Photoexcitation of methylene green dye in aqueous solution: TD-DFT study

Abstract

The vibronic absorption spectra of methylene green (MG) thiazine dye in an aqueous solution using X3LYP hybrid functional, the 6-31++G(d,p) basis set, and the SMD solvent model were calculated. It turned out that this theory level the choice of which is based on previous theoretical studies of thiazine dyes provided excellent agreement with the experiment in the positions of the main maximum and the short-wavelength shoulder. Our calculations showed that this shoulder is vibronic and is not caused by a separate electronic transition. At the same time, the shoulder intensity in the calculated spectrum turned out to be lower than in the experimental one. Various parameters of the MG cation in the ground and excited states (IR spectra, atomic charges, dipole moments, and transition moment) were calculated. The dipole moment of the dye molecule in the ground state turned out to be the same as in the excited one, contrary to previous studies, where the dipole moment of a thiazine dyes without a nitro group increases upon excitation. This could be explained by the influence of the MG nitro group in position 6, due to which electron density increases on both diametrically opposite N10 and S5 atoms of the central ring. At the same time, in other thiazine dyes (without a nitro group), photoexcitation leads to a one-sided increase in the electron density at the N10 atom, and as a result, the projection of the dipole moment directed across the chromophore increases significantly. Maps of the distribution of electron density and electrostatic potential have been built. The influence of the strong hydrogen bonds of the dye with three water molecules on the absorption spectrum was analyzed. It has been shown that these bonds are strengthened upon MG excitation. It was found that explicit assignment of water molecules strongly bound to the dye leads to a redshift of the spectrum as a whole and worsened its shape.