Fluorophoric Rotaxanes: Post-Synthetic Functionalization, Conformational Fluxionality and Metal Ion Chelation
Herein, we report an amino-ether fluorophoric macrocyclic wheel (MC) based multi-functional rotaxane. The said rotaxane is further functionalised to three different analogues as ROTa-c upon appending three different amide groups [-NC(O)R, where R = -CH3, 4-fluorophenyl and tertiary butyl respectively]. Such post-synthetically analogues are characterised via mass spectrometry, 1D/2D NMR and optical spectroscopy. Steric effects exerted by the tertiary amide groups on the rotamer induced dynamic motions among ROTa-c are investigated via variable temperature NMR spectroscopy. Although the tri-acetylated ROTa and tri-aryl substituted ROTb show rotamer induced conformational/ co-conformational diversities, one conformer is predominant in ROTc under identical experimental conditions due to the restricted rotation of bulky tertiary butyl groups. Interestingly, intramolecular exciplex formation between the wheel and the axle is exhibited by ROTa-c, which is established through detail emission studies in solution state. The exciplex emission generated by rotaxanes is monitored to predict the preferential coordination sites of metal ions (alkali and transition) in the above interlocked systems. Finally, chelating property of fluorophoric rotaxanes is investigated with different transition metal ions by absorption as well as emission spectroscopy studies.