One
of the major goals of modern supramolecular chemistry, with
important practical relevance in many technical fields, is the development
of synthetic host/guest partners with ultrahigh affinity and selectivity
in water. Currently, most association pairs exhibit micromolar affinity
or weaker, and there are very few host/guest systems with <i>K</i><sub>a</sub> > 10<sup>9</sup> M<sup>–1</sup>,
apparently
due to a barrier imposed by enthalpy/entropy compensation. This present
study investigated the threading of a water-soluble tetralactam cyclophane
by a deep-red fluorescent squaraine guest with flanking polyethylene
glycol chains, an association process that is dominated by a highly
favorable enthalpic driving force. A squaraine structure was rationally
designed to permit guest back-folding as a strategy to greatly expand
the hydrophobic surface area that could be buried upon complexation.
Guided by computational modeling, an increasing number of <i>N</i>-benzyl groups were appended to the squaraine core, so
that, after threading, the aromatic rings could fold back and stack
against the cyclophane periphery. The final design iteration exhibited
an impressive combination of fluorescence and supramolecular properties,
including ratiometric change in deep-red emission, picomolar affinity
(<i>K</i><sub>a</sub> = 5.1 × 10<sup>10</sup> M<sup>–1</sup>), and very rapid threading (<i>k</i><sub>on</sub> = 7.9 × 10<sup>7</sup> M<sup>–1</sup> s<sup>–1</sup>) in water at 25 °C. Similar excellent behavior
was observed in serum solution. A tangible outcome of this study is
a new cyclophane/squaraine association pair that will be a versatile
platform for many different types of fluorescence-based imaging and
diagnostics applications. From a broader perspective, guest back-folding
of aromatic groups is a promising new supramolecular stabilization
strategy to overcome enthalpy/entropy compensation and produce ultrahigh
affinity [2]pseudorotaxane complexes in water and biological media
