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.