Surface chemistry of 2,3-dibromosubstituted norbornadiene/quadricyclane as molecular solar thermal energy storage system on Ni(111)

Abstract

Dwindling fossil fuels force humanity to search for new energy production routes. Besides energy generation, its storage is a crucial aspect. One promising approach is to store energy from the sun chemically in strained organic molecules, so-called molecular solar thermal (MOST) systems, which can release the stored energy catalytically. A prototypical MOST system is norbornadiene/quadricyclane (NBD/QC) whose energy release and surface chemistry need to be understood. Besides important key parameters such as molecular weight, endergonic reaction profiles, and sufficient quantum yields, the position of the absorption onset of NBD is crucial to cover preferably a large range of sunlight’s spectrum. For this purpose, one typically derivatizes NBD with electron-donating and/or electron-accepting substituents. To keep the model system simple enough to be investigated with photoemission techniques, we introduced bromine atoms at the 2,3-position of both compounds. We study the adsorption behavior, energy release, and surface chemistry on Ni(111) using high-resolution X-ray photoelectron spectroscopy (HR-XPS), UV photoelectron spectroscopy, and density functional theory calculations. Both Br2-NBD and Br2-QC partially dissociate on the surface at ∼120 K, with Br2-QC being more stable. Several stable adsorption geometries for intact and dissociated species were calculated, and the most stable structures are determined for both molecules. By temperature-programmed HR-XPS, we were able to observe the conversion of Br2-QC to Br2-NBD in situ at 170 K. The decomposition of Br2-NBD starts at 190 K when C–Br bond cleavage occurs and benzene and methylidene are formed. For Br2-QC, the cleavage already occurs at 130 K when cycloreversion to Br2-NBD sets in.

Storing energy with molecular photoisomers

“…Fundamental understanding of these complex processes can be obtained by surface science studies in ultra-high vacuum (UHV). 107 In recent years, this approach was used to study energy release from the NBD/QC couple on Pt(111) 108,109 and Ni (111). 110,111 It was found that Pt( 111) is significantly more active for triggering the energy release than Ni (111).…”

“…107 In recent years, this approach was used to study energy release from the NBD/QC couple on Pt(111) 108,109 and Ni (111). 110,111 It was found that Pt( 111) is significantly more active for triggering the energy release than Ni (111). 108,110 On clean Pt (111), a monolayer of QC is immediately back-converted to NBD even at cryogenic temperatures (125 K).…”

“…110,111 It was found that Pt( 111) is significantly more active for triggering the energy release than Ni (111). 108,110 On clean Pt (111), a monolayer of QC is immediately back-converted to NBD even at cryogenic temperatures (125 K). The formed NBD chemisorbs onto the metal surface and was observed to be decomposed via a norbornadienyl intermediate at elevated temperatures.…”

The vacuum ultraviolet absorption spectrum of norbornadiene: Vibrational analysis of the singlet and triplet valence states of norbornadiene by configuration interaction and density functional calculations

“…Interconversions between NBD and QC, and their derivatives are very important, since a promising approach to energy storage from the sun, lies in the use of strained organic molecules. [4][5][6][7] This justifies a brief summary. These so-called molecular solar thermal (MOST) binary systems, absorb solar energy via the alkene form (here NBD) leading to the strained form (here QC); the stored energy can then be released from the latter catalytically, regenerating the NBD.…”

Accurate prediction of norbornadiene cycle enthalpies by DLPNO‐CCSD(T1)/CBS method

“…[1] Practical interest in norbornadiene thermochemistry is based on the potential use of the norbornadiene-quadricyclane isomerization as a solar energy storage system. [2][3][4][5] Although the enthalpies of formation (Δ f H ) of these compounds have been measured repeatedly (see References [1,[6][7][8][9] and references therein), there is still disagreement in their values. Only the experimental value of Δ f H (g) for norbornane recommended by…”