Effects of Substitution on the Phosphorescence Process of Aromatic Carbonyl Compounds

Phosphorescence and absorption spectra, phosphorescence- and excitation-polarization, spectra and phosphorescence lifetimes were measured at 77°K for benzaldehyde, acetophenone and their eight derivatives with a halogen atom, or a hydroxyl, methoxyl or amino group as a substituent at the para position. In chloro- and bromo-derivatives, the phosphorescent triplet state is of the (n,π*) type, and is mixed with the 1La(π,π*) state by direct spin-orbit interaction, as in the case of the parent carbonyls. In addition, owing to the heavy-atom effect, it gains its singlet character by the mechanism (Remark: Graphics omitted.) which accounts for the observation that the phosphorescence spectrum involves out-of-plane-polarized vibrational bands. Substitution of a more electron-donating hydroxyl, methoxyl, or amino group brings about a change of the phosphorescent state from the 3(n,π*) state to the; 3La(π,π*). The following mechanisms are shown to be important to the 3La(π,π*) state: (Remark: Graphics omitted.) These account for two progressions with mutually perpendicular polarizations observed in the phosphorescence spectrum. The lifetimes for the transitions from the 3La(π,π*) state to the ground state of the substituted carbonyls were found to be substantially shorter than the corresponding lifetimes in aromatic hydrocarbons. This fact is quantitatively interpreted as due to the presence of a low-lying 1(n,π*) state.