Excitation of rhodamine 800 in aqueous media: a theoretical investigation.

The main goal of this work was to obtain a calculated absorption spectrum of rhodamine 800 in an aqueous solution, which most accurately reproduces the experimental one. To achieve this result, I used the hybrid functionals supported by Gaussian 16 software package. In this case, the basis set (6-31++G(d,p)) and the solvent model (IEFPCM) were not varied. The B3PW91 functional gave the best agreement with the experimental absorption spectrum of the dye in an aqueous medium. B3P86, B971, B972, B98, X3LYP, APF, HSE06, and N12SX functionals also give good absorption energy coincidence. The B3PW91/6-31++G(d,p)/IEFPCM theory level chosen in this way made it possible to calculate the various characteristics of rhodamine 800 in the ground and excited states. An important result of this work was the establishment of the vibronic nature of the short-wavelength smaller maximum of the absorption spectrum. The influence of the strong H-bond of the exocyclic nitrogen atom with the water molecule on the dye excitation was analyzed.