Spatial Symmetry Operation Breaking Sulfur-Chiral Photoswitchable Ferroelectrics

Ferroelectrics with switchable spontaneous polarization have gained enormous attention for a century. However, the structural phase transitions of conventional ferroelectrics have been mainly limited to the ones with crystallographic point group changes accompanied by the breaking of spatial symmetry elements. Moreover, although chiral and photoswitchable ferroelectrics have both been of great interest in recent years, heteroatomic chiral photoswitchable ferroelectrics have never been reported. Herein, for the first time, we designed a pair of sulfur-chiral photoswitchable ferroelectrics (R_s and S_s)-N-(3-bromo-5-chloro-2-hydroxybenzylidene)-2-methylpropane-2-sulfinamide (1-R_s and 1-S_s). Notably, they undergo a structural phase transition from the chiral polar C2 to P2₁ space group of point group 2 at around 142 K, accompanied by the symmetry operation breaking from 2-fold rotation and screw rotation in C2 to only 2-fold screw rotation in P2₁. Such a structural phase transition with the same point group, while different space groups with the breaking of symmetry operations, is significantly distinct from the structural phase transition with different point groups and space groups in conventional ferroelectrics. Furthermore,1-R_s exhibits the highest piezoelectric d₃₃ value of 10 pC/N among organic photoswitchable ferroelectrics and shows optical control of ferroelectric polarization. To our knowledge, this is the first report of sulfur-chiral photoswitchable ferroelectrics showing phase transitions with only the space group change. This work enriches the family of chiral ferroelectrics and provides great inspiration for the exploitation of heteroatomic chiral photoswitchable ferroelectrics.