One challenge in the development of molecular photoswitches for solar energy storage is to increase the energy of the isomer formed upon irradiation. This work focuses on how to affect the relative stabilities of the isomers of the dihydroazulene/ vinylheptafulvene (DHA/VHF) couple by combining the light‐induced DHA to VHF conversion with the loss of aromaticity. We present a synthetic procedure for benzannulation of DHA at the C7−C8 bond, with the key steps being a stereoselective Diels–Alder cycloaddition followed by an oxidation. The optical properties and switching abilities of this compound are described in a combined theoretical and experimental study. Calculations support that benzannulation serves to increase the overall energy capacity of the photoswitch by stabilizing the DHA and destabilizing the VHF.
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.
Boron subphthalocyanines (SubPcs) are powerful chromophoric heterocycles that can be synthetically modified at both axial and peripheral positions. Acetylenic scaffolding offers the possibility of building large, unsaturated carbon-rich frameworks that can exhibit excellent electron-accepting properties, and when combined with SubPcs it presents a convenient method for preparing interesting chromophore-acceptor architectures. Here we present synthetic methodologies for the post-functionalization of the relatively sensitive SubPc chromophore via acetylenic coupling reactions. By gentle AlCl-mediated alkynylation at the axial boron position, we managed to anchor two SubPcs to the geminal positions of a tetraethynylethene (TEE) acceptor. Convenient conditions that allow for stepwise desilylations of trimethylsilyl (TMS) and triisopropylsilyl (TIPS) protected SubPc-decorated acetylenes using silver(i) fluoride were developed. The resulting terminal alkynes were successfully used as coupling partners in metal-catalyzed couplings, providing access to larger acetylenic SubPc scaffolds and multiple chromophore systems. Moreover, conditions allowing for the conversion of a terminal alkyne into an iodoalkyne in the presence of SubPc were developed, and the product was subjected to cross-coupling reactions affording unsymmetrical 1,3-butadiynes. The degree of interactions between two SubPc units as a function of the acetylenic bridge was studied by UV-Vis absorption spectroscopy and cyclic voltammetry. A TEE bridging unit was found to strongly influence the reductions and oxidations of the two SubPc units, while a more flexible bridge had no influence.
The boron subphthalocyanine (SubPc) fluorescence is reversibly modulated by photoisomerization of a covalently attached dihydroazulene (DHA) photoswitch into a vinylheptafulvene (VHF).
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