Molecular Binding Behaviors of Sulfonated Calixarenes with Phenanthroline-diium in Aqueous Solution and Solid State: Cavity Size Governing Capsule Formation

The
molecular binding behaviors of p-sulfonatocalix­[4]­arene
(SC4A), p-sulfonatocalix­[5]­arene (SC5A),
and p-sulfonatothiacalix­[4]­arene (STC4A)
with 5,6-dihydropyrazion­[1,2,3,4-lmn]­[1,10]­phenanthroline-4,7-diium
(DP2+) were systematically investigated by crystallography,
NMR spectroscopy, and microcalorimetry at pH 1–2. The obtained
results showed that, in both aqueous solution and the solid state,
DP2+ was immersed into the cavity of the sulfonated calixarene
host in a slantwise degree with the aromatic moiety being included
first. The different slantwise degree of the guest in the host cavity
determined whether the host–guest capsule could be formed in
the solid state. Furthermore, all three sulfonated calixarene hosts
showed high affinities with DP2+ in the magnitude of 105–106 M–1 in aqueous solution,
and the binding modes for host–guest complexation were explained
from a thermodynamic viewpoint.