A light-gated STOP-GO molecular shuttle

Degenerate [2]rotaxanes, with their two identical binding sites, generally exhibit equilibrium dynamics with free energies of activation (DeltaG(double dagger)) for the shuttling process starting as low as 10 kcal x mol(-1). This DeltaG(double dagger) value can be raised quite dramatically by inserting "speed bumps" in the form of steric and/or electrostatic barriers into the linkers between the two identical binding sites. In our more recent research targeted toward the exploitation of the 4,4'-azobiphenyloxy unit (ABP) as a light-operated gate, we decided to introduce (i) four methyl groups on the one hand and (ii) four fluorine atoms on the other, at the 3,5,3',5'-positions of the ABP units to curtail binding by the CBPQT(4+) ring if not sterically in the case of i, then electronically in the case of ii. The first approach led to a gate (ABP-Me(4)) that remains closed all the time, whereas the second approach affords a gate (ABP-F(4)) that we can close with UV light and open with visible light. Herein, we show how light can be used, in conjunction with thermal energy, to raise and lower the free energy barrier at will and, in so doing, impart STOP and GO instructions upon the operation of a molecular shuttle.