Excitations of Pyronin Y and Pyronin B Dyes in Aqueous Solution: Comparative Theoretical Analysis

Abstract

To elucidate the effect of side groups on the excitation of xanthene dyes, pyronin Y (PY) and pyronin B (PB) were studied by DFT/TD-DFT. The calculation results were compared with each other, as well as with the data for the acridine red dye previously studied. The O3LYP/6-31++G(d,p)/IEFPCM theory level reproduced well the experimental spectra of PY and PB aqueous solutions. According to calculations, the short-wavelength shoulders of these spectra are caused by vibronic transitions. The side groups significantly affect the set of vibronic transitions. The photoexcitation significantly changes the intensities of IR vibrations. The side groups of these xanthene dyes (aminodimethyl/aminodiethyl) strongly influence the vibrations of their chromophores. HOMOs cover the side groups of both dyes to a greater extent compared to LUMOs. The configurations of both frontier orbitals around the chromophores are identical for PY and PB. The dipole moment of the dye molecules in the excited state turned out to be greater than in the ground state. In the ground state, the dipole moment of the PB is 2.5 times greater than that of the PY, and in the excited states, it is 1.7 times less. The transition moments of PY and PB are almost identical to each other. Considering site-specific solute-solvent interactions in the form of an explicit assignment of water molecules that form strong hydrogen bonds with the dye cations was performed.