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Photochem 2022, 2 709 480 nm for VHF, which also includes an isosbestic point at 400 nm with a very low overlap between both species. The photoisomerization quantum yield is around 0.6, which could be considered a good value for practical applications, although clearly lower than the better examples of NBD/QC, but by far better in comparison with NBD (0.05) or azobenzene (0.33). However, the main drawback for this couple is the short half-life time of parent VHF, being only 10 h at 25 ◦C. This should rule out its use for a long-term storage application, but others implying daily cycles (as charging during the day and releasing at night) can be considered as an alternative. On the other hand, it has shown some promising features in certain application tests. For instance, it has been used on solar and fluidic devices to provide an efficiency of up to 0.13% in non-laboratory experiments, considering the total solar income harvested. In addition, DHA/VHF also has a very high robustness against degradation, featuring less than 0.01% degradation in 70 irradiation back conversion cycles in toluene solutions [2,17]. Another drawback of this system is the low-energy storage capacity of DHA/VHF, which present an energy difference of ca. 28 kJ/mol, which is quite modest in comparison with NBD derivatives. The general energy landscape of this couple can be seen in Figure 10. Upon light absorption, DHA isomerizes to VHF through a photoinduced carbon–carbon ring-opening reaction. In the excited state, DHA planarizes, allowing the stereochemical conditions to open the ring. This is favored by an increased steric hindrance due to the electronic reorganization. After the photochemical step, two possible isomers can be obtained. The initially formed cis-VHF can further evolve to the thermodynamically stable trans-VHF [38], with both species being in equilibrium through a small energy barrier [97]. This behaviour can facilitate an early back reversion to DHA, decreasing the lifetime of the photoisomer and affecting the energy density at the same time. In addition, the enthalpy of the photoreaction was measured to be ca. 35.2 kJ/mol in a vacuum [2]. Figure 10. Dihydroazulene (DHA)/vinylhetafulvene (VHF) couple and potential energy landscape. Adapted from Nielsen et al. [40]. Another relevant point is the large solvent effect that has been observed for the DHA/VHF couple. The photoreaction from DHA to VHF is faster in polar solvents than in apolar ones [98]. In contrast, the robustness through different irradiation conditions decreases notably in polar solvents (0.18–0.28%) compared with in apolar ones like toluene (0.01%). From these studies, some differences in the kinetic parameters have been invoked to explain the stabilization of charge-separated species, which acts as an intermediate during the photochemical transformation [39,98]. On the other hand, DHA is 10 times more soluble in toluene (apolar and aprotic) than in ethanol (polar and protic) and the photoisomerization quantum yield is larger (0.6) in toluene than in ethanol (0.5), being intermediate in acetonitrile (0.55). To sum up these effects, the more polar the solvent,PDF Image | Overview of Molecular Solar Thermal Energy Storage
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