Solvent effects on electronic transitions of highly dipolar dyes: A comparison of three approaches

The solvatochromism of several polar solutes, including some that contain both hydrogen bond-donating and -accepting properties (coumarins 1, 102, 120, 151, 152, and 153; nile red; and 4-aminofluorenone), is analyzed in terms of three models: the Reichardt single parameter E-T(N) polarity scale, the multiparameter Kamlet-Taft equation, and the reaction field model. We use a "step-forward" procedure to determine which terms of the Kamlet-Taft equation are statistically significant in fitting the data. These equations provide the best fits to the data in almost all cases. We also find a correlation between the parameters s and a, which quantify the effects on the transition energy related to the solvatochromic parameters pi* and alpha, respectively. This relationship suggests that the magnitude of a is not indicative of the strength of the hydrogen-bonding interaction, but rather reflects the additional field produced from the dipole moment of a hydrogen bond-donating molecule that is held in an orientation that roughly parallels the solute dipole.