Mechanically induced intramolecular electron transfer in a mixed-valence molecular shuttle

The kinetics and thermodynamics of intramolecular electron transfer (IET) can be subjected to redox control in a bistable [2]rotaxane comprised of a dumbbell component containing an electron-rich 1,5-dioxynaphthalene (DNP) unit and an electron-poor phenylenebridged bipyridinium (P-BIPY²⁺) unit and a cyclobis (paraquatp-phenylene) (CBPQT⁴⁺) ring component. The [2]rotaxane exists in the ground-state co-conformation (GSCC) wherein the CBPQT⁴⁺ring encircles the DNP unit. Reduction of the CBPQT ⁴⁺ leads to the CBPQT^2(•+) diradical dication while the P-BIPY²⁺ unit is reduced to its P-BIPY^•+ radical cation. A radical-state co-conformation (RSCC) results from movement of the CBPQT^2(•+) ring along the dumbbell to surround the P-BIPY ^•+ unit. This shuttling event induces IET to occur between the pyridinium redox centers of the P-BIPY^•+ unit, a property which is absent between these redox centers in the free dumbbell and in the 1:1 complex formed between the CBPQT^2(•+)ring and the radical cation of methyl-phenylene-viologen (MPV^•+). Using electron paramagnetic resonance (EPR) spectroscopy, the process of IET was investigated by monitoring the line broadening at varying temperatures and determining the rate constant (k_ET =1.33 × 10⁷ s^-1) and activation energy (ΔG^‡ = 1.01 kcal mol^-1) for electron transfer. These values were compared to the corresponding values predicted, using the optical absorption spectra and Marcus-Hush theory.