Molecular mechanisms of the photostability of indigo

The photophysics of indigo as well as of bispyrroleindigo, the basic chromophore of indigo, has been investigated with ab initio electronic-structure calculations. Vertical electronic excitation energies and excited-state potential-energy profiles have been calculated with the CASSCF, CASPT2 and CC2 methods. The calculations reveal that indigo and bispyrroleindigo undergo intramolecular single-proton transfer between adjacent N–H and CO groups in the 1ππ* excited state. The nearly barrierless proton transfer provides the pathway for a very efficient deactivation of the 1ππ* state via a conical intersection with the ground state. While a low-lying S1–S0 conical intersection exists also after double-proton transfer, the latter reaction path exhibits a much higher barrier. The reaction path for trans → cisphotoisomerizationvia the twisting of the central CC bond has been investigated for bispyrroleindigo. It has been found that the twisting of the central CC bond is unlikely to play a role in the photochemistry of indigo, because of a large potential-energy barrier and a rather high energy of the S1–S0 conical intersection of the twisted structure. These findings indicate that the exceptional photostability of indigo is the result of rapid internal conversion via intramolecular single-proton transfer, combined with the absence of a low-barrier reaction path for the generation of the cis isomer via trans → cisphotoisomerization.