The development of
photo-responsive ferroelectrics whose
polarization
may be remotely controlled by optical means is of fundamental importance
for basic research and technological applications. Herein, we report
the design and synthesis of a new metal-nitrosyl ferroelectric crystal
(DMA)(PIP)[Fe(CN)5(NO)] (1) (DMA = dimethylammonium,
PIP = piperidinium) with potential phototunable polarization via a
dual-organic-cation molecular design strategy. Compared to the parent
non-ferroelectric (MA)2[Fe(CN)5(NO)] (MA = methylammonium)
material with a phase transition at 207 K, the introduction of larger
dual organic cations both lowers the crystal symmetry affording robust
ferroelectricity and increases the energy barrier of molecular motions,
endowing1with a large polarization of up to 7.6 μC
cm–2and a high Curie temperature (Tc) of 316 K. Infrared spectroscopy shows that the reversible
photoisomerization of the nitrosyl ligand is accomplished by light
irradiation. Specifically, the ground state with the N-bound nitrosyl
ligand conformation can be reversibly switched to both the metastable
state I (MSI) with isonitrosyl conformation and the metastable state
II (MSII) with side-on nitrosyl conformation. Quantum chemistry calculations
suggest that the photoisomerization significantly changes the dipole
moment of the [Fe(CN)5(NO)]2–anion,
thus leading to three ferroelectric states with different values of
macroscopic polarization. Such optical accessibility and controllability
of different ferroelectric states via photoinduced nitrosyl linkage
isomerization open up a new and attractive route to optically controllable
macroscopic polarization.