Encapsulation of red sulfur chromophores in a zeolitic imidazolate framework (ZIF-8) via solvent assisted linker exchange

Abstract

Red color sulfur chromophores were entrapped inside the cages of zeolitic imidazolate framework ZIF-8 in order to prepare a ZIF analog of ultramarine red. ZIF-8 is made from Zn2+ and 2-methylimidazole linker and has sodalite topology. SIM-1, a ZIF isostructural to ZIF-8 that is made from Zn2+ and an imidazole linker containing a carboxaldehyde group was used as the starting material for the red pigment preparation. SIM-1 was converted to ZIF-8 via solvent assisted linker exchange (SALE) in DMF in the presence of sodium tetrasulfide as the sulfur source. The polysulfide was oxidized into the red sulfur chromophore S4 due to the presence of the carboxaldehyde group of the SIM-1 linker under the reaction conditions. During the transformation, the red sulfur species were successfully trapped inside the ZIF-8 cages generating a stable red solid pigment. To the best of our knowledge, this is the first example of low temperature ultramarine pigment preparation process. The red pigments were characterized by using PXRD, FTIR, UV–Vis, XPS and SEM.