Efficient solar energy storage is a key challenge in striving toward a sustainable future. For this reason, molecules capable of solar energy storage and release through valence isomerization, for so‐called molecular solar thermal energy storage (MOST), have been investigated. Energy storage by photoconversion of the dihydroazulene/vinylheptafulvene (DHA/VHF) photothermal couple has been evaluated. The robust nature of this system is determined through multiple energy storage and release cycles at elevated temperatures in three different solvents. In a nonpolar 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 into 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 by using a flow chip under simulated sunlight and under outdoor conditions by using a parabolic mirror. Device experiments demonstrated a solar energy storage efficiency of up to 0.13 % in the chip device and up to 0.02 % in the parabolic collector. Avenues for future improvements and optimization of the system are also discussed.

been widely implemented [1] but longterm storage of the heat at higher temperatures remains a challenge, limiting broader applications of solar heating. [2,3] Solar thermal energy can be stored for example as thermal energy in rocks or hot water, in phase change materials, [4,5] or in thermochemical energy storage materials. [6] A particular challenge is to identify compact thermal storage solutions capable of operating in the medium temperature range (100-180 °C). [2] In this context, one possible solution is to use molecular photoswitches where the solar energy is stored in high energy photoisomers. [7] This way of storing solar energy has been referred to as molecular solar thermal (MOST) [8] energy storage or solar thermal fuels. [9] MOST systems are based on a parent molecule, which upon irradiation photoisomerizes to a highenergy isomer. The stored energy can be released on demand by applying heat or by using a catalyst to trigger the thermal back isomerization (Scheme 1). [7,10,11] Various molecular designs have been explored in this context, including ruthenium compounds, [8,12] azobenzene derivatives, [13][14][15][16][17][18] norbornadiene (NBD) derivatives, [10,11,19] dihydroazulenes, [20] and others. [21] The requirements for an efficient MOST system [7,10,22] can be summarized as: (i) good spectral overlap of parent molecule absorptions with the solar spectrum, (ii) minimal absorption of the high energy isomer in the solar spectrum, (iii) high quantum yield for the photoisomerization, (iv) high energy storage density, (v) high kinetic stability of the metastable photoisomer, and (vi) high cyclability. It is also fundamental to obtain neat liquid or solid functional MOST materials (depending on the desired applications), since dilution in solvents or solid matrix lowers the energy storage density. A closed MOST system has been previously described, [23] which involves cycles of an MOST fluid in a solar collector for the photoconversion, a storage tank, and a heat extractor part (see Scheme 1).NBD (1 in Scheme 1) and its derivatives undergo photoisomerization to the highly strained quadricyclane (QC, 2 in Scheme 1) upon irradiation with UV or visible light. [24] By molecular design, the system has been optimized toward MOST applications, and the best NBD derivatives fulfil several of the requirements for a functional system, such as high photoisomerization quantum yield, redshifted absorption, high energy storage densities, and very long half-life of the 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 ...

Invited for this month′s cover are the groups of Kasper Moth‐Poulsen at Chalmers University of Technology and Mogens Brøndsted Nielsen at University of Copenhagen. The cover image shows a conceptual graphic of molecular solar thermal energy storage system (MOST) for outdoor facilities. The Full Paper itself is available at 10.1002/cssc.201700679.

Molecular photoswitches can be used for solar energy storage through daily, weekly or seasonal energy storage cycles. The cover for article number https://doi.org/10.1002/aenm.201703401 by Kasper Moth‐Poulsen and co‐workers illustrates a vision for future implementation combining solar energy capture in a liquid molecule system, long term storage and the release of the stored energy for domestic heating applications.

The Cover picture shows an implementation of molecular solar thermal energy storage (MOST) technology on a large scale. It illustrates how the yellow‐colored dihydroazulene (DHA) is converted into red‐colored vinylheptafulvene (VHF) upon irradiation. The robust nature of this system was demonstrated through multiple energy storage‐and‐release cycles. As a potential candidate for the MOST concept, the photoswitch was evaluated both under simulated sunlight conditions as well as under natural outdoor conditions, demonstrating a possible avenue for future lab‐to‐site transfer. More details can be found in the Full Paper by Wang et al. on page 3049 in Issue 15, 2017 (DOI: 10.1002/cssc.201700679).