Light-switched metal-tunneling across a π-basic tube of 1,3-alternate-calix[4]arenes

Three 1,3-alternate-calix[4]arenes (1-3) bearing two anthracenyl groups at one side and two propoxyl or ethoxyethoxyl groups at the ether side were synthesized. The aim of this molecular design is to reversibly change the metal-binding ability of the anthracene-linked side through their photoinduced cycloaddition and reverse thermal decomposition. Since certain metal cations tend to pass from one side to the other side across a π-basic calix[4]arene tube, one can expect that a light-triggered metal tunneling for this molecule. In compounds 1 and 2 the rate of the metal tunneling was faster than the ¹H NMR time-scale and the distribution of metal cations between two sides could not be determined. In a 3 · Ag⁺ complex the coalescence temperature appeared at -20°C and Ag⁺ is bound to the ethoxyethoxyl side more than to the anthracenyl side in the dark. After photoirradiation Ag⁺ was exclusively bound to the photo-cycloaddition side. These results support the view that compound 3 behaves like a mechanochemical 'syringe', the action of which is controllable by light.