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

Abstract

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 < (Z)-3 < (Z)-4). As a result of the complexation with H21 the thermal relaxation rate of (E) → (Z) isomerization decreased up to 3-fold.