Cyclophane-Sustained Ultrastable Porphyrins

We
report the encapsulation of free-base and zinc porphyrins by
a tricyclic cyclophane receptor with subnanomolar binding affinities
in water. The high affinities are sustained by the hydrophobic effect
and multiple [CH···π] interactions covering large
[π···π] stacking surfaces between the substrate
porphyrins and the receptor. We discovered two co-conformational isomers
of the 1:1 complex, where the porphyrin is orientated differently
inside the binding cavity of the receptor on account of its tricyclic
nature. The photophysical properties and chemical reactivities of
the encapsulated porphyrins are modulated to a considerable extent
by the receptor. Improved fluorescence quantum yields, red-shifted
absorptions and emissions, and nearly quantitative energy transfer
processes highlight the emergent photophysical enhancements. The encapsulated
porphyrins enjoy unprecedented chemical stabilities, where their D/H
exchange, protonation, and solvolysis under extremely acidic conditions
are completely blocked. We anticipate that the ultrahigh stabilities
and improved optical properties of these encapsulated porphyrins will
find applications in single-molecule materials, artificial photodevices,
and biomedical appliances.