Abstract
Molecular photoswitches that are capable of storing solar energy, so-called molecular solar thermal storage systems, are interesting candidates for future renewable energy applications. In this context, substituted norbornadiene-quadricyclane systems have received renewed interest due to recent advances in their synthesis. The optical, thermodynamic, and kinetic properties of these systems can vary dramatically depending on the chosen substituents. The molecular design of optimal compounds therefore requires a detailed understanding of the effect of individual substituents as well as their interplay. Here, we model absorption spectra, potential energy storage, and thermal barriers for back-conversion of several substituted systems using both single-reference (density functional theory using PBE, B3LYP, CAM-B3LYP, M06, M06-2x, and M06-L functionals as well as MP2 calculations) and multireference methods (complete active space techniques). Already the diaryl substituted compound displays a strong red-shift compared to the unsubstituted system, which is shown to result from the extension of the conjugated π-system upon substitution. Using specific donor/acceptor groups gives rise to a further albeit relatively smaller red-shift. The calculated storage energy is found to be rather insensitive to the specific substituents, although solvent effects are likely to be important and require further study. The barrier for thermal back-conversion exhibits strong multireference character and as a result is noticeably correlated with the red-shift. Two possible reaction paths for the thermal back-conversion of diaryl substituted quadricyclane are identified and it is shown that among the compounds considered the path via the acceptor side is systematically favored. Finally, the present study establishes the basis for high-throughput screening of norbornadiene-quadricyclane compounds as it provides guidelines for the level of accuracy that can be expected for key properties from several different techniques.
Highlights
The worldwide energy consumption is predicted to double within the 40 years requiring a shift toward widespread use of renewable energy.[1]
This implies that no triplet sensitizer is required and it is most likely that in the case of the substituted systems the relevant photocatalytic and thermal transitions occur exclusively on the singlet surface
The goal of this study was twofold: first, to gain insight into the mechanisms that govern the optical, kinetic, and thermodynamical properties of novel molecular solar thermal (MOST) compounds, and second to identify the level of computational methods sufficient for describing these properties
Summary
The worldwide energy consumption is predicted to double within the 40 years requiring a shift toward widespread use of renewable energy.[1]. An alternative is the direct conversion from solar energy to stored chemical energy. This can in principle be achieved via the conversion of water to hydrogen or the reduction of carbon dioxide to methanol,[4] which, involves gaseous species. Energy storage can instead be accomplished in closed cycle photo isomer systems. In these so-called molecular solar thermal (MOST) systems, the exposure of a low energy isomer to sunlight leads to its conversion into a high energy isomer.[5,6] The metastable high-energy isomer can be converted back to its low energy counterpart by heating or catalytic activation to release the stored energy. Several molecular and metal−organic systems have been explored in this context, notably stilbenes,[7,8] azobenzenes,[9−11] anthracenes,[12] ruthenium fulvalene compounds,[5,13−16] and norbornadiene-quadricyclane (N-Q) systems.[17−19] Both experimental and theoretical aspects of this research field have been reviewed recently.[6,18,20,21]
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