Conjugates of norbornadiene (NBD) and dihydroazulene (DHA) photoswitches were synthesised and subjected to isomerisation studies.

Dihydroazulene (DHA) is a molecular photoswitch that undergoes a ring-opening reaction upon irradiation to form a vinylheptafulvene (VHF) photoisomer. This VHF isomer will in time thermally return to the DHA isomer. As the isomerization is photo-induced only in one direction, the DHA – VHF couple has attracted interest as a molecular solar thermal energy storage device (MOST system). In this author review, we cover our systematic efforts to optimize the DHA – VHF couple for this purpose, with challenges being to achieve sufficiently high energy densities, to cover broad absorptions including the visible region, and to control the energy-releasing VHF-to-DHA back-reaction. By a combination of computations and experiments, we review the consequences of various structural modifications of the system (structure – property relationships), including the influence of donor-acceptor substitution at specific positions, benzannulations, and incorporation into macrocyclic structures. Synthetic protocols to reach the various modifications will also be discussed. The bibliography includes 60 references.

The gain and loss of aromaticity plays a key role in organic chemistry and in the prediction of rate-determining steps. Herein, we explore the concept of aromaticity in photoisomerization reactions. Benzannulated derivatives of the dihydroazulene-vinylheptafulvene (DHA-VHF) photoswitch were investigated using transient absorption spectroscopy and timedependent density functional theory to elucidate the effect of built-in aromaticity on the switching properties. We found that benzannulation hampered the switching ability by enhancing an already existing barrier on the excited state surface. This enhancement was found to arise from a significant loss of aromaticity in the DHA-to-VHF transition state on the excited state potential energy surface. The VHF was found to be highly aromatic on the excited state surface, showing a reversal of aromaticity compared to the ground state. The barrier was found to be dependent on the position of benzannulation, since one derivative was found to switch as fast as the nonbenzannulated molecule although with lower efficiency, whereas another derivative completely lost the ability to undergo reversible photoswitching. The findings herein provide novel principles for the design of molecular photoswitches, shedding new light on excited state aromaticity, as previous discussions have mainly considered excited state aromaticity to be beneficial to switching. Our findings show that this view must be reconsidered.[a] Mr.

Molecular photoswitches based on the norbornadiene–quadricylane (NBD–QC) couple have been proposed as key elements of molecular solar thermal energy storage schemes. To characterize the intrinsic properties of such systems, reversible isomerization of a charge-tagged NBD–QC carboxylate couple is investigated in a tandem ion mobility mass spectrometer, using light to induce intramolecular [2 + 2] cycloaddition of NBD carboxylate to form the QC carboxylate and driving the back reaction with molecular collisions. The NBD carboxylate photoisomerization action spectrum recorded by monitoring the QC carboxylate photoisomer extends from 290 to 360 nm with a maximum at 315 nm, and in the longer wavelength region resembles the NBD carboxylate absorption spectrum recorded in solution. Key structural and photochemical properties of the NBD–QC carboxylate system, including the gas-phase absorption spectrum and the energy storage capacity, are determined through computational studies using density functional theory.

Former work has improved the energy storage capacity of the dihydroazulene/vinylheptafulvene photo/thermoswitch by substitution with NH and NO in vacuum. This work extends the former by investigating the solvent effects systematically using cyclohexane, toluene, dichloromethane, ethanol, and acetonitrile and comparing them with the inclusion of vacuum calculations. The investigation includes more than 8000 calculations using density functional theory for comparison of energy storage capacities, activation energies for the thermal conversion of vinylheptafulvene to dihydroazulene, and UV-Vis absorption spectra. We thereby establish design and solvent guidelines in order to obtain an optimal performance of the dihydroazulene/vinylheptafulvene system for use in a solar energy harvesting and storing device.