Redox Reactivity of Methylene Blue Bound in Pores of UMCM-1 Metal-Organic Frameworks

Abstract

Redox processes were studied in the pores of a crystalline UMCM-1 metal organic framework (MOF) material. Methylene blue was employed as an absorbed redox active dye component. From the change in coloration during dye adsorption, it was concluded that an essentially irreversible adsorption process with high pore loading of the resulting MOF structure occurred. The adsorbed methylene blue remained redox active in the MOF pores and there was no evidence of significant losses during extended redox cycling. Due to the size of the pores, the reactivity of the pore-bound methylene blue is closely related to that expected for methylene blue in an aqueous solution. A study of the effect of solution pH on the voltammetric responses revealed an interesting gradual change in electrical pore conductivity form conducting poorly under acidic conditions to conducting very well under alkaline conditions. This is interpreted in terms of charge transport via single-electron hopping conduction in pores. An estimate of the apparent charge diffusion coefficient at pH 7, D_app = 1.4 × 10⁻¹⁵ m²s⁻², is obtained. Potential applications of the new family of redox active hybrid MOF materials are indicated.

Keywords:

• Carbon dioxide
• coordination polymers
• electrolyte
• electron transfer
• diffusion
• ion conduction
• junction
• metal-organic frameworks
• MOF
• pore
• pH
• sensors
• voltammetry

Acknowledgments

J.E.H. thanks the University of Bath and the EPSRC for a PhD stipend (Nanotechnology grand challenge: EP/H046305/1). Dr. Suguo Huo and Dr. Paul Warburton are gratefully acknowledged for allowing access to the EPSRC FIB service at UCL. C.Y.C. thanks the EPSRC for a studentship.