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

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

Synthesis and Characterization of Bola‐Amphiphilic Porphyrin‐Perylenebisimide Architectures

We report on the synthesis and characterization of a family of three water‐soluble bola‐amphiphilic zinc‐porphyrin‐perylenebisimide triads containing oligo carboxylic‐acid capped Newkome dendrons in the periphery. Variations of the perylenebisimide (PBI) core geometry and dendron size (G1 and G2 dendrons with 3‐ and 9‐carboxylic acid groups respectively) allow for tuning the supramolecular aggregation behavior with respect to variation of the molecular architecture. The triads show good solubility in basic aqueous media and aggregation to supramolecular assemblies. Theoretical investigations at the DFT level of theory accompanied by electrochemical measurements unravel the geometric and electronic structure of the amphiphiles. UV/Vis and fluorescence titrations with varying amounts of THF demonstrate disaggregation.We report the synthetic approach and the characterization of a family of highly water‐soluble porphyrin‐PBI donor‐acceptor amphiphiles by the utilization of oligo‐carboxylic acid capped Newkome dendrons. The amphiphiles form stable aggregates in basic aqueous solutions. The amphiphiles can be individualized by the addition of THF, which manifests itself, for example, in the reinstatement of fluorescence emission. image Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165

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

Electrochemically controlled energy release from a norbornadiene-based solar thermal fuel: increasing the reversibility to 99.8% using HOPG as the electrode material

Solar energy conversion using molecular photoswitches holds great potential to store energy from sunlight in the form of chemical energy in a process that can be easily implemented in a direct solar energy storage device. In this context, we investigated the electrochemically triggered energy release of a solar thermal fuel based on the norbornadiene (NBD)/quadricyclane (QC) couple by photoelectrochemical IR reflection absorption spectroscopy (PEC-IRRAS). We studied the photo-induced conversion of the energy-lean 2-cyano-3-(3,4-dimethoxyphenyl)-norbornadiene (NBD \u27) to the energy-rich 2-cyano-3-(3,4-dimethoxyphenyl)-quadricyclane (QC \u27) and the electrochemically triggered reconversion using highly oriented pyrolytic graphite (HOPG) as an electrode material. We compared our results with the results obtained previously using Pt(111) electrodes and we characterized the photochemical and electrochemical properties of the storage system. NBD \u27 can be photochemically converted and electrochemically reconverted with very high selectivity. HOPG largely suppresses the unwanted catalytic reconversion which was observed on Pt(111). We performed repetitive cycling experiments for 1000 cycles to determine the reversibility of the system. Our results show that it is possible to reach reversibility above 99.8% using HOPG as an electrode material

Tunable Energy Release in a Reversible Molecular Solar Thermal System br

Molecular solar thermal (MOST) systems open application fields for solar energy conversion as they combine conversion, storage, and release in one single molecule. For energy release, catalysts must be controllable, selective, and stable over many operation cycles. Here, we present a MOST/catalyst couple, which combines all these properties. We explore solar energy storage in a tailor-made MOST system (cyano-3-(3,4-dimethoxyphenyl)-norbornadiene/quadricyclane; NBD \u27/QC \u27) and the energy release heterogeneously catalyzed at a Au(111) surface. By photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS) and scanning tunneling microscopy, we show that Au triggers the energy release with very high activity. Most remarkably, the release rate of the heterogeneously catalyzed process can be tuned by applying an external potential. Our durability tests show that the MOST/catalyst system is stable over 1000 storage cycles without any decomposition. The surface structure of the catalyst is preserved, and its activity decreases by only 0.1% per storage cycle

Modifying the Electrocatalytic Selectivity of Oxidation Reactions with Ionic Liquids

Acidic supported ionic liquid phase catalysts were investigated for the synthesis of oxymethylene dimethyl ethers (OMEs). The application of OME1 and trioxane for the gas-phase synthesis of higher OMEs and in particular the generation of OMEn with n higher than 3 was successfully demonstrated. Raising the pressure led to an increase in the conversion of OME1 and to higher selectivity for higher molecular weight OMEs. Furthermore, a correlation between the ionic liquid’s acidity and the catalytic activity was shown with higher acidity leading to higher conversion of trioxane and OME1. Moreover, successful long-term operation for more than 200 h time on stream has been demonstrated with good catalyst system stability

Tunable Energy Release in a Reversible Molecular Solar Thermal System

Molecular solar thermal (MOST) systems open application fields for solar energy conversion as they combine conversion, storage, and release in one single molecule. For energy release, catalysts must be controllable, selective, and stable over many operation cycles. Here, we present a MOST/catalyst couple, which combines all these properties. We explore solar energy storage in a tailor-made MOST system (cyano-3-(3,4-dimethoxyphenyl)-norbornadiene/quadricyclane; NBD′/QC′) and the energy release heterogeneously catalyzed at a Au(111) surface. By photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS) and scanning tunneling microscopy, we show that Au triggers the energy release with very high activity. Most remarkably, the release rate of the heterogeneously catalyzed process can be tuned by applying an external potential. Our durability tests show that the MOST/catalyst system is stable over 1000 storage cycles without any decomposition. The surface structure of the catalyst is preserved, and its activity decreases by only 0.1% per storage cycle

Electrochemically Triggered Energy Release from an Azothiophene‐Based Molecular Solar Thermal System

Molecular solar thermal (MOST) systems combine solar energy conversion, storage, and release in simple one-photon one-molecule processes. Here, we address the electrochemically triggered energy release from an azothiophene-based MOST system by photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS) and density functional theory (DFT). Specifically, the electrochemically triggered back-reaction from the energy rich (Z)-3-cyanophenylazothiophene to its energy lean (E)-isomer using highly oriented pyrolytic graphite (HOPG) as the working electrode was studied. Theory predicts that two reaction channels are accessible, an oxidative one (hole-catalyzed) and a reductive one (electron-catalyzed). Experimentally it was found that the photo-isomer decomposes during hole-catalyzed energy release. Electrochemically triggered back-conversion was possible, however, through the electron-catalyzed reaction channel. The reaction rate could be tuned by the electrode potential within two orders of magnitude. It was shown that the MOST system withstands 100 conversion cycles without detectable decomposition of the photoswitch. After 100 cycles, the photochemical conversion was still quantitative and the electrochemically triggered back-reaction reached 94 % of the original conversion level