Thermal-induced reversible ferroelastic phase transition in a new bromethyl-substituted molecular rotor

A new bromethyl-substituted molecular rotor, [Cu(dabco CH2Br)(H2O)Br3](dabco CH2Br+=1-(2-bromethyl)-1,4-diazoniabicyclo[2.2.2]octane cation), which belongs to a family of halomethyl-substituted molecular rotors, was synthesized and structurally characterized. The reversible phase transition at ca. 250 K was well established for this molecular rotor by thermal analyses, variable-temperature X-ray diffraction, and variable temperature dielectric measurements. The order-disorder transformation of the rotator part(dabco moiety) causes ferroelastic phase transition with an Aizu notation of mmm F2/m from hightemperature orthorhombic phase(Pbnm) to low-temperature monoclinic phase(P21/n). More important, in reference to the density functional theory calculations and structural analyses, the key factors to tune the phase transition behaviors were discussed in detail for this family of halomethyl-substituted molecular rotors.