Optimization of Norbornadiene Compounds for Solar Thermal Storage by First‐Principles Calculations

Abstract

Molecular photoswitches capable of storing solar energy are interesting candidates for future renewable energy applications. Here, using quantum mechanical calculations, we carry out a systematic screening of crucial optical (solar spectrum match) and thermal (storage energy density) properties of 64 such compounds based on the norbornadiene-quadricyclane system. Whereas a substantial number of these molecules reach the theoretical maximum solar power conversion efficiency, this requires a strong red-shift of the absorption spectrum, which causes undesirable absorption by the photoisomer as well as reduced thermal stability. These compounds typically also have a large molecular mass, leading to low storage densities. By contrast, single-substituted systems achieve a good compromise between efficiency and storage density, while avoiding competing absorption by the photo-isomer. This establishes guiding principles for the future development of molecular solar thermal storage systems.

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

“…The shift of the absorption spectrum with the strength of the donor species is in line with our earlier calculations 10, 15. More interesting in the present context is the strong enhancement of the dipole oscillator strength (Table 5) that results from insertion of an ethynyl bridge on going from 4 a to 5 , and gives rise to a large increase in the extinction coefficient.…”

“…The spectra were subsequently obtained by averaging over the isomers weighted by their respective Boltzmann factors 15. All calculations were carried out using the NWChem 6.5 software package 31…”

Triazole‐Functionalized Norbornadiene‐Quadricyclane Photoswitches for Solar Energy Storage

“…Through this work, it was also found that the properties of NBD20 were much better than expected, as it appeared that the acetyl group causes additional broadening the absorbance peak, giving a better onset of absorbance; unfortunately, the attractive optical absorption properties were counterbalanced by a low quantum yield of 28 %. The kinetic data for NBD20 was interesting, having a ketone as an acceptor in conjunction with an anisole donor, where QC20 had a half‐life over one year at 25 °C, compared to less than four days for QC19 , which went against the trend typically found for NBD derivatives of this type and this observation verifies our earlier suggestion that broadening of the main optical transition could be an effective way to improve the absorption properties of this type of system. In summary, the 1,4‐triazole functionalizations were preferable over the 1,5‐triazole pattern, leading to NBD‐QC photoswitches with excellent thermal stability.…”

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

“…In the cases where the initial method did not work, this was supplemented by the reported procedure employing a palladium‐catalysed Suzuki reaction upon 2‐chloro‐3‐cyanonorbornadiene . A recent theoretical study showed that certain aromatic donor units could enhance the MOST properties, including dimethylaniline and thiophene, among other electron rich substituents, which were accordingly included in this series . Figure shows the X‐ray crystal structure for NBD2 h , which during the course of this investigation was shown to possess excellent absorption properties (additionally, the structure of NBD2 b can be found in the Supporting Information).…”