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

摘要

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 phenylene-bridged bipyridinium (P-BIPY~(2+)) unit and a cyclobis (paraquat-p-phenylene) (CBPQT~(4+)) ring component. The [2]rotaxane exists in the ground-state co-conformation (GSCC) wherein the CBPQT~(4+) ring encircles the DNP unit. Reduction of the CBPQT~(4+) leads to the CBPQT~(2(·+)) diradical dication while the P-BIPY~(2+) 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 × 10~7 s~(-1)) and activation energy (△G~* = 1.01 kcalmo~(-1)) for electron transfer. These values were compared to the corresponding values predicted, using the optical absorption spectra and Marcus-Hush theory.