Excitation of the C337, highly fluorescent dye in an aqueous solution was studied by the TD-DFT. Through the B3PW91/6-311++G(d, p)/IEFPCM theory level, we obtained the C337 vibronic absorption spectrum which exactly reproduces the experimental maximum. Significant differences are shown between the results of calculating the transition energies using the models of vertical and vibronic transitions. Upon excitation, the main shift of the electron density occurs from C10 to C4 neighboring atom. Thus, large-scale ICT not occur. The effect of excitation on the vibrational spectra of C337 was analyzed. A single C337 molecule and its five different hydration complexes with strongly bound water molecules have been analyzed. Various hydrate complexes of the dye have been studied, and the effect of hydration on its excitation vibrational properties has been elucidated. It was taken into account that a water molecule can attach to the nitrogen atom N1 ("A" type) and the C=O carbonyl ("B" type), and cyano C≡N ("C" type) groups of the dye.

The vibronic absorption spectrum of Toluidine blue O (TBO) dye 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 and 6-31++G(d,p) basis set with IEFPCM and SMD solvent models. The IEFPCM gave underestimated values of λmax in comparison with the experiment, what is a manifestation of the TD-DFT "cyanine failure". However, the SMD made it possible to obtain good agreement between calculated and experimental spectra. The best fit was achieved using the X3LYP functional. The dipole moments and atomic charges of the ground and excited states of the TBO 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 TBO molecule. Vibronic transitions play a significant role in the absorption spectrum of the dye.

The vibronic absorption spectra of Nile blue (NB) oxazine dye in an aqueous solution using 13 hybrid functionals, the 6-31++G(d,p) basis set, and the IEFPCM solvent model were calculated. It turned out that the O3LYP functional provided the best agreement with the experiment. Various parameters of the NB cation in the ground and excited states (IR spectra, atomic charges, dipole moments, and transition moment) were obtained. 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 two from these bonds were strengthened upon NB excitation and two ones were weakened. 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.