Excitation of neutral red dye in aqueous media: comparative theoretical analysis of neutral and cationic forms

The main goal of this work was the theoretical interpretation of the absorption spectra of neutral red in an aqueous solution (both neutral NR⁰ and protonated NR⁺ forms). To achieve this problem, TD-DFT/DFT calculations with different hybrid functionals, the IEFPCM solvent model, and the 6-31 +  + G(d,p) basis set were used. MN12SX functional provided the best agreement with the experiment for both dye forms. While the absorption band of the cationic form of the dye in the visible region of the spectrum is due to one transition S₀ → S₁ (HOMO-LUMO), for its neutral form, there are two transitions S₀ → S₁ (HOMO → LUMO) and S₀ → S₂ (HOMO-1 → LUMO), with the latter having a higher intensity. The protonation of the dye chromophore introduces significant changes in HOMO shape. At the same time, LUMOs are almost the same for the protonated and neutral forms of the NR. During the transition from NR⁰ to the S₁ state, its dipole moment increases more significantly than during the transition to the S₂ state. Calculations confirmed the assumption of Singh et al. about the existence of two closely spaced excited states of NR⁰, the first of which has a larger dipole moment. However, the hypothesis of these authors about the corresponding intramolecular charge transfer, as well as the huge value of the dipole moment of this excited state (~ 20 D) declared by them, was not confirmed by present calculations. It was shown that the photoinduced charge redistribution in both the neutral and cationic forms of the dye is local, and the corresponding dipole moment is ~ 10 D. This agrees with other early theoretical work by Aaron et al. The influence on the NR⁺ absorption spectrum of hydrating water molecules was also analyzed. It was found that the interplay of electrostatic and site-specific contributions leads to the fact that NR solvatochromism does not have a pronounced dependence on the polarity of the solvent.