Excitations of safranin and phenosafranin in aqueous solution: Comparative theoretical analysis

Abstract

In the present work, the excitations of safranin (SAF) and phenosafranin (PheSAF) in an aqueous solution are analyzed by DFT/TD-DFT, considering the vibronic transitions and hydration. The frontier orbitals, transitions between which cause the absorption and emission of dyes in the visible region of the spectrum, almost do not cover benzene rings and methyl groups of molecules. As a consequence, benzene rings, aromatic hydrogen atoms, as well as SAF methyl groups are not covered by the change in electron density. These features explain the same excitation energies of SAF and PheSAF observed experimentally. At the same time, during relaxation to an excited equilibrium state, the changes in the vast majority of atomic charges again turn out to be different for SAF and PheSAF, which causes a slight difference between the emission energies of dyes. Excitations of both dyes do not lead to twisting of their benzene rings relative to the chromophores - the dihedrals between them are strictly φ = 90°. The thermal twisting of these dihedrals is practically free up to Δφ∼±30°, and then it is sharply hampered. The heights of potential barriers separating their energy minima are very high (∼400 kT). The increase in the dipole moments upon excitation is due to the greatest extent to the significant increase in the electron densities at the N10 atoms of both dyes. All five H-bonds of each of the dyes with water are strengthened upon excitation. The strongest and most enhanced due to excitation are the H-bonds of waters with the N10 atoms. There is a strong photoinduced polarization of water molecules bound to the N10 atoms of the dyes. H-bonds with water of amino groups are stronger in PheSAF, and with the N10 atom, in SAF. The influence of hydrating water molecules leads to significant bathochromic shifts of the maxima of the calculated spectra (23 nm for SAF and 22 nm for PheSAF) relative to the purely implicit specification of the solvent.