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Photochem 2022, 2 708 storage lifetimes (t1/2 = 5408 h) [85,86]. In view of these results, different experimental groups carried out the synthesis of the photoswitches, obtaining good results [87,88]. Specif- ically, they obtained molecules with 11–12 carbon atoms per azobenzene and the hydrogen bond stabilization of the azobenzenes was confirmed by NMR, FT-IR, and DFT studies. In a different approach, the addition of azobenzene to macrocycles [89] has also been suggested to improve energy storage capabilities [90]. The use of rings formed by azobenzenes connected through a suitable linker agent increase the barrier of the back reaction (Figure 9c). In this approach, the formation of the macrocycle contributes to increasing the rigidity of the system. In addition, it is possible to add functional groups to the macrocycle, as done in the graphene, allowing to establish hydrogen bonds with the aim of obtaining longer lifetimes for the energy storage. Even if azobenzene-functionalized CNTs have increased the energy density, these systems cannot be deposited into uniform films. To avoid this problem, an azopolymer was prepared to act as solid-state solar-thermal fuel (STF) [91]. This novel design allows to prepare uniform films. This device allowed to increase the temperature of the heat generated in the back conversion by up to 180 ◦C [86]. Following this breakthrough, different studies have been conducted on azopolymers [92,93] because of the ease of application of the photoswitches in large areas. Additionally, different studies attempted to combine the rigidity obtained by coupling a polymer and the addition of functional groups to generate non-covalent interactions to the azobenzene, with the aim of stabilizing the azopolymers. Non-covalent interactions have also demonstrated an increase in the lifetime of the stored energy, such as π–π stacking and hydrogen bonds. Taking advantage of these interactions, a new mechanism to synthesize azopolymers controlling the thickness and film morphology by electrodeposition was described. Unfortunately, when the substitution of the polymers is modified, the energy storage is greatly affected [94]. It is known that preparing azobenzenes with very bulky substituents increases the rigidity and, therefore, the lifetime of energy storage. However, different and simpler systems continued to be sought in parallel. In 2017, Grossman and co-workers demon- strated that it was not necessary to use templates or even polymers to achieve SFT materials with high-energy density and thermal stability. In this study, they synthesized various azobenzenes substituted with bulky aromatic rings, obtaining an enthalpy difference be- tween the cis and trans isomers comparable to the unsubstituted azobenzene. A critical factor to improve the azobenzene properties was to distort the molecule to avoid planarity, demonstrating that using small molecules made it possible to generate thin films with excellent charging and cycling properties [95]. Lately, a novel approach implying the use of azoheteroarene photoswitches has been explored. In various experimental and computational studies, it was discovered that chang- ing the type of heteroaromatic ring or the position of the heteroatom with respect to the azo group had a crucial effect on the photoswitching properties [35,96]. Using this approach, it was possible to prepare molecules with a half-life of days or even years, providing an excellent alternative to use these compounds in a MOST system. The increase in the half- lifetime in the photoswitches is due to the formation of an intramolecular hydrogen bond affecting the energy difference between isomers, and thus the energy storage (Figure 9d). However, not all studied azoheteroarenes absorb in the visible region, making their use for energy storage problematic. Again, the long list of features that a system should fulfil to be of practical use in the MOST technology makes it extremely difficult to select the ideal candidate. However, the ever-growing list of available azoheteroarenes turns these compounds into excellent candidates to absorb and store sunlight in MOST devices. 3.3. Dihdroazulene–Vinylheptafluvene (DHA–VHF) Another system widely studied in the context of MOST applications is the dihdroazu- lene/vinylheptafulvene (DHA/VHF) couple. While these compounds have some practical issues that hamper their applied use, they also feature some interesting properties. The optical properties of this system imply a DHA absorption of ca. 350 nm and a band atPDF Image | Overview of Molecular Solar Thermal Energy Storage
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