doi:10.1016/j.cplett.2007.10.015 
Copyright © 2007 Elsevier B.V. All rights reserved.
The triplet state of indigo: Electronic structure calculations
Vu Thi Ngana, b, G. Gopakumara, Tran Thanh Hueb and Minh Tho Nguyena, 
, 
aDepartment of Chemistry and Institute for Nanoscale Physics and Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium
bFaculty of Chemistry, Hanoi National University of Education, Hanoi, Vietnam
Received 12 July 2007;
accepted 5 October 2007.
Available online 10 October 2007.
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Abstract
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 C
C 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.
Graphical abstract
The first excited state of indigo was characterized with a singlet–triplet energy gap of around 1.0 eV.
Fig. 1. HOMO (b) and LUMO (a) of ground state of indigo 1 were plotted at isovalue of 0.05. Red color represents positive phase, yellow color negative phase. Structure of indigo is in an opposite orientation with that in Scheme 1. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Selected (U)B3LYP/6-31G(d) optimized parameters of the ground singlet state S0 (left part) and lowest triplet state T1 (right part) of indigo 1. Bond distances are given in angstrom.
Fig. 3. NBO charges on atoms (left part, in au) and Wiberg indices (WI) of bonds (right part). Upper values are for ground singlet state and lower values for lowest triplet state.
Fig. 4. Isosurfaces of ELF-difference between vertical triplet and singlet states, where red represents the isosurface ΔELF = 0.03, and yellow represents the isosurface ΔELF = −0.03. Values of deviation of basin populations are given in au. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Isosurfaces of electron density difference (Δρ = ± 0.012) between α- and β-electrons of adiabatic triplet indigo, population of α-electrons (left part) and integrated spin densities (right part) on atoms are given in atomic units.
Table 1.
Singlet–triplet energy gaps (ΔET–S) and first ionization energies (IE) of indigo and related compounds, using the (U)B3LYP method with different basis sets
a (a) stands for adiabatic and (v) for vertical. Values are given in eV.
Table 2.
NICS(1) aromaticity indices (ppm) of indigo and smaller models using the GIAO-B3LYP/6-311++G(d,p) method
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