Tailoring Photoisomerization Pathways in Donor–Acceptor Stenhouse Adducts: The Role of the Hydroxy Group

Abstract

Donor-acceptor Stenhouse adducts (DASAs) are a rapidly emerging class of visible light-activatable negative photochromes. They are closely related to (mero)cyanine dyes with the sole difference being a hydroxy group in the polyene chain. The presence or absence of the hydroxy group has far-reaching consequences for the photochemistry of the compound: cyanine dyes are widely used as fluorescent probes, whereas DASAs hold great promise for visible light-triggered photoswitching. Here we analyze the photophysical properties of a DASA lacking the hydroxy group. Ultrafast time-resolved pump-probe spectroscopy in both the visible and IR region show the occurrence of E-Z photoisomerization on a 20 ps time scale, similar to the photochemical behavior of DASAs, but on a slower time scale. In contrast to the parent DASA compounds, where the initial photoisomerization is constrained to a single position (next to the hydroxy group), H NMR in situ-irradiation studies at 213 K reveal that for nonhydroxy DASAs E-Z photoisomerization can take place at two different bonds, yielding two distinct isomers. These observations are supported by TD-DFT calculations, showing that in the excited state the hydroxy group (pre)selects the neighboring C-C bond for isomerization. The TD-DFT analysis also explains the larger solvatochromic shift observed for the parent DASAs as compared to the nonhydroxy analogue, in terms of the dipole moment changes evoked upon excitation. Furthermore, computations provide helpful insights into the photoswitching energetics, indicating that without the hydroxy group the 4π-electrocyclization step is energetically forbidden. Our results establish the central role of the hydroxy group for DASA photoswitching and suggest that its introduction allows for tailoring photoisomerization pathways, presumably both through (steric) fixation via a hydrogen bond with the adjacent carbonyl group of the acceptor moiety, as well as through electronic effects on the polyene backbone. These insights are essential for the rational design of novel, improved DASA photoswitches and for a better understanding of the properties of both DASAs and cyanine dyes.

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

“…In conclusion, although solvents strongly influence the overall photoswitching of DASAs,t he kinetics of the actinic step is only slightly perturbed. Time-resolved spectroscopy suggests that the same key intermediate A' ' is produced across all solvents studied, giving credibility to the proposed photoswitching mechanism, [15,16,30] and showing that in the presented cases the thermal steps are likely rate-limiting.W ith af ull understanding of the actinic step that has now been obtained, it is clear that the focus of future studies will need to shift toward the thermal part of the reaction mechanism to further improve photoswitching and reduce detrimental solvent effects.W ef oresee immediate application of the lessons learned that make use of the peculiarities of DASAs, not seen in other photoswitches,t oi nspire the rapidly developing field of visible light molecular photoswitches and beyond.…”

“…Calculations not only account for the negative DASA solvatochromism ( Figures S4.1-S4.3, SI section 4), [30] but also for solvent-induced differences in the spectral shifts for A and A' ' observed in ultrafast and steady state measurements.T he spectral separation of the maximum absorption of A' ' (positive peak in the long-lived component, blue EADS) and A (negative band in short-lived component, black EADS) decreases with increasing solvent polarity,w ith concurrent broadening of the absorption band of the elongated form A in more polar protic solvents.This explains why abathochromically shifted absorption band was not observed in previous steady-state spectroscopic measurements performed under continuous illumination. [15] Aproton-transfer-the nature of which remains elusiveis expected to be involved in the photoswitching mechanism ( Figure 1b).…”

Inducing molecular isomerization assisted by water

“…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.…”

It's a Trap: Thiol‐Michael Chemistry on a DASA Photoswitch

“…Undero ptimized conditions,t he reactionp roceeds preferentially in the presenceo fl ight and base. [10][11][12][13] Clarification of the cyclization mechanism resulted in ap ool of DASA photoswitches bearing avariety of acceptorand donor functionalitiesaswell as an improvedk nowledge of factors influencing the switching process. [1] Light, as ar eadily available and adjustable reactiont rigger,i sp articularly interesting for implementing spatiotemporal control on reaction outcomes.…”

“…[2][3][4][5] Photoswitches, such as derivatives of the donor-acceptor Stenhouse adduct (DASA), coexisti na ne quilibrium between two isomeric forms.O ne isomer consists of an intensely colored (blue, purple or orange)n onpolar linear form, whereas the other isomer is cyclized, polar,c olorless and commonly zwitterionic (Scheme 1). [10][11][12][13] Clarification of the cyclization mechanism resulted in ap ool of DASA photoswitches bearing avariety of acceptorand donor functionalitiesaswell as an improvedk nowledge of factors influencing the switching process. The cyclization mechanism hasbeen clarified in ac areful study by Feringa and Buma,i nw hichd ensity functional theory calculationsa nd transient absorption kinetics measurements were employed.…”