Reorientational dynamics of methylbenzylammonium cations in the chiral metal halide composite MBAMnCl3·2H2O

Abstract

We report results from quasielastic neutron scattering (QENS) measurements of the organic cation dynamics in the manganese-based organic–inorganic chiral metal halide composites (R)-MBAMnCl₃·2H₂O and (S)-MBAMnCl₃·2H₂O, where (R)-/(S)-MBA (methylbenzylammonium) = C₆H₅CH(CH₃)NH₃⁺. Measurements upon heating from 44 to 375 K reveal how the dynamics evolve from (an)harmonic vibrations to localized, reorientational motions of the (R)-/(S)-MBA cations at approximately 270 K, for both materials. Analysis of the elastic incoherent structure factors (EISFs) of the materials suggests that the spatial geometry of the reorientational dynamics changes, and becomes more complex, with increasing temperature. Up to 325 K, the dynamics can be described as three-fold (C₃) jump-diffusion dynamics or continuous rotational diffusion of the methyl (–CH₃) and ammonium (–NH₃) groups of the (R)-/(S)-MBA cations. At higher temperatures, 350–375 K, the analysis points towards the additional presence of continuous rotational diffusion of the –C₆H₅ group of the (R)-/(S)-MBA cation around its long molecular axis, as well as a whole-cation reorientation, here described as two-fold (C₂) jump-diffusion dynamics between two different cation orientations. Such whole-cation dynamics have previously been observed mainly in materials with smaller organic cations. This suggests that the dynamics of the (R)-/(S)-MBA cation are relatively mobile in nature, with relatively weak interactions with the surrounding lattice.