Comparative Ab-Initio Study of Substituted Norbornadiene-Quadricyclane Compounds for Solar Thermal Storage

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.

Norbornadiene‐Based Photoswitches with Exceptional Combination of Solar Spectrum Match and Long‐Term Energy Storage

“…and approximate the biradical character using unrestricted open‐shell DFT . This approach yields Δ H ≠ values in good agreement with experiment and reference CASPT2 calculations for a series of diaryl‐substituted NBD compounds investigated earlier (Figure b) . While it also yields reasonable agreement for the “regular”, i.e., non‐high‐performing, molecules studied here, for several of the high‐performing compounds ( QC2 d , QC2 i , QC2 m ) the calculated and measured barriers are barely correlated (Figure b).…”

Low Molecular Weight Norbornadiene Derivatives for Molecular Solar‐Thermal Energy Storage

“…[8d] However,t he introduction of large substituents is at the expense of ah igh molecular weight, which in turn lowerst he energys torage density.C omputational work has shown that the molar storagee nergy is largely independento fs ubstitution pattern, implying that the molecular weighti st he most important optimization parameter in this regard. [10] In previouss tudies by our group,aseries of electron-donating and electron-accepting diaryl-substituted norbornadienesw ith an improved solars pectrum match compared to 1 was synthesized (Table 1). [11] The most red-shifted compound (3e)h as an absorption maximum at 365 nm and an absorption onset of 462 nm.…”

“…As mentioned above, ar ecent computational study suggestedt hat the molecular weighti sakeyo ptimizationp arameter when aiming for high energy density in these systems. [10] Inspired by this hypothesis, our approachh ere was to find ar obust low molecular weight electron-accepting substituent to replace the p-substituted phenylg roups in compounds 3a-e.H erein, we report as eries of new cyano-substituted norbornadiene derivatives with improvedp hotochemical properties yet significantly reduced molecular weight.…”

Energy Storage in Strained Organic Molecules: (Spectro)Electrochemical Characterization of Norbornadiene and Quadricyclane

“…In the photo(electro)chemical strategy,t he fuel generated enabless torage and controlled energy release at will. [3] The photochemical conversion of the polycyclic norbornadiene (NBD) to its saturated, strainedb ut metastable, valence isomer quadricyclane (QC), was described over half ac entury ago (see Figure 1), [4] was investigated by theoretical methods, [5] and has long been recognized as ap ossible avenue towards solar-to-fuel energyc onversions chemes. Indeed, these multistep reactions typically involvem ajor bond rearrangements between severalmolecules and/or ions, associated with the consecutive transfer of severale lectrons.…”