Photoresponsive Organic-Inorganic Hybrid Ferroelectric Designed at the Molecular Level

Molecular ferroelectrics are becoming an important area of research due to their ability to form a variety of structures exhibiting the desired properties. However, the precise control over the assembly of molecular building blocks for the design and synthesis of photoresponsive molecular ferroelectrics remains a considerable challenge. Here, we report a new hybrid high-temperature ferroelectric, (Me₂NH₂)[NaFe(CN)₅(NO)], by judiciously assembling inorganic photochromic nitroprusside anion, as the framework building block, and polar organic cation Me₂NH₂⁺, as the dipole-moment carrier, into the crystal lattice. Ferroelectricity arises through the synergetic ordering of Me₂NH₂⁺ below 408 K. Piezoresponse force microscopy witnessed the presence of 180° ferroelectric domains and evidenced polarization switching by repeatedly applying an external electric field. Irradiation of the N-bound nitrosyl ligand (ground state) leads to two different conformations: isonitrosyl O-bound (metastable state I) and side-on nitrosyl conformation (metastable state II). Such photoisomerization realized in solid-state molecular ferroelectrics allows for the photoswitching between the ferroelectric ground state and the metastable state. These results pave the way for new design approaches toward developing next-generation photostimulated ferroelectric materials at the molecular level.