Molecular tweezers as synthetic receptors: molecular recognition of cationic substrates; an insight into the mechanism of complexation

The results of 1H NMR binding studies of mole-cular tweezers 1a with various aliphatic and aromatic cations in organic solvents are described. By the use of the viologene substrates 11 and 12 bearing bulky endgroups and therefore having a dumb-bell topology it could be demonstrated that besides the obvious mechanism of complexation in which the guest molecule enters the tweezer's cavity through the open sides of the host, a second mechanism is available in which the substrate enters the cavity through a gap emerging after a substantial spreading of the tweezer's tips by at least of about 280 pm.