Thermally Activated Transient Dipoles and Rotational Dynamics of Hydrogen-Bonded and Charge-Transferred Diazabicyclo [2.2.2]Octane Molecular Rotors

We present here dielectric properties and rotational dynamics of cocrystals formed with either triphenylacetic acid (cocrystal I) or 9,10-triptycene dicarboxylic acid (cocrystal II), as hydrogen-bonding donors, and diazabicyclo[2.2.2]­octane (DABCO), as a ditopic hydrogen-bond acceptor. While cocrystal I forms discrete 2:1 complexes with one nitrogen of DABCO hydrogen bonded and the other fully proton transferred, cocrystal II consists of 1:1 complexes forming infinite 1-D hydrogen-bonded chains capable of exhibiting a thermally activated response in the form of a broad asymmetric peak at ca. 298 K that extends from ca. 200 to 375 K in both the real and imaginary parts of its complex dielectric. The state of protonation in cocrystal II at 298 and 386 K was established by CPMAS 15N NMR, which showed signals typical of a neutral hydrogen-bonded complex. Taken together, these observations suggest a dielectric response that results from a small population of transient dipoles thermally generated when acidic protons are transiently transferred to either side of the DABCO base. A potential order–disorder transition further explored by taking advantage of the highly sensitive rotational dynamics of the DABCO group using line-shape analysis of solid-state spin echo 2H NMR and 1H NMR T1 spin–lattice relaxation showed no breaks in the Arrhenius plot or Kubo-Tomita 1H T1 fittings, indicating the absence of large structural changes. This was confirmed by variable-temperature single-crystal X-ray diffraction analysis, which showed a fairly symmetric hydrogen bond in cocrystal II at all temperatures, suggesting that both nitrogen atoms may be able to adopt a protonated state.