A solvent-free neutral cobalt complex exhibiting macroscopic polarization switching induced by directional charge transfer

Materials that exhibit polarization switching induced by directional charge transfer under external stimuli have garnered significant interest due to their high switching rates and potential applications. Nonetheless, most of these crystals contain solvents or counter ions, and their effects during measurement or calculations are far from trivial. Therefore, synthesizing a solvent-free and neutral complex exhibiting polarization switching is highly desirable. Herein, we successfully observed directional charge transfer-induced electronic pyroelectricity in a solvent-free neutral cobalt complex, namely, Co(teeda)(3,6-dbq)₂ (teeda = N,N,N′,N′-tetraethylethane-1,2-diamine and 3,6-dbq = 3,6-di-tert-butylcatecholate or 3,6-di-tert-butylsemiquinonate). The charge transfer property was confirmed by magnetometry, infrared and UV-vis-NIR spectroscopy, and single-crystal X-ray diffraction (SCXRD) measurements. Furthermore, pyroelectric current was observed during the valence tautomerism (VT) process. Theoretical calculations demonstrated that polarization switching mainly originates from the charge transfer. These results indicate that Co(teeda)(3,6-dbq)₂ is a promising candidate for exploring new VT compounds that exhibit polarization switching.