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 ΔG^‡ 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 kcalmol^-1) and the DNP-containing rotaxanes (15 kcalmol^-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 (ΔH^‡) and entropic (ΔS^‡) 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.