Absorption and Emission Spectra of Aromatic Ketones and Their Medium Dependence. Excited States of Xanthone

A detailed examination of the absorption spectra, the polarized phosphorescence spectra, and the polarized excitation spectra of xanthone (C_2v symmetry) in polar and nonpolar media has been carried out at 77°K. Five electronic transitions in the absorption region between 27,000 and 44,000 cm^-1 have been assigned. They are, in order of increasing energy: ¹A₂ (n,π*) ← ¹A₁, ¹A₁ (π,π*) ← ¹A₁, ¹B₂ (π,π*) ← ¹A₁, ¹A₁ (CT) ← ¹A₁ (tentative), and ¹A₁ (π,π*) ← ¹A₁ (tentative). Lifetime and polarization measurements, as well as vibrational features, indicate that the configuration of the lowest triplet state is nπ* in the 3-methylpentane (3-MP) solvent but ππ* in polar solvents. In both EPA and 3-MP the phosphorescence kinetics of xanthone are nonexponential. Time-delayed phosphorescence and excitation spectra show that the mixed kinetics in 3-MP are due to a small amount of the same phosphorescent species (3π,π*) which is dominant in EPA. Moreover, polarization spectra indicate that a small amount of the phosphorescent species (3n,π*) which is dominant in 3-MP produces the deviation from the exponential decay in EPA. This phenomenon is suggested to arise mainly from two types of solute-solvent cage configurations.