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Applications of Photoswitches in the Storage of Solar Energy

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Abstract

Photoswitches are organic or organometallic chromophores that undergo a reversible chemical transformation upon absorption of light. Among the most commonly studied photoswitches are stilbenes and azobenzenes, capable of efficient interconversion between cis and trans isomers. When one isomer is significantly less thermodynamically stable than the other, photoisomerization of the stable to the metastable isomer converts a fraction of the absorbed photon energy into excess free energy (chemical potential). If the metastable isomer is sufficiently inert at room temperature, its photoconversion provides a means of storing solar energy, which is recovered by triggering heat‐releasing thermal conversion of the metastable to the stable isomer. In other words, such a photoswitch acts as a battery that captures solar energy, stores it as chemical potential and releases it on demand as heat. This process is known as molecular solar thermal energy storage or a molecular solar thermal battery. Unlike the more established conventional solar thermal storage, which uses sunlight to heat, melt or vaporize material, molecular solar thermal energy storage does not require thermal insulation to prevent discharge but relies on the kinetic activation barrier separating the two isomers. Unlike solar‐to‐chemical energy conversion by photosplitting of H2O or photoreduction of CO2, which comprise open‐system cycles, photoswitches are thermodynamically closed storage media. Successful deployment of molecular solar thermal energy storage requires new photoswitches that combine a seemingly contradictory set of molecular parameters: a large difference in the free energies of the two isomers separated by a large kinetic barrier; a high quantum yield of photogeneration of the metastable isomer that itself is either photochemically inactive or transparent to sunlight; highly selective isomerizations that allow many charge/discharge cycles without accumulation of side‐products even at high discharge temperatures. While the optimal photoswitch for molecular solar thermal energy storage remains to be invented, a large body of empirical observations acquired in the past decade provides several potentially valuable starting points for such a search. From light to heat through chemistry: A molecular photoswitch isomerizes when irradiated or heated. Photoswitches with a suitable energy profile provide a means of storing solar energy as metastable molecules in a thermodynamically closed operation, known as molecular solar thermal energy storage or a molecular solar thermal battery. This Review assesses progress towards the deployment of molecular solar thermal energy storage based on empirical, computational or theoretical research published since 2011.

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... The main drawbacks of NBD are the absorption onset in the UV region of the spectra and the low quantum yield of the direct photoisomerization, meaning that sunlight cannot drive the isomerization directly. [4] Hence, at riplet photosensitizer with E T (sensitizer) > E T (NBD),s uch as acetophenone or benzophenone, is neededt oe nable the isomerization of neat NBD to QC. [5] Alternatively,i th as been shown that, by functionalization of the NBD scaffold, it is possible to redshift the absorption and simultaneously increase the quantum yield. [6] However,f unctionalization of the NBD scaffoldnaturally is alwaysa ccompanied with an increased molecular weight and, therefore, al ower energy storage density.N ext to the photoisomerization, also the corresponding back reactionf rom QC to NBD mustb ec onsidered and optimized. ...
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... Where E storage is the energy stored after the photoisomerization process form NBD to QC, χ m is the molar fraction of the photoisomer QC, w p is the mass fraction of the parent molecule NBD in the material, and MW is the molecular weight of the parent molecule NBD. 39 Thus, a new generation of systems was designed where the acceptor p-aryl-substituted group (MW≈ 90 g/mol) was substituted by a cyano unit (MW ≈ 26 g/mol) in a series of compounds, of which NBD 4 is reported here (Figure 3). This set of low molecular weight compounds has red-shifted absorption onset (up to 456 nm for NBD 4, Figure 4a), the values of which are similar to those presented by the higher molecular weight NBD compounds such as NBD 3 (a variation of NBD 3 absorbs up to 462 nm). ...
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Article
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... The conflict between isomer energy difference and metastable isomer stability is known as a primary obstacle for molecular solar energy storage. 4,5 To address this issue, we developed pyrazolylazophenyl ethers 24 (pzAzo ethers, Figure 1b) as energy-harvesting molecules, which have a very long cis-isomer half-life t 1/2 of 3 months at 25°C (a few days for azobenzene) and also retain a relatively high ΔH isom of 52 kJ/mol (48 kJ/mol for azobenzene 25 ). Their favorable photon-harvesting ability is supported by high photoisomerization quantum yield ϕ trans−cis of 0.40−0.44 ...
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Article
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Azobenzene is one of the most studied light‐controlled molecular switches and it has been incorporated in a large variety of supramolecular systems to control their structural and functional properties. Given the peculiar isomeric distribution at the photoexcited state (PSS), azobenzene derivatives have been used as photoactive framework to build metastable supramolecular systems that are out of the thermodynamic equilibrium. This could be achieved exploiting the peculiar E/Z photoisomerization process that can lead to isomeric ratios that are unreachable in thermal equilibrium conditions. The challenge in the field is to find molecular architectures that, under given external circumstances, lead to a given isomeric ratio in a reversible and predictable manner, ensuring an ultimate control of the configurational distribution and system composition. By reviewing early and recent works in the field, this review aims at describing photoswitchable systems that, containing an azobenzene dye, display a controlled configurational equilibrium by means of a molecular recognition event. Specifically, examples include programmed photoactive molecular architectures binding cations, anions and H‐bonded neutral guests. In these systems the non‐covalent molecular recognition adds onto the thermal and light stimuli, equipping the supramolecular architecture with an additional external trigger to select the desired configuration composition. In control! Gaining control of the distribution and related properties of a given molecular population is a hot topic in systems chemistry. This can be achieved in a multi‐stimuli system in which the out‐of‐equilibrium population of a photoexcited photochromic switch is finely tuned by a second supramolecular interaction such as a reversibly guest binding process. This concept is discussed taking examples from the large family of supramolecular architectures incorporating an azobenzene moiety.
... The main drawbacks of NBD are the absorption onset in the UV region of the spectra and the low quantum yield of the direct photoisomerization, meaning that sunlight cannot drive the isomerization directly. [4] Hence, a triplet photosensitizer with E T (sensitizer) > E T (NBD), such as acetophenone or benzophenone, is needed to enable the isomerization of neat NBD to QC. [5] Alternatively, it has been shown that, by functionalization of the NBD scaffold, it is possible to redshift the absorption and simultaneously increase the quantum yield. [6] However, functionalization of the NBD scaffold naturally is always accompanied with an increased molecular weight and, therefore, a lower energy storage density. ...
Article
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We report on the synthesis and properties of various norbornadiene/quadricyclane (NBD/QC) fullerene hybrids. 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 the switching between the NBD and the QC state of the interconversion couple simply by changing the irradiation wavelength back and forth between 310 nm and 400 nm. 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.
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Article
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Due to their potential for solar energy harvesting and storage, molecular solar thermal energy storage (MOST) materials are receiving wide attention from both the research community and the public. MOST materials absorb photons and convert their energy to chemical energy, which is contained within the bonds of the MOST molecules. Depending on the molecular structure, these materials can store up to 1 MJ/kg, at ambient temperature and with storage times ranging from minutes to several years. This work is the first to thoroughly investigate the potential of MOST materials for the development of energy saving windows. To this end, the MOST molecules are integrated into thin, optically transparent films, which store solar energy during the daytime and release heat at a later point in time. A combined experimental and modeling approach is used to verify the system's basic functionality and identify key parameters. Multi-physics modeling and simulation were conducted to evaluate the interaction of MOST films with light, both monochromatic and the entire solar spectrum, as well as the corresponding dynamic energy storage. The model was experimentally verified by studying the optical response of thin MOST films containing norbornadiene derivatives as a functional system. We found that the MOST films act as excellent UV shield and can store up to 0.37 kWh/m² for optimized MOST molecules. Further, this model allowed us to screen various material parameters and develop guidelines on how to optimize the performance of MOST window films.
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Article
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Some molecular photoisomers can be isomerized to a metastable high-energy state by exposure to light. These molecules can then be thermally or catalytically converted back to their initial state, releasing heat in the process. Such a reversible photochemical process has been considered for developing molecular solar thermal (MOST) systems. In this review, we introduce the concept, criteria, and state-of-the-art of MOST systems, with an emphasis on the three most promising molecular systems: norbornadiene/quadricyclane, E/Z-azobenzene, and dihydroazulene/vinylheptafulvene. After discussing the fundamental working principles, we focus on molecular design strategies for improving solar energy storage performance, remaining challenges, and potential focus areas. Finally, we summarize the current molecular incorporation into functional devices and conclude with a perspective on challenges and future directions.
... Functionalized norbornadiene/quadricyclane (NBD/QC) couples have been identified as promising candidates for molecular solar thermal energy storage (MOST) systems that can undergo a closed energy cycle of light absorption, energy storage and energy release [1][2][3]. NBD undergoes a [2 + 2] cycloaddition upon irradiation, forming a strained high-energy QC isomer (Scheme 1). By introduction of donor-acceptor substituents on one or both double bonds of NBD, its absorption maximum can conveniently be redshifted from the UV to the visible region [4][5][6], thereby better matching the solar spectrum. ...
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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.
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Photoswitching dynamics of common organic photoisomers in solid state are often reduced compared to the solution state due to the close packing of molecules that limits structural changes. The facile solid-state switching of photoisomers has implications in developing novel light-controlled devices such as actuator, field-effect transistor, and photodetector, as well as photon energy storage materials that can be charged by light and release thermal energy upon triggering. Thus, the solid-state photoswitching of organic molecules has been studied in relation to the structural characteristics, and various effective methods to enhance the switching in condensed phases have been developed. This review highlights isomerization dynamics of common photoswitches in solid and then introduces important strategies for facilitating the switching dynamics, including the covalent functionalization methods for small-molecule photoswitches as well as the incorporation of various templates such as porous medium, nanoparticle, and host-guest structure. Furthermore, the solid-state switching of molecules at the interface with inorganic substrates including 2D materials and the microscopy techniques for the characterization will be further described.
... The intramolecular motion triggered by light or chemical fuels is a driving force of the artificial molecular switches. [1][2][3][4][5][6][7][8] The concept of controlled E/Z isomerization allows for a challenging design of molecular architectures performing specific functions such as unidirectional rotation and intramolecular cargo transport. [9][10][11][12][13] Recently, Leigh and co-workers [9] have demonstrated selective small-molecule cargo transport by repositioning of 3-mercaptopropanehydrazide in either direction between two sites 2 nm apart. ...
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Article
Multi-photochromic systems incorporating individually addressable switching units are attractive for development of advanced data storage devices. Here we present the synthesis and properties of a selection of such molecular systems incorporating the dihydroazulene/vinylheptafulvene (DHA/VHF) photo-/thermo-switch. The influence of the linker ( meta -phenylene vs azulene-1,3-diyl vs thiophene-2,5-diyl) separating two DHA units on the switching properties was investigated. An azulene-1,3-diyl spacer largely inhibited both the DHA-to-VHF photoisomerizations and the thermal VHF-to-DHA back-reactions; the latter occurred 10 times slower than for the related compound with a meta -phenylene spacer. A DHA trimer containing three DHA units around a central benzene ring was found to undergo stepwise DHA-to-VHF photoisomerizations, while the thermal back-reactions occurred at similar rates for the three VHF entities. A meta -phenylene bridged DHA dimer was subjected to further structural modifications at position C-1 of each DHA, having strong implications for the switching events, and synthetic steps for further functionalizations at position C-7 of each DHA were investigated. Finally, the molecular structure (from X-ray crystallographic analysis) between the meta- phenylene bridged DHA dimer and Cu(I) is presented.
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Photoswitchable catalysts based on phosphorus ligands have been overlooked so far, despite the growing current interest for catalysts control through external stimuli. This review surveys the current knowledge in the field, including synthetic approaches to photoswitchable phosphines and their transition metal complexes, and applications in rhodium, palladium, gold, ruthenium and copper catalysis. This survey demonstrates that both regulation of the catalytic activity and tuning of the chemo‐ and enantioselectivity can be achieved using irradiation as a non‐invasive external stimulus. Non‐catalytic uses of photoswitchable phosphines in organic synthesis are also mentioned.
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A series of liquid and photoliquefiable azobenzene (Azo) derivatives (Azo-Cn-Br) have been synthesized for molecular solar thermal fuels. Each of the liquid and photoliquefiable azo derivatives shows a high degree of isomerization, a fast isomerization rate, a long half-life, an appropriate energy storage density, and a solvent-free "charging" and "discharging" process. The photoliquefied azo derivatives can isomerize upon UV light irradiation at low temperatures to give the "UV-charged" azo ones. Therefore, the phase transition enthalpy is stored simultaneously along with the isomerization enthalpy. The "UV-charged" azo derivatives are capable of releasing heat under the manipulation of blue light.
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The kinetics of the thermal quadricyclane-to-norbornadiene (QC-to-NBD) isomerization reaction was studied for a large selection of derivatives where the one NBD double bond contains a cyano and aryl substituent of either electron-withdrawing or -donating character. While the kinetics data did not satisfy a linear-free-energy-relationship for all the derivatives based on Hammett σ values, we found individual linear relationships for derivatives containing either electron-withdrawing or electron-donating para substituents on the aryl group; with the most electron-witdrawing substituent in the one series and with the most electron-donating substituent in the other providing the fastest reaction (corresponding to opposite slopes of the Hammett plots). All data were well described, however, by a linear relationship when using Creary radical σC• values; the correlation could be slightly improved by using a combination of σ and σC• values (used in ratio of 0.104:1). The results imply a combination of polar and free radical effects for the isomerization reaction of this specific class of derivatives, with the latter playing the most significant role. The NBD derivatives were prepared by Diels-Alder cycloaddition reactions between cyclopentadiene and 3-arylpropiolonitriles, and in the case of bromophenyl derivatives further cyanation and Sonogashira coupling reactions were performed.
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Dihydroazulene (DHA) is a molecular photoswitch that undergoes a ring-opening reaction upon irradiation to form a vinylheptafulvene (VHF) photoisomer. This VHF isomer will in time thermally return to the DHA isomer. As the isomerization is photo-induced only in one direction, the DHA - VHF couple has attracted interest as a molecular solar thermal energy storage device (MOST system). In this author review, we cover our systematic efforts to optimize the DHA - VHF couple for this purpose, with challenges being to achieve sufficiently high energy densities, to cover broad absorptions including the visible region, and to control the energy-releasing VHF-to-DHA back-reaction. By a combination of computations and experiments, we review the consequences of various structural modifications of the system (structure - property relationships), including the influence of donor-acceptor substitution at specific positions, benzannulations, and incorporation into macrocyclic structures. Synthetic protocols to reach the various modifications will also be discussed. The bibliography includes 60 references. © 2020 Uspekhi Khimii, ZIOC RAS, Russian Academy of Sciences and IOP Publishing Limited.
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The performance of molecular solar thermal energy storage systems (MOST) depends i.a. on the amount of energy stored. Azobenzenes have been investigated as high‐potential materials for MOST applications. In the present study it could be shown that intermolecular attractive London dispersion interactions stabilize the (E)‐isomer in bisazobenzene linked by different alkyl bridges. Differential scanning ca­lo­rimetry (DSC) measurements revealed, that this interaction leads to an increased storage energy per azo‐unit of more than 3 kcal/mol compared to the parent azobenzene. The origin of this effect has been supported by computation as well as X‐ray analysis. In the solid state structure attractive London dispersion interactions between the C–H of the alkyl bridge and the π‐system of the azobenzene could be clearly assigned. This concept will be highly useful in designing more effective MOST systems in the future.
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We report on the immobilization of cobalt catalysts on the surface of iron oxide nanoparticles for the preparation of highly active quasi‑homogeneous catalysts towards an efficient release of photochemically stored energy in norbornadiene based photoswitches. The facile separation of the iron oxide nanoparticles, via 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 we achieved a 22.6‐fold increase in the catalytic efficiency of the QC‐NBD back‑conversion with an initial TOF of up to 3.64 s ‐1 and excellent TON of over 3305. In addition, we prepared a series of novel “push‐pull” functionalized norbornadiene derivatives that feature excellent absorption properties with maxima up to 366 nm, quantum yields around 70 %, high energy storage capacities of up to 98.4 kJ/mol and outstanding thermal stability with T 1/2 (25 °C) over 100 days. Finally, we harnessed the energy storage potential of these MOST systems 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.
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Azobispyrazoles are a family of bis-heteroaryl azo photoswitches that combine excellent E ⇆ Z photoisomerization yields and widely tunable Z-isomer thermal half-lives from hours to years, showing the great potential of Het-N=N-Het architecture in developing high-performance molecular photoswitches. Abstract Azobenzenes are classical molecular photoswitches that have been widely used. In recent endeavors of molecular design, replacing one or both phenyl rings with heteroaromatic rings has emerged as a strategy to expand molecular diversity and access improved photoswitching properties. Many mono-heteroaryl azo molecules with unique structures and/or properties have been developed, but the potential of bis-heteroaryl architectures is far from fully exploited. We report a family of azobispyrazoles, which combine (near-)quantitative bidirectional photoconversion and widely tunable Z-isomer thermal half-lives from hours to years. The two five-membered rings remarkably weaken the intramolecular steric hindrance, providing new possibilities for engineering the geometric and electronic structure of azo photoswitches. Azobispyrazoles generally exhibit twisted Z-isomers that facilitate complete Z→E photoisomerization, and their thermal stability can be broadly adjusted regardless of the twisted shape, overcoming the conflict between photoconversion (favored by the twisted shape) and Z-isomer stability (favored by the orthogonal shape) encountered by mono-heteroaryl azo switches.
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Molecular solar thermal energy storage systems (MOST) offer emission-free energy storage where solar power is stored via valence isomerization in molecular photoswitches. These photoswitchable molecules can later release the stored energy as heat on-demand. Such systems are emerging in recent years as a vibrant research field that is rapidly transitioning from basic research to applications. Since a major part of the attention is focused on molecular design and engineering, MOST-based device development has not been systematically summarized and introduced to a broad audience. This tutorial review will discuss the most commonly used and developed devices from a chemical engineering point of view. It is expected that future developers of MOST technology could be inspired by the existing devices, keeping in mind the summarized essential practical challenges towards large-scale implementations and more innovative applications. Key learning points 1. A brief history of MOST-based devices with a conceptual outline. 2. An introduction to different types of MOST-based devices. 3. An understanding of the current state-of-the-art of MOST-related devices. 4. An understanding of practical issues related to current MOST molecule-based devices. 5. Potential challenges and prospects of making efficient MOST devices.
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Chapter
Some knowledge of photophysics and photochemistry is required to design, study, and use a photoswitch. This chapter offers a short prologue to the basics of photophysics and photochemistry and introduces fundamental concepts such as quantum yield and photostationary state. On two examples of common photoswitches, the reader is acquainted with steady‐state and time‐resolved spectroscopic methods commonly used to study photochromic systems.
Chapter
Detailed studies of stilbenes advanced conceptually and practically diverse areas of sciences. Here we illustrate the use of stilbenes for fundamental understanding of mechanochemical energy transduction, on examples of polymer mechanochemistry and molecule machines. Quantitative molecular descriptions of mechanochemical conversion have been hampered by the multiscale nature of coupling between localized motion of the few atoms comprising a chemical reaction to synchronous translations of millions of surrounding atoms. In polymer mechanochemistry, stiff stilbene is used as a molecular force probe to precisely load individual reactive sites. In molecular machines, stilbenes enabled experimental realization of the minimal design of unidirectional motor.
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The conformational isomerism originated from the rotation of single bonds significantly affects the properties of conjugated molecules. However, the relatively low energy barrier of the rotation of single bonds often renders a dynamic equilibrium of multiple conformers, raising the challenges of distinguishing the accurate conformation and controlling the conformational interconversion. Here, multiple non-covalent interactions induced by the fluorine atom are introduced into the conjugated molecule to generate two metastable rotamers. The interconversion is readily controlled by solvent, wherein tetrahydrofuran and chloroform show split trends. The isomerically pure powder can be obtained from appropriate solutions. Furthermore, the film-formation process hardly affects the conformation because of the multiple non-covalent interactions. Eventually, this work unravels the effect of rotational isomerisms on optoelectronic properties in thin-film-based devices, paving the way to tune the performance of organic semiconductors by controlling the molecular conformation with multiple non-covalent interactions.
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We present a computational methodology for the screening of a chemical space of 10²⁵ substituted norbornadiene molecules for promising kinetically stable molecular solar thermal (MOST) energy storage systems with high energy densities that absorb in the visible part of the solar spectrum. We use semiempirical tight-binding methods to construct a dataset of nearly 34,000 molecules and train graph convolutional networks to predict energy densities, kinetic stability, and absorption spectra and then use the models together with a genetic algorithm to search the chemical space for promising MOST energy storage systems. We identify 15 kinetically stable molecules, five of which have energy densities greater than 0.45 MJ/kg and the main conclusion of this study is that the largest energy density that can be obtained for a single norbornadiene moiety with the substituents considered here, while maintaining a long half-life and absorption in the visible spectrum, is around 0.55 MJ/kg.
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A series of compact azobenzene derivatives were investigated as phase-transition molecular solar thermal energy storage compounds that exhibit maximum energy storage densities around 300 J/g. The relative size and polarity...
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Solar thermal fuels (STFs) offer a unique way of harnessing energy from the sun by absorbing photons and storing the energy in a metastable photoisomerized state. The reverse isomerization process can then be catalytically or thermally triggered to release the stored energy and return the fuel to its stable configuration. Functionalization of these compounds is necessary to reach practical energy storage densities, but substitutions that increase the energy storage density may adversely impact performance at other steps along the fuel cycle. Recent computational screening efforts to identify high-performance STF candidates have focused on properties that can be estimated from ground-state electronic structure methods. Here we argue that computational screening of STF candidates across the full fuel cycle benefits from a multifactor approach with excited-state properties like excitation energies and photoisomerization quantum yields addressed alongside key ground-state properties like energy storage densities and reverse isomerization barriers. As a critical step toward multifactor high-throughput screening and optimization of STFs, in this work we first systematically simulate the specific storage energy and excitation energy of substituted azobenzene- and norbornadiene-based STFs through electronic structure calculations. Density-functional tight-binding (DFTB) predictions are benchmarked against density functional theory (DFT) and experimental measurements where available. To encompass the complete solar thermal fuel cycle in these compounds, we then apply DFT methods to analyze the reverse isomerization process and its relationship to the photoisomerization quantum yield. We find that DFTB provides a useful balance between accuracy and computational efficiency for virtual screening of STF photoabsorption and energy storage, while isomerization barrier and quantum yield predictions require more sophisticated approaches.
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Development of photoactive chemical heat storage (PCHS) materials that can be isomerized without ultraviolet light and have outstanding storage performance as well as high rate heat output capability under low temperature conditions is a core issue for effective solar thermal conversion. In this study, we report a novel PCHS material by attaching ortho-tetrafluorinated azobenzene (AzoTF) onto reduced graphene oxide (rGO) for photothermal conversion and storage. By applying the strategy of separating the n → π* transition of two different configurations of AzoTF to the field of PCHS materials, this AzoTF-rGO composite not only realizes the isomerization with visible light for two configurations with a long life cycle, but also exhibits excellent fatigue resistance and low temperature high rate heat output capability, which greatly increases its exploitation value. Moreover, the AzoTF-rGO composite also shows remarkable heat storage density (Max. 345.8 kJ kg⁻¹), power density (Max. 2401.4 W kg⁻¹) attribute to the intermolecular hydrogen bond as well as the strong intermolecular interactions arise from the high attachment density. This new AzoTF-rGO PCHS material may paves a new way for more effective and efficient solar thermal energy conversion and storage.
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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, we investigate the fundamental processes within these hybrids. 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.
Thesis
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偶氮苯是一种经典的光开关分子, 它能发生可逆的 trans↔cis 光异构反应,且两个异构体之间具有大的结构、 性质差异, 在光功能分子材料的开发中受到了很大关注。 然而,传统的偶氮苯分子在 trans 和 cis 异构体之间的光异构转化率一般不高, 且 cis 异构体的热稳定性差(容易回复到热力学稳定的 trans 异构体),这限制了光开关能力的发挥。 同时, 虽然偶氮光开关在很多领域具有应用潜力,但材料或器件的性能还有很大的提升空间,离实际应用还有很长的距离。 鉴于此, 我们从结构设计出发,在光开关性质的优化、 分子材料性能的提升与潜在应用等方面开展了系统的研究。 Azobenzenes are a typical class of photoswitches, which can undergo reversible trans↔cis photoisomerizations with large differences of the isomers in molecular configuration and property, and they have received widespread interest in developing photo-functional molecular materials. However, traditional azobenzenes generally show incomplete photoconversions and low thermal stability of cis isomers (readily relax back to trans isomers), which limit their photoswitch performance. Moreover, although azo-switches have been used in a wide range of fields, performance of the currently developed photoactive materials or devices are still far from good enough for pratical applications. To deal with these problems, we have designed and synthesized novel azo molecules with improved photoswitch properties and developed high-performance azo-materials for potential applications.
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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.
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Materials with dynamically controlled electronic structures (i.e., upon external stimuli) are at the forefront of the renewable energy sector with applications as memory devices, smart supercapacitors, programmable solar cells, and field‐effect transistors. Moreover, their continued development as device components is critical for the field of optoelectronics since their performance is comparable, or could even surpass, the current benchmarks. Adaptive electronic properties are the main focus of this review that discusses recent developments in the modulation of electronic behavior that can be tuned using external stimuli in metal–organic frameworks (MOFs), covalent–organic frameworks (COFs), primarily inorganic hybrids, polymers, and graphitic‐type materials. Triggers to achieve “dynamic” behavior discussed within this manuscript are primarily light‐based switches that include different classes of photochromic molecules such as naphthalene diimide, viologen, diarylethene, azobenzene, and spiropyran. The effect of material dimensionality and photoswitch connectivity achieved through integration of photochromic moieties inside 0D, 1D, 2D, and 3D hybrid matrices is discussed. This review showcases the prospects of advancing the material and energy landscapes through employment of structural motifs with adaptive electronic structures occurring as a function of their dimensionality and connectivity. The desire to explore the convergence of electronic properties, dimensionality of materials, and connections of photoswitchable molecules has led to momentous progress in the field of stimuli‐responsive materials. Their applications as memory devices, smart supercapacitors, programmable solar cells, and field‐effect transistors can bring dynamically controlled photoresponsive motifs to the forefront of material engineering.
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The transition from fossil fuels to renewable energy sources requires technologies that are capable of both producing and storing energy. Attractive candidates that combine these two features are molecular photoswitches used for molecular solar thermal (MOST) energy storage. One of the most promising MOST systems is the norbornadiene/quadricyclane (NBD/QC) couple. We present the first density functional theory benchmark study of a NBD/QC derivative in vacuum with respect to novel gas-phase experiments. It was found that the M06-2X/def2-TZVP and M06-2X/6-311+G(d) level of theories produced the most accurate results with respect to the maximum absorption wavelength and the energy density.
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Diphosphines displaying azobenzene scaffolds and the corresponding bis-gold chloride complexes have been prepared and fully characterized by photophysical, spectroscopic and X-ray diffraction studies. DFT calculations provide complementary information on their electronic, structural and spectroscopic properties. Comparative investigations have been carried out on compounds featuring phosphorus functions in the meta- and para-positions, respectively, with respect to the azo functions, as well as on diphosphines with an ortho-tetrafluoro substituted azobenzene core. The effects of the substitution patterns on structural and spectroscopic properties are discussed.
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Azobenzenes are classical molecular photoswitches that have been widely used. In recent endeavors of molecular design, replacing one or both phenyl rings with heteroaromatic rings has emerged as a strategy to expand molecular diversity and access improved photoswitching properties. Many mono-heteroaryl azo molecules with unique structures and/or properties have been developed, but the potential of bis-heteroaryl architectures is far from fully exploited. We report a family of azobispyrazoles, which combine (near-)quantitative bidirectional photoconversion and widely tunable Z-isomer thermal half-lives from hours to years. The two five-membered rings remarkably weaken the intramolecular steric hindrance, providing new possibilities for engineering the geometric and electronic structure of azo photoswitches. Azobispyrazoles generally exhibit twisted Z-isomers that facilitate complete Z→E photoisomerization, and their thermal stability can be broadly adjusted regardless of the twisted shape, overcoming the conflict between photoconversion (favored by the twisted shape) and Z-isomer stability (favored by the orthogonal shape) encountered by mono-heteroaryl azo switches.
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Photoinduced tuning of (anti)aromaticity and associated molecular properties is currently in the focus of attention for both tailoring photochemical reactivity and designing new materials. Here, we report on the synthesis and spectroscopic characterization of diarylethene-based molecular switches embedded in a biphenylene structure composed of rings with different levels of local (anti)aromaticity. We show that it is possible to modulate and control the (anti)aromatic character of each ring through reversible photoswitching of the aryl units of the system between open and closed forms. Remarkably, it is shown that the irreversible formation of an annulated bis(dihydro-thiopyran) side-product that hampers the photoswitching can be efficiently suppressed when the aryl core formed by thienyl groups in one switch is replaced by thiazolyl groups in another.
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Due to its important physiological function, especially as molecular biomarkers of diseases, RNA has been the focus of biomedicine and biochemical sensing. Signal amplification detection has been put forward because of the needs for accurate identification of RNA within low expression levels, which is significant for the early diagnosing and therapy of malignant diseases. However, conventional amplification methods for RNA analysis depend on utilization of protein enzyme, fixation of cells and even operation of thermal cycle, which confine their performances in cell lysate or dead cells, thus imaging of RNAs in living cells remains to be explored. In recent years, the advance of isothermal amplification with nucleic acids opens up paths for meeting this need in living cells. This minireview tracks the development of in situ amplification assay of RNAs in living cells, and highlights the potential challenges regarding this field, aiming to improve the development of in vivo isothermal amplification as well as usher in new frontiers in this fertile research area.
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Use of solar energy as a source of heat is an important method of storing and providing clean energy for thermal management. However, the difficulties associated with combining the high-energy storage and high-rate heat release of solar thermal fuels (STF) lowers their ability to output the heat controllably, and thus, prevents their application to temperature-sensitive or temperature-responsive systems. Herein, we first report on the closed-cycle utilization of photo-thermal energy for thermochromic displays by optimizing their solid-state high-rate heat output. By controlling the molecular interaction, a tri-azobenzene (Azo)-based templated assembly can be made to combine a maximum energy density of 150.3 Wh kg-1, a long half-life (1250 h), and a high power density of 3036.9 W kg-1. The STF film can induce a reversible color change in a complicated thermochromic-patterned display by releasing the heat to increase the temperature by 6–7 ℃. We also realize variable heat release by controlling the heating rate and temperatures to utilize photo-thermal energy efficiently. Efficient cycling utilization of photo-thermal energy using tri-Azo assembly could be used to harness photo-thermal power for thermal management.
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The development of solar energy can potentially meet the growing requirements for a global energy system beyond fossil fuels, but necessitates new scalable technologies for solar energy storage. One approach is the development of energy storage systems based on molecular photoswitches, so-called molecular solar thermal energy storage (MOST). Here we present a novel norbornadiene derivative for this purpose, with a good solar spectral match, high robustness and an energy density of 0.4 MJ kg-1. By the use of heterogeneous catalyst cobalt phthalocyanine on carbon support, we demonstrate a record high macroscopic heat release in a flow system using a fixed bed catalytic reactor, leading to a temperature increase of up to 63.4 °C (83.2 °C absolute temperature). Successful outdoor testing shows proof of concept and illustrates that future implementation is feasible. The mechanism of the catalytic back reaction is modelled using density functional theory (DFT) calculations rationalizing the experimental observations.
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Molecular photoswitches can be used for solar thermal energy storage by photoisomerization into high-energy, meta-stable isomers; we present a molecular design strategy leading to photoswitches with high energy densities and long storage times. High measured energy densities of up to 559 kJ kg −1 (155 Wh kg −1), long storage lifetimes up to 48.5 days, and high quantum yields of conversion of up to 94% per subunit are demonstrated in norbornadiene/ quadricyclane (NBD/QC) photo-/thermoswitch couples incorporated into dimeric and trimeric structures. By changing the linker unit between the NBD units, we can at the same time fine-tune light-harvesting and energy densities of the dimers and trimers so that they exceed those of their monomeric analogs. These new oligomers thereby meet several of the criteria to be met for an optimum molecule to ultimately enter actual devices being able to undergo closed cycles of solar light-harvesting, energy storage, and heat release.
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Energy densities of ~510 J/g (max: 698 J/g) have been achieved in azobenzene-based syndiotactic-rich poly(methacrylate) polymers. The processing solvent and polymer-solvent interactions are important to achieve morphologically optimal structures for high-energy density materials. This work shows that morphological changes of solid-state syndiotactic polymers, driven by different solvent processings play an important role in controlling the activation energy of Z-E isomerization as well as the shape of the DSC exotherm. Thus, this study shows the crucial role of processing solvents and thin film structure in achieving higher energy densities.
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Mechanochemistry offers exciting opportunities for molecular-level engineering of stress-responsive properties of polymers. Reactive sites, sometimes called mechanophores, have been reported to increase the material toughness, to make the material mechanochromic or optically healable. Here we show that macrocyclic cinnamate dimers combine these productive stress-responsive modes. The highly thermally stable dimers dissociate on the sub-second timescale when subject to a stretching force of 1–2 nN (depending on isomer). Stretching a polymer of the dimers above this force more than doubles its contour length and increases the strain energy that the chain absorbs before fragmenting by at least 600 kcal per mole of monomer. The dissociation produces a chromophore and dimers are reformed upon irradiation, thus allowing optical healing of mechanically degraded parts of the material. The mechanochemical kinetics, single-chain extensibility, toughness and potentially optical properties of the dissociation products are tunable by synthetic modifications.
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Efficient solar energy storage is a key challenge in striving towards a sustainable future. For this reason, molecules capable of solar energy storage and release through valence isomerization, so-called Molecular Solar Thermal energy storage (MOST), have been investigated. Here, we evaluate the energy storage potential through the photoconversion of the dihydroazulene-vinylheptafulvene (DHA/VHF) photo-thermal couple. The robust nature of this system has been determined through multiple energy storage and release cycles at elevated temperatures in three different solvents. In a non-polar solvent such as toluene, the DHA/VHF system can be cycled more than 70 times with less than 0.01% degradation per cycle. Moreover, the [Cu(CH3CN)4]PF6 catalyzed conversion of VHF to DHA was demonstrated in a flow reactor. The performance of the DHA/VHF couple was also evaluated in prototype photoconversion devices, both in the laboratory using a flow chip under simulated sunlight, and under outdoor conditions by using a parabolic mirror. Device experiments demonstrated a solar energy storage efficiency up to 0.13% in the chip device and up to 0.02% in the parabolic collector, respectively. Avenues for future improvements and optimization of the system are discussed herein.
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We report a new molecular design for optically triggered nm-scale translation of a submolecular component relative to another. We used a rotaxane-like molecule terminated at one end with stiff stilbene...
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We have investigated the surface chemistry of the molecular solar thermal energy storage system of the valence isomer pair norbornadiene (NBD)/quadricyclane (QC) on Ni(111). Our multimethod approach includes UV-photoelectron spectroscopy (UPS), high-resolution X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) and density functional theory (DFT) calculations. The NBD/QC system holds the potential to be utilized in future storage technologies due to its comparably high gravimetric energy storage density, and the release of energy in a catalytic and sustainable cycle. UPS shows molecular adsorption of both compounds at 120 K, as is also predicted by DFT. NEXAFS and DFT suggest an adsorption geometry of NBD with both double bonds binding to the surface (η2:η2). For QC, no preference is found, and both the η2: η2 and the η2: η1 adsorption geometry are stable. The conversion of QC to NBD is thermally activated. From UPS, a reaction temperature of ~175 K is determined. Possible detrimental decomposition reactions of NBD were investigated by XPS. At 190 K, benzene (C6H6) and methylidyne (CH) are formed, and further react to C-H fragments at 330 K and finally leave carbide on the surface above 475 K.
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Due to high global energy demands, there is a great need for development of technologies for exploiting and storing solar energy. Closed cycle systems for storage of solar energy have been suggested, based on absorption of photons in photoresponsive molecules, followed by on‐demand release of thermal energy. These materials are called solar thermal fuels (STFs) or molecular solar thermal (MOST) energy storage systems. To achieve high energy densities, ideal MOST systems are required either in solid or liquid forms. In the case of the latter, neat high performing liquid materials have not been demonstrated to date. Here is presented a set of neat liquid norbornadiene derivatives for MOST applications and their characterization in toluene solutions and neat samples. Their synthesis is in most cases based on solvent‐free Diels‐Alder reactions, which easily and efficiently afford a range of compounds. The shear viscosity of the obtained molecules is close to that of colza oil, and they can absorb up to 10% of the solar spectrum with a measured energy storage density of up to 577 kJ/kg corresponding to 152 kJ mol–1 (calculated 100 kJ mol–1). These findings pave the way towards implementation of liquid norbornadienes in closed cycle energy storage technologies.
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The development of novel solar power technologies is considered to be one of many key solutions toward fulfilling a worldwide increasing demand for energy. Rapid growth within the field of solar technologies is nonetheless facing various technical barriers, such as low solar cell efficiencies, low performing balance-of-systems (BOS), economic hindrances (e.g., high upfront costs and a lack of financing mechanisms), and institutional obstacles (e.g., inadequate infrastructure and a shortage of skilled manpower). The merits and de-merits of solar energy technologies are both discussed in this article. A number of technical problems affecting renewable energy research are also highlighted, along with beneficial interactions between regulation policy frameworks and their future prospects. In order to help open novel routes with regard to solar energy research and practices, a future roadmap for the field of solar research is discussed.
Chapter
Solar energy is abundant all over the world, but to be useful, the energy received must either be transformed to electricity, heat or latent chemical energy. The latter two options have the advantages that the energy can be stored. In molecular solar-thermal energy storage (MOST), solar energy is stored in chemical bonds; this is achieved using compounds undergoing photoinduced isomerisation to metastable isomers. Using a catalyst, the isomer can be recycled to its original form and the stored energy released as heat. This chapter describes the principles of the MOST concept and goes into details about the most studied MOST systems. The last part of the chapter deals with the integration of MOST systems into operational devices.
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Efficient energy storage and release are two major challenges of solar energy harvesting technologies. The development of molecular solar thermal systems presents one approach to address these issues by tuning the isomerization reactions of photo-/thermoswitches. Here we show that the incorporation of photoswitches into macrocyclic structures is a particularly attractive solution for increasing the storage time. We present the synthesis and properties of a series of macrocycles incorporating two dihydroazulene (DHA) photoswitching subunits, bridged by linkers of varying chain length. Independent of ring size, all macrocycles exhibit stepwise, light-induced, ring-opening reactions (DHA-DHA to DHA-VHF to VHF-VHF; VHF = vinylheptafulvene) with the first DHA undergoing isomerization with a similar efficiency as the uncyclized parent system while the second (DHA-VHF to VHF-VHF) is significantly slower. The energy-releasing, VHF-to-DHA, ring closures also occur in a stepwise manner and were systematically found to proceed slower in the more strained (smaller) cycles, but in all cases with a remarkably slow conversion of the second VHF to DHA. We managed to increase the half-life of the second VHF-to-DHA conversion from 65 h to 202 h at room temperature by simply decreasing the ring size. A com-putational study reveals the smallest macrocycle to have the most energetic VHF-VHF state and hence highest energy density.
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Stretching polymer chains accelerates dissociation of a variety of internal covalent bonds, to an extent that correlates well with the force experienced by the scissile bond. Recent theory has also predicted scenarios in which applied force accelerates dissociation of unloaded bonds and kinetically strengthens strained bonds. We report here unambiguous experimental validation of this hypothesis: Detailed kinetic measurements demonstrate that stretching phosphotriesters accelerates dissociation of the unloaded phosphorus-oxygen bond orthogonal to the pulling axis, whereas stretching organosiloxanes inhibits dissociation of the aligned loaded silicon-oxygen bonds. Qualitatively, the outcome is determined by phosphoester elongation and siloxane contraction along the pulling axis in the respective rate-determining transition states. Quantitatively, the results agree with a simple mechanochemical kinetics model.
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The two valence isomers norbornadiene (NBD) and quadricyclane (QC) enable solar energy storage in a single molecule system. We present a new photoelectrochemical infrared reflection absorption spectroscopy (PEC-IRRAS) experiment, which allows monitoring the complete energy storage and release cycle by in-situ vibrational spectroscopy. Both processes were investigated, the photochemical conversion from NBD to QC using the photosensitizer 4,4′-bis(dimethylamino)benzophenone (Michler’s ketone, MK) and the electrochemically triggered cycloreversion from QC to NBD. Photochemical conversion was obtained with characteristic conversion times in the order of 500 ms. All experiments were performed under full potential control in a thin layer configuration with a Pt(111) working electrode. The vibrational spectra of NBD, QC, and MK were analyzed in the fingerprint region permitting quantitative analysis of the spectroscopic data. We determined selectivities for both the photochemical conversion and the electrochemical cycloreversion and identified the critical steps that limit the reversibility of the storage cycle.
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Developing norbornadiene-quadricyclane (NBD-QC) systems for molecular solar-thermal (MOST) energy storage is often a process of trial and error. By studying a series of norbornadienes (NBD-R2) doubly substituted at the C7-position with R = H, Me, and iPr, we untangle the interrelated factors affecting MOST performance through a combination of experiment and theory. Increasing the steric bulk along the NBD-R2 series gave higher quantum yields, slightly red-shifted absorptions, and longer thermal lifetimes of the energy-rich QC isomer. However, these advantages are counterbalanced by lower energy storage capacities, and overall R = Me appears most promising for short-term MOST applications. Computationally we find that it is the destabilization of the NBD isomer over the QC isomer with increasing steric bulk that is responsible for most of the observed trends and we can also predict the relative quantum yields by characterizing the S1/S0 conical intersections. The significantly increased thermal half-life of NBD-iPr2 is caused by a higher activation entropy, highlighting a novel strategy to improve thermal half-lives of MOST compounds and other photo-switchable molecules without affecting their electronic properties. The potential of the NBD-R2 compounds in devices is also explored, demonstrating a solar energy storage efficiency of up to 0.2%. Finally, we show how the insights gained in this study can be used to identify strategies to improve already existing NBD-QC systems.
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Stoichiometric copper(I) tetrakis(acetonitrile) is found to activate the thermal ring-closure reaction of a series of high-energy vinylheptafulvene isomers to the corresponding low-energy and photoactive dihydroazulenes, allowing the release of energy upon request.
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A closed-cycle system for light-harvesting, storage, and heat release is important for utilizing and managing renewable energy. However, combining a high-energy, stable photochromic material with controllable trigger of solid-state heat release remains greatly challenging for developing photothermal fuels (PTFs). This paper presents a uniform PTF film fabricated from an assembly of close-packed bisazobenzene (bisAzo) grafted onto reduced graphene oxide (rGO). The assembled rGO-bisAzo template exhibited a high energy density of 131 Wh kg-1 and a long half-life of 37 days owing to inter- or intramolecular H-bonding and steric hindrance. The rGO-bisAzo PTF film released and accumulated heat to realize a maximum temperature difference (DT) of 15 °C and a DT of over 10 °C for 30 min when the temperature difference of the environment was >100 °C. Controlling heat release in the solid-state assembly paves the way to develop highly efficient and high-energy PTFs for a multitude of applications.
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We have investigated the effects of substituents on the properties of the dihydroazulene/vinylheptafulvene photoswitch. The focus is on the changes of the thermochemical properties by placing electron withdrawing and donating groups on the monocyano and dicyano structures of the parent dihydroazulene and vinylheptafulvene compounds. We wish to increase the energy storage capacity, that is, the energy difference between the dihydroazulene and vinylheptafulvene isomers, of the photoswitch by computational molecular design and have performed over 9000 electronic structure calculations using density functional theory. Based on these calculations we obtain design rules for how to increase the energy storage capacity of the photoswitch. Furthermore, we have investigated how the activation energy for the thermally induced vinylheptafulvene to dihydroazulene conversion depends on the substitution pattern, and based on these results we have outlined molecular design considerations for obtaining new desired target structures exhibiting long energy storage times. Selected candidate systems have also been investigated in terms of optical properties to elucidate how sensitive the absorption maxima are to the functionalizations.
Article
We have investigated the surface chemistry of the polycyclic valence isomer pair norbornadiene (NBD) and quadricyclane (QC) on Pt(111). NBD/QC is considered a prototype system for energy storage in strained organic compounds. With our multimethod approach, including UV photoelectron spectroscopy, high resolution X-ray photoelectron spectroscopy, infrared reflec¬tion absorption spectroscopy and density functional theory calculations, we can unambiguously identify and differentiate between the two molecules in the multilayer phase which implies that the energy-loaded QC is stable in this state. Upon adsorption in the (sub)monolayer regime, the different spectroscopies yield identical spectra for NBD and QC at 125 K and also at 160 K, when multilayer desorption takes place. This behavior is explained by a rapid cycloreversion of QC to NBD upon contact with the Pt surface. The NBD adsorbs in a η2:η1 geometry with an agostic Pt-H interaction of the bridgehead CH2 subunit and the surface. Between 190 and 220 K, strong spectral changes are observed, because the hydrogen atom forming the agostic bond is broken. This reaction yields a norbornadienyl intermediate species which is stable up to ~ 380 K. At higher temperatures, the molecule dehydrogenates and decomposes into smaller carbonaceous fragments.
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A hybrid solar energy system consisting of a molecular solar thermal energy storage system (MOST) combined with a solar water heating system (SWH) is presented. The MOST chemical energy storage system is based on norbornadiene-quadricyclane derivatives allowing for conversion of solar energy into stored chemical energy at up to 103 kJ/mol (396 kJ/kg). It is demonstrated that 1.1 % of incoming solar energy can be stored in the chemical system without significantly compromising the efficiency of the solar water heating system, leading to efficiencies of combined solar water heating and solar energy storage of up to 80%. Moreover, prospects for future improvement and possible applications are discussed.
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A UV-sensitive azopolymer and a visible-light-sensitive azopolymer are combined with a fluorescent dye and a filter to create a four-layer solar thermal cell with record efficiency gravimetric energy density. While most azopolymers discharge under visible irradiation, this device can use a true solar spectrum because fluorescent down-conversion enables storage of light that would otherwise cause discharging.