On the picosecond kinetics and photostability of indigo and 6,6′-dimethoxyindigo

Kinetics of the ground state repopulation and the decay lifetime of the lowest excited singlet state of indigo were measured by the use of picosecond spectroscopy. The photostability of indigo and its ring-substituted homologues can be reconciled in view of the ultrafast internal conversion from the lowest excited singlet state to the ground state enhanced by the proton transfer in the excited state.