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Photochem 2022, 2 698 torically, the compounds designed to be used in MOST systems can be grouped into two main types [2,16], depending on the photochemical transformation that takes place. In this sense, the mechanism of the photochemical process transforming sunlight into chemical energy can be an isomerization or a cycloaddition. Other types of photochemically induced intramolecular rearrangements were also studied. As an example of some more complex rearrangements, organometallic diruthenium fulvalene’s have also been considered [30]. According to the photochemical transformation involved, the systems based on an isomerization are typical examples of molecular photoswitches, like stilbenes [31], azoben- zenes, retinal-based photoswitches [32], or other less-known families like hydantoins [33]. The main problem behind using traditional cis-trans photoswitches is the typical small energy gap between the two isomers, producing a small amount of energy storage. This problem has been overcome by two different strategies. Firstly, stabilization of the E-isomer usually occurs when increasing electronic delocalization. Secondly, with a destabiliza- tion of the Z-isomer attributed to vicinal groups, steric interactions are incurred. Com- bining those strategies, some stilbene derivatives could be designed to reach an energy storage of 100 kJ/mol higher than the original unsubstituted stilbene molecule, reaching 105 kJ/mol [34]. Comparably, following the same strategies, retinal-like systems were postulated for this application too, with more modest energy storage capacities [35]. The employment of systems based on a photochemical cycloaddition typically has better properties in terms of energy storage, but their optical properties (absorption spectra) are usually less tuneable as absorption usually lies in the high-energy region of the UV spectra. The main exponent of this approach is the norbornadiene (NBD)–quadricyclane (QC) couple [36], which has been studied since the 1980s and nowadays is a focus of most of the efforts from the community. One of the first proposals using cycloaddition reactions was the use of anthracene derivatives, thanks to their well-known intermolecular [4 + 4] cycloaddition. These compounds also present some problems, as the absorption usually occurs below 300 nm, meaning a low efficiency exposed to solar radiation. This was partially solved by adding (a) bridge group(s) to link two anthracene moieties, but in this case, the efficiency decreased drastically [37]. Another cycloaddition system used is the pair based on dihydroazulene (DHA) and vinylheptafulvene (VHF) [38,39]. Unfortunately, the parent compounds in this couple present a small energy difference between isomers, plus the tunability of the optical properties has already been exhaustively explored [40]. Other systems such as ruthenium fulvalene complexes have also been proposed and studied but have been discarded for practical applications because of the low efficiency and high preparation costs [30,41]. In summary, many molecular systems have been studied along the years as poten- tial MOST candidates. In the following, we will focus our attention on the three most promising families of MOST molecules to date, namely, norbornadiene, azobenzenes, and dihydroazulenes. These three families combine relatively good (or tuneable) properties and are synthetically attainable. 3.1. Norbornadiene/Quadricyclane Couple Among the previously mentioned MOST systems under investigation, the most pro- foundly explored is without a doubt the NBD to QC isomerization. Even if the foundations of the MOST concept did not begin with the NBD/QC photoswitch, it is nowadays the main area of research in the field. These molecules have reached energy density values close to the maximum energy density limit of a solar thermal battery at 1 MJ/kg [42]. In contrast, the absorption of unsubstituted NBD is within the UVC range (less than 267 nm) and does not overlap with the solar spectrum, which begins at 340 nm [26]. The ideal absorption scenario for molecular solar thermal energy storage systems is to use solar radiation, which reaches the Earth’s surface at high intensities [43]. Thus, targeting a photoisomerization induced reaction in the 350–450 nm range is highly desirable. In designing new NBD/QC molecules, the difference in the absorption maxima between the NBD and QC molecules needs to be large enough to minimize spectral overlap [25,44],PDF Image | Overview of Molecular Solar Thermal Energy Storage
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