Molecular Solar Thermal Energy Storage Research Articles

Second Generation Zwitterionic Aza‐Diarylethene: Photoreversible CN Bond Formation, Three‐State Photoswitching, Thermal Energy Release, and Facile Photoinitiation of Polymerization

Diarylethenes are a well‐studied and optimized class of photoswitches with a wide range of applications, including data storage, smart materials, or photocontrolled catalysis and biological processes. Most recently, aza‐diarylethenes have been developed in which carbon‐carbon bond connections are replaced by carbon‐nitrogen connections. This structural elaboration opens an entire new structure and property space expanding versatility and applicability of diarylethenes. In this work, we present the second generation of zwitterionic aza‐diarylethenes, which finally allows for fully reversible photoswitching and precise control over all three switching states. High‐yielding photoswitching between the neutral open form and a zwitterionic Z isomer is achieved with two different wavelengths of light. The third zwitterionic E isomeric state can be reached up to 87% upon irradiation with a third wavelength. Its high energy content of >10 kcal/mol can be released thermally by deliberate solvent change as trigger mechanism, rendering aza‐diarylethenes into interesting candidates for molecular solar thermal energy storage (MOST) applications. The third state further serves as locking state, allowing to toggle light‐responsiveness reversibly between thermally labile and thermally stable switching. Further, irradiation of the zwitterionic states leads to highly efficient photopolymerization of methyl acrylate (MA), directly harnessing the unleashed chemical reactivity of aza‐diarylethene in materials applications.

Second Generation Zwitterionic Aza-Diarylethene: Photoreversible CN Bond Formation, Three-State Photoswitching, Thermal Energy Release, and Facile Photoinitiation of Polymerization.

Diarylethenes are a well-studied and optimized class of photoswitches with a wide range of applications, including data storage, smart materials, or photocontrolled catalysis and biological processes. Most recently, aza-diarylethenes have been developed in which carbon-carbon bond connections are replaced by carbon-nitrogen connections. This structural elaboration opens an entire new structure and property space expanding versatility and applicability of diarylethenes. In this work, we present the second generation of zwitterionic aza-diarylethenes, which finally allows for fully reversible photoswitching and precise control over all three switching states. High-yielding photoswitching between the neutral open form and a zwitterionicZisomer is achieved with two different wavelengths of light. The third zwitterionicEisomeric state can be reached up to 87% upon irradiation with a third wavelength. Its high energy content of >10 kcal/mol can be released thermally by deliberate solvent change as trigger mechanism, rendering aza-diarylethenes into interesting candidates for molecular solar thermal energystorage (MOST) applications. The third state further serves as locking state, allowing to toggle light-responsiveness reversibly between thermally labile and thermally stable switching. Further, irradiation of the zwitterionic states leads to highly efficient photopolymerization of methyl acrylate (MA),directly harnessing the unleashed chemical reactivity of aza-diarylethene in materials applications.

Sustainable Heat Generation in Flow from a Molecular Solar Thermal Energy Storage System

As the global deployment of renewable technologies accelerate, finding efficient ways to store energy will aid in responding to shifting energy demands. A prospective option not only in harvesting solar energy but also in emission‐free heating is MOlecular Solar Thermal (MOST) energy storage. A central part of MOST applications is to develop methods to release the stored energy. Herein, the Quadricyclane (QC)‐to‐Norbornadiene catalyzed back reaction is explored in a specially designed packed‐bed reactor. Four distinctly sized and purposely synthesized platinum on activated carbon catalysts are studied to trigger the heat release from the energy‐dense QC isomer. The catalysts are fully characterized using a variety of structural, surface, and spectroscopic techniques. Parameters to optimize catalytic conversion and heat release in flow conditions are explored including particle size and packing behavior, flow rates, and molecular residence times. Moreover, using CO pulse chemisorption technique, site time yield values and a turnover number are reported. Complementary to the flow reactions, computational fluid dynamic simulations applying lattice Boltzmann methods to two catalytic packed beds of different size ranges are done to evaluate fluid‐dynamic behavior within the reactor bed to ascertain the ideal particle size and packing density for catalysis in MOST applications.

Triplet-Sensitized Switching of High-Energy-Density Norbornadienes for Molecular Solar Thermal Energy Storage with Visible Light.

Norbornadiene-based photoswitches have emerged as promising candidates for harnessing and storing solar energy, holding great promise as a viable solution to meet the growing energy demands. Despite their potential, the effectiveness of their direct photochemical conversion into the resulting quadricyclanes has room for improvement owing to (i) moderate quantum yields, (ii) poor overlap with the solar spectrum and (iii) photochemical back reactions. Herein, we present an approach to enhance the performance of such molecular solar thermal energy storage (MOST) systems through the triplet-sensitized conversion of aryl-substituted norbornadienes. Our study combines deep spectroscopic analyses, irradiation experiments, and quantum mechanical calculations to elucidate the energy transfer mechanism and inherent advantages of the resulting MOST systems. We demonstrate remarkable quantum yields using readily available sensitizers under both LED and solar light irradiation, significantly surpassing those achieved through direct excitation with photons of higher energy. In contrast to the conventional approach, light-induced back reactions of the high-energy products do not play any role, allowing quantitative switching within minutes. These results not only underscore the potential of triplet-sensitized MOST systems to leverage the high energy storage capabilities of multistate photoswitches but they might also stimulate the broader usage of sensitization strategies in photochemical energy conversion.

Triplet‐Sensitized Switching of High‐Energy‐Density Norbornadienes for Molecular Solar Thermal Energy Storage with Visible Light

Norbornadiene‐based photoswitches have emerged as promising candidates for harnessing and storing solar energy, holding great promise as a viable solution to meet the growing energy demands. Despite their potential, the effectiveness of their direct photochemical conversion into the resulting quadricyclanes has room for improvement owing to (i) moderate quantum yields, (ii) poor overlap with the solar spectrum and (iii) photochemical back reactions. Herein, we present an approach to enhance the performance of such molecular solar thermal energy storage (MOST) systems through the triplet‐sensitized conversion of aryl‐substituted norbornadienes. Our study combines deep spectroscopic analyses, irradiation experiments, and quantum mechanical calculations to elucidate the energy transfer mechanism and inherent advantages of the resulting MOST systems. We demonstrate remarkable quantum yields using readily available sensitizers under both LED and solar light irradiation, significantly surpassing those achieved through direct excitation with photons of higher energy. In contrast to the conventional approach, light‐induced back reactions of the high‐energy products do not play any role, allowing quantitative switching within minutes. These results not only underscore the potential of triplet‐sensitized MOST systems to leverage the high energy storage capabilities of multistate photoswitches but they might also stimulate the broader usage of sensitization strategies in photochemical energy conversion.

Strain Engineered Bridged Bicyclic Diene Photoswitches in the Race of Next‐Generation Molecular Solar Thermal Energy Storage

AbstractNorbornadiene/Quadricyclane (NBD/QC) is a prototypical bridged bicyclic diene (BBD)‐based photoswitch that has been well‐studied for molecular solar thermal energy storage (MOST). Inspired by the recent synthetically accessed BBDs, herein several photoswitches are rationally designed with modulated ring strain energies (RSE) in photoisomers to incorporate high energy storage density (ESD) and storage time in a single couple. The storage energy ( ) calculated at DLPNO‐CCSD(T)/Def2TZVP level is correlated with difference in RSE of two isomers (ΔRSE) whereas thermal back reaction (TBR) barrier calculated at (8,8)‐CASPT2/6‐311++G** shows correlation with RSE in metastable photoproduct. On the basis of these structure‐property‐RSE relationships, we recognized that two photoisomers need not to be highly strained. Instead, the RSE in the photoproduct and diene should be minimized while maintaining a large enthalpy difference between them to increase ESD and extend energy storage times in a single photoswitch. TBR barrier is governed by RSE in photoproduct and increasing strain in photoproduct may improve the but at the cost of the TBR barrier. Herein, the structural skeletons are explored that holds promise to remarkably improve thermochemical properties relative to the unsubstituted BBD‐based photoswitches reported so far. The BBD molecules with short saturated bridge length but elongated unsaturated bridges could bestow desirable thermochemical parameters and can be regarded as excellent candidates for MOST application. The work lays a theoretical foundation that guides to improve thermochemical properties via strain engineering of BBD‐based photoswitches and opens a new avenue for designing principles and future experimental investigations of MOST systems.

Self-activated energy release cascade from anthracene-based solid-state molecular solar thermal energy storage systems

Self-activated energy release cascade from anthracene-based solid-state molecular solar thermal energy storage systems

Hybrid solar energy device for simultaneous electric power generation and molecular solar thermal energy storage

Hybrid solar energy device for simultaneous electric power generation and molecular solar thermal energy storage

Multi‐azo Photoswitches for Improved Molecular Solar Thermal Energy Storage

AbstractMolecular solar thermal energy storage (MOST) based on photoisomerization represents a novel approach for the capture, conversion and storage of solar energy. Azo photoswitches can store energy by isomerization from their thermodynamically stable E isomers to higher energy metastable Z isomers. The energy density of azo photoswitches is crucial as it directly determines the storage capacity, but it is relatively low (160 J g−1). Therefore, enhancing the energy density through molecular structural design represents a central research focus in the field of MOST. A straightforward approach to enhance the energy density is to design multi‐azo photoswitches. This allows multiple azobenzene units to share a common framework while keeping the molecular weight as small as possible. In particular, when two azobenzene units are connected via a phenyl ring in a meta orientation, it facilitates efficient isomerization, thereby maximizing the energy density of the azo photoswitches (392 J g−1). This paper provides a brief overview of the development of multi‐azo photoswitches and highlights their outstanding performance as a MOST system. It also offers prospects for their future advancements in the field. We propose that, to further improve the energy density of multi‐azo photoswitches, one approach is to design wide spectrum of light photoisomerization of multi‐azo photoswitches. Additionally, introducing photo‐induced phase changes to multi‐azo photoswitches enables the simultaneous storage of both photon energy and ambient heat.

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 and Characterization of Carbon-Based Heterogeneous Catalysts for Energy Release of Molecular Solar Thermal Energy Storage Materials.

Molecular solar thermal energy storage (MOST) systems are rapidly becoming a feasible alternative to energy storage and net-zero carbon emission heating. MOST systems involve a single photoisomerization pair that incorporates light absorption, storage, and heat release processes in one recurring cycle. Despite significant recent advancements in the field, the catalytic back-reaction from MOST systems remains relatively unexplored. A wide range of applications is possible, contingent on the energy densities of the specific photoisomers. Here, we report platinum-, copper-, and nickel-based heterogeneous catalysts screened in batch conditions for the back-conversion reaction on the cyano-3-(4-methoxyphenyl)-norbornadiene/quadricyclane pair. Catalyst reactivities are investigated using structural characterization, imaging techniques, and spectroscopic analysis. Finally, the thermal stability is also explored for our best-performing catalysts.

Two-way photoswitching norbornadiene derivatives for solar energy storage.

Molecular photoswitches of norbornadiene (NBD) derivatives have been effectively applied in molecular solar-thermal energy storage (MOST) by photoisomerization of NBD to a quadricyclane (QC) state. However, a challenge of the NBD-based MOST system is the lack of a reversible two-way photoswitching process, limiting conversion from QC to thermal and catalytic methods. Here we design a series of NBD derivatives with a combination of acceptor and donor units to achieve two-way photoswitching, which can optically release energy by back-conversion from QC to NBD. Highly efficient photoconversion yields from NBD to QC and QC to NBD are up to 99% and 82%, respectively. The energy storage density of two-way photoswitching NBD is up to 312 J g-1 and optically controlled two-way photoswitching devices are demonstrated for the first time both in flow and in thin films, which illustrate a promising approach for fast and robust energy release in both solution and solid state.

Photoswitchable hydrazones with pyridine-based rotors and halogen substituents.

The Z,E-photoisomerization of pyridine-based hydrazone switches is typically suppressed due to the presence of pyridine-based rotors. The crystal structures of studied compounds were investigated using theoretical methods combining DFT and QT-AIM calculations to unveil the nature and properties of the intramolecular hydrogen bonding. In this study, we introduced a new series of pyridine-based hydrazones anchored with o-halogen substituents (2-X) and investigated their photoswitching abilities using 1H NMR and UV-Vis spectroscopy. The efficiency of the photoisomerization from initial 2-X-Z to the 2-X-E isomer varied, with the highest yield observed for 2-Cl-E (55%). Our findings, supported by DFT calculations, revealed the formation of a new diastereomer, 2-X-E*, upon back-photoisomerization. We demonstrated that hydrazones from the 2-X series can be reversibly photoswitched using irradiation from the UV-Vis range, and additionally, we explored the effect of the halogen atom on their switching capabilities and also on their thermodynamics and kinetics of photoswitching, determining their molecular solar thermal energy storage potential.

Aza-bicyclodiene based photoswitches for molecular solar thermal energy storage

The effect of N-substitution on the properties of bicyclodienes with different bridge lengths is analysed for energy storage application. The improvement in the properties with N-substitution is more prominent for bicyclodienes with longer bridge.

Large and long-term photon energy storage in diazetidines via [2+2] photocycloaddition.

We report a series of p-functionalized phenylbenzoxazoles that offer remarkable energy storage, exceeding 300 J g-1, for the first time among intermolecular cycloaddition-based molecular solar thermal energy storage systems. The [2 + 2] photocycloaddition of phenylbenzoxazoles generates diazetidine cycloadducts that store energy for up to 23 years in the solid state and release energy upon triggered cycloreversion. The solid-state phase transition contributes to increasing overall energy storage densities, and the dearomative cycloaddition process is revealed to be critical for maximizing the intrinsic energy storage capacities. The solvent-assisted cycloreversion is also used to accelerate the energy release from the emerging molecular scaffold.

Emerging solid-state cycloaddition chemistry for molecular solar thermal energy storage.

Recently discovered designs of solid-state molecular solar thermal energy storage systems are illustrated, including alkenes, imines, and anthracenes that undergo reversible [2 + 2] and [4 + 4] photocycloadditions for photon energy storage and release. The energy storage densities of various molecular designs, from 6 kJ mol-1 to 146 kJ mol-1 (or up to 318 J g-1), are compared and summarized, along with effective strategies for engineering their crystal packing structures that facilitate solid-state reactions. Many promising molecular scaffolds introduced here highlight the potential for achieving successful solid-state solar energy storage, guiding further discoveries and the development of new molecular systems for applications in solid-state solar thermal batteries.

Acid-sensitive photoswitches: towards catalytic on-demand release of stored light energy

Molecular solar thermal energy storage route using teralylenes with pyridine protonatable moieties for on-demand energy release achieved upon protonation with catalytic amounts of acid.

Site Selectivity of Peptoids as Azobenzene Scaffold for Molecular Solar Thermal Energy Storage.

Storing solar energy is a key challenge in modern science. MOlecular Solar Thermal (MOST) systems, in particular those based on azobenzene switches, have received great interest in the last decades. The energy storage properties of azobenzene (t1/2 <4 days; ΔH~270 kJ/kg) must be improved for future applications. Herein, we introduce peptoids as programmable supramolecular scaffolds to improve the energy storage properties of azobenzene-based MOST systems. We demonstrate with 3-unit peptoids bearing a single azobenzene chromophore that dynamics of the MOST systems can be tuned depending on the anchoring position of the photochromic unit on the macromolecular backbone. We measured a remarkable increase of the half-life of the metastable form up to 14 days at 20 °C for a specific anchoring site, significantly higher than the isolated azobenzene moiety, thus opening new perspectives for MOST development. We also highlight that liquid chromatography coupled to mass spectrometry does not only enable to monitor the different stereoisomers during the photoisomerization process as traditionally done, but also allows to determine the thermal back-isomerization kinetics.

Searching the Chemical Space of Bicyclic Dienes for Molecular Solar Thermal Energy Storage Candidates.

Photoswitches are molecular systems that are chemically transformed subsequent to interaction with light and they find potential application in many new technologies. The design and discovery of photoswitch candidates require intricate molecular engineering of a range of properties to optimize a candidate to a specific applications, a task which can be tackled efficiently using quantum chemical screening procedures. In this paper, we perform a large scale screening of approximately half a million bicyclic diene photoswitches in the context of molecular solar thermal energy storage using ab initio quantum chemical methods. We further device an efficient strategy for scoring the systems based on their predicted solar energy conversion efficiency and elucidate potential pitfalls of this approach. Our search through the chemical space of bicyclic dienes reveals systems with unprecedented solar energy conversion efficiencies and storage densities that show promising design guidelines for next generation molecular solar thermal energy storage systems.

Searching the Chemical Space of Bicyclic Dienes for Molecular Solar Thermal Energy Storage Candidates

AbstractPhotoswitches are molecular systems that are chemically transformed subsequent to interaction with light and they find potential application in many new technologies. The design and discovery of photoswitch candidates require intricate molecular engineering of a range of properties to optimize a candidate to a specific applications, a task which can be tackled efficiently using quantum chemical screening procedures. In this paper, we perform a large scale screening of approximately half a million bicyclic diene photoswitches in the context of molecular solar thermal energy storage using ab initio quantum chemical methods. We further device an efficient strategy for scoring the systems based on their predicted solar energy conversion efficiency and elucidate potential pitfalls of this approach. Our search through the chemical space of bicyclic dienes reveals systems with unprecedented solar energy conversion efficiencies and storage densities that show promising design guidelines for next generation molecular solar thermal energy storage systems.