Improving Fatigue Resistance of Dihydropyrene by Encapsulation within a Coordination Cage

Photochromic
molecules undergo reversible isomerization upon irradiation
with light at different wavelengths, a process that can alter their
physical and chemical properties. For instance, dihydropyrene (DHP)
is a deep-colored compound that isomerizes to light-brown cyclophanediene
(CPD) upon irradiation with visible light. CPD can then isomerize
back to DHP upon irradiation with UV light or thermally in the dark.
Conversion between DHP and CPD is thought to proceed via a biradical
intermediate; bimolecular events involving this unstable intermediate
thus result in rapid decomposition and poor cycling performance. Here,
we show that the reversible isomerization of DHP can be stabilized
upon confinement within a PdII6L4 coordination cage. By protecting this reactive intermediate using
the cage, each isomerization reaction proceeds to higher yield, which
significantly decreases the fatigue experienced by the system upon
repeated photocycling. Although molecular confinement is known to
help stabilize reactive species, this effect is not typically employed
to protect reactive intermediates and thus improve reaction yields.
We envisage that performing reactions under confinement will not only
improve the cyclic performance of photochromic molecules, but may
also increase the amount of product obtainable from traditionally
low-yielding organic reactions.