For hemi-indigos 2-4, similar shifts were observed upon TFA addition, whereas the imidazole-based derivatives 5 and 6 mainly showed a decrease in absorptivity with only a minimal change in absorption maximum. Nevertheless, in all cases, Z→E isomerization was accompanied by a large bathochromic shift and clear isosbestic points were observed, while basification caused hypsochromic shifts. a solution of (Z)-1 (0.76 mM in degassed CHCl3, 1 mm quartz cuvette) before and after addition of excess TFA (4.3 × 10 2 equiv.), followed by equilibration at rt for 15 h, treatment with Et3N (1.6 equiv.