Structural rigidity has been found to be advantageous for molecules if they are to find applications in functioning
molecular devices. In the search for an understanding of the relationship between the rigidity and complex stability in mechanically
interlocked compounds, the binding abilities of two π-electron-rich model compounds (2 and 4), where rigidity is introduced in the
form of phenylacetylene units, toward the π-electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT^(4+)),
were investigated. 1,4-Bis(2-(2-methoxyethoxy)ethoxy)-2,5-bis(2-phenylethynyl)benzene 2 and 1,5-bis(2-(2-methoxyethoxy)ethoxy)-
2,6-bis(2-phenylethynyl)naphthalene 4 were synthesized, respectively, from the appropriate precursor dibromides 1 and 3 of benzene
and naphthalene carrying two methoxyethoxyethoxy side chains. The rigid nature of the compounds 2 and 4 is reflected in the
reduced stabilities of their 1:1 complexes with CBPQT^(4+). Binding constants for both 2 (100 M^(-1)) and 4 (140 M^(-1)) toward CBPQT^(4+)
were obtained by isothermal titration microcalorimetry (ITC) in MeCN at 25 °C. Compounds 1-4 were characterized in the solid
state by X-ray crystallography. The stabilization within and beyond these molecules is achieved by a combination of intra- and
intermolecular [C-H· · · O], [C-H· · ·π], and [π-π] stacking interactions. The diethyleneglycol chains present in compounds 1-4
are folded as a consequence of both intra- and intermolecular hydrogen bonds. The preorganized structures in both precursors 1 and
3 are repeated in both model compounds 2 and 4. In the structures of compounds 2 and 4, the geometry of the rigid backbone is
differentsthe two terminal phenyl groups are twisted with respect to the central benzenoid ring in compound 2 and roughly
perpendicular to the plane central naphthalene core in compound 4. To understand the significantly decreased stabilities of these
complexes toward rigid guest molecules, relative to more flexible systems, we performed density functional theory (DFT) calculations
using the newly developed M06-suite of density functionals. We conclude that the reduced binding abilities are a consequence of
electronic and not steric factors, originating from the extended delocalization of the aromatic system
