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Photochem 2022, 2 706 π–π* band in the UV region and a weak n–π* band in the yellow visible region. (ii) Aminoa- zobenzenes, which can be ortho- or para-substituted with an electron-donating group (EDG), [(para- or ortho-(EDG)-C6H4-N=N-Ar)]. The π–π* and n–π* bands are very close or even overlap in the UV/Vis region; they are typically orange. (iii) Pseudo stilbenes, which have an electron-donating and an electron-withdrawing group (EWG) at the para position of the phenyl rings, thus being a push–pull system [(p-(EDG)-C6H4-N=N-C6H4-p-(EWG)]. They are typically red and the absorption band corresponding to the π–π* electronic tran- sition undergoes a bathochromic shift, overlapping bands or even changing the order of appearance with the corresponding band of the n–π* electronic transition. Figure 8. (a) Azobenzene classification by Rau et al. [77] helps describe the optical and physical properties of this family of compounds (i.e., their color). (b) Typical example of absorption spectra of the different azobenzene derivatives. Pseudo-stilbenes have a highly asymmetric electron distribution alongside the molecule, which turns into a large molecular dipole and anisotropic optical properties. In addition, this class also presents the best photo-response, which is mainly caused by an overlapped absorption spectrum of the E and Z forms, thus reaching a mixed photo-stationary state where the trans and cis isomers are continuously interconverting. Therefore, for pseudo- stilbenes, a single visible light wavelength can induce both forward and reverse isomeriza- tion. Thermal relaxation from cis to trans spans from milliseconds to seconds. On the other hand, in the other two classes of compounds, the absorption spectra usually do not overlap, meaning that two different wavelengths are needed to switch between the cis and trans forms, which is ideal for photoswitchable materials. Particu- larly, azobenzene-type molecules are proven to isomerize back from cis into trans isomers very slowly through thermal relaxation when bulky substituents are introduced into the structure, thus providing a thermal relaxation time from cis to trans isomers of hours for the azobenzene-type molecules and minutes for the amino-azobenzene-type molecules. Recently, it has even been proven that some stable cis-isomers can be isolated using the proper solvent, which leads to a stable two-state photoswitchable system. Even if the optical properties of azobenzenes discussed in previous paragraphs are quite interesting for MOST applications, the applicability of these compounds is somewhat limited by the energy density. The energy difference of a typical azobenzene is usuallyPDF Image | Overview of Molecular Solar Thermal Energy Storage
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