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

- Spectroelectrochemistry of V-Cr Prussian blue analogue.
- V oxidation state controls wavelength of metal-to-metal charge-transfer absorption.
- Electrochromic switching from blue to black of room-temperature molecular magnet.
- Colour change due to reduction of Cr(III) to Cr(II).

This paper describes spectroelectrochemical measurements of thin films of the VII/III-CrIII 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 [CrIII(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 π* ligand orbitals. The electrochromic coloration efficiency associated with switching from blue to black colour was found to be η = − 25.2 ± 0.2 cm2 C−1 with a switching time of 6.6 s at −1.3 V. These findings demonstrate the possibility to electrically control the optical properties of a room-temperature molecular magnet.

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