Expanding the Hammett Correlations for the Vinylheptafulvene Ring‐Closure Reaction

Abstract

A selection of 2,3‐diarylated photochromic dihydroazulenes (DHAs) was prepared by following two different protocols. The first protocol relies on the synthesis of a 3‐bromo‐substituted dihydroazulene, which is subjected to a Suzuki cross‐coupling reaction in the final step. In the second protocol, the two aryl substituents are introduced early in the synthesis. The DHAs are photoactive and undergo light‐induced ring‐opening reactions to form vinylheptafulvene (VHF) isomers. These VHFs are quickly converted back to DHAs at room temperature in thermal ring‐closure reactions. The kinetics of these reactions was studied at –20 °C for two series for which the substituent at the C‐2 position (DHA numbering) was kept constant, and a linear free‐energy relationship (Hammett correlation) was revealed in both cases. The relationships show that an electron‐donating substituent at C‐3 (the exocyclic VHF carbon atom) enhances the ring‐closure reaction, in contrast to its influence when placed at C‐2. In a computational study (density functional theory), three different methods were benchmarked against the experimental results. Overall, this work shows for the first time how the VHF to DHA switch is influenced by donor–acceptor substitution at the exocyclic heptafulvene carbon atom.

Storing energy with molecular photoisomers

“…43,65 The conversion can also be tuned by donor and acceptor groups at DHA positions 2, 3, and 7 (Figure 3, DHA-8); the half-life is, for instance, increased by adding a donor group at C2 and an acceptor group at C3 or C7. 66,67 By placing a cyano acceptor group at position C7, photoisomerization provides an exceptionally long-lived VHF that returns to DHA with a half-life of 21.7 h in acetonitrile at 25 C (ca. six times slower than for the parent system; Figure 3, DHA-9).…”

Prospects of Improving Molecular Solar Energy Storage of the Norbornadiene/Quadricyclane System through Bridgehead Modifications

“…The UV–vis absorptions of, for example, azobenzene, NBD/QC, and DHA/VHF systems have been red-shifted significantly by adding substituents that increase the conjugation length of the system or by introduction of donor/acceptor functionalization. − These efforts have provided great absorption overlaps with the solar spectrum. In addition to that, the thermal stabilities of the formed photoproducts have been engineered to feature storage times that range from a few seconds to years, which provides great tunability for specific applications. ,,− Yet, earlier studies that focused on increasing the energy storage densities for the azobenzene, NBD/QC, and DHA/VHF systems have found that it is difficult to increase the storage energy without compromising the storage time. ,− For a comprehensive review on MOST systems, we refer to the very recent review of Moth-Poulsen et al It is, however, clear that the development of MOST systems based on the above systems centers around discovering the perfect balance between different properties.…”

Data enhanced Hammett-equation: reaction barriers in chemical space

“…29 More recent approaches have also tried to apply the models to nonbenzyl systems. 9,[30][31][32] It is, however, less satisfying because the linear relationship postulated by Hammett lacks a motivation based on physical effects. Early attempts to explain the theory by electrostatic considerations 33,34 were successful for special cases only.…”