Why is Indigo Photostable over Extremely Long Periods?

This experimental work clarifies the mechanism for the Ultra-photostability of indigo carmine by direct observation of a transition state using a sub 5-fs laser pulse. The molecular structural changes during the photoexcited proton-transfer process were traced as the time-dependent instantaneous frequency of molecular vibrations. The mono-alcohol intermediate generated by proton transfer via stepwise path was found to be unstable, and it immediately reverted to indigo carmine. This regression to the reactant is the reason for the ultraphotostability of indigo over extremely long periods.