Quantifying Intrinsic
Ion-Driven Conformational Changes
in Diphenylacetylene Supramolecular Switches with Cryogenic Ion Vibrational
Spectroscopy
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Abstract
We report how two flexible diphenylacetylene (DPA) derivatives
distort to accommodate both cationic and anionic partners in the binary
X<sup>±</sup>·DPA series with X = TMA<sup>+</sup> (tetramethylammonium),
Na<sup>+</sup>, Cl<sup>–</sup>, Br<sup>–</sup>, and
I<sup>–</sup>. This is accomplished through theoretical analysis
of X<sup>±</sup>·DPA·2D<sub>2</sub> vibrational spectra,
acquired by predissociation of the weakly bound D<sub>2</sub> adducts
formed in a 10 K ion trap. DPA binds the weakly coordinating TMA<sup>+</sup> ion with an arrangement similar to that of the neutral compound,
whereas the smaller Na<sup>+</sup> ion breaks all intramolecular H-bonds
yielding a structure akin to the transition state for interconversion
of the two conformations in neutral DPA. Halides coordinate to the
urea NH donors in a bidentate H-bonded configuration analogous to
the single intramolecular H-bonded motif identified at high chloride
concentrations in solution. Three positions of the “switch”
are thus identified in the intrinsic ion accommodation profile that
differ by the number of intramolecular H-bonds (0, 1, or 2) at play