Energy generation and storage has become one of the major challenges in our society and are especially relevant for industry [1,2]^[1][2]. The current energy demand is continuously rising ^[3] each year by 1.3%, and this progression is expected to last at least until 2040 ^[4], even considering that many industries worldwide have been affected by COVID-19. According to the International Energy Agency, buildings are responsible for almost 30% of energy consumption and account for 28% of CO₂ emissions [4,5]^[4][5]. To avoid the environmental impact from conventional energy sources, the use of renewable electricity needs to augment considerably. However, wresearchers are not yet able to avoid our dependence on fossil fuels. Consequently, significant efforts to find better alternatives to generate and store energy are under exploration. This is especially relevant for solar energy use and storage ^[6], which has been envisioned as an abundant, clean, and promising energy source.

Using natural photosynthesis as a working model for solar energy use, scientists are designing and preparing chemical systems capable of capturing and storing solar energy. Nowadays, different alternatives to make use of sunlight are under research, including direct use of photonic solar power and heating capacity of solar radiation. The variability in solar income is a very significant drawback to solar energy, as the power of both types of energy (photonic and heating) is not constant during the four seasons of the year ^[7] and strongly depends on the weather and geographical factors. This non-constant power supply unequivocally demands a storage solution, which should allow wider usability under conditions such as night or winter. Consequently, different methodologies have been developed to exploit solar power such as underground solar energy storage (USES) and molecular solar thermal (MOST) systems.

The USES system mechanism consists of the storage of sun energy underground during summer months using a pile [8,9]^[8][9]. There are four basic types of USES systems: hot-water-thermal storage, borehole thermal storage, aquifer thermal storage, and water gravel pit storage ^[10]. This mechanism requires a plant of quite large dimensions, making it quite difficult to use this technique once the building has already been built ^[11]. Similarly, in these approaches, the thermal insulation requirements usually imply a challenge for long-term storage.

On the other hand, MOST technology has become a promising candidate to capture and store solar energy in a sustainable and efficient manner. These systems have been expanded significantly in the last decades ^[7], even though the first idea dates a while back ^[12]. The MOST approach is based on the storage of solar energy as chemical energy using a photoactive molecule, which, after being exposed to sunlight, isomerizes into a metastable high-energy photoisomer ^[13]. The release of chemical energy as heat can be performed during the back conversion step using an external stimulus (Figure 1a) either through heat, through a catalyst, or electrochemically ^[14].

Along the years, a large number of systems have been proposed as MOST candidates. There are at least six major requirements for a practical system, which makes the design of successful candidates a challenging task; this will be described further in the next section. Until now, none of the previously proposed compounds completely fulfils this list of requirements. Thus, the design and preparation of new alternatives to MOST technology is still a hot topic. In the following sections, the most relevant efforts to prepare suitable compounds according to these requirements will be discussed.

2. Requisites for MOST Systems: Optical Properties

The development of new MOST candidates is a challenging task, which has gained more attention and visibility in the last decades owing to the expectations raised by this technology. As mentioned above, the ideal compound for a MOST device is still unknown, so many different strategies combining experimental and computational tools are being used to assist in the molecular design. In order to maximize the efficiency of MOST systems, their optimization has received a great deal of attention; in light of this, the design of a suitable photoswitch must meet a large number of objectives to fulfil the ideal MOST system. Thus, the design criterion of a MOST system is subjected to several parameters involving both engineering and chemical challenges [18]^[16]. The first key step in the molecular solar thermal energy storage system is the absorption of light by the parent molecule, which undergoes a reversible photoisomerization reaction to its corresponding metastable isomer. This photoisomer should be stable enough to store the chemical potential for varying periods of time, depending on the envisioned application. Then, this stored energy should be released when and where required, in the form of heat. For the initial photoisomerization part of the MOST cycle to be successful, the photoswitch pair needs to fulfil a long list of features, in some cases even contradictory ^[3]:

• A large difference in the free energies of the parent molecule and its photoisomer, with a minimal increase in the molecular weight to maximize energy density [19,20]^[17][18].
• A moderately large kinetic barrier for back conversion [21,22]^[19][20].
• An absorption spectrum of the photoactive molecule matching the solar spectrum [22,23]^[20][21].
• A high quantum yield (Φ = 1) for the photogeneration of the metastable photoisomer.
• A photochemically inactive (or non-absorbing) photoisomer.
• Negligible degradation of both the photoactive molecule and its photoisomer after multiple cycles, especially moving towards higher temperatures.

These are the major requirements that should be optimized to improve the performance of any potential candidate. Furthermore, when considering a MOST molecule in an integrated device, the use of environmentally friendly compounds and solvents is desired to minimize the risks in case of leaching or losses to its surroundings. In thiRes review, weearcgers will focus on the optical properties. Thus, a more detailed explanation of some of the requirements related to the photochemistry of these compounds will be presented: the absorption spectrum (solar match), photoreaction efficiency (quantum yield), and energy storage capacity.

2.1. Solar Match

The photochemical reaction from a low (parent molecule, photoactive molecule) to a high energy configuration (photoisomer) is the central focus of the photochemical part of the MOST cycle. For this transformation to take place, the main requirement is the absorption of energy supply from the sun. Hence, the ideal MOST systems should absorb the maximum number of photons in the UV and visible range of the solar spectrum, 300–700 nm, which corresponds to the maximum intensity of sunlight. Ideally, the absorption spectrum of the lower-energy isomer should overlap with the most intense region of the solar energy window (solar match) and preferably in the solar spectrum range between 340–540 nm, where the solar radiation is relatively high [22]^[20]. Moreover, it is desirable that no absorption overlap between the initial isomer and photoisomer exists to avoid a non-desirable photon absorption competition between the two states. Most of the basic cores of the photoswitches reported to date do not show wavelengths going far beyond 350 nm (for instance, parent norbornadiene has a maximum at 236 nm and some ruthenium derivatives absorb at 350) [24]^[22]. This is a significant drawback as the solar photons’ flux at wavelengths below 330 nm is quite low. In this regard, both experimental [25,26]^[23][24] and computational [27]^[25] progress has been made in providing functionalized photoswitches that absorb at larger wavelengths. The most used and successful chemical strategy to shift the absorption of MOST compounds toward higher wavelengths is by creating a ‘push–pull’ effect through the introduction of donor–acceptor substituents and increasing the molecule’s conjugated π-system. Preferably, low molecular weight electron donor and acceptor groups are prominent targets for generating relevant photoswitches, as they cause a lower impact on the energy density (affected by both the difference in energy between isomers and the molecular weight). However, beyond the stored energy, the chemical modification of photoswitches may also negatively affect other relevant properties, making it clear that the optimization of a set of molecules for MOST applications is extremely challenging.

2.2. Quantum Yield

Once the parent molecule absorbs a photon from sunlight, the excitation from the ground state (S₀) to the excited state (S_n) takes place. Subsequently, a certain number of molecules will undergo photo-conversion, but a fraction could undergo relaxation, returning to their initial state. To quantify the fraction of molecules effectively performing the photoconversion from the photoactive molecule to the photoisomer, the quantum yield is measured. This dimensionless number provides the probability of a parent molecule to furnish the metastable high-energy photoisomer per absorbed photon. From an efficiency perspective, the photo-conversion that leads to the high isomer needs to be as high as possible, being close to unity if possible. This should allow for an efficient conversion of solar energy into chemical energy, hence avoiding other competitive processes such as radiative, non-radiative, or quenching.

2.3. Storage Energy Density

While it is not strictly a photochemical property, another crucial concern in MOST systems is the energy storage. MOST technology is designed for generating the greatest possible increase in temperature after releasing the stored chemical energy in the photoisomer as heat. In this way, the key property to achieve this goal is the enthalpy difference (ΔH) between photoisomers. This means that, the bigger the energy difference between the (not charged) metastable photoisomer and its parent state, the larger the energy storage density that will be accumulated in the system. As a rule of thumb, MOST systems should provide at least 0.3 MJ/kg to be of practical use, prior to the subsequent heat release. Then, heat could be released using an external stimulus like a thermal increment or via a catalyst. Thus, the photoisomer should not undergo back-conversion quickly at room temperature in order to store the energy for hours, days, or months (storage time) depending on the target application. Even if Ithis review is focused on the photochemical aspects of the MOST technology, it is also is relevant to mention that alternative cooling and heating methods are available. For instance, the use of phase transition materials and water adsorption in zeolites has been already commercialized [28,29]^[26][27].

3. Photoswitches Used in MOST Technology

As a brief introduction to the state-of-the-art of the historic development of MOST candidates, very different sets of families were considered at some point. However, most of them were discarded at a relatively early stage because of some practical reasons or flaws. Considering the most explored molecular systems, the main parts of them are photoswitches, as explained above. This is partly because of the considerable overlap between the requirements for photoswitches in general and the compounds used in the MOST concept (absorption, high quantum yields, photostability, and robustness). Historically, the compounds designed to be used in MOST systems can be grouped into two main types [2,16]^[2][28], depending on the photochemical transformation that takes place. In this sense, the mechanism of the photochemical process transforming sunlight into chemical energy can be an isomerization or a cycloaddition. Other types of photochemically induced intramolecular rearrangements were also studied. As an example of some more complex rearrangements, organometallic diruthenium fulvalene’s have also been considered [30]^[29]. According to the photochemical transformation involved, the systems based on an isomerization are typical examples of molecular photoswitches, like stilbenes [31]^[30], azobenzenes, retinal-based photoswitches [32]^[31], or other less-known families like hydantoins [33]^[32]. The main problem behind using traditional cis-trans photoswitches is the typical small energy gap between the two isomers, producing a small amount of energy storage. This problem has been overcome by two different strategies. Firstly, stabilization of the E-isomer usually occurs when increasing electronic delocalization. Secondly, with a destabilization of the Z-isomer attributed to vicinal groups, steric interactions are incurred. Combining those strategies, some stilbene derivatives could be designed to reach an energy storage of 100 kJ/mol higher than the original unsubstituted stilbene molecule, reaching 105 kJ/mol [34]^[33]. Comparably, following the same strategies, retinal-like systems were postulated for this application too, with more modest energy storage capacities [35]^[34]. The employment of systems based on a photochemical cycloaddition typically has better properties in terms of energy storage, but their optical properties (absorption spectra) are usually less tuneable as absorption usually lies in the high-energy region of the UV spectra. The main exponent of this approach is the norbornadiene (NBD)–quadricyclane (QC) couple [36]^[35], which has been studied since the 1980s and nowadays is a focus of most of the efforts from the community. One of the first proposals using cycloaddition reactions was the use of anthracene derivatives, thanks to their well-known intermolecular [4 + 4] cycloaddition. These compounds also present some problems, as the absorption usually occurs below 300 nm, meaning a low efficiency exposed to solar radiation. This was partially solved by adding (a) bridge group(s) to link two anthracene moieties, but in this case, the efficiency decreased drastically [37]^[36]. Another cycloaddition system used is the pair based on dihydroazulene (DHA) and vinylheptafulvene (VHF) [38,39]^[37][38]. Unfortunately, the parent compounds in this couple present a small energy difference between isomers, plus the tunability of the optical properties has already been exhaustively explored [40]^[39]. Other systems such as ruthenium fulvalene complexes have also been proposed and studied but have been discarded for practical applications because of the low efficiency and high preparation costs [30,41]^[29][40]. In summary, many molecular systems have been studied along the years as potential MOST candidates. In the following, wresearchers will focus ourthe attention on the three most promising families of MOST molecules to date, namely, norbornadiene, azobenzenes, and dihydroazulenes. These three families combine relatively good (or tuneable) properties and are synthetically attainable.