Different from so
far reported oxacalix[4]crown-based host–guest
motifs in which oxacalix[4]crowns act only as hydrogen bond acceptors,
a [2]pseudorotaxane-type tetranitro-oxacalix[4]crown/urea host–guest
recognition motif was developed in which tetranitro-oxacalix[4]crown
played a role as both a hydrogen bond donor and an acceptor to stabilize
the resulting supramolecular complex. Furthermore, on the basis of
a [2]pseudorotaxane complex formed from a tetranitro-oxacalix[4]crown
and an axle containing a secondary ammonium ion and a urea group,
a [2]rotaxane-based molecular switch was created, in which the oxacalix[4]crown
wheel was able to reversibly translocate between the secondary ammonium
binding site and the urea binding site of the axle under acid–base
stimulation