ACID-BASE CONTROLLABLE MOLECULAR SHUTTLES

Two novel [2]rotananes, comprised of a dibenzo[24]crown-8 (DB24C8) mac roring bound mechanically to a chemical ''dumbbell'' possessing two di fferent recognition sites-viz., secondary dialkylammonium (NH2+) and 4 ,4'-bipyridinium (Bpym(2+)) units-have been synthesized by using the s upramolecular assistance to synthesis provided by, inter alia, hydroge n bonding interactions. One of these rotaxanes bears a fluorescent and redox-active anthracene (Anth) stopper unit. NMR spectroscopy and X-r ay crystallography have demonstrated that the DB24C8 macroring exhibit s 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 di splacement of the DB24C8 macroring to the Bpym(2+) recognition site, a n outcome that can be reversed by acid treatment. The switching proces ses have been investigated by H-1 NMR spectroscopy and, for the Anth-b earing rotaxane, by electrochemical and photophysical measurements. Fu rthermore, it is possible to drive the DB24C8 macroring from the dumbb ell's Bpym(2+) unit, in the deprotonated form of the Anth-bearing rota xane, by destroying the stabilizing DB24C8-Bpym(2+) charge-transfer in teractions via electrochemical reduction. The photochemical and photop hysical properties of this rotaxane (in both its protonated and deprot onated states) have also been investigated.