Liquid Norbornadiene Photoswitches for Solar Energy Storage

Abstract

been widely implemented [1] but longterm storage of the heat at higher temperatures remains a challenge, limiting broader applications of solar heating. [2,3] Solar thermal energy can be stored for example as thermal energy in rocks or hot water, in phase change materials, [4,5] or in thermochemical energy storage materials. [6] A particular challenge is to identify compact thermal storage solutions capable of operating in the medium temperature range (100-180 °C). [2] In this context, one possible solution is to use molecular photoswitches where the solar energy is stored in high energy photoisomers. [7] This way of storing solar energy has been referred to as molecular solar thermal (MOST) [8] energy storage or solar thermal fuels. [9] MOST systems are based on a parent molecule, which upon irradiation photoisomerizes to a highenergy isomer. The stored energy can be released on demand by applying heat or by using a catalyst to trigger the thermal back isomerization (Scheme 1). [7,10,11] Various molecular designs have been explored in this context, including ruthenium compounds, [8,12] azobenzene derivatives, [13][14][15][16][17][18] norbornadiene (NBD) derivatives, [10,11,19] dihydroazulenes, [20] and others. [21] The requirements for an efficient MOST system [7,10,22] can be summarized as: (i) good spectral overlap of parent molecule absorptions with the solar spectrum, (ii) minimal absorption of the high energy isomer in the solar spectrum, (iii) high quantum yield for the photoisomerization, (iv) high energy storage density, (v) high kinetic stability of the metastable photoisomer, and (vi) high cyclability. It is also fundamental to obtain neat liquid or solid functional MOST materials (depending on the desired applications), since dilution in solvents or solid matrix lowers the energy storage density. A closed MOST system has been previously described, [23] which involves cycles of an MOST fluid in a solar collector for the photoconversion, a storage tank, and a heat extractor part (see Scheme 1).NBD (1 in Scheme 1) and its derivatives undergo photoisomerization to the highly strained quadricyclane (QC, 2 in Scheme 1) upon irradiation with UV or visible light. [24] By molecular design, the system has been optimized toward MOST applications, and the best NBD derivatives fulfil several of the requirements for a functional system, such as high photoisomerization quantum yield, redshifted absorption, high energy storage densities, and very long half-life of the Due to high global energy demands, there is a great need for development of technologies for exploiting and storing solar energy. Closed cycle systems for storage of solar energy have been suggested, based on absorption of photons in photoresponsive molecules, followed by on-demand release of thermal energy. These materials are called solar thermal fuels (STFs) or molecular solar thermal (MOST) energy storage systems. To achieve high energy densities, ideal MOST systems are required either in solid or liquid forms. In the case of the latter, neat high ...

Efficient Cyclization of the Norbornadiene‐Quadricyclane Interconversion Mediated by a Magnetic [Fe3O4−CoSalphen] Nanoparticle Catalyst

“…By extrapolating the data, it was also possible to determine the stability of QC1 at 25 °C, which shows a half‐life of approximately 450 days in deuterated tetrachloroethane. Compared to similar “push‐pull” substituted NBD derivatives the stability of NBD1 is outstandingly high . It has been shown before that by introducing steric bulk to the bridgehead carbon of the NBD framework or by ortho substitution of the adjacent phenyl ring the thermal stability can be increased, however, this is usually accompanied with a higher molecular weight and therefore, lower energy storage capacity .…”

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

“…Several photochromic motifs, such as tetracarbonyl‐fulvalene‐diruthenium compounds, azobenzenes, norbornadienes, and dihydroazulenes have been investigated, both in solution and neat forms, as well as appended to polymers or integrated onto solid supports . The focus of our group has been on assessing the potential of the norbornadiene (NBD)/quadricyclane (QC) couple, where light can be used to facilitate a [2+2π] intramolecular cyclization to form the high energy QC1 from NBD1 (Figure ).…”

Precious metal-free molecular machines for solar thermal energy storage

“…For a pure MOST system the maximal solar energy conversion efficiency was estimated to be 10.6% [8]. It was demonstrated experimentally that by merging a MOST-system [9] with a triplet–triplet annihilation photon energy upconversion system (TTA-UC) [10–11] an effective utilization of sub-bandgap photons is possible [12–13]. Nevertheless, the identification of a precious metal-free MOST system utilizing the visible part of the sun spectrum remains a considerable challenge.…”