Microwave-assisted reductive homocoupling of aromatic nitro monomers: synthesis of azo-linked porous organic polymers for CO2 capture

An optimized protocol for the rapid synthesis of azo-linked porous organic polymers (POPs) containing trigonal triphenylpyridine (AZO-P-M), triphenyltriazine (AZO-T-M), and tetragonal tetraphenylethylene (AZO-E-M) central units by microwave-assisted NaBH4-mediated reductive homocoupling of the corresponding aromatic nitro monomers is presented. The structural and functional features of the azo-linked polymers prepared under microwave heating were directly compared to their counterparts obtained by conventional synthesis. Similar to azo-linked polymers synthesized by conventional reductive homocoupling of nitro monomers with NaBH4, the polymers prepared under microwave irradiation are amorphous solids of good thermal stability showing moderate (e.g., AZO-E-M with BET surface area of 302.1 m2 g-1) to modest (e.g., AZO-P-M with BET surface area of 22.9 m2 g-1) porosities. Although the microwave-assisted procedure for the synthesis of azo-linked polymers described in this work did not result in systems with improved porosities, their efficiency for CO2 adsorption (up to 29 mg g-1 at 306 K) is comparable to that of POPs synthesized by conventional heating. Therefore, the herein reported protocol could be used for the fast and efficient synthesis of new azo-linked POPs with potential for CO2 capture.