Vibronic absorption spectrum and electronic properties of methylene blue in aqueous solution: TD-DFT study

Abstract

The vibronic absorption spectrum of methylene blue (MB) in an aqueous solution is calculated using the time-dependent density functional theory (TD-DFT). The results of calculations are analyzed using all hybrid functionals supported by Gaussian16, the 6–31++G(d,p) basis set and the IEFPCM and SMD solvent models. The solvent model IEFPCM gave significantly underestimated values of λmax in comparison with the experiment. This is a manifestation of the TD-DFT “cyanine failure”. However, the SMD model made it possible to obtain good agreement between the calculation results and experimental data. The best fit was achieved using the X3LYP functional. According to our calculations, the shoulder in the visible absorption spectrum of MB has a vibronic origin. Explicit assignment of a water molecule strongly bound to the N10 endocyclic nitrogen atom does not change the shape of the absorption band, but shifts its maximum by 7 nm to the long-wavelength region of the spectrum. The dipole moments and atomic charges of the ground and excited states of the MB molecule have been calculated. Photoexcitation leads to an increase in the dipole moment of the dye molecule. An insignificant photoinduced electron transfer was found in the central ring of the chromophore of the dye molecule. In accordance with our results, the absorption peak of the MB solution in the visible region of the spectrum is due to the π → π* type electronic transition. Vibronic coupling plays a significant role in the absorption spectra of MB.