Determination of the barrier height of a crystalline dipolar molecular rotor directly from temperature-dependent dielectric spectra

An explicit relation between half-width at half-maximum ΔT in a dissipation factor of dipolar molecular rotors in a solid phase and rotational barrier height U_b was derived within the Debye relaxation model. The formula for U_b is simply expressed as (Formula presented.). Here, T* is the temperature at the peak position in the dissipation factor, and k is the Boltzmann constant. The formula is an alternative to the familiar Arrhenius equation, from which U_b has so far been estimated in various dielectric experiments. The distinction of the proposed formula is the determination of U_b from a single peak in a dissipation factor induced by an electric field, while the Arrhenius equation needs data for the magnitudes of the dissipation factor and temperatures at several peaks. By applying the proposed formula to dipole molecular rotors (ROT-2F) in a single crystalline and polycrystalline, for which U_b values were reported (Horansky et al., Phys. Rev. B 74, 054306 [2006]), the calculated values of U_b were quantitatively reproduced. This indicates the validity of the proposed formula, and the formula is expected to be applied to the analysis of dielectric spectra of 2D dipolar molecular rotor systems.