Redox-Responsive Covalent Organic Nanosheets from Viologens and Calix[4]arene for Iodine and Toxic Dye Capture

Owing to their chemical and thermal stabilities, high uptake capacities, and easy recyclability, covalent organic polymers (COPs) have shown promise as pollutant sponges. Herein, we describe the use of diazo coupling to synthesize two cationic COPs, COP1(++) and COP2(++), that incorporate a viologen-based molecular switch and an organic macrocycle, calix[4]arene. The COPs form nanosheets that have height profiles of 6.00nm and 8.00nm, respectively, based on AFM measurements. The sheets remain morphologically intact upon one- or two-electron reductions of their viologen subunits. MD simulations of the COPs containing dicationic viologens indicate that the calix[4]arenes adopt a partial cone conformation and that, in height, the individual 2D polymer layers are 5.48 angstrom in COP1(++) and 5.65 angstrom in COP2(++), which, together with the AFM measurements, suggests that the nanosheets are composed of 11 and 14 layers, respectively. Whether their viologens are in dicationic, radical cationic, or neutral form, the COPs exhibit high affinity for iodine, reaching up to 200% mass increase when exposed to iodine vapor at 70 degrees C, which makes the materials among the best-performing nanosheets for iodine capture reported in the literature. In addition, the COPs effectively remove Congo red from solution in the pH range of 2-10, reaching nearly 100% removal within 15minutes at acidic pH.