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.