Precise Molecular Design of High-T 3D Organic-Inorganic Perovskite Ferroelectric: [MeHdabco]RbI (MeHdabco = N-Methyl-1,4-diazoniabicyclo[2.2.2]octane).

With the flourishing development of (CHNH)PbI, three-dimensional (3D) organic-inorganic perovskites with unique structure-property flexibility have become a worldwide focus. However, they still face great challenges in effectively inducing ferroelectricity. Despite the typical 3D perovskite structure and the ability of dabco (1,4-diazabicyclo[2.2.2]octane) to trigger phase transition, unfortunately [Hdabco]RbCl adopts a nonpolar crystal structure without ferroelectricity. Within the larger RbI framework, we assemble N-methyl-1,4-diazoniabicyclo[2.2.2]octane (MeHdabco) obtained by reducing the molecular symmetry of dabco into a new 3D organic-inorganic perovskite. As expected, MeHdabco bearing a molecular dipole moment turns out to be vital in the generation of polar crystal structure and ferroelectric phase transition occurring at 430 K. It is the first time that the dabco component has been successfully wrapped into a 3D cage to achieve ferroelectricity even through there is intensive research on dabco. This precise molecular design strategy based on the modification of molecular symmetry provides an efficient route to enrich the family of 3D organic-inorganic perovskite ferroelectrics. Intriguingly, the iodine-doped crystal can exhibit intense saffron yellow luminescence with a high quantum yield of 17.17% under UV excitation, extending its application in the field of ferroelectric luminescence and/or multifunctional devices.