increases; for one tautomeric form, the effect is twice as large as for the other. Both amino- and nitroporphycenes emit single fluorescence; previously reported dual emission of aminoporphycenes is attributed to a degradation product. Introduction of bulky t-butyl groups leads to a huge decrease in fluorescence intensity; this effect, arising from the interaction of the meso substituent with the adjacent t-butyl moiety, is particularly strong in the nitro derivative.

Single-molecule nano-optoelectronics: insights from physics

Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.

Nonequilibrium reaction rate theory: Formulation and implementation within the hierarchical equations of motion approach

The study of chemical reactions in environments under nonequilibrium conditions has been of interest recently in a variety of contexts, including current-induced reactions in molecular junctions and scanning tunneling microscopy experiments. In this work, we outline a fully quantum mechanical, numerically exact approach to describe chemical reaction rates in such nonequilibrium situations. The approach is based on an extension of the flux correlation function formalism to nonequilibrium conditions and uses a mixed real and imaginary time hierarchical equations of motion approach for the calculation of rate constants. As a specific example, we investigate current-induced intramolecular proton transfer reactions in a molecular junction for different applied bias voltages and molecule-lead coupling strengths.

Electronic energy levels of porphyrins are influenced by the local chemical environment

Self-assembled islands of 5,10,15,20-tetrakis(pentafluoro-phenyl)porphyrin (2HTFPP) on Au(111) contain two bistable molecular species that differ by shifted electronic energy levels. Interactions with the underlying gold herringbone reconstruction and neighboring 2HTFPP molecules cause approximately 60% of molecules to have shifted electronic energy levels. We observed the packing density decrease from 0.64 ± 0.04 molecules per nm² to 0.38 ± 0.03 molecules per nm² after annealing to 200 °C. The molecules with shifted electronic energy levels show longer-range hexagonal packing or are adjacent to molecular vacancies, indicating that molecule–molecule and molecule–substrate interactions contribute to the shifted energies. Multilayers of porphyrins do not exhibit the same shifting of electronic energy levels which strongly suggests that molecule–substrate interactions play a critical role in stabilization of two electronic species of 2HTFPP on Au(111).

Self-assembled islands of 5,10,15,20-tetrakis(pentafluoro-phenyl)porphyrin (2HTFPP) on Au(111) contain two bistable molecular species that differ by shifted electronic energy levels.

Anisotropic Ballistic Transport Revealed by Molecular Nanoprobe Experiments

Atomic-scale charge transport properties are not only of significant fundamental interest but also highly relevant for numerous technical applications. However, experimental methods that are capable of detecting charge transport at the relevant single-digit nanometer length scale are scarce. Here we report on molecular nanoprobe experiments on Pd(110), where we use the charge carrier-driven switching of a single cis-2-butene molecule to detect ballistic transport properties over length scales of a few nanometers. Our data demonstrate a striking angular dependence with a dip in the charge transport along the [11[over ¯]0]-oriented atomic rows and a peak in the transverse [001] direction. The narrow angular width of both features and distance-dependent measurements suggest that the nanometer-scale ballistic transport properties of metallic surfaces are significantly influenced by the atomic structure.

Addressing a lattice of rotatable molecular dipoles with the electric field of an STM tip

Functional molecular groups mounted on specific foot structures are ideal model systems to study intermolecular interactions, due to the possibility to separate the functionality and the adsorption mechanism. Here, we report on the rotational switching of a thioacetate group mounted on a tripodal tetraphenylmethane (TPM) derivative adsorbed in ordered islands on a Au(111) surface. Using low temperature scanning tunnelling microscopy, individual freestanding molecular groups of the lattice can be switched between two bistable orientations. The functional dependence of this rotational switching on the sample bias and tip-sample distance allows us to model the energy landscape of this molecular group as an electric dipole in the electric field of the tunnelling junction. As expected for the interaction of two dipoles, we found states of neighbouring molecules to be correlated.

What can single-molecule Fano resonance tell?

In this work, we showcase applications of single-molecule Fano resonance (SMFR) measurements beyond the determination of molecular excitonic energy and associated dipole orientation. We use the SMFR measurement to probe the local influence of a man-made single chlorine vacancy on the molecular transition of a single zinc phthalocyanine, which clearly reveals the lifting-up of the double degeneracy of the excited states due to defect-induced configurational changes. Furthermore, time-trace SMFR measurements at different excitation voltages are used to track the tautomerization process in a free-base phthalocyanine. Different behaviors in switching between two inner-hydrogen configurations are observed with decreasing voltages, which helps to reveal the underlying tautomerization mechanism involving both the molecular electronic excited states and vibrational excited states in the ground state.

2 + 2 Can Make Nearly a Thousand! Comparison of Di- and Tetra-Meso-Alkyl-Substituted Porphycenes

Two porphycenes, substituted at the meso positions with two and four methyl groups, respectively, reveal similar absorption spectra, but their photophysical properties are completely different. 9,20-dimethylporphycene emits fluorescence with about 20% quantum yield, independent of the solvent. In contrast, fluorescence of 9,10,19,20-tetramethylporphycene is extremely weak in nonviscous solvents, but it can be recovered by placing the chromophore in a rigid environment. We propose a model that explains these differences, based on calculations and structural analogies with other extremely weakly emitting derivatives, dibenzo[cde,mno]porphycenes. The efficient S₁ deactivation involves delocalization of two inner cavity protons coupled with proton translocation toward a high-energy cis tautomer. The latter process leads to distortion from planarity. The probability of deactivation increases with the strength of the intramolecular NH···N hydrogen bonds. The model also explains the observation of biexponential fluorescence decay in weakly emitting porphycenes. It can be extended to other derivatives, in particular, the asymmetrically substituted ones. We also point to the possibility of using specific porphycenes as viscosity sensors, in particular, when working in single molecule regime.

Influence of local microenvironment on the double hydrogen transfer in porphycene

We performed time-resolved transient absorption and fluorescence anisotropy measurements in order to study tautomerization of porphycene in rigid polymer matrices at cryogenic temperatures. Studies were carried out in poly(methyl methacrylate) (PMMA), poly(vinyl butyral) (PVB), and poly(vinyl alcohol) (PVA). The results prove that in all studied media hydrogen tunnelling plays a significant role in the double hydrogen transfer which becomes very sensitive to properties of the environment below approx. 150 K. We also demonstrate that there exist two populations of porphycene molecules in rigid media: "hydrogen-transferring" molecules, in which tautomerization occurs on time scales below 1 ns and "frozen" molecules in which double hydrogen transfer is too slow to be monitored with nanosecond techniques. The number of "frozen" molecules increases when the sample is cooled. We explain this effect by interactions of guest molecules with a rigid host matrix which disturbs symmetry of porphycene and hinders tunnelling. Temperature dependence of the number of hydrogen-transferring molecules suggests that the factor which restores the symmetry of the double-minimum potential well in porphycene are intermolecular vibrations localized in separated regions of the amorphous polymer.

Controlling Hydrogen-Transfer Rate in Molecules on Graphene by Tunable Molecular Orbital Levels

Molecular switches are building blocks of potential interest to store binary information, especially when they can be organized in periodic lattices. Among the variety of possible systems, switches based on hydrogen transfer are of special importance because they allow the switching operation to occur without severe conformational change that may interfere with neighboring molecular units. We have studied the excitation process of hydrogen transfer inside porphyrin molecules assembled on a graphene surface, using a low-temperature scanning tunneling microscope. We show that this hydrogen transfer is induced by an electronic resonant tunneling process through the molecular orbitals. Using nitrogen doping of graphene, we tune the rate of hydrogen transfer by shifting the molecular orbital energies owing to the charge transfer at nitrogen dopant sites in the graphene lattice. The control of the switching process allows the storage of information inside a molecular lattice, which is demonstrated by writing an artificial pattern inside a molecular island.

Deep Molecular Orbital Driven High-Temperature Hydrogen Tautomerization Switching

Hydrogen tautomerization molecular switches, a promising class of molecular components for the construction of complex nanocircuits, have been extensively studied using low-temperature scanning tunneling microscopy. However, these molecules are generally only reliably controllable in cryogenic environments, obstructing their utility in real devices. Here, we use dispersion-inclusive density functional theory and systematically investigate the adsorption and tautomerization behaviors of porphycene on six transition-metal surfaces. Among these surfaces, we found that hydrogen tautomerization on the Pt(110) surface corresponds to the largest switching barrier, allowing a controllable transition at high temperature. The switching behavior is closely related to the exceptional degree of charge transfer in the HOMO-2 orbital, illustrating the important role of deep orbital-surface interactions in porphycene molecular switching. Our work provides an in-depth understanding of the porphycene tautomerization mechanism and highlights new research avenues toward the practical application of molecular switches.

An sp-hybridized molecular carbon allotrope, cyclo[18]carbon

Carbon allotropes built from rings of two-coordinate atoms, known as cyclo[n]carbons, have fascinated chemists for many years, but until now they could not be isolated or structurally characterized because of their high reactivity. We generated cyclo[18]carbon (C18) using atom manipulation on bilayer NaCl on Cu(111) at 5 kelvin by eliminating carbon monoxide from a cyclocarbon oxide molecule, C24O6. Characterization of cyclo[18]carbon by high-resolution atomic force microscopy revealed a polyynic structure with defined positions of alternating triple and single bonds. The high reactivity of cyclocarbon and cyclocarbon oxides allows covalent coupling between molecules to be induced by atom manipulation, opening an avenue for the synthesis of other carbon allotropes and carbon-rich materials from the coalescence of cyclocarbon molecules.

Accessing a Charged Intermediate State Involved in the Excitation of Single Molecules

Intermediate states are elusive to many experimental techniques due to their short lifetimes. Here, by performing single-electron alternate charging scanning tunneling microscopy of molecules on insulators, we accessed a charged intermediate state involved in the rapid toggling of individual metal phthalocyanines deposited on NaCl films. By stabilizing the transient species, we reveal how electron injection into the lowest unoccupied molecular orbital leads to a pronounced change in the adsorption geometry, characterized by a different azimuthal orientation. This observation allows clarifying the nature of the toggling process, unveiling the role of transient ionic states involved into fundamental processes occurring at interfaces.

Exploring a Route to Cyclic Acenes by On-Surface Synthesis

A route to generate cyclacenes by on-surface synthesis is explored. We started by synthesizing two tetraepoxycyclacenes by sequences of Diels-Alder cycloadditions. Subsequently, these molecules were deposited onto Cu(111) and scanning-tunneling-microscopy(STM)-based atom manipulation was employed to dissociate the oxygen atoms. Atomic force microscopy (AFM) with CO-functionalized tips enabled the detailed characterization of the reaction products and revealed that, at most, two oxygens per molecule could be removed. Importantly, our experimental results suggest that the generation of cyclacenes by the described route might be possible for larger epoxycyclacenes.

Imprinting Directionality into Proton Transfer Reactions of an Achiral Molecule

Directionality is key for the functionality of molecular machines, which is often achieved by built-in structural chiralities. Here, we present a scanning tunneling microscopy study of achiral H₂Pc and HPc molecules that acquire chirality by adsorption onto a Ag(100) surface. The adsorption-geometry-induced chirality is caused by a -29° (+29°) rotation of the molecules with respect to the [011] substrate direction, resulting in tautomerization processes that preferentially occur in a clockwise (counterclockwise) direction. The directionality is found to be independent of the particular energy and location of charge carrier injection. In contrast to built-in structural chiralities that are fixed by the molecular structure, the direction of proton motion in HPc on Ag(100) can be inverted by a rotation of the molecule on the substrate.

Polyyne formation via skeletal rearrangement induced by atomic manipulation

Rearrangements that change the connectivity of a carbon skeleton are often useful in synthesis, but it can be difficult to follow their mechanisms. Scanning probe microscopy can be used to manipulate a skeletal rearrangement at the single-molecule level, while monitoring the geometry of reactants, intermediates and final products with atomic resolution. We studied the reductive rearrangement of 1,1-dibromo alkenes to polyynes on a NaCl surface at 5 K, a reaction that resembles the Fritsch–Buttenberg–Wiechell (FBW) rearrangement. Voltage pulses were used to cleave one C–Br bond, forming a radical, then to cleave the remaining C^•–Br bond triggering the rearrangement. These experiments provide structural insight into the bromo-vinyl radical intermediates, showing that the C=C^•–Br unit is nonlinear. Long polyynes, up to the octayne Ph–(C≡C)₈–Ph, have been prepared in this way. The control of skeletal rearrangements opens a new window on carbon-rich materials and extends the toolbox for molecular synthesis by atom manipulation.

Anharmonicity in a double hydrogen transfer reaction studied in a single porphycene molecule on a Cu(110) surface

Anharmonicity plays a crucial role in hydrogen transfer reactions in hydrogen-bonding systems, which leads to a peculiar spectral line shape of the hydrogen stretching mode as well as highly complex intra/intermolecular vibrational energy relaxation. Single-molecule study with a well-defined model is necessary to elucidate a fundamental mechanism. Recent low-temperature scanning tunnelling microscopy (STM) experiments revealed that the cis↔cis tautomerization in a single porphycene molecule on Cu(110) at 5 K can be induced by vibrational excitation via an inelastic electron tunnelling process and the N-H(D) stretching mode couples with the tautomerization coordinate [Kumagai et al. Phys. Rev. Lett. 2013, 111, 246101]. Here we discuss a pronounced anharmonicity of the N-H stretching mode observed in the STM action spectra and the conductance spectra. Density functional theory calculations find a strong intermode coupling of the N-H stretching with an in-plane bending mode within porphycene on Cu(110).

Analyzing the Wave Nature of Hot Electrons with a Molecular Nanoprobe

We report on a novel method, the molecular nanoprobe (MONA) technique, which allows us to measure the nanoscale quasiparticle transport between two arbitrary surface points. In these experiments, hot electrons are injected into the sample surface from the probe tip of a scanning tunneling microscope (STM) and detected by tautomerization switching events of a single deprotonated phthalocyanine (H₂Pc) molecule. By making use of atom-by-atom-engineered interferometers on a Ag(111) surface, we demonstrate that the quantum-mechanical wave nature of hot electrons leads to characteristic oscillations of the molecule tautomerization probability. Two interferometers can be combined to build an energy-dependent selector, which allows it to selectively switch one out of two molecules without changing the position of the STM tip. The MONA technique is compared with conventional d I/d U measurements, where the injection and detection point of hot electrons is intrinsically tied to the same tip location.

Direct observation of single-molecule hydrogen-bond dynamics with single-bond resolution

The hydrogen bond represents a fundamental interaction widely existing in nature, which plays a key role in chemical, physical and biochemical processes. However, hydrogen bond dynamics at the molecular level are extremely difficult to directly investigate. Here, in this work we address direct electrical measurements of hydrogen bond dynamics at the single-molecule and single-event level on the basis of the platform of molecular nanocircuits, where a quadrupolar hydrogen bonding system is covalently incorporated into graphene point contacts to build stable supramolecule-assembled single-molecule junctions. The dynamics of individual hydrogen bonds in different solvents at different temperatures are studied in combination with density functional theory. Both experimental and theoretical results consistently show a multimodal distribution, stemming from the stochastic rearrangement of the hydrogen bond structure mainly through intermolecular proton transfer and lactam-lactim tautomerism. This work demonstrates an approach of probing hydrogen bond dynamics with single-bond resolution, making an important contribution to broad fields beyond supramolecular chemistry.

Near-Field Enhanced Photochemistry of Single Molecules in a Scanning Tunneling Microscope Junction

Optical near-field excitation of metallic nanostructures can be used to enhance photochemical reactions. The enhancement under visible light illumination is of particular interest because it can facilitate the use of sunlight to promote photocatalytic chemical and energy conversion. However, few studies have yet addressed optical near-field induced chemistry, in particular at the single-molecule level. In this Letter, we report the near-field enhanced tautomerization of porphycene on a Cu(111) surface in a scanning tunneling microscope (STM) junction. The light-induced tautomerization is mediated by photogenerated carriers in the Cu substrate. It is revealed that the reaction cross section is significantly enhanced in the presence of a Au tip compared to the far-field induced process. The strong enhancement occurs in the red and near-infrared spectral range for Au tips, whereas a W tip shows a much weaker enhancement, suggesting that excitation of the localized plasmon resonance contributes to the process. Additionally, using the precise tip-surface distance control of the STM, the near-field enhanced tautomerization is examined in and out of the tunneling regime. Our results suggest that the enhancement is attributed to the increased carrier generation rate via decay of the excited near-field in the STM junction. Additionally, optically excited tunneling electrons also contribute to the process in the tunneling regime.

Unusual effects in single molecule tautomerization: hemiporphycene

Temporal evolution of the fluorescence spectra of individual hemiporphycene molecules reveals unusual double hydrogen transfer reaction dynamics.

Generation and Characterization of a meta-Aryne on Cu and NaCl Surfaces

We describe the generation of a meta-aryne at low temperature (T = 5 K) using atomic manipulation on Cu(111) and on bilayer NaCl on Cu(111). We observe different voltage thresholds for dehalogenation of the precursor and different reaction products depending on the substrate surface. The chemical structure is resolved by atomic force microscopy with CO-terminated tips, revealing the radical positions and confirming a diradical rather than an anti-Bredt olefin structure for this meta-aryne on NaCl.

Remote Single-Molecule Switching: Identification and Nanoengineering of Hot Electron-Induced Tautomerization

Molecular electronics where single molecules perform basic functionalities of digital circuits is a fascinating concept that one day may augment or even replace nowadays semiconductor technologies. The tautomerization of molecules, that is, the bistable functional position of hydrogen protons within an organic frame, has recently been intensively discussed as a potential avenue toward nanoscale switches. It has been shown that tautomerization can be triggered locally or nonlocally, that is, by a scanning tunneling microscope (STM) tip positioned directly above or in close vicinity to the molecule. Whereas consensus exists that local switching is caused by inelastic electrons that excite vibrational molecular modes, the detailed processes responsible for nonlocal tautomerization switching and, even more important in the context of this work, methods to control, engineer, and potentially utilize this process are largely unknown. Here, we demonstrate for dehydrogenated H₂Pc molecules on Ag(111) how to controllably decrease or increase the probability of nonlocal, hot electron-induced tautomerization by atom-by-atom designed Ag nanostructures. We show that Ag atom walls act as potential barriers that exponentially damp the hot electron current between the injection point and the molecule, reducing the switching probability by up to 83% for a four-atom wide wall. By placing the molecule in one and the STM tip in the other focal point of an elliptical nanostructure, we could coherently focus hot electrons onto the molecule that led to an almost tripled switching probability. Furthermore, single and double slit experiment based on silver atom structures were used to characterize the spatial extension of hot electron packets. The absence of any detectable interference pattern suggests that the coherence length of the hot electrons that trigger tautomerization processes is rather short. Our results demonstrate that the tautomerization switching of single molecules can remotely be controlled by utilizing suitable nanostructures and may pave the way for designing new tautomerization-based switches.

Disentangling electron- and electric-field-induced ring-closing reactions in a diarylethene derivative on Ag(1 1 1)

Using scanning tunneling microscopy and spectroscopy we investigate the adsorption properties and ring-closing reaction of a diarylethene derivative (C5F-4Py) on a Ag(1 1 1) surface. We identify an electron-induced reaction mechanism, with a quantum yield varying from 10^-14-10^-9 per electron upon variation of the bias voltage from 1-2 V. We ascribe the drastic increase in switching efficiency to a resonant enhancement upon tunneling through molecular orbitals. Additionally, we resolve the ring-closing reaction even in the absence of a current passing through the molecule. In this case the electric-field can modify the reaction barrier, leading to a finite switching probability at 4.8 K. A detailed analysis of the switching events shows that a simple plate-capacitor model for the tip-surface junction is insufficient to explain the distance dependence of the switching voltage. Instead, describing the tip as a sphere is in agreement with the findings. We resolve small differences in the adsorption configuration of the closed isomer, when comparing the electron- and field-induced switching product.

Intermode Coupling Drives the Irreversible Tautomerization in Porphycene on Copper(111) Induced by Scanning Tunnelling Microscopy

In this contribution, we develop a nonadiabatic theory that explains, from first-principles, the recently reported irreversible trans → cis tautomerization of porphycene on Cu(111) induced by a scanning tunnelling microscope at finite bias. The inelastic contribution to the STM current is found to excite a large number of skeletal vibrational modes of the molecule, thereby inducing a deformation of the potential energy landscape along the hydrogen transfer coordinate. Above a threshold bias, the stability of the tautomers is reversed, which indirectly drives the reaction via intermode coupling. The proposed potential deformation term accounts effectively for the excitation of all internal vibrational modes without increasing the dimensionality of the problem. The model yields information about reaction rates, explains the reaction irreversibility at low temperatures, and accounts for the presence of resonant processes.

Molecular and atomic manipulation mediated by electronic excitation of the underlying Si(111)-7x7 surface

We report the local atomic manipulation properties of chemisorbed toluene molecules on the Si(111)-7x7 surface and of the silicon adatoms of the surface. Charge injected directly into the molecule, or into its underlying bonding silicon adatom, can induce the molecule to change bonding site. The voltage dependence of the rates of these processes match closely with scanning tunnelling spectroscopy of the toluene and adatom species. The branching ratio between toluene molecules which are moved to a neighbouring site, or those that travel further is invariant to voltage, suggesting a common final manipulation step for both injection into the molecule and into the bonding adatom site. At low temperatures the rate of silicon adatom manipulation matches that of toluene manipulation, further suggesting that all these manipulation processes are driven by electronic excitation of the underlying silicon surface. Our results therefore suggest that a common non-adiabatic process mediates atomic and molecular manipulation induced by the STM on the Si(111)-7x7 surface and may also mediate similar manipulation induced by the laser irradiation of the Si(111)-7x7 surface.

Spectroscopic and microscopic investigations of tautomerization in porphycenes: condensed phases, supersonic jets, and single molecule studies

Tautomerization of porphycene, coherent in supersonic jets and a rate process in solutions, can be controlled for single molecules on surfaces.

Breaking Degeneracy of Tautomerization-Metastability from Days to Seconds

We present a detailed study of the tautomerization, that is, the switching of hydrogen protons, between different sites in the molecular frame of phthalocyanine (H₂Pc) on a Ag(111) substrate by means of scanning tunneling microscopy (STM) and STM-based pump-and-sample techniques. Our data reveal that the symmetry mismatch between the substrate and the molecular frame lifts the energetic degeneracy of the two H₂Pc tautomers. Their energy difference is so large that only one tautomer can be found in the ground state. Tip-induced tautomerization was triggered at sufficiently high bias voltages. The excited metastable H₂Pc tautomer was found to exhibit a lifetime of at least several days, as derived from the fact that the molecule did not change back to the ground state within experimentally accessible time scales as long as noninvasive tunneling parameters were used to probe the state of the molecule. By the controlled removal of a hydrogen proton from the molecule, a four-level system was created. Pump-and-sample experiments reveal that the lifetime of the metastable positions amounts to seconds only. Current- and bias-dependent studies indicate that the presence of the STM tip modifies the potential barrier, thereby allowing for a controlled tuning of the metastable tautomer's lifetime.

Non-classical effects in proton or hydrogen transfer

Results of thorough investigations of tautomerism in the ground and excited electronic states performed for various organic molecules under different regimes: condensed phases, isolated and single molecules, with a special emphasis on porphycene, a porphyrin isomer, demonstrate that, in order to precisely describe the proton/hydrogen transfer path, one has to explicitly consider tunneling. Tautomerization is a multidimensional process, controlled by excitation of specific vibrational modes. Vibrational excitation can both enhance or hinder the reaction. The role of specific vibrational modes can now be assessed, even at the level of single molecules.

Initiating and imaging the coherent surface dynamics of charge carriers in real space

The tip of a scanning tunnelling microscope is an atomic-scale source of electrons and holes. As the injected charge spreads out, it can induce adsorbed molecules to react. By comparing large-scale 'before' and 'after' images of an adsorbate covered surface, the spatial extent of the nonlocal manipulation is revealed. Here, we measure the nonlocal manipulation of toluene molecules on the Si(111)-7 × 7 surface at room temperature. Both the range and probability of nonlocal manipulation have a voltage dependence. A region within 5-15 nm of the injection site shows a marked reduction in manipulation. We propose that this region marks the extent of the initial coherent (that is, ballistic) time-dependent evolution of the injected charge carrier. Using scanning tunnelling spectroscopy, we develop a model of this time-dependent expansion of the initially localized hole wavepacket within a particular surface state and deduce a quantum coherence (ballistic) lifetime of ∼10 fs.

Single-Molecule Rotational Switch on a Dangling Bond Dimer Bearing

One of the key challenges in the construction of atomic-scale circuits and molecular machines is to design molecular rotors and switches by controlling the linear or rotational movement of a molecule while preserving its intrinsic electronic properties. Here, we demonstrate both the continuous rotational switching and the controlled step-by-step single switching of a trinaphthylene molecule adsorbed on a dangling bond dimer created on a hydrogen-passivated Ge(001):H surface. The molecular switch is on-surface assembled when the covalent bonds between the molecule and the dangling bond dimer are controllably broken, and the molecule is attached to the dimer by long-range van der Waals interactions. In this configuration, the molecule retains its intrinsic electronic properties, as confirmed by combined scanning tunneling microscopy/spectroscopy (STM/STS) measurements, density functional theory calculations, and advanced STM image calculations. Continuous switching of the molecule is initiated by vibronic excitations when the electrons are tunneling through the lowest unoccupied molecular orbital state of the molecule. The switching path is a combination of a sliding and rotation motion over the dangling bond dimer pivot. By carefully selecting the STM conditions, control over discrete single switching events is also achieved. Combined with the ability to create dangling bond dimers with atomic precision, the controlled rotational molecular switch is expected to be a crucial building block for more complex surface atomic-scale devices.

The butterfly - a well-defined constant-current topography pattern on Si(001):H and Ge(001):H resulting from current-induced defect fluctuations

Dangling bond (DB) arrays on Si(001):H and Ge(001):H surfaces can be patterned with atomic precision and they exhibit complex and rich physics making them interesting from both technological and fundamental perspectives. But their complex behavior often makes scanning tunneling microscopy (STM) images difficult to interpret and simulate. Recently it was shown that low-temperature imaging of unoccupied states of an unpassivated dimer on Ge(001):H results in a symmetric butterfly-like STM pattern, despite the fact that the equilibrium dimer configuration is expected to be a bistable, buckled geometry. Here, based on a thorough characterization of the low-bias switching events on Ge(001):H, we propose a new imaging model featuring a dynamical two-state rate equation. On both Si(001):H and Ge(001):H, this model allows us to reproduce the features of the observed symmetric empty-state images which strongly corroborates the idea that the patterns arise due to fast switching events and provides an insight into the relationship between the tunneling current and switching rates. We envision that our new imaging model can be applied to simulate other bistable systems where fluctuations arise from transiently charged electronic states.

Spectroscopy and Tautomerization Studies of Porphycenes

Tautomerization in porphycenes, constitutional isomers of porphyrins, is strongly entangled with spectral and photophysical parameters. The intramolecular double hydrogen transfer occurring in the ground and electronically excited states leads to uncommon spectroscopic characteristics, such as depolarized emission, viscosity-dependent radiationless depopulation, and vibrational-mode-specific tunneling splittings. This review starts with documentation of the electronic spectra of porphycenes: Absorption and magnetic circular dichroism are discussed, together with their analysis based on the perimeter model. Next, photophysical characteristics are presented, setting the stage for the final part, which discusses the developments in research on tautomerism. Porphycenes have been studied in different experimental regimes: molecules in condensed phases, isolated in supersonic jets and helium nanodroplets, and, recently also on the level of single molecules investigated by optical and scanning probe microscopies. Because of the rich and detailed information obtained from these diverse investigations, porphycenes emerge as very good models for studying the complex, multidimensional phenomena involved in the process of intramolecular double hydrogen transfer.

Single molecule Raman spectra of porphycene isotopologues

Single molecule surface-enhanced resonance Raman scattering (SERRS) spectra have been obtained for the parent porphycene (Pc-d0) and its deuterated isotopologue (Pc-d12), located on gold and silver nanoparticles. Equal populations of "hot spots" by the two isotopologues are observed for 1 : 1 mixtures in a higher concentration range of the single molecule regime (5 × 10(-9) M). For decreasing concentrations, hot spots are preferentially populated by undeuterated molecules. This is interpreted as an indication of a lower surface diffusion coefficient of Pc-d12. The photostability of single Pc molecules placed on nanoparticles is strongly increased in comparison with polymer environments. Trans tautomeric species dominate the spectra, but the analysis of time traces reveals transient intermediates, possibly due to rare cis tautomeric forms.

Direct Observation of Photoinduced Tautomerization in Single Molecules at a Metal Surface

Molecular switches are of fundamental importance in nature, and light is an important stimulus to selectively drive the switching process. However, the local dynamics of a conformational change in these molecules remain far from being completely understood at the single-molecule level. Here, we report the direct observation of photoinduced tautomerization in single porphycene molecules on a Cu(111) surface by using a combination of low-temperature scanning tunneling microscopy and laser excitation in the near-infrared to ultraviolet regime. It is found that the thermodynamically stable trans configuration of porphycene can be converted to the metastable cis configuration in a unidirectional fashion by photoirradiation. The wavelength dependence of the tautomerization cross section exhibits a steep increase around 2 eV and demonstrates that excitation of the Cu d-band electrons and the resulting hot carriers play a dominant role in the photochemical process. Additionally, a pronounced isotope effect in the cross section (∼100) is observed when the transferred hydrogen atoms are substituted with deuterium, indicating a significant contribution of zero-point energy in the reaction. Combined with the study of inelastic tunneling electron-induced tautomerization with the STM, we propose that tautomerization occurs via excitation of molecular vibrations after photoexcitation. Interestingly, the observed cross section of ∼10(-19) cm(2) in the visible-ultraviolet region is much higher than that of previously studied molecular switches on a metal surface, for example, azobenzene derivatives (10(-23)-10(-22) cm(2)). Furthermore, we examined a local environmental impact on the photoinduced tautomerization by varying molecular density on the surface and find substantial changes in the cross section and quenching of the process due to the intermolecular interaction at high density.