Correlation of Kondo effect and molecular conformation of the acceptor molecule in the TTF-TCNE charge transfer complex

A Kondo resonance has been observed on purely organic molecules in several combinations of charge transfer complexes on a metal surface. It has been regarded as a fingerprint of the transfer of one electron from the donor to the extended π orbital of the acceptor's LUMO. Here, we investigate the stoichiometric checkerboard structure of tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE) on a Au(1 1 1) surface using scanning tunneling and atomic force microscopy at 4.8 K. We find a bistable state of the TCNE molecules with distinct structural and electronic properties. The two states represent different conformations of the TCNE within the structure. One of them exhibits a Kondo resonance, whereas the other one does not, despite of both TCNE types being singly charged

Quantum tunneling in real space: Tautomerization of single porphycene molecules on the (111) surface of Cu, Ag, and Au

Tautomerization in single porphycene molecules is investigated on Cu(111), Ag(111), and Au(111) surfaces by a combination of low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations. It is revealed that the trans configuration is the thermodynamically stable form of porphycene on Cu(111) and Ag(111), whereas the cis configuration occurs as a meta-stable form. The trans → cis or cis → trans conversion on Cu(111) can be induced in an unidirectional fashion by injecting tunneling electrons from the STM tip or heating the surface, respectively. We find that the cis → cis tautomerization on Cu(111) occurs spontaneously via tunneling, verified by the negligible temperature dependence of the tautomerization rate below ∼23 K. Van der Waals corrected DFT calculations are used to characterize the adsorption structures of porphycene and to map the potential energy surface of the tautomerization on Cu(111). The calculated barriers are too high to be thermally overcome at cryogenic temperatures used in the experiment and zero-point energy corrections do not change this picture, leaving tunneling as the most likely mechanism. On Ag(111), the reversible trans → cis conversion occurs spontaneously at 5 K and the cis → cis tautomerization rate is much higher than on Cu(111), indicating a significantly smaller tautomerization barrier on Ag(111) due to the weaker interaction between porphycene and the surface compared to Cu(111). Additionally, the STM experiments and DFT calculations reveal that tautomerization on Cu(111) and Ag(111) occurs with migration of porphycene along the surface; thus, the translational motion couples with the tautomerization coordinate. On the other hand, the trans and cis configurations are not discernible in the STM image and no tautomerization is observed for porphycene on Au(111). The weak interaction of porphycene with Au(111) is closest to the gas-phase limit and therefore the absence of trans and cis configurations in the STM images is explained either by the rapid tautomerization rate or the similar character of the molecular frontier orbitals of the trans and cis configurations

Tautomerisierung von Porphycen Untersucht mit Rastersondenmikroskopie

In this thesis, single-molecule tautomerization on metal surfaces are studied. Tautomerization is a structural conversion between two or more isomeric forms of a molecule. Tautomerization is related to many important chemical and biological processes and combined studies of spectroscopic experiments with quantum chemical calculations have provided detailed insights into the correlation between a molecular structure and tautomerization dynamics for an isolated molecule, but the study on single-molecule tautomerization in condensed phase where the local environment could have a crucial impact has been scarce so far. In this research low-temperature scanning probe microscopy is used to investigate the tautomerization of single porphycene molecules on copper surfaces. Porphycene is a structural isomer of porphyrin with similar chemical properties but relatively strong hydrogen bonds in the inner cavity, which makes this molecule an intriguing model for studying hydrogen bond dynamics. Using scanning tunneling microscopy (STM) and atomic force microscopy (AFM) the thermally-, electron-, and force-induced tautomerization of porphycene on copper surfaces were investigated and the mechanisms of the tautomerization processes were elucidated at the single-molecule level. On the Cu(111) surface the hot carrier-induced trans to cis tautomerization and the thermally induced backward reaction were investigated with an STM. The hot carrier-induced process was observed not only in molecules underneath the STM tip, but also in molecules in a distance of up to 100 nm away from the tip. This nonlocal reaction is rationalized by traveling hot carriers along the surface and a more efficient transportation occurs for hot electrons traveling through the surface state than for hot holes. Additionally, a coupling between the surface state and the molecular adsorption was found to lead to a characteristic coverage dependence of the nonlocal tautomerization efficiency. On the Cu(110) surface the force-induced cis to cis tautomerization was investigated with AFM. The reaction was induced merely by bringing the tip closer to a molecule, suggesting that the interaction between the tip apex and the molecule triggers the process. Force spectroscopy revealed the force needed to induce the tautomerization and quantified a fraction of nano-Newton. Density functional theory simulations revealed that the tip proximity distorts the potential landscape along the tautomerization coordinate and significantly reduces the activation barrier. It was found that the tautomerization could not be induced by a Xe terminated tip, demonstrating the importance of the chemical nature of the tip to trigger the reaction via the distortion of the potential landscape.In dieser Arbeit wird die Tautomerisierung einzelner Moleküle auf Metalloberflächen untersucht. Tautomerisierung ist eine strukturelle Anpassung zwischen zwei oder mehr Isomeren eines Moleküls und ist in vielen wichtigen chemischen und biologischen Prozessen involviert. Spektroskopische Experimente und quantenchemische Rechnungen lieferten einen detaillierten Einblick in die Zusammenhänge zwischen der Molekülstruktur und der Dynamik der Tautomerisierung eines isolierten Moleküls. Bisher gibt es jedoch nur wenige Untersuchungen der Tautomerisierung von einzelnen Molekülen in der kondensierten Phase, wobei hier die lokale Umgebung einen entscheidenden Einfluss haben kann. In dieser Arbeit wird Tieftemperatur- Rastersondenmikroskopie angewendet, um die Tautomerisierung von einzelnen Porphycenmolekülen auf Kupferoberflächen zu erforschen. Porphycen ist ein Isomer von Porphyrin mit ähnlichen chemischen Eigenschaften, aber relativ starken Wasserstoffbrückenbindungen im Inneren des Moleküls, die Porphycen zu einem interessanten Modell für die Dynamik von Wasserstoffbrückenbindungen machen. Die temperatur-, elektronen- und kraftinduzierte Tautomerisierung sowie die zugrundeliegenden Mechanismen wurden mit Rastertunnelmikroskopie (STM) und Rasterkraftmikroskopie (AFM) untersucht. Die trans zu cis Tautomerisierung durch heiße Ladungsträger und die thermisch induzierte Rückreaktion wurden auf der Cu(111) Oberfläche mit STM erforscht. Der durch heiße Ladungsträger induzierte Prozess wurde nicht nur unter der STM Spitze beobachtet, sondern auch in Molekülen bis zu 100 nm weit entfernt. Diese nichtlokale Reaktion kann durch die Fortbewegung heißer Ladungsträger entlang der Oberfläche erklärt werden. Elektronen bewegen sich dabei effizienter fort als Löcher, indem sie den Cu(111)-Oberflächenzustand nutzen. Zusätzlich wurde eine Wechselwirkung zwischen dem Oberflächenzustand und der Adsorption der Moleküle nachgewiesen, die zu einer Abhängigkeit der nichtlokalen Tautomerisierungseffizienz von der Molekülbedeckung führt. Die kraftinduzierte cis zu cis Tautomerisierung wurde mit AFM auf Cu(110) untersucht. Die Reaktion wurde ausschließlich von der Annäherung der Spitze hervorgerufen, was die Wechselwirkung zwischen Spitze und Molekül als Auslöser nahelegt. Kraftspektroskopie ermittelte die zum Auslösen der Reaktion benötigte Kraft auf einen Bruchteil von Nanonewton. Dichtefunktionaltheorie-Simulationen zeigten, dass die Spitze die Potentiallandschaft entlang der Tautomerisierungskoordinate deformiert und die Aktivierungsbarriere erheblich reduziert. Die Tautomerisierung kann nicht durch ein Xenonatom an der Spitze ausgelöst werden, wodurch die Bedeutung der chemischen Reaktivität der Spitze demonstriert wird

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

Here, we report the study of tautomerization within a single porphycene molecule adsorbed on a Cu(111) surface using low-temperature scanning tunneling microscopy (STM) at 5 K. While molecules are adsorbed on the surface exclusively in the thermodynamically stable <i>trans</i> tautomer after deposition, a voltage pulse from the STM can induce the unidirectional <i>trans</i> → <i>cis</i> and reversible <i>cis</i> ↔ <i>cis</i> tautomerization. From the voltage and current dependence of the tautomerization yield (rate), it is revealed that the process is induced by vibrational excitation <i>via</i> inelastic electron tunneling. However, the metastable <i>cis</i> molecules are thermally switched back to the <i>trans</i> tautomer by heating the surface up to 30 K. Furthermore, we have found that the unidirectional tautomerization can be remotely controlled at a distance from the STM tip. By analyzing the nonlocal process in dependence on various experimental parameters, a hot carrier-mediated mechanism is identified, in which hot electrons (holes) generated by the STM travel along the surface and induce the tautomerization through inelastic scattering with a molecule. The bias voltage and coverage dependent rate of the nonlocal tautomerization clearly show a significant contribution of the Cu(111) surface state to the hot carrier-induced process

Force-induced tautomerization in a single molecule

Heat transfer, electrical potential and light energy are common ways to activate chemical reactions. Applied force is another way, but dedicated studies for such a mechanical activation are limited, and this activation is poorly understood at the single-molecule level. Here, we report force-induced tautomerization in a single porphycene molecule on a Cu(110) surface at 5 K, which is studied by scanning probe microscopy and density functional theory calculations. Force spectroscopy quantifies the force needed to trigger tautomerization with submolecular spatial resolution. The calculations show how the reaction pathway and barrier of tautomerization are modified in the presence of a copper tip and reveal the atomistic origin of the process. Moreover, we demonstrate that a chemically inert tip whose apex is terminated by a xenon atom cannot induce the reaction because of a weak interaction with porphycene and a strong relaxation of xenon on the tip as contact to the molecule is formed