Design of Improved Molecular Solar‐Thermal Systems by Mechanochemistry: The Case of Azobenzene

Abstract

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...

A Predictive Screening Tool to Evaluate the Efficiency of Z/E Photoisomerizable Molecular Switches

“…Numerous families of light-driven molecular switches are known and their performance has been studied, [13][14][15] based on different photoreactivities coupled to a vast variety of physicochemical properties and photo-modulable applications. [16][17][18] Among them, light-induced cyclization and Z/E isomerization are possibly the most applied photoswitching reactions: on one hand, photocyclization was invoked to obtain versatile molecular building blocks, [19][20][21] to propose various molecular solar thermal systems (MOST), [22][23][24][25][26][27][28] and to develop different generations of donor-acceptor Stenhouse adducts (DASA). [29][30][31][32][33][34][35][36] On the other hand, Z/E photoisomerization has been inspired by different sources: 16 biological chromophores, such as the retinal chromophore in rhodopsins [37][38][39][40][41] or the green fluorescent protein (GFP) chromophore, [42][43][44] but also artificial prototypes, such as the family of chiroptical switches designed by the group of Feringa, [45][46][47] stilbene-based [48][49][50] or hemithioindigo-based [51][52]…”

Electrocatalytic Energy Release of Norbornadiene‐Based Molecular Solar Thermal Systems: Tuning the Electrochemical Stability by Molecular Design

“…[ 20 , 21 , 22 , 23 ] However, this method lacks of controllability, and the fact that the triggering process consumes energy reduces the overall efficiency of the storage system. There are different ways to overcome these drawbacks, for example, molecular design[ 24 , 25 ] or choosing an appropriate trigger to release the energy. Commonly used triggers are metal complex catalysts,[ 26 , 27 , 28 , 29 , 30 , 31 ] copper(II) or tin(II) salts, [32] or heterogeneous catalysts.…”