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 C=O groups in the (1)ππ* excited state. The nearly barrierless proton transfer provides the pathway for a very efficient deactivation of the (1)ππ* state via a conical intersection with the ground state. While a low-lying S(1)-S(0) conical intersection exists also after double-proton transfer, the latter reaction path exhibits a much higher barrier. The reaction path for trans→cis photoisomerization via the twisting of the central C=C bond has been investigated for bispyrroleindigo. It has been found that the twisting of the central C=C 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 S(1)-S(0) 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→cis photoisomerization.