Simple molecular ferroelectrics: N,N′-dialkyl-terephthalamide derivatives in the solid phase

Abstract

Simple molecules of N,N′-dialkyl-1,4-benzenedicarboxamide (CnIPA) form a one-dimensional (1D) N–H⋯O hydrogen-bonding molecular assembly, which shows a solid–solid phase transition through the partial melting of alkyl chains before phase transition to an isotropic liquid without the formation of a liquid crystal phase. The phase transition, molecular assembly structure, dielectric constant, and ferroelectricity of CnTPA were evaluated by changing the alkyl chain length (n) of –CONHC_nH_2n+1 through n = 5–16, 18. The 1D hydrogen-bonded structures were further assembled into a bilayer arrangement of alkyl chains to form a lamellar-type molecular assembly, where the partial melting of two alkyl chains occurred at the high-T solid phase. An even–odd effect was observed in the phase transition behavior and the T-dependent dielectric constants of CnTPA, where the even-number derivatives had much larger motional freedom than those of the odd-number derivatives. The dielectric response was associated with the motional freedom of the polar structural unit of N–H⋯O hydrogen bonds, the dynamics of which were activated in the T range after the phase transition to the high-T solid phase. The collective dipole inversion of the N–H⋯O hydrogen-bonding chain was observed in the ferroelectric polarization–electric field (P–E) hysteresis curves of CnTPAs with n ≥ 11. The melting state of long alkyl chains in the high-T solid phase assisted the dipole inversion of N–H⋯O units to form hydrogen-bonded ferroelectrics.