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

Fully Reprogrammable 2D Array of Multistate Molecular Switching Units

Integration of molecular switching units into complex electronic circuits is considered to be the next step toward the realization of novel logic and memory devices. This paper reports on an ordered 2D network of neighboring ternary switching units represented by triazatruxene (TAT) molecules organized in a honeycomb lattice on a Ag(111) surface. Using low-temperature scanning tunneling microscopy, the bonding configurations of individual TAT molecules can be controlled, realizing up to 12 distinct states per molecule. The switching between those states shows a strong bias dependence ranging from tens of millivolts to volts. The low-bias switching behavior is explored in active units consisting of two and more interacting TAT molecules that are purposefully defined (programmed) by high-bias switching within the honeycomb lattice. Within such a unit the low-bias switching can be triggered and accessed by single-point measurements on a single TAT molecule, demonstrating up to 9 and 19 distinguishable states in a dyad and a tetrad of coupled molecules, respectively. High experimental control over the desired state, owing to bias-dependent hierarchical switching and pronounced switching directionality, as well as full reversibility, make this system particularly appealing, paving the way to design complex molecule-based memory systems.

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.

Probing Conical Intersection in the Multipathway Isomerization of CH3Cl Using Coulomb Explosion

Ubiquitous ultrafast isomerization is paramount in photoexcited molecules, in which non-adiabatic coupling among multiple electronic states can occur. We use the pump-probe Coulomb explosion imaging method to study the isomerization of CH3Cl molecules. We find that the isomerization under our strong field pump-probe scheme proceeds along multiple pathways, which are encoded in several distinct branches of the time-resolved kinetic energy release spectra for the CH2++HCl+ Coulomb explosion channel. Apart from the isomerized dissociative pathway in neutral and cationic excited states, the pump laser can also induce coherent vibrational dynamics in two coupled intermediate states and set up the initial conditions for the two concurrently proceeding isomerization pathways. The isomerization of CH3Cl provides an intriguing example of a chemical reaction consisting of multiple pathways and non-adiabatic dynamics.

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.

Probing Energy-Level Alignment in Molecular Multilayers by Frequency-Modulation Electrostatic Force Microscopy under Tapping-Mode-Combined Fowler-Nordheim Tunneling Spectroscopy

The alignment of molecular electronic levels in a molecular multilayer is of crucial importance to realize desired functions for molecular devices. Amplitude-modulation-feedback frequency-modulation electrostatic force microscopy combined with Fowler-Nordheim tunneling spectroscopy is utilized as a probe for the energy-level alignment in an organic multilayer. Bias-dependent electrostatic force spectra were examined for bilayers including a Ru complex as a benchmark multilayer system. Electrostatic properties in the low-bias region were captured well by a single-capacitor model, which indicates weak coupling at the bilayer interface between the Ru complex and self-assembled monolayer. In contrast, in the high-bias region, significant disagreement with the expected electrostatic force was recognized for the bilayers and evaluated as the loss of electrostatic energy through the Fowler-Nordheim tunneling process. Alignment of the lowest unoccupied molecular orbital (LUMO) level of the Ru complex was determined by Fowler-Nordheim emission through resonant tunneling. These results indicate an effective method to probe level alignment at interfaces inside multilayers and to provide the partition factor β that depicts a divided electric field.

Free-Base Corrole Anion

Free-base corroles have long been known to be acidic, readily undergoing deprotonation by mild bases and in polar solvents. The conjugate base, however, has not been structurally characterized until now. Presented here is a first crystal structure of a free-base corrole anion, derived from tris(p-cyanophenyl)corrole, as the tetrabuylammonium salt. The low-temperature (100 K) structure reveals localized hydrogens on a pair of opposite pyrrole nitrogens. DFT calculations identify such a structure as the global minimum but also point to two cis tautomers only 4-7 kcal/mol above the ground state. In terms of free energy, however, the cis tautomers are above or essentially flush with the trans-to-cis barrier so the cis tautomers are unlikely to exist or be observed as true intermediates. Thus, the hydrogen bond within each dipyrrin unit on either side of the molecular pseudo-C2 axis through C10 (i.e., between pyrrole rings A and B or between C and D) qualifies as or closely approaches a low-barrier hydrogen bond. Proton migration across the pseudo-C2 axis entails much higher activation energies >20 kcal/mol, reflecting the relative rigidity of the molecule along the C1-C19 pyrrole-pyrrole linkage.

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.

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.

Voltage-driven control of single-molecule keto-enol equilibrium in a two-terminal junction system

Keto-enol tautomerism, describing an equilibrium involving two tautomers with distinctive structures, provides a promising platform for modulating nanoscale charge transport. However, such equilibria are generally dominated by the keto form, while a high isomerization barrier limits the transformation to the enol form, suggesting a considerable challenge to control the tautomerism. Here, we achieve single-molecule control of a keto-enol equilibrium at room temperature by using a strategy that combines redox control and electric field modulation. Based on the control of charge injection in the single-molecule junction, we could access charged potential energy surfaces with opposite thermodynamic driving forces, i.e., exhibiting a preference for the conducting enol form, while the isomerization barrier is also significantly reduced. Thus, we could selectively obtain desired and stable tautomers, which leads to significant modulation of the single-molecule conductance. This work highlights the concept of single-molecule control of chemical reactions on more than one potential energy surface.

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.

Searching for correlations between geometric and spectroscopic parameters of intramolecular hydrogen bonds in porphyrin-like macrocycles

Chemical bond lengths and angles are characteristic structural parameters of a molecule. Similarly, the frequencies of the vibrational modes and the NMR chemical shifts are unique "chemical fingerprints" specific to a compound. These are the basic parameters describing newly obtained compounds and enabling their identification. Intramolecular hydrogen bonding significantly influences the physicochemical properties of macrocyclic compounds with a porphyrin-like structure. This work presents the verification for correlations between geometric and spectroscopic parameters related to hydrogen bonds in this type of macrocyclic compounds. In particular, such relationships were investigated for a large group of porphyrin, porphycene, and dibenzotetraaza[14]annulene derivatives and a group of other macrocycles with similar structure. A very strong linear correlation was found only between the vibrational frequencies of the NH groups involved in a hydrogen bond and the length of this bond, which applied to all macrocyclic compounds of this type. Several other relationships were found between spectroscopic (IR, Raman, NMR) and geometric (X-ray) parameters, highlighting differences and similarities between different families of macrocycles. Apart from providing a better understanding of the nature of hydrogen bonds and their characteristics in porphyrin-like macrocyclic compounds, these relationships will facilitate the identification of new macrocycles and the extrapolation of their spectroscopic properties.

Three-Bit Digital Comparator Based on Intracell Diffusion of Silver Single Atom

Using a single atom to construct electronic components is a promising route for the microminiaturization of electronic instruments. However, effective control of the intrinsic property in a molecular/atomic prototype component is full of challenges. Here, we present that the intracell diffusion behavior of a target Ag single atom within a unit cell of Si reconstruction is controllably modulated by constructing Ag nanoclusters and arrays in the neighboring cells. Moreover, a three-bit digital comparator device is fabricated on the basis of the diffusion time of a Ag single atom that can be effectively regulated by using the intercoupling between the target Ag monomer and the surrounding metal arrays.

Selectivity in single-molecule reactions by tip-induced redox chemistry

Controlling selectivity of reactions is an ongoing quest in chemistry. In this work, we demonstrate reversible and selective bond formation and dissociation promoted by tip-induced reduction-oxidation reactions on a surface. Molecular rearrangements leading to different constitutional isomers are selected by the polarity and magnitude of applied voltage pulses from the tip of a combined scanning tunneling and atomic force microscope. Characterization of voltage dependence of the reactions and determination of reaction rates demonstrate selectivity in constitutional isomerization reactions and provide insight into the underlying mechanisms. With support of density functional theory calculations, we find that the energy landscape of the isomers in different charge states is important to rationalize the selectivity. Tip-induced selective single-molecule reactions increase our understanding of redox chemistry and could lead to novel molecular machines.

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.

Inverted Conformation Stability of a Motor Molecule on a Metal Surface

Molecular motors have been intensely studied in solution, but less commonly on solid surfaces that offer fixed points of reference for their motion and allow high-resolution single-molecule imaging by scanning probe microscopy. Surface adsorption of molecules can also alter the potential energy surface and consequently preferred intramolecular conformations, but it is unknown how this affects motor molecules. Here, we show how the different conformations of motor molecules are modified by surface adsorption using a combination of scanning tunneling microscopy and density functional theory. These results demonstrate how the contact of a motor molecule with a solid can affect the energetics of the molecular conformations.

Determination of concerted or stepwise mechanism of hydrogen tunneling from isotope effects: Departure between experiment and theory

Isotope substitution is an important experimental technique that offers deep insight into reaction mechanisms, as the measured kinetic isotope effects (KIEs) can be directly compared with theory. For multiple proton transfer processes, there are two types of mechanisms: stepwise transfer and concerted transfer. The Bell-Limbach model provides a simple theory to determine whether the proton transfer mechanism is stepwise or concerted from KIEs. Recent STM experiments have studied the proton switching process in water tetramers on NaCl(001). Theoretical studies predict that this process occurs via a concerted mechanism, however, the experimental KIEs resemble the Bell-Limbach model for stepwise tunneling, raising question on the underlying mechanism or the validity of the model. We study this system using ab initio instanton theory, and in addition to thermal rates, we also considered microcanonical rates, as well as tunneling splittings. Instanton theory predicts a concerted mechanism, and the KIEs for tunneling rates (both thermal and microcanonical) upon deuteration are consistent with the Bell-Limbach model for concerted tunneling, but could not explain the experiments. For tunneling splittings, partial and full deuteration changes the size of it in a similar fashion to how it changes the rates. We further examined the Bell-Limbach model in another system, porphycene, which has both stepwise and concerted tunneling pathways. The KIEs predicted by instanton theory are again consistent with the Bell-Limbach model. This study highlights differences between KIEs in stepwise and concerted tunneling, and the discrepancy between theory and recent STM experiments. New theory/experiments are desired to settle this problem.

Internal Stark effect of single-molecule fluorescence

The optical properties of chromophores can be efficiently tuned by electrostatic fields generated in their close environment, a phenomenon that plays a central role for the optimization of complex functions within living organisms where it is known as internal Stark effect (ISE). Here, we realised an ISE experiment at the lowest possible scale, by monitoring the Stark shift generated by charges confined within a single chromophore on its emission energy. To this end, a scanning tunneling microscope (STM) functioning at cryogenic temperatures is used to sequentially remove the two central protons of a free-base phthalocyanine chromophore deposited on a NaCl-covered Ag(111) surface. STM-induced fluorescence measurements reveal spectral shifts that are associated to the electrostatic field generated by the internal charges remaining in the chromophores upon deprotonation.

The internal Stark effect, a shift of the spectral lines of a chromophore induced by electrostatic fields in its close environment, plays an important role in nature. Here the authors observe a Stark shift in the fluorescence spectrum of a phthalocyanine molecule upon charge modifications within the molecule itself, achieved by sequential removal of the central protons with a STM tip.

In-Core N4-Coordination of Palladium(II) in Dinaphthoporphycene: Synthesis, Structure, and Photophysical Studies

Dinaphthoporphycene (DNP) has emerged as a versatile ligand undergoing large out-of-plane distortion to form a cis-bimetallic complex with Pd(II) using Pd(OAc)₂ and out-of-plane monometallic complexes with Pd(acac)₂ and PtCl₂(PhCN)₂. Herein, we are finally able to synthesize the in-core complex with Pd(II) using PdCl₂(PhCN)₂ or PdCl₂. The crystal structure shows the palladium ion resides slightly above the N4-core, with the Pd(II) dimensionally dissenting with the typical square planarity displayed by the reported in-core DNP complexes with Ni(II) and Cu(II) ions. The deformed complex displays a blue shift in the absorption spectra compared to DNP and its metallo-derivatives. PdDNP exhibits a moderate singlet oxygen generation ability (18%).

3,6,13,16-Tetraalkylporphycenes: Synthesis and Exploration of Effect of Alkyl Groups on Structure, Photophysical Properties, and Basicity

Two new 3,6,13,16-tetraalkylporphycenes were synthesized following rational approach. The reason behind lower yield of the desired β,β'-bipyrroles was unraveled. The σ-donating effect of alkyl-substituents was more profound than reported positional...

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.

Facile room-temperature self-assembly of extended cation-free guanine-quartet network on Mo-doped Au(111) surface

Guanine-quadruplex, consisting of several stacked guanine-quartets (GQs), has emerged as an important category of novel molecular targets with applications from nanoelectronic devices to anticancer drugs. Incorporation of metal cations into a GQ structure is utilized to form stable G-quadruplexes, while formation of a cation-free GQ network has been challenging. Here we report the room temperature (RT) molecular self-assembly of extended pristine GQ networks on an Au(111) surface. An implanted molybdenum atom within the Au(111) surface is used to nucleate and stabilize the cation-free GQ network. Additionally, decoration of the Au(111) surface with 7-armchair graphene nanoribbons (7-AGNRs) enhances the GQ domain size by suppressing the influence of the disordered phase nucleated from Au step edges. Scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations confirm the formation of GQ networks and unravel the nucleation and growth mechanism. Our work, utilizing a hetero-atom doped substrate, provides a facile approach to enhance the stability and domain size of the GQ self-assembly, which would be applicable for other molecular structures.

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.

Lanthanide-porphyrin species as Kondo irreversible switches through tip-induced coordination chemistry

Metallosupramolecular chemical protocols are applied to in situ design dysprosium porphyrin complexes on Au(111) by sequential deposition of 2H-4FTPP species and Dy, resulting in the production of premetallated Dy-2H-4FTPP, partially metallated Dy-1H-4FTPP and fully metallated Dy-0H-4FTPP complexes, as determined by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. A zero bias resonance is found in the Dy-2H-4FTPP species which, upon study of its spatial distribution and behavior with temperature, is assigned to a Kondo resonance resulting from an unpaired spin in the molecular backbone, featuring a Kondo temperature (TK) of ≈ 21 K. Notably, the Kondo resonance can be switched off by removing one hydrogen atom of the macrocycle through tip-induced voltage pulses with submolecular precision. The species with this Kondo resonance can be laterally manipulated illustrating the potential to assemble artificial Kondo lattices. Our study demonstrates that the pre-metallation of macrocycles by lanthanides and their controlled manipulation is a novel strategy to engineer in situ tunable Kondo nanoarchitectures, enhancing the potential of coordination chemistry for spintronics.

Double intramolecular hydrogen transfer assisted dual emission in a carbazole-embedded porphyrin-like macrocycle

The introduction of a pyrrole ring at one of the meso positions of carbazole-based porphyrins lowers the structural symmetry and results in dual emission, which strongly depends on the excitation wavelength and temperature. The origin of dual emission induced by NH-tautomerism is confirmed via photophysical and DFT calculations.

Controlling Emissive Properties by Intramolecular Hydrogen Bonds: Alkyl and Aryl meso-Substituted Porphycenes

Porphycene, a porphyrin isomer, is an efficient fluorophore. However, four-fold meso substitution with alkyl groups decreases the fluorescence quantum yield by orders of magnitude. For aryl substituents, this effect is small. To explain this difference, we have synthesized and studied a mixed aryl-alkyl-substituted compound, 9,20-diphenyl-10,19-dimethylporphycene, as well as the 9,20-diphenyl and 9,20-dimethyl derivatives. Analysis of the structural, spectroscopic, and photophysical data of the six porphycenes, combined with quantum chemical calculations, shows a clear correlation between the strength of the intramolecular NH⋅⋅⋅N hydrogen bonds and the efficiency of the radiationless depopulation of the lowest-excited singlet state. This result led us to propose a model in which the delocalization of the inner protons in the cavity of the macrocycle is responsible for the nonradiative deactivation channel. The applicability of the model is confirmed by the literature data for other alkyl- or aryl-substituted porphycenes. The finding of a correlation between structural and emissive characteristics enables a rational design of porphycenes with desired photophysical properties.

Photoisomerization-mechanism-associated excited-state hydrogen transfer in 2'-hydroxychalcone revealed by on-the-fly trajectory surface-hopping molecular dynamics simulation

By performing global-switching on-the-fly trajectory surface-hopping molecular dynamics simulation at the OM2/MRCI (14,15) quantum level, we probed the S3(ππ*) photoisomerization mechanisms associated with excited-state intramolecular hydrogen transfer for 2'-hydroxychalcone (2HC) within the interwoven conical intersection networks from four singlet electronic states (S3, S2, S1, and S0). The simulated quantum yields of 0.03 for cis-to-trans and zero for trans-to-cis photoisomerization were due to almost all the conical intersections being localized either in the cis-2HC or in trans-2HC region, and there was little chance for sampling trajectories to reach the rotation conical intersection (S1/S0) in between cis-2HC and trans-2HC that is key for reactive isomerization. The potential energy well on the S1 state in the trans-2HC region prevents trajectories from trans-to-cis photoisomerization, while the fact there is no well on S1 state in cis-2HC region opens a few chances for trajectories to reach the rotation conical intersections. The present simulation found that excited-state intramolecular hydrogen transfers in 2HC have a negative impact for reactive isomerization, and that hydrogen transfers take place on the S1 state, while back-transfer on the S0 state prevents sampling trajectories reaching rotational conical intersections. It was realized that it could be possible to enhance the quantum yield of 2HC photoisomerization by suppressing the hydrogen transfer (such as by changing an electron-donating substitution or adjusting the substitution position to decrease the acidity of the phenol group). From a perspective view of the potential energy surfaces, the theoretical design of such 2HC derivatives could enhance (control) the quantum yield by shifting the conical intersections away from the cis- and trans-region.

Single hydrogen atom manipulation for reversible deprotonation of water on a rutile TiO2 (110) surface

The discovery of hydrogen atoms on the TiO₂ surface is crucial for many practical applications, including photocatalytic water splitting. Electronically activating interfacial hydrogen atoms on the TiO₂ surface is a common way to control their reactivity. Modulating the potential landscape is another way, but dedicated studies for such an activation are limited. Here we show the single hydrogen atom manipulation, and on-surface facilitated water deprotonation process on a rutile TiO₂ (110) surface using low temperature atomic force microscopy and Kelvin probe force spectroscopy. The configuration of the hydrogen atom is manipulated on this surface step by step using the local field. Furthermore, we quantify the force needed to relocate the hydrogen atom on this surface using force spectroscopy and density functional theory. Reliable control of hydrogen atoms provides a new mechanistic insight of the water molecules on a metal oxide surface.

Rectangular Holes in Porphyrin Isomers Act As Mono- and Binucleating Ligands: Stereochemistry of Mono- and Diboron Porphycenes and Their Protonation Behaviors

The first boron complexes of porphycenes, structural isomers of porphyrin, are reported. They are synthesized in good yields by reacting the free-base porphycene ligands with BF₃·Et₂O through a microwave-assisted method. Depending on the substituent group of porphycenes, two different coordination structures, mono- and diboron porphycenes, are obtained simultaneously. The single crystal structures and DFT calculations suggest that the boron atom of the monoboron porphycene is favorably coordinated on the dipyrroethene site, and the regioisomer of diboron porphycene is of cisoid stereochemistry, which is more stable than transoid. We also investigate the protonation behavior of boron porphycene complexes. Diboron porphycene does not undergo protonation, whereas monoboron porphycene undergoes protonation at the nonboron coordinating pyrroline site, resulting in a red shift in both absorption and emission spectra. Protonation and deprotonation of monoboron porphycene can be reversibly triggered using acids and bases.

Porphyrinoids with Vinylene Bridges

Porphyrinoids containing vinylene bridges, such as triphyrin(2.1.1), porphycene, porphyrin(2.1.2.1), and hexaphyrin(2.1.2.1.2.1), are a relatively new family of porphyrinoids. Vinylene bridges give porphyrinoids a lower symmetry and a flexibility of the framework and they permit cis/trans-isomerization reactions; these factors confer unique properties to these substances, such as coordination to metal ions and aromaticity switching. In this account, the synthesis, crystal structures, and properties of new porphyrinoids containing vinylene bridges are summarized.

1 Introduction

2 Triphyrin(2.1.1)

3 Porphycene

4 Porphyrin(2.1.2.1)

5 Hexaphyrin(2.1.2.1.2.1)

6 Conclusion

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.

3,6,13,16-Tetrasubstituted Porphycene: The Missing Link in Porphycene Chemistry

We have introduced the first 3,6,13,16-tetrasubstituted porphycene as its tetramethoxy analogue. This substitution pattern is one of the most general patterns yet missing in this isomeric porphyrin chemistry. This porphycene exhibits intense fluorescence along with the ability to coordinate with divalent metal ions; in particular, it forms the first stable Zn(II) complex among the tetrasubstituted porphycenes. Notably, the molecular structure of Zn1•Py displays supramolecular chirality.

NH Tautomerism of N-Confused Porphyrin: Solvent/Substituent Effects and Isomerization Mechanism

The effects of substituents and solvents on the NH tautomerism of N-confused porphyrin (2) were investigated. The structures, electronic states, and aromaticity of NH tautomers (2-2H and 2-3H) were studied by absorption and nuclear magnetic resonance (¹H, ¹³C, and ¹⁵N) spectroscopies, single-crystal X-ray diffraction analysis, and theoretical calculations. The relative stability of the tautomers is highly affected by solvents, with the 3H-type tautomer being more stable in nonpolar solvents, while the 2H-type tautomer being highly stabilized in polar solvents with high donor numbers such as N,N-dimethylformamide (DMF), pyridine, and acetone. Electron-withdrawing groups on the meso-aryl substituents as well as the methyl group at the ortho position also stabilize the 2H-type tautomer. Kinetically, the tautomerism rate is significantly influenced by solvent and concentration, and a particularly large activation entropy (ΔS^⧧) is obtained in pyridine. The first-order deuterium isotope effect on the reaction rates of NH tautomerism (k_H/k_D) is determined to be 2.4 at 298 K. On the basis of kinetic data, the mechanism of isomerization is identified as an intramolecular process, including the rotation of the confused pyrrole in pyridine/chloroform and DMF/chloroform mixed solvent systems, and as a pyridine-mediated process in pyridine alone.

Absence of Nonlocal Manipulation of Oxygen Atoms Inserted below the Si(111)-7×7 Surface

The injection of electrons from the scanning tunneling microscope tip can be used to perform nanoscale chemistry and study hot electron transport through surfaces. While nonlocal manipulation has been demonstrated primarily for aromatic adsorbates, here we confirm that oxygen atoms bonded to the Si(111) surface can also be nonlocally manipulated, and we fit the measured manipulation data to a single channel decay model. Unlike aromatic adsorption systems, oxygen atoms also insert below the surface of silicon. Although the inserted oxygen can be manipulated when the tip is directly over the relevant silicon adatom, it is not possible to induce nonlocal manipulation of inserted oxygen atoms at the same bias. We attribute this to the electrons injected at +4 eV initially relaxing to couple to the highest available surface state at +3.4 eV before laterally transporting through the surface. With a manipulation threshold of 3.8 eV for oxygen inserted into silicon, once carriers have undergone lateral transport, they do not possess enough energy to manipulate and remove oxygen atoms inserted beneath the surface of silicon. This result confirms that nonlocal nanoscale chemistry using the scanning tunneling microscope tip is dependent not only on the energy required for atomic manipulation, but also on the energy of the available surface states to carry the electrons to the manipulation site.

Atomic-Scale Dynamics Probed by Photon Correlations

Light absorption and emission have their origins in fast atomic-scale phenomena. To characterize these basic steps (e.g., in photosynthesis, luminescence, and quantum optics), it is necessary to access picosecond temporal and picometer spatial scales simultaneously. In this Perspective, we describe how state-of-the-art picosecond photon correlation spectroscopy combined with luminescence induced at the atomic scale with a scanning tunneling microscope (STM) enables such studies. We outline recent STM-induced luminescence work on single-photon emitters and the dynamics of excitons, charges, molecules, and atoms as well as several prospective experiments concerning light-matter interactions at the nanoscale. We also describe future strategies for measuring and rationalizing ultrafast phenomena at the nanoscale.

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.

Rotation of Ethoxy and Ethyl Moieties on a Molecular Platform on Au(111)

Molecular rotors have attracted considerable interest for their prospects in nanotechnology. However, their adsorption on supporting substrates, where they may be addressed individually, usually modifies their properties. Here, we investigate the switching of two closely related three-state rotors mounted on platforms on Au(111) using low-temperature scanning tunneling microscopy and density functional theory calculations. Being physisorbed, the platforms retain important gas-phase properties of the rotor. This simplifies a detailed analysis and permits, for instance, the identification of the vibrational modes involved in the rotation process. The symmetry provided by the platform enables active control of the rotation direction through electrostatic interactions with the tip and charged neighboring adsorbates. The present investigation of two model systems may turn out useful for designing platforms that provide directional rotation and for transferring more sophisticated molecular machines from the gas phase to surfaces.

Realizing Efficient Single Organic Molecular White Light-Emitting Diodes from Conformational Isomerization of Quinazoline-Based Emitters

Single pure organic molecular white light emitters (SPOMWLEs) are of significance as a new class of material for white lighting applications; however, few of them are able to emit white electroluminescence from organic light-emitting diodes. Herein, donor-π-acceptor conjugated emitters, 2PQ-PTZ and 4PQ-PTZ, were designed and synthesized as SPOMWLEs for white light emission considering the distinct advantages of their conformation isomers. The coexistence of conformational isomers in 2PQ-PTZ, which is the first experimental evidence of the coexisting quasi-axial and quasi-equatorial conformers, provides ideal flexibility to obtain white light emission from their simultaneous and well-separated fluorescence and thermally activated delayed fluorescence. With these remarkable properties, a 2PQ-PTZ-based white light-emitting diode (LED) with a CIE of (0.32, 0.34) and color rendering index (CRI) of 89 is demonstrated. Further, the white organic light-emitting diode (OLED) of 2PQ-PTZ exhibits a high external quantum efficiency (EQE) of 10.1%, which is the reported highest performance among SPOMWLE-based OLEDs.

Keto-enol tautomerization drives the self-assembly of leucoquinizarin on Au(111)

The self-assembly of leucoquinizarin molecules on Au(111) surfaces is shown to be characterized by the molecules mostly being in their keto-enolic tautomeric form, with evidence of their temporary switching to other tautomeric forms. This reveals a metastable chemistry of the assembled molecules, to be considered for their possible employment in the formation of more complex hetero-organic interfaces.

Tip-Induced Inversion of the Chirality of a Molecule's Adsorption Potential Probed by the Switching Directionality

The switching behavior of surface-supported molecular units excited by current, light, or mechanical forces is determined by the shape of the adsorption potential. The ability to tailor the energy landscape in which a molecule resides at a surface gives the possibility of imposing a desired response, which is of paramount importance for the realization of molecular electronic units. Here, by means of scanning tunneling microscopy, a triazatruxene (TAT) molecule on Ag(111) is studied, which shows a switching behavior characterized by transitions of the molecule between three states, and which is attributed to three energetically degenerate bonding configurations. Upon tunneling current injection, the system can be excited and continuously driven, showing a switching directionality close to 100%. Two surface enantiomers of TAT show opposite switching directions pointing at the chirality of the energy landscape of the adsorption potential as a key ingredient for directional switching. Further, it is shown that by tuning the tunneling parameters, the symmetry of the adsorption potential can be controllably adjusted, leading to a suppression of the directionality or an inversion of the switching direction. The findings represent a molecule-surface model system exhibiting unprecedented control of the shape of its adsorption potential.

Single-molecule tautomerization tracking through space- and time-resolved fluorescence spectroscopy

Tautomerization, the interconversion between two constitutional molecular isomers, is ubiquitous in nature¹, plays a major role in chemistry² and is perceived as an ideal switch function for emerging molecular-scale devices³. Within free-base porphyrin⁴, porphycene⁵ or phthalocyanine⁶, this process involves the concerted or sequential hopping of the two inner hydrogen atoms between equivalent nitrogen sites of the molecular cavity. Electronic and vibronic changes⁶ that result from this NH tautomerization, as well as details of the switching mechanism, were extensively studied with optical spectroscopies, even with single-molecule sensitivity⁷. The influence of atomic-scale variations of the molecular environment and submolecular spatial resolution of the tautomerization could only be investigated using scanning probe microscopes^3,8-11, at the expense of detailed information provided by optical spectroscopies. Here, we combine these two approaches, scanning tunnelling microscopy (STM) and fluorescence spectroscopy^12-15, to study the tautomerization within individual free-base phthalocyanine (H₂Pc) molecules deposited on a NaCl-covered Ag(111) single-crystal surface. STM-induced fluorescence (STM-F) spectra exhibit duplicate features that we assign to the emission of the two molecular tautomers. We support this interpretation by comparing hyper-resolved fluorescence maps^15-18(HRFMs) of the different spectral contributions with simulations that account for the interaction between molecular excitons and picocavity plasmons¹⁹. We identify the orientation of the molecular optical dipoles, determine the vibronic fingerprint of the tautomers and probe the influence of minute changes in their atomic-scale environment. Time-correlated fluorescence measurements allow us to monitor the tautomerization events and to associate the proton dynamics to a switching two-level system. Finally, optical spectra acquired with the tip located at a nanometre-scale distance from the molecule show that the tautomerization reaction occurs even when the tunnelling current does not pass through the molecule. Together with other observations, this remote excitation indicates that the excited state of the molecule is involved in the tautomerization reaction path.

Direct Visualization of Hydrogen-Transfer Intermediate States by Scanning Tunneling Microscopy

Hydrogen atoms bonded within molecular cavities often undergo tunneling or thermal-transfer processes that play major roles in diverse physical phenomena. Such transfers may or may not entail intermediate states. The existence of such fleeting states is typically determined by indirect means, while their direct visualization has not been achieved, largely because their concentrations under equilibrium conditions are negligible. Here we use density-functional-theory calculations and scanning-tunneling-microscopy (STM) image simulations to predict that, under specially designed nonequilibrium conditions of voltage-enhanced high transfer rates, the cis-intermediate of the two-hydrogen transfer process in metal-free naphthalocyanine molecules adsorbed on Ag(111) surfaces would be visualizable in a composite image of double-C morphology. As guided by the theoretical predictions, at adjusted scanning temperature and bias, STM experiments achieve a direct visualization of the cis-intermediate. This work demonstrates a practical way to directly visualize elusive intermediates, which enhances understanding of the quantum dynamics of hydrogen atoms.