An organic–inorganic hybrid indium molecular ferroelectric originating from a “butterfly-flapping” model of 1,5-diazabicyclo[3.3.0]octane

Abstract

Molecular ferroelectric materials exhibit enormous application potential in fields such as flexible electronics due to their structural tunability and multifunctional properties. Therefore, the efficient design and synthesis of molecular ferroelectrics has become one of the most important topics in ferroelectric chemistry. In this study, a new compound (BrCH₂-3.3.0-Dabco)[InBr₄] (1) (3.3.0-Dabco = 1,5-diazabicyclo[3.3.0]octane) was designed by chemically modifying [3.3.0-Dabco] to reduce molecular symmetry, which shows excellent dielectric, ferroelectric, and second-order nonlinear optical properties. It belongs to the order–disorder type of molecular ferroelectric, but X-ray diffraction experiments and variable-temperature Raman spectroscopy indicate that the occurrence of its phase transition does not solely depend on the overall rotation of the molecule, but rather on the phased changes in the vibration mode of the five-membered ring within the molecule. Based on its unique molecular shape and vibration mode, we innovatively proposed the “butterfly-flapping” model, which vividly describes the connection between the microscopic changes of molecules in the crystal and the crystal phase transition, and deepens the understanding of the cation state during the phase transition of the compound. The proposal of the “butterfly-flapping” model breaks through the traditional molecular rotation mechanism of order–disorder ferroelectrics and provides a new design idea for the design of molecular ferroelectrics.