Modeling of Electronic Spectra of Ionic Forms of Eosin and Erythrosin
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MODELING OF ELECTRONIC SPECTRA OF IONIC FORMS OF EOSIN AND ERYTHROSIN A. V. Rogova,1 F. N. Tomilin,1,2 M. A. Gerasimova,1 and E. A. Slyusareva1
UDC 544.18, 544.17, 535.34, 535.37
Multistage dissociation of fluoroscein dyes, widely used in biological labeling, yields a variety of ionic and tautomeric forms in a wide range of pH values. In contrast to well-studied absorption spectra, the emission spectra are not quite readily interpreted due to their strong overlapping and proton transfer in electronically excited states. The least studied are the fluorescent properties of eosin and erythrosin dyes containing heavy atoms (Br, I), in which the characteristics of the dianionic form only are reliably determined. In the framework of the density functional theory using the B3LYP-functional including nonequilibrium solvation, the geometries of the series of ionic forms of eosin and erythrosin in the ground and excited states are found, and the electronic spectra are calculated. Based on the identified linear regression of the calculated and experimental data for the earlier resolved electronic spectra, for the first time, the emission spectrum maxima of the monoanionic, neutral quinoid, and cationic forms of the dyes are determined. The spectral peculiarities (Stokes shifts) are discussed in terms of variation of the molecule and ion geometries in the ground and excited states. Keywords: eosin, erythrosin, ionic forms, non-stationary density functional theory TD-DFT, B3LYP, polarized continuum model, electronic spectra, absorption, fluorescence.
INTRODUCTION Fluorescein and its halogen-substituted (Br, I) homologs (eosin, erythrosin) are used for biological labeling and visualization of colorless systems (cells, proteins, etc.) [1, 2] and as acceptors and converters of the electronic excitation energy [3, 4] due to their strong absorption in the visible spectrum and their fluorescent, phosphorescent, and photochemical properties. As a result of a three-stage dissociation in aqueous solutions, fluorescein dyes can exist in dianionic, monoanionic neutral, and cationic forms and can additionally have a number of tautomers of one and the same ionic state. In view of the fact that protonation/deprotonation frequently occurs in a chromophoric group, such changes most radically influence the absorption and emission spectra. In contrast to well-studied absorption spectra of ionic fluorescein dyes [5–7], the complex structure of the emission spectra in a wide pH range is not quite clear due to the variety of these forms, overlapping of broad spectra, and proton transfer in the electronically excited states [8]. The least studied dyes are those containing heavy atoms (eosin, erythrosin), whose neutral forms are poorly soluble and whose constants of the dianionic-monoanionic and monoanionic-neutral equilibrium are close [5]. One of the ways of studying the energy structure of molecules in the cases where the experimental investigations are difficult is a quantum-chemical calculation approach. The purpose of this work is to determi
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