Solute relaxation on the solvatochromism of ortho-betaine dyes. A sequential Monte Carlo/quantum mechanics study

Molecular relaxation in solution, estimated by thermodynamic perturbation theory, is shown to be important even for a qualitative description of the solvent effects on the solvatochromic shifts of flexible betaine dyes. It is shown that the most stable conformer of the ortho-betaine has a dihedral angle φ = 30° in the gas phase and φ = 60° in aqueous solution. This relaxation alone is responsible for about 30% of the overall blue shift observed in the n → π* transition of betaine in water. In addition, it was observed that the π → π* transitions are actually red shifted in the gas phase due to internal rotation. The total solvatochromism of about 6000 cm−1 estimated with a Monte Carlo/quantum mechanics sequential methodology is in reasonable agreement with the experimental shift of the absorption spectra in water and in toluene.