TD-DFT/DFT study of thionine in aqueous solution: Vibronic absorption spectrum and electronic properties

Abstract

The vibronic absorption spectrum of thionine (TH) in an aqueous solution is calculated using the time-dependent density functional theory (TD-DFT). The calculations are performed using all hybrid functionals supported by Gaussian16, the 6-31++G(d,p) basis set with IEFPCM and SMD solvent models. IEFPCM gave significantly underestimated values of λmax in comparison with the experiment. However, SMD made it possible to obtain good agreement between the calculation results and experimental data. The best fit was achieved using the X3LYP functional. The shoulder in the visible absorption spectrum of TH has a vibronic origin. 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. The absorption peak of the TH 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 TH.