Encouraged by the computational results, we synthesized hemi-indigo 1 and its derivatives 2-6 containing different N-heterocycles (Fig. ). The imidazole rings in compounds 5 and 6 were methylated to avoid tautomerization. These derivatives were accessed through a condensation reaction of commercially available indoxyl acetate with the respective aldehydes (Supplementary section 1). The desired products were obtained in good yields (65-82%) as the thermodynamically most stable Z isomers (Z/E ratio >99%). Importantly, the 1 H NMR spectra recorded in CDCl3 revealed downfield-shifted NH-signals for all compounds ( = 10.47-9.14 ppm) with respect to parent hemi-indigo containing a phenyl instead of the N-heterocyclic ring (7,  = 6.82 ppm, Supplementary section 2), which indicates involvement of the NH proton in hydrogen bonding with the nearest nitrogen atom of the N-heterocycles.