TD‐DFT absorption spectrum of Azure A in aqueous solution: Vibronic transitions and electronic properties

The vibronic absorption spectrum of Azure A cation (AA+) in an aqueous solution was calculated using the time‐dependent density functional theory (TD‐DFT). The calculations were performed using all hybrid functionals supported by Gaussian16 software, 6‐31++G(d,p) basis set and polarizable continuum model with external integration formalism (IEFPCM) and solvation model based on solute electron density (SMD) solvent models. The IEFPCM gave significantly underestimated values of λmax in comparison with the experiment, what is a manifestation of the well‐known TD‐DFT “cyanine failure.” However, the 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 dipole moments and atomic charges of the ground and excited states of the AA+ molecule were 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. The vibronic absorption spectrum of Azure A cation in an aqueous solution was calculated using the TD‐DFT. All hybrid functionals with IEFPCM solvent model gave significantly underestimated theoretical λmax values. X3LYP functional with SMD solvent model and 6‐31++G(d,p) gave good agreement of theoretical λmax value with experimental one. Photoexcitation leads to an increase in the dipole moment of the dye. An insignificant photoinduced electron transfer was found in the central ring of the chromophore.