Solvent Effects on the Actinic Step of Donor–Acceptor Stenhouse Adduct Photoswitching

Abstract

Donor–acceptor Stenhouse adducts (DASAs) are negative photochromes that switch with visible light and are highly promising for applications ranging from smart materials to biological systems. However, the strong solvent dependence of the photoswitching kinetics limits their application. The nature of the photoswitching mechanism in different solvents is key for addressing the solvatochromism of DASAs, but as yet has remained elusive. Here, we employ spectroscopic analyses and TD‐DFT calculations to reveal changing solvatochromic shifts and energies of the species involved in DASA photoswitching. Time‐resolved visible pump‐probe spectroscopy suggests that the primary photochemical step remains the same, irrespective of the polarity and protic nature of the solvent. Disentangling the different factors determining the solvent‐dependence of DASA photoswitching, presented here, is crucial for the rational development of applications in a wide range of different media.

Inducing molecular isomerization assisted by water

“…Interestingly, after adding water into a toluene solution of linear DASA-N (toluene:water = 6:1, v/v), we found that the linear DASA-N spontaneously converts to cyclic DASA-N, even without light, this is in good accordance to the previous reports ( Fig. 2b) 1,2,[18][19][20] . The process is extremely slow compared with the light-induced isomerization, and onlỹ 30% of cyclic DASA-N can be obtained after 10 h, and the process is finished in 40 h ( Supplementary Fig.…”

“…Since the earliest report cyclic DASAs have been found to be more stable when water exists 1 . The linear-to-cyclic isomerization occurs under dark in aqueous environment was also noticed by Feringa's group earlier in 2016 and 2018 [18][19][20] . However, to our knowledge no research is focusing on the mechanism of the water-induced isomerization and the relationships between DASAs and H 2 O molecules, which are important for the understanding, development and application of DASAs.…”

“…22). The choice of this combination was used by several previous benchmarks that demonstrated their performances for weak interactions like hydrogen bonds 18,20,28 . Vibration frequencies were calculated to confirm if all of the structures optimized were at stationary points or true minima on the potential energy surfaces.…”

“…Feringa's group demonstrated a intermediates-included linearto-cyclic isomerization process of DASAs 18,20,25 . Linear DASAs (A1) switches to A2 via a light-induced C-C isomerization (Fig.…”

Learning from Spiropyrans: How to Make Further Developments of Donor‐Acceptor Stenhouse Adducts

“…(2) The excited state transfers to intermediate Aˈ through a C2‐C3 isomerization in nanoseconds as an actinic step, which has been observed by evolution‐associated difference spectra (EADS); [ 61‐63 ]…”