Investigation of the Structural and Thermochemical Properties of [2.2.2]-Bicyclooctadiene Photoswitches

Abstract

Molecular photoswitches can under certain conditions be used to store solar energy in the so-called molecular solar thermal storage systems, which is an interesting technology for renewable energy solutions. The current investigations focus on the performance of seven different density functional theory (DFT) methods (B3LYP, CAM-B3LYP, PBE0, M06-2X, ωB97X-D, B2PLYP, and PBE0DH) when predicting geometries and thermochemical properties of the [2.2.2]-bicyclooctadiene (BOD) photoswitch. We find that all of the investigated DFT methods provide geometries that are in good agreement with those obtained using coupled cluster singles and doubles (CCSD) calculations. The dependence on the employed basis set is not large when predicting geometries. With respect to the thermochemical properties, we find that the M06-2X, CAM-B3LYP, PBE0, and ωB97X-D functionals all predict thermochemical properties that are in good agreement with the results of the CCSD, the CCSD including perturbative triples (CCSD(T)), and the explicitly correlated CCSD-F12 and CCSD(T)-F12 models. Lastly, for energy calculations, we tested the newly developed fourth-order cluster perturbation theory singles and doubles CPS(D-4) model, which in this study provides energy differences that are of CCSD and sometimes also CCSD(T) quality at a relatively low cost. We find that the CPS(D-4) model is an excellent choice for further investigation of BOD derivatives because accurate energies can be obtained routinely using this methodology. From the results, we also note that the predicted storage energies and storage energy densities for the BOD photoswitch are very large compared to other molecular solar thermal storage systems and that these systems could be candidates for such applications.

Prospects of Improving Molecular Solar Energy Storage of the Norbornadiene/Quadricyclane System through Bridgehead Modifications

“…Furthermore, the systems have relatively long thermal back reaction times. However, we have recently reported that the storage energy can be significantly increased by altering the bridging unit of the NBD/QC system, a line of thought, which we will expand upon here. , …”

“…Calculations have been performed in Gaussian16 . This methodology is chosen as earlier studies have shown good performance for both NBD/QC and BOD/TCO systems. , Subsequently, we refine the electronic energy of the determined structures using the newly developed cluster perturbation theory CPS­(D-4) model in LSDalton, ,, which provides energies of CCSD quality at a low cost, while we utilize the vibrational and entropic energy contributions of the DFT calculations. The resulting storage energies are given in Table .…”

“…More specifically, we focus on scrutinizing the effects by elongating the bridging unit and on introducing heteroatoms. Our recent theoretical and experimental investigations have shown that elongating the bridge by a single carbon atom to obtain bicyclooctadiene (BOD) resulted in a 2–3-fold increase in the energy storage density of the system. , This increase in storage energy is likely to be caused by increased strain in the ring systems of the tetracyclooctane (TCO) photoproduct for which the bridging unit, contrary to the BOD, adopts a staggered conformation. The increased storage energy comes without compromising the linear optical properties, yet the thermal back reaction barriers were significantly reduced relative to the NBD/QC analogues.…”

“…However, we have recently reported that the storage energy can be significantly increased by altering the bridging unit of the NBD/QC system, a line of thought, which we will expand upon here. 45,46 In the current investigation, we report the theoretical prospects of modifying the bridging unit in the NBD/QC system in order to possibly improve on three crucial parameters of MOST systems: storage energy density, thermal back reaction barrier, and UV−vis absorption properties. More specifically, we focus on scrutinizing the effects by elongating the bridging unit and on introducing heteroatoms.…”

Synthesis, characterization and computational evaluation of bicyclooctadienes towards molecular solar thermal energy storage

“…The storage energy for the unsubstituted NBD/QC system is 0.97 MJ kg −1 while the corresponding value for the unsubstituted BOD/TCO is 1.77 MJ kg −1 . 24 Therefore, we became interested in evaluating BODs as candidates for MOST. Known BOD systems absorb in the UV range of the solar spectrum, 25 and molecular engineering is needed to red-shift the absorption spectrum towards more intense parts of the solar spectrum.…”