Photoexcitation of oxazine 4 dye in aqueous solution: TD-DFT study

The vibronic absorption spectra of oxazine 4 (OX4) dye in an aqueous solution using 40 hybrid functionals, the 6–31++G(d,p) basis set, and the SMD solvent model were calculated. It turned out that the X3LYP functional provided the best 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 OX4 cation in the ground and excited states (IR spectra, atomic charges, dipole moments, and transition moment) were calculated. Maps of the distribution of electron density and electrostatic potential have been built. The influence of four strong hydrogen bonds of the dye with water molecules on the absorption spectrum was analyzed. It has been shown that these bonds were strengthened upon OX4 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, but does not change its shape. Photoexcitation of the dye leads to a noticeable polarization of only one of the four considered water molecules (associated with the endocyclic nitrogen atom in the central ring of the chromophore, the electron density on which increases the most).