N Solid-State NMR as Bright Eyes to See the Isomerization of the Azo Bond: Revision of Tris(β-hydroxyl-azo)-benzene to Tris(β-ketohydrazo)-cyclohexane in Porous Organic Polymers

Porous organic polymers (POPs) have aroused great and wide attention from the materials community, while the identification of their precise structures is still very challenging. The well-defined structures are of great importance in understanding the relationship between the structure and function of the polymer materials, though they are sometimes ignored and do not receive enough attention. In this letter, an efficient N-15 labeling technique and N solid-state NMR ( N-SSNMR) were combined to obtain strong evidence for the presence of the azo bond and keto-hydrazone structure in the solid state. Thus, the structure of tris(beta-hydroxyl-azo)-benzene in previously proposed hydroxylazo-benzene polymers was revised to tris(beta-keto-hydrazo)-cyclohexane in TKH-POPs for the first time. In contrast, similar tautomerization did not occur in the azo coupling polymerization of 1,3,5-triaminobenzene and diazonium salts, i.e., tris(beta-amino-azo)-benzene remained in Azo-POPs. This work will open up a window to develop innovative porous organic polymers more efficiently with the aid of N-5-SSNMR.

Automated summary

Porous organic polymers (POPs) have attracted widespread attention, but identifying their precise structures remains challenging. By combining an efficient N-15 labeling technique and N solid-state NMR (N-SSNMR), strong evidence for the presence of the azo bond and keto-hydrazone structure in the solid state was obtained, leading to the discovery of a new structure in TKH-POPs. This finding will help develop innovative porous organic polymers more efficiently.