Redox-Active Monopyrrolotetrathiafulvalene-Based Rotaxane Incorporating the Dihydroazulene/Vinylheptafulvene Photo/Thermoswitch

A dumbbell-shaped molecule with a central monopyrrolo-tetrathiafulvalene (MPTTF) unit and a 1,1-dicyano-dihydroazulene (DHA) photoswitch as one of the two end-groups was prepared and subsequently converted into a [2]rotaxane with cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) as the ring component. Investigations revealed that both the DHA and its vinylheptafulvene (VHF) photoisomer prevent deslipping of CBPQT⁴⁺. It was found that the presence of the CBPQT⁴⁺ ring on the MPTTF-DHA dumbbell changes the rate of the thermal back-conversion of VHF into DHA, i.e., this conversion was found to be accelerated by the presence of the positively charged CBPQT⁴⁺ ring. Studies also showed that DHA forms a weak complex with CBPQT⁴⁺, which seems to originate from weak interactions between the cyano group located on DHA and the bipyridinium α-protons on CBPQT⁴⁺. The rotaxane was subjected to various oxidation experiments, but these were, unfortunately, accompanied by some decomposition of the molecule; yet, movement of the CBPQT⁴⁺ ring from the MPTTF unit towards the DHA moiety seems to occur upon oxidation of the MPTTF unit, promoted by the weak interaction between the DHA moiety and CBPQT⁴⁺.