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 dielec tric constant is small. This excess polarity is referred to as the "dioxane anomaly". Several explanations of this anomaly have been offered in the li terature including hydrogen bonding, conformational polarity, and quadrupol ar interactions. We have investigated the origins of this anomaly by employ ing the unique photochemical properties of the ADMA (1-(9-anthryl)-3-(4-dim ethylaniline) propane) molecule. ADMA forms an intramolecular exciplex in t he excited state. Solvent polarity governs the mechanism of exciplex format ion. In nonpolar solvents the decay of the locally excited-state population is monoexponential, while in polar solvents (epsilon > 5.4) the decay of t he locally excited-state population is biexponential. Although dioxane has a dielectric constant of 2.2, ADMA dissolved in dioxane. exhibits biexponen tial kinetics. From this result and from the solvatochromic shift of the AD MA exciplex peak in dioxane, we have concluded that the large nonideal quad rupolar charge distribution of 1,4-dioxane is responsible for its anomalous polarity.