Photoswitchable Molecular Rings for Solar-Thermal Energy Storage

Abstract

Solar-thermal fuels reversibly store solar energy in the chemical bonds of molecules by photoconversion, and can release this stored energy in the form of heat upon activation. Many conventional photoswichable molecules could be considered as solar thermal fuels, although they suffer from low energy density or short lifetime in the photoinduced high-energy metastable state, rendering their practical use unfeasible. We present a new approach to the design of chemistries for solar thermal fuel applications, wherein well-known photoswitchable molecules are connected by different linker agents to form molecular rings. This approach allows for a significant increase in both the amount of stored energy per molecule and the stability of the fuels. Our results suggest a range of possibilities for tuning the energy density and thermal stability as a function of the type of the photoswitchable molecule, the ring size, or the type of linkers. SECTION:Energy Conversion and Storage; Energy and Charge Transport E fficient utilization of the sun as a renewable and clean energy source is one of the greatest goals and challenges of this century due to the increasing demand for energy and its environmental impact. Numerous strategies exist to convert sunlight into useful forms of energy, including photocatalytic processes, artificial photosynthesis, 1,2 photothermal power plants, 3 and photovoltaic applications. 4 An alternative strategy to these is to convert and store the sun's energy directly in the chemical bonds of metastable photoisomers of suitable molecular systems. The stored energy can then be released as heat on demand by an external trigger. Ideally both the photisomerization and heat release reactions reversibly occur in a closed-cycle without changing the chemical composition. 5The clear advantage of such an approach is that the same material both converts and stores the sun's energy, providing a rechargeable fuel that can be safely transported and used ondemand; the materials used could, in principle, be cheap, nontoxic and abundant, and the cycle can be repeated thousands of times without any emission or waste.The idea of storing solar energy in chemical bonds was first introduced with the photoisomerization reactions of organic molecules (such as norbornadiene⇌quadricyclane 6,7 and the trans⇌cis isomers of azobenzene 8 ), although the concept was generally dismissed as unpractical 7 due to a number of reasons including degradation, instability of the photoinduced isomer (limited half-life or shelf life), low energy density, and cost. As such, the majority of recent experimental studies for such materials have focused on their photoswitchable properties, 9 as opposed to energy storage. The Ru-based organometallic complex (tetracarbonyl−diruthenium fulvalene) 10 has shown promise as a solar thermal fuel due to its high cyclability without degradation; however, substitutions for Ru must be found to reduce costs, and the volumetric energy density increased for practical applications. 11,12 Recently, a new s...

Azopolymer‐based micro‐ and nanopatterning for photonic applications

“…32,33 An intriguing prospect would be to use the light-controlled macroscopic actuation of azopolymers to study distancedependent physical phenomena, such as Casimir effect 229 and transition between the weak and strong electromagnetic interaction regimes. 230 Finally, the possibility for photonenergy harvesting 6,231 and optical-to-mechanical energy conversion 232 can offer significant advantages in energy storage and, for example, microrobotics, that warrant much further study. Azobenzene has many faces, but many of them are yet to be revealed.…”

Optimal Size Matching and Minimal Distortion Energy: Implications for Natural Selection by the Macrocycle of the Iron Species in Heme

“…Structural distortion plays a vital role in a variety of living processes and many coordination reactions . Processes induced by structural distortion include energy storage in photoactive chromophores, energy transfer in photoswitchable molecular rings, oxygen affinity in heme, exciton coupling effects in phthalocyanine, and spin‐crossover behavior in isomorphous complexes . A general principle is that in such processes the distortion energy should be minimized .…”

Intermolecular London Dispersion Interactions of Azobenzene Switches for Tuning Molecular Solar Thermal Energy Storage Systems

“…Feng and also later Grossmann used nanotemplating as a tool. Another approach is based on incorporating the azobenzene into macrocyclic structures to utilize strain energy in the ( Z )‐isomer . Alternatively, azobenzenes can be connected to carbon nanotubes to improve the storage energy …”

“…Another approach is based on incorporating the azobenzene into macrocyclic structures to utilize strain energy in the (Z)isomer. [21,22] Alternatively, azobenzenes can be connected to carbon nanotubes to improve the storage energy. [23] In contrast to altering the azobenzene moiety itself the interaction of the individual molecules with each other can also contribute to increase the overall storage energy ( Figure 1).…”