Yellow-to-brown and yellow-to-green electrochromic devices based on complexes of transition metal ions with a triphenylamine-based ligand

Abstract

Transmissive-to-colored electrochromism has been achieved by combination of MLCT of transition metal complexes with the electrochromic properties of ligand molecules. The color transitions were from yellow to dark brown for the Fe(II) complex, yellow to orange to bluish-green for the Co(II) complex and yellow to green for the Zn(II) complex. By using a metal ion−ligand coordination approach, the self-assembly of hydrazone-based ligands containing a triphenylamine group with appropriate metal salts (FeCl₂, Co(ClO₄)₂ and Zn(BF₄)₂) produced novel complexes of the general formula [ML₂]X₂. The isolated complexes were characterized by spectroscopic methods, and the Co(II) complex also by X-ray diffraction analysis. Thin films of the complexes have been obtained by a spray-coating method and they were used in the construction of electrochromic devices, which showed good electrochromic stability, a high color contrast of 47.5% for Fe(II), 37.2% for Co(II) and 33.7% for Zn(II) complexes and fast coloring and bleaching times.