The vibronic absorption spectra and electronic states of acridine orange in aqueous solution

[Display omitted] •The visible absorption of AOH+ is due to the HOMO → LUMO transition.•The visible absorption of AO0 is due to the (HOMO-1) → LUMO transition.•The vibronic coupling play a significant role in the absorption spectrum of the AO.•The best agreement with the experiment gives the X3LYP/6-31G(d,p) level of theory.•During photoexcitation, a small electron transfer occurs in the AO central ring. The time-dependent density functional theory (TD-DFT) was used to obtain vibronic absorption spectra of acridine orange dye (AO) in an aqueous solution that were in good agreement with the experiment. The protonated and neutral forms of the dye have been investigated. The results of calculations using various functionals and basis sets have been analyzed. The best agreement with experiment was given by the level of theory X3LYP/6-31G(d,p). AO molecular orbitals involved in electronic transitions due light absorption in the visible region of the spectrum have been obtained. The dipole moments and atomic charges of the ground and excited states of the AO molecule have been calculated. Maps of the electrostatic potential have been drawn. An insignificant photoinduced electron transfer was found in the central ring of the chromophore of the dye molecule. According to our calculations, the vibronic coupling and the Boltzmann distribution play a significant role in the absorption spectra of the AO.