Halochromism of Phenol Red in Aqueous Solution: A Comparative Theoretical Analysis of Excitations of Dianion and Anion

MN15/6‐31++G(d,p)/IEFPCM theory level gave a good agreement between the calculated vibronic and experimental absorption spectra of the dianion, in both the maximum position and the shape. Vibronic transitions of the dianion activate only two motions in the S 1 state, namely, torsional vibrations of two aromatic rings (A and B) and the SO 3 group attached to the third ring. Significant photoinduced distortions of the spatial structure of the anion (A and B rings, being almost parallel to each other in the ground state, become mutually perpendicular in the excited state) led to a failure of the Franck‐Condon computational procedure for calculating its vibronic spectrum. Photo‐induced shifts of the electron density are analyzed. The ring with the SO 3 group attached is not involved in these charge redistributions for both the dianion and the anion. The negative solvatochromism of the dianion and the positive one of the anion observed experimentally were explained both from the point of view of non‐specific dipole–dipole interactions with the solvent (changes in the dipole moments of the solutes upon excitation) and specific ones (strengthening/weakening of strong hydrogen bonds of the dianion and anion with water molecules).