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.

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

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...

Triplet-triplet annihilation photon upconversion (TTA-UC) can, through a number of energy transfer processes, efficiently combine two low frequency photons into one photon of higher frequency. TTA-UC systems consist of one absorbing species (the sensitizer) and one emitting species (the annihilator). Herein, we show that the structurally similar annihilators, 9,10-diphenylanthracene (DPA, 1), 9-(4-phenylethynyl)-10-phenylanthracene (2) and 9,10-bis(phenylethynyl)anthracene (BPEA, 3) have very different upconversion efficiencies, 15.2 ± 2.8%, 15.9 ± 1.3% and 1.6 ± 0.8%, respectively (of a maximum of 50%). We show that these results can be understood in terms of a loss channel, previously unaccounted for, originating from the difference between the BPEA singlet and triplet surface shapes. The difference between the two surfaces results in a fraction of the triplet state population having geometries not energetically capable of forming the first singlet excited state. This is supported by TD-DFT calculations of the annihilator excited state surfaces as a function of phenyl group rotation. We thereby highlight that the commonly used "spin-statistical factor" should be used with caution when explaining TTA-efficiencies. Furthermore, we show that the precious metal free zinc octaethylporphyrin (ZnOEP) can be used for efficient sensitization and that the upconversion quantum yield is maximized when sensitizer-annihilator spectral overlap is minimized (ZnOEP with 2).

Norbornadiene-quadricyclane (NBD-QC) photoswitches are candidates for applications in solar thermal energy storage. Functionally, they rely on an intramolecular [2+2] cycloaddition reaction, which couples the S landscape on the NBD side to the S landscape on the QC side of the reaction and vice-versa. This commonly results in an unfavourable correlation between the first absorption maximum and the barrier for thermal back-conversion. This work demonstrates that this correlation can be counteracted by using steric repulsion to hamper the rotational motion of the side groups along the back-conversion path. It is shown that this modification reduces the correlation between the effective back-conversion barrier and the first absorption maximum and also increases the back-conversion entropy. The resulting molecules exhibit exceptionally long half-lives for their metastable forms without significantly affecting other properties, most notably solar spectrum match and storage density.