Molecular shuttles based on tetrathiafulvalene units and 1,5-dioxynaphthalene ring systems

Six different degenerate [2]rotaxanes were synthesized and characterized. The rotaxanes contained either two tetrathiafulvalene (TTF) units or two 1,5-dioxynaphthalene (DNP) ring systems, both of which serve as recognition sites for a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. Three different spacer units were incorporated into the dumbbell components of the [2]rotaxanes between the recognition sites. They include a polyether chain, a terphenyl unit, and a diphenyl ether linker, all of which were investigated in order to probe the effect of the spacers on the rate of the shuttling process. Data from dynamic 1H NMR spectroscopy revealed a relatively small difference in the DeltaG++ values for the shuttling process in the [2]rotaxanes containing the three different spacers, in contrast to a large difference between the TTF-containing rotaxanes (18 kcal mol(-1)) and the DNP-containing rotaxanes (15 kcal mol(-1)). This 3 kcal mol(-1) difference is predominantly a result of a ground-state effect, reflecting the much stronger binding of TTF units to the CBPQT4+ ring in comparison with DNP ring systems. An examination of the enthalpic (DeltaH++) and entropic (DeltaS++) components for the shuttling process in the DNP-containing rotaxanes revealed significant differences between the three spacers, a property which could be important in designing new molecules for incorporation into molecular electronic and nanoelectromechanical (NEMs) devices.