The triplet state of indigo: Electronic structure calculations

The electronic structure of indigo was determined using B3LYP functional and different basis sets. The first triplet 3Au (C2h) state of indigo is characterized by the singlet–triplet gaps of ΔET–S(vertical) = 1.23 eV and ΔET–S(adiabatic) = 0.95 eV (recent experiment: 1.04 ± 0.1 eV). The vertical S1–T1 gap amounts to 1.0 eV (exptl: 0.91 ± 0.1 eV). The electron localization function (ELF) and spin density analysis show that the singlet–triplet excitation is accompanied by a migration of electrons from the CC bond and N-atoms to the adjacent C–C and C–N bonds. A low ionization energy is confirmed for indigo IEa = 6.9 eV.