Light-Emitting Molecular Machines: pH-Induced Intramolecular Motions in a Fluorescent Nickel(II) Scorpionate Complex

A cyclam-like macrocycle has
been synthesized with a pendant arm
containing a dansylamide group. In the
corresponding nickel() complex, binding
of the pendant arm to the metal is
pH controlled. In particular, at pH 4.3,
the sulfonamide group deprotonates and
coordinates the NiII center, giving rise to
a complex of trigonal bipyramidal stereochemistry,
as shown by X-ray diffraction
studies performed on the crystalline
complex salt. At pH 7.5, an OH ion
binds the metal and a six-coordinate
species forms. The binding-detachment
of the pendant arm to/from the NiII
center is signaled by changes in the
emission properties of the dansyl subunit
in the side chain; the fluorescence
of this side chain is high when the
pendant arm is not coordinated and
low when the sulfonamide group is
bound to the metal. The system investigated
represents the prototype of a
light-emitting molecular machine, driven
by a pH change.