2-(4’-pyridyl-N-oxide)-Substituted Hemithioindigos as Photoresponsive Guests for a Super Aryl-Extended Calix[4]pyrrole Receptor

Abstract

We report the synthesis of two 2-(4’-pyridyl-N-oxide)-substituted hemithioindigos. We probed their photoisomerization using UV-Vis and 1H NMR spectroscopic techniques. Light irradiation at 450 nm provoked the isomerization of the HTI Z isomer to the E counterpart in a large extent (~80 % at the PSS). 1H NMR titration experiments revealed the formation of thermodynamically and kinetically stable 1:1 inclusion complexes of the Z-HTI isomers with a super aryl-extended host (Ka > 104 M-1). Photoirradiation at 450 nm of the inclusion complexes induced the isomerization of the bound HTI N-oxide affording the E-HTI⊂calix[4]pyrrole complex. We determined accurate association constant values for the 1:1 inclusion complexes of the Z and E-HTI isomers using isothermal titration calorimetry experiments (ITC). The results showed a 2.2-2.8-fold decrease in the stability constants of the E-HTI complexes in comparison to the Z-HTI counterparts explaining the lack of light-induced release of the former to the bulk solution.