Quantifying intrinsic ion-driven conformational changes in diphenylacetylene supramolecular switches with cryogenic ion vibrational spectroscopy

We report how two flexible diphenylacetylene (DPA) derivatives distort to accommodate both cationic and anionic partners in the binary X ^±·DPA series with X = TMA⁺ (tetramethylammonium), Na⁺, Cl^-, Br^-, and I^-. This is accomplished through theoretical analysis of X ^±·DPA·2D₂ vibrational spectra, acquired by predissociation of the weakly bound D₂ adducts formed in a 10 K ion trap. DPA binds the weakly coordinating TMA⁺ ion with an arrangement similar to that of the neutral compound, whereas the smaller Na ⁺ 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.