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Photochem 2022, 2 711 The modulation on the stability of VHF and the back reversion speed can be controlled by the modification of positions 2, 3, and 7; herein, the effect of donor- and electron- withdrawing groups can drastically change some properties. The inclusion of a donor group at C2 and an acceptor group at C3 or C7 have some impact, increasing the lifetime of these derivatives [100,101]. The addition of electron- withdrawing groups (EWGs), like cyano, in position 7 can increase the half-life of VHF by up to six times, in some cases reaching an exceptionally long-lived VHF in acetonitrile. This behavior can be explained by the stabilization of the VHF form, increasing the energy needed to initiate the back-reaction, but, in contrast, having a low impact on the energy storage capacity. The inclusion of an amino group in position 3 yields a hydrogen bond between the amino and the electron-withdrawing groups (CN). This seems to stabilize the DHA system, increasing the storage energy, also blocking VHF in the s-cis-VHF conformer. This means that the s-cis-VHF becomes the most stable isomer of VHF. Thus, geometrically, it is more similar to the intermediate and, as expected, the back-reaction barrier becomes even lower. This behavior reduces its usability because of the fast reversion achieved despite the higher energy stored. In contrast, the addition of a donor group (amino) on position 3 and an acceptor group (NO2) at C2 produces an increase in the ∆G of the back-reaction, also lengthening the lifetime of the VHF form [40]. Some other modifications, like the condensed effect of adding a 9-anthryl group in position 2 and 3, were found to increase the storage density to 0.38 MJ/kg, but this facilitates the VHF-to-DHA ring closure in a few seconds [46]. The addition of bulky groups on the ortho position of the phenyl group in C2 can stabilize the cis conformer of VHF, increasing the energetic barrier of the back-reaction [102]. The quantum yields of photoisomerization for these ortho substituted derivatives range in acetonitrile close to DHA (0.55) values, thus improving the lifetime without perturbing the photoisomerization rate. This effect is only with an iodine atom; if the size of the added group increases, this effect is maximized, reaching a 60 times longer half-life and quantum yields slightly higher than DHA of ca. 0.6–0.7. The last modification that we will comment about is the preparation of multi-switcher devices; on these, the combination of two DHA moieties, connected through a bisacetylene bridge, can differ too much depending on the substitution in ortho, meta, and para, ranging from an increase in the lifetime from a few hours to a couple of days [102]. In addition, combinations with other MOST candidates were attempted, offering some promising results, especially the increase in storage density due to the charge of two molecules, and in case of DHA-NBD, the combined absorption is notably red-shifted and the spectral overlap is decreased [103]. Moreover, the use of an azobenzene derivative together with phenylene bridge bis-DHA moieties, all included in a macrocyclic structure, can yield a modification of the DHA isomerization because of the predominant azobenzene isomerization found. This can limit the possibilities of combining azobenzenes and DHA moieties [104]. A principal outlook towards the use of DHA systems presents a few advantages, such as efficient isomerization and good optical properties, as well as a series of drawbacks including the modest energy storage and short lifetimes. 4. Conclusions In this review, we have covered the three more relevant families of compounds that are under investigation as MOST systems, norbornadiene, azobenzene, and dihydroazulene derivatives. Every set of compounds has its own drawbacks and strengths, which should be carefully considered for any specific application. However, it is also relevant to recognize that we are still far from finding the ideal MOST candidate. The mentioned improvements in the molecular design constitute the basis for the future development of these systems. In addition, the use of molecular modelling and machine learning strategies can provide a fast and valuable input in this way [105]. A general increase in the performance of thePDF Image | Overview of Molecular Solar Thermal Energy Storage
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