15N Solid-State NMR as Bright Eyes to See the Isomerization of the Azo Bond: Revision of Tris(β-hydroxyl-azo)-benzene to Tris(β-keto-hydrazo)-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 15N labeling technique and 15N solid-state NMR (15N-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(β-hydroxyl-azo)-benzene in previously proposed hydroxylazobenzene polymers was revised to tris(β-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(β-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 15N-SSNMR.