Charge Transfer Kinetics and Solvatochromism of 1-(9-Anthryl)-3-(4-dimethylaniline) Propane in 1,4-Dioxane: Nonideal Quadrupolar Charge Distribution and the Origin of the Dioxane Anomaly

The solvent 1,4-dioxane behaves like a polar solvent even though its dielectric constant is small. This excess polarity is referred to as the “dioxane anomaly”. Several explanations of this anomaly have been offered in the literature including hydrogen bonding, conformational polarity, and quadrupolar interactions. We have investigated the origins of this anomaly by employing the unique photochemical properties of the ADMA (1-(9-anthryl)-3-(4-dimethylaniline) propane) molecule. ADMA forms an intramolecular exciplex in the excited state. Solvent polarity governs the mechanism of exciplex formation. In nonpolar solvents the decay of the locally excited-state population is monoexponential, while in polar solvents (ε > 5.4) the decay of the locally excited-state population is biexponential. Although dioxane has a dielectric constant of 2.2, ADMA dissolved in dioxane exhibits biexponential kinetics. From this result and from the solvatochromic shift of the ADMA exciplex peak in dioxane, we have concluded that the large nonideal quadrupolar charge distribution of 1,4-dioxane is responsible for its anomalous polarity.