For hemi-indigos 2-6, the thermal isomerization behavior was comparable (Supplementary Table ). The dimethylamino-and bromo-substituted pyridyl derivatives 2 and 3, as well as the quinolyl derivative 4, showed identically high conversion to the E isomer when protonated (>95%), while differences in isomerization rates among the different compounds were small. For imidazole-based derivatives 5 and 6, conversion to the E isomer was lower (85% and 88%, respectively), and while the former compound exhibited the slowest isomerization in the series, this process was the fastest for the latter. Where all compounds displayed quantitative isomerization back to their Z isomer after neutralization of the solution by Et3N addition, in this case large variations were observed in the E→Z isomerization rates (Supplementary Table ).