A notable reversible thermochromic (3,3-difluoropyrrolidinium)2[CuCl4] with ferroelectricity and ferroelasticity

The design and fabrication of sensitive and reversible thermochromics (TCs) remain challenging although they own enormous applications. Especially, there is a tough task to introduce multifold physical properties to them. Here, a temperature-response organic–inorganic metal halide hybrid (OIHM) (3,3-Difluoropyrrolidium)2CuCl4 ((DFP)2CuCl4) 1 is fabricated, utilizing a self-assembly method based on Cu2+ inorganic salt and fluoropyrrolidinium dexterously. It possesses two reversible phase transitions (PTs) comprising a ferroelectric to paraelectric PT at 194 K and a ferroelastic to paraelastic PT at 351 K. The ferroelastic and ferroelectric properties are mainly derived from the order–disorder transitions in the microstructure and the displacement of the charge center. Importantly, the high-temperature structural phase transition is accompanied by a reversible color change. Its crystals can be directly applied in reversible thermochromic materials, exhibit an obvious color change (green to yellow–brown at 351 K) with full reversibility, and are successfully used as inks for hand-painting. To the best of our knowledge, 1 is the first Cu(II)-based OIMH TC with both ferroelectricity and ferroelasticity. This work laid the foundation for further development of alternative TCs, ferroics and practical applications for TCs.