Estudo computacional da fotodegradação da molécula de índigo-carmim

Abstract

In this work, we present a computational investigation on the Indigo Carmine (IC) molecule. Structural and energetic properties for the ground and lowest-lying excited states of the IC were determined and used in the investigation of possible paths related to its photodegradation. For this purpose, the density functional theory (DFT) and its corresponding time-dependent formalism (TD-DFT) were used along with two exchange-correlation functionals: M06-2X and CAM-B3LYP. The 6-311+G(d,p) basis set was also used. All computations were performed considering water as a solvent. In general, the results obtained (for both the ground and excited states) at the M06-2X/6-311+G(d,p) and CAM-B3LYP/6- 311+G(d,p) presented good agreement. For example, the first singlet excited state was found at 2.28 eV (having oscillator strength of 0.4730) at the TD-DFT/M06-2X/6-311+G(d,p) level of thoery and at 2.19 eV (with an oscillator strength of 0.4669) at the TD-DFT/CAM-B3LYP/ 6- 311+G(d,p) level of theory. In addition, excellent agreement was observed from the comparison between the UV-Vis absorption spectra obtained through calculations with the corresponding obtained experimentally by collaborators, a fact that contributed to the validation of the choice of the theoretical approaches used here. The comparison between the structure obtained for the IC molecule in the ground state and that for its first excited state showed minor differences, suggesting that the molecule undergoes photodecomposition through a chemical reaction occurring in its excited state rather than via direct photolysis.