Rapid Redox Hopping Charge Transfer and Electrochromism in a Multivariate Metal–Organic Framework

Electrochromic materials exploit a change in molecular absorbance after an electrochemical redox event for applications, such as smart glass and segmented displays. Current applications use metal oxides; however, these materials are plagued by slow response times to potential changes. Herein, we investigate a metal-organic framework (MOF) film loaded with a molecular ruthenium redox carrier for its electrochromic capabilities. Upon the application of a potential jump, the native MOF was found to transport charge with an apparent diffusion coefficient, <i>D</i>_app of 8(±3) × 10^-9 cm²/s. That said, only 55% of the redox centers, primarily those at the MOF surface, were converted. Using a multivariate (MTV) approach to incorporate sulfonate groups into the backbone of the MOF in addition to the redox carrier allowed for charge transport throughout the MOF with a <i>D</i>_app of around 1(±1) × 10^-7 cm²/s and 100% conversion, the fastest reported diffusion coefficient in the redox hopping MOF literature to our knowledge. The sulfonated MOF exhibited a rapid electrochromic response, with coloration and bleaching times of approximately 1.29 and 1.33 s, respectively, at 400 mV overpotential. The sulfonate group is hypothesized to break ion pairs, allowing for higher ionic conductivity, which facilitates fast and complete charge transfer.