(372b) Electrochemical Switching of a Fluorescent Molecular Rotor Embedded within a Bistable Rotaxane.

I report how the constrained environment introduced by the mechanical bonds within an electrochemically switchable bistable [2]rotaxane controls the rotation of a fluorescent rotor, namely 8-phenyl-substituted boron dipyrromethene (BODIPY). The electrochemical switching of the bistable [2]rotaxane induces changes in the ground-state co-conformation and the corresponding excited-state properties of the BODIPY rotor. In the starting redox state, when no external potential is applied, the cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) ring component encircles the tetrathiafulvalene (TTF) unit on the dumbbell component, leaving the BODIPY rotor unhindered and exhibiting low fluorescence. Upon oxidation of the TTF unit to a TTF²⁺ dication the CBPQT⁴⁺ ring is forced toward the molecular rotor leading to an increased energy barrier for the excited-state to rotate the rotor into the state with the high non-radiative rate constant, resulting in an overall 3.4-fold fluorescent enhancement. On the other hand, when the solvent polarity is high enough to stabilize the excited charge transfer state between the BODIPY rotor and the CBPQT⁴⁺ ring, the movement of the ring towards the BODIPY rotor produces a strong fluorescent signal decrease. This fluorescence quenching results from photoinduced electron transfer from the BODIPY rotor to the CBPQT⁴⁺ ring, as demonstrated by femtosecond transient absorption spectroscopy. The modulation of switchable electrochromic behavior makes this bistable [2]rotaxane amenable to electro-optical applications.