Room-temperature bistability in a cobalt-octacyanidotungstate framework showing a charge-transfer phase transition with a red-blue color change

Abstract

A two-dimensional cyanido-bridged Co–W assembly, Cs⁺_0.1(H₅O₂⁺)_0.9 [Co(4-bromopyridine)_2.3{W(CN)₈}], was synthesized. This compound has a layer structure with H₅O₂⁺ and Cs⁺ ions in between the cyanido-bridged Co–W coordination sheets, and exhibits a charge transfer–induced phase transition between Co^II–W^V (red) and Co^III–W^IV (blue) states with a drastic color change. Upon cooling, the phase transition from the Co^II–W^V state to the Co^III–W^IV state occurs at 276 K, while the opposite transition from the Co^III–W^IV state to the Co^II–W^V state occurs upon heating at 329 K. The present compound shows a large thermal hysteresis loop with a width of 53 K and bistability at room temperature. Analysis of the thermal phase transition using the Slichter–Drickamer model reveals the thermodynamic parameters of the transition enthalpy, transition entropy, and interaction parameter.