Enhancing NMR Chemical Exchange Signals of Azo Compounds via Chemical-Shift Scaling.

Azo compounds offer wide-range applications due to their unique structures and properties. The N═N double bond in azo compounds can form a nonrotatable plane, frequently causing spins to experience different magnetic environments, and thus leading to chemical exchange. This may result in line broadening even invisibility in the nuclear magnetic resonance spectra, thus greatly hindering detection and kinetic analysis of azo compounds. Here, a new pulse sequence, termed as FasteR Exchange with chemical-Shift Scaling for Signal enHancement (FRES3H) is developed, which scales down the chemical shift by a factor of λ via manipulating indirect dimension evolution time and converts the spin system to fast-exchange regime, thus boosting chemical exchange signal. The method can recover invisible signals in intermediate-exchange regime, facilitating the analysis of azo compounds. It can detect trace amounts of azo initiators as low as 1 mM in 2D C-H coherence spectra, thus facilitating the quality control of polymer production. And it can confirm the fact of chemical exchange differences of tert-butyl groups in azobenzene cis- and trans-isomers, thus revealing the effect of exchange kinetics on electron transport properties and providing valuable guidance for the synthesis of functional photovoltaic materials.