Excitations of colored and colorless forms of Meldola's blue dye in aqueous solution: A comparative theoretical analysis

Abstract

TD-DFT calculations were performed for the colored (MB⁺) and colorless (МB⁰) forms of the oxazine dye. The nature of excitation of these forms of dye differs significantly: MB⁺ undergoes a local excitation, and МB⁰ has a significant charge transfer. In particular, the dipole moment of MB⁺ upon excitation slightly decreases while for MB⁰, on the contrary, it increases almost fourfold. Spatial structures and their photoinduced distortions also differ: while the MB⁺ chromophore remains flat in both the ground and excited states, MB⁰ has a significant chromophore kink along the NO line in the ground state and becomes flat in the excited state. The mechanisms of solvatochromic behavior of the dye - non-specific and specific interaction with the solvent - were also analyzed.