Real-Time Monitoring of Wheel Movement in a [2]Rotaxane Using a Competing Kinetic Pathway

The wheel movement of rotaxanes is typically fast in solution. Therefore, real-time monitoring of such movement is one of the challenging topics. Herein, we report real-time monitoring of wheel movement in a [2]rotaxane (comprising hydrophilic pillar[5]arene as a wheel and hydrophobic π-conjugated moiety with hydrophilic tetra(ethylene oxide) ends as an axle) via a competing kinetic pathway, i.e., aggregation/dissociation equilibrium. In non-aqueous solvent, the [2]rotaxane shows blue-purple locally excited emission from the π-conjugated moiety because the pillar[5]arene wheel shuttles over the whole axle. In aqueous solvent, the [2]rotaxane forms aggregates owing to its amphiphilic nature. Because the [2]rotaxane forms the kinetically stable aggregates, the location of the pillar[5]arene wheel on the axle is maintained, resulting in the locally excited emission even in the aqueous conditions. Surprisingly, the emission color gradually changes from blue-purple to light blue over 720 h because exposure of the monomeric [2]rotaxane generated by dissociation of the kinetically stable aggregates, to the aqueouspola environment triggers movement of the hydrophilic wheel to the hydrophobic π-conjugated moiety by the hydrophobic effect. After this movement, the monomeric [2]rotaxane forms the thermodynamically stable aggregates, which exhibit light-blue emission from the charge-transfer complex between the electron-donating pillar[5]arene wheel and the electron-accepting π-conjugated moiety.