Synchronous electrochromism and electrofluorochromism in a zirconium pyrenetetrabenzoate metal-organic framework

Redox-active materials which exhibit both electrochromism and electrofluorochromism have great potential as multifunctional elements in optoelectronics. Here we present in situ spectroelectrochemistry on NU-1000, a zirconium pyrenetetrabenzoate metal-organic framework. A thin film of NU-1000 exhibits reversible color changes between light yellow and dark blue when subjected to an alternating electrochemical potential. In situ fluorescence excitation-emission spectral mapping elucidates a dominant blue emission of highly fluorescent electrochromic NU-1000 that is being quenched via an oxidation reaction. Density-functional theory calculations reveal the the forbidden optical transition between the singly-occupied molecular orbital (SOMO) and the lowest unoccupied molecular orbital (LUMO) in the oxidized linker as the cause of the quenching. Double potential step chronoamperometry meaures response times as fast as a dozen seconds and excellent switching stability over 500 cycles without noticeable attenuation of the color contrast. These findings provide valuable insight into the electrochromism and electrofluorochromism in metal-organic frameworks, offering exciting opportunities for developing advanced multifunctional porous materials with potential applications in optoelectronics and sensing.