Enhancing NMR chemical exchange signal of azo compound 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 non-rotatable 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 significantly influencing detection and kinetic analysis of azo compounds. Here, a new pulse sequence, termed as FasteR Exchange with chemical-Shift Scaling for Signal enHance-ment (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 recovery invisible intermediate ex-change signals, helping determine the exact structure of azo compounds. It can detect trace amounts of azo initiators as low as 1 mM, thus en-hancing the quality of polymer production. And it can confirm the fact of chemical exchange differences of tert-butyl groups in azobenzene cis- and trans- isomers, thus explaining the effect of exchange kinetics on electron transport properties, and providing valuable guidance for the syn-thesis of functional photovoltaic materials.