Molecular solar thermal energy storage (MOST) systems based on photochromic molecules that undergo photoisomerization to high‐energy isomers are attractive for storage of solar energy in a closed‐energy cycle. One challenge is to control the discharge time of the high‐energy isomer. Here we show that incorporation of a strong acceptor substituent in the seven‐membered ring of the dihydroazulene/vinylheptafulvene (DHA/VHF) couple increases the half‐life of the energy‐releasing VHF‐to‐DHA back‐reaction from hours to more than a day in a polar solvent. For some derivatives, the absorption maximum of the photo‐active DHA is also significantly redshifted, thereby better matching the solar spectrum. Synthetic protocols and kinetics studies are presented together with a computational study of the energy densities of the systems and excitation spectra. The computations show that the increased lifetime of the high‐energy isomer is counter‐balanced by a lower energy storage capacity in vacuo than for the parent system, but a slightly higher energy density than for the parent system in a polar solvent.

Indenofluorene‐extended tetrathiafulvalenes (IF‐TTFs) comprise a class of π‐conjugated sensitizers that exhibit strong absorptions in the visible region and two reversible one‐electron oxidations. Herein we present the synthesis and optical as well as redox properties of novel IF‐TTF donor‐acceptor scaffolds that were integrated in dye‐sensitized solar cells (DSCs) via anchoring of a carboxylic acid end‐group on the scaffolds to TiO2. Synthetically, the scaffolds were constructed by Sonogashira coupling reactions between an iodo‐functionalized IF‐TTF and an acceptor moeity containing a terminal alkyne. These very first IF‐TTF based candidates for DSCs exhibited high performances, in particular a dye incorporating a benzothiadiazole acceptor moiety, showing a conversion efficiency of 6.4 %. This result signals that IF‐TTF derivatives present a promising class of compounds for further structural modifications. Such modifications will benefit from the readiness of the iodo‐funcitonalized IF‐TTF building block to undergo Pd‐catalyzed coupling reactions.

Orthogonal photoswitching is ac onvenient but challenging way of controlling multiple functions in a system by selectivep hotoisomerization of one unit before the other in any arbitrarily chosen sequence. Here, we present this concept for the norbornadiene/quadricyclane (NBD/ QC) photo/thermo-switch in the presence or absence of a coordinated metal ion. Thus, introducing two pyridyl ligands via ethyne-1,2-diyl bridges provides as ystem that by chelation of metal ions, such as Pd II ,h as alteredo pticala nd switching properties. Mixing the Pd II complex with its free ligandf urnishes af our-state system where NBD-to-QC photoisomerizations for complexed and uncomplexed species are controlledb yt he irradiation wavelength and can occur orthogonally,t hat is, the sequence of photoisomerizations can be swapped. Studies on Ag I and Pb II complexes, being less stable than the Pd II complex, are also presented;t hese exhibit like the Pd II complex significantly red-shifted NBD absorptions. Scheme1.The norbornadiene/quadricyclane (NBD/QC) interconversions. Scheme2.Metal complexationbyb identateb ipyridyl system and structure of NBD target molecule 1 NBD .M= metal.

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The Cover Feature shows how sunlight can convert a dihydroazulene derivative into a high‐energy vinyl­heptafulvene isomer, which in the dark returns to the original isomer, at the same time releasing the energy as heat. By introducing electron‐withdrawing groups (EWGs), the half‐life for the back‐reaction is significantly prolonged without compromising the energy density of the system, which is a key step towards using the couple as a molecular solar thermal energy storage system for light‐harvesting during daytime and heat release during nighttime. More information can be found in the Full Paper by M. Jevric, K. V. Mikkelsen, M. B. Nielsen et al.