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Photochem 2022, 2 696 is still a hot topic. In the following sections, the most relevant efforts to prepare suitable compounds according to these requirements will be discussed. For the scope of this review, we will describe the three types of systems that have been mainly used within the current framework of MOST technology: norbornadiene, azobenzene, and dihydroazulene derivatives [15–17] (Figure 1b). In addition, we will focus on the central role of the optical properties in the storage of solar energy. 2. Requisites for MOST Systems: Optical Properties The development of new MOST candidates is a challenging task, which has gained more attention and visibility in the last decades owing to the expectations raised by this technology. As mentioned above, the ideal compound for a MOST device is still unknown, so many different strategies combining experimental and computational tools are being used to assist in the molecular design. In order to maximize the efficiency of MOST systems, their optimization has received a great deal of attention; in light of this, the design of a suitable photoswitch must meet a large number of objectives to fulfil the ideal MOST system. Thus, the design criterion of a MOST system is subjected to several parameters involving both engineering and chemical challenges [18]. The first key step in the molecular solar thermal energy storage system is the absorp- tion of light by the parent molecule, which undergoes a reversible photoisomerization reaction to its corresponding metastable isomer. This photoisomer should be stable enough to store the chemical potential for varying periods of time, depending on the envisioned application. Then, this stored energy should be released when and where required, in the form of heat. For the initial photoisomerization part of the MOST cycle to be successful, the photoswitch pair needs to fulfil a long list of features, in some cases even contradictory [3]: • A large difference in the free energies of the parent molecule and its photoisomer, with a minimal increase in the molecular weight to maximize energy density [19,20]. • A moderately large kinetic barrier for back conversion [21,22]. • An absorption spectrum of the photoactive molecule matching the solar spectrum [22,23]. • A high quantum yield (Φ = 1) for the photogeneration of the metastable photoisomer. • A photochemically inactive (or non-absorbing) photoisomer. • Negligible degradation of both the photoactive molecule and its photoisomer after multiple cycles, especially moving towards higher temperatures. These are the major requirements that should be optimized to improve the perfor- mance of any potential candidate. Furthermore, when considering a MOST molecule in an integrated device, the use of environmentally friendly compounds and solvents is desired to minimize the risks in case of leaching or losses to its surroundings. In this review, we will focus on the optical properties. Thus, a more detailed explanation of some of the requirements related to the photochemistry of these compounds will be presented: the absorption spectrum (solar match), photoreaction efficiency (quantum yield), and energy storage capacity. 2.1. Solar Match The photochemical reaction from a low (parent molecule, photoactive molecule) to a high energy configuration (photoisomer) is the central focus of the photochemical part of the MOST cycle. For this transformation to take place, the main requirement is the absorption of energy supply from the sun. Hence, the ideal MOST systems should absorb the maximum number of photons in the UV and visible range of the solar spectrum, 300–700 nm, which corresponds to the maximum intensity of sunlight. Ideally, the absorption spectrum of the lower-energy isomer should overlap with the most intense region of the solar energy window (solar match) and preferably in the solar spectrum range between 340–540 nm, where the solar radiation is relatively high [22]. Moreover, it is desirable that no absorption overlap between the initial isomer and photoisomer exists to avoid a non-desirable photon absorption competition between the two states.PDF Image | Overview of Molecular Solar Thermal Energy Storage
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