Norbornadiene-Quadricyclane Photoswitches with Enhanced Solar Spectrum Match.

Herein, we report monomeric and dimeric norbornadiene-quadricyclane molecular photoswitch systems intended for molecular solar thermal applications. A series of six new norbornadiene derivatives conjugated with benzothiadiazole as the acceptor unit and dithiafulvene as the donor unit were synthesized and fully characterized. The photoswitches were evaluated by experimentally and theoretically measuring optical absorption profiles and thermal conversion of quadricyclane to norbornadiene. Computational insight by density functional theory calculations at the M06-2X/def2-SVPD level of theory provided geometries, storage energies, UV-vis absorption spectra, and HOMO-LUMO levels that are used to describe the function of the molecular systems. The studied molecules exhibit absorption onset ranging from 416 nm to 595 nm due to a systemic change in their donor-acceptor character. This approach was advantageous due to the introduction of benzothiadiazole and the dimeric nature of molecular structures. The best-performing system has a half-life of 3 days with quantum yields over 50 %.

Push-Pull Bis-Norbornadienes for Solar Thermal Energy Storage.

The norbornadiene/quadricyclane (NBD/QC) photoswitch pair represents a promising system for application in molecular solar thermal energy storage (MOST). Often, the NBD derivatives have very limited overlap with the solar spectrum, and substitution to redshift the absorption leads to a decrease in the gravimetric energy density. Dimeric systems mitigate this factor because two switches can 'share' a substituent. Here, we present five new NBD dimers with red-shifted absorption spectra. One dimer features the most red-shifted absorption onset (539 nm) and a significantly red-shifted absorption maximum (404 nm) for NBD systems reported so far, without compromising thermal half-life. Promising properties for high-performance MOST applications are demonstrated, such as high absorption onsets reaching 539 nm, and energy densities of 379 kJ/kg, while still maintaining long half-lives of the metastable isomer, up to 23 hours at 25 °C.

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.

A Norbornadiene-Based Molecular System for the Storage of Solar-Thermal Energy in an Aqueous Solution: Study of the Heat-Release Process Triggered by a Co(II)-Complex.

It is urgent yet challenging to develop new environmentally friendly and cost-effective sources of energy. Molecular solar thermal (MOST) systems for energy capture and storage are a promising option. With this in mind, we have prepared a new water-soluble (pH > 6) norbornadiene derivative (HNBD1) whose MOST properties are reported here. HNBD1 shows a better matching to the solar spectrum compared to unmodified norbornadiene, with an onset absorbance of λonset = 364 nm. The corresponding quadricyclane photoisomer (HQC1) is quantitatively generated through the light irradiation of HNBD1. In an alkaline aqueous solution, the MOST system consists of the NBD1-/QC1- pair of deprotonated species. QC1- is very stable toward thermal back-conversion to NBD1-; it is absolutely stable at 298 K for three months and shows a marked resistance to temperature increase (half-life t½ = 587 h at 371 K). Yet, it rapidly (t½ = 11 min) releases the stored energy in the presence of the Co(II) porphyrin catalyst Co-TPPC (ΔHstorage = 65(2) kJ∙mol-1). Under the explored conditions, Co-TPPC maintains its catalytic activity for at least 200 turnovers. These results are very promising for the creation of MOST systems that work in water, a very interesting solvent for environmental sustainability, and offer a strong incentive to continue research towards this goal.

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.

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.

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.

Accurate prediction of norbornadiene cycle enthalpies by DLPNO-CCSD(T1 )/CBS method.

The DLPNO-CCSD(T1 )/CBS method combined with simple reactions containing small reference species leads to an improvement in the accuracy of theoretically evaluated enthalpies of formation of medium-sized polyalicyclic hydrocarbons when compared with the widely used composite approach. The efficiency of the DLPNO-CCSD(T1 )/CBS method is most vividly demonstrated by comparing with the results of G4 calculations for adamantane. The most important factor in choosing appropriate working reaction is the same number of species on both sides of the equation. Among these reactions, the reactions with small enthalpy change usually provide a better cancellation of errors. The DLPNO-CCSD(T1 )/CBS method was used to calculate the enthalpies of formation of compounds belonging to the norbornadiene cycle (norbornadiene, quadricyclane, norbornene, nortricyclane, and norbornane). The most reliable experimental enthalpies of formation are recommended for these compounds by comparing calculated values with conflicting experimental data.

A Phosphinine-Derived 1-Phospha-7-Bora-Norbornadiene: Frustrated Lewis Pair Type Activation of Triple Bonds.

Salt metathesis of 1-methyl-2,4,6-triphenylphosphacyclohexadienyl lithium and chlorobis(pentafluorophenyl)borane affords a 1-phospha-7-bora-norbornadiene derivative 2. The C≡N triple bonds of nitriles insert into the P-B bond of 2 with concomitant C-B bond cleavage, whereas the C≡C bonds of phenylacetylenes react with 2 to form λ4 -phosphabarrelenes. Even though 2 must formally be regarded as a classical Lewis adduct, the C≡N and C≡C activation processes observed (and the mild conditions under which they occur) are reminiscent of the reactivity of frustrated Lewis pairs. Indeed, NMR and computational studies give insight into the mechanism of the reactions and reveal the labile nature of the phosphorus-boron bond in 2, which is also suggested by detailed NMR spectroscopic studies on this compound. Nitrile insertion is thus preceded by ring opening of the bicycle of 2 through P-B bond splitting with a low energy barrier. By contrast, the reaction with alkynes involves formation of a reactive zwitterionic methylphosphininium borate intermediate, which readily undergoes alkyne 1,4-addition.

Photoswitchable Norbornadiene-Quadricyclane Interconversion Mediated by Covalently Linked C60.

The synthesis and properties of various norbornadiene/quadricyclane (NBD/QC) fullerene hybrids are reported. By cyclopropanation of C60 with malonates carrying the NBD scaffold a small library of NBD-fullerene monoadducts and NBD-fullerene hexakisadducts was established. The substitution pattern of the NBD scaffold, as well as the electron affinity of the fullerene core within these hybrid systems, has a pronounced impact on the properties of the corresponding energy rich QC derivatives. Based on this, the first direct photoisomerization of NBD-fullerene hybrids to their QC derivatives was achieved. Furthermore, it was possible to use the redox-active fullerene core of a QC-fullerene monoadduct to enable the back reaction to form the corresponding NBD-fullerene monoadduct. Combining these two processes enables switching between NBD and QC simply by changing the irradiation wavelength between 310 and 400 nm. Therefore, turning this usually photo/thermal switch into a pure photoswitch. This not only simplifies the investigation of the underlying processes of the NBD-QC interconversion within the system, but also renders such hybrids interesting for applications as molecular switches.

Fast Oxygen Scavenging of Macroporous Poly(Norbornadiene) Prepared by Ring-Opening Metathesis Polymerization.

Emulsion templated norbornadiene is cured via ring-opening metathesis polymerization yielding macroporous poly(norbornadiene) foams of 76% porosity exhibiting appealing stiffness combined with considerable ductility. The foams are readily oxidized in the presence of air at room temperature exhibiting an oxygen uptake capacity of more than 300 mg O2 g-1 foam. In closed volumes of air, a final oxygen level of a maximum of 0.0005 vol%, that is, 5 ppm(v) can be achieved after several hours at room temperature. The synergism of the porous morphology and the chemical nature of the polymer allows for the first example of an organic oxygen scavenger material with properties distinctly surpassing the state-of-the art in the field.

Donor-Acceptor Substituted Benzo-, Naphtho- and Phenanthro-Fused Norbornadienes.

The photochromic norbornadiene/quadricyclane (NBD/QC) couple has found interest as a molecular solar thermal energy (MOST) system for storage of solar energy. To increase the energy difference between the two isomers, we present here the synthesis of a selection of benzo-fused NBD derivatives that contain an aromatic unit, benzene, naphthalene or phenanthrene, fused to one of the NBD double bonds, while the carbon atoms of the other double bond are functionalized with donor and acceptor groups. The synthesis protocols involve functionalization of benzo-fused NBDs with bromo/chloro substituents followed by a subjection of these intermediates to a cyanation reaction (introducing a cyano acceptor group) followed by a Sonogashira coupling (introducing an arylethynyl donor group, -CCC6H4NMe2 or -CCC6H4OMe). While the derivatives have good absorption properties in the visible region (redshifted relative to parent system) in the context of MOST applications, they lack the ability to undergo NBD-to-QC photoisomerization, even in the presence of a photosensitizer. It seems that loss of aromaticity of the fused aromatics is too significant to allow photoisomerization to occur. The concept of destroying aromaticity of a neighboring moiety as a way to enhance the energy density of the NBD/QC couple thus needs further structural modifications, in the quest for optimum MOST systems.

A novel and efficient synthesis of phenanthrene derivatives via palladium/norbornadiene-catalyzed domino one-pot reaction.

Herein we report a novel palladium-catalyzed reaction that results in phenanthrene derivatives using aryl iodides, ortho-bromobenzoyl chlorides and norbornadiene in one pot. This dramatic transformation undergoes ortho-C-H activation, decarbonylation and subsequent a retro-Diels-Alder process. Pleasantly, this protocol has a wider substrate range, shorter reaction times and higher yields of products than previously reported methods.

Norbornadiene-functionalized triazatriangulenium and trioxatriangulenium platforms.

Triazatriangulenium (TATA) and trioxatriangulenium (TOTA) ions are particularly suited systems to mount functional molecules onto atomically flat surfaces such as Au(111). The TATA and TOTA units serve as platforms that absorb onto the surface and form ordered monolayers, while the functional groups are protruding upright and freestanding from the central carbon atoms. Azobenzene derivatized TATA's are known to exhibit extremely fast cis→trans isomerization on metal surfaces, via a peculiar non-adiabatic singlet→triplet→singlet mechanism. We now prepared norbornadienes (NBD) and quadricyclanes (QC) attached to TATA and TOTA platforms which can be used to check if these accelerated rates and the spin change mechanism also apply to [2 + 2] cycloreversions (QC→NBD).

Low Molecular Weight Norbornadiene Derivatives for Molecular Solar-Thermal Energy Storage.

Molecular solar-thermal energy storage systems are based on molecular switches that reversibly convert solar energy into chemical energy. Herein, we report the synthesis, characterization, and computational evaluation of a series of low molecular weight (193-260 g mol(-1) ) norbornadiene-quadricyclane systems. The molecules feature cyano acceptor and ethynyl-substituted aromatic donor groups, leading to a good match with solar irradiation, quantitative photo-thermal conversion between the norbornadiene and quadricyclane, as well as high energy storage densities (396-629 kJ kg(-1) ). The spectroscopic properties and energy storage capability have been further evaluated through density functional theory calculations, which indicate that the ethynyl moiety plays a critical role in obtaining the high oscillator strengths seen for these molecules.

Flow-Integrated Preparation of Norbornadiene Precursors for Solar Thermal Energy Storage.

Molecular solar thermal (MOST) energy storage systems are getting increased attention related to renewable energy storage applications. Particularly, 2,3-difunctionalized norbornadiene-quadricyclane (NBD-QC) switches bearing a nitrile (CN) group as one of the two substituents are investigated as promising MOST candidates thanks to their high energy storage densities and their red-shifted absorbance. Moreover, such NBD systems can be prepared in large quantities (a requirement for MOST-device applications) in flow through Diels-Alder reaction between cyclopentadiene and appropriately functionalized propynenitriles. However, these acetylene precursors are traditionally prepared in batch from their corresponding acetophenones using reactive chemicals potentially leading to health and physical hazards, especially when working on a several-grams scale. Here, we develop a multistep flow-chemistry route to enhance the production of these crucial precursors. Furthermore, we assess the atom economy (AE) and the E-factor showing improved green metrics compared to classical batch methods. Our results pave the way for a complete flow synthesis of NBDs with a positive impact on green chemistry aspects.

The surface chemistry of norbornadiene and norbornene on Pd(111) and Pd(100): a comparative DFT study.

CONTEXT: The interaction of norbornadiene (NBD) and norbornene (NBE) with the palladium (111) and (100) surfaces have been investigated using density functional theory (DFT). Five configurations of adsorbed NBD may be formed on Pd(111): endo-tetra-σ, endo-di-σ,π, endo-di-π, exo-di-σ, and exo-π. The NBE molecule adsorbed on Pd(111) may exist in 4 configurations: endo-di-σ, endo-π, exo-di-σ, and exo-π. On Pd(100), a smaller number adsorption configurations of NBD and NBE are formed, since the double bonds of these molecules in the endo-orientation are bound only in a di-σ mode. The adsorption energy of NBD and NBE molecules on Pd(100) is noticeably higher compared to Pd(111), which is due to the surface geometry of Pd(100). The most stable configurations on both Pd facets are endo-tetra-σ for NBD and exo-di-σ for NBE. However, due to smaller adsorption area of the exo-di-σ configuration on Pd(111), a larger number of NBD molecules may adsorbed on the same surface area. Energetically favorable endo-tetra-σ (NBD) and exo-di-σ (NBE) configurations are very mobile on Pd(111). On Pd(100), only NBE molecules can migrate, while NBD migration is hindered due to the high activation barrier. METHODS: All DFT calculations were performed using the Perdew-Burke-Ernzerhof density functional (PBE) with the relativistic SBK effective core potential and TZ2P basis set in the PRIRODA program.

Investigating New Applications of a Photoswitchable Fluorescent Norbornadiene as a Multifunctional Probe for Delineation of Amyloid Plaque Polymorphism.

Amyloid beta (Aß) plaques are a major pathological hallmark of Alzheimer's disease (AD) and constitute of structurally heterogenic entities (polymorphs) that have been implicated in the phenotypic heterogeneity of AD pathology and pathogenesis. Understanding amyloid aggregation has been a critical limiting factor to gain understanding of AD pathogenesis, ultimately reflected in that the underlying mechanism remains elusive. We identified a fluorescent probe in the form of a turn-off photoswitchable norbornadiene derivative (NBD1) with several microenvironment-sensitive properties that make it relevant for applications within advanced fluorescence imaging, for example, multifunctional imaging. We explored the application of NBD1 for in situ delineation of structurally heterogenic Aß plaques in transgenic AD mouse models. NBD1 plaque imaging shows characteristic broader emission bands in the periphery and more narrow emission bands in the dense cores of mature cored plaques. Further, we demonstrate in situ photoisomerization of NBD1 to quadricyclane and thermal recovery in single plaques, which is relevant for applications within both functional and super-resolution imaging. This is the first time a norbornadiene photoswitch has been used as a probe for fluorescence imaging of Aß plaque pathology in situ and that its spectroscopic and switching properties have been studied within the specific environment of senile Aß plaques. These findings open the way toward new applications of NBD-based photoswitchable fluorescent probes for super-resolution or dual-color imaging and multifunctional microscopy of amyloid plaque heterogeneity. This could allow to visualize Aß plaques with resolution beyond the diffraction limit, label different plaque types, and gain insights into their physicochemical composition.

Synthesis and Diels-Alder cycloaddition reaction of norbornadiene and benzonorbornadiene dimers.

Dimeric forms of norbornadiene and benzonorbornadiene were synthesized starting with known monobromide derivatives. The Diels-Alder cycloaddition reaction of dimers with TCNE and PTAD was investigated and new norbornenoid polycyclics were obtained. All compounds were characterized properly using NMR spectroscopy.