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Photochem 2022, 2 701 As mentioned previously, it is also crucial to control the efficiency of the photochemical process, which starts with the absorption of a photon by NBD. The incoming photon can induce an electronic transition from the minimum in the ground state to the S1 state, causing a [2 + 2] intramolecular cycloaddition [51]. Under natural solar irradiation conditions, this photoconversion does not take place in the parent NBD, as just a few photons below 300 nm arrive at sea level. Therefore, the unsubstituted NBD/QC couple is chemically inactive to sunlight. Furthermore, in the event that the NBD absorbs a proper solar radiation, just a few molecules will undergo photoisomerization because of the quantum yield of this system being quite low (φ = 0.05). In consideration of these requirements, the NBD/QC system has been chemically modified in order to increase the quantum yield [18] as well as red-shift the wavelength of absorption. As shown in Figure 3, the introduction of donor–acceptor groups increases the quan- tum yield and makes the system capable of absorbing natural solar irradiation above 400 nm. Thus, when the incoming photon is absorbed by NBD, the photoisomerization transformation starts through a S0–S1 transition. The molecules will then undergo relax- ation in the excited state potential energy surface to reach the minimum energy conical intersection point (MECI) S0/S1, leading to the photoisomer QC [24]. In the heat release step, the thermodynamic driving force of the process (∆Hisom = 372 kJ/mol) pushes QC in forming the less-strained geometry (NBD), based on the cleavage of two single bonds of the four-membered ring and the transformation of the remaining bonds to double bonds, as seen in Figure 4. In this stage, the reaction releases a high amount of energy because of the high-standard enthalpy of the reversion of QC to NBD. This energy also depends on the gravimetric energy density (MJ/kg), hence systems with smaller molecular weights will hold comparatively greater energy densities. Thus, cyano-substituted NBD deriva- tives have been computational and experimentally studied as they show attractive energy densities (0.4–0.6 MJ/kg) [25]. Figure 3. Summary of the effect of electron donor–acceptor groups on the quantum yield and on the onset of the absorption wavelength. Ar1 means a phenyl group and Ar2 means a p-methoxyphenyl group. Data collected from [18,52–54].PDF Image | Overview of Molecular Solar Thermal Energy Storage
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