Engineering of Norbornadiene/Quadricyclane Photoswitches for Molecular Solar Thermal Energy Storage Applications

Abstract

Conspectus Renewable energy resources are mostly intermittent and not evenly distributed geographically; for this reason, the development of new technologies for energy storage is in high demand. Molecules that undergo photoinduced isomerization reactions that are capable of absorbing light, storing it as chemical energy, and releasing it as thermal energy on demand are referred to as molecular solar thermal energy storage (MOST) or solar thermal fuels (STF). Such molecules offer a promising solution for solar energy storage applications. Different molecular systems have been investigated for MOST applications, such as norbornadienes, azobenzenes, stilbenes, ruthenium derivatives, anthracenes, and dihydroazulenes. The polycyclic strained molecule norbornadiene (NBD), which photoconverts to quadricyclane (QC), is of great interest because it has a high energy storage density and the potential to store energy for a very long time. Unsubstituted norbornadiene has some limitations in this regard, such as poor solar spectrum match and low quantum yield. In the past decade, our group has developed and tested new NBD systems with improved characteristics. Moreover, we have demonstrated their function in laboratory-scale test devices for solar energy harnessing, storage, and release. This Account describes the most impactful recent findings on how to engineer key properties of the NBD/QC system (photochemistry, energy storage, heat release, stability, and synthesis) as well as examples of test devices for solar energy capture and heat release. While it was known that introducing donor–acceptor groups allows for a red-shifted absorption that better matches the solar spectrum, we managed to introduce donor and acceptor groups with very low molecular weight, which allowed for an unprecedented solar spectrum match combined with high energy density. Strategic steric hindrance in some of these systems dramatically increases the storage time of the photoisomer QC, and dimeric systems have independent energies barriers that lead to an improved solar spectrum match, prolonged storage times, and higher energy densities. These discoveries offer a toolbox of possible chemical modifications that can be used to tune the properties of NBD/QC systems and make them suitable for the desired applications, which can be useful for anyone wanting to take on the challenge of designing efficient MOST systems. Several test devices have been built, for example, a hybrid MOST device that stores sunlight energy and heat water at the same time. Moreover, we developed a device for monitoring catalyzed QC to NBD conversion resulting in the possibility to quantify a significant macroscopic heat generation. Finally, we tested different formulations of polymeric composites that can absorb light during the day and release the energy as heat during the night for possible use in future window coating applications. These lab-scale realizations are formative and contribute to pushing the field forward towar...

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

“…With t 1/2 (25 °C) between 103 and 611 days, all the QCs reported here are suitable for long‐term energy storage. Compared with the photochemical characteristics of other literature‐known NBD derivatives, our compounds are highly competitive; in particular, NBD5 combines an exceptional absorption profile with high thermal stability [11a] …”

Photoisomerization Efficiency of a Solar Thermal Fuel in the Strong Coupling Regime

“…The norbornadiene–quadricyclane (NBD–QC) photoswitch couple has been applied in molecular logics, [ 16 ] molecular electronics, [ 17 ] and molecular thermal energy storage schemes. [ 18 ] After excitation, NBD converts into its QC isomer by an intramolecular [2 + 2] cycloaddition (Figure 1c). The ring tension in QC leads to a higher ground state energy compared to NBD, thus, QC is meta‐stable and converts back to NBD thermally.…”

“…Therefore, large efforts have been made to synthesize derivatives showing a red shift in their absorption properties. [ 18 ] The most successful method involves introducing electron donating and electron withdrawing substitutes, resulting in a push‐pull conjugated system. [ 18 ] However, by shifting the absorption further using a strong push‐pull system, the thermal stability of the QC state in many cases becomes too low for the couple to be of practical use.…”

“…[ 18 ] The most successful method involves introducing electron donating and electron withdrawing substitutes, resulting in a push‐pull conjugated system. [ 18 ] However, by shifting the absorption further using a strong push‐pull system, the thermal stability of the QC state in many cases becomes too low for the couple to be of practical use. [ 20 ] While structural modifications of the system have shown that the thermal stability can be improved, [ 19 ] it is very intriguing to investigate if the thermal stability can be enhanced by other approaches.…”

Multifactor theoretical modeling of solar thermal fuels built on azobenzene and norbornadiene scaffolds

“…Finally, the energy barrier for the reverse isomerization must result in a half-life for the metastable conguration that lasts long enough for reliable storage, yet not be great enough to prohibit efficient reclamation of the chemical energy on demand by triggering the reverse isomerization either thermally or catalytically. 14 Two classes of STF candidates that have received considerable attention in the literature are the azobenzenes [15][16][17][18] and the norbornadiene-quadricyclane (NBD-QC) system, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] shown in Fig. 2.…”