Above Room Temperature Organic Ferroelectrics: Diprotonated 1,4-Diazabicyclo[2.2.2]octane Shifts between Two 2-Chlorobenzoates

A pure organic single crystal, [H₂dabco]·[2CB]₂ ([H₂dabco]²⁺ = diprotonated 1,4-diazabicyclo[2.2.2]octane, 2CB^- = 2-chlorobenzoate), which undergoes a ferroelectric-to-paraelectric phase transition above room temperature (∼323 K upon heating), was prepared and characterized. This ferroelectric crystal possesses a distinctive supramolecular architecture composed of discrete H-bonded trimeric units (two 2CB^- anions bridged by one [H₂dabco]²⁺ cation through N-H···O hydrogen bond interactions). In the paraelectric phase, the [H₂dabco]²⁺ cation is rotationally disordered and lies at the symmetric center of the trimer. Upon cooling, it is frozen in an ordered state and deviates toward a 2CB^- anion at one end along the H-bond. The collective displacement of the cations leads to a polarization of the single crystal along the crystallographic c axis, which is confirmed by the temperature dependence of the second harmonic generation and spontaneous polarization. A significant increase in the phase transition temperature of the deuterated analogue suggests that the proton plays an important role in the ferroelectric phase transition.