Shedding Light on the Photoisomerization Pathway of Donor–Acceptor Stenhouse Adducts

Abstract

Donor–acceptor Stenhouse adducts (DASAs) are negative photochromes that hold great promise for a variety of applications. Key to optimizing their switching properties is a detailed understanding of the photoswitching mechanism, which, as yet, is absent. Here we characterize the actinic step of DASA-photoswitching and its key intermediate, which was studied using a combination of ultrafast visible and IR pump–probe spectroscopies and TD-DFT calculations. Comparison of the time-resolved IR spectra with DFT computations allowed to unambiguously identify the structure of the intermediate, confirming that light absorption induces a sequential reaction path in which a Z–E photoisomerization of C2–C3 is followed by a rotation around C3–C4 and a subsequent thermal cyclization step. First and second-generation DASAs share a common photoisomerization mechanism in chlorinated solvents with notable differences in kinetics and lifetimes of the excited states. The photogenerated intermediate of the second-generation DASA was photo-accumulated at low temperature and probed with time-resolved spectroscopy, demonstrating the photoreversibility of the isomerization process. Taken together, these results provide a detailed picture of the DASA isomerization pathway on a molecular level.

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.…”

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

“…Furthermore,s olvents influence the band-overlap of A and A' ',potentially influencing the photostationary states reached and thus affecting overall cyclization kinetics. [15,16] and c) corresponding energy level diagrami nkcal mol À1 for 1 in selected solvents obtained at the M06-2X/6-31 + G(d)/SMDlevel of theory.Analogous diagrams for 2 and 3 are presented in the SI ( Figure S8.5). In the electronic density difference(EDD) plot (inset), the blue (red) regions correspond to adecrease (increase) in electron density upon electronic excitation.…”

Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light

“…Many potential applications of photoswitches, particularly in biological contexts require them to respond within the biooptical window, prompting synthetic efforts aimed at shifting their absorptions to longer wavelength. [4][5][6][7][8][9] Recently, new classes of photoswitches such as DASAs, [10][11][12] hydrazones, [13,14] and indigoid chromophores [15][16][17][18][19] have been developed with strong absorptions in the red region of the spectrum, and with high photoisomerization quantum yields.Indigoid photoswitches such as hemithioindigo (HTI) [20][21][22][23] and hemiindigo (HI) [24][25][26] are chromophores derived from the parent indigo dye. [27] Although HI molecules were first synthesized over a century ago [24] and their basic photoswitching characteristics have been known for at least two decades, [25] interest has recently grown due to the development of new derivatives that respond to red light, have high thermal stability of their switching states, are simple to synthesize and function-alize, and exhibit high photostability.…”

“…Many potential applications of photoswitches, particularly in biological contexts require them to respond within the biooptical window, prompting synthetic efforts aimed at shifting their absorptions to longer wavelength. [4][5][6][7][8][9] Recently, new classes of photoswitches such as DASAs, [10][11][12] hydrazones, [13,14] and indigoid chromophores [15][16][17][18][19] have been developed with strong absorptions in the red region of the spectrum, and with high photoisomerization quantum yields.…”