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 phi = 30degrees in the gas phase and phi = 60degrees in aqueous solution. This relaxation alone is responsible for about 30% of the overall blue shift observed in the n --> pi* transition of betaine in water. In addition, it was observed that the pi --> pi* 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.