Control of Photoisomerization of an Azoazacryptand by Anion Binding and Cucurbit[8]uril Encapsulation in an Aqueous Solution

Control of isomerization
of a receptor bearing multiple light-switchable
subunits in a confined space is critical for the design of synthetic
molecular machines. Toward this goal, a new azacryptand containing
three azobenzene subunits has been developed, and its photoisomerization
in an aqueous solution has been studied depending on anion coordination
and recognition by a larger hostcucurbit[8]­uril (CB[8]). The
cryptand in its hexaprotonated form shows considerable affinity for
fluoride and perchlorate, which in turn affects the isomer distribution
of the receptor under UV-light irradiation, stabilizing the isomers
of the cryptand with Z-configurations. CB[8] was
found to be able to encapsulate the isomers of the cryptand by forming
a Matryoshka-type complex. The irradiation of a 10:1 CB[8]–cryptand
mixture has led to a selective conversion of the cryptand to the E,E,Z isomer inside CB[8].
It has been demonstrated that the addition of fluoride to the resulted
complex induces the release of the cryptand as a major E,E,E isomer, while other studied
anions were ineffective in this reaction. To our knowledge, this work
presents a first example of a host-controlled photoisomerization of
an anion receptor bearing multiple switching azobenzenes that model
the function of naturally occurring chaperones.