On the intrinsic photophysics of indigo: a time-resolved photoelectron spectroscopy study of the indigo carmine dianion

The intrinsic photophysics of indigo has been studied using gas-phase time-resolved photoelectron imaging of the indigo carmine dianion (InC2-). The action spectrum reveals that the gas-phase absorption spectrum arising from the S-1 <- S-0 transition in InC2- has a similar solvent shift to that of neutral indigo. Femtosecond spectroscopy shows that the S-1 state decays on a 1.4 ps timescale. Through isotopic substitution, the primary mechanism on the S-1 excited state can be assigned to an intra-molecular proton transfer, which is the same as that which has been observed in solution. However, the excited state lifetime is significantly shorter in vacuum. These similarities and differences are discussed in terms of recent theoretical investigations of the S-1 excited state of indigo.