π‐Bridge Substitution in DASAs: The Subtle Equilibrium between Photochemical Improvements and Thermal Control**

Abstract

Donor–acceptor Stenhouse adducts (DASAs) are playing an outstanding role as innovative and versatile photoswitches. Until now, all the efforts have been spent on modifying the donor and acceptor moieties to modulate the absorption energy and improve the cyclization and reversion kinetics. However, there is a strong dependence on specific structural modifications and a lack of predictive behavior, mostly owing to the complex photoswitching mechanism. Here, by means of a combined experimental and theoretical study, the effect of chemical modification of the π‐bridge linking the donor and acceptor moieties is systematically explored, revealing the significant impact on the absorption, photocyclization, and relative stability of the open form. In particular, a position along the π‐bridge is found to be the most suited to redshift the absorption while preserving the cyclization. However, thermal back‐reaction to the initial isomer is blocked. These effects are explained in terms of an increased acceptor capability offered by the π‐bridge substituent that can be modulated. This strategy opens the path toward derivatives with infra‐red absorption and a potential anchoring point for further functionalization.

Stepping Out of the Blue: From Visible to Near‐IR Triggered Photoswitches

“…These chemotypes have attracted enormous attention owing to their facile preparation as well as their unique negative photochromic properties. [11,32,[156][157][158][159][160] DASAs are colored in the extended state and undergo Z-E isomerization and subsequent thermal conrotatory 4π electrocyclization to the compact state upon irradiation with visible light (Figure 17a,b). [161][162][163][164] Until now, three generations of DASAs have been developed (Figure 17c).…”

Improving the kinetics and dark equilibrium of donor–acceptor Stenhouse adduct by triene backbone design

“…The ΔΔ G between DASA derivatives with substituents in the 3- and 4-position compared to the unsubstituted derivative show a stabilization of the closed form in line with previously published results. 29 In contrast, we found that ΔΔ G CA is increasingly positive when steric bulk is introduced to the 5-position due to steric interaction between the methyl and donor group (Fig. S15, ESI†).…”

“…To expand our understanding of tuning the performance of DASA based systems, we targeted structural modifications on the triene backbone. Substitution on the 3-position of the backbone has been shown to favour the closed form both in donor–acceptor Stenhouse adducts as well as Stenhouse Salts, 28,29 however other substitutions have been largely overlooked. We envisioned that by finding a position on the triene backbone which increases the relative energy of the stable closed form ( i.e.…”

Mechanism of the Aza-Piancatelli Reaction: Scope and Limitations of Furan Substitution in Donor–Acceptor Stenhouse Adduct Synthesis

“…On one hand, the effect of substitution on the properties as a whole is hard to predict and may lead to improved optical properties ( i.e ., red-shifted absorption) but poorer performance as a photoswitch. 10 On the other hand, the preparation of new DASAs with different substituents in the bridge has encountered synthetic problems. The key step for the synthesis of these compounds is usually based on the initial work by Šafář et al, 11 where the opening of furan derivatives by amines leads to the open form of DASAs through the aza-Piancatelli reaction.…”

“… 16 Substituents in the bridge of the resulting DASA come from the substituents in positions 3 and 4 in the furan moiety. We successfully synthesized 10 different DASAs through the opening of 3-substituted furans, but all our efforts to open 3,4-disubstituted furan rings were unsuccessful. However, exploration of this substitution pattern could contribute to the better understanding of the effects of these structural changes on the performance of these switches ( Scheme 1 B).…”