Molecular solar–thermal systems (MOST) offer a promising alternative to store and release solar energy compared to more conventional solar cells. Especially, the norbornadiene-based system was found to be, until now, the most efficient organic-based MOST offering, when opportunely substituted, several promising candidates that could lead to commercial use. Nevertheless, all carried out experimental attempts largely lack a proper design, hence finding difficulties in improving selected MOST properties. Here, a novel theoretical approach is described and applied to the norbornadiene–quadricyclane system, with the goal to study the effect of applied mechanical external forces, which can be traduced into a steric substitution effect. Especially we found that, while the absorption energy cannot be almost modified, the stored energy can be increased by ca. 70 kJ/ mol and, at the same time, a consistent increase (ca. 10 kJ/ mol) of the quadricyclane activation energy can be obtained. Moreover, we show that optimal complex forces, resulting from a combination of different contributions, could be suitably converted into substitution patterns and allow further improvements of the MOST properties.

Direct 11-cis to all-trans retinal photoisomerization within rhodopsin is well known to be the initial chemical reaction triggering the process of vision in mammalians, such as bovine. Nevertheless, deep-sea fish are known to use chlorophyll derivatives as photosensitizers in order to see deep-red light, at wavelengths where retinal does not absorb. Also, some photodynamic therapy treatments were shown to enhance the vision of patients in dim light conditions. Energy transfer from the photosensitizer to rhodopsin was therefore proposed as a mechanism to populate the triplet state of the retinal chromophore. Herein, by means of hybrid quantum mechanicscoupled-molecular mechanics modeling techniques, we give insights into the possible energy mechanism and describe the retinal isomerization mediated by the lowest-lying triplet state. Especially, we show how a few kcal/mol energy barrier separates a T 1 minimum from a S 0 /T 1 intersection region, hence proposing an equilibrium between phosphorescence and isomerization processes. Moreover, the eventual self-production of singlet oxygen, constituting a potential danger for the integrity of rhodopsin, is discussed.

In this contribution, we studied the photophysics and photochemistry of an archetypal molecular switch in terms of solar energy storage and release. In detail, we characterized the valence and Rydberg states of norbornadiene (NBD) and quadricyclane (QC) by means of CASPT2//CASSCF theory level, finding a good agreement of the calculated vertical excitation energies with the experimental counterparts. The NBD $ QC thermal and photochemical valence isomerization [2π + 2π] reactions have been addressed and investigated. Low energy crossing points between excited states were identified for both the NBD $ QC photochemical reactions, through the calculation of minimum energy paths which revealed that the photochemistry is ruled by the deformation of two coupled reaction coordinates. Such coordinates were also used to build potential energy surfaces through relaxed energy scans with the goal of building a simplified, still accurate, model system able to catch the NBD $ QC photorelaxation. Also, the S 1 /S 0 respective conical intersection were characterized and related to the reaction quantum yield. Interestingly, we found that reverse photoisomerization from QC to the NBD progenitor could in principle occur by direct excitation of σ-3 s and σ-3 p Rydberg excited states in the UV spectral range. In both NBD and QC photoreactivity, the doubly excited valence state has shown to play a crucial role for reaching crossing points leading to nonadiabatic population transfers.

Molecular dynamics simulations provide fundamental knowledge on the reaction mechanism of a given simulated molecular process. Nevertheless, other methodologies based on the “static” exploration of potential energy surfaces are usually employed to firmly provide the reaction coordinate directly related to the reaction mechanism, as is the case in intrinsic reaction coordinates for thermally activated reactions. Photoinduced processes in molecular systems can also be studied with these two strategies, as is the case in the triplet energy transfer process. Triplet energy transfer is a fundamental photophysical process in photochemistry and photobiology, being for instance involved in photodynamic therapy, when generating the highly reactive singlet oxygen species. Here, we study the triplet energy transfer process between porphyrin, a prototypical energy transfer donor, and different biologically relevant acceptors, including molecular oxygen, carotenoids, and rhodopsin. The results obtained by means of nanosecond time-scale molecular dynamics simulations are compared to the “static” determination of the reaction coordinate for such a thermal process, leading to the distortions determining an effective energy transfer. This knowledge was finally applied to propose porphyrin derivatives for producing the required structural modifications in order to tune their singlet-triplet energy gap, thus introducing a mechanochemical description of the mechanism.

direct conversion of the solar energy to chemical stored energy in a single process. [1][2][3] The usage of the sun as energy source makes these devices more available in comparison to other methods (like hydropower or wind turbines, limited by their geographical location). Different approaches have been studied with the aim of directly convert and store sunlight, like CO 2 reduction, [4] induced water splitting, [5] and more biologically inspired approaches. MOST systems are a less explored option, but very up-and-coming. The mechanism of these devices is the conversion of a lowenergy isomer, irradiated by light, into a high-energy isomer. Afterward, by heating or catalytic conversion, the system comes back to his low-energy form and the energy is released. Another important advantage of these kind of systems is their sustainability (i.e., their cyclic use is in principle illimited), hence reducing the environmental impact. [6,7] The most important properties that MOST systems must fulfil are: i) the low-energy isomer must efficiently absorb a maximum of the solar spectrum to generate the high-energy isomer; ii) the photoproduced high-energy isomer has to be stable-both thermally and photochemically-in order to store for a long time the energy; iii) the cycle has to be sustainable, i.e., without byproducts. [8,9] Different organic compounds have been studied as MOST candidates: stilbenes, azobenzenes, anthracenes, [6] the dihydroazulene/vinylheptafulvene system, [10] and the norbornadiene-quadricyclane system. The latter was preferred among the others due to its low molecular weight and high storage enthalpy. Nevertheless, the maximum absorption of norbornadiene is centered at a maximum of 267 nm, [11] hence in the UV spectral region, losing most of the solar irradiation. A plethora of different chemical substitution patterns were therefore proposed to avoid this problem, nevertheless only attenuating it. [12] An alternative strategy to circumvent this problem is the application of external forces (i.e., mechanochemistry). Indeed, we have already shown that through mechanochemistry it is possible to modify, in principle, different molecular properties in the electronic ground-state, as well as in the excited-states (e.g., photo-mechanochemistry). [13][14][15][16][17] In the specific of MOST systems, we have recently applied our developed mechanochemistry methodology to the norbornadiene-quadricyclane system but, although storage and activation energies were predicted to be conveniently increased, the norbornadiene absorption energy could only be negligibly red-shifted. [12] Molecular solar-thermal systems (MOST) have emerged in these last years as a novel concept to store solar light. They rely on two state molecular switches that can absorb a photon to convert the initial state A to a higher-in-energy state B. The chemical energy stored by B can be then released to reconstitute A. Although simple in its principle, an optimal MOST needs to satisfy several requirements: incoming photon energy in the solar spectru...