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

Light-Controlled Multiconfigurational Conductance Switching in a Single 1D Metal-Organic Wire

Precise control of multiple spin states on the atomic scale presents a promising avenue for designing and realizing magnetic switches. Despite substantial progress in recent decades, the challenge of achieving control over multiconfigurational reversible switches in low-dimensional nanostructures persists. Our work demonstrates multiple, fully reversible plasmon-driven spin-crossover switches in a single π-d metal-organic chain suspended between two electrodes. The plasmonic nanocavity stimulated by external visible light allows for reversible spin crossover between low- and high-spin states of different cobalt centers within the chain. We show that the distinct spin configurations remain stable for minutes under cryogenic conditions and can be nonperturbatively detected by conductance measurements. This multiconfigurational plasmon-driven spin-crossover demonstration extends the available toolset for designing optoelectrical molecular devices based on SCO compounds.

Submolecular-scale control of phototautomerization

Optically activated reactions initiate biological processes such as photosynthesis or vision, but can also control polymerization, catalysis or energy conversion. Methods relying on the manipulation of light at macroscopic and mesoscopic scales are used to control on-surface photochemistry, but do not offer atomic-scale control. Here we take advantage of the confinement of the electromagnetic field at the apex of a scanning tunnelling microscope tip to drive the phototautomerization of a free-base phthalocyanine with submolecular precision. We can control the reaction rate and the relative tautomer population through a change in the laser excitation wavelength or through the tip position. Atomically resolved tip-enhanced photoluminescence spectroscopy and hyperspectral mapping unravel an excited-state mediated process, which is quantitatively supported by a comprehensive theoretical model combining ab initio calculations with a parametric open-quantum-system approach. Our experimental strategy may allow insights in other photochemical reactions and proof useful to control complex on-surface reactions.

Nuclear Quantum Effects in Proton or Hydrogen Transfer

Proton or hydrogen transfers, basic chemical reactions, proceed either by thermally activated barrier crossing or via tunneling. Studies of molecules undergoing single or double proton or hydrogen transfer in the ground or excited electronic state reveal that tunneling can dominate under conditions usually considered to favor the thermal process. Moreover, the tunneling probability strongly varies for excitation of certain vibrational modes, which changes the effective barrier and/or proton transfer distance. When the reaction is fast compared to vibrational relaxation, the mode selectivity can still be maintained for molecules in solutions at 293 K. These observations point to dangers of relating the calculated minimum energy paths and the associated barriers to the experimentally obtained activation energies. The multidimensional character of the reaction coordinate is obvious; it can dramatically change for slowly and rapidly relaxing environments. We postulate that the hydrogen bond definition should be extended by specifically including the role of molecular vibrations.

Unraveling the Mechanisms of On-Surface Photoinduced Reaction with Polarized Light Excitations

On-surface reaction has been shown as a powerful strategy to achieve atomically precise nanostructures. Numerous reactions have been realized on surfaces with thermal annealing as the primary excitation. In contrast, far fewer reactions have been triggered by light on surfaces despite its advantages due to the nonthermal process. This is possibly ascribed to our limited understanding on the excitation mechanisms of on-surface photoinduced reactions. In this work, we have studied the photoinduced debrominated coupling by using a linearly polarized light. We successfully achieved the reaction with no annealing process and obtained oligomers as the primary reaction products, which is in contrast with the formation of polymers with traditional thermal treatments. By exploring the dependence of reaction yield on the angle of incidence, we demonstrate an experimental method that can provide fundamental insights. The comparison with the theoretical approximation suggests indirect hot carrier excitation as the leading excitation mechanism. Our results not only provide fundamental insight into the surface photochemical reactions but also set the basis for harnessing light to construct unconventional nanomaterials.

Combining grating-coupled illumination and image recognition for stable and localized optical scanning tunneling microscopy

Combining scanning tunneling microscopy (STM) and optical excitation has been a major objective in STM for the last 30 years to study light-matter interactions on the atomic scale. The combination with modern pulsed laser systems even made it possible to achieve a temporal resolution down to the femtosecond regime. A promising approach toward a truly localized optical excitation is featured by nanofocusing via an optical antenna spatially separated from the tunnel junction. Until now, these experiments have been limited by thermal instabilities introduced by the laser. This paper presents a versatile solution to this problem by actively coupling the laser and STM, bypassing the vibration-isolation without compromising it. We utilize optical image recognition to monitor the position of the tunneling junction and compensate for any movement of the microscope relative to the laser setup with up to 10 Hz by adjusting the beamline. Our setup stabilizes the focus position with high precision (<1 μm) on long timescales (>1 h) and allows for high resolution STM under intense optical excitation with femtosecond pulses.

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.

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.

Orbital-resolved visualization of single-molecule photocurrent channels

Given its central role in utilizing light energy, photoinduced electron transfer (PET) from an excited molecule has been widely studied^1-6. However, even though microscopic photocurrent measurement methods^7-11 have made it possible to correlate the efficiency of the process with local features, spatial resolution has been insufficient to resolve it at the molecular level. Recent work has, however, shown that single molecules can be efficiently excited and probed when combining a scanning tunnelling microscope (STM) with localized plasmon fields driven by a tunable laser^12,13. Here we use that approach to directly visualize with atomic-scale resolution the photocurrent channels through the molecular orbitals of a single free-base phthalocyanine (FBPc) molecule, by detecting electrons from its first excited state tunnelling through the STM tip. We find that the direction and the spatial distribution of the photocurrent depend sensitively on the bias voltage, and detect counter-flowing photocurrent channels even at a voltage where the averaged photocurrent is near zero. Moreover, we see evidence of competition between PET and photoluminescence¹², and find that we can control whether the excited molecule primarily relaxes through PET or photoluminescence by positioning the STM tip with three-dimensional, atomic precision. These observations suggest that specific photocurrent channels can be promoted or suppressed by tuning the coupling to excited-state molecular orbitals, and thus provide new perspectives for improving energy-conversion efficiencies by atomic-scale electronic and geometric engineering of molecular interfaces.

Recent Advances in Photochemical Reactions on Single-Molecule Electrical Platforms

The photochemical reaction is a very important type of chemical reactions. Visualizing and controlling photo-mediated reactions is a long-standing goal and challenge. In this regard, single-molecule electrical detection with label-free, real-time and in situ characteristics has unique advantages in monitoring the dynamic process of photoreactions at the single-molecule level. In this Review, we provide a valuable summary of the latest process of single-molecule photochemical reactions based on single-molecule electrical platforms. The single-molecule electrical detection platforms for monitoring photoreactions are displayed, including their fundamental principles, construction methods and practical applications. The single-molecule studies of two different types of light-mediated reactions are summarized as below: i) photo-induced reactions, including reversible cyclization, conformational isomerization and other photo-related reactions; ii) plasmon-mediated photoreactions, including reaction mechanisms and concrete examples, such as plasmon-induced photolysis of S-S/O-O bonds and tautomerization of porphycene. In addition, the prospects for future research directions and challenges in this field are also discussed. This article is protected by copyright. All rights reserved.

Competition of Hydrogen Bonds and Coordinate Bonds Induces a Reversible Crystal Transformation

Achieving reversible molecular crystal transformation between coordinate aggregates and hydrogen bonded assemblies has been a challenging task because coordinate bonds are generally much stronger than hydrogen bonds. Recently, we have reported the incorporation of silver ions into the cyanuric acid-melamine (CAM) network, resulting in the formation of a 1D coordination polymer (crystal 1) through forming the κ₁N-Ag-κ₂N coordination bonds. In this work, we find crystal 1 will undergo reversible transformation to hydrogen bonded coordinate units (crystal 2) through the breaking of coordinate chains and then the addition of CAM hydrogen bonding motifs into the framework. Crystal 2 presents a pseudohexagonal arrangement comprised of the κ₁N-Ag-κ₂N units connected by two sets of the triple hydrogen bonds, which extends two-dimensionally and stacks into a layer-structured crystal. Light was shed on the tautomerization of CA and M ligands associated with the crystal transformations using single crystal X-ray diffraction and infrared spectroscopy by analyzing the bond lengths and vibrations. We also highlight that photoluminescence can be a useful tool to probe the tautomer conversions of conjugated molecules. Furthermore, crystal 1 demonstrates high flexibility and can be bent over 180° and recover to its original shape after stress release. Crystal 2, on the contrary, is brittle and shows distinct mechanical anisotropy along different crystal orientations, as unveiled by nanoindentation measurements. The elastic modulus is well correlated with the chemical bonding strength along each orientation, and it is noteworthy that the contribution of the triple hydrogen bonds is comparable to that of the coordination bonds.

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.

Voltage-Induced Bistability of Single Spin-Crossover Molecules in a Two-Dimensional Monolayer

Bistable spin-crossover molecules are particularly interesting for the development of innovative electronic and spintronic devices as they present two spin states that can be controlled by external stimuli. In this paper, we report the voltage-induced switching of the high spin/low spin electronic states of spin-crossover molecules self-assembled in dense 2D networks on Au(111) and Cu(111) by scanning tunneling microscopy at low temperature. On Au(111), voltage pulses lead to the nonlocal switching of the molecules from any─high or low─spin state to the other followed by a spontaneous relaxation toward their initial state within minutes. On the other hand, on Cu(111), single molecules can be addressed at will. They retain their new electronic configuration after a voltage pulse. The memory effect demonstrated on Cu(111) is due to an interplay between long-range intermolecular interaction and molecule/substrate coupling as confirmed by mechanoelastic simulations.

Hydrogen Bond Engineering Visualized by Picometer-Level Distortion of Planar Porphyrin Isomers

Directly investigating hydrogen bond (HB) dynamics in molecular materials is a challenging task. Here, we report a set of porphyrin isomers, porphycenes, that visualize slight changes on the order of picometers in the intramolecular HB dynamics. Intramolecular HBs of porphycenes were regulated by the systematic modification at meso positions with methyl (Me), cyclopentyl (Cy5), and cyclohexyl (Cy6) moieties. Notably, the quantum yields varied from 35 to 0.04% in chloroform, depending on a slight distortion in the porphycene framework. SC-XRD, XPS, and NMR clearly revealed that the Me and Cy6 moieties increased the nonradiative deactivation by strengthening the intramolecular NH···N HBs whereas Cy5 retained their photoluminescence properties. This is the first example of how the distortion of planar porphyrinoids at the picometer level along with the strength of the intramolecular NH···N HBs can drastically affect their optical properties. The results revealed new avenues of HB engineering based on porphyrinoids.

Significance Of Nuclear Quantum Effects In Hydrogen Bonded Molecular Chains

In hydrogen-bonded systems, nuclear quantum effects such as zero-point motion and tunneling can significantly affect their material properties through underlying physical and chemical processes. Presently, direct observation of the influence of nuclear quantum effects on the strength of hydrogen bonds with resulting structural and electronic implications remains elusive, leaving opportunities for deeper understanding to harness their fascinating properties. We studied hydrogen-bonded one-dimensional quinonediimine molecular networks which may adopt two isomeric electronic configurations via proton transfer. Herein, we demonstrate that concerted proton transfer promotes a delocalization of π-electrons along the molecular chain, which enhances the cohesive energy between molecular units, increasing the mechanical stability of the chain and giving rise to distinctive electronic in-gap states localized at the ends. These findings demonstrate the identification of a class of isomeric hydrogen-bonded molecular systems where nuclear quantum effects play a dominant role in establishing their chemical and physical properties. This identification is a step toward the control of mechanical and electronic properties of low-dimensional molecular materials via concerted proton tunneling.

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.

Multidimensional Hydrogen Tunneling in Supported Molecular Switches: The Role of Surface Interactions

The nuclear tunneling crossover temperature (T_{c}) of hydrogen transfer reactions in supported molecular-switch architectures can lie close to room temperature. This calls for the inclusion of nuclear quantum effects (NQEs) in the calculation of reaction rates even at high temperatures. However, computations of NQEs relying on standard parametrized dimensionality-reduced models quickly become inadequate in these environments. In this Letter, we study the paradigmatic molecular switch based on porphycene molecules adsorbed on metallic surfaces with full-dimensional calculations that combine density-functional theory for the electrons with the semiclassical ring-polymer instanton approximation for the nuclei. We show that the double intramolecular hydrogen transfer (DHT) rate can be enhanced by orders of magnitude due to surface fluctuations in the deep-tunneling regime. We also explain the origin of an Arrhenius temperature dependence of the rate below T_{c} and why this dependence differs at different surfaces. We propose a simple model to rationalize the temperature dependence of DHT rates spanning diverse fcc [110] surfaces.

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.

Temperature dependence of the vibrational spectrum of porphycene: a qualitative failure of classical-nuclei molecular dynamics

Approximate quantum dynamics succeed in predicting a temperature-dependent blue-shift of the high-frequency stretch bands that arise from vibrational coupling between low-frequency thermally activated modes and high-frequency quantized ones. Classical nuclei molecular dynamics fail and instead predict a red-shift.

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.

Scanning probe microscopy for real-space observations of local chemical reactions induced by a localized surface plasmon

Localised surface plasmon (LSP) resonance has attracted considerable attention in recent years as an efficient driving force for chemical reactions. The chemical reactions induced by LSP are classified into two types, namely, redox reactions based on plasmon-induced charge separation (PICS) and chemical reactions induced by the direct interaction between LSP and molecules (plasmon-induced chemical reactions). Although both types of reactions have been extensively studied, the mechanisms of PICS and plasmon-induced chemical reactions remain unexplained and controversial because conventional macroscopic methods can hardly grasp the local chemical reactions induced by LSP. In order to obtain mechanistic insights, nanoscale observations and investigations are necessary. Scanning probe microscopy (SPM) is a powerful experimental tool to investigate not only the surface morphology but also the physical and chemical properties of samples at a high spatial resolution. In this perspective review, we first explain SPM combined with optical excitation, and then, review the recent studies using SPM techniques for real-space observations of the chemical reactions induced by LSP.

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.

Near-Field Spectral Response of Optically Excited Scanning Tunneling Microscope Junctions Probed by Single-Molecule Action Spectroscopy

The near-field spectral response of metallic nanocavities is a key characteristic in plasmon-assisted photophysical and photochemical processes. Here, we show that the near-field spectral response of an optically excited plasmonic scanning tunneling microscope (STM) junction can be probed by single-molecule reactions that serve as a nanoscale sensor detecting the local field intensity. Near-field action spectroscopy for the cis ↔ cis tautomerization of porphycene on a Cu(110) surface reveals that the field enhancement in the STM junction largely depends on microscopic structures not only on the tip apex, but also on its shaft. Using nanofabrication of Au tips with focused ion beam, we show that the spectral response is strongly modulated through the interference between the localized surface plasmon in the junction and propagating surface plasmon polariton generated on the shaft. Furthermore, it is demonstrated that the near-field spectral response can be manipulated by precisely shaping the tip shaft.

Self-assembly of photochromic diarylethene-peptide conjugates stabilized by β-sheet formation at the liquid/graphite interface

Two-dimensional (2-D) self-assembly of diarylethene (DAE)-peptide conjugates at the octanoic acid/graphite interface was investigated by scanning tunnelling microscopy (STM). The open-ring isomer of a DAE-peptide conjugate formed a stable 2-D molecular assembly with an antiparallel β-sheet structure. Quantitative analysis of surface coverage depending on concentration revealed a stronger stabilization effect of the oligopeptide than that of the alkyl group with a similar side chain length.

Elucidating the Nuclear Quantum Dynamics of Intramolecular Double Hydrogen Transfer in Porphycene

We address the double hydrogen transfer (DHT) dynamics of the porphycene molecule, a complex paradigmatic system in which the making and breaking of H-bonds in a highly anharmonic potential energy surface require a quantum mechanical treatment not only of the electrons but also of the nuclei. We combine density functional theory calculations, employing hybrid functionals and van der Waals corrections, with recently proposed and optimized path-integral ring-polymer methods for the approximation of quantum vibrational spectra and reaction rates. Our full-dimensional ring-polymer instanton simulations show that below 100 K the concerted DHT tunneling pathway dominates but between 100 and 300 K there is a competition between concerted and stepwise pathways when nuclear quantum effects are included. We obtain ground-state reaction rates of 2.19 × 10¹¹ s^–1 at 150 K and 0.63 × 10¹¹ s^–1 at 100 K, in good agreement with experiment. We also reproduce the puzzling N–H stretching band of porphycene with very good accuracy from thermostated ring-polymer molecular dynamics simulations. The position and line shape of this peak, centered at around 2600 cm^–1 and spanning 750 cm^–1, stem from a combination of very strong H-bonds, the coupling to low-frequency modes, and the access to cis-like isomeric conformations, which cannot be appropriately captured with classical-nuclei dynamics. These results verify the appropriateness of our general theoretical approach and provide a framework for a deeper physical understanding of hydrogen transfer dynamics in complex systems.

Electronic structure of an iron porphyrin derivative on Au(1 1 1)

Surface-bound porphyrins are promising candidates for molecular switches, electronics and spintronics. Here, we studied the structural and the electronic properties of Fe-tetra-pyridil-porphyrin adsorbed on Au(1 1 1) in the monolayer regime. We combined scanning tunneling microscopy/spectroscopy, ultraviolet photoemission, and two-photon photoemission to determine the energy levels of the frontier molecular orbitals. We also resolved an excitonic state with a binding energy of 420 meV, which allowed us to compare the electronic transport gap with the optical gap.

Supersonic jet spectroscopy of parent hemiporphycene: Structural assignment and vibrational analysis for S0 and S1 electronic states

Hemiporphycene (HPc), a constitutional isomer of porphyrin, is studied under supersonic expansion conditions by means of laser-induced fluorescence, visible-visible hole-burning experiments, single vibronic level fluorescence techniques, and quantum chemical calculations. Only one trans form of jet-cooled HPc is observed, in contrast to solution studies that evidence a mixture of two trans tautomeric forms separated in energy by ∼1 kcal/mol. Reliable structural assignment is provided by simulating absorption and emission patterns at the density functional theory and time-dependent density functional theory levels of theory. The vibronic spectra are nicely reproduced for both electronic ground and lowest excited singlet states for the most stable trans form. In contrast to another porphyrin isomer, porphycene (Pc), no tunneling or photo-induced hydrogen transfer is detected. The lower symmetry of HPc compared with Pc and the concomitant non-equivalent positions of the inner-cavity nitrogen atoms result in a non-symmetric double minimum potential for tautomerization, larger energy barrier, and a longer tunneling distance, with the average intramolecular hydrogen bond length larger in HPc than in Pc. HPc readily forms hydrates that show red-shifted absorption relative to the bare molecule.

Supramolecular Assemblies on Surfaces: Nanopatterning, Functionality, and Reactivity

Understanding how molecules interact to form large-scale hierarchical structures on surfaces holds promise for building designer nanoscale constructs with defined chemical and physical properties. Here, we describe early advances in this field and highlight upcoming opportunities and challenges. Both direct intermolecular interactions and those that are mediated by coordinated metal centers or substrates are discussed. These interactions can be additive, but they can also interfere with each other, leading to new assemblies in which electrical potentials vary at distances much larger than those of typical chemical interactions. Earlier spectroscopic and surface measurements have provided partial information on such interfacial effects. In the interim, scanning probe microscopies have assumed defining roles in the field of molecular organization on surfaces, delivering deeper understanding of interactions, structures, and local potentials. Self-assembly is a key strategy to form extended structures on surfaces, advancing nanolithography into the chemical dimension and providing simultaneous control at multiple scales. In parallel, the emergence of graphene and the resulting impetus to explore 2D materials have broadened the field, as surface-confined reactions of molecular building blocks provide access to such materials as 2D polymers and graphene nanoribbons. In this Review, we describe recent advances and point out promising directions that will lead to even greater and more robust capabilities to exploit designer surfaces.

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).

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.

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.

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.

β-Hexaalkylporphycenes: Positional Effect of Alkyl Groups toward Design and Control of Structural and Photophysical Properties in Isomeric Hexaethylporphycenes

Two novel β-hexaalkylated porphycenes, i.e., 2,3,7,12,13,17- (HOT) and 2,3,6,12,13,16-hexaethylporphycenes (HIT) were introduced for the first time in porphycene chemistry. These were synthesized through McMurry coupling reactions of new isomeric unsymmetrically substituted triethylbipyrrole dialdehydes. The positional effects of alkyl groups could be manifested through significant alteration in structure of porphycene cores and, as a consequence their photophysical properties, not noticed in β-octaethylporphycene. HOT displays significant fluorescence accompanied by reasonable singlet oxygen generation ability.

The first synthesis of meso-dicycloalkylporphycenes: ring strain effects on structural and optical properties of isomeric porphyrins

Two novel meso-dicycloalkylporphycenes were synthesized for the first time, which showed remarkable ring-strain-induced switching of their fluorescence behaviors in the solution phase.

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.

Solid surface vs. liquid surface: nanoarchitectonics, molecular machines, and DNA origami

Comparisons of science and technology between these solid and liquid surfaces would be a good navigation for current-to-future developments.