Untersuchung von Strukturen auf Metalloberflächen gebildet durch die Adsorption organischer Moleküle mittels Dichtefunktionalmethoden

This thesis gives a short summary of the most important aspects of my work during the last years, which was mainly about density-functional theory (DFT) calculations of organic molecules adsorbed on metal substrates. A special focus of this work lies on the formed structures at the interface of the molecules with the metal surface. For this reason, molecule substrate as well as intermolecular interactions play a special role. At the beginning, a short historical introduction into surface science, catalysis, the role of DFT calculations, and the importance of interfaces between organic molecules and metal substrates is given. After introducing the formal basics of DFT and the possibilities to investigate interfaces with it, a short summary of the obtained results is given. First, a molecular solar thermal (MOST) system is considered that utilises the isomer pair norbornadiene (NBD) and quadricyclane (QC). Here, both molecules were investigated in regard of their adsorption on the catalytic Ni(111) surface. Thus, more insights into the energy release reaction taking place on this heterogenous catalyst could be acquired. The structures, namely locally bound quasi-isolated molecules, were found in two different geometries, influencing the reactivity. After the pristine molecules, also a modified NBD/QC system was investigated to obtain a better understanding of the influence of substituents red-shifting the molecules’ absorption towards the maximum of the sunlight spectrum. Following this, the results of self-assembled structures on Ag(111) and Au(111) surfaces are discussed. These monolayer structures are formed after the adsorption of 2,4,6-triethynyl-1,3,5-triazine (TET) and 1,3,5-triethynylbenzene (TEB) molecules. Different structures could be observed depending on different intermolecular interactions. Because of the possibility to use surface catalysed reactions to fuse the formed non-covalent structures to a graphyne network this system is of special interest. Afterwards, the results regarding the adsorption of trimesic acid (TMA) and halogen atoms in confined space, more precisely the pores of an organic network, are reported. This system was investigated because of the accompanying change in electronic properties interesting for possible future applications as a sensory or filter device. In the last section, a look at the complex system of intermolecular interactions at the interface of an ionic liquid (IL) on a catalytic transition metal surface is taken. As part of the solid catalysts with ionic liquid layers (SCILL) concept, the insight in the structure at this interface are important to tune catalytic activity and selectivity.Diese Arbeit soll die wichtigsten Aspekte meiner Ergebnisse aus den letzten Jahren zusammenfassen. Basiernd auf Dichtefunktionaltheorie (DFT) wurden Rechnungen für organische Moleküle, die auf metallischen Substraten adsorbiert sind, durchgeführt. Ein besonderes Augenmerk liegt dabei auf der Bildung von Strukturen an der Grenzfläche zwischen den Molekülen und der Metalloberfläche. Aus diesem Grund kommt der Beschreibung von Wechselwirkungen zwischen den Molekülen mit der Oberfläche, sowie den Molekülen untereinander eine besondere Rolle zu. Zunächst gebe ich eine kurze geschichtliche Einführung über die wissenschaftliche Untersuchung von Oberflächen und Katalyse, gehe etwas genauer auf die Bedeutung von DFT-Rechnungen ein und erläutere die besondere Rolle von Grenzflächen zwischen Metallen und organischen Molekülen. Anschließend berichte ich über die theoretischen Grundlagen und darüber hinausgehende Methoden zur Analyse und den Vergleich von experimentellen Daten mit DFT-Rechnungen. Danach fasse ich die Ergebnisse meiner wissenschaftlichen Arbeit kurz zusammen. Zunächst wird ein molekulares System zur Aufnahme, Speicherung und thermischen Freisetzung von Energie untersucht, das Isomerenpaar Norbornadien (NBD) und Quadricyclan (QC). Dazu werden beide Moleküle in Bezug auf die Adsorption auf einer katalytisch aktiven Ni(111)-Oberfläche analysiert, um Erkenntnisse über die Reaktion des Systems bei der Freisetzung von Energie, die auf dem heterogenen Katalysator stattfindet, zu gewinnen. Es können zwei Strukturen der lokal gebundenen, quasinicht- wechselwirkenden Moleküle ausgemacht werden. Dabei kann festgestellt werden, dass die Geometrien die Reaktivität des Systems beeinflussen. Neben den ursprünglichen Molekülen wird auch ein modifiziertes System mit substituierten Molekülen betrachtet. Durch die Substitution wird das Aborptionsspektrum rot verschoben, was eine bessere Überlagerung mit dem Spektrum des natürlichen Sonnenlichts ermöglicht. Als nächstes werden die Ergebnisse zu einer self-assembly Struktur, das heißt einer sich von selbst anordnenden Struktur, auf einer Au(111)- beziehungsweise auf einer Ag(111)-Oberfläche vorgestellt. Die dabei beobachteten, einlagigen Strukturen bilden sich nach der Adsorption der Moleküle 2,4,6-Triethynyl-1,3,5-triazin (TET) und 1,3,5-Triethynylbenzol (TEB) von selbst aus. Hier konnten, abhängig von den intermolekularen Wechselwirkungen der untersuchten Moleküle, unterschiedliche Strukturen beobachtet werden. Aufgrund der Möglichkeit, mithilfe der katalytischen Eigenschaften der Oberfläche, die lose miteinander gebundenen Moleküle zu einem Graphyn- Netzwerk zu verbinden, ist dieses System Gegenstand besonderen Intresses. Anschließend geht es um die Adsorption von Trimesinsäure (TMA) und Halogenatomen, innerhalb einer Pore eines organischen Netzwerks, die nur beschränkten Platz bietet. Bei diesem System spielt der Einfluss der Adsorbate auf die elektronische Struktur des Netzwerks eine besonderer Rolle, wodurch das Netzwerk unter Umständen als Sensor oder Filter Anwendung finden könnte. Das letzte Kapitel umfasst die Beschreibung des komplexen Zusammenspiels von Wechselwirkungen innerhalb der Grenzfläche zwischen einer ionischen Flüssigkeit und eines Übergangsmetallkatalysators. Als Teil des SCILL-Konzeptes, bei dem es um einen festen Katalysator, welcher mit einer ionischen Flüssigkeit beschichtet ist, geht, findet diese Grenzfläche besondere Beachtung. Wie sich zeigt, hat die Grenzflächenstruktur dabei einen entscheidenden Einfluss auf die Kontrolle der katalytischen Aktivität und die Selektivität

A Study on Free-floating Carsharing in Europe : Impacts of car2go and DriveNow on modal shift, vehicle ownership, vehicle kilometers traveled, and CO2_{2} emissions in 11 European cities

Free-floating carsharing, i.e., carsharing that allows pick-up and return of a car anywhere within a specified area in a city, has now been available in European cities for more than 10 years. As an important example of the sharing economy, carsharing strives for a more efficient use of resources with positive economic, social, and environmental impacts. After a decade of operation and user experience, an evaluation seems appropriate. car2go and DriveNow, who merged into SHARE NOW in 2019, are the largest carsharing operators in the world. They commissioned this study to identify the impact of carsharing on vehicle holdings, modal shift, vehicle kilometers traveled (VKT), and greenhouse gas emissions. The study was conducted in 2018 and 2019. It is based on a survey among car2go and DriveNow customers in 11 European cities. A previous study was performed by the University of California, Berkeley, for 5 North American cities in 2016 [7]. [...

Does free-floating carsharing reduce private vehicle ownership? The case of SHARE NOW in European cities

During the last decade, the use of free-floating carsharing systems has grown rapidly in urban areas. However, little is known on the effects free-floating carsharing offerings have on car ownership in general. Also the main drivers why free-floating users sell their cars are still rarely analysed. To shed some light on these issues, we carried out an online survey among free-floating carsharing users in 11 European cities and based our analysis on a sample of more than 10,000 survey participants. Our results show that one carsharing car replaces several private cars – in optimistic scenarios up to 20 cars. In Copenhagen (followed by Rome, Hamburg, and London) one carsharing car replaces about two times more private cars than in Madrid, the city with the lowest number. The main non-city specific influencing factor of shedding a private car due to the availability of the free-floating carsharing services seems to be the usage frequency of the service. The more kilometres users drive with these cars, the more likely it becomes that they sell a private car (or they sell their car and, therefore, use this service more often). Further memberships of bikesharing and other carsharing services, users that live in larger buildings as well as users that own several cars are more likely to reduce their number of cars, too. Finally, our findings are highly valuable for carsharing operators and (transport) policy makers when introducing free-floating carsharing systems in further cities. According to our results, all 11 cities show a reduced private car fleet due to members’ access to free-floating carsharing

Transcriptional Regulation of Liver-Type OATP1B3 (Lt-OATP1B3) and Cancer-Type OATP1B3 (Ct-OATP1B3) Studied in Hepatocyte-Derived and Colon Cancer-Derived Cell Lines

Due to alternative splicing, the SLCO1B3 gene encodes two protein variants; the hepatic uptake transporter liver-type OATP1B3 (Lt-OATP1B3) and the cancer-type OATP1B3 (Ct-OATP1B3) expressed in several cancerous tissues. There is limited information about the cell type-specific transcriptional regulation of both variants and about transcription factors regulating this differential expression. Therefore, we cloned DNA fragments from the promoter regions of the Lt-SLCO1B3 and the Ct-SLCO1B3 gene and investigated their luciferase activity in hepatocellular and colorectal cancer cell lines. Both promoters showed differences in their luciferase activity depending on the used cell lines. We identified the first 100 bp upstream of the transcriptional start site as the core promoter region of the Ct-SLCO1B3 gene. In silico predicted binding sites for the transcription factors ZKSCAN3, SOX9 and HNF1α localized within these fragments were further analyzed. The mutagenesis of the ZKSCAN3 binding site reduced the luciferase activity of the Ct-SLCO1B3 reporter gene construct in the colorectal cancer cell lines DLD1 and T84 to 29.9% and 14.3%, respectively. In contrast, using the liver-derived Hep3B cells, 71.6% residual activity could be measured. This indicates that the transcription factors ZKSCAN3 and SOX9 are important for the cell type-specific transcriptional regulation of the Ct-SLCO1B3 gene

Surface Chemistry of the Molecular Solar Thermal Energy Storage System 2,3-Dicyano-Norbornadiene/Quadricyclane on Ni(111)

Molecular solar thermal (MOST) systems are a promising approach for the introduction of sustainable energy storage solutions. We investigated the feasibility of the dicyano-substituted norbornadiene/quadricyclane molecule pair on Ni(111) for catalytic model studies. This derivatization is known to lead to a desired bathochromic shift of the absorption maximum of the parent compound. In our experiments further favorable properties were found: At low temperatures, both molecules adsorb intact without any dissociation. In situ temperature-programmed HR-XPS experiments reveal the conversion of (CN)2-quadricyclane to (CN)2-norbornadiene under energy release between 175 and 260 K. The absence of other surface species due to side reactions indicates full isomerization. Further heating leads to the decomposition of the molecular framework into smaller carbonaceous fragments above 290 K and finally to amorphous structures, carbide and nitride above 400 K. DFT calculations gave insights into the adsorption geometries. (CN)2-norbornadiene is expected to interact stronger with the surface, with flat configurations being favorable. (CN)2-quadricyclane exhibits smaller adsorption energies with negligible differences for flat and side-on geometries. Simulated XP spectra are in good agreement with experimental findings further supporting the specific spectroscopic fingerprints for both valence isomers

Au-Catalyzed Energy Release in a Molecular Solar Thermal (MOST) System: A Combined Liquid-Phase and Surface Science Study

Molecular solar thermal systems (MOSTs) are molecular systems based on couples of photoisomers (photoswitches), which combine solar energy conversion, storage, and release. In this work, we address the catalytically triggered energy release in the promising MOST couple phenylethylesternorbornadiene/quadricyclane (PENBD/PEQC) on a Au(111) surface in a combined liquid-phase and surface science study. We investigated the system by photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS) in the liquid phase, conventional IRRAS and synchrotron radiation photoelectron spectroscopy (SRPES) in ultra-high vacuum (UHV). Au(111) is highly active towards catalytically triggered energy release. In the liquid phase, we did not observe any decomposition of the photoswitch, no deactivation of the catalyst within 100 conversion cycles and we could tune the energy release rate of the heterogeneously catalyzed process by applying an external potential. In UHV, submonolayers of PEQC on Au(111) are back-converted to PENBD instantaneously, even at 110 K. Multilayers of PEQC are stable up to ~220 K. Above this temperature, the intrinsic mobility of the film is high enough that PEQC molecules come into direct contact with the Au(111) surface, which catalyzes the back-conversion. We suggest that this process occurs via a singlet–triplet mechanism induced by electronic coupling between the PEQC molecules and the Au(111) surface

Norbornadiene photoswitches anchored to well-defined oxide surfaces: From ultrahigh vacuum into the liquid and the electrochemical environment

Employing molecular photoswitches, we can combine solar energy conversion, storage, and release in an extremely simple single molecule system. In order to release the stored energy as electricity, the photoswitch has to interact with a semiconducting electrode surface. In this work, we explore a solar-energy-storing model system, consisting of a molecular photoswitch anchored to an atomically defined oxide surface in a liquid electrolyte and under potential control. Previously, this model system has been proven to be operational under ultrahigh vacuum (UHV) conditions. We used the tailor-made norbornadiene derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD) and characterized its photochemical and electrochemical properties in an organic electrolyte. Next, we assembled a monolayer of CNBD on a well-ordered Co3O4(111) surface by physical vapor deposition in UHV. This model interface was then transferred into the liquid electrolyte and investigated by photoelectrochemical infrared reflection absorption spectroscopy experiments. We demonstrate that the anchored monolayer of CNBD can be converted photochemically to its energy-rich counterpart 2-cyano-3-(4-carboxyphenyl)quadricyclane (CQC) under potential control. However, the reconversion potential of anchored CQC overlaps with the oxidation and decomposition potential of CNBD, which limits the electrochemically triggered reconversion

Electrocatalytic Energy Release of Norbornadiene‐Based Molecular Solar Thermal Systems: Tuning the Electrochemical Stability by Molecular Design

Abstract Molecular solar thermal (MOST) systems, such as the norbornadiene/quadricyclane (NBD/QC) couple, combine solar energy conversion, storage, and release in a simple one‐photon one‐molecule process. Triggering the energy release electrochemically enables high control of the process, high selectivity, and reversibility. In this work, the influence of the molecular design of the MOST couple on the electrochemically triggered back‐conversion reaction was addressed for the first time. The MOST systems phenyl‐ethyl ester‐NBD/QC (NBD1/QC1) and p‐methoxyphenyl‐ethyl ester‐NBD/QC (NBD2/QC2) were investigated by in‐situ photoelectrochemical infrared spectroscopy, voltammetry, and density functional theory modelling. For QC1, partial decomposition (40 %) was observed upon back‐conversion and along with a voltammetric peak at 0.6 Vfc, which was assigned primarily to decomposition. The back‐conversion of QC2, however, occurred without detectable side products, and the corresponding peak at 0.45 Vfc was weaker by a factor of 10. It was concluded that the electrochemical stability of a NBD/QC couple is easy tunable by simple structural changes. Furthermore, the charge input and, therefore, the current for the electrochemically triggered energy release is very low, which ensures a high overall efficiency of the MOST system

Selektivitätskontrolle in elektrokatalytischen Oxidationsreaktionen durch Ionische Flüssigkeiten

Der so genannte SCILL-Katalysator (englisch: solid catalyst with ionic liquid layer) beschreibt ein neues, äußerst erfolgreiches Konzept im Bereich der heterogenen Katalyse. Hierbei besteht die Grundidee darin, die Selektivität eines Katalysators durch die Beladung mit ionischen Flüssigkeiten drastisch zu erhöhen. In dieser Arbeit zeigen wir, dass das Konzept auf die Elektrokatalyse übertragbar ist und zur selektiven Umsetzung von organischen Verbindungen genutzt werden kann. Bei der hier untersuchten Elektrooxidation von 2,3-Butandiol können zwei Produkte entstehen. Das einfach oxidierte Acetoin und das zweifach oxidierte Diacetyl. Durch die Zugabe einer ionischen Flüssigkeit (1-Ethyl-3-methyl-imidazolium-trifluormethansulfonat, [C2C1Im][OTf]) kann die Selektivität des Katalysators zu Gunsten der Acetoinbildung drastisch erhöht werden. Der zugrundeliegende Mechanismus wurde dabei spektroskopisch in situ untersucht: Die Adsorption des Anions der ionischen Flüssigkeit verhindert die Wasseraktivierung. Dies unterbindet den zweiten Oxidationsschritt vom Acetoin zum Diacetyl und erhöht damit die Selektivität. Unsere Studie zeigt das große Potential elektrochemischer SCILL-Katalysatoren für die selektive Umsetzung von organischen Verbindungen

Solar energy storage at an atomically defined organic-oxide hybrid interface

Molecular photoswitches provide an extremely simple solution for solar energy conversion and storage. To convert stored energy to electricity, however, the photoswitch has to be coupled to a semiconducting electrode. In this work, we report on the assembly of an operational solar-energy-storing organic-oxide hybrid interface, which consists of a tailor-made molecular photoswitch and an atomically-defined semiconducting oxide film. The synthesized norbornadiene derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD) was anchored to a well-ordered Co3O4(111) surface by physical vapor deposition in ultrahigh vacuum. Using a photochemical infrared reflection absorption spectroscopy experiment, we demonstrate that the anchored CNBD monolayer remains operational, i.e., can be photo-converted to its energy-rich counterpart 2-cyano-3-(4-carboxyphenyl)quadricyclane (CQC). We show that the activation barrier for energy release remains unaffected by the anchoring reaction and the anchored photoswitch can be charged and discharged with high reversibility. Our atomically-defined solar-energy-storing model interface enables detailed studies of energy conversion processes at organic/oxide hybrid interfaces