Acid-base controllable molecular shuttles

Abstract

Two novel [2]rotaxanes, comprised of a dibenzo[24]crown-8 (DB24C8) macroring bound mechanically to a chemical 'dumbbell' possessing two different recognition sites - viz., secondary dialkylammonium (NH2+) and 4,4'-bipyridinium (Bpym2+) units - have been synthesized by using the supramolecular assistance to synthesis provided by, inter alia, hydrogen bonding interactions. One of these rotaxanes bears a fluorescent and redox- active anthracene (Anth) stopper unit. NMR spectroscopy and X-ray crystallography have demonstrated that the DB24C8 macroring exhibits complete selectivity for the NH2+ recognition sites, i.e., that the [2]rotaxanes exist as only one of two possible translational isomers. Deprotonation of the rotaxanes' NH2+ centers effects a quantitative displacement of the DB24C8 macroring to the Bpym2+ recognition site, an outcome that can be reversed by acid treatment. The switching processes have been investigated by 1H NMR spectroscopy and, for the Anth-bearing rotaxane, by electrochemical and photophysical measurements. Furthermore, it is possible to drive the DB24C8 macroring from the dumbbell's Bpym2+ unit, in the deprotonated form of the Anth-bearing rotaxane, by destroying the stabilizing DB24C8-Bpym2+ charge- transfer interactions via electrochemical reduction. The photochemical and photophysical properties of this rotaxane (in both its protonated and deprotonated states) have also been investigated.