[3]Rotaxanes: Two π-Systems at Close Quarters

Significance

The synthesis of [3]rotaxanes, in which two dumbbell-shaped molecules are threaded through a single macrocycle, is described. Rotaxane 2 was generated by Suzuki coupling of stilbene diboronate with iodoarene 1 in the presence of γ-cyclodextrin. Despite being occupied by a stilbene guest, rotaxane 2 tightly binds hydrophobic molecules such as cyanine 3a in aqueous solution. Thus, coupling of 3b with 1 yielded the hetero[3]rotaxane 4, in which two distinct π-systems are held in close proximity. Efficient energy transfer to the cyanine chromophore was observed upon excitation of the stilbene group.