Fluorophoric [2]rotaxanes: post-synthetic functionalization, conformational fluxionality and metal ion chelation

Abstract

Herein, we report the synthesis of an amino-ether fluorophoric macrocyclic wheel (MC)-based multi-functional [2]rotaxane. This rotaxane was further functionalised into three different analogues, which were denoted as ROTa–c, upon appending three different amide groups [–NC(O)R, where R = –CH₃, 4-fluorophenyl and tertiary butyl, respectively]. These post-synthetic analogues were characterised via mass spectrometry, 1D/2D NMR and optical spectroscopy. The steric effects exerted by the tertiary amide groups on the rotamer-induced dynamic motions in ROTa–c were investigated via variable-temperature NMR spectroscopy. Although the tri-acetylated ROTa and tri-aryl-substituted ROTb showed diverse rotamer-induced conformations/co-conformations, one conformer was predominant in ROTc under identical experimental conditions due to the restricted rotation of its bulky tertiary butyl groups. Interestingly, intramolecular exciplex formation between the wheel and the axle was exhibited by ROTa–c, which was established through detailed emission studies in the solution state. The exciplex emission generated by the rotaxanes was monitored to predict the preferential coordination sites of metal ions (alkali and transition) in the above interlocked systems. Finally, the chelating property of the fluorophoric rotaxanes was investigated with different transition metal ions via absorption and emission spectroscopy studies.