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

Abstract

Donor-acceptor Stenhousea dducts( DASA) are popularp hotoswitches capable of togglingb etween two isomersd epending on the light and temperature of the system.T he cyclized polar form is accessed by visible-light irradiation, whereas the linear nonpolar form is recovered in the dark. Upon the formation of the cyclized form,t he DASA contains ad ouble bond featuring a b-carbonp rone to nucleophilica ttack.H ere, an isomer selective thiol-Michael reaction betweenthe cyclized DASA and ab ase-activated thiol is introduced. The thiol-Michael addition was carried out with an alkyl (1-butanethiol) and an aromatic thiol (p-bromothiophenol) as reaction partners, both in the presence of ab ase. Undero ptimized conditions,t he reactionp roceeds preferentially in the presenceo fl ight and base. The current study demonstrates that DASAsc an be selectively trapped in their cyclized state.Molecular photoswitches undergo reversible isomerization upon irradiation with light, and sometimes in combination with ad ifferent stimulus, for example, temperature. [1] Light, as ar eadily available and adjustable reactiont rigger,i sp articularly interesting for implementing spatiotemporal control on reaction outcomes. For example, photoswitches are employed for biomedical andb iochemical processes including targetedd rug delivery and chemical/biological sensing applications. [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). [6][7][8][9] The equilibrium between the two forms is dependent on the solvente nvironment andt he structural motifs of the DASA derivatives, consistingo fa ne lectron donor and an acceptorm oiety,r esultingi napush-pull system.U pon irradiation, the equilibrium is shifted towardst he ring cyclized isomer (cyclized form) and, upon thermalr elaxation, it returns to the initialc onjugated state (linear form). 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. [32] The current consensus on the cyclization processi nvolves isomerizationo ft he double bonds, followedby ab ond rotation, and finallyathermal conrotatory4 p electrocyclization leading to the cyclized ring, commonly stabilizedb yi ts zwitterionic form through ap roton transfer. [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. [8,9,14,15] The inherent advantages of DASA photoswitches, which includesn egative photochromism, inexpensives tarting materials, modular synthesis, tunable absorption, visible-light switching, and good fatiguer esistance, are appealing for ar ...

Proton‐Gated Ring‐Closure of a Negative Photochromic Azulene‐Based Diarylethene

“…Stimuli‐responsive molecules are the basis of information processing in biological as well as synthetic complex systems. Among them, photochromic molecules are particularly attractive for the ability to convert between (at least) two states by noninvasive and rapid optical stimuli with a potential of high spatial and temporal resolution . Additionally, multiple photochromic molecules have been developed, which in addition to light respond to a secondary stimulus, such as the presence of ions, oxidants/reductants, or acids/bases .…”

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

“…From the application point of view, especially considering their capacity to be covalently anchored and their solvent dependence, π‐bridge substituted DASAs could be proposed as fine‐tuned molecular probes or as novel substrates for DASA reactivity [21] . Also, depending on the type of closed form that is generated (zwitterionic or non‐zwitterionic, see Figure 7), new applications could be envisioned taking advantage of the infrared DASA chemical transformation as a first step of a sequence.…”

“…Different experimental and computational studies have focused on distinct DASAs applications, [6–12] properties, [13–15] switching mechanism, [16–20] or reactivity [21] …”

Protonenvermittelter Ringschluss eines negativ photochromen, Azulen‐basierten Diarylethens

“…Auf Stimuli reagierende Moleküle sind die Grundlage der Informationsverarbeitung in biologischen wie auch synthetischen, komplexen Systemen. Unter ihnen sind photochrome Moleküle aufgrund ihrer Fähigkeit zur Umwandlung zwischen (mindestens) zwei Zuständen durch nicht‐invasive und schnelle optische Stimuli bei gleichzeitig potentiell hoher räumlicher und zeitlicher Auflösung besonders attraktiv . Darüber hinaus wurden photochrome Moleküle entwickelt, die zusätzlich zu Licht auf einen sekundären Stimulus reagieren, beispielsweise auf die Anwesenheit von Ionen, Oxidations‐/Reduktionsmitteln, oder Säuren/Basen .…”