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Photochem 2022, 2 704 The differences in spectroscopical properties for the cis and trans isomers and the distribution of excited state electronic levels allow them to undergo isomerization upon irradiation. This photochemical interconversion usually ends up providing a varying mixture of cis and trans isomers on the photostationary states (PSS). The ratio of isomers in the mixture depends on the substitution of the azobenzene core and the wavelength of irradiation. In turn, the excitation wavelength at which this process takes place depends on the nature of the substituents on the aryl groups of the azobenzene; although, usually, in the trans to cis isomerization, this process is promoted by wavelengths around 320–380 nm, while exposition to 400–450 nm wavelengths typically favors cis to trans isomerization. This reversion can also be induced thermally. Although both photochemical conversions take place in the order of picoseconds, the cis to trans thermal relaxation is in the order of seconds or even hours and can be tuned by substitution. Although several mechanistic studies have focused on the isomerization mechanisms of azobenzene, in fact, the effect of the substituents on the phenyl rings as well as the influ- ence of other parameters yields a complex mechanism [73–75]. In fact, several mechanisms of the isomerization of the azobenzene and its derivatives can take place depending on the nature of the substituents and the reaction conditions (see Figure 6): (i) The inversion of one of the N–C bonds corresponds to an in-plane inversion of the NNC angle between the azo bond and the first carbon of the benzene ring (angle Φ), reaching 180◦, while the angle of CNNC remains fixed at 0◦. (ii) Through the rotation of the N=N double bond. This mechanism is similar to the one in the stilbene isomerization [73]. The rotational pathway starts with the breakage of the N=N p-bond, thus becoming an N–N bond. After that, there is an out-of-plane rotation of the CNNC dihedral (φ) angle, while the CNN angle remains at 120◦. (iii) Through the concerted inversion [76], where there is a simultaneous increase in the NNC angles up to 180◦. (iv) Lastly, through inversion-assisted rotation mechanism, which implies changes in both CNNC and NNC angles at the same time. Figure 6. Schematic view of four different isomerization mechanisms of azobenzene described in the literature.PDF Image | Overview of Molecular Solar Thermal Energy Storage
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