Molecular mechanisms of the photostability of indigo

Abstract

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.