It is worth noting that for structurally related hemi-thioindigo, Hammett analysis indicated that electron-donating para-substituents can lower the thermal isomerization barrier, most likely because of the increase in donor-acceptor character. A similar effect may play a role here and, to summarize, the choice of N-heterocyclic ring can thus be used to tune the thermal isomerization rates. (400 MHz) of (Z)-1 (5.7 mM in CDCl3) before and after addition of TFA (32 equiv.) to give (Z)-1 . H + , followed by flame-sealing of the NMR tube and thermal equilibration at rt for 3 days to afford (E)-1 . H + , opening of the NMR tube with a diamond glass file and addition of Et3N (48 equiv.) to give (E)-1, and flame-sealing of the NMR tube and thermal equilibration at rt for 9 days to reobtain (Z)-1.