Efficient Cyclization of the Norbornadiene‐Quadricyclane Interconversion Mediated by a Magnetic [Fe 3 O 4 −CoSalphen] Nanoparticle Catalyst

Synthesis, Characterization and Interconversion of p-Tolylsulfone-Functionalized Norbornadiene/Quadricyclane Couples

We report the synthesis and characterization of library of new 2,3-disubstituted norbornadiene/quadricyclane couples. For the first time, the para-tolylsulfone moiety was employed as electron-withdrawing substituent in combination with a variety of different electron donors as counterparts. Comprehensive characterization was conducted for every interconversion couple. By comparison with structurally related molecules published before we established the tosyl moiety as suitable alternative to previously investigated ester functionalities by providing similar photophysical properties. The photo-induced interconversion behavior was investigated via UV/Vis- and NMR-spectroscopy. The UV/Vis experiments were carried out exclusively in acetonitrile, whereas several solvents were investigated in the NMR studies. A detailed description and comparison of the isomerization behavior is provided, while examining relevant optical properties like λmax and λonset. Thereby, an enhanced red-shift up to λmax=394 nm combined with an λonset value of 469 nm could be generated which is necessary for potential applications.

The Norbornadiene/Quadricyclane Pair as Molecular Solar Thermal Energy Storage System: Surface Science Investigations

For the transition to renewable energy sources, novel energy storage materials are more important than ever. This review addresses so-called molecular solar thermal (MOST) systems, which appear very promising since they combine light harvesting and energy storing in one-photon one-molecule processes. The focus is on norbornadiene (NBD), a particularly interesting candidate, which is converted to the strained valence isomer quadricyclane (QC) upon irradiation. The stored energy can be released on demand. The energy-releasing cycloreversion from QC to NBD can be initiated by a thermal, catalytic, or electrochemical trigger. The reversibility of the energy storage and release cycles determines the general practicality of a MOST system. In the search for derivatives, which enable large-scale applications, fundamental surface science studies help to assess the feasibility of potential substituted NBD/QC couples. We include investigations under well-defined ultra-high vacuum (UHV) conditions as well as experiments in liquid phase. Next to the influence of the catalytically active surfaces on the isomerization between the two valence isomers, information on adsorption geometries, thermal stability limits, and reaction pathways of the respective molecules are discussed. Moreover, laboratory-scaled test devices demonstrate the proof of concept in various areas of application.

Unveiling the Potential of Heterogeneous Catalysts for Molecular Solar Thermal Systems

Solar energy utilization has gained considerable attention due to its abundance and renewability. However, its intermittent nature presents a challenge in harnessing its full potential. The development of energy storing compounds capable of capturing and releasing solar energy on demand has emerged as a potential solution. These compounds undergo a photochemical transformation that results in a high-energy metastable photoisomer, which stores solar energy in the form of chemical bonds and can release it as heat when required. Such systems are referred to as MOlecular Solar Thermal (MOST)-systems. Although the photoisomerization of MOST systems has been vastly studied, its back-conversion, particularly using heterogeneous catalysts, is still underexplored and the development of effective catalysts for releasing stored energy is crucial. Herein we compare the performance of 27 heterogeneous catalysts releasing the stored energy in an efficient Norbornadiene/Quadricyclane (NBD/QC) MOST system. We report the first benchmarking of heterogeneous catalysts for a MOST system using a robust comparison method of the catalysts' activity and monitoring the conversion using UV-Visible (UV-Vis) spectroscopy. Our findings provide insights into the development of effective catalysts for MOST systems. We anticipate that our assay will reveal the necessity of further investigation on heterogeneous catalysis.

Driving the quadricyclane-to-norbornadiene isomerization by charge separation with perylenediimide as electron acceptor

Through comprehensive photo-assays, this study investigates the reaction coordinate governing the interconversion between quadricyclane (QC) and norbornadiene (NBD) upon photo-irradiation up to a wavelength of 550 nm. To harness this spectroscopic range for energy release, we link the NBD-core with a highly electron-accepting perylenediimide (PDI) with broad absorption, achieving strong electronic coupling between them. We detail the successful synthesis and present extensive DFT calculations to determine the amount of stored energy. By means of transient absorption spectroscopy, an oxidative electron transfer is observed during the QC-to-NBD isomerization following the initial PDI photoexcitation. This charge-separated state is key to triggering the back-isomerization with visible light excitation.

Surface science and liquid phase investigations of oxanorbornadiene/oxaquadricyclane ester derivatives as molecular solar thermal energy storage systems on Pt(111)

The transition to renewable energy sources comes along with the search for new energy storage solutions. Molecular solar thermal systems directly harvest and store solar energy in a chemical manner. By a suitable molecular design, a higher overall efficiency can be achieved. In this study, we investigate the surface chemistry of oxa-norbornadiene/quadricyclane derivatives on a Pt(111) surface. Specifically, we focus on the energy storage and release properties of molecules that are substituted with ester moieties of different sizes. For our model catalytic approach, synchrotron radiation-based x-ray photoelectron spectroscopy measurements were conducted in ultra-high vacuum (UHV) and correlated with the catalytic behavior in the liquid phase monitored by photochemical infrared reflection absorption spectroscopy. The differences in their spectral appearance enabled us to unambiguously differentiate the energy-lean and energy-rich isomers and decomposition products. Next to qualitative information on the adsorption motifs, temperature-programmed experiments allowed for the observation of thermally induced reactions and the deduction of the related reaction pathways. We analyzed the selectivity of the cycloreversion reaction from the energy-rich quadricyclane derivative to its energy-lean norbornadiene isomer and competing processes, such as desorption and decomposition. For the 2,3-bis(methylester)-substitution, the cycloreversion reaction was found to occur between 310 and 340 K, while the thermal stability limit of the compounds was determined to be 380 K. The larger 2,3-bis(benzylester) derivatives have a lower apparent adsorption energy and a decomposition onset already at 135 K. In the liquid phase (in acetonitrile), we determined the rate constants for the cycloreversion reaction on Pt(111) to k = 5.3 × 10-4 s-1 for the 2,3-bis(methylester)-substitution and k = 6.3 × 10-4 s-1 for the 2,3-bis(benzylester) derivative. The selectivities were of >99% and 98% for the two molecules, respectively. The difference in the catalytic behavior of Pt(111) for both derivatives is less pronounced in the liquid phase than in UHV, which we attribute to the passivation of the Pt(111) surface by carbonaceous species under ambient conditions.

Taking up the quest for novel molecular solar thermal systems: Pros and cons of storing energy with cubane and cubadiene

Molecular solar thermal (MOST) systems are working their way as a possible technology to store solar light and release it when necessary. Such systems could, in principle, constitute a solution to the energy storage problem characteristic of solar cells and are conceived, at a first instance, as simple molecular photoswitches. Nevertheless, the optimization of their different required properties is presently limiting their technological scale up. From the chemical perspective, we need to design a novel MOST system based on unconventional photoswitches. Here, by applying multi-configurational quantum chemistry methods, we unravel the potentialities of ad hoc-designed molecular photoswitches, which aim to photoproduce cubane or cubadiene as high-energy isomers that can be thermally (or eventually catalytically) reverted to the initial structure, releasing their stored energy. Specifically, while cubane can be photoproduced via different paths depending on the reactant tricycle diene conformation, an undesired bicyclic by-product limits its application to MOST systems. An evolution of this starting design toward cubadiene formation is therefore proposed, avoiding conformational equilibria and by-products, considerably red shifting the absorption to reach the visible portion of the solar spectrum and maintaining an estimated storage density that is expected to overcome the current MOST reference system (norbornadiene/quadricyclane), although consistently increasing the photoisomerization energy barrier.

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

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 V_fc , which was assigned primarily to decomposition. The back-conversion of QC2, however, occurred without detectable side products, and the corresponding peak at 0.45 V_fc 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.

Reversible Photoinduced Conversion of Unprecedented Norbornadiene-Based Photoswitches with Redox-Active Naphthalene Diimide Functionalities

An unprecedented compound class of functional organic hybrids consisting of a photoswitchable norbornadiene building block and a redoxactive chromophore, namely naphthalene diimide, were designed and synthesized. Within these structures the capability of rylene chromophores to function as a redox active catalyst upon their photoexcitation was utilized to initiate the oxidative back-conversion of the in situ formed quadricyclane unit to its norbornadiene analogue. In this way successive photoexcitation at two different wavelengths enabled a controlled photoswitching between the two isomerical states of the hybrids. Beyond this prove of concept, the dependency of the reaction rate to the intramolecular distance of the two functional molecular building blocks as well as the concentration of the photoexcited sample was monitored. The experimental findings and interpretations were furthermore supported by quantum chemical investigations.

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.

Metal-Catalyzed, Photo-Assisted Selective Transformation of Tertiary Alkylbenzenes and Polystyrenes into Carbonyl Compounds

Every year, thousands of tons of polystyrene are produced and discarded, filling landfills and polluting the marine environment. Although several degradation alternatives have been proposed, the need for an effective procedure for the chemical recycling of polystyrene still remains. Here, a vanadium-catalyzed reaction, assisted by visible light, promoted the direct, selective conversion of tertiary alkylbenzenes into acetophenone and other ketone derivatives. Likewise, standard polystyrene samples as well as polystyrenes from insulation and packaging waste could be chemically recycled into acetophenone in a scalable way regardless of their molecular weight, polydispersity, or form. Preliminary mechanistic investigations revealed the participation of singlet oxygen, superoxide, and hydroxyl radical species in this homogenously catalyzed process. Acetophenone could be used as an additive to accelerate the reaction and to increase the yields in some cases.

Optimization of the thermochemical properties of the norbornadiene/quadricyclane photochromic couple for solar energy storage using nanoparticles

In this paper, we present an investigation concerning the prospects of using nanoparticles to improve solar energy storage properties of three different norbornadiene/quadricyclane derivatives. Computationally, we study how different nanoparticles influence the properties of the systems that relate to the storage of solar energy, namely, the storage energy and the back reaction barrier. Our approach employs hybrid quantum mechanical/molecular mechanical calculations in which the molecular systems are described using density functional theory while the nanoparticles are described using molecular mechanics. The interactions between the two subsystems are determined using polarization dynamics. The results show that the influence of the nanoparticles on the thermochemical properties largely depends on the type of nanoparticle used, the relative orientation with respect to the nanoparticle, and the distance between the the nanoparticle and the molecular system. Additionally, we find indications that copper and/or titanium dioxide nanoparticles can lower the energy barrier of the back reaction for all of the studied systems without significantly lowering the storage capability of the systems. Consequently, the study shows that nanoparticles can potentially be employed in the optimization of molecular photoswitches towards solar energy storage.

The effects of solvation on the back reaction and storage capabilities of solar thermal energy storage systems

Solvent effects on molecular solar thermal energy storage systems have been investigated using density functional theory combined with solvent models describing the effects of viscosities and dielectric constants on chemical reaction rates. We have addressed the following issues concerning how solvents influence both the thermochemical properties and the thermal relaxation kinetics of the studied systems, how the friction of the solvent influences the recrossing of the reactions along with the dynamics and force constants of the transition state. We observe that the rate constants for the chemical reactions of the molecular solar thermal energy storage systems depend strongly on the dielectric solvent properties and the viscosities of the solvents.

Solar Energy Storage: Competition between Delocalized Charge Transfer and Localized Excited States in the Norbornadiene to Quadricyclane Photoisomerization

We describe for the first time the full reaction coordinate regarding the photoisomerization of red-absorbing norbornadienes (NBDs) to quadricyclanes (QCs). Our studies go beyond steady-state investigations by using an arsenal of time-resolved techniques. Importantly, the red absorption of NBDs is made possible by a different charge-transfer character; adjusting its strength enables control over the photoreversibility of the rearrangement. In the case of strong charge-transfer character (a weakly electron-withdrawing ester and a strongly electron-donating dimethylaniline), photoirradiation with visible light into the delocalized charge-transfer absorption of NBD affords QC reversibly. In stark contrast, UV photoirradiation into the NBD localized excited state leads to a photoinduced degradation and cannot be back-isomerized to NBD under any circumstances. If the charge-transfer character is weak (a weakly electron-withdrawing ester and a weakly electron-donating phenyl), reversibility is seen independently of the photoirradiation light.

Host-Guest Systems on the Surface of Functionalized Superparamagnetic Iron Oxide Nanoparticles (SPIONs) Utilizing Hamilton Receptors and Cyanurate Derivative Molecules

The study of hydrogen bonding interactions at the level of functionalized nanoparticles remains highly challenging and poorly explored area. In this work, superparamagnetic iron oxide nanoparticles (SPIONs) were orthogonally functionalized using receptors bearing multiple hydrogen bonding motifs. Pristine SPIONs were modified by wet chemical processes with Hamilton receptors (hosts), or cyanurate‐guest molecules linked to phosphonic acid moieties for monolayer functionalization. The modified surfaces were fully characterized and the number of attached ligands on the surface were determined. The host‐guest interactions on the interface of modified SPIONs were investigated by using UV‐Vis spectroscopic titrations. Functionalized SPIONs demonstrated two to three magnitudes stronger binding affinities as compared to the related molecular interactions in solution due to synergistic effects on complex surface environment. Higher supramolecular binding ratios of host‐guest interactions on the modified surface were emerged. These studies provide fundamental insights into supramolecular complexations on the surface at solid‐liquid interface systems with applications in engineered nanomaterials, nano‐sensing devices, and drug delivery systems.

Efficient Electrocatalytic Switching of Azoheteroarenes in the Condensed Phases

Azo-based photoswitches have shown promise as molecular solar-thermal (MOST) materials due to their ability to store energy in their metastable Z isomeric form. The energy is then released, in the form of heat, upon photoisomerization to the thermodynamically stable E form. However, obtaining a high energy density and recovering the stored energy with high efficiency requires the materials to be employed in the condensed phase and display a high degree of Z to E switching, both of which are challenging to engineer. Here, we show that arylazopyrazole motifs undergo efficient redox-induced Z to E switching in both the solution and the condensed phase to a higher completeness of switching than achieved photochemically. This redox-initiated pathway lowers the barrier of Z to E isomerization by 27 kJ/mol, while in the condensed phase, the efficiency of electrochemical switching is improved by over an order of magnitude relative to that in the solution state. The influence of the photoswitch's phase, electrical conductivity, and viscosity on the electrochemical switching in the condensed phase is reported, culminating in a set of design rules to facilitate further investigations. We anticipate the use of an alternative stimulus to light will facilitate the application of MOST materials in situations where phototriggered heat release is unachievable or inefficient, e.g., indoor or at night. Furthermore, exploiting the electrocatalytic mechanism, whereby a catalytic amount of charge triggers Z to E switching via a redox process, bypasses the need for fine tuning of the photoswitching chromophore to achieve complete Z to E switching, thus providing an alternative approach to photoswitch molecular design.

Tunable Photoswitching in Norbornadiene (NBD)/Quadricyclane (QC) - Fullerene Hybrids

With respect to molecular switches, initializing the quadricyclane (QC) to norbornadiene (NBD) back‐reaction by light is highly desirable. Our previous publication provided a unique solution for this purpose by utilizing covalently bound C₆₀. In this work, the fundamental processes within these hybrids has been investigated. Variation of the linker unit connecting the NBD/QC moiety with the fullerene core is used as a tool to tune the properties of the resulting hybrids. Utilizing the Prato reaction, two unprecedented NBD/QC – fullerene hybrids having a long‐rigid and a short‐rigid linker were synthesized. Molecular dynamics simulations revealed that this results in an average QC–C₆₀ distance of up to 14.2 Å. By comparing the NBD–QC switching of these derivatives with the already established one having a flexible linker, valuable mechanistic insights were gained. Most importantly, spatial convergence of the QC moiety and the fullerene core is inevitable for an efficient back‐reaction.

Promoting the thermal back reaction of vinylheptafulvene to dihydroazulene by physisorbtion on nanoparticles

We investigate the effects of nanoparticles on molecular solar thermal energy storage systems and how one can tune chemical reactivities of a molecular photo- and thermoswitch by changing the nanoparticles. We have selected the dihydroazulene/vinylheptafulvene system to illustrate the effects of the nanoparticles on the chemical reactivities of the molecular photo- and thermoswitch. We have utilized the following nanoparticles: a TiO₂ nanoparticle along with nanoparticles of gold, silver and copper. We calculate the rate constants for the release of the thermal energy utilizing a QM/MM method coupled to a transition state method. The molecular systems are described by density functional theory whereas the nanoparticles are given by molecular mechanics including electrostatic and polarization dynamics. In order to investigate whether the significant stabilization of the transitions state provided by the nanoparticles is general to the DHA/VHF system, we calculated the transition state rate constant of the parent- and 3-amino-substituted-DHA/VHF systems at 298.15 K in the four different orientations and at the three different separations. We observe that the transition state rate constant of the parent system is only increased as the cyano groups are oriented towards the nanoparticle while the presence of the nanoparticle actually impedes the reactions using the three other orientations. On the other hand, for the substituted system the nanoparticle generally leads to a significant increase in the rate of the reaction. We find that the nanoparticles can have a substantial effect on the calculated rate constants. We observe, depending on the nanoparticle and the molecular orientation, increases of the rate constants by a factor of 10⁶. This illustrates the prospects of utilizing nanoparticles for controlling the release of the stored thermal energy.

Non-Covalent Postfunctionalization of Dye Layers on TiO2 - A Tool for Enhancing Injection in Dye-Sensitized Solar Cells

We report on newly tailored dye layers, which were employed, on one hand, for covalent deposition and, on the other hand, for non-covalently post-functionalizing TiO2 nanoparticle films. Our functionalization concept enabled intermixing a stable covalent attachment of a first layer with a highly versatile and reversible hydrogen bonding through the Hamilton receptor-cyanuric acid binding motif as a second layer. Following this concept, we integrated step-by-step a first porphyrin layer and a second porphyrin/BODIPY layer. The individual building blocks and their corresponding combinations were probed with regard to their photophysical properties, and the most promising combinations were implemented in dye-sensitized solar cells (DSSCs). Relative to the first porphyrin layer adding the second porphyrin/BODIPY layers increased the overall DSSC efficiency by up to 43 %.

Molecular Solar Thermal Batteries through Combination of Magnetic Nanoparticle Catalysts and Tailored Norbornadiene Photoswitches

Cobalt catalysts are immobilized on the surface of iron oxide nanoparticles for the preparation of highly active quasi-homogeneous catalysts toward an efficient release of photochemically stored energy in norbornadiene-based photoswitches. The facile separation of the iron oxide nanoparticles through exploitation of the intrinsic magnetic properties of this material enables efficient cyclization of energy storage and release. Through the transition from cobalt (II) salphen to cobalt porphyrins, a 22.6-fold increase in the catalytic efficiency of the QC-NBD back-conversion is achieved, with an initial TOF of up to 3.64 s-1 and excellent TON of over 3305. In addition, a series of novel "push-pull" functionalized norbornadiene derivatives is prepared, featuring excellent absorption properties with maxima up to 366 nm, quantum yields around 70 %, high energy storage capacities of up to 98.0 kJ mol-1 , and outstanding thermal stability with t1/2 (25 °C) over 100 days. Finally, the energy storage potential of these molecular solar thermal (MOST) systems is harnessed in a heat release experiment. This demonstrates the potential of norbornadiene-based photoswitches in combination with efficient magnetic catalysts for the generation of environmentally benign process heat.

Shell-by-Shell Functionalization of Inorganic Nanoparticles

The current state of the hierarchical chemical functionalization of inorganic nanoparticles (NPs) by shell-by-shell (SbS)-assembly of organic layers around the NP cores is summarized. This supramolecular functionalization concept is based on two steps: 1) the covalent grafting of a first ligand-shell consisting of, for example, long chain phosphonic acids and 2) the noncovalent interdigitation of amphiphiles forming the second ligand shell. The latter process is guaranteed predominantly by solvophobic interactions. These highly order organic-inorganic hybrid architectures are currently an emerging field at the interface of synthetic chemistry, nanotechnology, and materials science. The doubly functionalized NPs display tunable materials properties, such a controlled dispersibility and stability in various solvents, highly efficient trapping of guest molecules in between the ligand shells (water cleaning) as well as compartmentalization and modification of electronic interactions between photoactive components integrated in such complex nano-architectures. Such SbS-functionalized NPs have a high potential as water-cleaning materials and also some first prototype applications as biomedicinal therapeutics have been presented.

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.