Reversible structural phase transition and dielectric behaviour associated with the ordering-disordering of molecular rotation in an organic supramolecular solid

Abstract

• A pure organic single crystal, [H 2 dabco][Ma] 2 (1) undergoes a reversible structural phase transition at ~250 K and possesses a unique three-dimensional network structure. • The origin of the phase transition and the dielectric anomaly is ascribed to the order-disorder transformation of [H 2 dabco] 2+ cations and the resulting relative molecular rotation from the equilibrium position. A pure organic single crystal, [H 2 dabco][Ma] 2 ( 1 ) ([H 2 dabco] 2+ = diprotonated 1,4-diazabicyclo[2,2,2]octane, [Ma] = malic acid), was prepared and characterised. Differential scanning calorimetry, potential energy calculations, and dielectric property measurements indicated the occurrence of a reversible structural phase transition at ~250 K, while single-crystal X-ray diffraction revealed that 1 possesses a unique three-dimensional network structure with the [H 2 dabco] 2+ cation bridged by two [Ma] − anions through N–H•••O hydrogen bonds. At 296 K, the [H 2 dabco] 2+ cation exists in a rotationally disordered state and is fixed into the two-dimensional layered structure of [Ma] − anions through N–H•••O hydrogen bonds. Upon cooling to 100 K, the above cation is frozen into an ordered state and shifts towards the top anion and the bottom anion at one end along the hydrogen bond. The origin of the phase transition and the dielectric anomaly is ascribed to the order-disorder transformation of [H 2 dabco] 2+ cations and the resulting relative molecular rotation from the equilibrium position. One pure organic single crystal, [H 2 dabco][Ma] 2 ( 1 ) ([H 2 dabco] 2+ = diprotonated 1,4-diazabicyclo[2,2,2]octane, [Ma] ‐ = malic acid), was synthesized and underwent the structural phase transitions from space group P 2 1 /c to C 2/c. The conformations of dabco rings in the RTP (296 K) and the LTP (100 K) are markedly different owing to the order-disorder rotation of [H 2 dabco] 2+ cations. This results in the phase transition and dielectric anomaly of compound and provides useful strategies of finding new organic dielectric materials.