Precise Design of Molecular Ferroelectrics with High T_C and Tunable Band Gap by Molecular Modification

Molecular ferroelectric materials are widely applied in piezoelectric converters, non-volatile memorizers, and photovoltaic devices due to their advantages of adjustable structure, lightweight, easy processing, and environmental friendliness. However, designing multifunctional molecular ferroelectrics with excellent properties has always been a great challenge. Herein, a multiaxial molecular ferroelectric is successfully designed by modifying the quasi-spherical cation dabco with CuBr₂ to obtain halogenated [Bretdabco]CuBr₄ (Bretdabco = N-bromoethyl-N'-diazabicyclo [2.2.2]octane), which crystallizes in polar point groups (C₆). Typical ferroelectric behaviors featured by the P-E hysteresis loop and switched ferroelectric domain are exhibited. Notably, the molecular ferroelectric shows a high T_C of 460 K, which is rare in the field and could greatly expand the application range of this material. In addition, the band gap is adjustable through the regulation of halogen. Both the UV absorption spectra and theoretical calculations indicate that the molecular ferroelectrics belong to a direct band gap (2.14 eV) semiconductor. This tunable and narrow band gap semiconductor molecular ferroelectric material with high T_C can be utilized more effectively in the study of optoelectronics and sensors, including piezoelectric energy harvesters. This research may provide a promising approach for the development of multiaxial molecular ferroelectrics with a tiny band gap and high T_C.