A Dynamic Tetracationic Macrocycle Exhibiting Photoswitchable Molecular Encapsulation

Designing macrocycles
with appropriate molecular recognition features that allow for the
integration of suitable external stimuli to control host–guest
processes is a challenging endeavor which enables molecular containers
to solubilize, stabilize, and separate chemical entities in an externally
controllable manner. Herein, we introduce photo- and thermal-responsive
elements into a semi-rigid tetra­cationic cyclophane, OPV­Ex2Box4+, that is composed of oligo­(p-phenylene­vinylene)
pyridinium units and the biphenylene-bridged 4,4-bipyridinium extended
viologens and adopts a rectangle-like geometry. It transpires that
when the photoactive oligo­(p-phenylene­vinylene)
pyridinium unit is incorporated in a macrocyclic scaffold, its reversibility
is dramatically improved, and the configurations of the cyclophane
can go back and forth between (EE)- and (EZ)-isomers upon alternating blue light irradiation and
heating. When the macrocycle is found in its (EE)-configuration,
it is capable of binding various π-electron-rich guestse.g.,
anthracene and peryleneas well as π-electron-deficient
guestse.g., 9,10-anthra­quinone and 5,12-tetracene­quinonethrough
charge-transfer and van der Waals interactions. When irradiated with
blue light, the (EE)-isomer of the cyclophane can
be transformed successfully to the (EZ)-isomer, resulting
in the switching off of the binding affinity for guest molecules,
which are bound once again upon heating. The use of light and heat
as external stimuli to control host–guest interactions involving
a multi-responsive host and various guests provides us with a new
opportunity to design and construct more-advanced molecular switches
and machines.