Electrochromic Thin Films of the V-Cr Prussian Blue Analogue Molecular Magnet

This paper describes spectroelectrochemical measurements of thin films of the V-II/III-Cr-III Prussian blue analogue (V-Cr PBA). We show that we can electrically control the optical properties of this room-temperature molecule-based magnet and reversibly switch the colour of the films from blue to black upon reduction. The cyclic voltammogram of the films showed a weak wave at -0.85 V and a stronger wave at -1.22 V. The spectroelectrochemical measurement in KCl indicate that the wave at -0.85 V is related to the reduction of V(III) to V(II) because of a shift in the absorption band associated with the metal-to-metal charge-transfer band. The wave at -1.22 V occurred simultaneously with the growth of a new absorption feature at 465 nm and is attributed to the reduction of [Cr-III(CN)(6)](3) sites to [CrII(CN)(6)](4). We have assigned the change to the optical spectrum occurring at 465 nm after electrochemical reduction to a red-shift of a metal-to-ligand charge-transfer (MLCT) transition from Cr2+ to empty pi* ligand orbitals. The electrochromic coloration efficiency associated with switching from blue to black colour was found to be eta = - 25.2 +/- 0.2 cm(2) c(-1) with a switching time of 6.6s at 1.3 V. These findings demonstrate the possibility to electrically control the optical properties of a room-temperature molecular magnet. (C) 2017 Elsevier Ltd. All rights reserved.