Self-assembling formation of a [2]catenane consisting of cyclobis (4,4″-azopyridinium-p-phenylene) and bis-p-phenylene-34-crown-10

A [2]catenane consisting of a π-electron-accepting tetracationic cyclophane of cyclobis(4,4′-azopyridinium-p-phenylene) and a π-electron-donating macrocyclic polyether of bis-p-phenylene-34-crown-10 was synthesized via a template-directed synthesis in 68% yield. The [2]catenane exhibited charge transfer bands with λ_max = 526 nm in CH₃CN. A precursor of the cyclophane, bis[4-(4-pyridylazo)pyridinium], spontaneously formed a charge transfer complex with the macrocyclic polyether. The investigation of the charge transfer complex using UV-visible and ¹H NMR spectroscopy revealed that the complex had a pseudo-rotaxane structure with a stability constant (K_a) of 120 dm³ mol^-1 at 25°C in CH₃CN. The highly efficient catenation of 68% yield was attributable to cooperative self-assembling processes derived from the strongly π-electron deficient 4,4′-azopyridinium and 4-(4-pyridylazo)pyridinium units. These results suggested that there was a new formation mechanism of the catenated structure through preorganization of the charge transfer complex.