Substituent Effect on Molecular Motions of m-Halogenated Anilinium/Dibenzo[18]crown-6 Supramolecular Cations in [Ni(dmit)₂]^- Crystals

Molecular motion within crystals can significantly affect their dielectric properties and functionalities. Crystals of (m-XAni+)(DB[18]crown-6)[Ni(dmit)2]- (m-XAni+ = m-halogenated anilinium (X = F, 1), (X = Cl, 2), (X = Br, 3), and (X = I, 4); DB[18]crown-6 = dibenzo[18]crown-6; dmit2- = 2-thioxio-1,3-dithiol-4,5-dithiolate) were synthesized, and the substituent effects of crystals 2-4 were discussed in terms of crystal structure, dielectric response, and molecular motion compared with crystal 1, which exhibited a ferroelectric transition due to the flip-flop motion of the m-FAni+ cation. Although crystals 2-4 did not show rotational disorder of the aryl groups due to the larger size of the halogen atoms, in- and out-of-plane pendulum motions were observed. Temperature-dependent crystallographic analysis revealed that crystal 2 underwent a symmetry reduction from space group P21/m at 293 K to P21 at 93 K with the disappearance of the mirror plane on m-ClAni+ associated with the stopping of the out-of-plane pendulum motion. In the case of crystals 3 and 4, an in-plane pendulum motion at small swing angles was suggested, which was further supported by potential energy calculations. Owing to the semiconducting nature of the crystals, these pendulum motions could not be detected by dielectric measurements.