Stabilization of thermally unstable photoisomers of pyridinium-functionalized hemithioindigo switches by host-guest complexation

• Published in: Tetrahedron 2021-12, Vol.102, p.132499, Article 132499
• Main Authors: Bruekers, Jeroen P.J., Bakker, Rob, White, Paul B., Tinnemans, Paul, Elemans, Johannes A.A.W., Nolte, Roeland J.M.
• Format: Article
• Language: English
• Subjects: Host-guest chemistry; Photostationary state; Porphyrins; Pyridinium compounds; Switches
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/j.tet.2021.132499

Pyridine-functionalized hemithioindigo photoswitches (Z)-6-(Z)-8 were obtained by an aldol condensation of benzothiophenone 5 with the appropriate pyridinecarbaldehyde. These compounds were methylated with methyl iodide or methyl triflate to obtain pyridinium-functionalized hemithioindigo photoswitches (Z)-2-(Z)-4. The resulting compounds (Z)-2-(Z)-4 and (Z)-6-(Z)-8 could be isomerized to the (E)-isomer with light of λ = 470 nm, with relative abundances of the photoisomers up to 96%. All photoswitches except (Z)-2 could be isomerized back to the (Z)-state with light of longer wavelengths. For (Z)-2 this process of adopting a new photostationary state was slower compared to the thermal relaxation. Switches (Z)-2-(Z)-4 were capable of binding in a porphyrin cage host (H21) with association constants that increased with the distance between the positive charge and the central double bond of the switch: ((Z)-2